Many facilities are too small to have their own chiller. Instead, they have a number of packaged DDX units. These have unique energy-saving opportunities, either in retrofit projects or the modification to the existing equipment.
The ideal system is designed for several stages of cooling, either by part-loading a compressor or by part-loading and sequencing several compressors. It’s essential to have an accurate thermostat to take advantage of these features, so the unit generates the exact cooling required to match the load.
Larger areas with several air handlers have additional opportunities for energy savings. As an example, consider a large gymnasium with ceiling fans, recently retrofitted with two high-tonnage air conditioning systems. The optimum arrangement uses a single two-stage thermostat to control the fans and the air conditioners, in the following responses to increasing space temperature:
• Operate the ceiling fans at all times, interlocked with the lights, to provide mixing of air throughout the space. The fans of both air handlers are on whenever the ceiling fans operate.
• Operate one of the air handlers in the cooling mode.
• Operate both air handlers as the load peaks.
• For areas without ceiling fans, the fans of both air handlers can be run with the compressors staged as above.
Many designers would provide a thermostat for each air handler serving the gymnasium. However, this can cause control problems. If one unit over-cools then, the other may start heating, and vice versa in the heating season. Also, at low-load conditions, the units may cycle on and off frequently (short cycling), which uses extra energy and causes unnecessary wear and tear on the motors and compressors. It’s best to control both units from a centrally located two-stage thermostat, with a remote averaging thermostat at the return-air duct, to adequately monitor a space that does not have ceiling fans to mix the room air thoroughly.
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Excerpt from Energy Conservation Projects.