Question 14.9: Simplified Part-Load Performance Model Find the part-load CO......

The chiller COP at part load COP_{part-load} is given by

COP_{part-load} = \frac{\dot{Q}_{cool}}{\dot{W}_{comp}}          (14.35)

The denominator of Equation 14.35 can be found from Equation 14.32 at given levels of cooling load \dot{Q}_{cool}. Specifically, the expression for COP_{part-load} is found to be:

\dot{W}_{comp} = \frac{\dot{Q}^{*}_{cool}}{COP^{*}} [a + b(PLR) + c(PLR)^{2}]          0.1 \lt PLR \leq 1.0         (14.32)

COP_{part-load} = \frac{COP^{*} \times PLR }{a  +  b \times PLR  +  c \times PLR^{2}}             (14.36)

Table 14.11 assembles the results, and Figure 14.23 shows the COP as a function of PLR.
Comments
The performance of this chiller is relatively uniform in the PLR range between 0.3 and 1.0 in contrast to boilers and furnaces described in Chapter 15 where efficiency drops strongly for PLR < 1. Note that the COP at part load appears to be larger than at full load. This is an artifact of using PLR coefficients based on constant evaporator and condenser temperatures. The reader should be aware that the part-load data used in this example are not universally applicable to this generic chiller type. Manufacturer’s data must be used for reliable assessments of part-load performance.
When several chillers of different sizes are available in a building, part-load information can be used to determine which chillers should be operating (Austin, 1991). Proper sequencing of chillers to minimize energy use is the job of the control system, discussed in Chapter 21.