ASHRAE Journal - September 2020 - 56

2020

ASHRAE TECHNOLOGY AWARD CASE STUDIES

pumps for the HX2 and self-contained
unit (SCU) loop, used to run continuously
without regard for if the HX2 leaving water
temperature was below its setpoint. Pump 8
operation was changed to have a minimum speed of 15%. The programming was
changed to shut the pump off if the leaving
water temperature from HX2 dropped 10°F
(5.5°C) below its setpoint. Pump 8 will not
cut on again until the HX2 leaving water
temperature raises 5°F (2.7°C) above its
setpoint. Pump 10 operation was changed
so that it runs at a constant 20% instead of
35%.
* HX1 operation. Used only when the
The East Tower chiller plant.
chiller plant has the waterside econoclosed as the discharge air temperature dropped below
mizer mode enabled, which occurs when the
outside air temperature is below waterside economizer its setpoint.
These programming modiﬁcations allows an addimode enabled setpoint. Once the waterside econotional 45,000 cfm (21 238 L/s) of cool outside air,
mizer is enabled, the chillers are shut off and isolation
which will reduce CO2 levels and increase indoor
valves on the condenser and chilled water side close
air quality during early morning operation of the
to isolate the chillers. Next, the cooling tower fans
ramp to a speed of 100%. The cooling tower attempts to building.
See the Maintenance and Operation section for discusproduce condenser water at temperatures of near 35°F
sion on occupant comfort.
(1.6°C). With the changes made to the airside economizer operation, the plant can stay inside waterside
Innovation
economizer mode for much longer stretches of time.
To address the single setpoint that added a chiller regard* Airside Economizer Operation. See indoor air
less of time of day or day of the week, the add sequence was
quality section.
changed to take into account the load pattern throughIndoor Air Quality
out the day. From Figure 2, it is observed that the load of
The original operation of the air handlers would
the building varies throughout the day.
allow for the units to go into economizer mode if it was
Monday thru Friday 4 a.m. to 8 a.m.: A second chiller
Monday through Friday between 7 a.m. and 5 p.m. and
will be added if the CHWST exceeds the chilled water
the air handler was in cooling mode with a return air
supply setpoint by 3.5°F (1.9°C), the third at 7.0°F
dew-point temperature greater than the outside air dew- (3.9°C). The time from 4 a.m. to 8 a.m. shows a sharp
point temperature but less than 50°F (10°C), outside air
increase in the rate of change of the building load due to
dry-bulb between 65°F (18.3°C) and 40°F (4.4°C) and the
equipment turning on (Figure 2).
chilled water valve greater than 10% open. The updated
Monday thru Friday 8 a.m. to 5 p.m: A second chiller
sequence was modiﬁed to allow for airside economizer
will be added if the CHWST exceeds the chilled water
operation if the air handler is occupied and in airside
supply setpoint by 4.5°F (2.5°C), the third at 7.5°F
economizer mode, regardless of time of day and day of
(4.1°C). The time from 7 a.m. to 5 p.m. shows a gradual
week, and the outside air dry bulb is less than 68°F (20°C). increase in the rate of change. This is partly due to the
In addition, instead of having the air handler come out typically increasing outside air temperature.
of economizer mode when the outside air temperature
All other times: A second chiller will be added if the
is below 40°F (4.4°C), the sequence was modiﬁed so
CHWST exceeds the chilled water supply setpoint by
that the outside air damper for the unit would modulate 6.5°F (3.6°C) the third at 8.5°F (4.7°C). These times were
56

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ASHRAE Journal - September 2020

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