ASHRAE Journal - July 2021 - 41

TECHNICAL FEATURE
43.3°F (6.3°C) chilled water temperature. Design
day conditions still required 39°F (3.9°C), however.
Estimated annual savings were modeled with hourly
weather data (TMY3) using a reset schedule developed at
an adjacent semiconductor factory on-site that had previously
implemented this strategy.
Option 3: Centrifugal Chillers Converted to Heat Pumps
The chiller supplier modeled the existing chillers
operating at constant 78% load. Design condenser leaving
water temperature is 76.5°F (24.7°C) (allowing 1°F
[0.56°C] temperature loss to the heat exchanger inlet).
Currently, these chillers operate 39% load at 0.59 kW/ton
(0.17 kW/kW). Operating as 78% load heat pumps with
the stated condenser leaving water temperature would
yield average annual specifi c chiller power between
0.437 kW/ton and 0.465 kW/ton (0.124 kW/kW and
0.132 kW/kW). The remaining chillers will then be able
to operate at a lower condenser water temperature as
described in Option 4 and a higher chilled water temperature
most of the year as described in Option 2. Some
of the modeled results are summarized in Table 1.
Bypasses installed at each heat exchanger balance the
heat rejected by the heat pumps, sending condenser
water directly to the cooling towers as necessary to
maintain industrial city water temperature at 74.3°F
(23.5°C) minimum while industrial city water fl ow fl uctuates
through one or two heat exchangers (up to the
design fl ow). The industrial city water inlet temperature
varies throughout the year (Figures 1 and 2).
Option 4: Reset Condenser Water Temperature
Before the system reconfi guration, the chiller condenser
entering water setpoint was maintained at a high
enough temperature that allowed condenser water to be
the primary heat source warming industrial city water
used as makeup to the reverse osmosis/deionized system.
After the re-piping shown in Figure 2, the remaining
chillers are operated with signifi cantly colder condenser
water resulting in approximately 7% lower average
annual chiller kW/ton.
Table 2 represents some typical modeled chiller performance
points derived from supplier data for a range of
operating conditions. The complete set of performance
points were graphed to provide " best fi t " equations used
to predict kW/ton for the plant load every hour of the
year based on past load history after fi rst optimizing the
TABLE 1 Heat pump chiller performance modeled by vendor (constant speed).
EVAPORATOR
(OUTLET/INLET)
43°F/58.9°F
39°F/54.9°F
39°F/54.9°F
39°F/54.9°F
CONDENSER (INLET/OUTLET)
65°F/76.5°F
65°F/76.5°F
60°F/71.5°F
85°F/98.1°F
KW/TON
0.437
0.465
0.430
0.629
TABLE 2 Modeled chiller performance (chilled water and condenser water temperature
reset).
EVAPORATOR
(LWT)
CONDENSER
(EWT)
39°F
39.2°F
40°F
43.1°F
43.5°F
43.5°F
43.5°F
79.3°F
75.7°F
77.1°F
71.3°F
65.4°F
60°F
60°F
KW/TON
0.53
0.51
0.51
0.47
0.44
0.43
0.41
CHILLER %
OF FULL LOAD
65
71
73
71
70
69
60
required number of running chillers. This data does
not consider the impact of fi tting variable speed drives
to the chillers. The minimum condenser water temperature
was governed by the chiller manufacturer's
recommendation.
Option 5: Installed Chiller Compressor VSDs
The energy saved by fi tting VSDs was modeled by the
consultant based on chiller supplier performance data
for a range of loads, chilled water temperatures, condenser
water temperatures and for both constant speed
motors and VSD fi tted motors. The performance data
from the chiller supplier was reduced to a number of
equations that were selected within the spreadsheet
cells for each annual hour, the corresponding chiller
load, leaving chilled water temperature (CHW LWT)
and entering condenser water temperature (CW EWT)
throughout the year, giving the watt/ton reduction if a
VSD was fi tted. Hourly entering condenser water temperature
was predicted based on a derived cooling tower
performance equation deduced from vendor-supplied
cooling tower performance graphs and hourly wet-bulb
temperatures.
Chiller performance equations derived are shown in
Table 3. (Negative values indicate power savings with
chiller compressor VSD installed compared to no VSD.)
Due to high 4,160 volt chiller compressor motor VSD
cost, the minimum number of VSDs were installed to
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