Cooling and Heating 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:
- Infiltration load – outdoor air temperatures that are transmitted through cracks in walls, roofs, doors, etc.
- Solar load – radiant heat from the sun transmitted through windows, skylights, glass doors, etc.
- Internal load – heat generated by people, equipment, lights, etc.
- Ventilation load – heat from outdoor air needed for ventilation.
For cooling applications, there are two types of systems that are used: a chilled water system or 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 and the heat is transferred from the air to the fluid in the coil, whether it be water for a chilled water system or refrigerant for a DX system. Another term for the coil is "heat exchanger" as it is a device that exchanges heat between the fluid temperature and the air temperature.
As a chilled water system, the chilled water flows through the heat exchanger through a series of small pipes with fins. An external chiller supplies the water to the units in the building. Chilled water units are typically indoor units. A DX system uses an evaporator coil which has a distributer to spray the refrigerant into the cooling coil. These heat exchangers are constructed of aluminum fins and usually utilize a stainless steel casing.
Many times in a commercial HVAC system, a heating coil can also be used to control the temperature of the air being delivered to the space. Similar to a chilled water coil, a hot water coil uses hot water supplied by an external boiler to flow through it. Steam can also be used through a heating coil as a heat transfer medium. The overall goal of an HVAC system is to heat or cool the air entering the heat exchanger and provide warm or cool air to the space.
Commercial HVAC Coil Design and Construction
A coil is comprised of 4 main components: tubes, fins, casing, and headers.
- Tubes – pipes run the length of the heat 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 cooling capabilities.
- 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 pressure drop across the coil. Most coils use a range of 8 to 16 FPI. Aluminum fins are most commonly used to deliver high quality performance but copper fins can also be selected as well. The fin material can play a role in the heat transfer effectiveness.
- 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.
- Headers (also called the manifolds) – larger pipe that connects all the tubes within the coil. There is a supply header that introduces the cold medium to the tubes and a return header that routes the medium away from the coil after passing through the tubes. The headers are essentially the connection between the coil and the rest of the cooling system.
A coil can be constructed as a single circuit or multiple circuit configuration. Multi circuit selections are great for part load capacity control. The configuration will also adjust the number of passes. A pass is the number of times the fluid moves from one end to the other. Commonly heard terms would are “single row”, “three row” or “six row” evaporator.
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 as well. 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 FPI, and tube size can all be common factors to adjust when designing for the 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. Sizing the coil is extremely important to provide the maximum life and performance for the system.
Cooling Coil Applications
Choosing the Right Cooling Coil Solution
Cooling coil selection is extremely important for certain applications. For example, natatoriums and coastal climates will have high humidity loads, so it would be important to ensure you had a six row or higher coil that can pull more moisture out of the space. Options such as polymer e-coating can help extend the life of the coil as well. 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, the priorities of the project, such as a focus on dehumidifying or cooling, are all things to consider when working with the engineer to develop the best system for the job.
Natatorium