The evaporator is the component that absorbs heat into the refrigeration system. It is a refrigeration coil made of aluminum or copper with aluminum fins attached to the coil of either type to give more surface area for better heat exchange. There are several designs for airflow through the evaporator coil, and for draining the condensate water from the coil, depending on the installation. The different designs are known as the A coil, the slant coil, and the H coil.
The A coil is used for upflow, downflow, and horizontal-flow applications. It consists of two coils with side-by-side circuits that are spread apart at the bottom in the shape of the letter A. In the upflow or downflow applications, the condensate pan is at the bottom of the A pattern. When used for horizontal flow, a pan is placed at the bottom of the coil and the coil is turned on its side. With the two coils in series, the airflow in the A coil is through the core of the coil; it cannot be from the side. When horizontal airflow is required, slant or H coils may be more desirable.
The slant coil is a one-piece coil mounted in the duct on an angle (usually 60 degrees), or slant, to give the coil more surface area. The slant of the coil causes the condensate water to drain to the condensate pan located at the bottom of the slant. The coil can be used for upflow, downflow, or horizontal flow when designed for these applications.
The H coil is normally used in horizontal applications, although it can be adapted to vertical applications using special drain pan configurations. The drain is normally at the bottom of the H pattern.
All of the aforementioned coils may have more than one circuit for the refrigerant. When a coil becomes too long, and excessive pressure drop occurs, it is advisable to have more than one coil in parallel. The coil may have as many circuits as necessary to do the job. However, when there is more than one circuit, a distributor must be used to distribute the correct amount of refrigerant to the individual circuits.
Efficient heat exchange between the refrigerant and air has been improved over the past couple of years by means of different coil designs. In the past, all coils have been copper or aluminum tubes with fins attached to the tubes. Coil sizes were limited to the tube sizes typically 3/8 inch and 5/16 inch.
New coil designs have been developed that resemble automobile radiators. Small passages with fins, called microchannel or flat-plate-and-fin coils, have made it possible to build a smaller coil with more heat-exchange surface that requires less refrigerant charge. Some manufacturers are using these coils for evaporators and condensers.
Remember, when smaller, more-efficient equipment can be built, it is a savings in materials, manufacturing time, storage space, and shipping costs that helps all the way from material purchasing to installation.
This article is an excerpt from Refrigeration & Air Conditioning Technology, seventh edition, published by Delmar Cengage Learning, co-authored by John Tomczyk, Bill Whitman, Bill Johnson, and Eugene Silberstein. For more information, visit http://amzn.to/13YVT9M.
Publication date: 6/10/2013