Chem Eng: Calculate Heat Req for Temp Change from 25°C to 250°C

In summary, the conversation discusses the process of heating air from ambient temperature to 250 °C in a combustion furnace. The specific enthalpy change is calculated to be 3745 J/mol and the exit pressure is 150 kPa with a flow rate of 1.3 m3/min. Using the ideal gas law, the exit molar density is determined to be 34.387 mol/m^3 and the flow rate in moles/min is calculated to be 44.7031 mol/min. The amount of heat being added per minute is also discussed. The question of whether the specific enthalpy for the entering flow needs to be calculated is posed.
  • #1
kp3legend
8
0

Homework Statement



Prior to entering a combustion furnace, air is heated up from ambient temperature (25 °C) to 250 °C. This change in temperature causes the specific enthalpy to change to 3745 J/mol. Assume that the pressure of the air leaving the heater is 150 kPa and the flow rate is 1.3 m3 /min. Assuming ideal behaviour, calculate the heat required in units of kW.

Homework Equations



PV=nRT
H=U+PV
dH=Q

The Attempt at a Solution


I can find P1 and V1 from the equation T1/T2=P1/P2 and V1/V2=P2/P1
P1=85.487 kpa
V1=136.863m^3
 
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  • #2
Since you know the exit temperature and exit pressure, you should be able to determine the exit molar density using the ideal gas law. What is the exit molar density (moles/m^3)?
 
  • #3
It will be 34.487 mol/m^3
 
  • #4
kp3legend said:
It will be 34.487 mol/m^3
If the exit molar density is 34.387 moles/m^3, and the exit flow rate is 1.3 m^3/min, what is the flow rate in moles/min?
 
  • #5
It should be 1.3 x 34.387 = 44.7031 mol/min. I guess
 
  • #6
kp3legend said:
It should be 1.3 x 34.387 = 44.7031 mol/min. I guess

In the molar flow rate is 44.7 mol/min and the change in enthalpy per mole is 3745 Joules, how much heat is being added per min? How much heat is being added per second?

Chet
 
  • #7
Omg thank you very much now I get it . but do you think I have to find H for the entering flow ?
 
  • #8
kp3legend said:
Omg thank you very much now I get it . but do you think I have to find H for the entering flow ?
What do you think?
 
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FAQ: Chem Eng: Calculate Heat Req for Temp Change from 25°C to 250°C

What is the formula for calculating the heat required for a temperature change?

The formula for calculating heat required for a temperature change is Q = mcΔT, where Q is the amount of heat required, m is the mass of the substance, c is the specific heat capacity of the substance, and ΔT is the change in temperature.

What units are used for the variables in the heat calculation formula?

The units used for the variables in the heat calculation formula are:Q (heat) in joules (J)m (mass) in kilograms (kg)c (specific heat capacity) in joules per kilogram per degree Celsius (J/kg°C)ΔT (change in temperature) in degrees Celsius (°C)

How do I calculate the specific heat capacity of a substance?

The specific heat capacity can be calculated by dividing the heat required (Q) by the mass (m) and the change in temperature (ΔT). So, c = Q/(mΔT). This value can also be found in reference tables or through experimentation.

What is the difference between specific heat capacity and heat capacity?

Specific heat capacity (c) is the amount of heat required to raise the temperature of 1 kilogram of a substance by 1 degree Celsius, while heat capacity (C) is the amount of heat required to raise the temperature of a given mass of a substance by a certain amount. In other words, specific heat capacity is an intensive property while heat capacity is an extensive property.

Can the heat required for a temperature change be negative?

Yes, the heat required can be negative if the temperature change is negative. This means that the substance is losing heat and its temperature is decreasing instead of increasing. However, the absolute value of the heat required will remain the same regardless of the sign.

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