Heating and Cooling Coil Options for HVAC Systems

Proper coil selection ensures efficient cooling, moisture removal, and durability across different environments.

Cooling Coil Fundamentals

Many HVAC systems are typically focused on cooling applications.  Cooling, in terms of building HVAC, is the extraction of heat to maintain a desired space temperature setpoint. To reduce the space temperature, more heat must be extracted than is being produced.

There are four primary sources of heat that affect the space:

1. Envelop load – conductive heat transfer through the walls, roof, doors due the difference between indoor and outdoor temperatures.

2. Solar load – radiant heat from the sun transmitted through windows, skylights, glass doors, etc.
3. Internal load – heat generated by people, equipment, lights, etc.

4. Outdoor air load – heat from outdoor air as the result of ventilation or infiltration.

For cooling applications, there are two types of mediums of heat exchange that are used: chilled water or refrigerant for a direct expansion (DX) system. Each system contains a cooling coil in which the warm air from the space passes through the coil by use of a supply fan. The heat is transferred from the air to the fluid in the coil. Another term for the coil is "heat exchanger" as it is a device that exchanges heat between the fluid and the a

In a chilled water system, the chilled water flows through a series of small pipes that comprise the chilled water coil. These pipes have fins increasing the surface area to promote heat transfer. A DX system uses an evaporator coil which has a distributer to spray the refrigerant into the cooling coil. Both types of heat exchangers are constructed of aluminum fins and usually utilize a stainless steel casing.  

Heating Coil Fundamentals

Commercial HVAC systems not only use coils to cool the space during the warmer seasons, they also use heating coils to heat the space in cool seasons or for reheat in the case of low air temperatures associated with dehumidification. Similar to a cooling coil, water or refrigerant can be used as the medium routed through the coil. Hot water or steam is supplied by a boiler to serve a hot water coil. In a DX system, a device called a reversing valve can be used to switch the flow of refrigerant in the system, sending hot gas to the heat exchanger in the supply air stream instead of the cold refrigerant.

Commercial HVAC Coil Design and Construction

A coil is comprised of 4 main components: tubes, fins, casing, and headers.

1. Tubes – copper pipes that run the length of the coil and circulate the medium being used for heat transfer. Tubes are oriented horizontally in the coil to increase the contact with the air passing over it. Number of rows, tube diameter, and thickness of the coil can affect the how well the medium transfers heat with the air.
2. Fins – small plates that are attached to the tubes to extend the surface area beyond that of the tube itself. Fin density is measured as fins per inch (FPI). Increasing fin density can increase the capacity of the system, but also increases the airside pressure drop across the coil. Most fin count is between the range of 8 to 16 FPI. Aluminum fins are most commonly used to deliver high quality performance but copper fins can also be selected. The fin material plays a role in the heat transfer effectiveness, corrosion resistance, and longevity of the coil. 

3. Casing – the frame housing the tubes and fins that provides structural rigidity to the coil and helps protect it from being damaged. This casing is commonly constructed as a stainless steel casing.

4. Headers (also called the manifolds) – larger pipe that connects all the tubes within the coil.  There is a supply header that introduces the heat exchange fluid to the tubes and a return header that routes the fluid away from the coil after passing through the tubes. The headers are essentially the connection between the coil and the rest of the hydronic or DX system.

Coil Circuitry

A coil can be constructed as a single circuit or multi-circuit configuration. Multi-circuit selections are great for part load capacity control. These heat exchangers contain rows, which refer to the number of horizontal tubes (or passes) that the refrigerant or chilled water flows through in the heat exchanger. A pass is the number of times the fluid moves from one end to the other. Common terminology in reference to the coil configuration are “single row”, “three row” or “six row.”

Construction of a Common Evaporator

Rows and Passes of a Heat Exchanger

Cooling Coil Considerations and Performance

The function of the cooling coil is the air is not just cooled, but moisture is also removed. This happens as the air is warmer than the fluid in the coil, so moisture from the air condenses on the outer surface and dryer air is supplied on the other side. There are many factors that affect the performance. Number of rows, fin material and density, and tube size can all be common factors to adjust when designing for an application. For example, the tube size will affect the velocity of the fluid moving through them. If they are too large, the fluid velocity will be low, which can decrease the thermal transfer effectiveness. Whereas, being too small, results in high velocity and increased fluid pressure drop. Correctly sizing a coil is extremely important to optimize the performance of a system.

Cooling Coil Applications

Choosing the Right Cooling Coil Solution

Coil selection is extremely important. For example, high humidity applications such as natatoriums and buildings located in coastal climates may require a coil with six or more rows be selected to increase the amount of moisture that can be pulled from the space. Options such as polymer e-coating can help extend the life of the coil in these applications as well. The coating is an epoxy based coating that is applied to protect the heat exchanger from corrosive elements like moisture, salt, and chemicals. Material options for the fins and casing may also need to be considered to improve the heat transfer effectiveness of the coil. Understanding the application, the type of system, and the priorities of the project, are all necessary when working with the engineer to determine the best coil for the job.

Natatorium

Coastal Cities