Components Used in Air Conditioning (Cooling Cycle).

TM 55-1905-223-24-17

(7)

Air Flow Switch. This is a device designed to prevent the heater from operating when there is no

air in the duct. The pressure differential air flow switch measures the total pressure difference

between the inside of the duct and the outside. The total pressure is the summation of the static

pressure and velocity pressure. I n critical low pressure situations, it is important to turn the

sensing tube into the air stream so that the velocity pressure is also measured. Since the

manufacturer may not know the direction of air flow, this is a field adjustment. The air flow

switch is connected in the control circuit in series with the control thermostat and automatic reset

thermostat controlling the operating contactor.

(8)

Overcurrent Protective Device. Overcurrent protective devices are required for any circuit that is

over 48 amperes. This protection is in the form of fuses and resettable circuit breakers. The

overcurrent protection devices may be built into the terminal box of the heater or in a remote

control panel.

(a)

Safety Disconnect Switch. This is a switch that interlocks with the box or panel door. The

switch insures that the terminal box is not "live" electrically when the door is opened.

Components Used in Air Conditioning (Cooling Cycle).

(1)

Thermostats. Refer to a.(5) and (6) of this paragraph.

(2)

Contactors. Refer to a.(4) of this paragraph.

(3)

Time Delay Device.

(a)

A time guard circuit prevents compressor short cycle by providing a 5 minute delay before

restarting the compressor after shutdown for any reason. On startup, the timer causes a

15-second delay before the compressor can start.

(b)

On compressor shutdown, the timer runs for 4 minutes 45 seconds. During this time the

compressor cannot start.

(4)

High and Low Pressure Controls. The high pressure control switch is a safety device which stops

the compressor in the event of excessive pressure in the high (discharge) side of the system.

The low pressure control switch is a safety device which stops the compressor in the event of low

pressure in the low (suction) side of the system. Either switch prevents the compressor from

restarting until the pressure returns to its cut-in setting (no low pressure control on self-contained

units).

(5)

Filter Dryer. The filter dryer is installed in the liquid line, and is filled with activated alumina or

silica gel which has the ability to attract and hold moisture. It is also a filter preventing

contaminants from entering the expansion valve, or capillaries, and the rest of the system.

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