ASHRAE Journal - January 2024 - 30

TECHNICAL FEATURE
FIGURE 3 FCU selection. Inlet CHWT 7°C (45°F) for on coil 23°C DB/16.6°C WB (73°F DB/62°F WB) off coil
13.5°C DB/12.9°C WB (56°F DB/55°F WB).
Psychrometric Chart
Metric Units
101 325 Pa
Sea Level
70
60
30%
Enthalpy,
kJ/kg
40
30
10
-10
-15
-10
-5
5
10
15
Temperature °C (Tdb)
20
25
30
35
40
1
Off Coil 13.5°C DB/12.9°C WB
20
50
On Coil 23°C DB/53% RH
Moisture Condensed Out 0.3 gr/kg
From Process 0 to Point 1
10%
90
80
90% 70%
50%
26
24
22
20
18
16
14
12
10
8
6
4
2
pumps. Over the course operation
(10 years), the building will save
$32,692 in electricity costs. This
is the simple cost for accurate
economic analysis discount factor
to be applied for future cost to
present. However, higher delta T
increases the FCUs' cooling coils
rows, and the pressure drop across
the coils. Online Table 1 shows
that fan energy consumption will
increase from 543,490 kWh (1.8 ×
109 Btu) to 571,655 kWh (1.9 × 109
Btu), but pump energy savings
equivalent to 32,692 kWh (1.1 ×
108 Btu) will be higher than the
fan energy. The amount of carbon
embodied in pumps will decrease,
whereas carbon embodied in FCUs
and AHUs will rise due to bigger
cooling coils.
For compliance with
TABLE 3 CHW pump energy consumption.
DESCRIPTION
COOLING
Chilled Water Pump
Chilled Water Pump
Chilled Water Pump
CAPACITY, kW
2,838
2,838
2,838
FLOW
RATE, L/s
96.99
84.86
75.43
PUMP,
kW
25
22
19
FIGURE 4 Chiller carbon emissions.
Chiller Carbon Emissions kg CO2
4,100,000.0
4,050,000.0
4,000,000.0
3,950,000.0
3,900,000.0
3,850,000.0
3,800,000.0
4,063,360
3,990,988.8
3,920,090.6
5.5°C
6°C
Chilled Water Supply Temperature
to 16°F) to evaluate the annual energy consumption of
CHW pumps. In this study, chilled water pumps used
116,632 kWh (3.9 × 108 Btu), which is a reduction of
149,324 kWh (5.1 × 108 Btu) on an annual basis (Table 3).
If the average cost per kWh of electricity is $0.10, the
annual operating cost difference would be $3,269 for the
30
ASHRAE JOURNAL ashrae.org J A N U ARY 2024
6.5°C
Carbon Emissions
Each kWh saved will have an economic impact,
reduce carbon emissions, and limit global warming.
Based on a hypothetical office building model with
a higher chilled water supply temperature of 6.5°C
(44°F), the chiller annual energy consumption is
reduced compared to a low supply chilled water
temperature of 5.5°C (42°F). As a result, carbon
emissions could be reduced to 143,270 kg CO2 annually
(Figure 4).6 Similarly chilled water pumps' energy
consumption is reduced with a higher temperature
differential of 9°C (16°F) compared to a temperature
differential of 7°C (13°F).6 As a result, operational
energy carbon emissions could be reduced to
PUMP
HEAD, kPa
198.2
198.2
198.2
DELTA
T, °C
7
8
9
NUMBER
OF PUMPS
2
2
2
TOTAL PUMPS
ENERGY, kWh
149,324
131,231
116,632
ANNUAL PUMP
ENERGY COST, $
14,932
13,123
11,663
ASHRAE/IES Standard
90.1-2016, the chilled
water temperature
differential must be a
minimum of 8.3°C (15°F)
or higher.4,5 Therefore,
the CHW temperature
differential of 9°C (16°F) complies with Standard
90.1-2016 requirements, and significant energy savings
can be achieved throughout the project's life cycle.
Abs. Humidity gr/kg (X)
Carbon Emissions kg C02
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ASHRAE Journal - January 2024

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