Trane TRG-TRC004-EN manual Preventing Evaporator Freeze-Up

period three

The Compressor in a System

notes	In a VAV chilled water system (shown in Figure 46), the capacity of the chilled-
	water coil is controlled to maintain the desired supply air temperature. By
	sensing the supply air temperature, a controller varies the flow of water
	through the coil by modulating the valve. Varying the water flow maintains the
	temperature of the air as the flow rate of the air changes to match the space
	load.
	In a constant-volume chilled-water system, the capacity of the chilled-water coil
	is controlled by directly sensing space temperature and varying the flow of
	water through the coil by modulating the valve. Varying the water flow changes
	the temperature of the air leaving the coil to match the space load.
	In either case, the capacity of the compressor is generally controlled by sensing
	the temperature of the water leaving the evaporator and comparing it to the set
	point.

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Preventing Evaporator Freeze-Up

In addition to unloading the compressor in order to match the ever-changing system load, a second system-related concern involves maintaining the suction temperature above the conditions where evaporator freeze-up may occur. This can be illustrated by returning to an earlier example. Assume that, in response to a decreasing load, the capacity of the 40-ton [105.5-kW] scroll-compressor unit is progressively reduced to a minimum of 8 tons [28.1 kW], corresponding to a suction temperature of 28°F [-2.2°C] (H). If the load on the evaporator decreases no further, the suction temperature is maintained within safe operating limits. However, if the system must be operated at loads below this minimum stage of unloading, the suction temperature may fall to the point (I) where evaporator freeze-up can occur.

In a direct-expansion (DX) application, where the refrigerant in the evaporator is cooling air, a suction temperature of approximately 28°F [-2.2°C] can cause the moisture that condenses out of the air to form frost on the surface of the evaporator coil. In a chilled-water application, where the refrigerant in the evaporator is cooling water, a suction temperature of approximately 30°F [-1.1°C] can cause the water to freeze inside the evaporator. (This minimum

TRG-TRC004-EN 33