Steam Heat Ex Changers Are Under Worked and Over-Surfaced _ TLV

8/2/2019 Steam Heat Ex Changers Are Under Worked and Over-Surfaced _ TLV

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One of the more difficult chal-lenges facing process and

maintenance engineers is tofully understand the steam-pressure dynamics in over-

surfaced heat exchangers using modu-lating control with varying loadconditions. And this challenge can becommon because indirect heating bysteam is often employed for heatingprocess streams. Heat exchangers em-ployed in this service are known asprocess heaters.

The stall chart, which comparessteam pressure at various steam-

heater turndown ratios with the sys-tem back pressure, provides a generaloverview of a given steam heating ap-plication. A basic understanding ofthis industry tool is necessary, and canbe acquired from a variety of sources[1]. However, the basic stall chart as-sumes that the steam heater has notbeen oversized, which is usually notthe case; heat exchanger area is gen-erally over-surfaced for fouling factorsand capacity considerations.

An effective alternative is the ex-tended stall chart (ESC). Easy to em-ploy, this plot readily accommodatesoversizing.

Review of the stall chartAs the demand load varies in any heatexchanger using inlet-modulatingsteam control, the delivered steam-pressure profile will correspondinglychange. As an example, consider theheat exchanger (process heater) shownin Figure 1. For this setup, the steam-supply pressure (P

1

) can be relativelyconstant at a typical value of 150 psig.But the pressure (P2) of the steam de-livered directly to the heat exchanger

by the control valve may vary greatly.That means the inlet pressure (P3) tothe condensate-drainage solution(CDS) will also vary considerably. (Forsimplicity, it is assumed that no pres-sure drop occurs across the heat ex-changer, so P3 = P2).

If the pressure P3 falls below theback pressure (P

B

), there will be anegative pressure differential acrossthe CDS. When this situation arises,the condensate cannot drain out, so it

Engineering Practice

58 ChemiCal engineering www.Che.Com november 2004

Condensate return header

Steam inlet

Process heater

Reservoir

Product inlet

Grade

T1

L

P3

P1 P2

T2

PB

FH CDS

Figure 1. If the inlet pressure (P3) to the condensate-drainage solution (CDS)drops below the back pressure (PB), there will be a negative pressure differentialacross the CDS, and a stall condition will occur

Steam Heat ExchangersAre UnderworkedAnd Over-surfaced

An

Extended Stall Chartdelivers effective

condensate drainage

James R. Risko

TLV CorporaTion


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backs up into the steam side of theheater. This state of affairs is com-monly refered to as a system stall.

The first condition to understand

when making the stall chart is that,even under full demand load, themaximum P2 pressure will be the P1pressure minus the control valve pres-sure drop. If, for this example, thecontrol valve pressure drop is 25 psi,and the steam-supply utility line lossequals 10 psi, then the delivered-steam pressure (P2) will be a maxi-mum of 115 psig.

Therefore, a P2 of 115 psig is thepressure that should be converted tosteam temperature and it is this valuethat is input into the stall chart as thesteam profile at 100% demand, not thesupply steam pressure, P1, of 150 psig.

The 115 psig value for P2 is shown inthe stall chart in Figure 2.

The second condition to review isthe effect that varying load conditions

have on the supply steam require-ments for pressure and temperature.The control system adjusts the steampressure and temperature to balancethe delivered heat with the load. So, ifthe demand load drops, the steampressure will also drop. Since less tem-perature is needed with decreased de-mand, lower temperature can balancethe need and this causes the steampressure to lower.

This can be seen by looking at thebasic heat-transfer equation:

Q = (UA)(LMTD)

where UA is the characterizing coeffi-

cient (the product of the heat-transfercoefficient and surface area), and

LMTD is the logarithmic mean tem-perature difference across the heat

exchanger. Because the product UA isrelatively constant, the steam temper-ature must drop when the load drops.

For this example, suppose that theback pressure (PB) is 20 psig (solidgreen line) and that the product tem-perature increases from 50F (T1) to150F (T2). The mean temperature ofthe process fluid, Tm, is 100F, whichis entered on the right. (Note that the

values of Tm and T2 are found on theleft axis but plotted on the right axis.)

Control adjustments affect the

steam-pressure profile, and the result-ing steam pressure for a load percent-age is recorded on the stall chart asthe descending solid red line on Figure2. It is formed by connecting the pointsP2 (actually, its equivalent on the tem-perature scale) and TM.

The stall point (S) occurs where thered and green lines intersect, or at at65% load for this example. The dashedorange vertical line through the De-mand section indicates the caution fora stall condition.

The demand load can also be graph-ically represented as a product tem-perature increase on the same stallchart as the ascending solid dark blueline, connecting the temperaturepoints T1 and T2. And the equipmentsheat-transfer capability from surfacearea is noted by the left hand side ofthe bottom hoizontal axis where theload condition is 100%.

To summarize, the basic stall chartactually represents three different en-

vironments in one. Those are:

Theproduct-demandload,shownbythe solid blue line, usually starting inthe lower left of the Demand section

Theheat suppliedby thecombina-tion of steam supply and the equip-ment heat transfer surface, shownby the solid red line starting inupper left of the Supply section

Thebackpressureofthereturnsys-tem, shown by the solid green line,usually drawn in the Supply section

Such a stall chart is relatively com-pact, due to the hypothetical situationwhere the demand heating require-ment exactly meets the installed heatexchanger surface area. [In such a

ChemiCal engineering www.Che.Com november 2004 59

400 233

STALL CHART

0

181

138P2

TM

T2

T1

PB

103

75

52

35

20

Positivepressure,psig

Vacuump

ressure,

in.H

g

10

3

6

15

20

24

26

380

360

340

320

300

280

260

240

220

200

Temperature,

F

180

160

140

120

100

80

60

40

20

0

100 90 80 70 60 50

Load, percent

Produ

ctde

mand

load

Hea

tsupplied

Back P of return systemS

40 30 20 10 0

DEMAND

SUPPLY

Figure 2. The stall point for this example, described in the text, occurs at 65% load


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case,MCp(T2 T1) = Q = (UA)(LMTD),where M is mass flowrate and Cp isheat capacity at constant pressure,and the exchanger has no excess sur-face area.] While nearly impossible toachieve, the heat and surface area sup-plied are exactly equal to 100% of thedemand, and the three environmentsare superimposed in the 100% scalestall chart shown in Figure 2.

The extended stall chartHeat-exchanger area is generally over-surfaced for fouling factor and capac-

ity considerations. The stall chart dis-cussed above does not provide forrecording the amount of over-surfac-ing. Therefore, a new stall chart isneeded; one where over-surfacing canbe graphically demonstrated in an ad-ditional quadrant to account for extrasurface area.

This new, extended stall chart(ESC) improves the estimation ofavailable pressure differential acrossthe condensate-drainage device. Theconditions of Figure 2 can be inputand graphically demonstrated for

comparison on the ESC (Figure 3). Thesurface area equal to only 100% of thedemand, with no over-surfacing, is in-dicated by the dashed light-blue lineon the left hand vertical axis of theSupply quadrant. So far, the results ofFigures 2 and 3 look similar.

However, the ESC can be used topredict extremely valuable informa-tion. A leftward ascending line can bedrawn from the point T

M

on the righthand axis of the Demand quadrantthrough an intersection of the PB linewith the left hand axis of the Supply

Engineering Practice


400 233

EXTENDED STALL CHART (ESC)

0

181

138

P2

TM

T2

T1

PB

103

75

52

35

20

Po

sitivepressure,psig

Vacuump

ressure,

in.

Hg

10

3

6

15

20

24

26

380

360

340

320

300

280

260

240

220

200

Tem

perature,

F

180

160

140

120

100

80

60

P1 = Supply steam pressure, psig

P2 = Delivered steam pressure, psig

P3 = CDS inlet pressure, psig

T1 = Product inlet temperature, F

T1 = Product outlet temperature, F

TM= Mean product temperature, F

PB = Total system back pressure, psig

O/S% = Percent oversurfacing

40

20

0

100 90 80 70 60 50

Load, percent

40 30 20 10 0

100

150

115

115

50

150

100

20

0

90 80 70 60

Oversurfacing, percent

Conditions

50 40 30 20 10

DEMAND

SUPPLYOVERSURFACING

Prod

uctd

eman

dloa

d

Heatsupplied


Figure 3. The extended stall chart (ESC) is formed by adding an additional, equally scaled quadrant (Oversurfacing) in the

upper left-hand side. This new quadrant begins at PB, as shown here. The surface area equal to 100% of the demand, with noover-surfacing, is indicated by the dashed light blue line on the left hand vertical axis of the Supply quadrant


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quadrant, and continue into the Over-surfacing quadrant to intersect withthe P2 pressure line. This is shown bythe dashed red line in Figure 4.

The intersection of the dashed redline with the back pressure at 100%load provides a useful caution. It iden-tifies the amount of over-surfacingand the pressure profile where theequipment is always under a stallcondition. With this over-surfacing,the equipment will stall even with100% demand, so this is called a full-load stall (FLS). By extending this

dashed red line into the Oversurfacingquadrant, one can see in this examplethat 55% over-surfacing will lead to aFLS. Therefore, the amount of over-surfacing causing a FLS is shown witha vertical cautionary dashed yellowline in the Oversurfacing quadrant.

So, the ESC can be used to warn de-signers if the selected over-surfacingamount will lead to a FLS. Addition-ally, the ESC can predict the actualstall point more accurately than canthe older style stall chart.

For example, suppose that the heat

exchanger of the previous example is40% over-surfaced. That supply capa-bility of the heat exchanger now can bequickly drawn graphically in the Over-surfacing quadrant (the verticaldashed light blue line in Figure 5). Al-though the exchanger is over-surfaced,the demand does not increase. There-fore, the Demand Load (solid dark blueline) is input on the 100% scale, butthe Supply Steam (solid red line) isinput on the 40% over-surfacing scale.

The design impact is significant.When the design conditions were input

ChemiCal engineering www.Che.Com november 2004 61

400 233


0

181

138

P2

TM

T2

T1

PB

103

75

52

35

20

P

ositivepressure,psig

Vacuump

ressure,

in.

Hg

10

3

6

15

20

24

26

380

360

340

320

300

280

260

240

220

200

Temperature,

F

180

160

140

120

100

80

60









40

20

0

100 90 80 70 60 50

Load, percent

40 30 20 10 0

100

150

115

115

50

150

100

20

0

90 80 70 60


Conditions

50 40 30 20 10

DEMAND

SUPPLYOVERSURFACING

Prod

uctd

eman

dloa

d

Heatsupplied

Fullloadstall

Back P of return system

Figure 4. A leftward ascending line can be drawn from the TM point on the right hand axis of the Demand quadrant through an

intersection of the PB line on the left hand axis of the Supply quadrant, and continue into the Oversurfacing quadrant to intersectwith the P2 pressure line


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in Figure 2 or 3, the stall point was es-timated to occur at 65% of full load.However, when the over-surfacing

amount is considered, a more accuratestall point is estimated to occur at 90%of the full load condition. This knowl-edge guides the design engineer tomore accurately select a proper CDS.With the ESC, correctly identifyingparticular equipments propensity tostall will help eliminate poor processcontrol, waterhammer, and stratifica-tion issues in more installations.

ConclusionOver-surfacing must be consideredwhen selecting condensate drainagesolutions (such as traps, pump-traps,and level-pot control) for exchangers

with modulating control. An over-sur-faced exchanger is underworked andtends to lower pressure. So, the steam

pressure drops and can stall at higherdemand load than expected.

An engineer can predict the stallpoint with greater accuracy by the useof the extended stall chart. That keycapability enables him or her to selectthe optimal condensate-drainage so-lution. Simply stated, the equipmentsheat transfer and control performancewill be optimized by selecting a steamtrap or level pot control to drain con-densate when a positive pressure dif-ferential exists, or a combinationpump-trap for a negative pressuredifferential.

Edited by Gerald Ondrey


400 233


0

181

138

P2

TM

T2

T1

PB

103

75

52

35

20

Positivepressure,psig

Vacuump

ressure,in

.Hg

10

3

6

15

20

24

26

380

360

340

320

300

280

260

240

220

200

Temperature,

F

180

160

140

120

100

80

60









40

20

0

100 90 80 70 60 50

Load, percent

40 30 20 10 0

100

150

115

115

50

150

100

20

0

90 80 70 60


Conditions

50 40 30 20 10

DEMAND

SUPPLYOVERSURFACING

Prod

uctd

eman

dloa

d

Heatsupplied


Author

James R. Risko, C.E.M., isthe president of TLV Corp.(13901 South Lakes Drive,Charlotte, N.C. 28273; Phone:704-597-9070; Email: [email protected]. Previ-ous positions with TLV in-clude, vice president for sales,and national sales manager.He is active in the Fluid Con-trols Institute, where he iscurrently serving as chairman

of the association for 20042006. He has previ-ously served as Standards Chair, and Chair of theSecondary Pressure and Steam Trap Sections. Heis a dually-certified energy manager (A.E.E. &

NCSU), and holds two B.S. degrees from Kutz-town University (Kutztown, Pa.): in mathematicsand education and in business administrationand accounting. He also holds an M.B.A. from Wil-kes University (Wilkes-Barre, Pa.).

Figure 5. When the oversurfacing amount is considered, a moreaccurate stall point is estimated to occur at 90% of a full load condition

Reference:1. Risko, James R., Steam heaters need com-

plete condensate drainage, Chem. Eng., pp.

114119, July, 1996.

Additional Information:For further information, please contact the au-thor or TLVs Consulting & Engineering ServicesDepartment at (800) 858-8727. [800 TLV-Trap].

Documents

Steam Heat Ex Changers Are Under Worked and Over-Surfaced _ TLV