- #1
SweetBabyLou
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Hello all,
Its review time again for another Thermodynamics midterm. As such, I have a practice exam to try for optional extra review work. I've come across a problem that I'm somewhat stumped on. I've tried the problem, but I feel as though I've made too many assumptions in trying to solve the problem. Here's what I got
Air, an ideal gas, with temperature-dependent heat capacities, is to be compressed from 100 kPa to 1500 kPa. The air enters the compressor at a temperature of 300K, and the compressor has an isentropic efficiency of 85%.
How much power is required to operate the compressor adiabatically using an inlet air flow rate of 2.0 m3/s? (Calculate Power in kW)
p1 = 100 kPa
T1 = 300 K
p2 = 1500 kPa
T2 = ?
ηisentropic=.85
min = 2.0m3/s
To start the equation off, I set up the relation p2/p1 = pr2/pr1
The problem gave us values for both p1 and p2.
I looked up the value of temperature-dependent pr1 which turned out to be:
pr1 = 1.3860
While I was looking at the air tables, I also took the enthalpy value at 300K, which turned out to be:
h1 = 300.19
I plugged pr1 into the pressure/pressure-reduced relationship, found pr2 to come out to 20.79, roughly the 20.64 found at 640K on the same air table. From this, I pulled the secong enthalpy value:
h2s = 649.22
From here, I try to solve for h2 I set up the equation for isentropic efficiency:
η = (h1 - h2)/(h1 - h2s)
Solving for h2, I get:
h2 = 596.8655
Finally, I solve for power.
The equation I use is:
[itex]\dot{Q}[/itex] - [itex]\dot{W}[/itex] + (mi(hi)-me(he)
**I canceled out the kinetic and potential energy terms of the original equation**
Since the process is adiabatic, I eliminate the [itex]\dot{Q}[/itex].
Here's where I think I may have gone wrong
I then assume that the process is running at steady state, thus mi = me = 2.0m3/s
Plugging 2 in for m and the rest of the values in for hi, he, I find that [itex]\dot{W}[/itex] = -593.351kW
...this CAN'T be right. I'm quite possibly doing this whole problem wrong. Thus, I turn to the almighty physicsforums.com for help. I know this problem and my solution might be long-winded, so I appreciate all patience with this problem in advance. I also DEFINITELY appreciate any and all help/advice sent my way.
Thank you for your time
-Lou
Its review time again for another Thermodynamics midterm. As such, I have a practice exam to try for optional extra review work. I've come across a problem that I'm somewhat stumped on. I've tried the problem, but I feel as though I've made too many assumptions in trying to solve the problem. Here's what I got
Homework Statement
Air, an ideal gas, with temperature-dependent heat capacities, is to be compressed from 100 kPa to 1500 kPa. The air enters the compressor at a temperature of 300K, and the compressor has an isentropic efficiency of 85%.
How much power is required to operate the compressor adiabatically using an inlet air flow rate of 2.0 m3/s? (Calculate Power in kW)
Homework Equations
p1 = 100 kPa
T1 = 300 K
p2 = 1500 kPa
T2 = ?
ηisentropic=.85
min = 2.0m3/s
The Attempt at a Solution
To start the equation off, I set up the relation p2/p1 = pr2/pr1
The problem gave us values for both p1 and p2.
I looked up the value of temperature-dependent pr1 which turned out to be:
pr1 = 1.3860
While I was looking at the air tables, I also took the enthalpy value at 300K, which turned out to be:
h1 = 300.19
I plugged pr1 into the pressure/pressure-reduced relationship, found pr2 to come out to 20.79, roughly the 20.64 found at 640K on the same air table. From this, I pulled the secong enthalpy value:
h2s = 649.22
From here, I try to solve for h2 I set up the equation for isentropic efficiency:
η = (h1 - h2)/(h1 - h2s)
Solving for h2, I get:
h2 = 596.8655
Finally, I solve for power.
The equation I use is:
[itex]\dot{Q}[/itex] - [itex]\dot{W}[/itex] + (mi(hi)-me(he)
**I canceled out the kinetic and potential energy terms of the original equation**
Since the process is adiabatic, I eliminate the [itex]\dot{Q}[/itex].
Here's where I think I may have gone wrong
I then assume that the process is running at steady state, thus mi = me = 2.0m3/s
Plugging 2 in for m and the rest of the values in for hi, he, I find that [itex]\dot{W}[/itex] = -593.351kW
...this CAN'T be right. I'm quite possibly doing this whole problem wrong. Thus, I turn to the almighty physicsforums.com for help. I know this problem and my solution might be long-winded, so I appreciate all patience with this problem in advance. I also DEFINITELY appreciate any and all help/advice sent my way.
Thank you for your time
-Lou