Ch 1. Properties of Steam

8/3/2019 Ch 1. Properties of Steam

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Properties of steam

Prof. Medhat Sorour



Why Use Steam in IndustrialHeating?

Steam is produced by the evaporationof water which is relatively cheap andplentiful commodity in most parts ofthe world.

Steam temperature can be adjustedvery accurately by the control of its

pressure, using simple valves.




Steam carries relatively large amounts ofenergy in small mass, and when it is

encouraged to condense back to water,high rates of energy flow (into thematerial being heated) are obtained.

The steam heating plant does not have tobe very large.




Steam is inherently safe and suitable foruse in all zones.

Steam, unlike electricity, is unable to emitelectrical charges which may result insparking and large scale damage to both

personnel and property.



Advantages of steam heating

Efficiency

Flexibility

Economics

Controllability

Reliability



Efficiency

Steam is a very efficient carrier of heatenergy.

Steam tracing systems for example areusually ideally suited to using excessprocess steam that would otherwise go to

waste.



Flexibility

It is often necessary to steam clean aprocess pipe prior to the beginning of anew process.

Process pipes that are fitted with steamtracers may easily be steam cleaned

without the fear of damaging the actualtracing system.



Flexibility

In the case of electric trace heating,however, many of the available tracingcables are unable to withstand the hightemperatures that are experienced duringthe cleaning process.

Cables that are able to withstand high

temperatures (200C) are considerablymore expensive.



Economics

The system is easy to design and install and uses simple mechanical componentswhich require no external power source.

All of these factors combine to suggestthat the labor costs for installation andmaintenance are likely to be less than for

more complex heating applications.



Controllability

In saturated steam, temperature andpressure are directly related, sotemperature can be regulated by the

straightforward control of pressure. The ability to easily raise the steam

temperature allows adjustments to bemade to the heat output from theapplication in the inevitable event ofinsulation degradation.



Controllability

Inexpensive self acting temperature

controls are available; these sense

ambient temperature or thetemperature of the process itselfand regulate the output of the steam

accordingly.



Controllability

For larger duties or where a moresophisticated control is desired,pneumatically operated controls aresimple to select and source.

A limited form of temperature regulation

can also be achieved by the prudentselection of automatic steam traps.



Reliability

Steam systems are extremely strong.

Their operation is not affected by adverseweather conditions and they can easilywithstand the normal day to day knocksthat occur in a process plant.



The cost of production of Steam

The main cost element with heating systemsis the running cost; cost of the energyconsumed.

As products, pipelines and insulation typescome in a variety of shapes and sizes, andbecause ambient conditions vary from onelocation to another, a practical approach is to

review the cost of a GJ of energy .

The cost of the GJ can then be used as anindex for comparison purposes.



The cost of steam depends on:

The feedwater temperature.

Boiler pressure, and the percentage of

condensate return.

other components such as:

o the cost of water,

o water treatment,

o effluent discharge and

o the cost of the fuel itself.





Some of these components vary

considerably from site to site; it is

therefore necessary to define a ‘standard’

set of criteria:

Boiler feed water temperature = 80C Mass of Steam = 1,000 kg

Boiler efficiency = 80%

Condensate return = 100%

Fuel = Heavy fuel oil (C.V=42,500kJ/kg)



Example

From steam tables

Total enthalpy at 7 bar g = 2,769 kg/kg

Enthalpy of water at 80C = 335 kJ/kg

Difference = 2,434 kJ/kg

Therefore

energy to raise 1000 kg of steam

= 2,434 kJ/kg x 1,000 kg

= 2,434,000 kJ



Example

Each kg of fuel provides 42,500 kJ/kg,

therefore the mass of fuel required to raise 1000

kg of steam

= 2,434,000 kJ = 57.3 kg

2,500 kJ of energy,



Example

The efficiency of the boiler is 80%

Therefore more fuel is required. 57.3 = 71.6 kg fuel

0.8

Ton of fuel oil = 330 L.E

The cost of 71.6 kg = 15.5 L.E

The cost of 1GJ = 15.5 = 6.36 L.E

2.434



Example

It must be realized that additional 20%

must be added to the cost of steam for

water treatment and blowdown. This isstill very cheap compared to the cost of

electricity which is 50 L.E/ GJ



THE HEATINGPROPERTIES OF STEAM



USE OF LOW PRESSURE STEAM

For heating and process purposessteam should be always used at

the lowest possible pressure.



THE HEATING VALUE OF LOW

PRESSURE STEAM

What can be done to make use of the fact

that the latent heat of steam is greater the

lower the pressure?

When steam is used inside a heating

surface, coil, pipe or jacket, the steam

condenses and gives up its latent heat.

All the liquid or sensible heat remains inthe condensate which is removed by the

trap.



THE HEATING VALUE OF LOWPRESSURE STEAM

Temperature enthalpy phase diagram



Extract from the saturated steam tables




Low pressure steam

Higher pressure




The lower the pressure the greater the

latent heat and the less the sensible heat.

The greatest amount of heat can beobtained from the condensation of heating

steam by using the lowest possible

pressure.




PRESSURE STEAM

It is known from the steam table thatthe steam at 1.39 atm contains 5percent more latent heat than steam at4.54 atm .

If a plant could be adapted to use thelower pressure we should use 5 percent

less steam . In addition as this lower pressure

contains 2.5 percent less total heat,giving a total of 7.5 percent.




PRESSURE STEAM

Cases where advantages cannot be taken of

this useful steam property.

1. The heating surface in the plant may betoo small to give a proper heat rate.

2. Some process requires a certain

minimum temperature, and consequently

a sufficient pressure must be used to give

such special temperatures.




PRESSURE STEAM

Disadvantage of very low pressures is the

need for larger steam mains.

Steam at 1.397atm has more than threetimes the volume of steam at 4.54 atm.

In addition the steam flow at 1.397 atm

will be about half that at 4.54 atm.

Such a reduction in pressure may therefore

not be possible.



USE OF LOW PRESSURE STEAM



Approximate weight in kg/min that will flowthrough 30 m of various sizes of piping with aloss of 0.068 atm (1psi)

Diameter of pipe in Inches

12” 10” 8” 5” 3” 1” PRESSURE

325.22 195.0 112.0 34.00 9.38 0.53 0.34 atm

478.54 286.6 164.20 49.89 13.74 0.757 2.04 atm

607.82 364.2 208.6 63.50 17.55 0.984 4.08 atm

816.47 491.2 281.2 85.27 23.58 1.32 8.16 atm

1011.52 604.6 347.9 105.68 29.12 1.66 13 atm

1192.96 716.6 410.5 124.74 34.47 1.96 20.4 atm



Flash Steam



Flash steam

The term ‘flash steam’ is traditionally used

to describe steam issuing fromcondensate receiver vents and open-

ended condensate discharge lines fromsteam traps.

How can steam be formed from waterwithout adding heat?



Flash steam

Flash steam occurs whenever water athigh pressure (and a temperature higherthan the saturation temperature of the low-

pressure liquid) is allowed to drop to alower pressure.

Conversely, if the temperature of the high-

pressure water is lower than the saturationtemperature at the lower pressure, flashsteam cannot be formed.



Flash steam

In the case of condensate passingthrough a steam trap, it is usually the

case that the upstream temperature ishigh enough to form flash steam..



Flash steam

Consider a kilogram of condensate at 5 bar g and asaturation temperature of 159°C passing through asteam trap to a lower pressure of 0 bar g.

The amount of energy in one kilogram ofcondensate at saturation temperature at 5 bar g is671 kJ.



Flash steam

In accordance with the first law ofthermodynamics, the amount of energycontained in the fluid on the low-pressureside of the steam trap must equal that onthe high-pressure side, and constitutesthe principle of conservation of energy.



Flash steam

Consequently, the heat contained in onekilogram of low-pressure fluid is also 671kJ. However, water at 0 bar g is only able

to contain 419 kJ of heat, subsequentlythere appears to be an imbalance of heaton the low-pressure side of671 – 419 = 252 kJ,

which, in terms of the water, could beconsidered as excess heat.



Flash steam

This excess heat boils some of thecondensate into what is known as flashsteam and the boiling process is calledflashing.

Therefore, the one kilogram ofcondensate which existed as onekilogram of liquid water on the high

pressure side of the steam trap now partlyexists as both water and steam on thelow-pressure side.



The amount of flash steam produced atthe final pressure (P2) can be determinedusing Equation

Where:

P1 = Initial pressureP2 = Final pressurehf = Liquid enthalpy (kJ/kg)hfg = Enthalpy of evaporation (kJ/kg)

Flash steam



Example :The case where the high pressure condensatetemperature is higher than the low pressure saturation

temperature

Consider a quantity of water at a pressure of 5 barg, containing 671 kJ/kg of heat energy at itssaturation temperature of 159°C.

If the pressure was then reduced down toatmospheric pressure (0 bar g), the water couldonly exist at 100°C and contain 419 kJ/kg of heatenergy.

This difference of 671 - 419 = 252 kJ/kg of heatenergy, would then produce flash steam atatmospheric pressure.



Example :The case where the high pressure condensatetemperature is higher than the low pressure saturation

temperature

The proportion of flash steam produced can be

thought of as the ratio of the excess energy to theenthalpy of evaporation at the final pressure.



The principle of energy conservation betweentwo process states



Example : The case where the high pressure condensatetemperature is lower than the low pressure saturationtemperature.

Consider the same conditions, with theexception that the high-pressurecondensate temperature is at 90°C, that

is, sub-cooled below the atmosphericsaturation temperature of 100°C.

Note: It is not usually practical for such a

large drop in condensate temperaturefrom its saturation temperature (in thiscase 159°C to 90°C);



No flash steam formed because T1 < T2

Example :The case where the high pressurecondensate temperature is lower than the low

pressure saturation temperature.



FLASH FOR 1 KG CONDENSATE COLLECTEDAT 100C

HEAT IN FLASH

IN KJ WEIGHT OF

FLASH

IN KG

PRESSURE

IN ATM

42.980.03540.34atm 185.180.15252.04atm

296.730.24444.08atm

357.140.29405.59atm

409.390.33736.98atm



FLASH STEAM

The table shows the amount of heat that

will be liberated as flash if condensate is

reduced to atmospheric pressure.

It shows clearly how the latent heat rises

and the flash drops with lower pressures.



FLASH STEAM

Flash is a good valuable heat and should

not be wasted, but it is difficult to collect.

If steam can be used for heating at

atmospheric pressure , not only is there a

bigger supply of latent heat than with

pressure-steam, but there will be no flashto waste or collect.



FLASH STEAM

Sometimes the hot condensate can bereturned under pressure to the boilers, whenthere is of course no loss by flash and highpressure steam can then be used in heatingsurfaces with the advantage of higher rate of heat transfer.

If the pressure on the condensate is reduced

but not to atmospheric pressure the followingtable shows the percentage of flash steam.



PERCENT OF FLASH STEAM %

0 1.4

atm 2.04

atm 4.08

atm

8.16

atm 13

atm

FINALPRESSURE

INITIAL PRESSURE

20.7616.414.9311.626.872.3517.5atm

18.7314.3012.819.384.61-13.0 atm

13.278.677.103.52--7.0 atm

11.346.685.091.46--5.2 atm

8.984.252.62---3.5 atm 7.823.041.41---2.8 atm

6.471.65----2.0 atm

5.60-----1.4 atm

The final pressure being the gauge pressure



FLASH STEAM

When arrangements can be made to collect

and use flash steam there is also no loss in

using high pressure steam.

Both these arrangement require more and

special plant and their application is

limited.

High pressure mean high temperature sothe heat losses will always be higher.



High pressure versus low pressure

The use of high pressure steam can only be

justified if process temperature call for it.

High pressure steam should, if possible,

pass through a turbine to generate power,

exhaust steam then being used for heating.



THROTTLING

What happens if the pressure on dry saturated

steam is reduced ?

Suppose we allow dry saturated steam at1MPa (10 atm) to pass through a reducing

valve in to a low pressure main at 0.3MPa ( 3

atm).

Saturated steam at 10 atm contains 2778.1 kgof total heat

THROTTLING



THROTTLING

In expanding through the reducing valve the

steam does no work so it still holds this

energy. If we look up 3 atm steam in the steam tables

we see that 2725.3 kJ/kg is the enthalpy of

dry saturated steam.

The steam is superheated at this pressure.



THROTTLING

To find the superheated temperature the steam

tables show that:

h (kJ/kg)TC2761.0 150

2778.1 x

2865.6 200

Thenx = 158C

THROTTLING



THROTTLING

The temperature of saturated steam at 3

atm is equal to 133.5C.

Reducing the pressure has added 24.5 C of superheat although the actual

temperature had fallen from 179.91 C to

158C.



THROTTLING

This superheating by expansion only occurs

to saturated steam below 31.5atm.

At higher pressures there are other effects

because the reduction in latent heat becomesmore rapid than the increase in liquid heat.

For example saturated steam at 52.5 atm

blown through a reducing valve gets wetterdown to 31.5 atm and then gets drier until

about 16.8 atm it is again dry and saturated

steam. If it is still allowed to expand it superheats

itself.



31.5atm

DESUPERHEATING



Steam used in a heating surface should not

be superheated. Superheated steam is a

dry gas and it parts with its heat slowly.

Steam used for direct heating in a blower

or injector often gets superheated by

expansion in the blower and great lossescan occur this way.

DESUPERHEATING



DESUPERHEATING

Superheated steam is almost always bad

steam to use for heating.

It may therefore be necessary to desuperheat

it by passing it through a desuperheater

which adds a spray of distilled water to the

steam.



DESUPERHEATING

The superheat gives itself up in evaporating

some of the sprayed water.

The water first takes sensible heat energy tobe heated to the boiling temperature

corresponding to the steam pressure, and

then it takes the latent heat of evaporation at

this pressure to reach the saturated conditionof this pressure.



DESUPERHEATING

The amount of water sprayed depends on the

degree of superheat since the total energy

change is zero by the first low of

thermodynamics.

If the losses is neglected the energy gained

by the water is equal the energy loss by the

superheated steam.

This process is accompanied by an increase in

weight of the saturated steam leaving the

desuperheater.

SUPERHEAT AND STEAM



SUPERHEAT AND STEAM

DISTRIBUTION

In certain circumstances the superheat

given to steam by reducing its pressure

may be very useful.

Steam is sometimes very wet.

If this wet steam goes into the heating

surface of a piece of plant the extra

condensate is not convenient.

SUPERHEAT AND STEAM



SUPERHEAT AND STEAM

DISTRIBUTION

It has little heating value, but it coats theheating surface with an additional water filmand the extra water has to be handled by the

trap, and the amount of flash steam isincreased.

If such wet steam can be expanded as soon aspossible on its journey to the heating process,

the superheat due to expansion will help todry it.

SUPERHEAT AND STEAM



SUPERHEAT AND STEAM

DISTRIBUTION

This increase the amount of steam reaching

the process plant and, as the steam is at a

lower pressure, it will have a higher latent

heat.

DISADVANTAGES OF



DISADVANTAGES OF

SUPERHEATED STEAM

Superheated steam is a dry gas, and it

parts with its heat by conduction.

When the layer of steam which is in

contact with the heating surface has parted

with some heat the heat from the body of

the stream has to pass by conduction

through the very bad conducting gas toreach the outer layer.

DISADVANTAGES OF



DISADVANTAGES OF

SUPERHEATED STEAM

Saturated steam gives up its latent heat bycontact with heating surface.

When the contact layer of steam has condensedmore steam flows towards the surface to takeits place.

The heat does not have to flow. The steamflows.

DISADVANTAGES OF



DISADVANTAGES OF

SUPERHEATED STEAM

Objections to using superheated steam for process.

The temperature of the heating surface is not

definite. The bulk of the heat transfer takes place at

saturated temperature but some of it may takeplace at some higher temperature.

This may be undesirable if a material is to beheated up and then maintained at a particulartemperature.

DISADVANTAGES OF



DISADVANTAGES OF

SUPERHEATED STEAM

Objections to using superheated steam for process.

If circulation of the process material is not

strong, there may be local overheating withdamage to the product.

Another disadvantage of superheated steam for

process is that large temperature stresses mayoccur in the plant.

DISADVANTAGES OF



SUPERHEATED STEAM

During the heating-up process the heat

transfer is rapid and most of the heat

transfer will take place at saturation

temperature.

When the material is up to the required

temperature and the steam supply is reduced

a large part of the heating surface may be atsuperheated temperature.

EXAMPLE



EXAMPLE:

A desuperheater , in which superheated

steam at 3.5 MPa, 400C enters at the rate of

0.5kg/s, and the liquid water enters the unit

at 3.5 MPa,40C. If saturated vapor exits at3MPa, determine the required rate of liquid

water input to the desuperheater unit.

M1+ M2=M3

(M1 h1) + ( M2 h2) =( M3 x h3)

(0.5 3222.3) + (M2 170.7) =

(0.5+M2) 2804.2

M2 = 0.079 kg/s

The effect of air



The effect of air

If air is mixed with steam and flows alongwith it, pockets of air will remain at theheat exchange surfaces where the steam

condenses.

Gradually, a thin layer builds up to form

an insulating blanket, hindering heattransfer.

The effect of air



The effect of air



The effect of air

Air is widely used as an insulator because of its lowconductivity

Most insulating material is made up of millions ofmicroscopic air cells, within a matrix of fibre glass,mineral wool, or polymer-type material.

The air is the insulator and the solid material simply

holds it in position. Similarly, a film of air on the steam side of a heat

transfer surface is resistive to the flow of heat,reducing the rate of heat transfer.

The effect of air



The thermal conductivity of air is 0.025W/m °C, while the corresponding figurefor water is typically 0.6 W/m °C, for ironit is about 75 W/m °C and for copperabout 390 W/m °C.

A film of air only 1 mm thick offers aboutthe same resistance to heat flow as awall of copper some 15 meters thick.

The effect of air

The heat content of a given



e eat co te t o a g evolume of the mixture

When air is added to steam, the heatcontent of a given volume of the

mixture is lower than the samevolume of pure steam, so the mixtemperature is lowered.

Dalton's Law of Partial Pressures



Dalton s Law of Partial Pressures

In a mixture of steam and air, the totalpressure is the sum of the partial pressureeach gas would exert, when occupying the

total volume on its own.

For example, if the total pressure of asteam / air mixture at 2 bar (absolute) is

made up of 3 parts steam to 1 part air byvolume, then:

D l ' L f P i l P



= 0.5 bar a= ¼ x 2 bar aPartial pressureof air

=1.5 bar a= ¾ x 2 bar aPartial pressureof steam

= 2 bar a(1 bar g)

= 0.5 + 1.5 bar aTotal pressureof mixture





The pressure gauge would indicate apressure of 1 bar g, inferring acorresponding temperature of 120°C to the

observer. However, the partial pressure due to the

amount of steam present in the mixture is

only 0.5 bar g (1.5 bar a), contributing atemperature of only 111.6°C.

Dalton s Law of Partial Pressures



Effect of air on steam temperature

Effect of air on steam



The presence of air has a double effect:

1. It offers a resistance to heat transfer via itslayering effect,

2. It reduces the temperature of the steam spacethus reducing the temperature gradient acrossthe heat transfer surface.

The overall effect is to reduce the heat transfer

rate below that which may be required by a criticalprocess, and in worst cases may even prevent a

final required process temperature being reached.

temperature

Effect of air on steam



In many processes, a minimum temperatureis needed to achieve a chemical or physicalchange in a product, just as a minimum

temperature is essential in a sterilizer.

The presence of air is particularly

problematic because it will cause a pressuregauge to mislead. It follows that thetemperature cannot be inferred from thepressure.

Effect of air on steamtemperature

Air in the system



Air in the system

Air is present within steam pipes and steamequipment at start-up. Even if the system were filledwith pure steam when used, the condensing steamwould cause a vacuum and draw air into the pipesat shutdown.

Air can also enter the system in solution in thefeedwater. At 80°C, water can dissolve about 0.6% of its volume, of air. The solubility of oxygen isroughly twice that of nitrogen, so the 'air' whichdissolves in water contains nearly one part ofoxygen to two of nitrogen rather than the one part tofour parts in atmospheric air.

Carbon dioxide has a higher solubility, roughly 30 times greater than oxygen.

Ai i B il f d



Boiler feedwater, and condensate exposed to theatmosphere, can readily absorb these gases.

When the water is heated in the boiler, the gases arereleased with the steam and carried into the

distribution system. Unless boiler 'make-up' water is fully dematerialized

and degassed, it will often contain soluble sodiumcarbonate from the chemical exchange of watertreatment processes.

The sodium carbonate can be released to someextent in the boiler and again carbon dioxide isformed.

Air in Boiler feedwater




With higher pressure boilers, the feedwater is oftenpassed through a deaerator before it is pumped tothe boiler.

The best deaerators can reduce oxygen levels to 3parts per million (ppm) in water.

This residual oxygen can then be dealt with bychemical treatment.

However, such an amount of oxygen will beaccompanied by about 6 ppm of nitrogen, which thechemical treatment ignores.

If the boiler is of a moderate size producing 10 000kg per hour of steam, it uses about 10 000 litres perhour of water, in turn producing 60 cm³ of nitrogen.This will cumulate over time with a significant effecton heat transfer if not removed from the system.


Signs of air



Signs of air

A gradual fall off in the output of any

steam heated equipment.

Air bubbles in the condensate.

Corrosion.

C d t t



Condensate return

Unless some contamination is likely(perhaps due to the process), thiscondensate is ideal boiler feedwater.

It makes economic sense, therefore, toreturn as much as possible for re-use.

In reality, it is almost impossible to returnall the condensate; some steam may havebeen injected directly into the process for

applications such as humidification andsteam injection, and there will usually bewater losses from the boiler itself, forinstance, via blowdown.

Condensate return



Make-up (chemically treated) water willtherefore have to be introduced to the systemto maintain the correct working levels.

The return of condensate represents hugepotential for energy savings in the boilerhouse.

Condensate has a high heat content andapproximately 1% less fuel is required forevery 6°C temperature rise in the feed tank.



Comparison of energy to raise



p gysteam at 10 bar g

Figure (a) shows the formation of steam at 10bar g when the boiler is supplied with coldfeedwater at 10°C.

The portion at the bottom of the diagramrepresents the enthalpy (42 kJ / kg) available inthe feedwater.

A further 740 kJ / kg of heat energy has to be

added to the water in the boiler beforesaturation temperature at 10 bar g is reached.

Comparison of energy to raise



p gysteam at 10 bar g

Figure (b) again shows the formation of steam at 10 barg, but this time the boiler is fed with feedwater heated to70°C by returning more condensate.

The increased enthalpy contained in the feedwatermeans that the boiler now only has to add 489 kJ / kg ofheat energy to bring it up to saturation temperature at 10bar g.

This represents a saving of 9.2% in the energy needed toraise steam at this same pressure.

The returned condensate is virtually pure water and thissaves not only on water costs but also on watertreatment chemicals, which reduces the lossesassociated with blowdown.

Ch 1. Properties of Steam

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Transcript of Ch 1. Properties of Steam