2-40 HP Portable Chillers Chapter 2: Functional Description 12 of 70
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.