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View Subscription Details: Letter ( 04/30/2014 ) - Plain Text Format Good and bad investments in energy efficiency (3)
Condenser fan VFDs

At low outside air wet bulb temperature, capacity of an evaporative condenser will increase and it will become oversized. In this case, the heat rejection capacity of the evaporative condenser needs to be modified to maintain the head pressure at optimum level. Optimum condensing temperature (pressure) can be determine as wet bulb temperature of outside air plus optimum wet bulb approach. It is very important to know this optimum approach, which can vary from 8 degF to 20 degF. Condenser efficiency (energy use per unit of heat rejection) and level of outside air wet bulb temperature are two major factors that determine how great optimum wet bulb approach should be.
Control of airflow through the condenser is the preferred method to change capacity of evaporative condensers. Typically, on/off fan control and condenser fan motor VFDs are used to modulate air flow. Variable frequency drive (VFD) will continuously modulate fan speed to maintain a stable and efficient system operation. VFD is more efficient than fan cycling because of an affinity law. According to this law condenser fan energy use varies with cube of the fan speed. Assume that 50% of condenser capacity is required. Single speed condenser fan will cycle on/of and operating time of this fan will be around 50%. It means that 20HP fan on average will use 10HP of energy. The same fan with VFD will be operated at 50% speed and its energy use will be 15% of nominal energy or 3HP. At 50% of condenser capacity, energy saved by condenser fan VFD will be 10HP -3HP = 7HP.
To estimate potential annual energy savings of condenser fan VFD, someone should determine average condenser capacity and average fan speed during periods of cool weather. Energy use of a fan with VFD should be compared to energy use by fan cycling and multiplied by operating hours. In our example assume that average fan speed will be 50% and operating hours will be 5000. Total annual energy savings will be 7HP x 0.75 x 5000 = 26,250 kWh.
Additionally, condenser fan VFDs can be very useful to provide wet operation of the evaporative condensers at subzero temperatures. Some condensers with single speed fans can not operate at wet mode during cold weather because fan blades can be frozen. Continuous operation of the condenser fans with VFDs can solve this issue. Some people claim that dry operation of the evaporative condensers is more efficient than wet operation if outside temperature is below 0 degC. I do not agree with this statement. You may save $1 on water and chemicals but you will spend additional $3 - 4 on energy costs.
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