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