Retrotec USACE - Manual (Page 377)

J1
Appendix J
Some Chilled Water Systems Retrofi t
Design Guidance
J.1 Use Variable Frequency Drives (VFDs) for Primary
Chilled Water Pumps above 5 hp
A VFD allows equipment speed to be controlled to match the need of the loads
it serves, rather than running at full speed any time it is running.
Installation of VFDs on the primary chilled water pumps will allow the
speed of the pumps to be varied in response to changes in the cooling loads
and chilled water system temperature differentials. Since the power demand
of the primary chilled water pump motor varies exponentially with speed by
approximately a power of 2.5 (it is not quite a cubic relationship in the installa-
tion world), reducing the pump speed to 70% when the chiller load is around
70% will result in a primary chilled water pump energy savings of approxi-
mately 55–60%. At low loads, reducing the speed to 50% when the chiller load
is around 40% or less will result in a primary chilled water pump energy sav-
ings of approximately 80%.
Since the chillers spend the majority of their time operating below 70% of
their design cooling capacity, the system energy savings can be substantial.
J.2 Use VFDs for the Condenser Water Pumps
A VFD allows the equipment speed to be controlled to match the need of the
loads it serves, rather than running at full speed any time it is running.
Installation of VFDs on the condenser water pumps will allow the speed
of the pumps to be varied in response to changes in the cooling loads. Since
the power demand of the condenser water pump motor varies approximately
to the second power with speed (due to the fi xed minimum lift conditions of
an atmospheric cooling tower, the savings does not correspond to a cubic rela-
tionship), reducing the speed to 70% when the chiller load is around 70% will
result in a condenser water pump energy savings of approximately 50%. At low
loads, reducing the speed to 50% when the chiller load is around 50% or less
will result in a condenser water pump energy savings of approximately 75%.
We typically do not recommend taking the condenser water pump speed down
below approximately 50%, as the savings are minimal below that level, and
we need to pay attention to the minimum allowable fl ow rate over the cooling
towers, which is typically in the 50% of design fl ow range.