What Is the Mass Flow Rate at Exit in a Steady Flow System?

[madzior771]

Homework Statement

Air flows steadily into a well isnulated piping junction through the two pipes and is heated by an electric resistor at uknown rate befoe exiting through the pipe. Pressure renaubs approcimetaly constant
p=0.1 MPa, first inlet: volumetric flow rate = 10m^3/s, t1= 20 C, 2nd inlet: volumetric flow rate = 30m^3/s, t2= 30C. At the outlet: t3= 55C, cross section area = 2m^2. Assume the effects of kinetic and potential energy are negligible and air behaves as a perfect gas iwth Cp= 1,0kJ/(kgK). Find the : mass flow rate at exit, heat rate of electric heater and exite velocity of air.

Homework Equations

The Attempt at a Solution

:
m1+m2=m3
Q-W= m1*ht1 + m2*ht2 - m3*ht3
W=0
Q=m1ht1+m2ht2-(m1+m2)ht3
m=A*ro*velocity
[/B]

 

[Chestermiller]

Homework Statement

Air flows steadily into a well isnulated piping junction through the two pipes and is heated by an electric resistor at uknown rate befoe exiting through the pipe. Pressure renaubs approcimetaly constant
p=0.1 MPa, first inlet: volumetric flow rate = 10m^3/s, t1= 20 C, 2nd inlet: volumetric flow rate = 30m^3/s, t2= 30C. At the outlet: t3= 55C, cross section area = 2m^2. Assume the effects of kinetic and potential energy are negligible and air behaves as a perfect gas iwth Cp= 1,0kJ/(kgK). Find the : mass flow rate at exit, heat rate of electric heater and exite velocity of air.

Homework Equations

The Attempt at a Solution

:
m1+m2=m3
Q-W= m1*ht1 + m2*ht2 - m3*ht3
W=0
Q=m1ht1+m2ht2-(m1+m2)ht3
m=A*ro*velocity[/B]

This looks OK so far, except for the signs of the right hand side...they are all opposite to what they should be. Please show some more detail.