Critical parameters for fair comparisons call for a number
of input assumptions. Some of the data may be readily available, some not so
available. As many of the following factors as possible should be considered
in conducting a proper evaluation:
1. Electric, steam and fuel rate schedules, including demand and energy
charges segregated by applicable seasonal or time-of-use criteria and
appropriate fuel adjustment charges.
2. Chiller type, size and full load efficiency: for electric chillers
consider the kW per ton; for natural gas fueled check the Btu per ton-hour,
or the steam pressure at the unit for steam chillers.
3. Consider the size cooling tower required to reject the building's heat
plus the work added to do the cooling that ends up in the chiller's
condenser.
4. The chiller unit electric auxiliaries: for electric chillers these are
included in the kW per ton; for non-electric chillers this kW per ton should
include all the solution, refrigerant, jacket water, lube oil or other pumps
(as applicable) and the control power.
5. The chiller system electric auxiliaries in kW per ton These include
the condenser water pumps and the cooling tower fans plus any added fans or
other power use's applicable to one type chiller but not another.
6. The costs (per 1,000 gallons) to acquire makeup water for the cooling
tower, to chemically or otherwise treat this water, and to dispose of the
tower overflow and the blowdown needed to maintain an acceptable
concentration of dissolved solids.
7. The projected annual operating hours of the chiller and the load
profile it is designed to serve. For detailed analyses, the chiller's
operating schedule including the utility's seasonal and on-peak time
definitions must be taken into account. For less detail analyses, the
concept of Equivalent Full Load Hours and Integrated Part Load Value can be
used.
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