Heat Transfer and the 2nd Law of Thermodynamics

Click For Summary

Discussion Overview

The discussion revolves around a homework problem involving heat transfer in water transitioning from a saturated liquid to a saturated vapor at a constant temperature of 300°C. Participants are exploring the necessary calculations for initial and final pressures, as well as the heat transferred per kilogram of water, utilizing steam tables.

Discussion Character

  • Homework-related
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant expresses uncertainty about determining the initial and final pressures and suggests using steam and compressed liquid tables.
  • Another participant emphasizes the importance of understanding the phase change from liquid to vapor and recommends consulting saturated steam tables.
  • Several participants discuss the need to know the pressure at saturation and question how to identify the correct pressure from the saturated vapor tables.
  • A participant proposes that the pressure at 300°C saturated water is approximately 85.81 bar, suggesting that values for entropy and enthalpy can be directly read from the tables.
  • Another participant indicates that if the value for 300°C is indeed 85.81 bar, then it can be used, but notes the need for superheated vapor tables for further calculations.
  • There is a discussion about the variety of pressures listed in the saturated vapor tables and how to determine which to use, with a suggestion to interpolate if necessary.
  • One participant mentions finding similar temperatures around 295.06°C and 311.06°C and questions the validity of interpolating directly between different sets of tables.
  • Another participant asserts that interpolation between sets of tables is permissible.

Areas of Agreement / Disagreement

Participants generally agree on the need to use steam tables for the calculations, but there is disagreement regarding the specifics of interpolation and the correct pressures to use for the calculations.

Contextual Notes

Participants express uncertainty about the interpolation process between different sets of tables and the implications of using specific values from those tables. There is also a lack of consensus on the exact pressures corresponding to the given temperature.

aznkid310
Messages
106
Reaction score
1

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

 
Physics news on Phys.org
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.
 
Wouldnt i also need to know the pressure?
 
aznkid310 said:
Wouldnt i also need to know the pressure?

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

CS
 
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?
 
aznkid310 said:
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
 
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?
 
aznkid310 said:
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
 
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?
 
  • #10
aznkid310 said:
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
 

Similar threads

Replies
22
Views
4K
  • · Replies 13 ·
Replies
13
Views
3K
  • · Replies 6 ·
Replies
6
Views
3K
Replies
1
Views
3K
  • · Replies 15 ·
Replies
15
Views
3K
  • · Replies 11 ·
Replies
11
Views
8K
Replies
7
Views
2K
  • · Replies 1 ·
Replies
1
Views
4K
  • · Replies 2 ·
Replies
2
Views
4K
Replies
1
Views
2K