Calculate Thermal Power Output of Electric Furnace

[Windseaker]

1. Heating element in an electric furnace draws 65amps on a 240v single phase circuit witha .98 ele.to thermal eff. The volume flow rate of air across the element is 1200CFM. the incoming air temp to the element is 65degree and the element temp is 75degree.
Whats the thermal power output in Btu/hr.?



2. Q=1.08xCFMxdeltaT and eff=Power(out)/Power(in)



3. Q=1.08(1200CFM)(72-65), Q= 9072 Btu/hr
Power(out)=(eff)(Power(in) P(out)= (.98)(9072) = 8890btu/hr

The problem is delta T , I am assuming 72degree the temp in but it says incoming temp is 65degree?? I am missing somthing here either the real delta T or Power In--somthinks not right??

 

[Andrew Mason]

1. Heating element in an electric furnace draws 65amps on a 240v single phase circuit witha .98 ele.to thermal eff. The volume flow rate of air across the element is 1200CFM. the incoming air temp to the element is 65degree and the element temp is 75degree.
Whats the thermal power output in Btu/hr.?
2. Q=1.08xCFMxdeltaT and eff=Power(out)/Power(in)
3. Q=1.08(1200CFM)(72-65), Q= 9072 Btu/hr
Power(out)=(eff)(Power(in) P(out)= (.98)(9072) = 8890btu/hr

The problem is delta T , I am assuming 72degree the temp in but it says incoming temp is 65degree?? I am missing somthing here either the real delta T or Power In--somthinks not right??

The thermal power output is $\eta \times P_{in} = .98 * 65 * 240 = 15.288 Kw = 52165 Btu/Hr.$

This question does not make much sense. It is obvious that the temperature of the heating element cannot be 75 degrees F.

AM