Air Handlers are manufacturers of Heating, Ventilating and Air Conditioning Equipment for the building services industry

Air Handlers Celebrating 25+ Years of Success 1989 - 2015

Operating and Maintenance Manual

Heating and Cooling Coils


Coils are normally designed as either cartridge arrangement where they are fitted inside the casework of the air handling unit, or bolted directly between two sections of the AHU. All LPHW and chilled water coils are fitted with an air vent and drain plug on the header connections.

Drain Connections
All cooling coils include a condensate drain tray fitted with a drain connection to be connected to an appropriate trapping system. Care must be taken to ensure that the correct type of drain trap is used depending on positive / negative air pressure within the coil section. Drain lines from the trap must be pitched downwards, a slope of 1 : 25 is recommended.

Installation & Maintenance of Coils
During installation care must be taken to prevent damage being caused to fins, tubes, headers & return bends of the coil. Damage to fins can be corrected by use of a fin comb (available from Air Handlers) all coils should be installed level and upright, unless otherwise specified.
Care must be taken to ensure the following conditions are satisfied :-

- Water flow & return connections are correctly connected.
- All connecting Pipework is independently supported with adequate mountings.
- Any pipe movement caused by expansion or contraction must be absorbed by    flexible joints.
- Coils located at high points of the system should be regularly vented, other wise    coils may become air locked causing a reduction in duty.
- When connecting screwed fittings, it is necessary to restrain the back nut to avoid    damage to the coil.

Coils should be inspected every three(3) months to check for build up of foreign matter between the fins, and that coil and connections are free from leaks. Should any foreign matter be found cleaning should be carried out with a high pressure air line directed at the air off face of the coil. Alternatively the coil can be washed down with a mild solution of detergent and water, after which the coil should be thoroughly rinsed with clean water. Eliminators should be cleaned down using a soft brush and hot water, any sediment cleared out of drain tray and drain pipe, and if necessary repair any areas of corrosion.

Condensate Removal From Steam Coils
When air, water, or another product is heated, a modulating steam pressure control valve can regulate the temperature or heat transfer rate. Since pressure and temperature do not vary at the same rate as load, the steam trap capacity, which is determined by the pressure differential between the trap inlet and outlet, may be adequate at full load, but not at some lesser load.

Analysis shows that steam pressure must be reduced dramatically to achieve a slight lowering of temperature. In most applications this can result is sub-atmospheric pressure in the coil, while as much as 75% of full condensate load has to be handled by the steam trap. This is especially important for coils exposed to outside air, since sub-atmospheric conditions can occur in the coil at outside temperatures below 32oF (0oC) and the coil will freeze if the condensate is not removed. There are detailed methods for determining condensate load under various operating conditions. However , on most cases, this load does not need to be calculated if the coils are piped as shown in the figure below and this procedure is followed.

The steam trap should be 1 to 3ft (0.3 to 0.9m) below the bottom of the steam coil to provide a hydraulic head of approximately 0.5 to 1.5 psig (3.5 to 10.3kpa above atm.). Location of the trap at less than 12in (300mm) minimum usually results in improper drainage and operating difficulties.

q Vacuum breakers must be installed between the coil and the trap inlet to ensure that the hydraulic head drain the coil when it is at atmospheric or sub-atmospheric pressure. The vacuum breaker should respond to a differential pressure of no greater than 3in. of water (750pa). For atmospheric returns, the vacuum breaker should be opened to the atmosphere, and the return system must be designed to ensure no pressurization of the return line. In vacuum return systems, the vacuum breaker should be piped to the return line.

Discharge from the trap must flow by gravity, without any lifts in the piping, to the return system, which must be vented properly to the atmosphere to eliminate any back pressure that could prevent the trap from draining the coil. Where the return main is overhead, the trap discharge should flow by gravity to a vented receiver, from which it is then pumped to the overhead return.

Traps must be designed to operate at maximum pressure at the control valve inlet and sized to handle the full condensate load at a pressure differential equal to the hydraulic head between the trap and coil. Since the actual condensate load can vary from the theoretical design load because of the safety factors used in coil selection and the fact that condensate does not always form at a uniform steady rate, steam traps should be sized according to the following:-

0-15pisg (0-100kpa above atm.) at the control valve inlet: size the trap for twice the full condensate load (coil condensing rate at maximum design conditions) at 0.5psi (3kpa) pressure differential.

16-30psig (110 to 200kpa above atm.) at the control valve inlet: size the trap for twice the full condensate load at 2psi (14kpa) pressure.

31psig (210kpa above atm.) and over at the control valve inlet: size the trap to handle triple the full condensate load at a pressure differential equal to half the control valve inlet pressure.

The steam supply must be drained of entrained condensate prior to entering the coil, preferably, just before the regulating valve.