Chilled water plants are generally one of two types: variable flow or constant flow. To reduce installation and operation cost, it is best to vary the chilled water flow throughout the building. Doing this allows the chilled water temperature rise to remain constant during partial load conditions and chilled water flow to vary in proportion with the load. By varying the flow, smaller pumps and piping can be used. The main circulating pump must be variable flow. Multiple pumps can be tagged on, but variable speed AC drives are commonly used.
Chilled water supply temperatures typically range from 5 to 10 degrees C higher. This temperature increase is the key parameter for the variable flow system. Once the temperature is selected, all equipment must be designed to operate within that range. This means the chillers, air handling unit coils, and the control valves.
This system for space cooling of several rooms with chilled water circulating through air terminal device (6) and thermostat valve (5) in each room. The return water is circulated by a chiller water pump (1) through the chiller evaporator (3). The water temperature is reduced in the evaporator by about 5 degrees C and it is supplied back to the rooms. The variable water circulation is achieved by the AC drive (2) and the control parameter is the water pressure at the most distant room to keep the pressure high enough for the thermostat valves.Â The pressure is measured by the transmitter (4).
Performance in Creating comfort zone
The use of variable speed AC drives improves the controllability of the whole chilled water system, causing good possibilities to maintain the Comfort Zone.
The use of variable speed drives usually increases the first cost of investment but through sav
ings in electrical energy in motors alone the investment in VSD has pay back time between 0.2 – 3 years.
In addition to savings in electrical energy, there are several important customer benefits speaking for VSD:
- Fast control to maintain the comfort zone conditions
- Accurate control to keep the desired air quality
- Reduced pump and pipe size
- Reduced consumption of electrical energy
- Only one set of pumps is needed