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The design operating conditions were a chiller-module size of 600 tons of cooling capacity, an evaporator design based on 1,200 gpm of chilled water cooled from 54°F to 42°F, and a condenser design based on 85°F condenser water available at a flow rate capable of producing a 12°F rise in the condenser-water stream.

Only full-load conditions were compared. For each chiller, the following were considered:

• Input energy -- the electricity, steam, or natural gas used to power the chiller and auxiliary equipment (e.g., pumps, cooling tower) or the natural gas used to power an engine creating electricity to power the chiller and auxiliary equipment.

• Recovered heat -- heat recovered from the condenser-water stream or the engine/generator producing the input electricity.

Input energy and recovered heat were determined using manufacturer data. Cooling-tower-fan power is the catalogued power needed for heat rejection. For absorption chillers, input energy included catalogued power needs for solution, refrigerant, and vacuum pumps. For chilled-water and condenser-water pumps, chiller input-power contribution was calculated based on catalogued evaporator or condenser head loss and the following formula:

Pump power (kw) = (gpm x ΔH x 0.746) �÷ (3,960 x η_pump x η_motor)

where:

ΔH = evaporator or condenser head loss (feet) catalogued by chiller manufacturer

η_pump = pump efficiency (70 percent assumed)

η_motor = motor efficiency (90 to 93 percent assumed, depending on motor size)

The catalogued values for each of the chiller options are given in Table 1.

High-efficiency electric centrifugal chiller

Figure 1 shows the pounds of CO₂ resulting from the production of 1 ton-hr of cooling by a high-efficiency electric centrifugal chiller. Power for the chiller and auxiliary equipment is on the left, with cooling output on the right and rejected heat above. (For clarity, chiller ton-hours produced is symbolized by an arrow leaving the process; more accurately, this is heat energy removed from the chilled-water system by the chiller.) This chiller is the yardstick by which the others will be compared.

Standard-efficiency electric centrifugal chiller

Figure 2 shows the pounds of CO₂ resulting from the production of 1 ton-hr of cooling by a standard-efficiency electric centrifugal chiller. Six-percent-higher kilowatts per ton, 6-percent-higher emissions ¡ª no surprise here.