Air Handling Unit Coil Heat Transfer Capacity and Design Considerations

Coil is the area in an Air Handling Unit (AHU) where heat transfer takes place. Construction and design of a coil determine heat transfer capacity. Mechanically bonded aluminum fins are used on copper tubes. These fins provide extended surface for transfer of heat from one fluid to other. Water, refrigerant or glycol solution circulates inside copper tubes. These circulating fluids cool or heat air which comes in contact with aluminum fins.
During cooling operations, dehumidification takes place due to temperature of fluid in tubes being lower than dew point of air. This condensate is collected under the coil and is drained to nearest drain point. In order to avoid foul sewer smell to mix with air conditioning air water seal is used before connecting drain lines. Drain pan and drain line is insulated to avoid condensation and freezing. There is always a chance of corrosion on coil surface and in drain pan. Protective coatings are used for this purpose to get maximum performance form the coil throughout its life cycle. Stainless steel drain pans provide maximum protections against corrosion.
Number of fins per inch can be increased to get more capacity. With increasing fin density, cleaning of the coil becomes difficult as spacing gets narrower. Moreover, with high fin density, air side pressure drop increases. It means more fan power to keep required air flow through the system. System consumes more energy.
Balance is kept when selecting coil fin spacing and height, to keep overall system performance at best operating point. Velocity of air through coil is kept at 400-500 fpm. Above this level, condensate moves with air from coil to ducting system. In extreme cases, this may travel through duct joints and damage insulations and ceilings. To keep air velocities within these limits, dimension of the coil is increased. It requires more room space. When space is limited, water eliminators are used. Air after passing through coil, crosses a bank of these eliminators. As air changes its direction, water droplets come in contact with the metal surface. These are removed from air and collected in drain pan for removal.
Fan can be installed before or after the coil. Fan downstream of the coil has more chances of carryover at high velocity, but has balanced air flow across the face of the coil. In this case sensible load of fan motor is added after the coil operation. Fan upstream of coil may cause unequal flow pattern across coil face. Equalizing plate in between fan and coil is used to distribute air equally throughout its dimension to get maximum heat transfer efficiency.