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Effectiveness = ( M oa / M min) * {(T1 - T2) / (T1 - T3)}
Where:
M oa = mass flow rate of outdoor air (fresh air)
M min = Smaller mass flow rate ( fresh air or exhaust air which is smaller)
T1 = outdoor air temperature.
T2 = air temperature after basing recovery unit.
T3 = exhaust air temperature before basing recovary unit.
μe = (h2 - h1) / (h3 - h1) (3)
where
μe = enthalpy transfer efficiency
h1 = enthalpy outside make-up air before the heat exchanger (kg/kg)
h2 = enthalpy outside make-up air after the heat exchanger (kg/kg)
h3 = enthalpy outlet air before the heat exchanger (kg/kg)
if you want that for air ,so will be as the following :
H = 1.08 q (t2 - t1) η
where
H = heat transferred (Btu/h)
q = quantity of airflow (cfm)
1.08 = a constant for sensible heat equations
t1 = air temperature of the exhaust 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 oF) - (75 oF)) 0.73
= 473,040 (Btu/h)
