12 Jan 98
Sample Economic Comparisons
C-1. OBJECTIVE. The objective of this Appendix is to perform a sample
economical analysis to determine the optimum efficiency to specify for a
specific chiller capacity. The type and size of chiller to be evaluated
is an electrically-driven, water-cooled 200 ton centrifugal type unit.
First cost and efficiency values used in the calculations and presented
in Table C1 below are approximations. This data should not be used in
any other economic comparisons.
Table C1. Typical Data for a 200 Ton
Centrifugal Type Chiller
C-2. CALCULATIONS. Two example efficiency comparison calculations are
provided in this Appendix. Example 1 calculations, as presented on
pages C-4 through C-9, are based on energy costs without any demand
charge factors. Example 2 calculations, as presented on pages C-10
through C-15, are based on energy costs which include demand charge
factors. The terms and definitions used in the calculations are
explained in the following paragraph. The calculations provide the most
economical solutions for variable energy costs and variable chiller
C-3. DEFINITIONS. The following is a list of terms and definitions
used throughout the calculations.
a. Energy Costs. Optimum chiller efficiencies were determined using
energy costs equal to 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08 and 0.09
$/kWh. These energy costs are typical values for what can be
encountered across the country.
b. Equivalent Full-Load Hours (EFLH). Over a year's period a
chiller will operate at partial load the majority of the time. Full-
load conditions are experienced only a small percentage of the time.
For this exercise, the only way to estimate a chiller's annual energy
usage is to estimate the chiller's annual full-load run hours or EFLH.
For most typical chiller applications the EFLH can be approximated to be
equal to 50% of a chiller's estimated run hours over a year's period
(e.g., a chiller that is estimated to run approximately 8000 hours/year
will have an EFLH of 4000). Optimum chiller efficiencies were
determined in the attached calculations using EFLH equal to 1000, 2000,
3000, and 4000. Note: Instead of using the EFLH method to determine
chiller energy usage, the ideal way to determine actual energy usage
would be to model the installation using an annual energy load
calculation program such as BLAST or Trane's Trace 600.