System Economics - Chiller Efficiency
Chiller operating efficiency is the major component in the annual energy cost. In the past energy was cheap and plentiful, and efficiency received little attention. Older chillers can be quite inefficient. In fact, some chiller replacements will payback quite quickly just due to significantly reduced operating cost at the higher efficiency of the new unit. For analysis purposes, chillers are typically compared on the basis of their ARI Standard Rating - Water cooled, using 44°F leaving chilled water and 85°F inlet condenser water.
All chillers require electric power to operate their auxiliaries (solution, refrigerant, and lube pumps, controls, and so on). These energy costs must be included in the economic comparison, as well as the cost of water required for the cooling tower. The chilled water pump consumption of electricity is common to all chillers, so this power input can be either included or omitted since it almost never affects the outcome of the analysis.
Typical Chiller Energy Operating Costs
Electric Chiller kW/ton-hr
| Chiller | New Chiller | Existing | |
| Reciprocating | .78 to .85 | .90-1.2 or higher | |
| Screw | .62 to .75 | .75-.85 or higher | |
| Centrifugal | High | .50 to .62 | NA |
| Moderate | .63 to .70 | .70-.80 or higher | |
= (kW/ton-hr x 3,413 Btuh/kW x 0.92) + 12,000 Btuh/ton
where the 0.92 factor makes an 8% allowance for the losses to ambient.
Heat-Driven Chiller:
| Steam input | HHV input | Heat rejection | |||
| @ Nom. psig |
Btu/ton-hr
|
Btu/ton-hr | Temp.Diff. | ||
| Absorption | |||||
| 1 stage steam | 18 pph | 22,000 | 29,000 | 15°F | |
| 2 stage steam | 10 pph | 12,200 | 22,300 | 10°F | |
| Exhaust Gas Fired (EG) | Varies with EG temp.* | 22,900 | 10°F | ||
| Direct Fired | NA | 12,000 | 22,900 | 10°F | |
| Natural Gas Engine Driven Compressor | |||||
| Reciprocating | NA | 9,300 | 16,900 | 10°F | |
| Rotary Screw | NA | 8,600 | 16,500 | 10°F | |
| Centrifugal | NA | 7,760 | 16,300 | 10°F | |
*Tons Cooling = pph EG flow x (EG temp. - 375) / 40,950
The heat rejection values shown represent the approximate amount of heat that must be rejected to the atmosphere by the cooling tower. This value includes the 12,000 Btu per ton hour of cooling plus the Btu per ton-hour of energy input to the chiller, less an allowance for motor, drive, and radiation losses.Cooling Tower Fans & Pumps
| Cooling Tower Fans | Condenser Water Pump* | |
| Water-cooled Chiller |
kW/ton
|
kW/ton |
| Reciprocating | .083 | .057 |
| Centrifugal | .079 | .048 |
| Absorption 1-stage steam | .138 | .110 |
| Absorption 2-stage (all models) | .113 | .096 |
| Natural Gas Engine | .087 | .054 |
*These figures are based on efficiencies of 0.70 pump and 0.90 motor.
Condenser fan power is typically included in chiller rated input kW in packaged air-cooled units. If data is not available, estimate it at 0.128 kW/ton.Typical Chiller System Makeup Water Operating Cost Parameters
| Chiller Type | Gallons per ton |
| Electric Chillers | 4.0 |
| Absorption 1-stage | 8.0 |
| " 2-stage | 6.2 |
| Natural gas-driven | 4.3 |
The typical cost to acquire and chemically treat incoming water and to dispose of tower bleed-off (blowdown) is $4.00 per 1,000 gallons.
Typical Chiller Unit Auxiliaries
Electric Chillers - unit auxiliary energy included in chiller package rated kW/ton
Heat-activated Chillers - in absence of manufacturer's catalog data, use these approximations:
| Added kW/ton | |||||
| Nat. Gas-Engine Driven | Recip. Compr. | Screw Compr. | Centrifugal Compr. | ||
| 0.040 | 0.033 | 0.014 | |||
| Absorption | 1-Stage Steam | 2-Stage Steam | Direct-Fired | ||
| 0.014 | 0.021 | 0.024 | |||
