Process Cooling

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Posted on: 03/01/2004

Evaporators for Process Cooling SystemsAsh Bhadsavle, Art Stipanovic and Teresa Sauble, FES Systems

Understanding the types of heat exchangers used as evaporators

in process cooling systems can help you select the one that isbest for your particular process.

Depending on the application, itis important to use the properheat exchanger for a givenprocess cooling requirement.Both shell-and- tube and plate-and-frame heat exchangers canbe used as evaporators in aprocess cooling system. Thetypes of heat exchangers usedas evaporators include dry

expansion, flooded shell-and-tube and thermosyphon (floodedtube) evaporators as well asplate-and-frame heatexchangers.

Dry Expansion Evaporator.Introduced in 1937 by Acme Industries of Jackson, Mich., the now-common dry expansion evaporator appeared under the registeredname of Dry-Ex. In a dry expansion evaporator (figure 1), the fluid tobe cooled is in the shell side of the heat exchanger while the refrigerantis in the tubes. The refrigerant enters as a two-phase mixturedownstream of a thermoexpansion valve. The design is based on therefrigerant exiting the tube side of the heat exchanger totally vaporizedand superheated. Control of the process refrigeration load is achievedby controlling the refrigerant flow through a thermo-expansion valve,

which maintains a preset superheat -- typically 6 to 12oF (3.3 to 6.7oC).

This type of evaporator is suitable for small to medium refrigerationloads (1 to 250 tons) for cooling water or glycol in a relatively steady-load application. A dry expansion evaporator is the heat exchanger ofchoice for cooling heat transfer fluids with viscosity greater than 6 cP ina steady-load application. Such applications include cooling 40 percent

ethylene glycol to 0oF (-18oC) when viscosity (cP) is 10, or cooling 40

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Figure 1. Dry expansion

evaporators are suitable for small

to medium (1 to 250 ton)refrigeration loads for cooling

water or glycol in a relativelysteady-load application. They

typically are used to cool heattransfer fluids with viscosity

greater than 6 cP.

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percent propylene glycol to 10oF (-12oC) when cP is 25.

Direct expansion evaporators arenot recommended for use inbatch cooling or other unsteadyapplications. Experience hasshown that the dry expansiondesign, with its control scheme,

has difficulty tracking the varyingloads of a batch cooling processand therefore does not satisfythe cooling requirements.

For process chillers with dryexpansion evaporators, a suctionaccumulator typically is providedfor compressor protection. Alsoin process chilling, a water/glycol surge tank should be includedbetween the process load and the chiller to dampen any upsetcondition.

Typical materials of construction are carbon steel for the shell andcopper for the tubes. In ammonia systems, the tubes also are carbonsteel.

Flooded Evaporator. Flooded evaporators (figure 2) are the norm forwater and light glycol chillers with steady-state refrigeration loadsgreater than 300 tons. The flooded evaporator is suitable for water andlight glycol chilling for batch-type refrigeration loads -- both large andsmall. Batch-type refrigeration loads are common in thepharmaceutical and petrochemical industries.

With a flooded evaporator, a pool of refrigerant is maintained in theshell, submerging the tubes to a set level. As the refrigeration load

varies, a refrigerant-level control valve acts to maintain the liquid levelin the shell. The pool of refrigerant in the shell behaves as a flywheel,allowing the controls of the flooded evaporator to successfully track thevarying load of a batch process.

The flooded evaporator with the process fluid in the tubes is used tochill a range of heat transfer fluids, glycols and brines as well as water.The flooded evaporator is best suited for chilling a process fluid with aviscosity of approximately 6 cP or less. At higher viscosities, the tube-side Reynolds number becomes low, promoting boundary layer growthand inhibiting heat transfer. Examples of process fluids in the range of

6 cP are 30 percent ethylene glycol cooled to 15oF (-9oC), 30 percent

propylene glycol cooled to 30oF (-1oC), and 22 percent calcium

chloride cooled to 10o

F.

Figure 2. Flooded evaporatorsgenerally are used for water and

light glycol chillers with steady-state refrigeration loads greater

than 300 tons.




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Flooded evaporators are designed with a range of tube-side materials,including copper-based materials, carbon steels, stainless steels andtitanium. This variety in materials allows for cooling many types ofprocess fluids through a range of operating temperatures and

pressures. Flooded evaporators are used to cool various gases suchas air and nitrogen and also are used in condensing gases such aschlorine and hydrocarbons.

Flooded evaporators often are utilized when the process fluid is dirtyand prone to fouling. The evaporator tubes are accessible for cleaningon a scheduled maintenance program or as required. They aredesigned with vapor/liquid disengagement space for compressorprotection. Disengagement space is accomplished by allowing theappropriate space above the tube bundle in a single-shell evaporator(common in water chillers for air-conditioning service) or adding anintegral horizontal accumulator vessel above the tube bundleconnected by risers.

Thermosyphon Evaporator.Thermosyphon evaporators(figure 3) are a common choicefor large and small batch-typerefrigeration loads where theprocess fluid is clean but viscous(viscosity greater than 6 cP). Inthe thermosyphon evaporatordesign, the inside of the tubesare flooded with refrigerant, andthe process fluid is on the shellside. Liquid refrigerant is fed bygravity from an overhead

accumulator vessel to the tubeside of the evaporator. Therefrigerant in the tubes picks upheat from the process fluid, anda two-phase vapor/liquid refrigerant mixture exits the tube side of theevaporator and returns to the overhead accumulator vessel. The liquidseparates from the vapor in the accumulator vessel, and the vapor isdrawn off to the refrigerant compressor (figure 4).

As the refrigerant load varies, a refrigerant liquid control valve acts tomaintain a liquid seal level in the accumulator vessel. The tubes

Use this table to determine what type of heat exchanger design isbest for process cooling applications that use ammonia as a

refrigerant.

Figure 3. Thermosyphon

evaporators are suitable for large

and small batch-type refrigerationloads with process fluid that is

clean but viscous.




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flooded with refrigerant are the flywheel, allowing the controls of thethermosyphon evaporator to track the varying load of a batch process.

This type of evaporator is not as well known in the industry as the

direct expansion and flooded designs. However, the thermosyphonconcept is used extensively with oil coolers for screw compressorpackages in the food and beverage industries.

Plate-and-Frame Evaporators. In the past 10 years, plate heatexchanger technology has advanced. Laser welding of plates haseliminated 95 percent of gaskets on the refrigerant side. Clip-ongaskets on the process fluid side are easy to install (without glue) andare reliable. The flow path for the process fluid between plates isnarrow vs. a shell-and-tube design. For dirty process fluids and fluidsthat contain particulates, a shell-and-tube design is recommended, butplate evaporators are suitable for closed-loop, clean service,nonfouling process systems.

Use this table to determine what type of heat exchanger design isbest for process cooling applications that use R22, R134a, R404 or

R507.




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The plate evaporator (figure 5) can be employed in a dry expansiondesign similar to the dry expansion shell-and-tube evaporator bututilizing a thermoexpansion valve. Control of the refrigeration load isachieved through the thermoexpansion valve, which maintains a presetsuperheat. Dry expansion plate evaporators are recommended forsmall to medium steady-state refrigeration loads. For varying or batch-type loads, a flooded plate-and-frame evaporator is preferred.

Recommended for large refrigeration loads (approximately 300 tonsand greater), the flooded plate evaporator operates in a thermosyphonmode, with liquid refrigerant being fed by gravity from an overheadaccumulator vessel into the lower port of the plate evaporator. Therefrigerant picks up heat from the process fluid, and a two-phaseliquid/vapor refrigerant mixture exits the upper port of the evaporatorand returns to the overhead accumulator vessel. The liquid and vaporseparate in the accumulator, and the vapor is drawn off by the

Figure 4. Liquid refrigerant is fed by gravity from an overheadaccumulator vessel to the tube side of the evaporator. The refrigerant

in the tubes picks up heat from the process fluid, and a two-phase

vapor/liquid refrigerant mixture exits the tube side of the evaporatorand returns to the overhead accumulator vessel. The liquid separates

from the vapor in the accumulator vessel, and the vapor is drawn offto the refrigerant compressor.




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compressor.

Plate evaporators often are usedin ammonia process chillers andare popular in the food andbeverage industries. The lowmass flow of ammonia per ton ofrefrigeration suits the plate

design. Also, plate evaporatorsrequire less refrigerant chargevs. a comparable shell-and-tubeevaporator, thereby minimizingthe amount of ammonia in aprocess chiller.

One particular feature of the plate evaporator is the close approach

temperature that can be achieved (approximately 4 or 5oF [2.2 to

2.8oC]) in viscous glycol services such as 40 percent propylene glycol

at 10oF. The close approach temperature typically allows the user toselect a smaller-sized compressor for the process chiller, resulting inless brake horsepower (bhp) and therefore better brake horsepower

per ton.

Generally, plate heat exchangers are not recommended for use asrefrigerant condensers. Plates tend to foul and corrode if placed inservice with open-loop cooling tower water, well water or river water,which commonly are used in water-cooled condensers. A shell-and-tube design with proper tube-side materials should be used for water-cooled condenser service.

Several types of evaporators can be used for process cooling, so it isimportant to use the correct evaporator for an economical and reliablecooling system.

Ash Bhadsavle is product manager heat exchangers, Art Stipanovic isapplication engineering manager, and Teresa Sauble is marketingmanager at FES Systems Inc., York, Pa. For more information, call(800) 888-4337.

Figure 5. Plate-and-frameevaporators should be applied in

closed-loop, clean service,

nonfouling process systems.

Copyright 2005 by BNP Media.


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