THE STEAM-HEATED shell-and-tube hear

exchanger is the workhorse of the chemical industry.

Effective control of such an exchanger's liquid oudet

temperature often is crucial ro plant operation. In

the first article in this series (www.C hemicalProcess­

ing.com/articles/2008/041.html ), we focused on use

of a con trol valve on the steam supply. This article

will examine control via a valve on the condensate.

Fu ture articles will look at cascade control and liqu id

bypass with one or two control valves .

In many applications, a control valve on rhe steam

supply poses a major d isadvantage for condensate

return, especially when the pressure within the shell of

the exchanger provides the d riving force fo r the con­

densate ro flow back to the boiler feed-water makeup

equipment. Should rhe pressure in the shell d rop below

rhar required fo r condensate return, the exchanger fills

with condensate, leading to the cycling described in the

previous article. Condensate control avoids this.

H owever, install ing the control valve in the

condensate return li ne instead of the steam supply

line completely changes the mechan ism fo r vary­

ing rhe hear transfer rate (UAt:.T). With a valve on

steam supply (previous a rticle), altering the valve

pos it ion affects the shell pressu re, which affects

the shell tem peratu re, wh ich a ffects rhe hear tra ns­

fer rate (through the t:.T term). In contrast, w ith a

valve on condensate (Figure 1), changing the valve

posit ion a ffects t he level o f condensate w ithin the

exchanger, which affects the effective heat transfer

area, which affects the heat transfer rate (th rough

the A term). The effect ive area for heat tra nsfer is

the heat tran sfer sur face exposed to condensing

vapo rs; the submerged heat transfer a rea mainly

cools the condensate with little contribution to the

total heat t ransfer rate.

With regard to condensate return , the pressure

within the shell always is the steam supply pressure.

The process designers must size the condensate retu rn

piping so that this pressure is adequate to return the

condensate to the boiler house.

EXCHANGER RESPONSE

The configurat ion shown in Figu re I o ften raises

an issue of dynamics. Th e exchanger responds

more slowly co control actions than does an

exchanger w ith the control valve on the steam

supply. C hanges in the condensate valve posit ion

must fi rst a ffect the level within the exchanger

-causing a slower response. However, fo r most

applicat ions, t he response of the excha nger is suf­

ficiently rapid co provide acceptable perfo rma nce.

@ -<-------------

Steam

Condensate

Al though equal-percen tage valves a re gener­

a lly insta lled for condensate con t rol , an a rgu­

ment ca n be m ade fo r linear va lve cha rac te ri st ics .

The d riv ing force fo r fluid f low is p rovided by

the steam supply press ure . T he hydrostat ic head

of the condensate with in the excha nge r is neg­

ligible in comparison. T he pressure d ro p in the

condensate re turn sys tem also usually is smal l.

(O versizing isn 't lim ited co control va lves.) Under

these conditions, t he pressure drop across t he

cont rol valve in the condensate line is essentia lly

constan t. M ost guideli nes suggest linea r valve

charac teris tics fo r instal lations w here the pres­

su re drop across the control va lve is consta nt.

H owever, these guidelines a re n't always fo llowed.

Figure 2 presents the process operating lines

for both the linear va lve a nd the equal-percentage

valve . W ith a linear valve, the liquid outlet tem­

perature is linea rly related co the valve posit ion.

The process operating line for the equal-percent­

age valve clea rly reflects t he inherent cha racteris ­

t ics of such a va lve.

300 288

~ 260

~ .a 220 E "' Q.

E "' ... 180 "0 ·:; CT ::; 140 Equal·percentage valve

100~---~--~-~54~~--~8~4~-~ 0 20 40 60 80 100

Condensate valve position, %

Figure 1. A change in valve position d irect ly affects t he available heat-trans­fer area.

Figure 2. A linear valve can be a good choice for control of a condensate line.

25 CHEMICALPROCESSING.COM e JULY 2008

Figure 3. Adding a steam trap to the con­densate line keeps the exchanger f rom blowing steam.

Steam

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@ .c-------------

Liquid out

Liquid in

Condensate

HEAT TRANSFER

Theoret ically, the m inimum heat transfer rate is

zero. W ith the control va lve closed, the exchanger

completely fi ll s with condensate, resulting in no heat tra nsfer.

The maximum heat transfer occurs when the exchanger is completely drained of condensate. The

value for the maximum heat transfer rate is the same,

regardless of whether the control valve is on the steam

supply or the condensate. H owever, the consequences of attempts to exceed

the maximum are very different. The exchanger in Figu re I has the potential to "blow steam" into the condensate return system. As the control valve

opens, the condensate level within the exchanger

d rops. I f the valve is opened too much, the level drops ent irely out of the exchanger and steam Aows in to the condensate return system, a somewhat

unpleasant situation.

Both operating lines in Figure 2 terminate at a liquid outlet temperature of288°F. At this point, the exchanger is completely drained of condensate and the entire tube area is exposed to condensing steam.

Figure 2 suggests that this occurs at a valve opening

of 54% for the linear valve and 84% fo r the equal-per­centage valve. W hat about valve openings greater than these values? In practice, the liquid outlet temperature usually d rops. W hen the exchanger blows steam, the

shell pressure usually d rops below the steam supply

pressure, giving a lower liquid outlet temperature.

Grounded by conventional wisdom?

A SIMPLE STEP

Inserting a steam trap into the condensate li ne up­

stream of the control valve (Figure 3) wi ll prevent

release of steam in to the condensate return piping.

As long as there's some condensate within the ex­

changer, the trap has no effect on condensate flow.

However, should the condensate completely drain

from the exchanger, the trap prevents steam from

flowing into the condensate return system.

Unfortunately, the con figuration in Figure 3

exposes the liquid outlet temperature controller to

windup. The test for the possibility of windup is very

simple: Are t here situations where changes in the

controller output (the condensate valve position) have

no effect on the controlled variable (the liquid outlet

temperature)? Here, the answer is "yes"- whenever

the trap is preventing the steam from flowing into the

condensate return system.

As customarily configured, the windup pre­

vention mechanisms provided by d igital control

systems are ineffective for this situation . These

mechanisms are invoked when the contro ller our-

. . @<~ - - - - - - - - - - - - - - - -

Steam

Condensate

Figure 4. Such a configuration also can prevent the exchanger from releasing steam.

liquid in

put attains its upper limi t (normally set at a value

above 100%). Based on the operating li nes in Fig­

ure 2, the appropriate upper output limit is 54%

for a linear valve and 84% for an equal-percentage

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Steam

Condensate

Figure 5. This provides ano ther opt ion to forestall release of steam.

Figure 6. Equal-per­centage valve better suits the varying p ressure d rop across t he valve.

Liquid out

Liquid in

valve. H owever, these limits depend on operating

variables, especially throughput.

The windup prevention should be invoked at the

instant the condensate is completely drained from

the exchanger, which is when the steam trap begins

to block rhe steam from flowing into the condensate

return system. Unfortunately, with the exchanger in­

strumented as in Figure 3 there's no way to detect this

event (the exchanger completely drained of condensate).

However, there's a way to detect this- by

equipping the exchanger with instrumentation not

customarily provided. For instance, a level switch or

level transmitrer fo r the condensate could indicate

when the exchanger is drained of condensate and the

maximum heat transfer rate is attained. Under these

conditions, the liquid outlet temperature controller

should be inhibited from increasing its output. D igital

systems certainly are capable of this- but only, of

cou rse, if the necessary information is available.

MORE OPTIONS

Figure 4 illustrates an override configuration fo r

300-.-----------------, 288

260

220

180

140

I

Equal-percentage valve:

Condensate valve posit ion , %

JULY 2008 e CHEMICALPROCESSING .COM 28

preventing the exchanger from blowing steam . It

requires a measurement device for the condensate

level with in the shell of the exchanger bur all other

components are implemented in software.

As long as the condensate level exceeds its set

point, the liquid outlet temperature controller

determines the control valve position. However,

if condensate level drops to its set point, then the

condensate level controller takes over. This is imple­

mented by using a low select to choose between the

outputs of the liquid outlet temperature controller

and the condensate level controller.

Another way ro prevent the exchanger from

blowing steam is to install a condensate pot down­

stream (Figure 5). The steam pressure in the shell

of rhe exchanger and in the condensate pot is the

steam supply pressure. Therefore, the full steam

supply pressure is available for condensate rerum .

A con troller ma in ta ins the desired liquid level in

the condensate pot . The condensate line from the

exchanger enters below the liquid level in the con­

densate pot, so the excha nger can' t blow steam.

The condensate flows by gravity from the exchang­

er to the condensate pot. So, the condensate level in

the exchanger must be above that in the condensate

pot. The hydrostatic head provided by this difference

is the drivi ng force for condensate to flow from the

exchanger to the condensate pot.

The maximum driving force is when the ex­

changer is completely full of condensate; the con­

densate flow under these conditions would be zero.

The minimum driving force is when the exchanger

is completely empty of condensate; the maximum

condensate flow occurs under these conditions.

Figure 6 presents the process operating li nes for

both a linear and an equal-percentage valve. The pres­

sure drop across the control valve isn't constant, which

favors using the equal-percentage valve. The operating

lines in Figure 6 confirm this. There's only a modest

departure from linearity from a control valve opening

ofO% to 78%.

Above a valve opening of78o/o for the equal­

percentage valve (44% for the linear valve), the control

valve has no effect on the liquid outlet temperature

because no condensate remains within the exchanger.

So, opening the valve further doesn't impact the heat

transfer rate or, consequently, the liquid outlet tem­

perature. This exposes the liqu id outlet tem perature

controller ro windup.

The physical locat ions of the exchanger a nd

the condensate pot affect the degree of windup

that's possible. The maximum possible level in

the condensate pot corresponds

to the bottom of the exchanger.

If the condensa te pot level is

above the bottom of the ex­

changer, the exchanger can't be

completely drai ned of conden­

sate. If the condensate pot level

is exactly at the bottom of the

exchanger, there would be no

hydrostatic head for condensate

flow when the exchanger is

completely drained of conden­

sa te (and the condensate flow

is at its m aximum). As the

condensate pot level is dropped

further below the bottom of the

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exchanger, the hydrostat ic head

for fluid flow increases - bu t

this also ra ises the possibility

for w indup in the liquid outlet

tem perature controller.

Windup can't occur if the con­

trol valve is either perfecrly sized or

undersized. However, as previously

noted, most valves are oversized

at least to some degree. In this

case, an additional consequence of

oversizing the valve is the possibil­

ity of windup in the liquid ourlet

temperature controller.

This is another case where

the windup prevention mecha­

nisms as customarily configured

in dig ital systems are ineffective.

Figure 6 indicates that the upper

output limit could be set at 78%

(for an equal-percentage valve)

but this value is neither precise

nor constant. Instead , the wind­

up prevention should be invoked

at the instant the condensate is

completely drained from the ex­

changer. Detecting this requires

addi t ional instrumentation (such

as a level switch) to provide the

necessary information so the

system can properly initiate its

w indup protection mechanisms.

In large production facilit ies

where multiple exchangers can

be physically located within a

reasonable proximity, condensate

pots are commonly installed to

address the condensate return

issues. (Installing a condensate

pot for an individual exchanger

is diffi cult to justify.) There are

competing designs for condensate

pot arrangements but in most the

pressure in the condensate pot

is the steam supply pressure and

the condensate flows from the

exchangers to the condensate pot

by gravity. e

CECIL L. SMITH is president of Cecil L.

Smith, Inc., Baton Rouge, La. E-mail him at

cecilsmith@cox.net.

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