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An overview of the units of measurement used inthe Steam and Condensate Loop includingtemperature, pressure, density, volume, heat, workand energy.

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Engineering Units

Throughout the engineering industries, many different definitions and units have been proposed and usedfor mechanical and thermal properties.

The problems this caused led to the development of an agreed international system of units (or SI units:Systme International d'Units). In the SI system there are seven well-defined base units from which theunits of other properties can be derived, and these will be used throughout the Steam Engineering Tutorials.

The SI base units include length (in metres), mass (in kilograms), time (in seconds) and temperature (inkelvin). The first three will hopefully need no further explanation, while the latter will be discussed in moredetail later.

The other SI base units are electric current (in amperes), amount of substance (in moles) and luminousintensity (in candela). These may be familiar to readers with a background in electronics, chemistry andphysics respectively, but have little relevance to steam engineering nor the contents of these tutorials.

Table 2.1.1 shows the derived units that are relevant to this subject, all of which should be familiar to thosewith any general engineering background. Some of these quantities have been assigned special namesafter famous pioneers in the development of science and engineering.

Table 2.1.1Named quantities in derived SI units

There are many other quantities that have been derived from SI base units, which will also be of significanceto anyone involved in steam engineering. These are provided in Table 2.1.2.

Steam EngineeringPrinciples and HeatTransferEngineering Units

What is Steam?

Superheated Steam

Steam Quality

Heat Transfer

Methods of Estimating SteamConsumption

Measurement of SteamConsumption

Thermal Rating

Energy Consumption of Tanksand Vats

Heating with Coils and Jackets

Heating Vats and Tanks bySteam Injection

Steam Consumption of Pipesand Air Heaters

Steam Consumption of HeatExchangers

Steam Consumption of PlantItems

Entropy - A BasicUnderstanding

Entropy - Its Practical Use

Related ContentUnit ConvertersA comprehensive list ofconversion units.

Steam TablesA comprehensive set ofsteam tables is availablehere.

The Steam andCondensate Loop BookA comprehensive bestpractice guide to savingenergy and optimising plantperformance, this bookcovers all aspects of steamand condensate systems.

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Table 2.1.2Other quantities in derived SI units

Dot notationThis convention is used to identify a compound unit incorporating rate, for example:

m = Mass (e.g. kg) = Mass flow per time unit (e.g. kg/h) = Mass flowrate

Multiples and submultiplesTable 2.1.3 gives the SI prefixes that are used to form decimal multiples and submultiples of SI units. Theyallow very large or very small numerical values to be avoided. A prefix attaches directly to the name of a unit,and a prefix symbol attaches directly to the symbol for a unit.

In summary: one thousand metres may be shown as 1 km, 1000 m or 10 m.

Table 2.1.3Multiples and submultiples used with SI units

Special abbreviations used in steam flowmetering applicationsFor historical reasons, International Standard ISO 5167 (supersedes BS 1042) which refers to flowmetering,use the following abbreviations in Table 2.1.4.

Table 2.1.4Symbols used in flowmetering applications

STP - Standard temperature and pressureThese are the standard conditions for measurement of the properties of matter. The standard temperature isthe freezing point of pure water, 0C or 273.16K. The standard pressure is the pressure exerted by acolumn of mercury (symbol Hg) 760 mm high, often designated 760 mm Hg. This pressure is also called oneatmosphere and is equal to 1.01325 x 106 dynes per square centimetre, or approximately 14.7 lb per squareinch. The density (mass per volume) of a gas is usually reported as its value at STP. Properties that cannotbe measured at STP are measured under other conditions; usually the values obtained are thenmathematically extrapolated to their values at STP.

SymbolsTable 2.1.5 shows the symbols and typical units used in the Steam Engineering Tutorials.

Table 2.1.5Symbols and units of measure used in

the Steam Engineering Tutorials

Subscripts used with propertiesWhen using enthalpy, entropy and internal energy, subscripts as shown below are used to identify thephase, for example:

Subscript f = Fluid or liquid state, for example hf: liquid enthalpy

Subscript fg = Change of state liquid to gas, for example hfg: enthalpy of evaporation

Subscript g = Total, for example hg: total enthalpy

Note that, by convention, the total heat in superheated steam is signified by h.

It is also usual, by convention, to signify sample quantities in capital letters, whilst unit quantities are signifiedin lower case letters.

For example:Total enthalpy in a sample of superheated steam - H kJSpecific enthalpy of superheated steam - h kJ/kg

TemperatureThe temperature scale is used as an indicator of thermal equilibrium, in the sense that any two systems incontact with each other with the same value are in thermal equilibrium.

The Celsius (C) scaleThis is the scale most commonly used by the engineer, as it has a convenient (but arbitrary) zerotemperature, corresponding to the temperature at which water will freeze.

The absolute or K (kelvin) scaleThis scale has the same increments as the Celsius scale, but has a zero corresponding to the minimumpossible temperature when all molecular and atomic motion has ceased. This temperature is often referredto as absolute zero (0 K) and is equivalent to -273.16C.

The two scales of temperature are interchangeable, as shown in Figure 2.1.1 and expressed in Equation2.1.1.

Fig. 2.1.1Comparison of kelvin and Celsius temperatures

Equation 2.1.1

The SI unit of temperature is the kelvin, which is defined as 1 273.16 of the thermodynamic temperature ofpure water at its triple point (0C). An explanation of triple point is given in Tutorial 2.2.

Most thermodynamic equations require the temperature to be expressed in kelvin. However, temperaturedifference, as used in many heat transfer calculations, may be expressed in either C or K. Since both scaleshave the same increments, a temperature difference of 1C has the same value as a temperature differenceof 1 K.

PressureThe SI unit of pressure is the pascal (Pa), defined as 1 newton of force per square metre (1 N/m). As Pa issuch a small unit the kPa (1 kilonewton/m) or MPa (1 Meganewton/m) tend to be more appropriate to steamengineering.

However, probably the most commonly used metric unit for pressure measurement in steam engineering isthe bar. This is equal to 105 N/m, and approximates to 1 atmosphere. This unit is used throughout thesetutorials.

Other units often used include lb/in (psi), kg/cm, atm, in H2O and mm Hg. Conversion factors are readilyavailable from many sources.

Fig. 2.1.2Comparison of absolute and gauge pressures

Absolute pressure (bar a)This is the pressure measured from the datum of a perfect vacuum i.e. a perfect vacuum has a pressure of 0bar a.

Gauge pressure (bar g)This is the pressure measured from the datum of the atmospheric pressure. Although in reality theatmospheric pressure will depend upon the climate and the height above sea level, a generally acceptedvalue of 1.013 25 bar a (1 atm) is often used. This is the average pressure exerted by the air of the earth'satmosphere at sea level.

Gauge pressure = Absolute pressure - Atmospheric pressure

Pressures above atmospheric will always yield a positive gauge pressure. Conversely a vacuum or negativepressure is the pressure below that of the atmosphere. A pressure of -1 bar g corresponds closely to aperfect vacuum.

Differential pressureThis is simply the difference between two pressures. When specifying a differential pressure, it is notnecessary to use the suffixes 'g' or 'a' to denote either gauge pressure or absolute pressure respectively, asthe pressure datum point becomes irrelevant.

Therefore, the difference between two pressures will have the same value whether these pressures aremeasured in gauge pressure or absolute pressure, as long as the two pressures are measured from thesame datum.

Density and specific volumeThe density of a substance can be defined as its mass (m) per unit volume (V). The specific volume (vg) isthe volume per unit mass and is therefore the inverse of density. In fact, the term 'specific' is generally usedto denote a property of a unit mass of a substance (see Equation 2.1.2).

Equation 2.1.2

Where:

= Density(kg/m)m = Mass (kg)V = Volume(m)vg = Specific volume (m/kg)The SI units of density () are kg/m, whilst conversely the units of specific volume (vg) are m/kg.

Another term used as a measure of density is specific gravity. It is a ratio of the density of a substance (s)and the density of pure water (w) at standard temperature and pressure (STP). This reference condition is

usually defined as being at atmospheric pressure and 0C. Sometimes it is said to be at 20C or 25C and isreferred to as normal temperature and pressure (NTP).

Equation 2.1.3

The density of water at these conditions is approximately 1 000 kg/m. Therefore substances with a densitygreater than this value will have a specific gravity greater than 1, whereas substances with a density lessthan this will have a specific gravity of less than 1.

Since specific gravity is a ratio of two densities, it is a dimensionless variable and has no units. Therefore inthis case the term specific does not indicate it is a property of a unit mass of a substance. The specific gravityis also sometimes known as the relative density of a substance.

Heat, work and energyEnergy is sometimes described as the ability to do work. The transfer of energy by means of mechanicalmotion is called work. The SI unit for work and energy is the joule, defined as 1 N m.

The amount of mechanical work done can be determined by an equation derived from Newtonianmechanics:

Work = Force x Displacement

It can also be described as the product of the applied pressure and the displaced volume:

Work = Applied pressure x Displaced volume

Example 2.1.1An applied pressure of 1 Pa (or 1 N/m) displaces a volume of 1 m. How much work has been done ?

Work done = 1 N/m x 1 m = 1 N m (or 1 J)

The benefits of using SI units, as in the above example, is that the units in the equation actually cancel out togive the units of the product.

The experimental observations of J. P. Joule established that there is an equivalence between mechanicalenergy (or work) and heat. He found that the same amount of energy was required to produce the sametemperature rise in a specific mass of water, regardless of whether the energy was supplied as heat or work.

The total energy of a system is composed of the internal, potential and kinetic energy. The temperature of asubstance is directly related to its internal energy (ug). The internal energy is associated with the motion,interaction and bonding of the molecules within a substance. The external energy of a substance isassociated with its velocity and location, and is the sum of its potential and kinetic energy.

The transfer of energy as a result of the difference in temperature alone is referred to as heat flow. The watt,which is the SI unit of power, can be defined as 1 J/s of heat flow.

Other units used to quantify heat energy are the British Thermal Unit (Btu: the amount of heat to raise 1 lb ofwater by 1F) and the kilocalorie (the amount of heat to raise 1 kg of water by 1C). Conversion factors arereadily available from numerous sources.

Specific enthalpyThis is the term given to the total energy, due to both pressure and temperature, of a fluid (such as water orsteam) at any given time and condition. More specifically it is the sum of the internal energy and the workdone by an applied pressure (as in Example 2.1.1).

The basic unit of measurement is the joule (J). Since one joule represents a very small amount of energy, itis usual to use kilojoules (kJ) (1 000 Joules).

The specific enthalpy is a measure of the total energy of a unit mass, and its units are usually kJ/kg.

Specific heat capacityThe enthalpy of a fluid is a function of its temperature and pressure. The temperature dependence of theenthalpy can be found by measuring the rise in temperature caused by the flow of heat at constant pressure.The constant-pressure heat capacity cp, is a measure of the change in enthalpy at a particular temperature.

Similarly, the internal energy is a function of temperature and specific volume. The constant-volume heatcapacity cv, is a measure of the change in internal energy at a particular temperature and constant volume.

Because the specific volumes of solids and liquids are generally smaller, then unless the pressure is extremely high, the work done by an applied pressure can be neglected. Therefore, if the enthalpy can berepresented by the internal energy component alone, the constant-volume and constant-pressure heatcapacities can be said to be equal.

Therefore for, solids and liquids: cp cv

Another simplification for solids and liquids assumes that they are incompressible, so that their volume isonly a function of temperature. This implies that for incompressible fluids the enthalpy and the heat capacityare also only functions of temperature.

The specific heat capacity represents the amount of energy required to raise 1 kg by 1C, and can bethought of as the ability of a substance to absorb heat. Therefore the SI units of specific heat capacity arekJ/kg K (kJ/kg C). Water has a very large specific heat capacity (4.19 kJ/kg C) compared with many fluids,which is why both water and steam are considered to be good carriers of heat.

The amount of heat energy required to raise the temperature of a substance can be determined fromEquation 2.1.4.

Equation 2.1.4

Where:

Q = Quantity of energy (kJ)m = Mass of the substance(kg)cp = Specific heat capacity of the substance (kJ/kgC)T = Temperature rise of the substance (C)This equation shows that for a given mass of substance, the temperature rise is linearly related to theamount of heat provided, assuming that the specific heat capacity is constant over that temperature range.

Example 2.1.2Consider a quantity of water with a volume of 2 litres, which is raised from a temperature of 20C to 70C.

At atmospheric pressure, the density of water is approximately 1 000 kg/m. As there are 1 000 litres in 1 m,then the density can be expressed as 1 kg per litre (1 kg/l). Therefore the mass of the water is 2 kg.

The specific heat capacity for water can be taken as 4.19 kJ/kg C over low ranges of temperature.

Therefore: Q = 2 kg x 4.19 kJ/kg C x (70 - 20)C = 419 kJ

If the water was then cooled to its original temperature of 20C, it would also provide this amount of energyin the cooling application.

Entropy (S)Entropy is a measure of the degree of disorder within a system. The greater the degree of disorder, thehigher the entropy. The SI units of entropy are kJ/kg K (kJ/kg C).

In a solid, the molecules of a substance arrange themselves in an orderly structure. As the substancechanges from a solid to a liquid, or from a liquid to a gas, the arrangement of the molecules becomes moredisordered as they begin to move more freely. For any given substance the entropy in the gas phase isgreater than that of the liquid phase, and the entropy in the liquid phase is more than in the solid phase.

One characteristic of all natural or spontaneous processes is that they proceed towards a state ofequilibrium. This can be seen in the second law of thermodynamics, which states that heat cannot pass froma colder to a warmer body.

A change in the entropy of a system is caused by a change in its heat content, where the change of entropyis equal to the heat change divided by the average absolute temperature, Equation 2.1.5.

Equation 2.1.5

When unit mass calculations are made, the symbols for entropy and enthalpy are written in lower case,Equation 2.1.6.

Equation 2.1.6

To look at this in further detail, consider the following examples:

Example 2.1.3A process raises 1 kg of water from 0 to 100C (273 to 373 K) under atmospheric conditions.

Specific enthalpy at 0C (hf) = 0 kJ/kg (from steam tables)Specific enthalpy of water at 100C (hf) = 419 kJ/kg (from steam tables)Calculate the change in specific entropy

Since this is a change in specific entropy of water, the symbol 's' in Equation 2.1.6 takes the suffix 'f' tobecome sf.

Example 2.1.4A process changes 1 kg of water at 100C (373 K) to saturated steam at 100C (373 K) under atmosphericconditions.

Calculate the change in specific entropy of evaporation

Since this is the entropy involved in the change of state, the symbol 's' in Equation 2.1.6 takes the suffix 'fg' tobecome sfg.

Specific enthalpy of evaporation of steam at 100C (373 K) (hfg) = 2 258 kJ/kg (from steam tables)

Specific enthalpy of evaporation of water at 100C (373 K) (hfg) = 0 kJ/kg (from steam tables)

The total change in specific entropy from water at 0C to saturated steam at 100C is the sum of the changein specific entropy for the water, plus the change of specific entropy for the steam, and takes the suffix 'g' tobecome the total change in specific entropy sg.

Example 2.1.5A process superheats 1 kg of saturated steam at atmospheric pressure to 150C (423 K). Determine thechange in entropy.

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Equation 2.1.6

As the entropy of saturated water is measured from a datum of 0.01C, the entropy of water at 0C can, forpractical purposes, be taken as zero. The total change in specific entropy in this example is based on aninitial water temperature of 0C, and therefore the final result happens to be very much the same as thespecific entropy of steam that would be observed in steam tables at the final condition of steam atatmospheric pressure and 150C.

Entropy is discussed in greater detail in Tutorial 2.15, Entropy - A Basic Understanding, and in Tutorial 2.16,Entropy - Its Practical Use.

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The properties of steam explained here, includingthe ability of steam under pressure to carry, andthen give up, large amounts of energy. Topicsinclude saturated steam tables, dryness fractionand flash steam.

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You are here: Home Resources Steam Engineering TutorialsSteam Engineering Principles and Heat Transfer What is Steam?

What is Steam?

A better understanding of the properties of steam may be achieved by understanding the general molecularand atomic structure of matter, and applying this knowledge to ice, water and steam.

A molecule is the smallest amount of any element or compound substance still possessing all the chemicalproperties of that substance which can exist. Molecules themselves are made up of even smaller particlescalled atoms, which define the basic elements such as hydrogen and oxygen.

The specific combinations of these atomic elements provide compound substances. One such compound isrepresented by the chemical formula H2O, having molecules made up of two atoms of hydrogen and one atomof oxygen.

The reason water is so plentiful on the earth is because hydrogen and oxygen are amongst the most abundantelements in the universe. Carbon is another element of significant abundance, and is a key component in allorganic matter.

Most mineral substances can exist in the three physical states (solid, liquid and vapour) which are referred toas phases. In the case of H2O, the terms ice, water and steam are used to denote the three phasesrespectively.

The molecular arrangement of ice, water, and steam is still not fully understood, but it is convenient to considerthe molecules as bonded together by electrical charges (referred to as the hydrogen bond). The degree ofexcitation of the molecules determines the physical state (or phase) of the substance.

Triple pointAll the three phases of a particular substance can only coexist in equilibrium at a certain temperature andpressure, and this is known as its triple point.

The triple point of H2O, where the three phases of ice, water and steam are in equilibrium, occurs at atemperature of 273.16 K and an absolute pressure of 0.006 112 bar. This pressure is very close to a perfectvacuum. If the pressure is reduced further at this temperature, the ice, instead of melting, sublimates directlyinto steam.

IceIn ice, the molecules are locked together in an orderly lattice type structure and can only vibrate. In the solidphase, the movement of molecules in the lattice is a vibration about a mean bonded position where themolecules are less than one molecular diameter apart.

The continued addition of heat causes the vibration to increase to such an extent that some molecules willeventually break away from their neighbours, and the solid starts to melt to a liquid state. At atmosphericpressure, melting occurs at 0C. Changes in pressure have very little effect on the melting temperature, and formost practical purposes, 0C can be taken as the melting point. However, it has been shown that the meltingpoint of ice falls by 0.0072C for each additional atmosphere of pressure. for example, a pressure of 13.9 bar gwould be needed to reduce the melting temperature by 0.1C.

Heat that breaks the lattice bonds to produce the phase change while not increasing the temperature of theice, is referred to as enthalpy of melting or heat of fusion. This phase change phenomenon is reversible whenfreezing occurs with the same amount of heat being released back to the surroundings.

For most substances, the density decreases as it changes from the solid to the liquid phase. However, H2O isan exception to this rule as its density increases upon melting, which is why ice floats on water.


WaterIn the liquid phase, the molecules are free to move, but are still less than one molecular diameter apart due tomutual attraction, and collisions occur frequently. More heat increases molecular agitation and collision,raising the temperature of the liquid up to its boiling temperature.

Enthalpy of water, liquid enthalpy or sensible heat (hf) of waterThis is the heat energy required to raise the temperature of water from a datum point of 0C to its currenttemperature.

At this reference state of 0C, the enthalpy of water has been arbitrarily set to zero. The enthalpy of all otherstates can then be identified, relative to this easily accessible reference state.

Sensible heat was the term once used, because the heat added to the water produced a change intemperature. However, the accepted terms these days are liquid enthalpy or enthalpy of water.

At atmospheric pressure (0 bar g), water boils at 100C, and 419 kJ of energy are required to heat 1 kg ofwater from 0C to its boiling temperature of 100C. It is from these figures that the value for the specific heatcapacity of water (cp) of 4.19 kJ/kg C is derived for most calculations between 0C and 100C.

SteamAs the temperature increases and the water approaches its boiling condition, some molecules attain enoughkinetic energy to reach velocities that allow them to momentarily escape from the liquid into the space abovethe surface, before falling back into the liquid.

Further heating causes greater excitation and the number of molecules with enough energy to leave the liquidincreases. As the water is heated to its boiling point, bubbles of steam form within it and rise to break throughthe surface.

Considering the molecular arrangement of liquids and vapours, it is logical that the density of steam is muchless than that of water, because the steam molecules are further apart from one another. The spaceimmediately above the water surface thus becomes filled with less dense steam molecules.

When the number of molecules leaving the liquid surface is more than those re-entering, the water freelyevaporates. At this point it has reached boiling point or its saturation temperature, as it is saturated with heatenergy.

If the pressure remains constant, adding more heat does not cause the temperature to rise any further butcauses the water to form saturated steam. The temperature of the boiling water and saturated steam within thesame system is the same, but the heat energy per unit mass is much greater in the steam.

At atmospheric pressure the saturation temperature is 100C. However, if the pressure is increased, this willallow the addition of more heat and an increase in temperature without a change of phase.

Therefore, increasing the pressure effectively increases both the enthalpy of water, and the saturationtemperature. The relationship between the saturation temperature and the pressure is known as the steamsaturation curve (see Figure 2.2.1).

Fig. 2.2.1Steam saturation curve

Water and steam can coexist at any pressure on this curve, both being at the saturation temperature. Steam ata condition above the saturation curve is known as superheated steam:

Temperature above saturation temperature is called the degree of superheat of the steam.Water at a condition below the curve is called sub-saturated water.

If the steam is able to flow from the boiler at the same rate that it is produced, the addition of further heat simplyincreases the rate of production. If the steam is restrained from leaving the boiler, and the heat input rate ismaintained, the energy flowing into the boiler will be greater than the energy flowing out. This excess energyraises the pressure, in turn allowing the saturation temperature to rise, as the temperature of saturated steamcorrelates to its pressure.

Enthalpy of evaporation or latent heat (hfg)This is the amount of heat required to change the state of water at its boiling temperature, into steam. Itinvolves no change in the temperature of the steam/water mixture, and all the energy is used to change thestate from liquid (water) to vapour (saturated steam).

The old term latent heat is based on the fact that although heat was added, there was no change intemperature. However, the accepted term is now enthalpy of evaporation.

Like the phase change from ice to water, the process of evaporation is also reversible. The same amount ofheat that produced the steam is released back to its surroundings during condensation, when steam meetsany surface at a lower temperature.

This may be considered as the useful portion of heat in the steam for heating purposes, as it is that portion ofthe total heat in the steam that is extracted when the steam condenses back to water.

Enthalpy of saturated steam, or total heat of saturated steamThis is the total energy in saturated steam, and is simply the sum of the enthalpy of water and the enthalpy ofevaporation.

Equation 2.2.1

Where:

hg = Total enthalpy of saturated steam (Total heat) (kJ/kg)hf = Liquid enthalpy (Sensible heat) (kJ/kg)hfg = Enthalpy of evaporation (Latent heat) (kJ/kg)The enthalpy (and other properties) of saturated steam can easily be referenced using the tabulated results ofprevious experiments, known as steam tables.

The saturated steam tablesThe steam tables list the properties of steam at varying pressures. They are the results of actual tests carriedout on steam. Table 2.2.1 shows the properties of dry saturated steam at atmospheric pressure - 0 bar g.

Table 2.2.1Properties of saturated steam at atmospheric pressure

Example 2.2.1At atmospheric pressure (0 bar g), water boils at 100C, and 419 kJ of energy are required to heat 1 kg ofwater from 0C to its saturation temperature of 100C. Therefore the specific enthalpy of water at 0 bar g and100C is 419 kJ/kg, as shown in the steam tables (see Table 2.2.2).

Another 2 257 kJ of energy are required to evaporate 1 kg of water at 100C into 1 kg of steam at 100C.Therefore at 0 bar g the specific enthalpy of evaporation is 2 257 kJ/kg, as shown in the steam tables (seeTable 2.2.2).

However, steam at atmospheric pressure is of a limited practical use. This is because it cannot be conveyedunder its own pressure along a steam pipe to the point of use.

Note: Because of the pressure/volume relationship of steam, (volume is reduced as pressure is increased) it isusually generated in the boiler at a pressure of at least 7 bar g. The generation of steam at higher pressuresenables the steam distribution pipes to be kept to a reasonable size.

As the steam pressure increases, the density of the steam will also increase. As the specific volume isinversely related to the density, the specific volume will decrease with increasing pressure.

Figure 2.2.2 shows the relationship of specific volume to pressure. This highlights that the greatest change inspecific volume occurs at lower pressures, whereas at the higher end of the pressure scale there is much lesschange in specific volume.

Fig. 2.2.2Steam pressure/specific volume relationship

The extract from the steam tables shown in Table 2.2.2 shows specific volume, and other data related tosaturated steam.

At 7 bar g, the saturation temperature of water is 170C. More heat energy is required to raise its temperatureto saturation point at 7 bar g than would be needed if the water were at atmospheric pressure. The table givesa value of 721 kJ to raise 1 kg of water from 0C to its saturation temperature of 170C.

The heat energy (enthalpy of evaporation) needed by the water at 7 bar g to change it into steam is actuallyless than the heat energy required at atmospheric pressure. This is because the specific enthalpy ofevaporation decreases as the steam pressure increases.

However, as the specific volume also decreases with increasing pressure, the amount of heat energytransferred in the same volume actually increases with steam pressure.

Table 2.2.2Extract from the saturated steam tables

Dryness fractionSteam with a temperature equal to the boiling point at that pressure is known as dry saturated steam.However, to produce 100% dry steam in an industrial boiler designed to produce saturated steam is rarelypossible, and the steam will usually contain droplets of water.

In practice, because of turbulence and splashing, as bubbles of steam break through the water surface, thesteam space contains a mixture of water droplets and steam.

Steam produced in any shell-type boiler (see Block 3), where the heat is supplied only to the water and wherethe steam remains in contact with the water surface, may typically contain around 5% water by mass.

If the water content of the steam is 5% by mass, then the steam is said to be 95% dry and has a drynessfraction of 0.95.

The actual enthalpy of evaporation of wet steam is the product of the dryness fraction () and the specificenthalpy (hfg) from the steam tables. Wet steam will have lower usable heat energy than dry saturated steam.

Equation 2.2.2

Therefore:

Equation 2.2.3

Because the specfic volume of water is several orders of magnitude lower than that of steam, the droplets ofwater in wet steam will occupy negligible space. Therefore the specific volume of wet steam will be less thandry steam:

Equation 2.2.4

Where vg is the specific volume of dry saturated steam.

Example 2.2.2Steam at a pressure of 6 bar g having a dryness fraction of 0.94 will only contain 94% of the enthalpy ofevaporation of dry saturated steam at 6 bar g. The following calculations use figures from steam tables:

The steam phase diagramThe data provided in the steam tables can also be expressed in a graphical form. Figure 2.2.3 illustrates therelationship between the enthalpy and temperature of the various states of water and steam; this is known as aphase diagram.

Fig. 2.2.3Temperature enthalpy phase diagram

As water is heated from 0C to its saturation temperature, its condition follows the saturated water line until ithas received all of its liquid enthalpy, hf, (A - B).

If further heat continues to be added, the water changes phase to a water/vapour mixture and continues toincrease in enthalpy while remaining at saturation temperature ,hfg, (B - C).

As the water/vapour mixture increases in dryness, its condition moves from the saturated liquid line to thesaturated vapour line. Therefore at a point exactly halfway between these two states, the dryness fraction () is0.5. Similarly, on the saturated steam line the steam is 100% dry.

Once it has received all of its enthalpy of evaporation, it reaches the saturated steam line. If it continues to beheated after this point, the pressure remains constant but the temperature of the steam will begin to rise assuperheat is imparted (C - D).

The saturated water and saturated steam lines enclose a region in which a water/vapour mixture exists - wetsteam. In the region to the left of the saturated water line only water exists, and in the region to the right of thesaturated steam line only superheated steam exists.

The point at which the saturated water and saturated steam lines meet is known as the critical point. As thepressure increases towards the critical point the enthalpy of evaporation decreases, until it becomes zero atthe critical point. This suggests that water changes directly into saturated steam at the critical point.

Above the critical point the steam may be considered as a gas. The gaseous state is the most diffuse state inwhich the molecules have an almost unrestricted motion, and the volume increases without limit as thepressure is reduced.

The critical point is the highest temperature at which water can exist. Any compression at constant temperatureabove the critical point will not produce a phase change.

Compression at constant temperature below the critical point however, will result in liquefaction of the vapour

as it passes from the superheated region into the wet steam region.

The critical point occurs at 374.15C and 221.2 bar a for steam. Above this pressure the steam is termedsupercritical and no well-defined boiling point applies.

Flash steamThe term 'flash steam' is traditionally used to describe steam issuing from condensate receiver vents andopen-ended condensate discharge lines from steam traps. How can steam be formed from water withoutadding heat?

Flash steam occurs whenever water at high pressure (and a temperature higher than the saturationtemperature of the low-pressure liquid) is allowed to drop to a lower pressure. Conversely, if the temperatureof the high-pressure water is lower than the saturation temperature at the lower pressure, flash steam cannotbe formed. In the case of condensate passing through a steam trap, it is usually the case that the upstreamtemperature is high enough to form flash steam. See Figure 2.2.4.

Fig. 2.2.4Flash steam formed because T1 > T2

Consider a kilogram of condensate at 5 bar g and a saturation temperature of 159C passing through a steamtrap to a lower pressure of 0 bar g. The amount of energy in one kilogram of condensate at saturationtemperature at 5 bar g is 671 kJ. In accordance with the first law of thermodynamics, the amount of energycontained in the fluid on the low-pressure side of the steam trap must equal that on the high-pressure side,and constitutes the principle of conservation of energy.

Consequently, the heat contained in one kilogram of low-pressure fluid is also 671 kJ. However, water at 0 barg is only able to contain 419 kJ of heat, subsequently there appears to be an imbalance of heat on the low-pressure side of 671 - 419 = 252 kJ, which, in terms of the water, could be considered as excess heat.

This excess heat boils some of the condensate into what is known as flash steam and the boiling process iscalled flashing. Therefore, the one kilogram of condensate which existed as one kilogram of liquid water onthe high pressure side of the steam trap now partly exists as both water and steam on the low-pressure side.

The amount of flash steam produced at the final pressure (P2) can be determined using Equation 2.2.5:

Equation 2.2.5

Where:

P1 = Initial pressureP2 = Final pressurehf = Liquid enthalpy (kJ/kg)hfg = Enthalpy of evaporation (kJ/kg)Example 2.2.3 The case where the high pressure condensate temperature is higherthan the low pressure saturation temperatureConsider a quantity of water at a pressure of 5 bar g, containing 671 kJ/kg of heat energy at its saturationtemperature of 159C. If the pressure was then reduced down to atmospheric pressure (0 bar g), the watercould only exist at 100C and contain 419 kJ/kg of heat energy. This difference of 671 - 419 = 252 kJ/kg ofheat energy, would then produce flash steam at atmospheric pressure.

The proportion of flash steam produced can be thought of as the ratio of the excess energy to the enthalpy ofevaporation at the final pressure.

Example 2.2.4 The case where the high pressure condensate temperature is lowerthan the low pressure saturation temperatureConsider the same conditions as in Example 2.2.3, with the exception that the high-pressure condensate

temperature is at 90C, that is, sub-cooled below the atmospheric saturation temperature of 100C. Note: It isnot usually practical for such a large drop in condensate temperature from its saturation temperature (in thiscase 159C to 90C); it is simply being used to illustrate the point about flash steam not being produced undersuch circumstances.

In this case, the sub-saturated water table will show that the liquid enthalpy of one kilogram of condensate at 5bar g and 90C is 377 kJ. As this enthalpy is less than the enthalpy of one kilogram of saturated water atatmospheric pressure (419 kJ), there is no excess heat available to produce flash steam. The condensatesimply passes through the trap and remains in a liquid state at the same temperature but lower pressure,atmospheric pressure in this case. See Figure 2.2.5.

Fig. 2.2.5No flash steam formed because T1 < T2

The vapour pressure of water at 90C is 0.7 bar absolute. Should the lower condensate pressure have beenless than this, flash steam would have been produced.

The principles of conservation of energy and mass between two process statesThe principles of the conservation of energy and mass allow the flash steam phenomenon to be thought offrom a different direction.

Consider the conditions in Example 2.2.3.

1 kg of condensate at 5 bar g and 159C produces 0.112 kg of flash steam at atmospheric pressure. This canbe illustrated schematically in Figure 2.2.6. The total mass of flash and condensate remains at 1 kg.


An explanation of the properties and uses ofsuperheated steam (such as for electricitygeneration). Including explanations of the Rankineand Carnot thermodynamic cycles, superheatedsteam tables and the Mollier (H-S) chart.



Fig. 2.3.1Steam and force on a turbine blade

You are here: Home Resources Steam Engineering TutorialsSteam Engineering Principles and Heat Transfer Superheated Steam

Superheated Steam

If the saturated steam produced in a boiler is exposed to a surface with a higher temperature, its temperaturewill increase above the evaporating temperature.

The steam is then described as superheated by the number of temperature degrees through which it hasbeen heated above saturation temperature.

Superheat cannot be imparted to the steam whilst it is still in the presence of water, as any additional heatsimply evaporates more water. The saturated steam must be passed through an additional heat exchanger.This may be a second heat exchange stage in the boiler, or a separate superheater unit. The primaryheating medium may be either the hot flue gas from the boiler, or may be separately fired.

Superheated steam has its applications in, for example, turbineswhere the steam is directed by nozzles onto a rotor. This causes therotor to turn. The energy to make this happen can only have comefrom the steam, so logically the steam has less energy after it hasgone through the turbine rotor. If the steam was at saturationtemperature, this loss of energy would cause some of the steam tocondense.

Turbines have a number of stages; the exhaust steam from the firstrotor will be directed to a second rotor on the same shaft. This meansthat saturated steam would get wetter and wetter as it went throughthe successive stages. Not only would this promote waterhammer,but the water particles would cause severe erosion within the turbine. The solution is to supply the turbinewith superheated steam at the inlet, and use the energy in the superheated portion to drive the rotor until thetemperature/pressure conditions are close to saturation; and then exhaust the steam.

Another very important reason for using superheated steam in turbines is to improve thermal efficiency.

The thermodynamic efficiency of a heat engine such as a turbine, may be determined using one of twotheories:

The Carnot cycle, where the change in temperature of the steam between the inlet and outlet iscompared to the inlet temperature.The Rankine cycle, where the change in heat energy of the steam between the inlet and outlet iscompared to the total energy taken from the steam.

Note: The values used for the temperature and energy content in the following examples are from steamtables.

Example 2.3.1A turbine is supplied with superheated steam at 90 bar a @ 450C.The exhaust is at 0.06 bar a (partial vacuum) and 10% wet.Saturated temperature = 36.2C.

2.3.1.1 Determine the Carnot efficiency (C)

Equation 2.3.1


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2.3.1.2 Determine the Rankine efficiency (R)For the theoretical Rankine Cycle, Figure 2.3.2, it is assumed that there are no frictional losses in the turbine,perfect expansion of the steam occurs in the turbine (isentropic), and ignores energy added by thefeedpump returning condensate to the boiler.

Fig. 2.3.2 Basic Rankine cycle

Using Example 2.3.1 where:- A turbine is supplied with superheated steam at 90 bar a @ 450C. - The exhaust is at 0.06 bar a (partial vacuum) and 10% wet. - Saturated temperature = 36.2C.This data can be plotted on the temperature / enthalpy curve as illustrated in Figure 2.3.3:

Fig. 2.3.3 Temperature / enthalpy curve + figures from example 2.3.1

Equation 2.3.2

Examination of the figures for either of the cycles indicates that to achievehigh efficiency:The temperature or energy at the turbine inlet should be as high as possible. This means as high apressure and temperature as is practically possible.

Superheated steam is the simplest way of providing this.The temperature or energy in the exhaust must be as low as possible. This means as low a pressureand temperature as is practically possible, and is usually achieved by a condenser on the turbineexhaust.

Notes:

The figures calculated in Examples 2.3.1.1 and 2.3.1.2 are for thermodynamic efficiency, and mustnot be confused with mechanical efficiency.Although the efficiency figures appear to be very low, they must not be viewed in isolation, but ratherused to compare one type of heat engine with another. For example, gas turbines, steam enginesand diesel engines.

Superheated steam tablesThe superheated steam tables display the properties of steam at various pressures in much the same wayas the saturated steam tables. However, with superheated steam there is no direct relationship betweentemperature and pressure. Therefore at a particular pressure it may be possible for superheated steam toexist at a wide range of temperatures.

In general, saturated steam tables give gauge pressure, superheated steam tables give absolute pressure.

Table 2.3.1Extract from superheated steam tables

Example 2.3.2How much more heat does superheated steam with a temperature of 400C and a pressure of 1.013 bar a (0bar g) have than saturated steam at the same pressure?

hg for saturated steam at 1.013 bar a = 2 676 kJ/kg (from saturated steam tables)hg for steam at 1.013 bar a and 400C = 3 278 kJ/kg (from superheated steam tables)Enthalpy in the superheat = 3 278 kJ/kg - 2 676 kJ/kg:Enthalpy in the superheat = 602 kJ/kgThis may sound a useful increase in energy, but in fact it will actually make life more difficult for the engineerwho wants to use steam for heating purposes.

From the energy in the superheat shown, the specific heat capacity can be determined by dividing this valueby the temperature difference between saturation temperature (100C) and the superheated steamtemperature (400C):

However, unlike the specific heat capacity of water, the specific heat capacity for superheated steam variesconsiderably with pressure and temperature and cannot be taken as a constant.

The value of 2.0 kJ/kg C given above is therefore only the mean specific heat capacity over the specifiedtemperature range for that pressure.

There is no direct relationship between temperature, pressure and the specific heat capacity of superheatedsteam. There is, however, a general trend towards an increase in specific heat capacity with increasingpressure at low degrees of superheat, but this is not always the case.

Typical value range: 2.0 kJ/kg C at 125C and 1.013 bar a (0 bar g)3.5 kJ/kg C at 400C and 120 bar a.

Can superheated steam be used in process heat exchangers and other heatingprocesses?Although not the ideal medium for transferring heat, superheated steam is sometimes used for processheating in many steam plants around the world, especially in the HPIs (Hydrocarbon Processing Industries)which produce oils and petrochemicals. This is more likely to be because superheated steam is alreadyavailable on site for power generation, being the preferred energy source for turbines, rather than because ithas any advantage over saturated steam for heating purposes. To be clear on this point, in most cases,saturated steam should be used for heat transfer processes, even if it means desuperheating the steam todo so. HPIs often desuperheat steam to within about ten degrees of superheat. This small degree ofsuperheat is removed readily in the first part of the heating surface. Greater amounts of superheat are more

difficult, and often uneconomic to deal with and (for heating purposes) are best avoided.

There are quite a few reasons why superheated steam is not as suitable for process heating as saturatedsteam:

Superheated steam has to cool to saturation temperature before it can condense to release its enthalpy ofevaporation. The amount of heat given up by the superheated steam as it cools to saturation temperature isrelatively small in comparison to its enthalpy of evaporation.

If the steam has only a few degrees of superheat, this small amount of heat is quickly given up before itcondenses. However, if the steam has a large degree of superheat, it may take a relatively long time to cool,during which time the steam is releasing very little energy.

Unlike saturated steam, the temperature of superheated steam is not uniform. Superheated steam has tocool to give up heat, whilst saturated steam changes phase. This means that temperature gradients over theheat transfer surface may occur with superheated steam.

In a heat exchanger, use of superheated steam can lead to the formation of a dry wall boiling zone, close tothe tube sheet. This dry wall area can quickly become scaled or fouled, and the resulting high temperature ofthe tube wall may cause tube failure.

This clearly shows that in heat transfer applications, steam with a large degree of superheat is of little usebecause it:

Gives up little heat until it has cooled to saturation temperature.Creates temperature gradients over the heat transfer surface as it cools to saturation temperature.Provides lower rates of heat transfer whilst the steam is superheated.Requires larger heat transfer areas.

So, superheated steam is not as effective as saturated steam for heat transfer applications. This may seemstrange, considering that the rate of heat transfer across a heating surface is directly proportional to thetemperature difference across it. If superheated steam has a higher temperature than saturated steam at thesame pressure, surely superheated steam should be able to impart more heat? The answer to this is 'no'.This will now be looked at in more detail.

It is true that the temperature difference will have an effect on the rate of heat transfer across the heat transfersurface, as clearly shown by Equation 2.5.3.

Equation 2.5.3

Where:

= Heat transferred per unit time (W)U = Overall thermal transmittance (heat transfer coefficient) (W/m2C)A = Heat transfer area (m2)T = Temperature difference between primary and secondary fluid (C)Equation 2.5.3 also shows that heat transfer will depend on the overall heat transfer coefficient 'U', and theheat transfer area 'A'.

For any single application, the heat transfer area might be fixed. However, the same cannot be said of the 'U'value; and this is the major difference between saturated and superheated steam. The overall 'U' value forsuperheated steam will vary throughout the process, but will always be much lower than that for saturatedsteam. It is difficult to predict 'U' values for superheated steam, as these will depend upon many factors, butgenerally, the higher the degree of superheat, the lower the 'U' value.

Typically, for a horizontal steam coil surrounded with water, 'U' values might be as low as 50 to 100 W/m2)Cfor superheated steam but 1 200 W/m2)C for saturated steam, as depicted in Figure 2.3.4.

For steam to oil applications, the 'U' values might be considerably less, perhaps as low as 20 W/m2)C forsuperheated steam and 150 W/m2)C for saturated steam.

In a shell and tube heat exchanger, 100 W/m2)C for superheated steam and 500 W/m2)C for saturatedsteam can be expected. These figures are typical; actual figures will vary due to other design andoperational considerations.

Figure 2.3.4Typical U values for superheatedand saturated steam coils in water

Although the temperature of superheated steam is always higher than saturated steam at the samepressure, its ability to transfer heat is therefore much lower. The overall effect is that superheated steam ismuch less effective at transferring heat than saturated steam at the same pressure. The next Section'Fouling' gives more detail.

Not only is superheated steam less effective at transferring heat, it is very difficult to quantify using Equation2.5.3, = U A T, as the temperature of the steam will fall as it gives up its heat while passing along theheating surface.

Predicting the size of heat transfer surfaces utilising superheated steam is difficult and complex. In practice,the basic data needed to perform such calculations is either not known or empirically obtained, putting theirreliability and accuracy in doubt.

Clearly, as superheated steam is less effective at transferring heat than saturated steam, then any heatingarea using superheated steam would have to be larger than a saturated steam coil operating at the samepressure to deliver the same heat flowrate.

If there is no choice but to use superheated steam, it is not possible to maintain steam in its superheatedstate throughout the heating coil or heat exchanger, since as it gives up some of its heat content to thesecondary fluid, it cools towards saturation temperature. The amount of heat above saturation is quite smallcompared with the large amount available as condensation occurs.

The steam should reach saturation relatively soon in the process; this allows the steam to condense toproduce higher heat transfer rates and result in a higher overall 'U' value for the whole coil, see Figure 2.3.5.

To help to enable this, superheated steam used for heat transfer purposes should not hold more than about10C of superheat.

Figure 2.3.5Less superheat allows the steam to condense

in the major part of the coil thus increasingthe overall U value approaching that of saturated steam.

If this is so, it is relatively easy and practical to design a heat exchanger or a coil with a heating surface areabased upon saturated steam at the same pressure, by adding on a certain amount of surface area to allowfor the superheat. Using this guideline, the first part of a coil will be used purely to reduce the temperature ofsuperheated steam to its saturation point. The rest of the coil will then be able to take advantage of thehigher heat transfer ability of the saturated steam. The effect is that the overall 'U' value may not be muchless than if saturated steam were supplied to the coil.

From practical experience, if the extra heating area needed for superheated steam is 1% per 2C ofsuperheat, the coil (or heat exchanger) will be large enough. This seems to work up to 10C of superheat. It

is not recommended that superheated steam above 10C of superheat be used for heating purposes due tothe probable disproportionate and uneconomic size of the heating surface, the propensity for fouling by dirt,and the possibility of product spoilage by the high and uneven superheat temperatures.

FoulingFouling is caused by deposits building up on the heat transfer surface adding a resistance to heat flow.Many process liquids can deposit sludge or scale on heating surfaces, and will do so at a faster rate athigher temperatures. Further, superheated steam is a dry gas. Heat flowing from the steam to the metal wallmust pass through the static films adhering to the wall, which resist heat flow.

By contrast, the condensation of saturated steam causes the movement of steam towards the wall, and therelease of large quantities of latent heat right at the condensing surface. The combination of these factorsmeans that the overall heat transfer rates are much lower where superheated steam is present, even thoughthe temperature difference between the steam and the secondary fluid is higher.

Example 2.3.3 Sizing a tube bundle for superheated steamSuperheated steam at 3 bar g with 10C of superheat (154C) is to be used as the primary heat source for ashell and tube process heat exchanger with a heating load of 250 kW, heating an oil based fluid from 80Cto 120C (making the arithmetic mean secondary temperature (T AM) 100C). Estimate the area of primarysteam coil required.

(Arithmetic mean temperature differences are used to keep this calculation simple; in practice, logarithmicmean temperatures would be used for greater accuracy. Please refer to Tutorial 2.5 'Heat Transfer' for detailson arithmetic and logarithmic mean temperature differences).

First, consider the coil if it were heated by saturated steam at 3 bar g (144C).

The 'U' value for saturated steam heating oil via a new carbon steel coil is taken to be 500 W/m2C.

Other applications using superheated steamAll the above applies when steam is flowing through a relatively narrow passage, such as the tubes in ashell and tube heat exchanger or the plates in a plate heat exchanger.

In some applications, perhaps a drying cylinder in a paper machine, superheated steam is admitted to agreater volume, when its velocity plummets to very small values. Here, the steam near the wall of thecylinder quickly drops in temperature to near saturation and condensation begins. The heat flow through thewall is then the same as if the cylinder were supplied with saturated steam. Superheat is present only withinthe 'core' in the steam space and has no discernible effect on heat transfer rates.

There are instances where the presence of superheat can actually reduce the performance of a process,where steam is being used as a process material.

One such process might involve moisture being imparted to the product from the steam as it condenses,such as, the conditioning of animal feedstuff (meal) prior to pelletising. Here the moisture provided by thesteam is an essential part of the process; superheated steam would over-dry the meal and make pelletisingdifficult.

The effects of reducing steam pressure

In addition to the use of an additional heat exchanger (generally called a 'superheater'), superheat can alsobe imparted to steam by allowing it to expand to a lower pressure as it passes through the orifice of apressure reducing valve. This is termed a throttling process with the lower pressure steam having the sameenthalpy (apart from a small amount lost to friction in passing through the valve) as the upstream highpressure steam. However, the temperature of the throttled steam will always be lower than that of the supplysteam.

The state of the throttled steam will depend upon:

The pressure of the supply steam.The state of the supply steam.The pressure drop across the valve orifice.

For supply steam below 30 bar g in the dry saturated state, any drop in pressure will produce superheatedsteam after throttling. The degree of superheat will depend on the amount of pressure reduction.

For supply steam above 30 bar g in the dry saturated state, the throttled steam might be superheated, drysaturated, or even wet, depending on the amount of pressure drop. For example, dry saturated steam at 60bar g would have to be reduced to approximately 10.5 bar g to produce dry saturated steam. Any less of apressure drop will produce wet steam, while any greater pressure drop would produce superheated steam.

Equally, the state of the supply steam at any pressure will influence the state of the throttled steam. Forexample, wet steam at a pressure of 10 bar g and 0.95 dryness fraction would need to be reduced to 0.135bar g to produce dry saturated steam. Any less of a pressure drop would produce wet steam while anygreater pressure drop would superheat the throttled steam.

Example 2.3.4 Increasing the dryness of wet steam with a control valveSteam with a dryness fraction () of 0.95 is reduced from 6 bar g to 1 bar g, using a pressure reducing valve.

Determine the steam conditions after the pressure reducing valve.

This quantity of heat energy is retained by the steam as the pressure is reduced to 1 bar g.

As the actual enthalpy of the steam at 1 bar g is less than the enthalpy of dry saturated steam at 1 bar g, thenthe steam is not superheated and still retains a proportion of moisture in its content.

Since the total enthalpy after the pressure reducing valve is less than the total enthalpy of steam at 1 bar g,the steam is still wet.

Example 2.3.5 Superheat created by a control valveSteam with a dryness fraction of 0.98 is reduced from 10 bar g down to 1 bar g using a pressure reducingvalve (as shown in Figure 2.3.6).

Determine the degree of superheat after the valve.

As in the previous example (2.3.3), the specific enthalpy of dry saturated steam (hg) at 1 bar g is 2 706.7kJ/kg.

The actual total enthalpy of the steam is greater than the total enthalpy (hg) of dry saturated steam at 1 bar g.The steam is therefore not only 100% dry, but also has some degree of superheat.

The excess energy = 2 741.7 - 2 706.7 = 35 kJ/kg, and this is used to raise the temperature of the steam fromthe saturation temperature of 120C to 136C.

Fig. 2.3.6The creation of superheat by pressure reduction

The degree of superheat can be determined either by using superheated steam tables, or by using a Mollierchart.

The Mollier chartThe Mollier chart is a plot of the specific enthalpy of steam against its specific entropy (sg).

Fig. 2.3.7Enthalpy - entropy or Mollier chart for steam

Figure 2.3.7 shows a simplified, small scale version of the Mollier chart. The Mollier chart displays manydifferent relationships between enthalpy, entropy, temperature, pressure and dryness fraction. It may appearto be quite complicated, due to the number of lines:

Constant enthalpy lines (horizontal).Constant entropy lines (vertical).The steam saturation curve across the centre of the chart divides it into a superheated steam region,and a wet steam region. At any point above the saturation curve the steam is superheated, and atany point below the saturation curve the steam is wet. The saturation curve itself represents thecondition of dry saturated steam at various pressures.Constant pressure lines in both regions.Constant temperature lines in the superheat region.

Constant dryness fraction () lines in the wet region.

A perfect expansion, for example within a steam turbine or a steam engine, is a constant entropy process,and can be represented on the chart by moving vertically downwards from a point representing the initialcondition to a point representing the final condition.

A perfect throttling process, for example across a pressure reducing valve, is a constant enthalpy process. Itcan be represented on the chart by moving horizontally from left to right, from a point representing the initialcondition to a point representing the final condition.

Both these processes involve a reduction in pressure, but the difference lies in the way in which this isachieved.

The two examples shown in Figure 2.3.8 illustrate the advantage of using the chart to analyse steamprocesses; they provide a pictorial representation of such processes. However, steam processes can alsobe numerically represented by the values provided in the superheated steam tables.

Fig. 2.3.8 Examples of expansion and throttling

Example 2.3.6 Perfect isentropic expansion resulting in workConsider the perfect expansion of steam through a turbine. Initially the pressure is 50 bar a, the temperatureis 300C, and the final pressure is 0.04 bar a.

As the process is a perfect expansion, the entropy remains constant. The final condition can then be foundby dropping vertically downwards from the initial condition to the 0.04 bar a constant pressure line (seeFigure 2.3.9).

At the initial condition, the entropy is approximately 6.25 kJ/kg C. If this line is followed vertically downwardsuntil 0.04 bar a is reached, the final condition of the steam can be evaluated. At this point the specificenthalpy is 1 890 kJ/kg, and the dryness fraction is 0.72 (see Figure 2.3.9).

The final condition can also be determined by using the superheated steam tables.

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Fig. 2.3.9Enthalpy - entropy or Mollier chart for steam - Example

Since the entropy of dry saturated steam at 0.04 bar a (8.473 kJ/kgC) is greater than the entropy of thesuperheated steam at 50 bar a/300C (6.212 kJ/kgC), it follows that some of the dry saturated steam musthave condensed to maintain the constant entropy.

As the entropy remains constant, at the final condition:

Note: The values used for the temperature and energy content in the following examples are from steamtables.

These answers correspond closely with the results obtained using the Mollier chart. The small difference invalue between the two sets of results is to be expected, considering the inaccuracies involved in reading offa chart such as this.

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Steam should be available at the point of use in thecorrect quantity, at the correct pressure, clean,dry and free from air and other incondensablegases. This tutorial explains why this is necessary,and how steam quality is assured.

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You are here: Home Resources Steam Engineering TutorialsSteam Engineering Principles and Heat Transfer Steam Quality

Steam Quality

Steam should be available at the point of use:

In the correct quantityAt the correct temperature and pressureFree from air and incondensable gasesCleanDry

Correct quantity of steamThe correct quantity of steam must be made available for any heating process to ensure that a sufficientheat flow is provided for heat transfer.

Similarly, the correct flowrate must also be supplied so that there is no product spoilage or drop in the rateof production. Steam loads must be properly calculated and pipes must be correctly sized to achieve theflowrates required.

Correct pressure and temperature of steamSteam should reach the point of use at the required pressure and provide the desired temperature for eachapplication, or performance will be affected. The correct sizing of pipework and pipeline ancillaries willensure this is achieved.

However, even if the pressure gauge is correctly displaying the desired pressure, the correspondingsaturation temperature may not be available if the steam contains air and/or incondensable gases.

Air and other incondensable gasesAir is present within the steam supply pipes and equipment at start-up. Even if the system were filled withpure steam the last time it was used, the steam would condense at shutdown, and air would be drawn in bythe resultant vacuum.

When steam enters the system it will force the air towards either the drain point, or to the point furthest fromthe steam inlet, known as the remote point. Therefore steam traps with sufficient air venting capacitiesshould be fitted to these drain points, and automatic air vents should be fitted to all remote points.

However, if there is any turbulence the steam and air will mix and the air will be carried to the heat transfersurface. As the steam condenses, an insulating layer of air is left behind on the surface, acting as a barrierto heat transfer.


What is Steam?

Superheated Steam

Steam Quality

Heat Transfer



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Related ContentStrainersStrainers improve steamquality by arresting pipedirt.

SeparatorsSeparators improve steamquality by removingmoisture from wet steam.

Air VentsAir vents improve steamquality by removing airfrom steam pipes andsystems.

WaterhammerSee how we can eliminatewaterhammer.

Steam TablesInvestigate steam qualitybu using the wet steamtable.


Fig. 2.4.1Steam process equipment with anautomatic air vent and strainers

Steam and air mixturesIn a mixture of air and steam, the presence of air will cause the temperature to be lower than expected. Thetotal pressure of a mixture of gases is made up of the sum of the partial pressures of the components in themixture.

This is known as Dalton's Law of Partial Pressures. The partial pressure is the pressure exerted by eachcomponent if it occupied the same volume as the mixture:

Equation 2.4.1

Note: This is a thermodynamic relationship, so all pressures must be expressed in bar a.

Example 2.4.1Consider a steam/air mixture made up of steam and air by volume. The total pressure is 4 bar a.

Determine the temperature of the mixture:

Therefore the steam only has an effective pressure of 3 bar a as opposed to its apparent pressure of 4 bara. The mixture would only have a temperature of 134C rather than the expected saturation temperature of144C.

This phenomena is not only of importance in heat exchange applications (where the heat transfer rateincreases with an increase in temperature difference), but also in process applications where a minimumtemperature may be required to achieve a chemical or physical change in a product. For instance, aminimum temperature is essential in a steriliser in order to kill bacteria.

Other sources of air in the steam and condensate loopAir can also enter the system in solution with the boiler feedwater. Make-up water and condensate,exposed to the atmosphere, will readily absorb nitrogen, oxygen and carbon dioxide: the main componentsof atmospheric air. When the water is heated in the boiler, these gases are released with the steam andcarried into the distribution system.

Atmospheric air consists of 78% nitrogen, 21% oxygen and 0.03% carbon dioxide, by volume analysis.However, the solubility of oxygen is roughly twice that of nitrogen, whilst carbon dioxide has a solubilityroughly 30 times greater than oxygen!

This means that 'air' dissolved in the boiler feedwater will contain much larger proportions of carbon dioxideand oxygen: both of which cause corrosion in the boiler and the pipework.

The temperature of the feedtank is maintained at a temperature typically no less than 80C so that oxygenand carbon dioxide can be liberated back to the atmosphere, as the solubility of these dissolved gasesdecreases with increasing temperature.

The concentration of dissolved carbon dioxide is also kept to a minimum by demineralising and degassingthe make-up water at the external water treatment stage.

The concentration of dissolved gas in the water can be determined using Henry's Law. This states that themass of gas that can be dissolved by a given volume of liquid is directly proportional to the partial pressureof the gas.

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This is only true however if the temperature is constant, and there is no chemical reaction between theliquid and the gas.

Cleanliness of steamLayers of scale found on pipe walls may be either due to the formation of rust in older steam systems, or toa carbonate deposit in hard water areas. Other types of dirt which may be found in a steam supply lineinclude welding slag and badly applied or excess jointing material, which may have been left in the systemwhen the pipework was initially installed. These fragments will have the effect of increasing the rate oferosion in pipe bends and the small orifices of steam traps and valves.

For this reason it is good engineering practice to fit a pipeline strainer (as shown in Figure 2.4.2). Thisshould be installed upstream of every steam trap, flowmeter, pressure reducing valve and control valve.

Fig. 2.4.2A pipeline strainer

Steam flows from the inlet A through the perforated screen B to the outlet C. While steam and water willpass readily through the screen, dirt will be arrested. The cap D can be removed, allowing the screen to bewithdrawn and cleaned at regular intervals.

When strainers are fitted in steam lines, they should be installed on their sides so that the accumulation ofcondensate and the problem of waterhammer can be avoided. This orientation will also expose themaximum strainer screen area to the flow.

A layer of scale may also be present on the heat transfer surface, acting as an additional barrier to heattransfer. Layers of scale are often a result of either:

Incorrect boiler operation, causing impurities to be carried over from the boiler in water droplets.Incorrect water treatment in the boiler house.

The rate at which this layer builds up can be reduced by careful attention to the boiler operation and by theremoval of any droplets of moisture.

Dryness of steamIncorrect chemical feedwater treatment and periods of peak load can cause priming and carryover of boilerfeedwater into the steam mains, leading to chemical and other material being deposited on to heat transfersurfaces. These deposits will accumulate over time, gradually reducing the efficiency of the plant.

In addition to this, as the steam leaves the boiler, some of it must condense due to heat loss through thepipe walls. Although these pipes may be well insulated, this process cannot be completely eliminated.

The overall result is that steam arriving at the plant is relatively wet, and the droplets of moisture carriedalong with the steam can erode pipes, fittings and valves especially if velocities are high.

It has already been shown that the presence of water droplets in steam reduces the actual enthalpy ofevaporation, and also leads to the formation of scale on the pipe walls and heat transfer surface.

The droplets of water entrained within the steam can also add to the resistant film of water produced as thesteam condenses, creating yet another barrier to the heat transfer process.

A separator in the steam line will remove moisture droplets entrained in the steam flow, and also anycondensate that has gravitated to the bottom of the pipe.

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In the separator shown in Figure 2.4.3 the steam is forced to change direction several times as it flowsthrough the body. The baffles create an obstacle for the heavier water droplets, while the lighter dry steamis allowed to flow freely through the separator.

The moisture droplets run down the baffles and drain through the bottom connection of the separator to asteam trap. This will allow condensate to drain from the system, but will not allow the passage of any steam.

Fig. 2.4.3A steam separator

WaterhammerAs steam begins to condense due to heat losses in the pipe, the condensate forms droplets on the inside ofthe walls. As they are swept along in the steam flow, they then merge into a film. The condensate thengravitates towards the bottom of the pipe, where the film begins to increase in thickness.

The build up of droplets of condensate along a length of steam pipework can eventually form a slug ofwater (as shown in Figure 2.4.4), which will be carried at steam velocity along the pipework (25 - 30 m/s).

Fig. 2.4.4Formation of a solid slug of water

This slug of water is dense and incompressible, and when travelling at high velocity, has a considerableamount of kinetic energy.

The laws of thermodynamics state that energy cannot be created or destroyed, but simply converted into adifferent form.

When obstructed, perhaps by a bend or tee in the pipe, the kinetic energy of the water is converted intopressure energy and a pressure shock is applied to the obstruction.

Condensate will also collect at low points, and slugs of condensate may be picked up by the flow of steamand hurled downstream at valves and pipe fittings.

These low points might include a sagging main, which may be due to inadequate pipe support or a brokenpipe hanger. Other potential sources of waterhammer include the incorrect use of concentric reducers andstrainers, or inadequate drainage before a rise in the steam main. Some of these are shown in Figure 2.4.5.

The noise and vibration caused by the impact between the slug of water and the obstruction, is known aswaterhammer.

Waterhammer can significantly reduce the life of pipeline ancillaries. In severe cases the fitting may fracturewith an almost explosive effect. The consequence may be the loss of live steam at the fracture, creating ahazardous situation.

The installation of steam pipework is discussed in detail in Block 9, Steam Distribution.

Fig. 2.4.5Potential sources of waterhammer

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Steam is often generated to provide heat transferto a process. Modes of heat transfer (conduction,convection, radiation) within or between media areexplained, together with calculations and otherissues such as heat transfer barriers.



You are here: Home Resources Steam Engineering TutorialsSteam Engineering Principles and Heat Transfer Heat Transfer

Heat Transfer

In a steam heating system, the sole purpose of the generation and distribution of steam is to provide heat atthe process heat transfer surface. If the required heat input rate and steam pressure are known, then thenecessary steam consumption rate may be determined. This will allow the size of the boiler and the steamdistribution system to be established.

Modes of heat transferWhenever a temperature gradient exists, either within a medium or between media, the transfer of heat willoccur. This may take the form of either conduction, convection or radiation.

ConductionWhen a temperature gradient exists in either a solid or stationary fluid medium, the heat transfer which takesplace is known as conduction. When neighbouring molecules in a fluid collide, energy is transferred from themore energetic to the less energetic molecules. Because higher temperatures are associated with highermolecular energies, conduction must occur in the direction of decreasing temperature.

This phenomenon can be seen in both liquids and gases. However, in liquids the molecular interactions arestronger and more frequent, as the molecules are closer together. In solids, conduction is caused by theatomic activity of lattice vibrations as explained in Tutorial 2.2.

The equation used to express heat transfer by conduction is known as Fourier's Law. Where there is a lineartemperature distribution under steady-state conditions, for a one-dimensional plane wall it may be written as:

Equation 2.5.1

Where:

= Heat transferred per unit time (W)k = Thermal conductivity of the material (W/m K or W/mC)A = Heat transfer area (m)T = Temperature difference across the material (K or C)

= Material thickness (m)Example 2.5.1Consider a plane wall constructed of solid iron with a thermal conductivity of 70 W/mC, and a thickness of25 mm. It has a surface area of 0.3 m by 0.5 m, with a temperature of 150C on one side and 80C on theother.

Determine the rate of heat transfer:

The thermal conductivity is a characteristic of the wall material and is dependent on temperature. Table 2.5.1shows the variation of thermal conductivity with temperature for various common metals.


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Table 2.5.1Thermal conductivity (W/m C)

Considering the mechanism of heat transfer in conduction, in general the thermal conductivity of a solid willbe much greater than of a liquid, and the thermal conductivity of a liquid will be greater than of a gas. Air hasa particularly low thermal conductivity and this is why insulating materials often have lots of air spaces.

ConvectionThe transfer of heat energy between a surface and a moving fluid at different temperatures is known asconvection. It is actually a combination of the mechanisms of diffusion and the bulk motion of molecules.

Near the surface where the fluid velocity is low, diffusion (or random molecular motion) dominates. However,moving away from the surface, bulk motion holds an increasing influence. Convective heat transfer may takethe form of either forced convection or natural convection. Forced convection occurs when fluid flow isinduced by an external force, such as a pump or an agitator. Conversely, natural convection is caused bybuoyancy forces, due to the density differences arising from the temperature variations in the fluid.

The transfer of heat energy caused by a phase change, such as boiling or condensing, is also referred to asa convective heat transfer process.

The equation for convection is expressed by Equation 2.5.2 which is a derivation of Newton's Law ofCooling:

Equation 2.5.2

Where:

= Heat transferred per unit time (W)A = Heat transfer area of the surface (m)h = Convective heat transfer coefficient of the process (W/m K or W/mC)T = Temperature difference between the surface and the bulk fluid (K or C)Example 2.5.2Consider a plane surface 0.4 m by 0.9 m at a temperature of 20C. A fluid flows over the surface with a bulk temperature of 50C.The convective heat transfer coefficient (h) is 1 600 W/mC.

Determine the rate of heat transfer:

RadiationThe heat transfer due to the emission of energy from surfaces in the form of electromagnetic waves is knownas thermal radiation. In the absence of an intervening medium, there is a net heat transfer between twosurfaces of different temperatures. This form of heat transfer does not rely on a material medium, and isactually most efficient in a vacuum.

The general heat transfer equationIn most practical situations, it is very unusual for all energy to be transferred by one mode of heat transferalone. The overall heat transfer process will usually be a combination of two or more different mechanisms.

The general equation used to calculate heat transfer across a surface used in the design procedure andforming a part of heat exchange theory is:

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Feature

Equation 2.5.3

Where:

= Heat transferred per unit time (W (J/s))U = Overall heat transfer coefficient (W/m K or W/mC)A = Heat transfer area (m)T = Temperature difference between the primary and secondary fluid (K or C)Note:

will be a mean heat transfer rate ( M) if T is a mean temperature difference (T LM or T AM).

The overall heat transfer coefficient (U)This takes into account both conductive and convective resistance between two fluids separated by a solidwall. The overall heat transfer coefficient is the reciprocal of the overall resistance to heat transfer, which isthe sum of the individual resistances.

The overall heat transfer coefficient may also take into account the degree of fouling in the heat transferprocess. The deposition of a film or scale on the heat transfer surface will greatly reduce the rate of heattransfer. The fouling factor represents the additional thermal resistance caused by fluid impurities, rustformation or other reactions between the fluid and the wall.

The magnitude of the individual coefficients will depend on the nature of the heat transfer process, thephysical properties of the fluids, the fluid flowrates and the physical layout of the heat transfer surface.

As the physical layout cannot be established until the heat transfer area has been determined, the design ofa heat exchanger is by necessity, an iterative procedure. A starting point for this procedure usually involvesselecting typical values for the overall heat transfer coefficient of various types of heat exchanger.

An accurate calculation for the individual heat transfer coefficients is a complicated procedure, and in manycases it is not possible due to some of the parameters being unknown. Therefore, the use of establishedtypical values of overall heat transfer coefficient will be suitable for practical purposes.

Temperature difference TNewton's law of cooling states that the heat transfer rate is related to the instantaneous temperaturedifference between the hot and the cold media. In a heat transfer process, this temperature difference willvary either with position or with time. The general heat transfer equation was thus developed as anextension to Newton's law of cooling, where the mean temperature difference is used to establish the heattransfer area required for a given heat duty.

Mean temperature difference T MThe determination of the mean temperature difference in a flow type process like a heat exchanger will bedependent upon the direction of flow. The primary and secondary fluids may flow in the same direction(parallel flow/co-current flow), in the opposite direction (countercurrent flow), or perpendicular to each other(crossflow). When saturated steam is used the primary fluid temperature can be taken as a constant,because heat is transferred as a result of a change of phase only. The

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