Waste Heat Fired Absorption Chillers
Waste steam from a cogeneration system obviously produces the same level of cooling as boiler generated steam, Low pressure waste steam sources (say 14 psig) typically require 18-20 pounds of steam per hour to produce one ton of cooling in a single-stage absorption chiller. That performance improves to 10-12 pounds per ton-hour of steam when steam pressures are in the 50 to 130 psig range and used in a 2-stage (double effect) absorption chiller.
Steam absorption chillers are nominally rated as follows:
- Single stage: 9 psig at generator flange
- Two stage: 114 psig steam input pressure.
Capacity ratings are decreased as steam pressure drops below nominal. For example, a nominal 100-ton unit's capacity will drop to 84 tons with 78.5 psig steam.
Direct-fired absorption chillers can often be modified to accept hot air or exhaust from a gas turbine or engine. Performance is almost totally dependent upon air temperature, For example, waste heat air temperatures °F or higher offer performance similar to direct-fired absorption chillers where every 13,000 Btu of heat recovered produces one ton of cooling. When calculating heat recovery, remember to assume waste heat leaving the absorption chiller at 375° to 400°F (this means the absorption chiller will not reclaim all of the waste heat potential).
For exhaust gas heat recovery
Chilling capacity (tons) = m x (Tg - 375)
Heating capacity (BTUH) = m x (Tg - 375) x 0.257
where m = exhaust gas flow rate in pounds per hour
Tg = exhaust gas inlet temperature in °F
40,950 = cooling constant representing average gas specific heat, interconnect efficiency, cooling COP and the conversion from BTUH to tons
0.257 = heating constant representing average gas specific heat and the interconnect efficiency
375 = minimum temperature of exhaust gas leaving chiller in °F.