ASHRAE Journal - July 2021 - 39

TECHNICAL FEATURE
FIGURE 1 Original configuration (simplified).
In: 73°F+
From Cooling Towers
In: 56°F
CHWR from FAB
Existing
Condensers
Existing
Evaporator
Out: 39°F to 43.6°F
CHWS to FAB
Pumps
(Typical 2)
ICW/RO
Heat
OFA Compressors
CHWR from FAB
CHWS to FAB
Exchanger
1,200 gpm, Max. Ea., 1,880 gpm Total,
Max. to RO, 74.3°F Min.
ICW, 36°F Design Min., 54°F Avg.
Note: Primary/Secondary Pumping and Controls Not Shown for Simplicity
1. Chilled water provided by centrifugal chillers and
associated cooling towers with approximately 12,000 ton
(42,190 kW) peak cooling load. For reference, the chilled
water loop is confi gured with primary-secondary pumping,
and all chillers are piped in parallel.
2. Compressed air with 2,400 hp (1790 kW) average
power consumption, cooled by chilled water entering
at 39°F (3.9°C), requiring a minimum of 421 tons
(1480 kW) of cooling.
3. Industrial city water for the deionized/reverse
osmosis makeup water system fl owing an average of
1,430 gpm (325 m3/h); two (each) plate-and-frame heat
exchangers with heat provided by the condenser water
loop maintained at a minimum of 73°F (22.8°C) leaving
all cooling towers. Since condenser water was used to heat
industrial city water, the condenser water loop temperature
setpoint was governed by the deionized water plant,
preventing a reduction below the value given above.
Description of Evaluated Options
Option 1
Modify piping to cool the air compressors with secondary
chilled water return (SCHWR) at 56°F (13.3°C) rather
than secondary chilled water supply (SCHWS) at 39°F
(3.9°C).
Add external compressed air aftercoolers to achieve
the original design leaving air dew point.
Route cooling water exiting the air compressors to
a new plate-and-frame heat exchanger to preheat
approximately 60% of the industrial city water for the
deionized/reverse osmosis makeup water system by
recovering heat from the air compressor cooling water.
Cooling water exits the air compressors at ~76°F
(24.4°C) and the plate-and-frame heat exchanger at
55°F (12.8°C). This temperature is controlled by varying
the industrial city water fl ow through the heat
exchanger cold side.
Option 2
Reset chilled water supply temperature up to 43.5°F
(6.4°C) in accordance with outdoor air dew point since
there are relatively few hours each year requiring 39°F
(3.9°C). Chilled water is used primarily to cool/dehumidify
outdoor air and for process equipment sensible
cooling.
Option 3
Convert several chillers to heat pumps or, essentially
heat recovery chillers, but without a double-bundle condenser,
providing up to 100% of the heat required by the
existing deionized/reverse osmosis makeup water heat
exchangers.
Use the existing cooling towers dedicated to these
chillers to balance the load (remove heat not required by
the industrial city water heat exchangers). Add a bypass
line and a motor-actuated control valve at each heat
exchanger, diverting fl ow from the heat exchangers.
Preferentially load the heat pumps to a fi xed minimum
load, ensuring heat is always available to meet the
process demand; the leaving condenser water temperature
is controlled to 76.5°F (24.7°C) by varying condenser
water fl ow. Add variable speed drives to the heat pump
primary water pumps, providing suffi cient cooling load
while maintaining the leaving chilled water setpoint.
Option 4
Reset condenser water inlet temperature to all other
chillers from an average of 73.8°F to 65.7°F (23.2°C to
J U LY 2 0 2 1 ashrae.org ASHRAE JOURNAL
39
Out: 77.8°F to 87.4°F
To Cooling Towers
CWR
To Condensers
Cooling Towers
73°F, Min.
CHW = Chilled Water
CW = Condenser Water
ICW = Industrial City Water
OFA = Oil Free Air
RO = Reverse Osmosis
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ASHRAE Journal - July 2021

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