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Engineering ToolBox > Heat-Recovery
A heat-recovery system captures a building exhaust air and reuses some of the energy to precondition the make up air before supplying it to the building.
Air-to-air heat-recovery systems for comfort-to-comfort applications can in general be categorized as
A sensible heat-recovery systems will transfer sensible (dry-bulb) heat from the exhaust air to the supply air entering the building. Rotary wheel heat exchangers and cross flow heat exchangers are some of the common designs used in typical comfort-to-comfort sensible heat-recovery applications.
The amount of heat transferred can be expressed as
H = 1.08 q (t2 - t1) η (1)
where
H = heat transferred (Btu/h)
q = quantity of airflow (cfm)
1.08 = a constant for sensible heat equations
t1 = air temperature of outside air before the heat recovery unit (oF)
t2 = air temperature of outside air after the heat recovery unit (oF)
η = heat-recover efficiency
The outside air temperature is 90ºF and the exhaust air temperature is 75ºF. The system operates at 40,000 cfm at 73 percent efficiency.
The sensible heat-recovery can be expressed as
H = 1.08 40,000(cfm) (90 - 75)(ºF) 0.73
= 473,040 Btu/h
The calculator below can be used to calculate the sensible heat recovery where the air flow, outside and inside temperature and recovery efficiency are known.
For total enthalpy heat-recovery systems the heat recovered can be expressed as
H = 4.5 q dh η (2)
where
4.5 = a constant for total heat equations
h1 = air enthalpy of outside air before the heat recovery unit (Btu/lbºF)
h2 = air enthalpy of outside air after the heat recovery unit (Btu/lbºF)
The outside air wet-bulb temperature is 70ºF (enthalpy 34.1 Btu/lbºF) and the exhaust-air wet-bulb temperature is 60ºF (enthalpy 26.5 Btu/lbºF). The system operates at 40,000 cfm at 73 percent efficiency.
The total heat-recovery can be expressed like
H = 4.5 40,000 ( cfm) (34.1 - 26.5)(Btu/lbºF) 0. 73
= 998,640 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.