I simulated one electric process heater with flow simulation, air will pumped into inlet of heater at constant temp and pressure, some wattage will be generated via heating elements, the outlet face will be  goal, after simulation is finished, I found the following calculation formula is not correct? The formula:  (specific heat X mass flow X (Outlet temp-inlet temp) is not match  generate heat by heating elements?  Could you tell me the reasons? 

BTW, the inlet and outlet is same size.Outlet temperature is bulk AV temperature

Wall condition:

     default outer wall thermal condition: adiabatic wall

      default wall radiative surface: No-radiating surface

     default outer wall radiative surface: No-raditing surface