UC Los Angeles Campus & Medical Center Strategic Energy Plan ...

8.8.5 UCLA Custom Project 5. Install VFDs on Cooling Towers
This project considers installing Variable Frequency drives (VFD) on Cooling Tower Fan. With
VFD, fan speed is regulated to meet the required cooling needs. Since cooling tower is
designed based on outside air wet bulb temperature, variation in wet bulb temperature affects
tower performance. Highest wet bulb temperature for a particular location occurs only few hours
per year; and it is not required to run tower fan at design speed (100%) for remaining hours. Fan
speed can be reduced to meet cooling needs and therefore allowing significant electric energy
and demand savings. VFD allows soft start and reduces wear and tear of fan belts, bearings.
The power requirement of cooling tower fan varies as a cube of its speed. The VFD allows the
cooling tower fan to operate at whatever speed is required to meet the cooing needs. Typically
the fan runs at only 40 - 70% of design speed, thus saving 50% or more of the energy
consumed by fixed speed system. Every time the actual wet bulb temperature is lower than the
design wet bulb, an energy saving potential exists. Changes in the ambient wet bulb
temperatures allow the fan speed to be regulated. Running fan at lower speed reduces energy
consumed by the fan’s motor. The magnitude of this saving potential depends upon wet bulb
temperature fluctuations. In addition to fan energy savings, running fan at lower speed reduces
evaporation rate of treated water.
8.8.6 UCLA Custom Project 6. Variable Speed Drives on Pumps
This project considers installing Variable Frequency drives (VFD) on HVAC centrifugal pumps to
vary pump speed by varying electrical frequency input to pump motor in relation to system
heating and cooling load. This project focuses on pumps which run at fixed speed and do not
alter flow of water in accordance with the HVAC system load requirements. In other words these
pumps run at design speed (100%) independent of system characteristics wasting pumping
energy and imposing electric demand charges on the campus. Generally pumps are designed
to run at rated speed to meet the peak HVAC system demand which occurs only few hours per
year. Rest of the time, due to the diversity in the system, the system does not require pump to
deliver design flow (since the peak for each zone does not occur at the same time). Therefore
system requires less flow than the design flow providing an opportunity to save pump energy by
reducing pump speed to meet reduced system flow requirements. Speed reduction results in a
more significant energy reduction. The larger the flow reduction from the designed operating
point, the larger the energy savings. Since flow rate is directly proportional to pump speed and
the differential pressure is directly proportional to the square of the pump speed, power usage is
directly proportional to the cube of the pump speed. For example, reducing speed by 50%
requires only 12.5% of the power needed at full speed.
The project involves installing Variable Speed Drives on pumps and differential pressure
transmitter in HVAC loop to control and monitor pump speed. Pump VFD and differential
pressure transmitter will be connected to existing building energy management system.
Also existing bypass valves in loop will be disabled for successful implementation of this ECM.
This ECM assumes that existing pump motors are inverter duty type.
2413.01/Reports/UC SEP Final Report – UCLA.doc 8-20 December 31, 2008
Newcomb | Anderson | McCormick