Application Principle
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 stagged on, but variable
speed AC drives are commonly used. |
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Chilled water system with variable speed
drives |
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.
Detailed Description
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.
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 savings in electrical energy in motors
alone the investment in VSD has pay back time between 0.2 - 3 years. |
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A chiller unit on the roof of an industry
building (Guangzhou, China) |
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
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