2. Operation of Boiler Feed Pumps Etc

7/29/2019 2. Operation of Boiler Feed Pumps Etc

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2. Operation of boiler feed pumps, condensate pumps, and feed water heaters1. Boiler feed pumps

A boiler feed pump ensures continuous supply of feed water to the boiler, preventing

overheating and damage to the boiler. The boiler feed pump is an essential part of the boiler feed

system. The article explains the construction and working of a boiler feed pump.

1.1 IntroductionA feed pump supplies feed water to the boiler as and when required. An essential part of

the Boiler feed water system, a boiler feed pump is selected according to the quantity and the

amount of pressure required by the boiler. The feed pump raises the pressure of the feed water

to a level high enough for the water to enter the boiler. The type of the boiler also plays an

important role in selecting a feed water pump. For example, in case of auxiliary boilers, where

the amount of feed water required is less, a steam driven reciprocating positive displacement

pump is generally used.

1.2Reciprocating Boiler Feed PumpThe positive displacement reciprocating pumps are double acting pumps; which means that

the liquid enters the boiler feed pump on either sides of the pump piston. Thus, when the piston

moves up, suction is taken in the area below the piston and the liquid is drawn in. During the

intake of the liquid, only the suction valve is open and the design is made in such a way that

discharge valve remains shut. As the piston moves up, discharge valve on the top side of the

piston opens, pushing the water out, while the suction valve remains closed. As the piston

moves in the downward direction, the same operation is repeated in the upper part of the

piston. In this way, the suction and discharge occurs on the opposite sides of the piston of the

boiler feed pump.

All the positive displacement reciprocating pumps are self priming pumps, which produce

the pressure required by the system. The pump is not affected by the intrusion of vapors or

gases inside the pump system. Yet the pump is always provided with an air vessel at thedischarge pipe to reduce the effects created by the pressure variations at the time of discharge.

Due to high occurrences of pressure fluctuation inside the pump, the pump system is fitted with

a relief valve in between the suction and discharge chambers in order to protect the pump.


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1.3Turbo-Feed Boiler PumpThe systems having steam turbine driven feed pumps usually have a high pressure water

tube installations. The turbo-feed pump is a two stage horizontal centrifugal pump, which is

driven by an impulse turbine. The whole arrangement includes a diffuser and a volute casing

along with the impulse turbine. The pump runs with the help of direct steam from the high

pressure boiler. The exhaust steam from the turbine is send to the pre-heater for feed heating.

The bearings of the pump are lubricated with the help of filtered water supplied from an inlet

opening provided in the first stage of the pump.

In most of the systems, the feed water supply is controlled with the help of a feed water

controlled system, also known as the cascade system. The pressure of the feed discharged is

controlled with the help of a governor. For the safety of the system, over-speed protection trips

are also provided.

Another type of feed pump known as "electro feeder" is generally used in packaged boiler

installations. An electro feeder type of a pump is a multistage centrifugal pump, driven by a

constant speed electric motor. The number of stages required by the boiler feed pump depends

on the feed quantity and discharge pressure.

2. Condensate pumpsCentrifugal pumps are almost exclusively given the job of condensate pumping. For one thing,

the self-regulating character of submergence control simplifies many condensate pumping

installation, particularly those from vacuum regions.

The single-acting, double suction pump has the widest range of application and can serve for all

condensate drainage except those cases with high discharge pressure. Then 2-stage axially balanced

pumps are needed.

Another application of the centrifugal condensate pump is the integrated condensation

pumping set. These sets are employed to receive assorted flows of condensate from traps, heating


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systems, etc., and deliver the condensate to the feedwater tank. On a common base will be

mounted a motor-driven centrifugal pump, a receiver tank of steel or cast iron, and the necessary

valves, strainers, controls. Usually the control is a simple, float-operated switch that starts the motor

when the tank has been nearly pumped out. By venting the tank, hot trap discharges are cooled to

212 F through flashing; hence the pump must be designed to operate on a few inches suction

submergence, although pumping hot water.

3. Feedwater heatersFeedwater heaters have two primary functions in power plants: (1) to provide the means for

increasing the feedwater temperature, which improves the overall plant efficiency, and (2) to

minimize the thermal effects in the boiler. Feedwater heaters use steam from selected turbine

extraction points to preheat the feedwater from the condenser prior to it entering the economizer

or boiler drum.

The number and type of feedwater heaters used depend on the steam cycle, the operating

pressure of the cycle, and the plant economics, i.e., where lower operating costs can offset the

additional capital cost expenditure. In general, smaller plants have fewer units. In utility and large

industrial plants, five to seven stages of feedwater heaters are often part of the design. Feedwater

heaters are classified as either closed or open designs and are designed for operating at low or highpressure.

3.1Closed feedwater heatersClosed feedwater heaters are specialized shell and tube heat exchangers. The steam flows

from an extraction stage of the turbine and condenses on the shell side of the feedwater heater,

while the feedwater flows inside the tubes and absorbs heat and thereby increases its

temperature.

Most closed feedwater heaters are composed of bundles of a large number of tubes that are

bent in the form of a U, and therefore, this type of design is called a U-tube heat exchanger or

feedwater heater. The tubes are either expanded or welded into tube sheets at one end of the

shell. A series of baffles and tube support plates are used to direct flow, minimize tube

vibration, reduce erosion, and promote high heat transfer. The lowest-cost closed feedwaterheaters are typically long, horizontal, two-pass designs with high water velocities.

Low-pressure feedwater heaters are located prior to (upstream) the boiler feed pump (see

Figure 1). They are generally designed for tube side pressures of less than 900 psig for utility

boiler designs. The location of the feedwater heater relative to the boiler feed pump generally

defines whether it is called a low- or high-pressure heater no matter what the actual pressure is.

High-pressure heaters of a plant cycle are those heaters which are located after (downstream)

the boiler feed pump.


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The illustration shown in Figure 2 is a U-tube feedwater heater design. The feedwater enters

the lower portion of the heater through the inlet nozzle and proceeds to the feedwater outlet

nozzle through the horizontal U-tubes in basically a two-pass system. Steam from extraction

stages in the steam turbine enters at the top of the heater and flows in a counterdirection, alsoin a two-pass flow arrangement, and flows over the tubes where the steam condenses and

leaves the feedwater heater through the drain outlet nozzle. Since the system is closed, these

condensate drains cascade through other feedwater heaters and eventually to the boiler feed

pumps, where all the condensate becomes the feedwater that is recycled continually to the

boiler. Rollers are provided on this design to handle the expansion of the feedwater heater

during operation.

The closed feedwater heater shown in Figure 3 is different from a U-tube design in that

straight tubes are used between two tube sheets. Feedwater heaters of this type are classified

as one-, two-, three-, or four-pass designs depending on the number of times the water passes

the length of the unit before it is discharged. The design shown in Figure 3 is baffled and

provides four passes for the water.


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As noted previously, the U-tube design of a feedwater heater is the preferred choice of heaters

in todays power plants. This design has minimized the expansion problems of tubes being

connected to two tube sheets. The tubes are bent into the form of the letter U, and these tubes are

frequently referred to as hairpin tubes. Both ends of these tubes are expanded into the same tube

sheet, and the water box is baffled to direct the flow of water through the tubes from one pass to

the other. Although both ends of these tubes are rigidly attached to the tube sheets, their shape

permits free expansion. However, because of their U shape, these tubes cannot be cleaned easily by

mechanical means. But by having good water quality, the need for this cleaning has been reduced

significantly. Also, because of concerns about potential transport of copper from the feedwater

heaters into the boiler and turbine, stainless steel and carbon steel tubes generally are used in

feedwater heaters.

The tube sections of closed feedwater heaters must withstand the pressure of the water, and

the shell must withstand the pressure of the steam. The water is forced through one or moreheaters by a single pump. High-pressure steam plants use heaters between the boiler feed pumps

and the boilers, exposing the water side of the heater to the full boiler feed pump pressure. These

are called high-pressure heaters. The low-pressure heaters are located between the condensate

pumps and the boiler feed pumps. (Refer to Figure 1.) Safety measures in the form of relief valves

must be installed to prevent the possibility that either the water or the steam pressure will exceed

that for which the heater was built.

3.2Open feedwater heaters (deaerators)These feedwater heaters are also called deaerators, and they serve the dual purpose of heating

the feedwater to improve plant efficiency and deaerating the feedwater to remove gases that could

cause corrosion of equipment and piping systems. Deaerators also provide the storage of high-quality feedwater for the boiler feed pump. Several deaerator arrangements are shown in Figures. 4

and 5.


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Heat transfer in deaerators is by direct contact between the feedwater and the turbine

extraction steam, and various design techniques are used such as bubbling, tray, spray, or

various combinations of these. The drains from the high-pressure heaters usually flow into the

deaerator, and noncondensable gases are vented to the atmosphere.

Two types of deaerating heaters are used most often in todays power plants: (1) the spray-

tray type and (2) the spray-scrubber type. Schematics of these designs are shown in Figure 6.


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The feedwater that must be treated by the deaerator can vary significantly depending onthe plants application. A closed-loop system found in a typical utility has feedwater makeup

requirements of only a few percent, while an industrial application such as a pulp and paper

plant has requirements for feedwater makeup of 40 to 70 percent of the total flow.

Condensate that returns from a turbine is usually high in temperature and low in O2 and

CO2. For industrial plants where makeup water may come from a number of sources, this water

is usually much cooler and can contain high levels of dissolved O2 and CO2. Each situation

requires a careful design and selection of the proper equipment.

3.2.1 Spray-type deaeratorThe spray-type deaerator has three main sections, as shown in Figure 6a:

a.

A water box at the top, where water enters the unit through valves or nozzlesb. A spray area immediately below, where sprayed water interacts with steam to do 95percent of the deaerating and heating of the feedwater

c. A tray section, where the final 5 percent of the deaeration and heating takes placeBelow the tray tank is a temporary storage area for oxygen-free water. In most large

units, the entire heater is mounted above a storage tank for volume retention. In some

smaller units, the bottom of the heater serves as the storage area. The water box holds


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incoming water so that it may be evenly sprayed to the chamber below. Because of the

potential for corrosion, the water box is constructed of stainless steel or lined with

stainless steel.

The heater valves are held open by the pressure of water from above. Increasing

feedwater flow forces the valves open further until full capacity is reached. These valves

are also stainless steel. The spray chamber is the large area directly below the water

box, where water meets the upward moving steam for the first time.

The operation of the tray area may seem of minor importance,since only 5 percent

of the deaeration and heating takes place in the trays. The importance of the trays

becomes apparent when it is realized that a deaerator heater must ensure minimum O2

levels in the feedwater of about 7 parts per billion (ppb). It is in these trays that the

cleanest steam meets the almost completely cleaned water for final polishing. These

trays are also made from stainless steel for corrosion protection.

Below the tray station, at the bottom of the heater tank, there is a section where

deaerated water is briefly retained before it drops into a separate storage tank. In one-

piece heaters, the heater storage volume must be sufficient to permit water to be

drawn off and directed to the boiler feed pumps.

3.2.2 Spray-scrubber deaerator.In this deaerator design, a scrubber section is used in place of trays, as shown in

Figure 6b. This design accomplishes the same thing as the tray design by removing

residual dissolved gases from water that has passed through the spray chamber. While

the steam can be counterflow, cross flow, or parallel flow to the water in tray-type

deaerators, it is parallel flow in the scrubber section of spray scrubbers, as shown in

Figure 6b.

Turbulent mixing and scrubbing take place as water and steam are in intimate

contact and rise through this region. Assisted by the turbulence caused by steam

condensation, this results in a highly effective gas stripping action.

Spray-scrubber units are generally less expensive than tray types. However, they do

have a disadvantage when compared with tray types in that their turndown range(design flow/low flow) is about 5:1, whereas a tray type can operate efficiently in a flow

range of 10:1. Selection of the type of deaerator is often based on the load variations

that are expected at the plant.

3.3Operation and maintenance of feedwater heatersDamage to feedwater heaters has involved primarily tube failures, which have been caused by

the following:

a. Erosion from steam impingement

b. Tube vibration

c. Erosion and corrosion on the inlet tube end

d. Oxygen pitting

e. Stress corrosion cracking

In addition, failures of tube joints, improper plugging, and poor maintenance all lead to

downtime and repairs of feedwater heaters. Generally, feedwater heaters require no daily

maintenance; however, associated valves need to be given attention. During scheduled outages,

nondestructive examination (NDE) techniques are used for tube-side and shell-side inspections.

Since tube conditions are critical, eddy current and ultrasonic testing (UT) are used to evaluate

tube integrity. Eddy current testing determines wall thinning and identifies cracks that have

occurred.


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When leaks are detected, tubes are usually plugged with tapered or mechanical plugs and

are expanded in the tubesheet. Explosive plugs are also used. Units can have 10 to 30 percent of

their tubes plugged and still meet thermal performance, although the pressure drop does

increase and the condensate and boiler feed pumps must be designedto handle this additional

pressure requirement.

The feedwater heater system has a significant impact on steam system performance. Much

of the deposits found in steam-generating systems come from corrosion products and

contaminants whose source is the feedwater heater system. Copper-based tubing has been

replaced by other materials in order to reduce the carry-over of dissolved copper. Proper

chemistry control and deaeration operation can minimize corrosion in the feedwater heater

system and reduce corrosion product carry-over into the boiler. The proper control of oxygen is

very important so that corrosion of the boiler system is minimized.

Feedwater heaters may be taken out of service for maintenance during plant operation by

bypassing the feedwater around them and shutting off the extraction steam to them. However,

this requires additional piping and valves, and the system must be designed to accommodate

the bypassing.

4. Compressed LiquidA compressed liquid is one whose pressure is higher than the saturation pressure corresponding

to its temperature. A subcooled liquid is one whose temperature is below the saturation

temperature corresponding to its pressure. These two definitions define identical states, mean the

same thing, and the names are customarily used interchangeably. In Figure 7, imagine a saturated

liquid in state d, and let it cool at constant pressure to either state B, c or b. It has become

subcooled. On the other hand, imagine a saturated liquid at a, Figure 7; let it be pumped to a higher

pressure bcBd. If it is pumped isothermally, the end state is b; if isentropically, the end state is c; if

isometrically (v= C), the end state is B. Each of these states represents compressed liquid.


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Typically, vapor tables do not give properties of compressed liquid (an exception, the ASME

S.T.), which means that a good estimate is convenient. The easiest assumption is that the liquid is

incompressible, in which case, points b, c, and B are quite close together. First for a reversible

pumping where = 0, P = 0, we have

dhdW or shW

where the subscript s indicates constant entropy, process ac, Figure 7. Then with Tds = 0, we havevdpdh or pvdpvh

Observe that vp is the rectangular area naBm, Figure 7(a); v= va = vfis the saturated liquid volume

available in vapor tables. Let pa = psat, meaning the saturation pressure from which the liquid was

imagined to have been pumped to the actual pressurepact. Then

satactffBc ppvhhh or satactffB ppvhh

an approximation of the state after isentropic pumping if the liquid is nearly incompressible.

In dealing with compressed liquid, decide first whether or not any adjustment of saturated-state

properties is needed. If the answer to this question is yes, then decide whether the approximation

involved in the above equation is appropriate. These are engineering decision easily made with a

background of experience. In the meantime, for pedagogical purposes for H2O, let us say: when p

400 psia, use saturated liquid properties, at the specified temperature, for compressed liquid

properties; when p > 400 psia, make an accurate or an approximate correction, depending on the

accuracy needed and the facilities at hand.

Example No. 1

Water at 1300 psia is delivered to a steam generator. The exit state from the superheater is defined

by 1100 psia and 1100 F. The condenser pressure is 2.223 psia. The pump receives the liquid at 110

F. (a) For a pump efficiency of 70%, what is the specific pump work? The heat supplied in the steam

generator? (b) Compute the approximate pump work needed to deliver 3.6 x 106

lb/hr.

Given:

Steam generator = 1300 psia

Superheater = 1100 psia, 1100 FCondenser pressure = 2.223 psia.

m = 3.6 x 106

lb/hr

p = 70%

Required:

(a) Specific pump work, Heat supplied in the steam generator.(b) Approximate pump workSolution:


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At state 1, 1100 psia, 1100 F

h1 = 1559.4 Btu/lb

At state 3, 2.223 psia, sat.

h3 = hf= 97.95 Btu/lb

v3 = 0.0016247 ft3/lb

At state 4, 1300 psia

By approximation.

34334ppvhh

Btulbft

ftinpsialbftlbBtuh

16778

14422321300001624709597

223

4.

...

lbBtuh 34984

.

At state 4.

34

34

hh

hhp

9597

9597349870

4

.

...

h

lbBtuh 51984

.

(a) Specific pump work = h4h3 = 98.51 97.95 = 0.56 Btu/lbHeat supplied in the steam generator = h1h4 = 1559.4 98.51 = 1460.9 Btu/lb

(b) Pump work hrBtuhhmWp 00001625601063 634 ,,..

5. Specific Speed and Impeller ConfigurationsA dimensionless index of pump type known as specific speed has been developed for pump

design and selection to show the relationship between pump capacity, head, and impeller speed.

Specific speed of an impeller is defined as the speed in revolutions per minute at which a

geometrically similar impeller would operate to deliver 1 gpm at a developed head of 1 ft. Specificspeed is algebraically defined as

4

3

h

QnNs

where

Ns = specific speed;

n = pump speed, rpm;

Q= pump flow at best efficiency point (), gpm; and

h= pump developed head at , ft.

Specific speed characterizes the shape and configuration of an impeller. Since the ratios of the

major impeller dimensions vary uniformly with specific speed, specific speed is useful to the pump

designer in determining impeller proportions and dimensions required, as well as to the application

engineer in checking suction limitations of the pump.

It should be noted that the above equation for specific speed is written for single-stage pump

applications. For multistage pumps, the head per stage is used to calculate specific speed.

Generalizing, the head per stage is found by dividing the total head of the pump by the number of

stages of the pump.


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Impeller form and proportions vary with specific speed, as shown in Figure 8. Each type of

impeller has been found to have a range of specific speeds in which it will give the best

performance. Radial-vane area impellers have specific speeds up to about 1,000, whereas Francis-

vane area impeller specific speeds go up to 4,000 to 4,500. The specific speeds of mixed flow area

impellers range from that of the Francis-vane impellers up to about 9,500 to 10,000. The specific

speeds of axial flow area impellers range from 10,000 to 14,000. Generally, as flow goes up and

pump head decreases, specific speed increases.

For power station centrifugal pump applications, the following specific speeds and impeller

profiles are generally seen:

In addition, the impeller configuration and specific speed characterize the shape of the pump

operating head-capacity curve, as shown in Figure 9.


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6. Affinity LawsAnother important tool used by pump designers and application engineers is Affinity Laws.

These laws express the mathematical relationship and illustrate the effect of changes in pump

operating conditions or pump performance variables such as pump head, flow, speed, horsepower,

and pump impeller diameters. The affinity laws are

Flow

1

2

1

2

12D

D

n

nQQ

Head2

1

2

2

1

2

12

D

D

n

nhh

Horsepower

3

1

2

3

1

2

12

D

D

n

nhphp

where

Q = pump flow, gpm (m3/h);


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n = pump speed, rpm;

D = impeller diameter, in. (m);

h = pump head, ft (m);

hp = pump brake horsepower, bhp (kw); and

Efficiency12 pp

,

(Nearly constant for pump speed changes and small changes in impeller diameter)

7. Regenerative Feedwater HeatingIncreasing the average temperature of heat addition can also be accomplished by increasing the

temperature of the feedwater entering the boiler. To realize a gain in efficiency, heat from within

the cycle is used to elevate the feedwater temperature. This can be done by extracting a portion of

the partially expanded steam from the turbine and directing it to a heat exchanger that heats the

feedwater to the boiler. This process is called regenerative feedwater heating. Figure 10 shows the

equipment arrangement and T-s diagram for the regenerative Rankine cycle. Steam enters the

turbine at state 1 and is partially expanded to state 2. A portion of the steam is extracted at state 2

and sent to a feedwater heater operating at state 6. The remainder of the steam expands through

the steam turbine to state 3. Heat is rejected as the turbine exhaust steam is condensed in process

3-4. The condensate is pumped to the feedwater heater and mixed with the turbine extraction

steam to become saturated liquid at state 6. The feedwater is pumped to the boiler pressure (state

8), heated to saturation, and evaporated in the boiler to reenter the turbine at state 1. The low

temperature heat addition into the cycle (5-6) is avoided and the improvement in efficiency comes

from the increase in the average temperature of heat addition. This is difficult to show graphically

on the T-s diagram. Because the flow rates are not equal at all of the state points on the T-s diagram,

the areas do not represent the total work and heat rejected of the cycle. Rather they represent the

work and heat rejected per pound of steam.


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Example No. 2

There are received 150,000 lb/hr of steam by an ideal regenerative engine, having only one heater,

of which the heater receives 33,950 lb/hr; the condenser receives the remainder at 1 psia. If the

heater pressure is 140 psia, find the state (quality or Sh) of the steam (a) at the heater entrance, (b)

at the condenser entrance.

Given:

Steam entering engine = 150,000 lb/hr

Steam extracted = 33,950 lb/hr

Condenser pressure = 1 psia

Heater pressure 140 psia

Required:

Quality or SH of the steam (a) at the heater entrance, (b) at the condenser pressure.

Solution:


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Letm = 150,000 lb/hr

2263330000150

95033

1.

,

,m

At state 6, 140 psia

h6 = hf= 324.96 Btu/lb

At state 4, 1 psia

h4 = hf= 69.72 Btu/lb

v4 = 0.0016136 ft3

/lbAt state 5,

45445ppvhh

Btulbft

ftinpsialbftlbBtuh

16778

1441140001613607269

223

5.

..

lbBtuh 76695

.


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Heat balance in the heater:

62151

1 hhmhm

9632422633307669226333012

.... h

lbBtuh 311972

.

(a) At the heater entrance, 140 psia, tsat= 353.04 Chf= 324.96 Btu/lb

hg = 1192.96 Btu/lb

hfg = 868 Btu/lb

Since h2 > hg, there is a superheat.

Temperature = 360.13 C

Superheat = 360.13 353.04 = 7.09 C

Then s2 = 1.58047 Btu/lb-R(b) At the condenser entrance, 1 psia, s3 = s2 = 1.58047 Btu/lb Rsf=0.13262 Btu/lb-R

sfg = 1.84553 Btu/lb-R

For qualityx,

%...

..457878450

845531

1326205804713

fg

f

s

ssx

8. Heat Balance Calculations for Surface HeatersTerminal difference for a surface heater is defined in the same way as the difference between

the saturation temperature of the steam in the heater and the temperature of the water leaving theheater. Saturation temperature is always used, even though the steam may be entering the heater

in a superheated state.

Steam requirements for a heater may be determined by the Law of Conservation of Energy; or,

all the energy entering a system during a given period of time must be equal the energy leaving

during the same period when the process is steady flow.


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Making the energy balance around the heater, Figure 6, we get:

Leaving

d

Entering

s hWhShWhS 21

With one equation we may have one unknown. Usually, but not always, the unknown is the

steam required, S. Solving for S,

ds hh

hhWS

12

where

S = steam flow, lb per hr

W= water flow, lb per hr

hs = enthalpy of the steam, Btu per lb

hd= enthalpy of drains corresponding to saturated liquid at steam pressure, Btu per lb

h1 = enthalpy of water entering corresponding to temperature t1, Btu per lb

h2= enthalpy of water leaving, corresponding to temperature t2 = tsatTD , Btu per lb

p = steam pressure, psiatsat= saturation temperature of steam, F

t1= temperature of water entering, F

t2= temperature of water leaving, F

TD = terminal difference, F

Example No. 3

Find the amount of steam needed by the heater, Figure 11, at 75 psia, 350 F, with 5 F TD, 350,000 lb

per hr of water entering at 260 F.

Given:

Steam at 75 psia, 350 F

TD = 5 F

W= 350,000 lb per hr

t1 = 260 F

Required: S


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ds hh

hhWS

12

hs = h at 75 psia and 350 F = 1205.3 Btu per lb

tsat= 307.6 F

t2= tsat TD = 307.6 5 = 302.6 F

hd= hfat 75 psia = 277.4 Btu per lb

h1 = hfat 260 F = 228.6 Btu per lb

h2 = hfat 302.6 F = 272.3 Btu per lb

hrlbS 48016

427731205

62283272000350,

..

..,

9. Heat Balance Calculations for Deaerators and Contact Heaters.Heat-balance calculations for a deaerator are no different from those for a contact heater. The

calculations assume that the heater is perfectly insulated, that the process is one of steady flow, and

that the process is one of steady flow, and that the loss of energy from the vent to atmosphere or

some other region is negligible. If a vent condenser is used with the equipment, it is considered a

part of the heater for heat-balance calculations.

The same two laws that define the performance of all heat-exchange equipment are used for

contact-heater calculations. The Law of Conservation of Energy applies to the energy exchange and

the Law of Conservation of Mass applies to the weights flowing. Refer to Figure 11.

Energy entering = energy leaving

2211 hWhWhS s

where

S = steam flow, lb per hr

W1 = water entering, lb per hr

W2 = water leaving, lb per hr

hs = steam enthalpy, Btu per lb

h1 = enthalpy of entering water, Btu per lb


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h2 = enthalpy of water leaving, Btu per lb

Regardless of the number of fluids entering the heater, the energy balance must always hold

true. The enthalpy of the water leaving should be taken at zero terminal difference. A typical

application of this equation would contain two unknowns, S and W1. But, applying the second of the

two laws, we get

21WWS

Solving for steam quantity,

1

122

hh

hhWS

s

Example No. 4

Steam enters a contact type of heaters at 145 psia and 1264.0 Btu per lb. The heater discharges

550,000 lb per hr of water; the entering temperature is 286 F. Find the steam required.

Given:Steam pressure = 145 psia

hs = 1264.0 Btu per lb

W2 = 550,000 lb per hr

tsat 145 psia = 355.8 F

h1 = hfat 286.0 F = 255.2 Btu per lb

h2 = hfat 355.8 F = 327.7 Btu per lb

Required: Steam flow, S

Solution:

1

122

hhhhWS

s

hrperlbS 53039

225501264

22557327000550,

..

..,

hrperlbSWW 47051053039000550

21,,,

- End -

Documents

2. Operation of Boiler Feed Pumps Etc