What is the contact factor for heating coil, if td1 = temperature at entry, t2 = temperature at exit and td3 = coil temperature?

What is the contact factor for heating coil, if td1 = temperature at entry, t2 = temperature at exit and td3 = coil temperature? Correct Answer td2 – td1 / td3 – td1

Contact factor or efficiency of the coil is 1 – By-pass factor. So, for heating coil, ηH = 1 – BPF = 1 – = td2 – td1 / td3 – td1.

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The bypass factor (B.P.F.) in case of sensible heating of air is (Where td1 = Dry bulb temperature of air entering the heating coil, td2 = Dry bulb temperature of air leaving the heating coil and td3 = Dry bulb temperature of heating coil)