Heating & Ventilation, Air Conditioning & Refrigeration: Chapter 2

OVERVIEW This chapter deals with the science and mechanics that underpin the actions that occur when designing, installing and maintaining building services within the MES sector.

At the end of this chapter you should understand the basic principles and calculations relating to:

• physical quantities and SI units

• force, energy, work and power

• mechanical principles and simple machines

• pressure

• heat

• chemistry

• electricity.

Identify the physical forces that have an impact on the MES sector

35

chapter2

36

Heating and Ventilation, Air Conditioning and Refrigeration

Physical quantities and SI unitsIn the UK there are two principal systems of measurement, metric and imperial. The units of measurement you will come across should be metric although you will still hear references to imperial units or find them on older equipment. Some materials are still sold in them, notably refrigeration copper tube.

SI units are now the standard international measurement system. Table 2.01 (overleaf) shows that, for our purposes, there are only six basic units in the SI system, and all other units are derived from these. For example, the derived unit of speed is metres per second.

Table 2.02 shows the multiples and sub-multiples of SI units and the symbols that are used to indicate these.

Prefix Symbol Factor

giga G 109

mega M 106

kilo k 103

hecto h 102

deca da 101

deci d 10–1

centi c 10–2

milli m 10–3

Table 2.02 Multiples and sub-multiples of SI units

Why do I need to know all about this science?This all effects the way in which HVACR systems are chosen, installed and work, so it is essential to have a good awareness of the key science principles.

FAQ

37

Chapter 2 Identify the physical forces that have an impact on the MES sector

Attribute SI unit Abbreviation Imperial Unit(s)

Imperial abbreviation

Conversion

Length metre m inches, feet, miles

ins, ft 1 in = 2.54 cms1 ft = 0.3048 m

Mass kilogram kg ounces, pound

oz, lb 1 oz = 28.35 g1 lb = 0.4536 kg

Time second s 60 secs = 1 min60mins = 1 hour

Electric current

ampere A

Temperature kelvin K degrees centigrade (°C)K = °C + 273.15

degrees Fahrenheit

°F °C = 5/9 (°F – 32)

Angle radian rad 1 rad = 57 degrees (°)1 degree = 60 mins (')1 min = 60 seconds ('')

Area square metres

m2 1 hectare = 10,000 m2

square inches, acres

Volume cubic metres

m3 cc = cm3 cubic inches

1 m3 = 1,000,000 cc

Capacity litre l 1 ml = 1 cc or cm3

pints, gallons

1 pint = 0.5663 l

Speed metres per second

m/s miles per hourfeet per second

mphfps

Acceleration metres per second per second

m/s2

Force Newtons N pounds wt lb wt

Table 2.01 SI Units


38

Force, energy, work and power

ForceForces cannot be seen, but their effects can. When something is weighed on a spring balance, the forces cause the spring to extend, i.e. change its shape. Changes in movement, for example from rest to other kinds of movement, and changes in shape are signs that forces are acting. There are many different kinds of force, such as wind force, water power, the force of gravity or centrifugal force.

The SI unit of force is the newton (N), which is the force necessary to accelerate a mass of one kilogram to a speed of one metre per second in one second. It is used as a measure of the weight of an object. A 1 kg mass is the same everywhere, whether it is on the earth, the moon or in a satellite orbiting the earth. However, its weight varies depending on the amount of gravity pulling on it. On earth, all objects are being accelerated towards its centre due to the planet’s gravitational pull. Therefore, if a newton is the SI unit of measurement equal to 1 metre per second on one kilogram of mass then:

Weight (on earth) = mass × acceleration due to gravity = kg × 9.81 m/s2

Therefore 1 kg weight = 9.81 newtons (often rounded up to 10 N)

Energy and workEnergy is measured in joules and is the ability to do work. It can occur in many different forms: heat, light, sound, a coiled spring etc. Hence, it can cause something to move or provide the ability to cause change. Machines cannot work without energy and we are unable to get more work out of a machine than we put into it. In mechanical machines this is due mainly to friction, which occurs when two surfaces rub together, as no surfaces are ever perfectly smooth.

Energy can be transferred from one form to another but cannot be created or destroyed. The loss of energy by friction usually ends up as heat.

If an object is moved, work is said to be done. The unit of work is the same as energy, the joule (J), and is expressed as:

Work done (J) = force (N) × distance moved (m)

For example:

An object with a mass of 50 kg has to be moved 10 m. The work done will be:

Work = force × distance

= (50 × 9.81) × 10

= 490.5 × 10

= 4905 J

Remember

Mass and weight are not the same thing. Mass is the amount of material in an object. Weight is a force. This means that a person would weigh a different amount on the earth than the moon, because of the decrease in the force of gravity, but their mass would still be the same

Remember

Acceleration due to gravity does not equal = 10 m/s2. It is in fact 9.81m/s2. Only take it as 10m/s2 if you are told to do so or when making a rough calculation


39


39

PowerWhen we do work in a mechanical system, the energy we put into the system does not appear instantaneously. It takes a certain time to move an object, lift a weight etc. The power that we put into a system depends not only on the amount of work we do but also how fast we carry it out.

We can state this as:

Power = the rate of doing work = energy used

time taken to do the work

The SI unit for work is the joule (J) and time is the second (s). Hence, power is measured in joules per second (J/s) or watts (W), where 1000 watts = 1 kilowatt.

Using the previous example:

The power used to move our 50 kg object by 10 m in 20 seconds would be:

Power = work done in moving the load 10 m

time taken

= 4905

20

= 245.25 W

Mechanical principles and simple machinesOver the years man has developed simple machines to help perform work more easily. A machine allows us to use a smaller force to overcome a larger force, and can also help us to change direction of the force and to work faster. The most common mechanical machines are: screws, wheel and axle, gears, levers, inclined planes and pulleys. We will look in more detail at the last three but, before doing so, we need to understand some simple mechanical principles.

Mechanical advantageThe common theme behind simple machines is the ability to gain advantage over nature. Mechanical advantage is the relationship between the effort needed to move or lift something and the load itself. Consequently, when a machine can put out more force than is put in, it is said to have good mechanical advantage. Mechanical advantage can be calculated by dividing the load by the effort. There are no units for mechanical advantage, just a number.

Remember

To make a simple machine work for us we need to apply a force to it


40

Velocity ratioSometimes machines translate a small amount of movement into a larger amount (or vice versa). For example, a small amount of movement on a piston can cause a load to move a much greater distance. This property is known as the velocity ratio, and is found by dividing the distance moved by the effort by the distance moved by the load. Like mechanical advantage there are no units, just a number.

Velocity ratio = Distance effort movesDistance load moves

Theory of momentsWhen we turn a nut with a spanner, from experience we know that the length of the spanner determines the turning force we can apply to tighten the nut and, hence, the amount of effort used. If the spanner is long, a small effort is sufficient. Instead of the words ‘turning effect’ we use the term ‘moment of a force’.

Moment = force × length

Figure 2.01 Moment of a force

Example

N is used to move a load of 50 N. What is the mechanical advantage of the lever?

MA = Load = 5010 = 5

times larger. To summarise:

Where MA is greater than 1: The

force (e.g. a class 1 lever)

Where MA is equal to 1: The machine is normally used to change the

Where MA is less than 1: The machine is used to increase the distance an object moves or the speed at which it moves (e.g. the siege machine).

J6990HED Mechanical Eng BW PDFAW_122AW by HL Studios

Force

length

Movement


41


41

The SI unit of moment is the newton metre (Nm) (i.e. newton × metre). Note that we can have identical moments by using a large force and short length, or small force and long length. A good example is a see-saw, where a small child sitting at one end can balance a large adult sitting closer to the pivot point.

Action and reaction When an object is weighed on a spring balance, the action of the force is vertically downwards. It makes no difference whether the object is suspended from the balance itself or from a string attached to it, the forces act in the same vertical line.

‘A force acts and is dispersed in its own line of action.’

Figure 2.02 Action of a force on a spring balance

When things are in balance or equilibrium the forces acting on them cancel each other out. Consider an object placed on a table. The force that the object exerts on the table is called the force of action and the force exerted by the table to hold the object up is called the force of reaction. In equilibrium, action equals reaction and so a body is able either to remain at rest or, if already moving, continue to do so without a change in velocity.

Figure 2.03 Action and reaction


Force

Force


action

reaction

Did you know?

A force applied to an object has the effect of either changing its shape or causing movement.


42

Centre of gravityA body can be divided into a number of equal parts. All of the parts are subject to the same gravitational pull from the earth. Therefore forces act on each part of the body and the resultant of all of these forces is the weight of the body. The point of application of the resultant force is the centre of gravity. Hence, the centre of gravity is the point through which all the weight of an object acts.

EquilibriumFigure 2.05 shows an object that is currently in equilibrium. It can be rotated about S (a point near its top), U (a point near its bottom or O (its centre of gravity). W shows the force of the object’s weight.

Figure 2.05 Equilibrium states

If able to rotate about S and we start to push it, the centre of gravity O would move above its original position and, if we let go, it would swing back down. The object is said to be in ‘stable equilibrium’.

If the object was rotated about U, the centre of gravity O would start to move below its original position and, if we let go, it would continue to move down and the object would fall over. It is said to be in ‘unstable equilibrium’. It would need to lean only a small amount before the centre of gravity is moving downwards – causing the object to fall

In the third case, it does not matter which way the object is rotated, the centre of gravity stays at the same height. It is said to be in a ‘neutral equilibrium’.

If we laid the object down on its long side on a flat surface, we would find that it was stable. The centre of gravity is now much lower and, if we attempt to tip it over, the centre of gravity would have to be lifted up (i.e. stable equilibrium) and, if we let go, the object would fall back.


weight

O


OO

s

a b c

u WW

Stable Unstable Neutral

W

OI

OI

O = S

Figure 2.04 Centre of gravity of an object


43


43

Hence, we can state three general rules of stability, such that a body is more stable:

the lower its centre of gravity

if its base is wider than its height

if the weight is concentrated at its base.

Rules of stability come into play when lifting and moving objects, especially when using cranes, chains and lifting tackle.

LeversLevers use the theory of moments to let us use a small force to apply a large force to an object. They are divided into classes of lever, dependent upon the position of the load, fulcrum and the applied force.

•

•

•

On the Job: EquilibriumA tall hot water storage cylinder, measuring 2.4 m high and 1.0 m in diameter, has to be moved into a plant room. When upright, the centre of gravity is likely to be near the middle of the cylinder (it might even be higher, depending on what fitments are attached to it). Hence, it would only need to lean a small amount before the centre of gravity is beyond the vertical line from the bottom edge of the cylinder, around which the cylinder is now turning. The centre of gravity would then be moving downwards (unstable equilibrium), causing the cylinder to topple over. Therefore, the engineers doing the work might consider moving the cylinder in the horizontal position. The centre of gravity would now be very low and the cylinder would have to lean a very long way before it would tip over. Until that point, the centre of gravity would be rising (stable equilibrium).

Remember

The fulcrum, or pivot, is the point around which the lever rotatesClass 1 lever

Class 2 lever Class 3 lever

Figure 2.06 Classes of lever


44

A small force at a long distance will produce a larger force close to the pivot.

Figure 2.07 Large force close to a pivot

Using the lever in Figure 2.07, if we want to know the maximum force exerted at the force end of the lever it can be calculated using our theory of moments:

Force × 0.5 m = 10 N × 2 m

If we now rearrange the formula we get:

Force = 10 N × 2 m ÷ 0.5 = 40 N

This is the maximum force that we can achieve with the lever in this configuration.

The inclined planeThe inclined plane is the simplest machine of all, as it is basically a ramp or sloping surface. If you imagine the height of a mountain, the shortest distance is straight up from the bottom to the top. However, we always build roads on a mountain as a slowly winding inclined plane from bottom to top. The road allows us to reach the top more easily but at the price of having to travel further. A machine screw is an inclined plane wound around a central cylinder.

Pulley systemsA pulley is made from a rope, belt or chain wrapped around one or more wheels, and can be used to lift heavy objects.

Imagine that, as shown in Figure 2.08 you have a 20 N weight that is suspended from a rope attached to a platform 3 m above it, but resting on the ground. If we had to lift the weight to the platform then a 20 N upward force would be required as we pulled up 3 m of rope. If we now add a pulley to the system at the platform height, and pass the rope over it so that it reaches back to the ground, we have not really done much to help ourselves, except that it is easier to pull down than lift upwards. A 20 N force is still required but we now need 6 m of rope, though we only have to pull 3 m of it through the pulley.

To gain an advantage we need to add a second moveable pulley, with the rope passing around both pulleys before reaching us. This new arrangement changes things in our favour, because now the load is suspended by two ropes, each holding 10 N instead of one holding 20 N. The force required now to lift the 20 N load is only 10 N but, to move the load through 3 m, we have to pull 6 m of rope. The more pulleys we add to the system the easier it is to lift heavy loads, as shown in Figure 2.08. As the rope is pulled from the top wheel, the bottom pulley wheel is lifted.


45


45

Figure 2.08 Pulley systems

To calculate the effort required to lift the load we divide the load by the number of pulley wheels used. Figure 2.08 shows a four pulley system, where the person lifting the 200 kg mass (i.e. 2000 N load) has to exert a pull equal to only 500 N, i.e. 2000 divided by 4.

PressurePressure is defined as force applied per unit area and is measured in newtons per square metre (N/m2), a unit also known as the pascal (Pa). The pascal is a very small unit and for, most practical purposes, the kilopascal (kPa) is used together with the ‘bar’. This is because the ‘bar’ is roughly the average pressure of the earth’s atmosphere at sea level.

1 bar = 100,000 N/m2 = 100,000 Pa = 100 kPa

Pressure, therefore, is the concentration of force on an area. To find the pressure we need to know two quantities:

the size of the force involved (in newtons)

the area over which the force is acting (usually in mm2 or m2).

Pressure of a solid object on a flat surfaceIf a solid object with a flat base is pressing on a surface, either due to its weight or because of a force acting on it, then the pressure on the surface is the total force divided by the area over which it is acting, as shown in Figure 2.09.

•

•

Singlefixedpulley

appliedforce

appliedforce

3m

appliedforce

4m

1m200kg


forcepressure

area

area

weight

Figure 2.09 Pressure on a flat surface from a solid object


46

If we keep the same area but increase the force, then the pressure will also increase.

Similarly, if we keep the same force but reduce the area over which it acts, the pressure on that area increases (think of a lady standing on stiletto heels and the damage it can do to a floor). Conversely if we can spread the load out over a larger area, we can reduce the pressure on each unit area (which is why we can move across soft snow on skis, without sinking in, or a heavy military tank can move across soft ground on its large tracks).

Figure 2.10 Reducing or increasing the surface area over which the force acts

Hydrostatic pressure (pressure exerted by liquids)Liquids act in a different way to solids. In a container a liquid exerts a pressure on the bottom but also the sides. This pressure has nothing to do with the total weight of the liquid. Instead it depends upon the density of the liquid and depth of liquid in the container. The pressure exerted at any point by the liquid is the same in all directions. (Density is discussed further below.)

Figure 2.11 The effects of pressure from solids and liquids


low pressure

weight weight

high pressure

Did you know?

An approximation of the pressure exerted at the bottom of a 10 m high vertical column of cold water is 1 bar

HeinemannNVQ2 Plumbing9pt Zurich BTfig001906/04/05James Petrie

LiquidSolid


47


47

Pressure exerted by gases confined within a containerGas molecules are always on the move, so they press against the walls of a container. Thus a force acts on all the walls of the container and we call this the gas pressure, or air pressure if we are dealing with air. When a tyre is inflated, we are increasing the quantity of air in it by compressing the air and, hence, increasing the pressure the air is exerting on the inside of the tyre. On something small, like a bicycle wheel, if we want to know whether the tyre is hard enough, we can test with our fingers whether we can compress the air any more; otherwise we need a pressure gauge.

Atmospheric pressureRound the earth there is a layer of air, the atmosphere, which is held in place by the pull of gravity and, hence, has weight. It, therefore, exerts a pressure on the earth’s surface. To measure this pressure we assume that we have a vertical column of air above us. This vertical column will be different in different parts of the world. The higher we go, like up a mountain, the lower the pressure because the column of air above us is shorter.

At sea level the average pressure is about 760 mm of mercury using a fluid barometer or, using an aneroid barometer, it will register about 101,325 N/m2 (approximately 1 bar). The actual pressure of the atmosphere is constantly changing due to weather patterns around the world. Meteorologists look at these pressures to decide the positions of high and low pressure areas and help forecast the weather. Atmospheric pressures are normally quoted in millibars (mb). Note, that 1 bar equals 1000 mb and atmospheric pressure will fluctuate around this figure.

The measurement of pressureIn practice, when we take pressure readings in an enclosed vessel, using devices like the Bourdon type pressure gauge or the fluid manometer, we are taking readings that are above atmospheric pressure. Some calculations to do with building services require readings in absolute pressure. Absolute pressure is gauge pressure plus atmospheric pressure. Therefore, if a negative reading is found, then the internal pressure is lower than atmospheric pressure. This is normally called a (partial) vacuum.

On the Job: PressureThe pressure from a pump supplying a cold water system in a high rise building is 2.0 bar. The designer of the system is worried that pressure at the draw off points on the top floor might not be enough. We know that hydrostatic pressure cancels out the output from the pump at the rate of 1 bar for every 10 m in height. The pump, therefore, will only be able to push the water up to 2 × 10 m, which equals 20 m. Beyond this height the pressure in the system would be zero.

Remember

Two things about gases: they exert pressure and they can be compressed


48

Figure 2.12 Measurement of pressure

Heat

The properties of heatHeat is a form of energy possessed by every substance as molecular vibration: the more heat, the more vibration. Temperature is a measure of the speed of this vibration and it determines the direction of heat flow to or from a body. Heat will always flow from a warm body to a cold body and the rate of heat transfer will be directly proportional to the temperature difference.

Temperature is, thus, the degree of hotness of a substance and is measured in degrees Celsius (°C) or degrees Kelvin (K). (You may also hear reference to degrees ‘centigrade’, which is an old name for Celsius, or degrees Fahrenheit (°F), a scale which should no longer be used but may still be found.)

Why two scales?One degree Kelvin = one degree Celsius, so why not just use one scale? The answer is convenience. When considering temperature measurement, it is common to use the boiling point and freezing point of water as references. Using the Celsius scale, 0°C is defined as the freezing point of water and 100°C its boiling point, so many of the temperatures we deal with are easily expressed in Celsius.

However, some calculations relating to heat require a different approach. The temperature point at which all molecular vibration ceases is defined as, and called, absolute zero. The Kelvin scale has this point as zero degrees Kelvin (0 K) with the freezing point of water as 273 K, the boiling point of water being 100 degrees higher at 373 K. The scale is compatible with the Celsius scale in that absolute zero on the Celsius scale is –273°C, with the other two points at 0°C and 100°C respectively. When calculating cooling loads the Kelvin scale is used to avoid using negative quantities.

The temperature scales are shown in Figure 2.13. The Fahrenheit scale has been included for comparison, as it may still be found on older equipment.


(a) fluid manometer

gas

(b) Bourdon gauge (c) Schiffer and Budenberg gauge

20

10

0

Remember

Temperature in Kelvin = temperature in °C + 273; temperature in °C = temperature in Kelvin – 273


49


49

Figure 2.13 Temperature scales

Specific heat capacityHeat is the amount of energy that is contained in a substance and is measured by the same units as other forms of energy, i.e. the joule. To calculate a quantity of heat, we refer to the SI unit for specific heat capacity, which is the amount of heat required to raise 1 kg mass of a material by 1°C (kJ/kg/°C), or in some cases 1 K. The amount of heat required differs from material to material. For example, while it would require 4.186 kJ to raise the temperature of 1 kg of water by 1°C, it would only take 0.385 kJ to raise the same amount of copper by 1°C. Table 2.03 shows the specific heat values for some common materials. For most heat calculations the specific heat capacity for water is rounded to 4.2 kJ/kg/°C.

Latent and sensible heatIf water is heated in an open container from freezing point to boiling point (i.e. through 100°C), 420 kJ are added for every 1 kg of water (i.e. 4.2 × 1 × 100). However, if the supply of heat is then maintained, the temperature does not rise further but the water changes state from liquid to steam. The first stage of heating (temperature rise


Celsius Kelvin

Degrees

Fahrenheit

100 373 212

–100 173 –148

–200 73 –328

–273 0 –460

C K F

50 122

0Freezing pointof water

Boiling pointof water

273 32

Material kJ/kg °C

Water 4.186

Aluminium 0.887

Cast Iron 0.554

Zinc 0.397

Lead 0.125

Copper 0.385

Mercury 0.125

Table 2.03 Specific heat values

Remember

°F can be converted into °C by using the formula °F – 32 divided by 1.8. °C can be converted to °F by using (°C × 1.8) + 32.


50

without a change of state), is named ‘sensible heat’. The second stage (no temperature rise but a change of state) is called ‘latent heat’. The quantity of heat required to effect a change of state is extremely large in comparison to that required to warm the water. It requires 2300 kJ to change 1 kg of water to steam and is referred to as the ‘specific latent heat of evaporation’. As steam cools and changes back to water the same amount of heat is given off. We exploit this in a closed container to produce a steam engine.

There is also a latent heat of fusion when ice is converted to water, or the reverse, and this has a value of 330 kJ/kg. All materials have a latent heat of fusion and of evaporation.

Heat and gases The perfect gas conforms to Boyle’s and Charles’ law, which respectively state that the volume varies inversely with the pressure when the temperature is constant, and the volume varies with the absolute temperature when the pressure is constant. The gas laws can be combined to give:

absolute pressure × volume

absolute temperature = a constant

Gases have two specific heat capacities according to whether the pressure or the volume is kept constant while the temperature is raised.

Heat and states of matterEvery known substance is made up of molecules that are very tiny particles of matter, which can only be seen using a very powerful microscope. Molecules are always in constant motion and the ease at which they move around determines the form of the substance they make up.

When molecules are densely packed together their movement is restricted and the form that the substance takes is called a solid.

As the molecules move further apart they are able to move so freely that the substance takes on a new form called a liquid.

Finally, a substance which allows the molecules unrestricted movement is known as a gas.

These are the ‘three states of matter’. The application of heat affects the molecular structure of a substance, which in turn affects the state of that substance. As water in its solid form of ice is heated it changes to liquid and with more heat it becomes a gas (steam).

Heat and densityThe density of solid or liquid substances, which have the same size and shape, can frequently have a completely different mass. This relative lightness or heaviness is referred to as density, where:

Density = mass

volume

•

•

•

Did you know?

Every substance can exist as a solid, liquid or gas; we are just used to them in the states they adopt at temperatures we normally experience


51


51

The application of heat also affects the density of a substance, so that they become lighter, volume for volume, when heated. Water, however, is at its maximum density at 4°C and above or below this value it is less dense.

Relative density (RD), also called specific gravity, is an effective way of expressing the density of a substance or object by comparing it, volume for volume, with that of water. Hence, water has a relative density of 1.0. Other substances will either be less or greater than this. For instance, steel has an RD of 7.7, meaning that it has 7.7 times the density of water; and fuel oil has an RD of 0.7, meaning that it is lighter than water so will float on top of it.

Gas densities are expressed in the same way, although they are compared with air, air being 1.0.

Heat transferWe have already said that heat moves from a warm body to a cold body. This is known as thermal transfer and has three methods associated with it: conduction, convection and radiation.

Conduction This is heat energy transfer by contact. It takes place as a result of the increased vibration of molecules, which occurs when materials are heated. The vibrations from the heated material are passed on to the adjoining material, which in turn heats up. Metals tend to be good conductors of heat and materials that are poor conductors of heat are referred to as thermal insulators. Gases and liquids are generally poor conductors of heat.

ConvectionThis is the transfer of heat by movement. As a liquid or gas is heated it expands and, therefore, volume for volume it is less dense. This makes it rise to be displaced by cooler liquid or gas. Eventually thermal currents are set up, which allow a continuous flow upwards from the source of heat.

RadiationThis is the transfer of heat by waves. Heat waves are part of the electromagnetic spectrum. At one end there are very short waves in the form of gamma rays, X-rays, ultraviolet rays, light rays (the rays we can see) and infrared rays (those we feel as heat on the skin). At the other end we have much longer waves, some of which we use for radio transmission etc. This form of heat transfer travels in straight lines and only warms surfaces that the waves impinge upon, like the sun’s rays warming the earth.

Did you know?

Water is almost the only substance that expands when it changes from a liquid to a solid state; the colder the water the greater the expansion. Hence, the ability of water to break down rocks in nature or split unlagged pipes in houses!


52

Expansion and contractionMost materials expand when heated and contract when cooled. This is because as heat is applied the molecular structure vibrates and the individual molecules move apart. On cooling they move back together again. The amount of expansion and contraction can be predicted and experiments with different materials have produced a formula for linear expansion.

Expansion = length × temperature rise or fall × coefficient of linear expansion

The building services industry, in particular, has to make allowances for expansion and contraction by installing such things as flexible connections between heating/cooling plant and pipe work. Coefficients of linear expansion for some common materials are given in Table 2.04.


Boiler

Flow pipe

Warm water rises

Cool water sinks back down to the boiler

Hot storagecylinder

Returnpipe


Convected heat 85%

Radiated heat 15%

Figure 2.14 Examples of heat transfer

Remember

A coefficient is a constant, which varies for different materials


53


53

Material Coefficient

Plastic 0.00018

Zinc 0.000029

Lead 0.000029

Aluminium 0.000026

Tin 0.000021

Copper 0.000016

Cast iron 0.000011

Mild steel 0.000011

Invar 0.0000009

ChemistrySubstances exist as elements, mixtures and compounds. They may be in the state of a gas, liquid or solid.

Elements are simple substances that cannot be reduced any further. They are the building blocks of all other substances. Examples of elements are hydrogen, oxygen, carbon or iron.

Mixtures are combinations of elements, which retain their original characteristics and can still be separately identified. For instance, under a microscope some of the carbon and iron that make up cast iron can be distinguished.

Compounds are chemical and metallurgical combinations in which two or more elements come together to form new substances. For example, hydrogen and oxygen can combine in the right quantities, and with the right conditions, to form water.

On the Job: Repairing a componentA heating system is designed to allow for the expansion of the water contained in the system. If the water is heated from 6°C to 100°C then the total expansion would be just over 4% of the volume. Allowances must be made in feed water cistern size and expansion vessel size to accommodate this.

Did you know?

There are 92 naturally occurring elements

Table 2.04 Coefficients of linear expansion for some common materials


54

Chemical symbolsWe use chemical symbols as a form of shorthand writing to refer to combinations of chemical elements and their resultant products. Some of the commonest elements and their symbols are in Table 2.05.

Hydrogen H Aluminium Al

Carbon C Sulphur S

Nitrogen N Iron Fe

Oxygen O Copper Cu

Sodium Na Zinc Zn

Magnesium Mg Mercury Hg

Table 2.05 Some common elements and their chemical symbols

When we write a symbol down we use a letter or letters and a subscript, like CO

2. In chemistry this way of writing has a meaning. In this case we have one

molecule of carbon dioxide. What has actually happened is that one atom of carbon (C

1) has combined with two atoms of oxygen (O

2) to form a new

substance. You will notice that, when we only have one atom of anything, we do not put the subscript 1 after it; it is written as, in this case, C.

Chemical reactionsWhen oxygen combines with a substance, an oxide of that substance is formed. This is called oxidation. Single atoms of oxygen combining with some other substance are called monoxides, two atoms a dioxide and three a trioxide. Three example oxides common to the MES industry would be carbon monoxide (CO), carbon dioxide (CO

2) and sulphur trioxide (SO

3). All three are

the result of the combustion of hydrocarbon (H + C) fuels.

The opposite of oxidation is reduction, which is the removal of oxygen from an oxide, as in the process of smelting iron.

The results of chemical compounds can often be toxic, flammable, corrosive, irritant or oxidising to human or animal life.

WaterWater is a chemical compound of two gases: hydrogen and oxygen (H

2O) and

is formed when hydrogen gas is burned in the presence of oxygen. One of the most important properties of water is its power to dissolve gases or solids. It is one of the few substances that we can readily find as a solid (i.e. ice), liquid or gas (steam).

As pure rainwater falls through the atmosphere it absorbs gases such as nitrogen, oxygen, sulphur dioxide and carbon dioxide. The once pure water now will either be acidic or alkaline, which will increase the water’s ability to dissolve other substances. Once on the ground water will further absorb or dissolve other chemicals from the earth. When water contains calcium

Did you know?

The elements can be grouped by chemical and electronic similarities, known as the Periodic Table, which you might like to look at on the World Wide Web


55


55

carbonates and sulphates from limestone and chalk it becomes alkaline, which we call ‘hard water’. If the water falls on areas containing lots of vegetation it becomes acidic, which we call ‘soft water’.

The term pH value, or hardness value, refers to this level of acidity or alkalinity. The level of pH affects the water’s ability to cause corrosion. Therefore, it is in our interests to use water in a neutral state, around 6 or 7 on the pH scale.

Figure 2.15 The pH scale of acidity

Chemistry of oilsPetroleum and other oils are essentially hydrocarbons; that is they are made up from hydrogen and carbon atoms, of which there are many combinations. The process of producing the various grades of oil products is called fractionation. Crude oil is heated and the vapour produced is fed into a large tower called a fractionating column, or cracking column; with different products condensing out at different temperatures and at different levels within the column. The lightest oils appear at the top and the thickest or heaviest at the base.

The chemical differences between the fractions result from the number of carbon atoms present in the molecules; heavy oils having more atoms and lighter oils having less. The fractioning process can be further carried out, so that some of the lighter oils are broken down to form a series of ethanes, including, methane and propane (liquid petroleum gases).

PlasticsPlastics, or polymers, are also products of the petroleum industry. Ethene (C

2H

4) is a colourless hydrocarbon gas that combines with oxygen or other

atoms to produce chains of molecules. Simple chains, called monomers, can link together in long chains or polymers, which, when heated under pressure with a catalyst, become the building blocks for products like polyethylene (polythene). This process is known as polymerisation. There may be over a thousand carbon atoms forming one of these chains.

The chains do not lie in straight lines but are curled and twisted together, a bit like a plate of spaghetti. At this stage the molecules can slide about in relation to one another and it is this ability to move about that gives the material the property of plasticity. These plastics are referred to as thermoplastics and can be heated many times and reformed into different shapes. Most piping products fall into this category.


1 2 3 4 5 6 7 8 9 10 11 12

Acid Neutral Alkaline

The pH scale of acidity

Remember

A molecule of a substance is made up from a combination of atoms, which is unique to that substance


56

Replacement of one of the hydrogen atoms with another atom or molecule produces other plastics. In some cases the long chains form cross links which, after the application of heat, cannot be undone. These plastics are known as thermosetting plastics.

Thermosetting plastics are generally used for mouldings. They soften when first heated and formed into shapes but, when set, they cannot be reformed. Most hard plastics are in this category, typically used for electrical products.


Petroleum Gas

Bubble cap

Petroleum

Paraffin

Diesel

Lubricating oil

Heavy fuel oil

Bitumen

Steamenters here

Crudevapour fromthe furnaceenters here

Temperatureis hottest atthe base ofthe tower, andcools risingtowards thetop

Figure 2.16 Fractionating column

Did you know?

A catalyst is a chemical added to a process, which helps a reaction occur between two or more other chemicals without itself undergoing any permanent chemical change


57


57

Chemistry of metalsMetals are rarely found in their pure form but exist in the earth as oxides of the metal. To produce a usable metal from an ore a smelting process is necessary. This takes away the oxygen (i.e. reduction) and other impurities, which may be replaced with other elements to suit the needs of the metal.

An example of smelting is the production of iron in which iron oxide is loaded into a blast furnace along with carbon and limestone. As very hot air is blasted through the furnace, the carbon reacts with the oxygen from the air to form carbon monoxide, which in turn reacts with the iron oxide to form carbon dioxide and pure iron. The iron formed by this process requires further processing where other elements are added or taken away to improve the characteristics of the final product.

The mechanical properties of metals are shown in Table 2.06.

Property Characteristic

Strength Ability to withstand being pulled apart

Ductility Ability to be bent without breaking

Hardness Ability to withstand abrasion

Brittleness Ability to withstand bending before fracturing

Elasticity Ability to be stretched and regain original shape

Malleability Ability to be hammered into shape

Table 2.06 Mechanical properties of metals

Categories of metalsMetals fall into two basic categories: ferrous metals, those that contain iron, or non-ferrous, those that do not contain iron. Iron in its pure form is referred to as wrought iron. It has little use today and is normally alloyed with other elements, such as carbon, to produce steel. If manganese is added, the steel will resist wear and abrasion. If the steel contains chromium and nickel, it will resist rusting. Stainless steels are grouped into three broad categories:

ferritic stainless steel, containing 16% to 30% chromium and producing a heat resisting steel used for the manufacture of high temperature boiler parts

martensitic stainless steel, containing 11% to 18% chromium, used in the production of cutting tools

austenitic stainless steel, containing 14% to 30% chromium and 6% to 30% nickel; which at the lower end of the range (about 18% Cr and 8% Ni) is used in the production of pipes, valves and items like kitchen sinks.

Non-ferrous alloys are base metal elements with other elements added. The typical alloys used for building services are derived from copper or aluminium;

•

•

•

Did you know?

A metal alloy is a combination of a base metal with additions of small amounts of other elements added to improve the properties of the base metal


58

although copper is also used in its pure state, with just small amounts of non-metallic elements added to improve its workability characteristics. Common alloys are listed in Table 2.07.

Some commonly used alloys

Brass Copper and zinc Used for electrical contacts and corrosion-resistant fixings (screws, bolts etc.) and pipe fittings

Bronze Copper and tin Used for decorative or artistic purposes and corrosion-resistant pumps

Solder Lead and tin, tin and copper

Used for electrical connectionsUsed as a jointing material

Duralumin Aluminium, magnesium, copper and manganese

Used in aircraft production

Gunmetal Copper, tin and zinc Used for underground corrosion-resistant fittings

Table 2.07 Alloys

Heat treatment of ferrous metalsAs a metal cools from a molten state the molecules of the metal form into grains. The individual grains swell until they meet the next grain. The size of the grain structure in most metals will determine the final strength of the cooled metal. Uniform slow cooling produces a large grain structure, which results in a metal with poor strength and ductility.

Figure 2.17 Slow cooling of a pure metal


(a) The metal is molten (b) The metal cools and the molecules form grains

(c) The grains swell untilthey meet other grains

(d) Size of grains when cooldetermine strength of metal


59


59

On the other hand, uniform fast cooling tends to produce a metal that has a finer grain structure with greater strength and hardness but at the expense of being more brittle. Rolling or forging steel alters the grain shape and can introduce stresses within the steel, further changing its characteristics. Heat treatment of iron and steel can also alter mechanical properties for the better. Heat treatments for steels include: recrystallisation, annealing, normalising, hardening and tempering.

The chemistry of combustionThe combustion of a fuel is a rapid chemical reaction, which is accompanied by the liberation of heat (and light). Combustion reactions can only take place at a high temperature known as the ignition temperature, which varies between 400°C and 600°C according to the type of fuel.

For complete combustion to take place there must be present three elements:

fuel + oxygen + ignition

Figure 2.19 Combustion process

The complete combustion of gases, oils and solid fuels produce similar products of combustion, namely carbon dioxide and water vapour. Oils and solid fuels contain more impurities than gases. This affects the products of combustion, particularly producing sulphur, as sulphur dioxide and sulphur

On the Job: Cooling of materialsEngineers, shaping and forming steel stock for brackets and components, allow the finished products to cool slowly in air instead of quenching them in water. This action prevents the material becoming hard and brittle, which reduces the possibility of the component failing in service.

NVQ PL3. AW_0415. By Stephen Hibberd

8m3 of nitrogen

2m3 of oxygen10m3 of air

1m3 of carbon dioxide

8m3 of nitrogen

2m3 of water vapour

1m3 of methane


60

trioxide, and unburned carbon in the form of soot. Incomplete combustion means that not all the fuel is burned and carbon monoxide is formed as a by-product. This poisonous gas can be a serious health hazard, if released in an enclosed space. The effects on adults are shown in Table 2.08.

% CO Symptom

0 to 10% No obvious symptoms

10 to 20% Tightness across the forehead, yawning

20 to 30% Flushed skin, headache, breathlessness and palpitation on exertion, slight dizziness

30 to 40% Severe headache, dizziness, nausea, weakness of the knees, irritability, impaired judgement, possible collapse

40 to 50% Symptoms as above with increased respiration and pulse rates, collapse on exertion

50% to 60% Loss of consciousness, coma

60% to 70% Coma, weakened heart and respiration

70% or more Respiratory failure and death

Table 2.08 Effects of carbon monoxide on adults

Corrosion

Atmospheric corrosionPure air and pure water have little effect, but together in the form of moist air they can attack ferrous metals such as steel and iron. The process of oxygen combining with a substance produces an oxide of that substance. When iron and oxygen combine they form iron oxide, which is known as rust. This process can completely destroy a metal.

Coastal areas suffer from increased atmospheric corrosion due to the amount of sodium chloride (salt) from the sea, which becomes dissolved into the local atmosphere. Non-ferrous metals, such as copper, aluminium and lead have significant protection against atmospheric corrosion. Protective barriers (usually sulphates) form on these metals to prevent further corrosion. This protective barrier is known as patina.

Corrosion by waterHeating and cooling systems are made up of various metals, amongst which the ferrous materials are particularly vulnerable to corrosion caused by water. The effects of internal corrosion can be seen as a black oxide, formed as a residue in radiators, refrigeration system heat exchangers and pipe work. A by-product of this reaction is the build up of hydrogen gas within a system.


61


61

Figure 2.19 Corrosion effect in steel panel radiators

Electrolytic corrosionElectrolytic action occurs when two dissimilar metals become immersed in an electrolyte (weak acid or salt solution). They effectively become a battery. One of the metals becomes an anode and the other a cathode and between the two an electrical current is generated. The effect of this reaction is known as electrolysis, with the result that the metal that is forming the anode dissolves away. When a combination of metals is in an electrolyte, the one that will become the anode depends on the relative positions of the metals in the electromotive series, as shown in Table 2.09.

The elements further up the table will destroy those lower down, and the further apart they are, the faster the reaction. Anodic protection can be provided to cope with this form corrosion.


Coldarea

Hydrogen gas Air releasevalve

Flow

Ferrous oxide

Copper Cathodic

Anodic

↓Tin

Lead

Nickel

Cadmium

Iron

Chromium

Zinc

Aluminium

Magnesium Anodic

Table 2.09 The electromotive table

Did you know?

A simple battery uses electrolysis to produce electricity

On the Job: Electrolytic corrosionTo prevent electrolytic corrosion from harming the metals within a heating or cooling system it is normal practice to install a sacrificial anode somewhere within the system. The anode, typically made from zinc which is low down on the electromotive table, is dissolved away in preference to other metal parts of the system.


62

ElectricityThis section starts to look at the principles that form the basis for electrical studies. They are the foundation for further work later on when these principles are interpreted in a practical way.

What is electricity?We should have learned so far that every substance is composed of molecules, which in turn are made up of atoms. Atoms are not solid but are made up of even smaller matter. The centre of each atom has a nucleus made up of protons and neutrons. Protons possess a positive charge while the neutron is electrically neutral. The neutron can be considered the glue that holds the nucleus together.

The remaining particles in an atom are known as electrons. These circle in orbits around the nucleus and have a negative charge. While there are an equal number of positive and negative charges in the atom, it is stable. But in some cases it is possible to remove or add an electron to a neutral atom and leave it with a net positive or negative charge. These atoms are then called ions.

Under normal conditions electrons are arranged in layers, or shells, at varying distances from the nucleus. If the outer shell is full, the electrons are bound tightly to the atom. However, if there are many gaps in the outer shell, the

Aluminium (Al) Low cost and weightNot very flexibleUsed for large power cables

•••

Brass (alloy of copper and zinc)

Easily machinedCorrosion resistantUsed for terminals and plug ins

•••

Carbon (C) HardLow friction in contact with other materialsUsed for machine brushes

•••

Copper (Cu) Good conductorSoft and ductileUsed in most cables and busbar systems

•••

Iron/Steel (Fe) Good conductorCorrodesUsed for conduit, trunking and equipment enclosure

•••

Lead (Pb) FlexibleCorrosion resistantUsed as an earth and as a sheath of cable

•••

Table 2.10 Common conductors


63


63

electrons there are easily moved from their orbits, so are free to join those of another atom whose own outer electrons may in turn leave to join another atom, and so on. It is these wandering or free electrons moving about the molecular structure of a material that give rise to a flow of electricity.

We call a material that allows the free passage of electrons a conductor (see Table 2.10) and one that does not, an insulator (Table 2.11).

Rubber/plastic Very flexibleEasily affected by temperatureUsed in cable insulation

•••

Impregnated paper Stiff and hygroscopicUnaffected by moderate temperaturesUsed in large cables

•••

Magnesium oxide Powder, therefore requires a containing sheathVery hygroscopicResistant to high temperatureUsed in cables for alarms and emergency lighting

••••

Mica Unaffected by high temperatureUsed for kettle and toaster elements

••

Porcelain Hard and brittleEasily cleanedUsed for carriers and overhead line insulators

•••

Rigid plastic Less brittle and less costly than porcelainUsed in manufacture of switches and sockets

••

Table 2.11 Common insulators

Electric currentAn electron is a very small item and therefore it would not be practical to try and measure movement of one electron. So we group a number of electrons together and then measure the number of groups. This grouping is known as a coulomb, which comprises millions of electrons. The flow rate of coulombs along a conductor is the current and is defined as one coulomb per second or one ampere (amp or A for short).

Hence the ampere is a measure of the quantity of electricity flowing in circuit.

Basic electric circuitIn order to cause electron flow an electromotive force (e.m.f.) is required. Any apparatus that produces an e.m.f, such as a battery or generator, is called a power source. It requires wires or cables to be attached to its terminals to form a basic circuit. Electromotive force is the pressure available to cause electron flow and is measured in volts (or V for short).

Definition

Hygroscopic means that it absorbs water easily


64

For practical purposes a working circuit should:

have a power source or supply

have a device like a fuse to protect the circuit

contain conductors through which the current can flow

be a complete circuit

have a load, such as a lamp, that needs current to make it work

have a switch to control the supply to the load.

•

•

•

•

•

•

1. Name the three states of matter.

2. Why does oil float on water?

3. What does the term ‘absolute zero’ mean?

4. List the three forms of heat transfer.

5. Name two forms of corrosion.

6. What is an alloy?

7. State the SI unit of force.

8. Explain ‘centre of gravity’.

9. Explain how a pulley system makes work easier.

10. Define the SI unit of power.

11. Explain the term ‘pressure’.

12. State the products of complete combustion.

Knowledge check


OVERVIEW Effective communication takes place when the sender’s message is understood by the recipient. There are many ways to represent technical information and this chapter looks at ways of presenting and using technical instructions found in the MES industry. Drawings are only one way of communicating technical information. Others include: specifications, data sheets, manuals and manufacturers’ instructions.

At the end of this section you should be able to:

• identify the common types of engineering drawing

• identify the more common graphical symbols and abbreviations used on MES drawings

• describe how specifications and standards are used to communicate information

• understand the coding used to identify main services within the MES sector

• understand how computers and information technology (IT) are used in the MES sector.

Interpret drawings, specifications and data, and describe the use of IT in the sector

65

chapter3

66


Engineering drawingsEngineering drawings should provide accurately scaled views of an object and contain all necessary information for understanding the drawing. Hence, they follow various conventions on display and content, particularly on how images of the object are projected on to the drawing.

Title block The first thing to be looked at on any drawing is the title block, which in most cases is situated in the bottom right corner of the sheet. It is there to identify the drawing with the project. For example look at Figure 3.01.

J6990 HED Mechanical Eng BW PDFAW_068_J6990 AW by HL Studios

PROJECT

C.M.NICHOLS LTD. AND SON

BUILDING SERVICES ENGINEERING CONTRACTORS

15 – 17 THE TERRACEANYTOWN

TITLE

DATE JAN 2000 XREF 454–3 SCALE 1:50DRG SIZE A1CAD 545011

CHECKED DRG NO

DRAWN SAL/DJR

THIRD FLOOR – HEATINGAND WATER SERVICES

590049/11C

Figure 3.01 Title block

Try to identify the scale, date, project title and the title of the part of the job the drawing relates to.

Note also the drawing number (590049/11 C). This is very important as, in this example, it carries the drawing revision letter ‘C’, showing that this is the third drawing to be issued with updated information. Look above the title block and you will see what has been changed and when.

On the job: RevisionsMark has been given two sets of drawings for his job, which seem the same but he looks at the title block and sees, by the drawing numbers that there has been a revision. He sorts out the latest revision and destroys the earlier drawings.

Remember

It is good practice to destroy or file away all previous drawings when a new revision is issued, so that out-of-date drawings are not used by mistake

67

Chapter 3 Interpret drawings, specifications and data, and describe the use of IT in the sector

Scaled viewsEngineering drawings follow rules of orthographic projection for displaying scaled views. This assumes that we have a scale model of the object, which we look at from different sides and draw parallel lines from it on to a sheet of paper. Hence, we end up with different views of the object. If it was a building, we would expect to see at least a front view (elevation), top view (plan) and views from both sides.

There are then different ways of laying out the views on the drawing. Engineering drawings typically use ‘third angle projection’, where the front view is in the centre, the plan view drawn above it and each end view next to the end that it refers to.

Figure 3.02 illustrates this. Note how the views of the object have a relationship to the front view. Extra, or auxiliary, views can be added as required and would be positioned in relation to the view they are developed from. Auxiliary views include sectional, three-dimensional and exploded views.

Sectional viewA sectional view is drawn as though the component has been sawn through at a particular point to show hidden detail or how items are assembled. The main view will be annotated to show the section line and direction of view.

Three-dimensional views Three-dimensional views are drawn using isometric projection, pictorial views or oblique views. They help in understanding how objects look to the eye and may be used to show how generally things relate to each other:

• For an isometric projection, the object is drawn at an angle where one corner of the object is closest to the viewer. Vertical lines remain vertical. Horizontal lines are drawn at an angle of 30° to the horizontal.

• Oblique views are similar to isometric, except that one side is square on to the viewer and other lines are at 45° to the horizontal.

• A pictorial view looks at one side square on and the other sides are drawn so that the lines meet at some distant point, called the ‘vanishing point’.

J6990 HED Mechanical Eng BW PDF AW_069_J6990 AW by HL Studios

Isometric projection of shaped block

Drawn on the left is what would be seen from the left

Drawn underneath is what would be

seen from underneath

Drawn above is what would be

seen from above

This is selected to be the front view

Drawn on the right is what would be seen from the right

Figure 3.02 Third angle orthographic projection

J6990HED Mechanical Eng BW PDFAW_070_J6990AW by HL Studios

Figure 3.03 Sectional views

68



FRONT

a pictorial

b isometric

Vanishingpoint

c oblique

30° 30°45°

FRONT

FRONT

SIDE

SIDE

SIDE

PLAN

PLAN

PLAN

Figure 3.04 Three dimensional views

Exploded viewsExploded views are very useful to show in detail how components are assembled. They are found in most service manuals and tend not to follow strict rules of projection.


Stator windingElectrical regulator

Capacitor

Terminal boxand gland

Stainless steelcapped bearingbracket

Stainlesssteelimpeller

Joint ringposition

Bearings

Bolt andwasher

Pumpcasing Vent

screws

Rotor

Shaft

Locknutand washer

Figure 3.05 Exploded view

On the job: Exploded viewsDave is servicing a pump, which has a leaking gland. He thinks that the pump has been assembled incorrectly some time ago. He uses his laptop computer to access the Internet and download a manual for the pump. One of the pages shows an exploded view of the pump assembly and he can see he is correct. He reassembles the pump in the right way and finishes the job.

69



RS05OFFICE:

3 PERSONSRS06FEMALETOILETS

disabledrefuge

RS01LIFTLOBBY

RS04DIS WC

22 HTG F&R TO LLCS5 & IV IN VERTICAL28 CWS FB 22 TA22 HWS FB 22 TA15 BRANCHES AT LL

35 HTG F&RFB TA

RISER DETAILSDWG No 20

R16

R14

22

22

CS5

15

15 RS11LOBBY

RE-USE EXISTING NOTCHESIN JOISTS FOR PIPE RUNS

UNDER FLOOR

Figure 3.06 Typical MES drawing

MES drawingsMES drawings do not necessarily adopt all engineering drawing conventions for projections or lay out. They can be just plans of a building with auxiliary views added somewhere convenient on the sheet, rather like a street map or atlas. True engineering drawings are used in the MES sector, but largely as reference material for components.

Have a look at the sample MES drawing in Figure 3.6. Note how the design engineer is providing information to the site staff. It almost looks like a foreign language, doesn’t it?

70


Drawing symbols and abbreviationsIn order to reduce the amount of text that is contained on a drawing, symbols and abbreviations are used that can be understood by the majority of skilled personnel.

It is common practice to print a key to the main symbols on drawings, as well as abbreviations. An individual company may develop their own range of symbols; however those in use will normally be based upon British or International standards. The most common standards are:

• BS 1553 ‘Graphical symbols for general engineering. Part 1 Piping symbols and plant’

• BS 1192 part 3 ‘Construction drawing practice – Recommendations for symbols and other graphic conventions’.

A selection of symbols used in MES are shown in Figure 3.07.

Pipework symbols

Figure 3.07 MES symbols

Pipework symbols

On the job: InstallationsKaren is installing some radiators in a new office block and has to decide which radiators go in which room. She looks at the drawing to identify each radiator, by number, and then looks up the schedule in the specification, which gives her the information she needs.

71


Typical abbreviations are shown in Table 3.01.

Symbol Item Symbol Item

CWS cold water supply HWS hot water supply

DTB drop to below LL low level

F flow R return

FB from below RTA rise to above

Table 3.01 Some common MES drawing abbreviations

Specifications and technical standardsIn addition to graphical representations of a job, a specification is also published. It describes the scope of the contract, listing in detail all of the works that are to be completed. It is effectively a reference book for the job, covering every aspect from boiler size to the colour of protective paint that should be used. There may be a schedule of heat emitting and evaporator cooling units, giving size, type and piping connection size. There will certainly be references to national or international standards, codes of practice and possibly statutes.

These references are there to protect subsequent users of the buildings, or any other items produced under the contract, by ensuring that work is carried out correctly.

Codes of practice and standards are created by groups of people who decide on the best methods for a particular operation, to ensure safety during construction and that the final product is fit for purpose. Sometimes reference may be made to statutoryinstruments, which are standards agreed on and serious enough to be enforceable by law. Every item and practice used will have a standard associated with it and the engineer is expected to become familiar with them.

Coding used to identify main services within the MES sector With the introduction of complex mechanical services within modern buildings there is a need to identify each of the piping systems, electrical cabling and gas cylinders. This is achieved by colour coding according to agreed standards. An engineer in unfamiliar plant rooms or service ducts will be able to use this colour coding to trace piping or cabling.

Definition

A statute or statutory instrument defines something in writing that has to be obeyed by law

72


Identification of pipelinesBS 1710 is the normative recommendation for piping services and consists of identification bands painted around the relevant supply pipe. The bands are of a specific size, being 400 mm base colour band with a 100 mm safety code colour band in the centre, as in Table 3.02. A general list would include, but not limited to, the following.

Item Colour code

Potable water green/blue/green

Cooling water green/white/green

Chilled water white/green/white

LPHW heating blue/crimson/blue

HTHW heating crimson/blue/crimson

Cistern fed cold water white/blue/white

Hot water supply white/crimson/white

Fire fighting red no band

Natural gas canary yellow

Table 3.02 Pipeline colour codes

On the job: Identifying pipe linesAli is working in the plant room and is trying to locate a chilled water circuit from a roof top chiller unit. He looks for pipe work that has white/green/white identification bands and arrows pointing in the direction of flow.

Identification of compressed gas cylindersThe MES engineer will, at some time or other, become involved with compressed gas cylinders. These, too, are colour coded to make them readily identifiable. Some gases will be more hazardous than others but all compressed gases should be treated with care and respect. Information on handling and storage should be sought from gas suppliers before use.

Colour coding conforms to either BS 349 or EN 1089-3.

73


Identification of electrical cablingColour coding conforming to BS 7671 is used to identify particular electrical circuits and cabling. Some examples are given in Tables 3.03 and 3.04.

Item Colour code

Live brown

Neutral blue

Protective earth green/yellow

Table 3.03 Colour codes for alternating current single-phase power supply

Item Colour code

Protective earth green/yellow

Phase 1 brown

Phase 2 black

Phase 3 grey

Neutral blue

Table 3.04 Colour codes for alternating current three-phase power supply

74


Information technology within the MES sectorInformation technology (IT) has transformed working practices for all sectors of industry, and continues to do so as new technology is developed. The MES sector is no exception. All of us are familiar with the rapid exchange of information available almost everywhere via the mobile phone. Computers, also, have moved from the office to the work site, including laptop PCs and special purpose computers for operating in harsh environments.

Computer technology is also embedded in an increasing amount of equipment that we now use. In addition, it is enabling the introduction of smart measuring instruments and new ones like laser levels. However, we will look here at the impact of IT on working practices in the industry.

In the officeThe modern PC has many thousand times the computing power of the most expensive main frame computer of 10 years ago, and massive storage capacity. The information does not even need to be available on each computer, as computers can be networked so they can share information. Broadband connection to the Internet, over standard telephone lines, has opened up almost unlimited information, which can be looked at and, if required, down loaded for use.

Many of us are familiar with software packages like word processing. They come complete with automatic checking of spelling and grammar, and can use templates to help generate complex documents quickly and accurately, with company logos and a company style. Printers are also now so effective that high quality documentation, including photographs and graphics, can be generated within the office. The typewriter is already a rarely used item. There are then hundreds of specialist packages to help even very small firms to carry out functions, which previously needed specialist support or had to be contracted out. These range from accountancy to assistance with planning and design. Powerful ‘search engines’ also enable rapid retrieval of information from within a computer system, or the Internet.

Most firms now have their own website, which advertises their capabilities, products and services, and can provide rapid ordering and payment facilities. It can be a powerful marketing tool for a company, and streamline the process of acquiring products for a task.

Within the office, some of the main changes affecting the MES industry are as follows, though you can probably think of more:

• reduction in paper storage, and rapid retrieval of information

• project planning and scheduling, including the ability to ask ‘What if?’ questions and make adjustments

75


• generation, updating, printing and electronic distribution of specifications and drawings

• ready access to standards, with search/help facilities

• version control on drawings and specifications

• access to the Internet to research and purchase items like fixings

• marketing via a website to assist in gaining further work

• internal production of sales literature

• rapid exchange of information, internally and externally, by email

• monitoring, controlling and rapidly adjusting the activities of MES engineers on site, to make best use of their time and capabilities

• even using webcams to provide visual as well as verbal links with staff, to improve communication.

Use of IT on sitePCs and printers can be taken on site and, provided there is access to a telephone line, or “wifi” (wireless fidelity) facility, they can be electronically linked to the office and the Internet. Linkage can be via a mobile phone, with many of these functions, including Internet access, available now on mobile phones, though necessarily limited by the size of the screen.

Hence, almost all office functions could be carried out on site; and some smaller MES engineering organisations effectively do run their business from a ‘virtual office’ on site. However, changes of most relevance to the MES engineer operating out of a base office are probably:

• access to project information, including the latest versions of drawings and plans

• access to British Standards, legislation and all the various guides that may be needed to complete a job, with search/help facilities

• ability to communicate easily with the office to clarify queries and seek advice, including downloading graphical and textual information

• rapid exchange of information with third parties to resolve problems

• data capture and recording progress on site, with rapid transmission back to the office

• access to the Internet to research and even purchase alternative items like fixings

• use of digital cameras to report and record progress on site, including records for inspections and reports.

76


Building management systems (BMS)IT is also being built into new buildings to provide life control and maintenance, reducing both energy and maintenance costs. They are called building management systems (BMS) and comprise a computerised intelligent network of electronic devices that monitor and control mechanical features, lighting and sometimes the security of buildings. The MES engineer may be involved in their installation, and can expect to use them during maintenance and repair.

A typical system comprises a PC controller connected to various sensors and control devices throughout the building. These could be:

• occupancy sensors to monitor building use

• lighting controllers that switch on or off to a schedule

• air conditioning and heating controllers, which monitor and control the central heating plant, air outlets, chilled water and heat circulation.

There will often be feedback on the efficiency of the building environment, either produced automatically or on demand, such as temperature and humidity of nominated areas. The system may monitor and report the condition of filter systems, circulating pump efficiency or indicate which circuits are open and shut. Analysis of performance over a given period is possible, enabling predictions of component failure.

In the event of a breakdown, data can also be directed to alarm systems on site, or remotely sent via pagers or mobile phone to building managers.

Networking can allow several buildings, such as a hospital complex, to be monitored by one PC controller. Smaller systems may have an access point for connection of a laptop computer, so that a mobile service engineer can carry out diagnostics of the system and make adjustments.

The downside of ITIT is not the answer to every prayer. If not correctly exploited it can create more problems than it solves:

• Computers can crash, through malfunction of either hardware or software. The more software that is put on to a system, the more risk there is of unwanted reactions between them.

• Data can be corrupted, deliberately or accidentally.

• Internet access and email provide the opportunity for malicious damage via computer viruses.

• It is easy to generate a surfeit of data, making it difficult to control, often referred to as ‘information overload’.

• Tight control is needed, especially of contractually important documents such as drawings, to ensure that only the latest versions are available to those needing access.

77


• A balance must be struck between giving access and ensuring security of information. Tight controls must ensure that data is not changed by those unauthorised to make changes. Connecting a computer system externally to the Internet, if only to send emails, can allow unauthorised access to a company’s files to extract information or amend data. It can be a problem internally, with temporary staff of particular concern, as a large amount of a company’s commercially sensitive information can be rapidly downloaded on to storage medium small enough to hide in a pocket.

All of these risks and problems can be minimised by good procedures. There are many specialist software packages available, especially designed to help manage data and combat malicious attack by computer viruses, but they only work if used correctly. Continual vigilance is needed. An effective system must also be in place to back up and retrieve data.

78


1. What information would you expect to see in the title block of an engineering drawing?

2. When would you expect to use an exploded view of something and where could you find it?

3. State the colour of live, neutral and earth cables found on a 230 V supply.

4. Describe the contents of a typical contract specification.

5. Describe what the initials BMS refer to.

6. List the advantages of using an IT based management system for a contract.

Knowledge check

Date post:	20-Feb-2015
Category:	Documents
Upload:	pearson-schools-and-fe-colleges
View:	1,176 times
Download:	6 times

Heating & Ventilation, Air Conditioning & Refrigeration: Chapter 2

Documents