Operating in this temperature range provides a safety buffer in the event that the air
conditioning system goes down for a period of time.
The ambient relative humidity range of 45 to 50 percent is suitable for safe data
processing operations. Most computer equipment can operate in a wide range (20 to
80 percent), but the range of 45 to 50 percent is recommended for the following
reasons:
•
Optimal range helps protect computer systems from corrosion problems
associated with high humidity levels.
•
Optimal range provides the greatest operating time buffer in the event of air
conditioner control failure.
•
This range helps avoid failures or temporary malfunctions caused by intermittent
interference from static discharges that may occur when relative humidity is too
low.
Note:
Electrostatic discharge (ESD) is easily generated, and hard to dissipate in
areas of low relative humidity, such as below 35 percent. ESD becomes
critical when humidity drops below 30 percent. It is not difficult to maintain
humidity in a data center because of the high-efficiency vapor barrier and low
rate of air changes normally present.
1.6 Ventilation and Cooling Requirements
Always provide adequate space in front and behind the rack to allow for proper
ventilation.
Do not obstruct the front or rear of the rack with equipment or objects that might
prevent air from flowing through the rack. Rack-mountable servers and equipment
typically draw cool air in through the front of the rack and let out warm air out the rear
of the rack. There is no air flow requirement for the left and right sides due to front-to-
back cooling.
If the rack is not completely filled with components, then cover the empty sections with
filler panels. Gaps between components can adversely affect air flow and cooling
within the rack.
Relative humidity is the percentage of the total water vapor that can exist in the air
without condensing, and is inversely proportional to air temperature. Humidity goes
down when the temperature rises, and goes up when the temperature drops. For
example, air with a relative humidity of 45 percent at a temperature of 24 degrees
Celsius (75 degrees Fahrenheit) has a relative humidity of 65 percent at a temperature
of 18 degrees Celsius (64 degrees Fahrenheit). As the temperature drops, the relative
humidity rises to more than 65 percent, and water droplets are formed.
Air conditioning facilities usually do not precisely monitor or control temperature and
humidity throughout an entire computer room. Generally, monitoring is done at
individual points corresponding to multiple exhaust vents in the main unit, and other
units in the room. Special consideration should be paid to humidity when using
underfloor ventilation. When underfloor ventilation is used, monitoring is done at each
Chapter 1
Ventilation and Cooling Requirements
1-45
