Trane TRG-TRC004-EN manual 6HQVLQJ6XFWLRQ7HPSHUDWXUH

period three

The Compressor in a System

notes	suction temperature for a specific application depends on the system operating
	conditions and the evaporator design.)
	Evaporator freeze protection in a chilled-water application is accomplished by
	sensing the temperature of the water in the evaporator. If the water approaches
	32°F [0°C], the compressor is shut off to protect the evaporator from freezing.
	Most chilled water-equipment includes this protection as part of the controls for
	the equipment.
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	In a direct-expansion (DX) application, where the refrigerant in the evaporator is
	cooling air, frost protection can be accomplished in a number of ways. As
	mentioned, if the surface temperature of the coil gets too cold, the moisture
	that condenses out of the air can form frost on the surface of the coil. This “coil
	frosting” is detrimental to system performance and compressor reliability.

Historically, in DX air-conditioning applications, hot gas bypass, coil pressure regulators, and defrost cycles initiated by a timer, pressure sensor, or temperature sensor are a few of the methods that have been used to prevent evaporator frosting. This clinic will focus on two of these—a defrost cycle initiated by a temperature sensor and hot gas bypass.

A temperature sensor on the suction line leaving the evaporator is used to determine if the coil reaches a frosting condition. Compressors are turned off and the supply fan continues to run to de-ice the coil. Timers prevent the compressors from rapid cycling.

This control scheme (referred to by Trane as FROSTAT™) is especially well suited for equipment using scroll compressors, which are designed to start and stop much more often than large reciprocating compressors.

34 TRG-TRC004-EN