System Controls

Variable condenser water flow

Chiller-tower-pump balance

There are times when a system designer may choose to vary the condenser water flow in addition to, or instead of, the cooling-tower fan speed. This may be beneficial in systems with high pumping power. If a variable-speed drive is installed, the flow may be reduced and the pump power can be reduced substantially—approximately with the cube of the speed. Attempting to vary both the pump and the tower fan speeds is complex and requires adequate time for design and implementation.

Keep the flow through the condenser above the minimum allowable flow rate for the chiller’s condenser. The operator should regularly log the condenser approach temperature (the temperature difference between the condenser’s refrigerant temperature and the condenser-water leaving temperature) to ensure that the tubes are not becoming fouled. The approach temperature may be monitored using a chiller plant management system.

Tower and/or tower nozzle design can affect the allowable condenser-water flow. If the flow drops below the manufacturer’s specified limit, the water is no longer evenly distributed over the tower fill. This results in a decrease in cooling-tower heat-transfer effectiveness. In extreme cases, it can also result in water freezing in the cooling tower. If variable tower flow is a consideration, contact the cooling-tower manufacturer to determine the flow limit and possibly choose nozzles or cooling-tower configurations that can handle variable-water flow.

Most water-cooled, chilled-water systems use a constant condenser water flow rate. However, the condenser water flow rate can be varied between the minimum and maximum flows allowed for the specific chiller (refer to product catalog or selection program).

But reducing the condenser water flow rate affects the power consumption of the pumps, chiller, and cooling tower, as described below:

•Condenser water pump: Pump power is reduced because both the flow rate and the pressure drop through the piping and condenser are reduced.

•Chiller: Compressor power is increased because, as the flow rate decreases, the temperature of the water leaving the condenser increases. At a given load, this increases the compressor lift and, therefore, its energy use.

•Cooling tower: As explained above, the temperature of the water returning to the cooling tower is warmer. This increases the effectiveness of the heat exchanger. But the water flow rate is decreased, which can either improve or reduce the effectiveness of the cooling tower. So, for a given load, reducing the flow rate through the cooling tower sometimes decreases and sometimes increases energy use.

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Chiller System Design and Control

SYS-APM001-EN

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Trane SYS-APM001-EN manual System Controls Variable condenser water flow, Chiller-tower-pump balance
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SYS-APM001-EN specifications

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