2-4 Safety Devices and Interlocks

High/Low Thermostat Control

The high/low thermostat is a cutout device that protects the chiller system in two ways. Stage one of the thermostat shuts down the compressor if the chilled process water/glycol temperature approaches the particular mixture’s freezing point. The chilled process water/glycol pump will continue to run.

The low thermostat stage one cutout temperature is factory-set at 42ºF (5.5ºC). This is correct for a supply water temperature of 50ºF (10ºC), the rated capacity operating temperature. If you want lower chilled water temperatures, mix process water with glycol to provide protection down to 20ºF (11ºC) below the operating “To Process” temperature you want. You can then reset the high/low thermostat cutout temperature to a temperature 8ºF (4.4ºC) below the operating temperature you want.

Stage two of the high thermostat shuts down the pumps, therefore shutting off the chiller, if the chilled process water/glycol temperature rises above the selected cutout point. This prevents possible damage to the complete process piping system due to overheating of the components by the process water/glycol. The thermostat stage two cutout temperature is factory set at 100ºF (38ºC). See Section 4-6 on page 36 for information about changing the thermostat settings.

Caution! Protect the system from freezing with glycol 20ºF below the leaving water temperature set point. Condensation may form inside the pump tank and dilute the mixture, therefore the freezing point should be verified periodically. See Figure 6 on page 26 for the correct mixture.

Caution! Set low thermostat cutout temperature 8ºF below the set point leaving water temperature.

Crankcase Heater

5 hp through 40 hp portable chillers have a crankcase heater. It is wired through the control transformer that operates continuously whenever power is applied to the chiller.

Caution! Energize the crankcase heater for at least 24 hours before initial startup to drive dissolved refrigerant from the compressor oil. Failure to do so will damage the compressor.

High Pressure Cutout

This electro-mechanical cutout device opens the compressor control circuit if the refrigeration system compressor discharge pressure exceeds 360 psi for an air-cooled chiller and 290 psi for a water-cooled chiller.

Note: The high-pressure cutout is a manual reset device typically mounted on the compressor discharge line inside the mechanical cabinet. Call a refrigeration service technician to analyze the problem and reset the control.

2-40 HP Portable Chillers

Chapter 2: Functional Description

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Sterling A0554831 manual Safety Devices and Interlocks, High/Low Thermostat Control, Crankcase Heater, High Pressure Cutout
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A0554831 specifications

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