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62 TRG-TRC016-EN
notes
period three
System Variations
Realize, however, that the chiller is only one component of the chilled-water
system. Although chiller efficiency is important, overall system efficiency is
more important because the building owner pays to operate the entire system,
not just the chiller. Said another way, “The meter is on the building!”
With this in mind, many system design engineers are looking for ways to
optimize the efficiency of the entire system, not just the chiller.
One approach to increase overall system efficiency has been to reduce pump
and cooling-tower energy by reducing the amount of water being pumped
through the system. In the past, the conditions shown in the center column of
the table in Figure 70 were often used when designing a water-cooled, chilled-
water system. These flow rates result in a 10°F [5.6°C] temperature difference
(∆T) through both the evaporator and the condenser. In fact, they are the
standard conditions at which electric, vapor-compression chillers are rated by
Greater Focus on System Efficiency
Figure 69
Trend Toward Lower Flow Rates
yesterday
evaporator
flow rate
leaving
chilled-water
temperature
condenser
flow rate
entering
condenser-water
temperature
2.4 gpm/ton
[0.043 L/s/kW]
44°F
[6.7°C]
3.0 gpm/ton
[0.054 L/s/kW]
85°F
[29.4°C]
today
1.5 gpm/ton
[0.027 L/s/kW]
41°F
[5°C]
2.0 gpm/ton
[0.036 L/s/kW]
85°F
[29.4°C]
electric-driven chiller
Figure 70