February 2018 — An evaporator is a device that absorbs the heat surrounding it to cause the liquid refrigerant inside it to boil until the refrigerant leaves as a superheated (saturated) gas. Each evaporator operates according to the same basic formula: it absorbs heat while boiling off the liquid refrigerant.
Remember that heat travels from a warmer substance to a cooler one. Consequently, the evaporator must be at a lower temperature than its surroundings. For example, a home air conditioner would have an evaporator coil temperature of 40°F, which is its boiling temperature at 68.5 psig when using refrigerant-22 (R-22). As long as air moving across the evaporator stays above 40°F, the refrigerant in the evaporator will boil, absorbing the heat from the room air and transferring it to the refrigerant.
When designing an evaporator, an engineer must ask and answer the following questions about the design of the system.
- What material should be used to construct the evaporator? Cost and location are two key factors that will influence the decision.
- Will the coil be around acidic conditions, such as a location near saltwater? Some materials would corrode very rapidly in such an environment.
- Should steel coil be used? If so, the steel coil will not exchange heat as fast as copper or aluminum. It will also require a larger coil for the same capacity.
- Will the evaporator be used for temperatures below 32°F? If so, spacing of fins on the evaporator must accommodate defrosting.
- What is the tonnage of the system? A correct coil will have to be chosen to match the other system components.
The three design applications for evaporators—bare tube, plate type, and finned tube—can each be used in a wide variety of circumstances, such as within multiple evaporators of large systems. They are also categorized by temperature ranges: low temperature (32°F and below), medium temperature (32°F to 40°F), and high temperature (40°F to 50°F).
The bare tube is used in refrigeration storage because it is easy to clean and defrost. A bare tube evaporator is nothing more than a bent tube. This type of coil was typically found without any type of fan. It used natural convection in that cool air would fall and warm air would rise, producing the necessary air motion. This made the velocity of the air moving across the evaporator very slow, which required the evaporator to be large. Later on, when fans were installed to increase efficiency and lower costs, the coil size was reduced.
The plate type evaporator is made up of two plates—usually made of aluminum because it conducts heat well—that have been seamed together. Liquid refrigerant flows through a tube going through the plates. The plate type was designed to improve the efficiency of the bare tube design. It is likewise used in refrigeration storage and is easy to clean and defrost. This design may be used in a home refrigerator where food would actually be put on the evaporator. By doing this, it would increase the efficiency of the heat transfer rate. The plate type may use natural draft or forced air.
The finned tube is essentially a bare tube coil, usually copper, with aluminum fins pressed on it. It is the most widely used type of evaporator. These particular metals are chosen because of their heat transfer abilities and cost. The fins absorb the heat moving across them (convection) and, through conduction, transfer the heat to the copper. Adding the fins to the pipe increases the effective surface area, leading to increased heat transfer and evaporator capacity, but at the same time creating a greater air flow restriction.
The two general types of evaporators used in today’s refrigeration systems are flooded evaporators and dry, or direct expansion (DX), evaporators.
As the name implies, a flooded evaporator is always filled with liquid refrigerant. Depending on the refrigeration system, certain flooded evaporators can be fitted with a device (sometimes called a surge chamber) that prevents liquid from going back to the compressor and return liquid from going back to the evaporator. In larger systems the flooded evaporators are commonly found on centrifugal chillers.
The flooded evaporator is sometimes called a cooler and, as such, contains tubes (the number varies greatly with design and capacity). The refrigerant circulates around the tubes and is called the primary refrigerant. The secondary refrigerant is R-718, more commonly known as water. This water is sent through the tubes and then circulated to various chilled-water coils to remove heat.
Dry, or Direct Expansion, Evaporators
Although it’s important for building engineers to be familiar with flooded evaporators, often they will work with dry, or direct expansion (DX), evaporators. As its name implies, the dry, or DX, evaporator, under normal conditions, does not allow the evaporator coil to be full of liquid refrigerant. This type of evaporator is designed to take so much superheat at its outlet that no liquid goes back to the compressor. This design is very popular for home air conditioning units, rooftop units, and package units because of lower initial cost and ease of operations for units under 200 tons of capacity.
In a dry evaporator, as the refrigerant enters the metering device, it is 100 percent liquid and the subcooling is generally around 10°F. As it moves through the metering device, 20 percent of the liquid flashes into a gas, lowering the temperature of the remaining liquid. The pressure is now at 68.5 psig, which corresponds to a saturation temperature of 40°F when using R-22. As the refrigerant passes through the coil, it remains at 68.5 psig; no pressure drop is shown. At the end of the coil, the temperature is 52°F. Now there is a safe superheat of 12°F going back to the compressor.
In some smaller chiller applications, the dry, or DX, evaporator will be used to cool water using the same principle as the flooded evaporator: there is a primary refrigerant, and the water circulating through the tubes is the secondary refrigerant. The main difference between dry and flooded evaporators is that the flooded evaporator is full of liquid refrigerant. Liquid refrigerant levels are maintained by using a float device to regulate the metering device. Vapor refrigerant is typically taken from the top of the evaporator and sent to the suction side of the compressor. The dry evaporator design is the most common: the liquid refrigerant enters and flows through the evaporator and is slowly boiled off (evaporated), leaving the evaporator as a vapor.
This article is adapted from BOMI International’s Refrigeration Systems and Accessories course, part of the SMA and SMT designation programs. More information regarding this course or BOMI International’s new High-Performance Sustainable Buildings credential (BOMI-HP™) is available by calling 1-800-235-2664. Visit BOMI International’s website, www.bomi.org.