SYS-APM001-EN
System Configurations
30
Temperature control can be executed in several ways. Figure 10 shows a
strategy where the controller on each chiller is set at the system design set
point. Either chiller can be used to meet the system demand for up to
50 percent of the system load. At system loads greater than 50 percent, the
upstream chiller is preferentially loaded because it will attempt to produce the
design leaving chilled-water temperature. Any portion of the load that remains
is directed to the downstream chiller.
If chiller set points are staggered (upstream at 49°F [9.4°C] and downstream at
44°F [6.7°C]), the downstream chiller is loaded first. The upstream machine then
meets any portion of the system load that the downstream chiller cannot meet.
This control strategy offers several benefits. The first is that the upstream chiller
is always operating at an elevated temperature. This allows it to operate at a
higher efficiency. Also, placing an absorption chiller in the upstream position
increases its capacity. As an example, an absorption chiller that can produce
500 tons [1,760-kW] at a leaving chilled-water temperature of 44°F [6.7°C] may
produce 600 tons [2,110-kW] at 50°F [10°C]. Centrifugal, helical-rotary,
reciprocating, and scroll chillers experience capacity and efficiency changes to
a lesser degree. By judicious use of the series configuration, these benefits can
provide reduced installed cost and fuel flexibility to the building owner.
Equal loading of the two chillers may be accomplished by using a chiller plant
management system to reset the upstream chiller's set point in response to
changes in system load.
Summary :
If chiller set points are staggered (upstream at 49°F [9.4°C] and downstream at 44°F [6.7°C]), the downstream chiller is loaded first. Equal loading of the two chillers may be accomplished by using a chiller plant management system to reset the upstream chiller's set point in response to changes in system load.
Tags :
system,upstream,downstream,temperature,set,load,chillers,produce,44f,absorption,67c,tons,portion