VSD Centrifugal Chiller Performance EIR-f(PLR, dT) and EIR-f(t1, t2)

1 post / 0 new

Hello Equest users,

I have a question regarding variable speed centrifugal chillers and
generating the EIR performance curves.

The DOE-2 help information regarding the EIR-f(PLR) curve explains that it
is equally important to include the "dT" term when considering variable
speed chillers due to the relationship between the impeller speed and the
suction/discharge pressure (or temperature) differential. This affect can
be seen when comparing a VFD chiller with condenser water temperature reset
to a constant speed chiller with condenser water reset. Without the
condenser water reset, there is little efficiency gain from the variable
speed compressor.

The second EIR performance curve is the EIR-f(T evap leaving, T cond
entering) or EIR-FT. This curve adjusts the EIR based on the leaving
chilled water temperature and the entering condenser water temperature.

It seems that if the "dT" term is included in the EIR-f(PLR) curve, the
efficiency increase due to a reduced condenser water temperature is being
double counted since Equest calculates the chiller demand by multiplying the
design EIR by these two results together (plus the capacity curves). Can
anyone provide some insight on this topic? What is the difference between
the affects of the "dT" in the EIR-f(PLR) and the EIR-f(T evap leaving, T
cond entering) on the design EIR?

Thanks for your thoughts,

John T. Forester, LEED AP

John Forester's picture
Joined: 2011-09-30
Reputation: 400