Heat Transfer and the 2nd Law of Thermodynamics

[aznkid310]

Homework Statement

Heat is transferred into saturated liquid water until it is saturated vapor. The water remains at 300 C throughout the process.

a. What is the initial pressure?

b. What is the final pressure?

c. How much heat is transferred per kg of water?

I'm having trouble with a and b. I assume i need to use the steam and compressed liquid tables, but I am not sure how to use this given info to get it.

Homework Equations

part c: energy balance gives Q/m = u1 - u2

Q = heat transfer
u = specific internal energy

I can simply use tables to find the u values once parts a and b are found.

The Attempt at a Solution

 

[Astronuc]

What is the significance of "saturated liquid water until it is saturated vapor", i.e. this is just a change of phase from pure liquid to pure vapor.

Take a look at saturated steam tables.

 

[aznkid310]

Wouldnt i also need to know the pressure?

 

[stewartcs]

Wouldnt i also need to know the pressure?

What must the pressure be if the water is at a saturation point?

CS

 

[aznkid310]

looking at the saturated liquid-vapor table, is it 85.81 bar?

If so, it also gives values for entropy, enthalpy... for saturated liquid and vapor. I could just use those values right?

 

[stewartcs]

looking at the saturated liquid-vapor table, is it 85.81 bar?

If so, it also gives values for entropy, enthalpy... for saturated liquid and vapor. I could just use those values right?

Since I don't have your table I can't say. However, if the value listed for 300 C saturated water is 85.81 bar then I would say yes. Essentially you will just read it off the table since the problem tells you it is saturated. You'll need to use the super-heated vapor table for part b.

CS

 

[aznkid310]

It says saturated liquid vapor mixture.

When I am looking at the sat. vapor tables, there are a wide variety of pressures. How do i know which to use?

 

[stewartcs]

It says saturated liquid vapor mixture.

When I am looking at the sat. vapor tables, there are a wide variety of pressures. How do i know which to use?

Most superheated vapor tables list the saturation temperature as well. So just find the one with the pressure that corresponds to 300 C and that will be the saturation pressure of the superheated vapor. Note: you may have to interpolate if 300 C is not listed.

The pressure should be around 8.5 MPa for 300 C superheated vapor based on my tables.

T= 295.01 C, P = 8.0 MPa

and

T = 303.35 C, P = 9.0 MPa

So just interpolate to find 300 C exactly.

Hope this helps.

CS

 

[aznkid310]

I have found similar temperatures: 295.06 and 311.06, but we can't interpolate directly between sets of tables, right?

So should I, say, find specific volume v in one set (at 295.09 C for example), do the same for the other, then interpolate these v and T values to find the actual v at 300C so that I can use the ideal gas equation?

 

[stewartcs]

I have found similar temperatures: 295.06 and 311.06, but we can't interpolate directly between sets of tables, right?

Sure you can.

CS