Energy Experts cases

From Platte River Power Authority

The following is a question from a Western customer to the Energy Experts Hotline.

Question:

One of our customers has a 265-ton chiller and wants to use an "oversized" cooling tower of 325-ton capacity. Is that going to be energy-efficient?

Answer:

A 265-ton chiller must dissipate its 265 tons of cooling capacity, plus the heat of compression--another 25 percent. The chiller in question requires a cooling tower with 332 tons of capacity, so the proposed 325-ton cooling tower may actually be slightly undersized.

Unlike many other air conditioning system components, over-sizing the cooling tower can save significant amounts of energy if done carefully. The cooling tower is the sink for the energy being removed from an air-conditioning system. The cooler the temperature of the sink, the less energy is required for the compressor to move heat from the inside of the building to the environment. Over-sizing the cooling tower provides more surface over which to dissipate heat from the chiller, resulting in lower condensing temperatures and higher coefficients of performance at the compressor. The trade-off is increased cooling tower fan and pump energy consumption.

Using a VFD on the tower motor helps minimize fan energy during part-load operation, mitigating the energy penalty due to over-sizing. Increasing the size of the cooling tower by 15 percent can drop condensing temperatures by 3 percent, reducing cooling costs by 8 percent or more under certain conditions. The key to cooling tower sizing is to balance the benefits of lower condensing temperature (from increasing the size of the tower), against the extra fan and pumping costs, to minimize the overall cooling systems over the life cycle of the system.

Need answers about energy-efficiency or renewable-energy technology in a real-life situation? Contact the Energy Experts Hotline (form) at 800-769-3756.