Calculating Thermal Energy of Hot Air Flow

[wat681]

haw can i calculate the thermal energy (watt or BTU ) of the hot air (or the exhaust air of the chimney) according to the following :

Temp. : 250 C
hot air flow rate : 11 000 m3/h


thanks

 

[MrMatt2532]

You need to multiply the mass flow rate by the specific internal energy.

Mass flow rate equals the volumetric flow rate times the density, which is a function of pressure and temperature. Specific internal energy is a function of pressure and temperature, or just a function of temperature for an ideal gas.

 

[russ_watters]

You alsoneed to subtract the temp of the outside air.

 

[wat681]

thanks for reply
i got the following formula for the thermal energy :

Flow rate (m3/h) * Density (Kg/m3) * specific energy (KJ/Kg K) * temp. deference (K)

my question , what is the temperature deference ? is it between the hot air and the atmosphere temperature ?
because i don't want to heat this air i Have the hot air at 200 C and want to calculate the thermal energy of it


thanks

 

[MrMatt2532]

If you just want the thermal energy of the exhaust, do what I originally said. You need to multiply the mass flow rate by the specific internal energy [kJ/kg], which is just a function of the temperature for an ideal gas (u=2.5*R*T for a diatomic gas). The temperature you should use is the exhaust temperature.

Now, on the other hand, if you are looking for the heat from combustion, you can apply the first law for an open system and find the Q=m_dot*(h_out-h_in)=m_dot*c_p*(T_out-T_in), where T_in would be the intake temperature, and T_out would be the exhaust temperature and this is likely the temperature difference russ was referring to. But it sounds like that is not what you are looking for, so just refer to my original response.