Retrotec USACE - Manual (Page 378)

J2 ENERGY & PROCESS ASSESSMENT PROTOCOL
The pump energy savings ratio for the condenser water pump is slightly
different than it is for the primary chilled water pumps, as the condenser water
pumps have a fi xed minimum lift required to move water up over the top of the
cooling towers, and there is no static regain for water falling through a cooling
tower as there is with primary chilled water pumps operating in a closed loop.
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.3 Cooling Load–Based Optimization
Cooling load–based optimization strategies should be incorporated into the
control routines for the chilled water supply (CHWS) temperature setpoint,
the chilled water differential pressure setpoint, and the condenser water tem-
perature setpoint (for water cooled equipment) to reduce chiller and pump-
ing system energy waste while improving the control system response at the
cooling loads. The load-based optimization strategies should also incorporate
chiller staging routines that take best advantage of the installed equipment.
Self-tuning loops should be incorporated that will adjust the system to accom-
modate continuous changes in the load and weather.
One such system that incorporates these concepts is the Load-Based Op-
timization System (LOBOS), which takes data from the air handling units that
are controlled from the same direct digital control (DDC) system as the chiller
plant and uses this data to raise the chilled water supply temperature as high
as possible to save chiller energy, while lowering the chilled water system dif-
ferential pressure as much as possible to save pumping system energy.
These resets are accomplished while still maintaining the required supply
air temperatures at the air handling units—feedback from the cooling loads is
required for best system performance.
The system also resets the condenser water supply temperature setpoint to
reduce chiller energy by lowering the condensing pressure of the refrigerant
without wasting cooling tower fan energy.
Increasing the chilled water supply temperature (within limitations of
reason, comfort, and humidity control) can reduce energy consumption on
constant speed centrifugal chillers by up to 20%, and by up to 40% or more on
variable speed centrifugal chillers.
Reducing the chilled water system differential pressure (within limitations
of reason, comfort, and humidity control) can reduce energy consumption on
variable speed chilled water pumping systems by up to 50% at light loads.
Differential pressure is what forces cold water from the chiller plant
through the chilled water distribution piping system to the air conditioning
systems to provide cooling to the end use loads. A system does not need the
same differential pressure when it is 21.1 °C (70 °F) and dry outside as it does
when it is 32.2 °C (90 °F) and raining outside.