ASHRAE Journal - February 2024 - 42

TECHNICAL FEATURE
FIGURE 4 Cooling conditions and concepts for IT halls.
Return Air
EC Fan
Hot Aisle Containment
Supply Air
24°C
(Primary) Plant System
The IT hall HVAC system can
Rack-Servers
28°C Out
18°C In
Fan Wall Unit
Chilled Water
The fan of a CRAC/H unit scales with the same input
power if the efficiency of both the motor and belt drive
is kept constant. Additionally, a cooling tower equipped
with a fan inverter can conserve energy by adjusting
the airflow rate based on the chiller's operating load
factor, and HSP operation can be achieved by operating
redundant cooling towers.
Case Study: Data Center A
The case analysis is based on the standard capacities
of the IT equipment, cooling system and power system
in Data Center A, which will comprise five floors of data
halls with a total IT load of 30 MW. The 30 MW IT load
is evenly distributed across five floors, resulting in a
power allocation of 6 MW per floor. The IT equipment
is designed based on the global average rack density of
8.8 kW/rack,11 with approximately 680 rack-servers on
each floor, for a total of 3,400 units that will need to be
cooled. Online Table 1 presents the complete configuration
of the cooling system for Data Center A. (Visit https://
tinyurl.com/JournalExtras to view Online Table 1.)
(Secondary) HVAC System
As more than 95% of the power consumed by IT
equipment is typically converted into heat, the cooling
load for each floor's IT equipment on the secondary
system exceeds 5,700 kW (1,621 tons). Therefore,
the total size of the secondary units was chosen to be
6,600 kW (1,877 ton), including a safety factor. Each
floor will have a total of 24 fan wall units, consisting
of 20 active units with a capacity of 345 kW (98 ton)
each and four spares. To enhance energy efficiency,
the chilled water inlet and outlet temperatures were
set at 18.0°C/28.0°C (64°F/82°F), while the load-side
42
ASHRAE JOURNAL ashrae.org FEBRUARY 2 0 2 4
support 100 active operational fan
walls with a total equipment capacity
of 34,500 kW (9,810 tons), while the
primary plant side must supply a total cooling load
of 35,000 kW (9,952 tons), which includes cooling for
the UPS/battery room and the DOAS system. This size
accommodates a maximum operating load factor of
95% and considers piping heat losses. To achieve this,
six variable speed turbo (centrifugal) chillers with a
size of 7,050 kW (2,005 tons) were installed, consisting
of five active and one redundant chiller with N + 1
redundancy. These chillers have a high COP between
40% and 80% operating load factors and are regulated
to operate at those load factors by quantity control.
The chillers can function with chilled water inlet and
outlet temperatures of 28.0°C/18.0°C (82°F/64°F)
and condenser water inlet and outlet temperatures of
27.0°C/36.3°C (81°F/97.3°F), with a maximum COP of
15.09 and are most efficient at partial loads of 50% to
60%.
Data Center A has a total of six cooling towers,
including one spare, with a capacity of 8,836 kW
(2,512 tons) on an N + 1 basis. These open circuit,
induced-draft cross flow cooling towers operate in a
1:1 ratio with the chillers. The condenser water inlet
and outlet temperature conditions of 36.3°C/27.0°C
(97.3°F/81°F) are attainable, based on an outside wetbulb
temperature of 29.3°C (84.7°F). The cooling tower
fans use variable air volume technology to regulate
airflow rates based on the chiller's operating load factor.
(Distribution) Pump System
The water flow rate in the chilled water and condenser
water system varies depending on the operating load
factor of the variable speed chiller. As a result, the
chilled water circulation pump supplying the secondary
side fan wall units and the condenser water circulation
36°C
supply air/return air conditions were
set at 24°C/36°C (75°F/97°F), both at a
substantially reduced level. Moreover,
the fan wall units are equipped with
electronically commutated fans that
can adjust the airflow rate based on the
operating load factor (Figure 4).
Filter
Heat Exchanger
http://tinyurl.com/JournalExtras http://tinyurl.com/JournalExtras http://www.ashrae.org
ASHRAE Journal - February 2024

Table of Contents for the Digital Edition of ASHRAE Journal - February 2024
