Knowledgebase: Central Plant > Chillers
How to calculate Generator HIR for Chiller:Absorption:Indirect
Posted by Michael J. Witte on 05 August 2010 10:05 PM
Question: Please explain how to calculate the Generator Heat Input Function of Part Load Ratio Curve (GenHir curve) for Chiller:Absorption:Indirect.

Answer: Referring to the equations in the Engineering Reference section "Indirect Absorption Chiller" starting on pdf p. 487 (v5.0), with some additional notes:

PLR: The part-load ratio of the indirect absoprtion chiller’s evaporator is simply the actual cooling
effect required (load) divided by the maximum cooling effect available.

PLR = Qevap/Qevapmax

where
PLR = part-load ratio of chiller evaporator
Qevap = chiller evaporator operating capacity [W] (current load on the chiller)
Qevapmax = chiller evaporator available capacity [W] (Nominal capacity times temperature correction curves)

Qgenerator = GeneratorHIR * Qevapmax * GenfCondT * GenfEvapT * CyclingFrac

So, GeneratorHIR = Qgenerator / (Qevapmax * GenfCondT * GenfEvapT * CyclingFrac)

If operating at nominal condenser and evaporator temepratures such that GenfCondT = 1 and GenfEvapT = 1, and if operating above PLRmin such that CyclingFrac = 1, then:

GeneratorHIR = Qgenerator / Qevapmax

Since PLR = Qevap/Qevapmax, then Qevapmax = Qevap/PLR

GeneratorHIR = Qgenerator*PLR/Qevap

or, stated another way GeneratorHIR = PLR/COP

So, GeneratorHIR(fPLR) is *not* normalized by a nominal COP or HIR. When PLR = 1.0, GeneratorHIR = 1/COP.

Also, because the denominator is Qevapmax, GeneratorHIR approaches zero as PLR approaches zero.

For example, for a perfect chiller which has constant efficiency at all PLRs, with a nominal COP of 0.5 at full load at rated temperatures, the GeneratorHIR(fPLR) values would be:

PLR = 0.0, GeneratorHIR = 0.0 (PLR/COP = 0.0/0.5 = 0.0)
PLR = 0.2, GeneratorHIR = 0.4 (PLR/COP = 0.2/0.5 = 0.4)
PLR = 0.5, GeneratorHIR = 1.0 (PLR/COP = 0.5/0.5 = 1.0)
PLR = 1.0, GeneratorHIR = 2.0 (PLR/COP = 1.0/0.5 = 2.0)

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