Trane ctv-prc007-en Unit, Options, Free Cooling, Operation, A liquid refrigerant storage vessel

Unit

Options Free Cooling

Temperature and humidity control requirements are important considerations when evaluating the use of CenTraVac free cooling. Low temperature outside air (from the outside air economizer) often requires a large amount of energy for humidification purposes. Free cooling operation helps to reduce these humidification costs on many applications.

It is important to note that those applications which require extremely precise humidity control typically cannot tolerate warmer than design chilled water temperatures. Therefore, since free cooling chillers normally deliver warmer than design chilled water temperatures, free cooling operation is usually not applicable with systems which require precise humidity control.

Also, free cooling is generally not used in conjunction with heat recovery systems, since mechanical cooling must be used to recover heat that will be used elsewhere in the building for simultaneous heating.

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:

•A refrigerant gas line, including an electrically actuated shutoff valve, installed between the evaporator and condenser.

•A valved liquid return line including an electrically activated shutoff valve, between the condenser sump and evaporator.

•A liquid refrigerant storage vessel.

•Added refrigerant charge.

•Manual free cooling controls on the unit control panel.

For specific information on free cooling applications, contact the local Trane sales office.

Figure O-5 — Compressor Operation Schematic

Figure O-6 — Free Cooling Operation Schematic

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