controlling flow rates in a shell and tube heat exchanger

8/14/2019 Controlling Flow Rates in a Shell and Tube Heat Exchanger

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Controlling Flow Rates InA Shell And Tube Heat

Exchanger

WHICH STREAM DO WE MANIPULATE?Exchangers have four ports and involve two different

fluids, either of which may change phase. The former

feature alone allows eight different valve arrangements.Term the two streams the "process" side and the "heatexchange medium" side.

a - Process side, outlet throttling.b - Process side, inlet throttling.c - Process side, bypass with outlet restriction.

d - Process side, bypass with inlet restriction.e - Medium side, outlet throttling.f - Medium side, inlet throttling.g - Medium side, bypass with outlet restriction.h - Medium side, bypass with inlet restriction.


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Various Methods Applied For Flow RateManipulation:

1. THROTTLING THE PROCESS FLUID. It is quitemeaningless to attempt to control the processtemperature by throttling either the inlet or the outletof the process fluid. The desired process flow rate isset by other requirements and these would beinterfered with by manipulating the process flow.

Temperature will change somewhat since flowreduction increases the residence time of the fluid

and the outlet temperature will more closelyapproach the inlet temperature of the medium. Onthe other hand, variations in process flow, caused bysome external influence, is one of the major causesof temperature variation. It is often the reason whywe must manipulate some other parameter tomaintain constant temperature.

2. BYPASSING THE PROCESS FLUID. Processtemperature can be controlled by manipulating

process flow if a bypass is installed. As the outlettemperature rises (assume this is a heater), morefluid is bypassed around the ex-changer withoutbeing heated. As the two streams are blendedtogether again, the correct temperature is achieved.


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3. SPLIT RANGE. Bypass manipulation sounds simplebut there are a few tricks to it. Firstly, there are twoways of arranging the valve controls: We can attemptto minimize pressure drop at all times, or we can

attempt to keep the pressure drop constant. In neithercase do we want to interrupt the total flow. If we wishto minimize pressure drop, a butterfly valve is thelikeliest choice. However, even a wide open butterflyhas some pressure drop. It may be greater than thatof the heat exchanger itself. This means that evenwhen the valve is wide open only half the flow, orless, will bypass the exchanger. To accomplish agreater degree of bypass, a restriction must be placed

on the flow through the exchanger. The restrictionshould be adjustable since conditions change and wedo not want more restriction than necessary. Theeasiest way to do this is with a hand valve. Sincethese valves are often in relatively inaccessibleplaces, remote actuators may be added. Once that isdone it becomes an obvious matter to arrangeautomatic controls so that once the bypass is fullyopen, the restriction valve starts to close, and vice

versa.


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4. CROSS EXCHANGERS. When the heat in oneprocess stream is to be exchanged with anotherprocess stream, the flow on neither side may beinterfered with while controlling the temperature. An

example is when distillation tower bottoms are crossexchanged with the tower feed. The tower requires ahigh temperature at the bottom in order to functionbut the heat is not "consumed" by the process nor isit needed in the product. It is returned from thebottom product back to the feed. This is a commonand extremely effective energy conservationmeasure. As with all energy recovery arrangements,the key to success is to control the heat recovery

without disturbing the process. That is, the flow ofneither of the two process streams may be interferedwith. The solution is to manipulate the heat transferby bypassing one of the two streams around theexchanger. Most often control is exercised on thetube side. The failure modes of the valves are chosento prevent overheating and flow blockage.

5. UNCONTROLLED HEAT EXCHANGE. In some crossexchange applications it is desired to recover all the

heat (or cold) content of the product stream and totransfer it to the feed. In such cases the exchangerneeds no controls at all. The feed stream usually hasa second exchanger downstream of the first to boostthe temperature to the required level. This exchangeris the one that is manipulated.

6. AERIAL COOLERS. As mentioned earlier, aerialcoolers can be considered a special type of shell andtube exchanger in which the shell is the shell of thecooler. A large fan is used to blow air, usually frombelow, past the tubes. As with other exchangers it ispossible to control the temperature by manipulatingthe process or the medium flows. The normal way toprovide accurate temperature control is to useprocess flow bypass valves. In addition there arethree means of manipulating the medium: Louver ordamper control, fan pitch control and variable speed.

7. FAN PITCH CONTROL. This is an obvious means ofcontrolling the temperature. It has the advantage ofreducing horsepower as the cooling demand is


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reduced. As with every control technique, there arelimitations. Firstly, the turndown is rather poor. This isespecially important in a northern climate where itmay not be possible to turn down the fans sufficiently

in winter. The spinning blades still stir up the air evenwhen the pitch is zero. Natural draft alone mayprovide more cooling than is required. Secondly, thepitch control mechanism can be a maintenanceheadache. The control system engineer mustexamine the equipment drawings to be sure that themechanisms allow easy access for lubrication andrepair. It is a wise idea to put separate I/Ps on eachfan. Long strings of tubing with many tees make leak

detection a nightmare. Each I/P requires a separateoutput from the control system as the current loopwill not work if there are more than two in series.Note that if a single controller drives multiple fanpitch controls, the process gain of the loop isproportional to the number of fans in service. If thecontroller is tuned with only half the fans running, itmay go unstable when the rest are turned on. Acontroller that is tuned with all fans running will be

sloppy if some are turned off. It is, of course, possibleto configure automatic gain compensation within aDCS. (Remember to check for divide by zero when nofans are running.)

8. VARIABLE SPEED. Fully variable fan speed control isbecoming more common on aerial coolers. The fanmotors are often quite numerous but not extremelylarge. However, the cost of the electronics has comedown considerably in recent years. One way to cutcosts is to connect both fans of one bay to the sameset of VFD electronics. On the other hand, two-speedfans have always been quite common. This isespecially true in climates with extreme seasonaltemperature swings. Reducing the fan to half speedresults in an 85% reduction in electric powerdemand. (Remember that electrical power varies asthe cube of fan speed.) Cutting the speed of anelectric motor in half requires only a reconnection ofthe wiring to a multipole stator. This can beaccomplished by having two electrical starters wired


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in different ways. The increase in cost is not verylarge. A variation of the two-speed motor is toarrange for reverse flow. This can be extremely usefulin climates where icing is a problem. Reversing the

flow blows warm air through the inlet louvers andserves to melt any accumulated ice. This should bedone before ice build-up is too large or large chunksof ice may be sent crashing down onto other piecesof equipment or even personnel.

9. LOUVER CONTROL. Automatic louver control hassimilar problems as fan pitch control. There is anadditional problem of hysteresis. The louvers seldommove smoothly for long and it becomes very difficult

to maintain stable control as dirt and wearaccumulate over time, especially in sandy or dustyenvironments. In climates with strong seasonaltemperature swings, it is possible to stabilise the airtemperature and prevent icing by controlling internalrecirculation. The exchanger is fitted with a ductleading from the top outlet to the bottom inlet of theunit. Dampers are placed in this duct and at the airintake. A temperature controller senses the air above

the fan and controls it by opening the recirculationduct and simultaneously closing the intake. Note thatan opposite action arrangement, as described above,is appropriate. Figure 3-6 shows a possiblearrangement using both outlet louver control fromthe process and recirculation control off the internalair temperature.


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10.ADVANCED TRICKS FEEDFORWARD. Large heat

exchangers have both dead time and considerablethermal inertia. These two factors can make control

difficult. Feedforward can be usefully applied if loadchanges are a problem. Since the heat demand isproportional to the process flow rate, other thingsbeing equal, a flow rate measurement can be used.Figure 3-7 shows a typical arrangement. Note thatthe output of the TC is multiplied by the flow signal.

That is because the heating medium flow rate mustbe roughly proportional to the feed flow. This worksbest if the installed characteristic of the valve is

linear. If the exchanger is very large, it may benecessary to insert a lag or some other form of delayinto the flow signal to prevent it from acting too soonand causing a reverse spike to appear in thetemperature. Note that the dead time is inverselyproportional to the flow rate and some "typical" valuemust be used. Some brands of DCS have the optionof a variable delay time. This allows delay to beinversely proportional to flow rate.

11.TEMPERATURE OPTIMIZATION. Another"Advanced Trick" involves optimization of a fired


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heater. Heat is being supplied to the reboiler of adeethanizer as shown in Figure 3-8. It is required tokeep the temperature at the bottom of the towerconstant. The heating medium is hot oil which is

being heated by a fired heater and circulated by apair of pumps. Since the tower bottoms is beingboiled, and is also very clean, it goes on the shellside. The oil goes through the tube side where theoutlet is throttled by a butterfly valve. A positiontransmitter has been added to the valve. Its outputgoes to a Position Controller with a setpoint of about80% open. The output of the Position Controller iscascaded to the setpoint of the Temperature

Controller of the furnace. The effect is to maintainthe furnace, and the hot oil, at the lowesttemperature consistent with the heat demand of thetower. It works as follows:

a) As the heat demand rises, the valve opens further.b) When the valve is open beyond 80%, the setpoint to the

furnace Temperature Controller is raised.c) As the temperature of the hot oil rises, the valve closes

to near the 80% value.

d) As the heat demand of the tower falls, the valve closesbelow 80%.e) As the valve closes, the setpoint to the furnace is

lowered until the valve is once again at its 80%target.

In this way the temperature of the hot oil system is kept atits lowest acceptable value and a minimum of heat is lostby the furnace or the piping.


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12.COMBINATION CONTROL. Sometimes a heatexchanger is used to heat, or cool, a fluid whose totalflow is being controlled by some other parameter.

The most straightforward way of controlling this is touse a three-way valve, or two butterflies, to controlthe heat exchange and to use another valve tocontrol the total flow. The flow control valve must beon the common line either upstream or downstreamof the exchanger. This arrangement has two valves in

series and cries out for a way of eliminating one ofthem. If the positions of the inlet and bypass valvesare controlled separately so that the total Cv iscontrolled by the Flow Controller and the differencebetween the Cvs is controlled by the TemperatureController, complete control can be achieved withonly two valves. Figure 3-9 shows how this can bedone. Theexample uses boiler feedwater to cool a sulphur

condenser at the same time the water is beingpreheated. The sulphur vapour, being the moredifficult fluid, is in the tubes. The water is in the shell.Since we want to make certain that the water doesnot boil, we will put the valves on the outlet side. Thevalve controlling the outlet of the exchanger receivesa signal equal to half the sum of the two controlleroutputs. The valve controlling the bypass receiveshalf of the difference between the two controlleroutputs. Assuming that the installed characteristic ofboth valves is linear, the combined flow of the two


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valves is then dependent entirely on the FlowController. The difference between the two flows isdependent on the Temperature Controller. In thisparticular situation it is desirable that both valves are

fail open. If the failure mode of either, or both, valvesis fail closed, the signs of the summing/scalers UY-Aand UY-B will have to be changed to give the properresult.

13.EQUIPMENT PROTECTION. The usual shell andtube exchanger has no moving parts nor any in-putof external energy. There are few machineryprotection issues. Severe corrosion is sometimes aproblem. If so, corrosion detection devices may beinstalled. These consist of a thin wire or film of thesame material as the exchanger. The wire is held in aholder that is inserted through a nozzle into the

exchanger. Two electrical contacts are accessiblefrom the outside. When the resistance is measured,the extent of corrosion can be determined directly.

These devices are not normally connected into a datalogging network. The usual practice is to make themeasurements with a portable monitor on a regularbasis. Intrinsically safe monitors are available forhazardous locations.

Aerial coolers require protection from the energy

introduced by the electric motors. The most serioushazard is a thrown blade. The resulting vibration is quite


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severe and can cause extensive damage. A simple seismicvibration switch mounted on the structure that holds thelower bearing of each fan is quite sufficient. It works byhaving a small weight held in place by a magnet against

the force of a spring. A "bump" dislodges the weight fromthe magnet and allows it to open the shutdown contact.

The usual method of "calibration" is a light whack with ahammer. A button allows the operator to reset the switchby pushing the weight back against the magnet. Switcheswith remote electrical reset can be bought but it is alwaysbest for an operator to look at the machine and determinethe cause of the shutdown before restarting theequipment. Precautions must be taken when reversing a

motor that has been running. Such a change is aconsiderable shock to the machinery. The usual approachis to provide a time delay interlock so that sufficient timehas elapsed to be certain that the fan has stopped rotatingbefore the motor can be started in the opposite direction.If this is not done the fan will most likely trip on vibration.

The nuisance of resetting locally mounted vibrationswitches will encourage the operators to be more carefulin the future.

controlling flow rates in a shell and tube heat exchanger

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