Choosing Between Room, Row, and Rack-based Cooling for Data Centers
Schneider Electric – Data Center Science Center White Paper 130
Rev 2 2
Nearly all of the electrical power delivered to the IT loads in a data center ends up as waste
heat that must be removed to prevent over temperature conditions. Virtually all IT equipment
is air-cooled, that is, each piece of IT equipment takes in ambient air and ejects waste heat
into its exhaust air. Since a data center may contain thousands of IT devices, the result is
that there is thousands of hot airflow paths within the data center that together represent the
total waste heat output of the data center; waste heat that must be removed. The purpose of
the air conditioning system for the data center is to efficiently capture this complex flow of
waste heat and eject it from the room.
The historical method for data center cooling is to use perimeter cooling units that distribute
cold air under a raised floor with no form of containment. This is known as targeted supply
and flooded return air distribution as discussed in White Paper 55, The Different Types of Air
Distribution for IT Environments. In this approach, one or more air conditioning systems,
working in parallel, push cool air into the data center while drawing out warmer ambient air.
The basic principle of this approach is that the air conditioners not only provide raw cooling
capacity, but they also serve as a large mixer, constantly stirring and mixing the air in the
room to bring it to a homogeneous average temperature, preventing hot-spots from occurring.
This approach is effective only as long as the power needed to mix the air is a small fraction
of the total data center power consumption. Simulation data and experience show that this
system is effective when the average power density in data is on the order of 1-2 kW per
rack, translating to 323-753 W/m² (30-70 W/ft²). Various measures can be taken to increase
power density of this traditional cooling approach, but there are still practical limits. More
information on the limitation of using traditional cooling can be found in White Paper 46
“Cooling Strategies for Ultra-High Density Racks and Blade Servers”. With power densities of
modern IT equipment pushing peak power density to 20 kW per rack or more, simulation data
and experience show traditional cooling (no containment), dependent on air mixing, no longer
functions effectively.
To address this problem, design approaches exist that focus on room, row, and rack-based
cooling. In these approaches the air conditioning systems are specifically integrated with the
room, rows of racks, or individual rack in order to minimize air mixing. This provides much
better predictability, higher density, higher efficiency, and a number of other benefits. In this
paper, the various approaches are explained and contrasted. It will be shown that each of
the three approaches has an appropriate application, and in general a trend toward row-
based cooling for smaller data centers and high density zones and toward room-based
cooling with containment for larger data centers should be expected.
Every data center air conditioning system has two key functions: to provide the bulk cooling
capacity, and to distribute the air to the IT loads. The first function of providing bulk cooling
capacity is the same for room, row, and rack-based cooling, namely, that the bulk cooling
capacity of the air conditioning system in kilowatts must exhaust the total power load (kW) of
the IT equipment. The various technologies to provide this function are the same whether the
cooling system is designed at the room, row, or rack level. The major difference between
room, row, and rack-based cooling lies in how they perform the second critical function,
distribution of air to the loads. Unlike
power distribution, where flow is constrained to
wires
and clearly visible as part of the design, airflow is only crudely constrained by the room
design and the actual air flow is not visible in implementation and varies considerably
between different installations. Controlling the airflow is the main objective of the different
cooling system design approaches.
The 3 basic configurations are shown in the generic floor plans depicted in Figure 1. In the
figure, black square boxes represent racks arranged in rows, and the blue arrows represent
Introduction
Room, row,
and rack-
based cooling
The Different Types of Air
Distribution for IT Environments
Link to resource
White Paper 55
Cooling Strategies for Ultra-High
Density Racks and Blade Servers
Link to resource
White Paper 46