Thermo King – Modulation Function

The modulation system functions to control the box temperature of fresh perishables without excess heating or cooling. It improves the humidity and temperature conditions desirable for fresh products.

A modulation valve, or ETV, and a hot gas bypass valve work together in the refrigeration system to provide precise control of box temperature. Modulation begins when the box temperature is near set point. Full modulation and opening of the hot gas bypass solenoid occur when box temperature comes to within one degree of setpoint.

Modulation functions under the following operating conditions:
1. The unit must be operating in continuous run.
2. The set point must be set above heat lockout 24 F (15 C).
3. The box temperature must be within approximately 10 F (6 C) of setpoint.

Modulation Valve – This valve is located in the suction line between the evaporator and compressor. It closes to its maximum at a box temperature within one degree of setpoint. During modulation the flow of suction gas to the compressor is restricted. This results in a lower cooling effect and is seen as a lowering of the suction pressure at the compressor.

Electronic Throttling Valve (ETV) – This valve is located in the suction line between the evaporator and compressor. It performs the function of the modulation valve and replaces it. The ETV also acts as the throttling valve and replaces it.

Hot Gas Bypass Valve – When box temperature is within about one degree of setpoint, the hot gas bypass valve may be energized (opened). Its function is to minimize the cooling effect of the evaporator’s outlet air while allowing the ETV or modulation valve to remain partially open.

Modulation systems reduce suction pressure returning to the compressor during the modulation mode. The modulation valve is normally in the open position and may be disabled for refrigeration system troubleshooting.

The ETV does not automatically open when de-energized. Refer to appropriate Maintenance and Diagnostic manuals for proper service procedures.

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