For total enthalpy heat-recovery systems the heat recovered can be expressed as
H = 4.5 q (h 2 - h 1 ) η (2)
4.5 = (0.075 lb m /ft 3 ) (60 min/h)
h 1 = air enthalpy of the exhaust air before the heat recovery unit (Btu/lb)
h 2 = air enthalpy of outside air after the heat recovery unit (Btu/lb)
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The outside air wet-bulb temperature is 70ºF ( enthalpy 34 Btu/lb ) and the exhaust-air wet-bulb temperature is 60ºF ( enthalpy 26.4 Btu/lb ). The system operates at 40,000 cfm at 73 percent efficiency.
The total heat-recovery can be expressed like
H = 4.5 (40000 cfm) ((34 Btu/lb) - (26.4 Btu/lb)) 0. 73
= 998640 (Btu/h)
The calculator below can be used to calculate the total (latent and sensible) heat recovery where the air flow, enthalpy of outside and inside air and the recovery efficiency are known.
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