65
THE COOLING
CAPACITY FACTOR
The Cooling Capacity Factor is
calculated by dividing the total rated
cooling capacity (kW) by 110% of the
IT critical load (kW).
The total rated cooling capacity is the
sum of the running cooling units’ rated
capacities. If all cooling units are
running, then this will be the same
value as the total installed rated
cooling capacity. For example, if there
are 10 cooling units installed with a
rated capacity of 30 tons each and
seven are running, then the total
running cooling capacity is 739 kW (7
x 30 tons = 210 tons, 210 tons x 3.52
= 739 kW). To convert tons to kW,
multiply tons by the constant 3.52.
The IT critical load in the room is
equal to the Uninterruptable Power
Supply (UPS) output(s) for the room.
Ensure that the UPS output used is
only for the room being calculated. If
the UPS feeds other rooms, then those
loads must be subtracted from the
total UPS output. To account for
typical room load not reflected in the
UPS output, add 10% for lights,
people, and building envelope.
Cooling units are typically referred to
by the manufacturer’s stated capacity,
such as a 30-ton or 20-ton cooling
unit. If the manufacturer’s stated
cooling capacity is unknown, record
the model number on the cooling unit
nameplate and search online or call
the manufacturer.
It is important to know that the
manufacturer’s stated capacity refers to
the unit’s total capacity at standard
conditions, which refer to the
temperature and relative humidity of
the air returning to the cooling unit.
Typically, the standard conditions are
75° F (24° C) and 45% relative
humidity (RH). If the return air
conditions differ, the unit will have a
different cooling capacity. If the return
air is cooler and/or moister than the
standard condition, the unit will deliver
less than the stated capacity. If the
return air is warmer and/or drier than
the standard condition, the unit will be
capable of more than the stated
capacity.
Calculation of CCF uses the
manufacturer’s stated cooling capacity
because if the room layout is good
and airflow management (AFM) best
practices have been implemented
well, then it will be possible for the
unit to deliver at least the rated
capacity. However, in their current
state, many cooling units have a lower
return air temperature set-point than
the standard condition. The difference
between the delivered capacity at
current conditions and the potential
capacity at the standard condition, or
higher return temperature, is a form of
stranded capacity.
HOW TO INTERPRET
YOUR CCF AND
DETERMINE THE
AMOUNT OF
OPPORTUNITY AT
YOUR SITE
For rooms with a CCF of
1.0
to
1.1
,
there is little to no redundant cooling
capacity. It is critical that AFM
fundamentals be thoroughly
implemented in these rooms to make
available any redundant capacity and
keep IT intake air temperatures as low
as possible. It will likely be necessary
to install an additional cooling unit(s)
to have redundant capacity. AFM
improvements will likely improve IT
equipment intake temperatures and
create an environment where cooling
unit set-points can be raised, which
increases cooling unit efficiency and
capacity. However, there is no
opportunity to turn off cooling units or
reduce fan speeds.
For rooms with a CCF of
1.1
to
1.2
,
the number of running cooling units is
very closely coupled to the heat load
in the room. If IT intake temperatures
are not outside of the ASHRAE
recommended range then AFM
fundamentals have been thoroughly
implemented. There is approximately
one redundant cooling unit for every
10 units running. In some cases, this is
sufficient to maintain the room
temperatures if a cooling unit fails.
Cooling units should not be turned off
unless the room has 24-hour-by-
forever monitoring and staffing.
For rooms with a CCF of
1.2
to
1.5
,
there is moderate opportunity to
realize savings from turning off cooling
units or reducing fan speeds. This can
often only be done once AFM
improvements have been effectively
implemented. This does not require
full containment strategies, but does
require thorough sealing of raised
floor penetrations and open spaces in
racks, and best practice placement of
perforated tiles and grills.
A CCF of
1.5
to
3.0
is most common.
These rooms have substantial
opportunity to reduce operating cost,
improve the IT environment, and
increase the IT load that can be
effectively cooled. Rooms in this range
often have significant stranded cooling
capacity that can be released by
improving AFM.
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