ASHRAE Journal - April 2014 - 84

Column Data centers
Donald l. Beaty

Part Two

IT Equipment Load Trends
By DonalD l. Beaty, P.e., Fellow asHrae

Using simple rules of thumb for the IT load can readily result in significant underdesign/over-design in today's data centers. The TC 9.9 Power Trends Charts represent
a maximum condition of potential loads intended as a worst-case guide. Part 1 (March
2014) introduced the IT Load Trends by providing a general history of its development
and the basic origination of TC 9.9. Part 2 examines the variables of a typical server
rack and provides some insight as to why the Power Trends Chart values can be misinterpreted if their application is not clearly understood.
The first variable is as basic as the occupancy of a
given server rack. Figure 1 shows a comparison of an
empty rack and fully configured rack. As the occupancy
of the rack decreases from fully configured, the published values in the Power Trends Charts
begin to decrease. This column will examine other variables that can cause further
decreases from the maximum published
trend values.

The height of a rack can be expressed dimensionally,
but it is more common to express it in terms of rack units
or "U." One rack unit is dimensionally 1.75 in. (44.5 mm)
and although the width is standardized, the height of
volume servers varies in multiples of U's
(e.g., 1U, 2U, 3U, 7U, 10U, etc.), depending
on their architecture and specifications.
Consequently, server rack heights are
described in terms of the number of total Us
they can accommodate (e.g., some common
Server racks
rack heights are 42U, 48U, 55U).
Volume servers from all of the major IT
A summary of the server rack dimensions
equipment manufacturers have physical
can be seen in Figure 2.
dimensions that comply with a standard
The standardization of rack-mounted
chassis configuration. This server conservers means that end users are able to
Figure 1 Visual comparison of an
figuration enables servers to be installed
seamlessly mix and match their IT equipempty rack to a fully configured
rack.
within a structural frame, called a server
ment (hardware) within a given rack.
rack, which is specifically designed to
They can mix and match different server
house multiple servers irrespective of the IT equipment
models from a given manufacturer or even multiple
manufacturer.
manufacturers, which makes for a limitless number of
The typical server rack width is expressed as a frame
occupancy configurations (Figure 3).
size with typical widths being 19 in. and 23 in. (482 mm
Server Configuration
and 584 mm). This does not represent the actual physiIf you have purchased a computer, you know that there
cal width of the rack, but it is the more critical dimenare a number of configuration options for any one make
sion since it determines the IT hardware mounting
and model, the most common options being the quancriteria. Racks can vary in their depth anywhere from
tity and speed of the central processing unit (CPU), the
24 in. to 48 in. (610 mm to 1220 mm), depending on the
hard drive size, the amount of memory (random access
different types of servers, connection architecture and/
or the housing (standard racks are open on all sides, but memory [RAM]) and so on. The resultant performance
some racks are enclosed on all sides and are typically
Donald l. Beaty, p.E., is president of DlB Associates Consulting Engineers, in Eatontown,
referred to as cabinets).
N.J. He is publications chair of ASHrAE TC 9.9.
84

ASHRAE JouRnAl

ashrae.org

Apri l 2014

ASHRAE Journal - April 2014

Table of Contents for the Digital Edition of ASHRAE Journal - April 2014