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Variable Speed Control of Brine Pumps

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  rev. 2008-11-25        

Existing Conditions

A brine solution is presently circulated under the arena floor by a xxx hp centrifugal pump.  The pump is sized to deliver the required cooling during extreme heat load conditions.

The brine flows are designed to maintain adequate ice sheet cooling during flooding and during warm weather.  Most of the time, however, the flows are much higher than is necessary.  The operation of the pump also transfers energy (heat) to the brine, resulting in further inefficiencies.

The refrigeration compressors are activated in sequence to control the temperature of the brine returning from the ice sheet.  This results in a relatively large swing in the ice surface temperature and a lower than required brine temperature set point under low load conditions.  In addition, brine temperature is not effective in responding to changes in load at the ice surface.

For example, the greatest refrigeration requirement occurs during and immediately after flooding.  However, the ice acts as an insulator, so that brine temperature does not show this added load for several minutes.

Retrofit Conditions

We recommend installing a variable speed drive on the brine pump and controlling the brine flow based on ice temperature and an occupancy schedule.  This will reduce the required pump energy and the required refrigeration plant energy, allowing brine flow to match partial load conditions.

We recommend installing an infrared temperature sensor mounted on the ceiling to provide the control feedback for ice surface temperature. Because these temperature sensing methods are more reflective of the ice surface condition than brine temperature, they will allow the brine pump and refrigeration plant to react more quickly to floods, and thus reduce the swing in ice temperature.

This in turn allows the average ice surface temperature to be raised, further reducing the load on the refrigeration plant.

The refrigeration compressors should be locked out below a minimum brine pump speed, since evaporator heat transfer drops off quickly with laminar flow in the evaporator tubes.  With very low evaporator heat transfer, the refrigeration plant operation could become unstable, with surging and short cycling of the compressors.

Some experimentation will be required to determine this speed setting.  Once determined, the minimum speed setting should be entered at the VFD panel rather than in the controller.

Further Benefits




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