In the HVAC and Refrigeration field, cooling and heating coils are used to exchange heat to/from air to a heat exchange fluid. A heat exchange fluid is flown through the coil and as air is passed over the coil, the air is either heated or cooled. Coils consist of a metal box framing, which holds a series of copper tubes in staggered rows and columns.
The amount of heat that is transferred is related to the amount of surface area that the air is in contact with. In order to increase surface area, the size of the tubes may be decreased and more tubes can be provided, the number of rows increased or the amount of fins per inch are increased. Aluminum or copper fins are provided on each tube to increase the amount of surface area. Coils are rated by the height of the fins and the number of fins per inch.
Cooling and Heating Coil Fluids There are several different types of heat exchange fluids used in cooling/heating coils.
Refrigerant: Hot refrigerant gas or cool refrigerant liquid can be used in a coil to provide either heating or cooling. In a heating-coil, cool air is passed over a coil containing hot gas. Heat is exchanged to the cool air, which warms the air. The heat lost by the refrigerant gas causes it to condense to a liquid. In a cooling-coil, warm air is passed over a coil containing cool refrigerant liquid. Heat is exchanged to the cool refrigerant liquid, causing it to evaporate. The warm air loses heat, thereby decreasing the air temperature.
Water: Chilled water or hot water can be used in a coil to provide either heating or cooling. The air temperature is either raised or lowered as heat is transferred to raise or lower the temperature of the chilled or hot water.
Steam: Steam can be provided to a coil to provide heating. Steam enters the coil and as the air passes over the coil its air temperature increases. As the steam loses heat, it condenses to its liquid form.
Cooling and Heating Coil Terms
It is important to be able to understand the following terms, (1a) Apparatus Dew Point or (1b) Effective Surface Temperature and the (2) Bypass Factor
Apparatus Dew Point or Effective Surface Temperature is the temperature at which all air would be cooled to if the cooling coil was 100% effective. The ADP must be located on the saturation curve, refer to the psychrometric chart below. The ADP, leaving coil conditions and the entering coil conditions are located on the same line.
How close the leaving coil condition is to the apparatus dew point is a function of the bypass factor. The bypass factor describes the percentage of air that is not cooled to the ADP. The air that is bypassed remains unchanged from the entering coil conditions. The bypass factor is a function of the airflow, number of rows, surface temperature, number of fins per inch, height of fins and many other construction attributes of coils. The origin of the bypass factor is not important, but the use of the bypass factor in calculations is important. The bypass factor can be found through the use of (a) enthalpy, (b) dry bulb temperature or (c) humidity ratio.