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40 TRG-TRC010-EN
notes
period five
Application Considerations
1) The water flow rate through the chiller evaporator must stay between the
minimum and maximum flow rates for the evaporator bundle, as specified by
the chiller manufacturer. These limits depend on the specific design variables of
the actual evaporator bundle such as the number of tubes, number of passes,
and geometry. Implementation of a method for sensing evaporator water flow
through each chiller is the only way to make sure that the water flow rate stays
within these limits.
2) The rate of change for the evaporator water flow rate must not exceed a
specified level, depending on the level of protection desired. For example, the
maximum rate of change to maintain the chilled water set point is more
stringent than the maximum rate of change to keep the chiller on line. There are
3 common levels of protection desired: maintaining chilled water set point
control, keeping the chiller on line, and protecting the chiller from damage.
The limits for these different levels of protection should be obtained from the
chiller manufacturer.
Short Evaporator-Water Loops
Proper chilled water temperature control requires that the temperature of the
chilled water returning to the evaporator not change any faster than the chiller
controls can respond. The volume of water in the evaporator loop acts as a
buffer, ensuring that the return water temperature changes slowly and,
therefore, providing stable temperature control. If there is not a sufficient
volume of water in the loop to provide an adequate buffer, temperature control
can be lost, resulting in erratic system operation.
The chiller manufacturer should be consulted for volume requirements of the
evaporator-water loop.
Short Evaporator-Water Loops
load
load
chilled
chilled
water
water
pump
pump
evaporator
evaporator
Figure 56
trgtrc010_book.book Page 40 Wednesday, September 1, 1999 10:42 AM