What is the exit steam velocity?

  • #1
Nemo's
69
0

Homework Statement


Steam, at 15 bar and 280oC, enters a nozzle with an initial velocity of 125 m/s. The steam enthalpy at the exit section is 2800 kJ/kg and the heat loss is 25 kJ/kg. What is the exit steam velocity?.

Homework Equations


1)Qin - Qout = m [h2-h1+((c22-c12))/2]
2)h = Pv + u
3)A1c1/ρ1 = A2c2/ρ2

The Attempt at a Solution


0-25k = m((2800-h1)+-125^2/2)

Steam is not an ideal gas so I can't use Pv=mRT. I have two unknowns and I don't know where to start. I hope you can help.
 

Answers and Replies

  • #2
22,063
5,027
For steam at 15 bar and 280 C, from your steam tables, what is the specific volume and the specific enthalpy?
 
  • #3
SteamKing
Staff Emeritus
Science Advisor
Homework Helper
12,798
1,670

Homework Statement


Steam, at 15 bar and 280oC, enters a nozzle with an initial velocity of 125 m/s. The steam enthalpy at the exit section is 2800 kJ/kg and the heat loss is 25 kJ/kg. What is the exit steam velocity?.

Homework Equations


1)Qin - Qout = m [h2-h1+((c22-c12))/2]
2)h = Pv + u
3)A1c1/ρ1 = A2c2/ρ2

The Attempt at a Solution


0-25k = m((2800-h1)+-125^2/2)

Steam is not an ideal gas so I can't use Pv=mRT. I have two unknowns and I don't know where to start. I hope you can help.
1. What's the enthalpy of the steam in the nozzle inlet?
2. What have you done with the heat loss of 25 kJ/kg which occurs while the steam passes thru the nozzle?
 
  • #4
Nemo's
69
0
For steam at 15 bar and 280 C, from your steam tables, what is the specific volume and the specific enthalpy?
Thanks for letting me pay attention to the tables.
h1=2789.88
v=0.132
Now if I can only get the area I can use m= A*c1/ρ1 to get m and then substitute in 1 to get c2.
 
  • #5
22,063
5,027
Thanks for letting me pay attention to the tables.
h1=2789.88
v=0.132
Now if I can only get the area I can use m= A*c1/ρ1 to get m and then substitute in 1 to get c2.
You don't need to determine m or A, because you are going to solving the problem per unit mass. The 25 kJ/kg is already per unit mass. So the m should not be in your equation.
 
  • #6
Nemo's
69
0
You don't need to determine m or A, because you are going to solving the problem per unit mass. The 25 kJ/kg is already per unit mass. So the m should not be in your equation.
O.k thanks a lot
 
  • #7
Nemo's
69
0
I solved it . Thank you anyway
 

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