Efficient Steam Generation & Distribution

Dr. Bipin ThapliyalScientist

Central Pulp & Paper Research Institute, Saharanpur

Contents

Steam and its propertiesCondensate and Flash SteamEfficient Steam GenerationSteam Distribution

Why Steam is so popular as heat conveying media in industry?

Highest specific heat and latent heat Highest heat transfer coefficient Easy to control and distribute Cheap and inert

1. Energy • Energy can neither created nor destroyed.• It can always be accounted for, and if it disappears at point. A then it reappears in

equal amount at point B. • The steam tables can be relied on always to provide information on the properties

of steam.

2. Heat Flow• A temperature difference is necessary for heat to flow. Heat flows from higher-

temperature to lower-temperature.• The rate of flow varies directly with temperature differences, and inversely with

any resistances to this flow.!

3. Fluid Flow • Any fluid tends to flow from high-pressure to lower-pressure regions because of

the effects of friction,• The rate of flow varies directly with pressure differences and inversely with any

resistances to this flow. • Gravity acts downwards! the denser constituents in a mixture often tend to move to

the bottom of a space, unless other forces acting on them oppose such motion.

Saturated SteamSuper Heated SteamCondensateFlash Steam

What is steam ?

Adding heat energy to water raises its temperature: some 419.04 KJ will raise 1 kg to 1000C, any further addition of heat evaporates the water.

If 2257 KJ are added to each kg of water, then all the water becomes the dry gas, steam.

• In case, if only part of this extra energy is added –say, 90% - then 90% of the water evaporates and the other 10% remains liquid.

• The specific volume of steam at atmospheric pressure is 1.673 m3/ kg, so the mixture 1 kg steam containing 90% steam and 10% water would occupy a volume of (0.9x 1.673)+ (0.1x 0.001) = 1.5057 m3.

• This mixture would be described as steam with a dryness fraction of 0.9.

Specific Volume of Steam

• If the water is kept at a pressure above atmospheric, its temperature can be raised above 1000C before boiling begins.

• At 10 bar gauge, for example, boiling point is at about 184.10C.

• The extra energy needed to convert water at this pressure and temperature into steam (the enthalpy of evaporation) is now rather less at 2000.1 KJ/ kg, while the volume of 1 kg of pure steam is only 0.177 m3.

• When steam at the saturation temperature contacts a surface at a lower temperature, and heat flows to the cooler surface, some of the steam condenses to supply the energy.

• The pressure and temperature of the steam remain constant after condensing, due to a sufficient supply of steam moving into the volume which had been occupied by the steam and is now condensed.

Heat & Mass Flow of Steam

• The condensate produced within the heat exchangers, as also within the steam lines, is initially at the saturation temperature and carries the same pressure.

• If it is discharged to a lower pressure, through a manual or automatic drain valve (steam trap) or even through a leak, it then contains more energy than water is able to hold at the lower pressure if it is to remain liquid.

• If steam at the saturation temperature were to contact a surfaceat a higher temperature, as in some boilers, its temperature could be increased above the evaporation temperature and the steam would be described as superheated.

• Superheated steam is very desirable in turbines, where its use allows higher efficiencies to be reached, but it is much less satisfactory than saturated steam in heat exchangers.

• It behaves as a dry gas, giving up its heat content rather reluctantly as compared with saturated steam, which offers much higher heat transfer coefficients.

• The condensate often contains excess of energy.

• If the excess energy amounts to, say, 5% of the enthalpy of evaporation at the lower pressure, then 5% of the water would be evaporated. The steam released by this drop in pressure experienced by high–temperature water is usually called flash steam.

• Recovery and use of this low-pressure steam, released by flashing, is one of the easiest ways of improving the efficiencyof steam-utilization systems.

Condensate & Flash Steam

• It is equally true that condensate, even if it has been released to atmospheric pressure, carries the same 419.04 KJ/kg of heat energy that any other water at the same temperature would hold.

• Condensate is a form of distilled water, requiring little chemical feed treatment of softening. And it already holds energy which may amount 15% of the energy which would have to be supplied to cold make-up feed water, even in relatively low-pressure systems.

Condensate & Flash Steam

Steam Generation

Steam Generation: BoilerA boiler is an enclosed vessel that provides a means for combustion heat to be transferred into water until it becomes heated water or steam. The hot water or steam under pressure is then usable for transferring the heat to a process.When water is boiled into steam its volume increases about 1,600 times, producing a force that is almost as explosive as gunpowder. This causes the boiler to be extremely dangerous equipment that must be treated with utmost care.

Boiler Types and Classifications

Fire tube or “fire in tube”boilers;contain long steel tubes through which the hot gasses from a furnace pass and around which the water to be converted to steam circulates.

There are virtually infinite numbers of boiler designs but generally they fit into one of two categories:

Fire tube boilers, typically have a lower initial cost, are more fuel efficient and easier to operate, but they are limited generally to capacities of 25 tons/hr and pressures of 17.5 kg/cm2.

Boiler Types and Classifications

Water tube or “water in tube”boilers are in which the the water passes through the tubes and the hot gasses passes outside the tubes.

These boilers can be of single- or multiple-drum type. These boilers can be built to any steam capacities and pressures, and have higher efficiencies than fire tube boilers.

Boiler Types and Classifications

Packaged Boiler: The packaged boiler is so called because it comes as a complete package.Once delivered to site, it requires only the steam, water pipe work, fuel supply and electrical connections to be made for it to become operational. Package boilers are generally of shell type with fire tube design so as to achieve high heat transfer rates by both radiation and convection

Stoker Fired Boiler:

Stokers are classified according to the method of feeding fuel to the furnace and by the type of grate. The main classifications are:

1. Chain-grate or traveling-grate stoker and 2. Spreader stoker

Chain-Grate or Traveling-Grate Stoker Boiler

Spreader Stoker Boiler

Spreader stokers utilize a combination of suspension burning andgrate burning.

The coal fines are burned in suspension; the larger particles fall to the grate, where they are burned in a thin, fastburning coal bed.

This method of firing provides good flexibility to meet load fluctuations, since ignition is almost instantaneous when firing rate is increased.

Hence, the spreader stoker is favored over other types of stokers in many industrial

Pulverized Fuel Boiler

The coal is ground (pulverised) to a fine powder, so that less than 2% is +300 micro metre (µm) and 70-75% is below 75 microns, for a bituminous coal.

The pulverised coal is blown with part of the combustion air into the boiler plant through a series of burner nozzles.

Secondary and tertiary air may also be added. Combustion takes place at temperatures from 1300-1700°C, depending largely on coal grade.One of the most popular systems for firing pulverized coal is the tangential firing using four burners corner to corner to create a fireball at the center of the furnace

FBC Boiler

Air & finely divided bed of solid particles (sand) are supported on a fine mesh. At a particular air velocity, a stage is reached when the individual particles are suspended in the air stream.

Further, increase in velocity gives rise to bubble formation, vigorous turbulence and rapid mixing and the bed is said to be fluidized.

The sand in a fluidized state is heated to the ignition temperature of the coal and the coal is injected continuously in to the bed. The coal burns rapidly, and the bed attains a uniform temperature due to effective mixing.

• Fuel flexibility,

• Reduced emission of noxious pollutants such as SOx and NOx,

• Compact boiler design and

• Higher combustion efficiency.

Advantages of Fluidised bed combustion over conventional firing systems

The various efficient steam generation opportunities in boiler system are related to;

• Combustion of fuel,

• Heat transfer,

• Avoidable losses,

• High auxiliary power consumption,

• Water quality and

• Blowdown.

Efficient Steam Generation

Examining the following factors can indicate if a boiler is being run to maximize its efficiency:

1. Stack Temperature

Stack temperatures greater than 200°C indicates potential for recovery of waste heat.

It also indicates the scaling of heat transfer/recovery equipment and hence the urgency of taking an early shut down for water / flue side cleaning.

Efficient Steam Generation

2. Feed Water Preheating using Economiser

The potential for energy saving depends on the type of boiler installed and the fuel used.

For a typically older model shell boiler, with a flue gas exit temperature of 260 °C, an economizer could be used to reduce it to 200 °C, increasing the feed water temperature by 15 °C. Increase in overall thermal efficiency would be in the order of 3%.

For a modern 3-pass shell boiler firing natural gas with a flue gas exit temperature of 140 °C a condensing economizer would reduce the exit temperature to 65 °C increasing thermal efficiency by 5%.

Efficient Steam Generation

3. Combustion Air Preheat

Combustion air preheating is an alternative to feedwaterheating.

In order to improve thermal efficiency by 1%, the combustion air temperature must be raised by 20 °C.

Most gas and oil burners used in a boiler plant are not designed for high air preheat temperatures.

Modern burners can withstand much higher combustion air preheat, so it is possible to consider such units as heat exchangers in the exit flue as an alternative to an economizer, when either space or a high feed water return temperature make it viable

Efficient Steam Generation

4. Incomplete Combustion

In the case of oil and gas fired systems,

• CO or smoke with normal or high excess air indicates burner system problems.

• poor mixing of fuel and air at the burner.

• Poor oil fires can result from • improper viscosity, • worn tips, • carbonization on tips and • deterioration of diffusers or spinner plates.

Efficient Steam Generation

5. Excess Air Control

Excess air is required in all practical cases to ensure complete combustion, to allow for the normal variations in combustion and to ensure satisfactory stack conditions for some fuels.

The optimum excess air level for maximum boiler efficiency occurs when the sum of the losses due to incomplete combustion and loss due to heat in flue gases is minimum.

This level varies with furnace design, type of burner, fuel and process variables. It can be determined by conducting tests with different air fuel ratios.

Efficient Steam Generation

Relation Between CO2 and Excess Air for Fuel Oil

Relation Between Residual Oxygen and Excess Air

Various methods available to control the excess air are:

• Portable oxygen analysers and draft gauges - Excess air reduction up to 20% is feasible.

• Continuous oxygen analyzer with a local readout mounted draft gauge, by which the operator can adjust air flow. A further reduction of 10–15% can be achieved.

• Damper control by continuous oxygen analyzer. This enables an operator to remotely control a number of firing systems simultaneously.

• The automatic fan speed control from O2 analyser feed, It’s cost is really justified only for large systems.

Efficient Steam Generation

6. Radiation and Convection Heat Loss

With modern boiler designs, this may represent only 1.5% on the gross calorific value at full rating, but will increase to around 6%, if the boiler operates at only 25 percent output.

Repairing or augmenting insulation can reduce heat loss through boiler walls and piping.

7. Automatic Blowdown Control

Uncontrolled continuous blowdown is very wasteful. Automatic blowdown controls can be installed that sense and respond to boiler water conductivity and pH.

A 10% blow down in a 15 kg/cm2 boiler results in 3%

Efficient Steam Generation

8. Reduction of Scaling and Soot Losses

Elevated stack temperatures may indicate excessive soot buildup or scaling on the water side.

When the flue gas temperature rises about 20 °C above the temperature for a newly cleaned boiler, it is time to remove the soot deposits.

It is estimated that 3 mm of soot can cause an increase in fuel consumption by 2.5% due to increased flue gas temperatures.

Periodic off-line cleaning of radiant furnace surfaces, boiler tube banks, economizers and air heaters may be necessary to remove stubborn deposits

Efficient Steam Generation

9. Reduction of Boiler Steam Pressure

This is an effective means of reducing fuel consumption, if permissible, by as much as 1 to 2%.

Lower steam pressure gives a lower saturated steam temperature and without stack heat recovery, a similar reduction in the temperature of the flue gas temperature results.

Pressure should be reduced in stages, and no more than a 20 percent reduction should be considered.

Efficient Steam Generation

10. Variable Speed Control for Fans, Blowers and Pumps

In general, if the load characteristic of the boiler is variable, the possibility of replacing the dampers by a VSD should be evaluated.

11. Effect of Boiler Loading on Efficiency

The maximum efficiency of the boiler does not occur at full load, but at about two-thirds of the full load. If the load on the boiler decreases further, efficiency also tends to decrease.

Efficient Steam Generation

12. Proper Boiler Scheduling

It is usually more efficient, on the whole, to operate a fewer number of boilers at higher loads, than to operate a large number at low loads.

13. Boiler Replacement

A change in a boiler can be financially attractive if the existing boiler is :

•old and inefficient•not capable of firing cheaper substitution fuel•over or under-sized for present requirements•not designed for ideal loading conditions

Replacement must be carefully studied.

Efficient Steam Generation

Efficient Steam Utilization and Distribution

Energy in Fuel Purchased

Cost of Energy into Factory

Heat Generation

Distribution

Cost of Energy To Process

Utilisation in Production Process

Final Utilisation Cost

Heat to Product

Energy in Fuel to Useful Energy

Example of System LossThe typical steam system overall efficiency is about 35% as follows:

Generationefficiency80%

Distribution efficiency= 83%(including conden-sate return)

Utilisationefficiency47%

OIL Boiler Steam Mains

P

R

O

C

E

S

S

P

R

O

D

U

C

T

Condensate Return System

100% 80% 75% 35%

20%

5%

25%

5%

15%10%

Efficient Steam UtilisationAvoid steam leakagesProvide dry steam for processUtilising steam at the lowest possible pressureInsulation of steam pipelines and hot process equipmentMinimising barriers to heat transferCondensate recoveryFlash steam recoveryProper selection and maintenance of steam trapsProper sizing of steam and condensate pipingReducing the work to be done by steam

Avoiding Steam Leakages

Leaking Steam Pipe / Valve

Weak whistlingAlmost invisible steam jet

800 litre oil per year800 litre oil per year

Audible Leak

2,000 to 4,000 litre oil per year2,000 to 4,000 litre oil per year

Visible Leak

Weak hissingVisible steam jet

Provide dry steam for the process

Disadvantages of wet steamLess heat content, Extended process time, Irregular heating, Barrier to heat transfer, Overloading of steam traps

Disadvantages of superheated steamPoor heat transfer coefficient, takes time to give up superheat by conduction

Benefits of dry steamHeat transfer is rapid and regular

Providing Dry Steam for Process

Use Dry Saturated steam for processes

Steam Separators to be fitted at point of steam use

Provide a little superheat to ensure dry saturated steamat the process end

Utilising steam at the lowest possible pressure

2151.3 KJ/kg

579.4 KJ/kg

2054 KJ/kg

716.8 KJ/kg

2730.7 KJ/kg 2770.8 KJ/kg Total Heat

Latent Heat

Sensible Heat

2.4 bar, 121.5oC 6.8 bar, 164.3oC

Steam should always be generated and distributed at the highest possible pressure but utilised at the lowest practicable pressure

Optimal Insulation

50 mm insulation compared with an uninsulated pipe: 320 - 29 = 291 W per m263 litre oil per year

50 mm insulation compared with 100 mm insulation: 29 - 19 = 10 W per m9 litre oil per year

Heat loss, 89 mm black steel pipe, 90 oC

Uninsulated320 W/m

100 mm insulation19 W/m

50 mm insulation29 W/m

...But don’t Over-Insulate:There is always an optimum insulation level (1-3 years payback)

Direct Utilization of Steam

Direct Steam use involves both Latent Heat and Sensible Heat

Use temperature controller in Direct Use to avoid steam wastage

Minimising barriers to heat transferSt

eam

Air f

ilm

Cond

ensa

te fil

mSc

ale

Meta

l wall

Scale Stag

nant

pr

oduc

tpr

oduc

t

Resistance to heat transfer of water is 60 – 70 times more than steel and 500 – 600 times than copper

Resistance to heat transfer of Air is 1500 times more than steel and 19,000 times than copper

Effect of air and water filmSt

eam

Air f

ilm

Cond

ensa

te fil

mM

etal w

allSt

agna

nt

prod

uct

prod

uct

Stea

mAi

r film

Cond

ensa

te fil

mM

etal w

allSt

agna

nt

prod

uct

prod

uct

250OC

240OC

210OC 210OC

Steam at 1 kg/cm2Steam at 0.75 kg/.cm2:Air and water film reduced by 50 % ; Quicker process time

Boiler Fuel Saving by Condensate ReturnSaving in percent if condensate is returned to the boiler instead of draining

02468

101214

0 20 40 60 80 100

Condensate return temp. oC

Percentage saved 50 % returned

100% returned

For every 6OC rise in boiler feed water temperature, there is a 1 % raise in boiler efficiency

Reducing the work to be done by steam

Have shortest route of pipingRemove moisture mechanically to the fullest before steam drying / avoid bone dryingOptimise humidity of drier exhaustExplore process integrationUse thermostatic controlsRemove / blank redundant linesProductive use of machinery (Maximise equipment loading)

Look for cheaper alternatives of doing the job (waste heat boilers, thermic fluid heater etc)

Steam Piping : Featureswhile laying new pipes ,it is a compromise between aesthetic design and architect’s plans.Steam pipes should be laid by the shortest possible distance.Provision for proper draining of condensate.For example, a 100mm well lagged pipe of 30-meter length carrying steam at 7 Kg/cm2 pressure can condense nearly 10 Kg. of water in the pipe in one hour unless it is removed from the pipe through traps. The pipes should run with a fall (slope)of not less than 12.5 mm in 3 meter in the direction of flow.

Large pockets in the pipes to enable water to collectDrain pockets should be provided at every 30 to 50 meters and at any low point in the pipe network.Expansion loops are required to take care of the expansion of pipes when they get heated up.Automatic air vents should be fixed at the dead end of steam mains, which will allow removal of air, which will tend to accumulate.

Steam Pipe Sizing and Design

1. Pipe Sizing

Proper sizing of steam pipelines help in minimizing pressure drop.

The velocities for various types of steam are:Superheated 50-70 m/secSaturated 30-40 m/secWet or Exhaust 20-30 m/sec

The steam piping should be sized, based on permissible velocity and the available pressure drop in the line. A higher pipe size will reduce the pressure drop and thus the energy cost. However, higher pipe size will increase the initial installation cost.

By use of smaller pipe size, even though the installation cost can be reduced, the energy cost will increase due to higher-pressure drop.Pressure drop change is inversely proportional to the 5th power of diameter change.Hence, care should be taken in selecting the optimum pipe size.

2) Pipe Redundancy3) Drain Points

These points help in removing water in the pipes due to condensation of steam.The presence of water causes water hammering.A steam trap must be provided at the drain points to avoid leakage of steam.

Steam average velocity (in m/s)Nominal pipe size (in mm) Below 50 50 to 150 200 & above

Saturated steam at sub-atmospheric pressure - 10 - 15 15 - 20

Saturated steam at 0-1 kg/cm2.(g) 15 - 20 17 - 30 20 - 30

Saturated steam at 1.1 - 7 kg/cm2.(g) 15 - 22 20 - 33 25 - 43

Saturated steam over 7 kg/cm2.(g) 15 - 25 20 - 35 30 - 50

Superheated steam at 0 - 7 kg/cm2.(g) 20 - 30 25 - 40 30 - 50

Superheated steam at 7.1-35 kg/cm2.(g) 20 - 33 28 - 43 35 - 55

Superheated steam at 35.1 - 70 Kg/cm2.(g) 22 - 33 30 - 50 40 - 61

Superheated steam over 70 kg/cm2.(g) 22 - 35 35 - 61 50 - 76

Steam Traps?A steam trap is a valve device that discharges condensate and air from the line or piece of equipment without discharging the steam.The purpose of installing the steam traps is to obtain fast heating of the product and equipment by keeping the steam lines and equipment free of condensate, air and non-condensable gases.Functions

To discharge condensate as soon as it is formed Not to allow steam to escape.To be capable of discharging air and other incondensable gases

Types of Steam Traps

To discharge condensate as soon as it is formed Not to allow steam to escape.To be capable of discharging air and other incondensable gases

G r o u p P r in c ip le S u b -g r o u p M e c h a n ic a l tra p D if fe re n c e in d e n s ity

b e tw e e n s te a m a n d c o n d e n s a te .

B u c k e t ty p e - O p e n b u c k e t - In v e r te d b u c k e t ,

w ith le v e r , w ith o u t le v e r

- F lo a t ty p e - F lo a t w ith le v e r - F re e f lo a t

T h e rm o d y n a m ic tra p

D if fe re n c e in th e rm o d y n a m ic p ro p e r t ie s b e tw e e n s te a m a n d c o n d e n s a te

D is c ty p e O rif ic e ty p e

T h e rm o s ta tic tra p

D if fe re n c e in te m p e ra tu re b e tw e e n s te a m a n d c o n d e n s a te

B im e ta ll ic ty p e m e ta l e x p a n s io n ty p e .

Flash Steam

Flash steam available in % - S1 - S2

L2S1 - Sensible heat of high pressure condensateS2 - Sensible heat of steam at lower pressure (at which it is flashed)L2 - Latent heat of flash steam at lower pressure

VAPOR ABSORPTIONREFRIGERATION

CONTINUOUS STERILISER

DEAERATOR

FERMENTATION

PILOT PLANT

MICRO BIOLOGYLAB

EXTRACTION

SOLVENTRECOVERY

FUEL OIL TANKFARM

7 TPH

Bleed

Boiler Steam

DG Set WHR Steam

PRDV12 Bar

DistributionHeader

2 TPH

24 TPH

8 Bar

4 TPH

PRDV

3 Bar

0.5TPH

11 TPH

0.5 TPH

2.5 TPH

0.5 TPH

3 TPH

MediaSterilisation

GerminatorSterile vesselsPre fermentor

4 TPH

4 TPH

8 bar

Make a steam balance

Specific Steam Consumption

0

50

100

150

200

250

300

Sep'96 Oct'96 Nov'96 Dec'96 Jan'97 Feb'97 Mar'97 Apr'97 May'97 Jun'97 Jul'97 Aug'97

Month

Pro

duct

ion

(Ton

nes)

0

510

15

20

25

3035

40

KL / Tonne of P

E

Production (Total PE)

Steam (T) /Ton of PE

Conduct Steam audit

ConclusionAt your plant;

Ensure proper sizing of steam lines Select right type of trapsTest and identify malfunctioning trapsQuantify steam leakagesDetermine heat loss from leakagesQuantify flash steam and its recoveryIdentify energy saving opportunities in steam distribution and utilization systems