Computer Room Air Conditioning
| Date | 01 March 1989 |
| Published date | 01 March 1989 |
| DOI | https://doi.org/10.1108/eb047764 |
| Pages | 29-47 |
| Author | Bennett J. Price |
| Subject Matter | Information & knowledge management,Library & information science |
Computer Room Air Conditioning
Bennett
J.
Price
This article discusses the basics of computer-room
air conditioning, an important component of the
special environment required by mainframe com-
puters and many mini-computers as well. Computer
room air conditioners differ in some significant
ways from "comfort" air-conditioners, which are
designed for the comfort of people rather than
machines. These differences make it less than ideal
to use air conditioning systems designed for human
comfort for computer cooling. The author describes
several different types of air-conditioners, con-
siderations related to the construction of a com-
puter room, and factors that determine air-con-
ditioning requirements.
This article discusses the basics of computer-
room air conditioning, an important component
of the special environment required by mainframe
computers and many mini-computers as well. It
does not, however, address chillers for water cooled
mainframes. The purpose of the article is to acquaint
those responsible for facility operations with the
essentials of the topic. The material presented below
will not obviate the need for experts, whether mech-
anical or professional engineer; sheetmetal or heating,
ventilation, and air conditioning (HVAC) contractor.
Indeed, the installation of computer room air con-
ditioning is not a do-it-yourself job. Rather, the
article should help provide library staff members
with a basis for better understanding their needs,
constraints, and options and allow them to more
intelligently converse with consultants or experts.
Typically, computer manufacturers state in
their site preparation manuals that a computer
room must be maintained between 65 and 75 °F
and between 40 and 60 percent Relative Humidity.1
Failure to maintain such conditions may void warran-
ties and in all likelihood will adversely affect the
reliability of the equipment.
The manufacturers of computers, disk and
tape drives, and other pieces of electronic equipment
often install fans in their cabinets. The fans push
or pull air through the cabinet to insure proper
ventilation. Typically, cool air is drawn through
vents on the bottom or lower edges of the equipment
while the heated air is exhausted near or at the
top.
This arrangement is used, of course, because
cool air is denser than warm air and thus a room
is coolest at floor level. Additionally, convection,
the movement or circulation of air due to its own
Price is a principal systems analyst at the
University of California, San Francisco.
ISSUE 27 29
Sidebar 1: Some Fundamentals
This sidebar explains some fundamental
properties of heat, air, and water and then
briefly defines some of the major components
of all air conditioners.
Heat-Sensible, Latent and Specific
Temperature is what we measure with a
thermometer. Water freezes at 32 °F and boils
at 212 °F. Heat is more complex; it is not syn-
onymous with temperature. Consider which would
be more comforting after a day's skiing: a kit-
chen match or a warm bath. The match is hot-
ter; the bath has more heat. Similarly, after
a day in the sun a lukewarm 60 °F shower will
be more comforting than a glass of ice water.
The shower has less heat (or more "cold") than
the glass of water even though the shower is
warmer.
Heat--or, simpler to grasp, the heat content
of an object--is comprised of several factors:
its temperature, its mass (how much there is
of it), and its specific heat (the object's ability
to hold heat or energy). Water is defined as
having a specific heat of 1.00. Glass, as it hap-
pens,
has a specific heat of about 0.2. Thus
a pound of water at 150 °F would warm up
a chilly room five times as much as a pound
of 150 °F glass.
Heat is measured in British Thermal Units
(BTUs). A BTU is the heat necessary to raise
one pound of water one °F. (From this definition
water is assigned a specific heat of 1.00; thus
0.2 BTU will increase the temperature of a pound
of glass by one degree.) As in heating, to cool
down 5 pounds of water by ten degrees °F re-
quires that 50 BTUs be removed from the water.
(5 lbs x 10 °F x s.h.
1.00).
Oddly enough, ice
has a specific heat of 0.5; to heat a pound of
10 °F ice to 11 °F requires .5 BTU; to heat
a pound of water from 40 to 41 °F requires
one BTU.
In addition to the examples above of "sen-
sible"
(or tangible or easily perceivable) heat,
there is another form of heat--latent (or hidden)
heat. This is the heat required to bring about
a change of state without a change in tempera-
ture.
Changes in state are transitions from ice
to water and water to vapor (and back again).
Such changes, from solid to liquid to vapor,
can also take place in substances other than
water.
For instance, to boil away a pound of 211
°F water requires one BTU to heat the water
to 212 °F, the boiling point of water, and then
yet another 970 BTUs to change the pound
of 212 °F water to 212 °F steam. This extra
heat is called the latent heat of vaporization;
for water it is 970 BTUs per pound. Just as
it takes 970 BTUs to vaporize a pound of boiling
water, so too an air conditioner must remove
970 BTUs from the like amount of vapor to
cause it to condense into liquid.1
The discussion so far has been couched
in terms of 212 °F vapor but, in fact, the water
vapor at any temperature, including room temp-
erature, holds or "binds" this latent heat of
vaporization; or, expressed differently, water
vapor exists because it has latent heat. For
this reason, dehumidification, when accomplish-
ed by condensing water vapor into a liquid,
requires considerable air conditioning.
Understanding the difference between
sensible heat and latent heat is necessary in
order to understand why computer room air
conditioners are designed so differently than
comfort air conditioners. As you sit reading
this article you are generating about 400 BTU/hr;
60 percent of this heat is sensible, mostly heat
carried to the skin's surface by blood vessels
and lost to the air. The balance is latent heat,
vapor from perspiration and respiration. If
you are reading this on an exercycle and working
hard, you are generating some 1500 BTU/hr;
about 2/3 of this is latent heat. The heat that
comes from computers, on the other hand, is
all sensible. If the computer room were perfectly
sealed, the only vapor and latent heat in the
room would come from the computer operators.
In sum, a computer room generally has a far
higher ratio of sensible heat to latent heat
than does the general office environment.
Humidity, Dewpoint, and Condensation
Dehumidification is the removal of water
vapor from the air. Vapor is so removed by
cooling the air to its dewpoint temperature.
The dewpoint temperature is that at which
vapor condenses into a liquid. The dewpoint
varies with the amount of water vapor in the
air and the air temperature. The higher the
humidity and the higher the air temperature,
the higher the dewpoint.
For example, vapor in 70 °F air with
a relative humidity of 70 percent will condense
(form dew) when it touches a 60 °F surface.
It is this dewpoint phenomenon that causes
30 LIBRARY HI TECH
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