| Features
and Benefits Unit Options — Free Cooling
Operation Free cooling operates on the principle that
refrigerant flows to the area of lowest temperature in
the system. The Tracer® system/Chiller Plant Manager (CPM) can be
used for automatic free cooling control. When condenser
water is available at a temperature lower than the required leaving chilled water temperature, the CPM starts the free cooling
cycle. If the load cannot be satisfied with free
cooling, the CPM or a customer supplied system can
automatically switch to the powered cooling mode. If desired, the chiller can be
manually switched to the free cooling mode at the unit
control panel.
Upon changeover to free cooling, the
shutoff valves in the liquid and gas lines are opened and a lockout circuit prevents
compressor energization. Liquid refrigerant drains by
gravity from the storage tank into the evaporator, flooding the tube bundle. Since the refrigerant temperature and pressure
will be higher in the evaporator than in the condenser,
due to the water temperature difference, the refrigerant gas boiled off in the evaporator will flow to the condenser. The gas
then condenses and flows by gravity back to the
evaporator. This automatic refrigeration cycle is
sustained as long as a temperature difference exists between the condenser water
and evaporator water.
The difference in temperature between
the condenser and evaporator determines the rate of refrigerant flow between
the two shells and hence the free cooling capacity.
If the system load becomes greater
than the free cooling capacity either the operator manually stops free cooling, a binary input from a customer-supplied system
disables free cooling or the CPM can automatically
perform this function. The gas and liquid valves
close and the compressor starts. Refrigerant gas is drawn out of the evaporator
by the compressor, compressed and introduced into the
condenser. Most of the condensed liquid first takes the path of least resistance
by flowing into the storage tank which is vented to the
high pressure economizer sump by a small bleed line. When the storage tank is filled, liquid refrigerant must flow through
the bleed line restriction. The pressure drop through
the bleed line is greater than that associated with the
orifice flow control device, hence liquid refrigerant flows normally from the condenser through the orifice system and into
the economizer.
The free cooling accessory consists
of the following factory-installed or supplied components:
1 A refrigerant gas line, including an electrically actuated shutoff valve, installed
between the evaporator and condenser.
2 A valved liquid return line including an electrically activated shutoff valve, between
the condenser sump and evaporator.
3 A liquid refrigerant storage vessel.
4 Added refrigerant charge.
5 Manual free cooling controls on the unit control panel. For specific information
on free cooling applications, contact the local Trane
sales office.
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