Evaporators for Process Cooling Systems
Understanding the types of heat exchangers used as evaporators in process cooling systems can help you select the one that is best for your particular process.
Depending on the application, it is important to use the proper heat exchanger for a given process cooling requirement. Both shell-and- tube and plate-and-frame heat exchangers can be used as evaporators in a process cooling system. The types of heat exchangers used as evaporators include dry expansion, flooded shell-and-tube and thermosyphon (flooded tube) evaporators as well as plate-and-frame heat exchangers.
Dry Expansion Evaporator. Introduced in 1937 by Acme Industries of Jackson, Mich., the now-common dry expansion evaporator appeared under the registered name of Dry-Ex. In a dry expansion evaporator (figure 1), the fluid to be cooled is in the shell side of the heat exchanger while the refrigerant is in the tubes. The refrigerant enters as a two-phase mixture downstream of a thermoexpansion valve. The design is based on the refrigerant exiting the tube side of the heat exchanger totally vaporized and superheated. Control of the process refrigeration load is achieved by 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 refrigeration loads (1 to 250 tons) for cooling water or glycol in a relatively steady-load application. A dry expansion evaporator is the heat exchanger of choice for cooling heat transfer fluids with viscosity greater than 6 cP in a steady-load application. Such applications include cooling 40 percent ethylene glycol to 0oF (-18oC) when viscosity (cP) is 10, or cooling 40 percent propylene glycol to 10oF (-12oC) when cP is 25.
Direct expansion evaporators are not recommended for use in batch cooling or other unsteady applications. Experience has shown that the dry expansion design, with its control scheme, has difficulty tracking the varying loads of a batch cooling process and therefore does not satisfy the cooling requirements.
For process chillers with dry expansion evaporators, a suction accumulator typically is provided for compressor protection. Also in process chilling, a water/glycol surge tank should be included between the process load and the chiller to dampen any upset condition.
Typical materials of construction are carbon steel for the shell and copper for the tubes. In ammonia systems, the tubes also are carbon steel.
Flooded Evaporator. Flooded evaporators (figure 2) are the norm for water and light glycol chillers with steady-state refrigeration loads greater than 300 tons. The flooded evaporator is suitable for water and light glycol chilling for batch-type refrigeration loads -- both large and small. Batch-type refrigeration loads are common in the pharmaceutical and petrochemical industries.
With a flooded evaporator, a pool of refrigerant is maintained in the shell, submerging the tubes to a set level. As the refrigeration load varies, a refrigerant-level control valve acts to maintain the liquid level in the shell. The pool of refrigerant in the shell behaves as a flywheel, allowing the controls of the flooded evaporator to successfully track the varying load of a batch process.
The flooded evaporator with the process fluid in the tubes is used to chill 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 a viscosity of approximately 6 cP or less. At higher viscosities, the tube-side Reynolds number becomes low, promoting boundary layer growth and 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 10oF.
Flooded evaporators are designed with a range of tube-side materials, including copper-based materials, carbon steels, stainless steels and titanium. This variety in materials allows for cooling many types of process fluids through a range of operating temperatures and pressures. Flooded evaporators are used to cool various gases such as air and nitrogen and also are used in condensing gases such as chlorine and hydrocarbons.
Flooded evaporators often are utilized when the process fluid is dirty and prone to fouling. The evaporator tubes are accessible for cleaning on a scheduled maintenance program or as required. They are designed with vapor/liquid disengagement space for compressor protection. Disengagement space is accomplished by allowing the appropriate space above the tube bundle in a single-shell evaporator (common in water chillers for air-conditioning service) or adding an integral horizontal accumulator vessel above the tube bundle connected by risers.
Thermosyphon Evaporator. Thermosyphon evaporators (figure 3) are a common choice for large and small batch-type refrigeration loads where the process fluid is clean but viscous (viscosity greater than 6 cP). In the thermosyphon evaporator design, the inside of the tubes are flooded with refrigerant, and the process fluid is on the shell side. Liquid refrigerant is fed by gravity from an overhead accumulator 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 evaporator and returns to the overhead accumulator vessel. The liquid separates from the vapor in the accumulator vessel, and the vapor is drawn off to the refrigerant compressor (figure 4).
As the refrigerant load varies, a refrigerant liquid control valve acts to maintain a liquid seal level in the accumulator vessel. The tubes flooded with refrigerant are the flywheel, allowing the controls of the thermosyphon 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 thermosyphon concept is used extensively with oil coolers for screw compressor packages in the food and beverage industries.
Plate-and-Frame Evaporators. In the past 10 years, plate heat exchanger technology has advanced. Laser welding of plates has eliminated 95 percent of gaskets on the refrigerant side. Clip-on gaskets on the process fluid side are easy to install (without glue) and are reliable. The flow path for the process fluid between plates is narrow vs. a shell-and-tube design. For dirty process fluids and fluids that contain particulates, a shell-and-tube design is recommended, but plate evaporators are suitable for closed-loop, clean service, nonfouling process systems.
The plate evaporator (figure 5) can be employed in a dry expansion design similar to the dry expansion shell-and-tube evaporator but utilizing a thermoexpansion valve. Control of the refrigeration load is achieved through the thermoexpansion valve, which maintains a preset superheat. Dry expansion plate evaporators are recommended for small 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 tons and greater), the flooded plate evaporator operates in a thermosyphon mode, with liquid refrigerant being fed by gravity from an overhead accumulator vessel into the lower port of the plate evaporator. The refrigerant picks up heat from the process fluid, and a two-phase liquid/vapor refrigerant mixture exits the upper port of the evaporator and returns to the overhead accumulator vessel. The liquid and vapor separate in the accumulator, and the vapor is drawn off by the compressor.
Plate evaporators often are used in ammonia process chillers and are popular in the food and beverage industries. The low mass flow of ammonia per ton of refrigeration suits the plate design. Also, plate evaporators require less refrigerant charge vs. a comparable shell-and-tube evaporator, thereby minimizing the amount of ammonia in a process 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 to select a smaller-sized compressor for the process chiller, resulting in less brake horsepower (bhp) and therefore better brake horsepower per ton.
Generally, plate heat exchangers are not recommended for use as refrigerant condensers. Plates tend to foul and corrode if placed in service 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 is important to use the correct evaporator for an economical and reliable cooling system.