Theory Of Operation; Cool Mode - Thermo NESLAB HX 150 Service Manual

THEORY OF OPERATION

The chiller operates in two modes: COOL or HEAT/IDLE. The
controller compares setpoint to actual temperature and decides
which mode is required. This is a thermostatic (not proportional)
control system. The compressor, pump, and fan motor all run
continuously, regardless of mode.
The thermal transfer of the HEAT/IDLE mode is not as efficient
as the COOL mode. This results in units running at little or no
load to spend most of their time in the HEAT/IDLE mode with
brief periodic COOL periods. A duty cycle of 20% COOL / 80%
HEAT/IDLE may be typical for units running under no load. This
duty cycle will reverse as increasing load is applied, reaching
100% COOL / 0% HEAT/IDLE at full load.
This system results in tighter temperature control than continu-
ally cycling the compressor on and off, as is typical in a house-
hold refrigerator.
The default mode is HEAT/IDLE. A failure of the temperature
sensor or controller will generally result in the HEAT/IDLE
mode. This prevents a possible freezing of the cooling fluid and
resulting damage from fluid expansion.

COOL MODE

If the setpoint is below the actual temperature, the controller is
in COOL mode. The controller triac output provides a closure
to supply line voltage to the solenoid coil. The solenoid valve,
which is normally-closed, is then pulled open by the coil and a
standard refrigeration cooling loop exists.
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