What is the equation of effectiveness for heat recovery unit ?

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

Example - A Sensible Heat Recovery System

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)