ASHRAE Journal - July 2021 - 42

TECHNICAL FEATURE
meet the payback criteria. The feasibility study considered
providing VSDs for two larger base loaded chillers
acting as heat pumps and three smaller non-heat recovery
(or cooling only) chillers, but during detailed design,
only three smaller chillers were fitted.
The average annual modeled improvement for the
cooling only chillers was 0.12 kW/ton (0.034 kW/kW)
with the chillers operating between 55% and 90% load
(average 69%). VSDs provide an average 27% improvement
since drive speed can be reduced due to reduced
compressor lift whenever cooling tower temperature
is below design, chilled water setpoint is reset above
design and/or cooling demand is below capacity.
Overall Results
Costs
Net cost, after energy reduction incentives, was
approximately $5 million. The costs included engineering,
construction, commissioning, heat exchanger
upgrades and air compressor aftercoolers, providing a
4.7-year simple payback based on the expected (modeled)
savings of $1,082,000. However, the actual energy
cost savings to date, annualized, is $555,000.
To achieve the modeled savings requires completing
system control changes and final tuning, increasing
preferential heat pump chiller loading to at least 78%
of capacity ($168,000 additional savings), increasing
VSD fitted chiller loading to 66% minimum while
operating those chillers a minimum 6,500 hours per
year ($230,000 additional savings) and completing condenser
water reset implementation.
After factoring in the 10% cooling load reduction that
occurred between the study and implementation, the
expected savings should nearly reach expectation. This
reduction is not reflected in the projections.
Performance After Partial Commissioning
The following figures are representative of performance
before and after energy conservation measures
were implemented. Six to 11 months operating data has
been collected since implementation.
Option 1: Figure 3 shows chilled water cooling load
reduction from using chilled water return flow to cool
air compressors followed by absorbing air compressor
heat with cold industrial city water (reverse osmosis/
deionized makeup water). Load reduction is valued at
$257,000 savings per year.
42
ASHRAE JOURNAL ashrae.org J U LY 2021
TABLE 3 Typical power reduction by fitting variable speed drive to 1,280 ton chillers.
CHILLER LOAD
80%
70%
60%
TEMPERATURE (°F)
W/ton = 3.421 (ΔT) - 164.2
W/ton = 3.891 (ΔT) - 198.4
W/ton = 5.345 (ΔT) - 259.1
Where ΔT = condenser water entering water temperature minus chilled water leaving
water temperature.
FIGURE 3 Oil free air cooling savings provided by heating industrial city water.
OFA Compressor " Free " Cooling Provided by Heating City Water, Average = 579 Tons
900
800
700
600
500
400
300
200
Oct 27
2018
Dec 16
|
Feb 4
Mar 26
2019
Options 2, 3, 4, and 5: Figure 4 shows the combined
benefit of condenser water and chilled water setpoint
optimization, converting two chillers to heat pumps
and adding three chiller compressor motor variable
speed drives to non-heat pump chillers. Average plantspecific
power reduced from 0.81 kW/ton to 0.71 kW/ton
(0.23 kW/kW to 0.20 kW/kW), a 12% reduction to date.
Option 4: Figure 5 shows condenser water temperature
reduction (minimum 60°F) (15.6°C) for non-heat recovery
chillers. As stated previously, condenser water reset
results in approximately 7% lower average annual chiller
energy consumption (as modeled).
Options 4 and 5: Figure 6 shows a 0.25 kW/ton
(0.071 kW/kW) average (six month) chiller improvement
for chillers fitted with compressor motor variable speed
drives and reduced condenser water temperature and
partial chilled water reset upward during low outdoor
dew point.
Other Savings
Based on Northwest United States power (60% hydro,
30% CO2 emitting), an approximate value of 0.5 lb
(0.227 kg) CO2 per kWh can be used. The modeled
energy saved would reduce CO2 emissions by 4,100 tons
(3727 metric tons) per year.3 If CO2 emissions are
May 15
Jul 4
Cooling (Tons Refrigeration)
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ASHRAE Journal - July 2021

Table of Contents for the Digital Edition of ASHRAE Journal - July 2021

Contents
ASHRAE Journal - July 2021 - Intro
ASHRAE Journal - July 2021 - Cover1
ASHRAE Journal - July 2021 - Cover2
ASHRAE Journal - July 2021 - 1
ASHRAE Journal - July 2021 - Contents
ASHRAE Journal - July 2021 - 3
ASHRAE Journal - July 2021 - 4
ASHRAE Journal - July 2021 - 5
ASHRAE Journal - July 2021 - 6
ASHRAE Journal - July 2021 - 7
ASHRAE Journal - July 2021 - 8
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