TECHNICAL FEATURE

Optimizing Chilled
Water Design in
High-Rises in Hot,
Humid Climates
BY MUHAMMAD OMER SAFDAR, MEMBER ASHRAE

Chilled water cooling systems consume nearly 50% of building energy required to
meet cooling loads, especially in hot, humid climates.1 Chiller COP can be improved
with the optimal chilled water supply temperature, and significant amounts of system
energy can be reduced. Similarly, chilled water distribution pumps designed with
high temperature delta T will have less annual energy consumption compared to low
delta T chilled water system design. The optimal chilled water supply temperature
needs to be determined based on space dehumidification factor, since a high chilled
water supply temperature leads to a decrease in the cooling coil latent load capacity
and a potential increase in the size of cooling coils. This study examined the impact of
chilled water (CHW) supply temperature on the chiller’s efficiency, the effects of CHW
temperature differentials on pump energy consumption and the most suitable CHW
supply temperature for space dehumidification.
Methodology (624,306.8 ft2) with a mechanical floor on the 20th floor.
We consider a hypothetical office building model Office space occupancy density is based on 1 person per
as a case study for optimizing the performance of the 20 m2 (215 ft2) per ASHRAE Standard 62.1-2022.2 The
chilled water system in a high-rise 40 floor office chilled water system is designed to meet the cooling
building in Abu Dhabi (ASHRAE Climate Zone 1A,1B). load of the occupied spaces. Software was used to build
The conditioned area of the office building is 58,000 m2 a 3D geometry (Online Figure 1) for a hypothetical office

Muhammad Omer Safdar is CEO at OS ENGINEERING CONSULTANTS LLC, Dubai-UAE.

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building and also used to run a FIGURE 1 Chilled water schematic.

thermal load and energy simulation.
15.5°C CHWR
A chilled water (CHW) distribution 40th Floor 160 m Building
system consists of two water-cooled CHWS Cooling
6.5°C Equipment
centrifugal chillers, each with a
50% design capacity of 2838 kW CHWR 15.5°C
(807 tons). A variable primary flow
design uses three variable chilled CHWR 15.5°C
CHWS
water pumps (two working + one CHWP-01
6.5°C
standby) to circulate the chilled PCHWP-02
Chiller-01 CHWS
water from chillers to cooling
equipment through two CHW risers.
The CHW riser circulates water 20th Floor 80 m Chiller-02 CHWP-03
from water cooled chillers located
on the mechanical floor (20th floor)
to the ground floor, while a second
riser distributes water from chillers
(mechanical floor) to the 40th floor.
Fan coil units (FCU) and air- CHWR
15.5°C
handling units (AHU) are designed Building
to meet the cooling load for the office Cooling
Ground Floor CHWS Equipment
spaces in the hypothetical building 6.5°C
model. Water-cooled centrifugal
chillers are located on 20th floor,
and two cooling towers with counterflow, induced draft that the chillers’ annual electrical energy consumption
axial fans are located on the roof. FCU and AHU cooling decreased from 8,627,093 kWh (2.9 × 1010 Btu) to
coils, valves and fittings from the roof to the 7th floor are 8,322,910 kWh (2.8 × 1010 Btu), with the CHW supply
designed for pressure rating PN16. Cooling equipment temperature increasing from 5.5°C (42°F) to 6.5°C
coils, valves and fittings from the 6th floor to the ground (44°F) (Table 1). If the average cost per kWh of electricity
floor are designed for pressure rating PN20 due to the is $0.10, the annual operating cost difference would be
hydrostatic pressure of the building (Figure 1) $30,418. Over the course of 10 years, the building will
save $304,183 in electricity costs. A higher chilled water
Analysis supply temperature will improve the chillers efficiency,
As the supply chilled water temperature increases, but space-designed relative humidity levels need to be
the efficiency of the water-cooled centrifugal chillers considered as the latent load capacity for the FCU coil
increase. Energy simulations have been performed to decreases. The amount of carbon embodied in chillers
evaluate the chillers’ annual energy consumption based will decrease at a high chilled water supply temperature.
on different CHW supply temperatures such as 5.5°C Using an FCU unit to maintain 23°C (73°F) dry-bulb
(42°F), 6°C (43°F), 6.5°C (44°F). The analysis shows (DB)/53% relative humidity (RH) in the occupied

TABLE 1 Water-cooled centrifugal chiller energy consumption.

COOLING CHWS CHWR ELECTRICAL ANNUAL

ELECTRICAL
DESCRIPTION CAPACITY, TEMPERATURE, TEMPERATURE, COP QUANTITY CONSUMPTION, OPERATION
POWER, kW
kW °C °C kWh COST, $

Water-Cooled Centrifugal Chiller 2,838 5.5 14.5 499.6 5.68 2 862,7093 862,709
Water-Cooled Centrifugal Chiller 2,838 6 15 490.1 5.79 2 8,473,437 847,343
Wate- Cooled Centrifugal Chiller 2,838 6.5 15.5 481.9 5.89 2 8,322,910 832,291

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TECHNICAL FEATURE

TABLE 2 FCU schedule.

CHW CHW OFF COIL AIR OFF COIL AIR MOISTURE

SENSIBLE LATENT AIRFLOW ON COIL AIR
CAPACITY, INLET OUTLET TEMPERATURE, °C TEMPERATURE, °C CONDENSED
DESCRIPTION CAPACITY, CAPACITY, RATE, TEMPERATURE, °C
kW TEMP TEMP PER DESIGN PER SELECTION OUT
kW kW m 3/s
°C °C DB WB DB WB DB WB GR/KG
FCU 6 5.2 0.78 0.4 5.5 14.5 23 16.6 12.5 12 12.2 11.7 0.9
FCU 6 5.2 0.78 0.4 6 15 23 16.6 12.5 12 12.5 12 0.7
FCU 6 5.2 0.78 0.4 6.5 15.5 23 16.6 12.5 12 12.9 12.4 0.5
FCU 6 5.2 0.78 0.4 7 16 23 16.6 12.5 12 13.5 12.9 0.3
space, a sensible load of 5.2 kW (1.5 ton), a latent load https://tinyurl.com/JournalExtras, moisture condensed
of 0.8 kW (0.2 ton) and an airflow rate of 0.4 m3/s out from indoor air decreases from 0.9 gr/kg (0.4 gr/lb)
(848 cfm), a cooling coil supply air temperature of 12.5°C to 0.3 gr/kg (0.1 gr/lb) with an increase in the inlet CHW
(55°F) DB/12°C (54°F) WB is considered to maintain temperature from 5.5°C (42°F) to 7°C (45°F).
design conditions in the occupied office space. A CHW supply temperature at 6.5°C (44°F) condensed
As the inlet CHW temperature rises from 5.5°C (44°F) out moisture of 0.5 gr/kg (0.2 gr/lb), and the latent
to 7°C (45°F), the FCU meets the sensible load, but the load will decrease slightly. The relative humidity of the
latent load capacity decreases. As a result, the higher the space increases; however, it will remain below 60% RH
supply air temperature from the FCU cooling coil, the at a 6.5°C (44oF) inlet CHW temperature, within the
less moisture condenses out from the indoor air. Based acceptable limits per Standard 62.1-2022.
on the FCU schedule in Table 2 and the psychrometric Considering the office space dehumidification from
charts in Figures 2 and 3, as well as Online Figures 2 and 3 at the FCU cooling coil and chillers’ efficiency (Table 1

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FIGURE 2 FCU selection. CHWT inlet 5.5°C (42°F) for on coil 23°C DB/16.6°C WB (73°F DB/62°F WB); off coil and Table 2), the optimum chilled
12.2°C DB/11.7°C WB (54°F DB/53°F WB). water supply temperature from
90% 70% 26 the chillers is 6.5°C (44°F) for the
Psychrometric Chart 90 cooling equipment serving from
Metric Units 50% 24
101 325 Pa the 20th floor to the ground floor
Sea Level 80 22
and from the 20th floor to the 40th
20
floor cooling equipment in the
70
18 proposed chilled water system.
60 16 A higher chilled water

abs. Humidity gr/kg (X)

30% 14 temperature differential between
Enthalpy, 50
kJ/kg 12
the leaving and entering water
40 On Coil 23°C DB/53% RH temperature (delta T) can have
10
0 significant impact on pump
30 1 8
Off Coil 12.2°C DB/11.7°C WB energy consumption. In the case
20 6 of low delta T, there will be higher
Moisture removed 0.9 gr/kg from point 0 to point 1
10 10% 4 flow, higher energy consumption
0 rate and a higher capital cost for
-10 2
pumps, pipes and valves.3 An
0
-15 -10 -5 0 5 10 15 20 25 30 35 40 energy simulation was performed
Temperature °C (Tdb) by varying the CHW temperature
differential from 7°C to 9°C (13°F

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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 pumps. Over the course operation
13.5°C DB/12.9°C WB (56°F DB/55°F WB). (10 years), the building will save
90% 70% 26 $32,692 in electricity costs. This
Psychrometric Chart 90
Metric Units 50% 24
is the simple cost for accurate
101 325 Pa economic analysis discount factor
Sea Level 80 22
to be applied for future cost to
20
70 present. However, higher delta T
18 increases the FCUs’ cooling coils
60 16 rows, and the pressure drop across

Abs. Humidity gr/kg (X)

30% 14 the coils. Online Table 1 shows
Enthalpy, 50
kJ/kg 12 that fan energy consumption will
40 On Coil 23°C DB/53% RH increase from 543,490 kWh (1.8 ×
10
30 1 0 109 Btu) to 571,655 kWh (1.9 × 109
Off Coil 13.5°C DB/12.9°C WB 8
Btu), but pump energy savings
20 6
Moisture Condensed Out 0.3 gr/kg equivalent to 32,692 kWh (1.1 ×
10 From Process 0 to Point 1 10%
4 108 Btu) will be higher than the
0
-10 2 fan energy. The amount of carbon
0
embodied in pumps will decrease,
-15 -10 -5 0 5 10 15 20 25 30 35 40 whereas carbon embodied in FCUs
Temperature °C (Tdb)
and AHUs will rise due to bigger
cooling coils.
For compliance with
TABLE 3 CHW pump energy consumption. ASHRAE/IES Standard
COOLING FLOW PUMP, PUMP DELTA NUMBER TOTAL PUMPS ANNUAL PUMP 90.1-2016, the chilled
DESCRIPTION
CAPACITY, kW RATE, L/s kW HEAD, kPa T, °C OF PUMPS ENERGY, kWh ENERGY COST, $
water temperature
Chilled Water Pump 2,838 96.99 25 198.2 7 2 149,324 14,932
differential must be a
Chilled Water Pump 2,838 84.86 22 198.2 8 2 131,231 13,123
minimum of 8.3°C (15°F)
Chilled Water Pump 2,838 75.43 19 198.2 9 2 116,632 11,663
or higher.4,5 Therefore,
the CHW temperature
FIGURE 4 Chiller carbon emissions. differential of 9°C (16°F) complies with Standard
90.1-2016 requirements, and significant energy savings
Chiller Carbon Emissions kg CO2
can be achieved throughout the project’s life cycle.
4,100,000.0
4,063,360
4,050,000.0
Carbon Emissions
Carbon Emissions kg C02

4,000,000.0 3,990,988.8
Each kWh saved will have an economic impact,
3,950,000.0 3,920,090.6 reduce carbon emissions, and limit global warming.
3,900,000.0 Based on a hypothetical office building model with
3,850,000.0 a higher chilled water supply temperature of 6.5°C
3,800,000.0 (44°F), the chiller annual energy consumption is
5.5°C 6°C 6.5°C
Chilled Water Supply Temperature reduced compared to a low supply chilled water
temperature of 5.5°C (42°F). As a result, carbon
to 16°F) to evaluate the annual energy consumption of emissions could be reduced to 143,270 kg CO2 annually
CHW pumps. In this study, chilled water pumps used (Figure 4).6 Similarly chilled water pumps’ energy
116,632 kWh (3.9 × 108 Btu), which is a reduction of consumption is reduced with a higher temperature
149,324 kWh (5.1 × 108 Btu) on an annual basis (Table 3). differential of 9°C (16°F) compared to a temperature
If the average cost per kWh of electricity is $0.10, the differential of 7°C (13°F).6 As a result, operational
annual operating cost difference would be $3,269 for the energy carbon emissions could be reduced to

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38,764 kg CO2 annually (Online Figure 4). Optimizing pump energy cost and comply with Standard 90.1-
chiller COP and using a higher temperature 2016 standard requirements. Proposed chilled water
differential could reduce 182,034 kg CO2 through design energy efficiency measures implemented
annual operational energy. This strategy could mitigate on new building designs at the community level
1,820.3 metric tons of CO2 from the atmosphere will develop climate resilient cities. Energy
over the period of 10 years of operation. The amount efficient system design will help meet the building
of carbon embodied in chillers and pumps will be decarbonization target by 2050.
reduced whereas FCUs and AHUs’ embodied carbon
will increase due to an increase in cooling coil size. References
1. Saidur, R. et al. 2011. “Chillers energy consumption, energy
Conclusion savings and emission analysis in an institutional buildings.” Energy
36(8):5233 – 5238.
An increase in chilled water supply temperature 2. ANSI/ASHRAE Standard 62.1-2022, Ventilation and Acceptable
of 1°C (1.8°F) can achieve 3.5% savings in chiller Indoor Air Quality.
annual energy consumption. The optimal chilled 3. Simmonds, P., 2020. ASHRAE Design Guide for Tall, Supertall,
and Megatall Building Systems, 2nd Ed, Chap. 11, Water Distribution
water supply temperature for a high-rise building is Systems, pp. 157 – 165. Atlanta: ASHRAE.
to be evaluated for every project based on the space- 4. Murphy, J. 2019. “Selecting chilled-water coils for ASHRAE 90.1’s
designed humidity levels and building static height. new 15°F delta T requirement.” Trane Engineers Newsletter, 48–2.
This will help achieve the designed relative humidity 5. ANSI/ASHRAE/IES Standard 90.1-2016, Energy Standard for
Buildings Except Low Rise Residential Buildings.
levels and comfortable indoor environment for
6. Statista. Undated. “Emissions Intensity from Electricity
occupants. The chilled water temperature differential Generation in the United Arab Emirates from 2011 to 2020.”
delta T = 9°C (16°F) or higher can significantly reduce Statista. https://tinyurl.com/mtrfd4fs.

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