Direct expansion, chilled water, and hot water coils should always be piped for counterflow. (Fluid should enter the coil at the leaving-air side.) Steam coils must have the condensate connection at bottom of coil.
To determine intervals for cleaning coils in contaminated air oper- ations, pressure taps should be installed across the coils and checked periodically. Abnormal air pressure drop will indicate a need for cleaning the coils.
Annual maintenance should include:
There are additional precautions and control strategies, as found in various catalogs and in the ASHRAE Fundamentals Handbook and in the Carrier System Design Guide — Piping Section, when the entering-air temperature to the coil falls below 35°F. These conditions occur when IDT coils are used for pre-heat and/or face and bypass applications.
In spite of the precautions listed above, a coil may still freeze up. An oversize capacity coil, at partial load, with a modulating steam control valve will occasionally freeze. Freezing occurs in the 20°F to 35°F range of entering-air temperatures. A better installation would be an undersize coil, with an on/off control valve with ther- mostatic control in the outside air, set at 35°F air temperature, in- stalled downstream of the first coil; or setting the minimum steam pressure at 5 psig.
Source: MaintainX (Community Member)