There are numerous applications for two levels of controls. One example is a large room served by two air handlers. Instead of having both controlled by a single thermostat (which may resort to short- cycling and excess energy use) or controlled by separate thermostats, a single controller can energize one unit, then both, as the space load demands. Many manufacturers have basic nonprogrammable thermostats (and programmable, if the staff are comfortable with them) with this function built in, for control of two-stage compressors.
Another great application for this simple device is to control a two- speed motor of an air handler. The controlling function can be static pressure in the discharge duct to a variable air volume (VAV) system. This is an inexpensive option to inlet vanes or a variable- speed drive and is a good compromise for system retrofits when a VFD is too costly. If the new two-speed motor is a high-efficiency model, there may be nearly as much savings as from installation of a VFD anyway, depending on the number of exterior and interior zones served by the air handler. If an air handler serves only a few zones, then the two-speed motor can be interlocked with a space-temperature sensor; that is, if the air distribution is not overly affected at the new airflow rate for a constant- volume system. An ideal application of this method is a system serving two operating rooms in a hospital or two classrooms in an academic building. If only one of the spaces is in use, the air ducted to the other can be dampered off, the motor put on low speed, and the system made to operate at half capacity to adequately condition just the one room.
Another simple application of the two-stage controller is cooling-tower fans, according to the return-water temperature to the condenser. A single two-speed fan accomplishes the same for a one-cell tower. Either way, the fan(s) operate only enough to provide the necessary cooling. They use less energy and, by providing a more constant temperature of condenser water to the chiller, improve the chiller’s operating efficiency, in addition to conserving fan horsepower.
There are many other applications of this simple method, which can be configured to a variety of setpoints and can be used for temperature, pressure, or humidity control. They’re ideal for smaller installations with older equipment that’s not ready for replacement with new models that have some of these controls already built in. A carefully designed control harness can increase efficiency, even approaching the efficiency of the latest technology, and at a reasonable cost.
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Excerpt from Energy Conservation Projects.