CondenSAVE

CondenSave is a control solution that minimizes energy use in water-cooled chiller systems. Typically, these systems include a chiller, cooling tower fan, evaporator pump and condenser circulation pump. In most instances, these items are operating with constant speed motors which consume their full-load electrical power all year round.

Why look at Pumps and Fans?

While these motors at full-load are not typically as large as the motor which drives the compressor in the chiller, they prove to be sources of significant energy savings. Because of “affinity laws” which dictate an exponential relation between pump / fan performance and energy use, finding savings is easy. Reducing the speed of the pump impeller by just 25% results in a power consumption reduction of 42%. Trying to save this magnitude of energy on the chillers’ compressor would be nearly impossible.

Optimizing cooling tower fans

A simplistic control for the cooling tower fan involves adjusting the fan speed to maintain its condenser supply to a constant temperature. While this may save energy, setting this temperature too low will result in the fan operating at its full speed most of the time. This is not necessarily the most efficient scenario. CondenSave operates the fan at its lowest possible speed to maintain the theoretically lowest condenser temperature possible based on the cooling tower performance and outdoor humidity conditions.

Optimizing condenser pumps

Most condenser pumps are sized based on theoretical information and operate at a constant speed for their entire life. The addition of a VFD to these pumps can help save energy where these pumps are oversized and delivering more flow than required. Implementing CondenSave in addition to the VFD minimizes energy consumption of these pumps by reducing their speed when the chiller is at part-load.

Won’t the chiller use more energy?

Lowering the flow in the condenser loop can lead to higher condenser water temperatures and therefore, higher head pressure on the chiller. This can increase the power consumption of the chiller. CondenSave resolves this issue by monitoring the cooling tower fan, condenser pump motor, and chiller compressor simultaneously to ensure the lowest power consumption scenario is found.

Operating the cooling tower and condenser pump in such a way to minimize the condenser water temperature is not the most energy efficient scenario. Let's look at an example:

A 95 ton chiller with a VFD screw compressor, 10 HP condenser pump operating at 285 GPM and 90’ of head pressure. If the chiller operates at 50% load, we have 2 options; continue to operate the pump at 100% flow to reduce the head pressure on the chiller as much as possible, or, operate the pump at 50% flow to maintain 85 / 95 F in the condenser loop.

Even though the chiller will consume more power due to the higher condenser temperature, the savings in pump power is more significant. In this example, 2.5 kW of power are saved!

Conclusion

In the example, the ratio of pump power to chiller power was about 1:10. With this ratio, it made more sense to reduce the flow rate of the pump in order to achieve the optimized solution. On the other hand, if the condenser loop is short and the pump is very small, then operating the pump at a higher speed might be the solution.

The most effective operation of the system involves evaluating the power consumption of the compressor, condenser pump, and cooling tower fan simultaneously. This is exactly what CondenSAVE achieves.