The Definition

Effectiveness epsilon = actual heat transferred / maximum possible heat transferred. Maximum possible = C_min x (T_hot_in - T_cold_in), where C_min is the smaller heat-capacity rate.

Worked Example

Exhaust 20 C, supply inlet -2 C, 70 percent effectiveness. Recovered temperature rise = 0.70 x (20 - (-2)) = 15.4 K, so supply leaves at about 13.4 C.

Check the Power

Q = m_dot x cp x 15.4. For 10,000 m3/h (3.33 kg/s): Q = 3.33 x 1.006 x 15.4 is about 51.6 kW.

Compare to Rating

Always compare your hand calc to the manufacturer's EN 308 or AHRI 1060 rating at the same face velocity.