# Heat Transfer and the 2nd Law of Thermodynamics

• aznkid310
In summary, at 300 C, saturated liquid water will become saturated vapor. The final pressure will be based on the saturation temperature and the pressure used to obtain the saturation temperature.
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

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?

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

## 1. What is heat transfer?

Heat transfer is the process of thermal energy being transferred from one object or system to another due to a temperature difference. It can occur through three main mechanisms: conduction, convection, and radiation.

## 2. What is the 2nd Law of Thermodynamics?

The 2nd Law of Thermodynamics is a fundamental principle in physics that states that in any natural process, the total entropy (or disorder) of a closed system will always increase or remain constant. This means that energy tends to disperse, and systems tend to move towards a state of thermal equilibrium.

## 3. What is the difference between conduction, convection, and radiation?

Conduction is the transfer of heat through a material or between two materials in physical contact. Convection is the transfer of heat through the movement of fluids (liquids or gases). Radiation is the transfer of heat through electromagnetic waves, such as infrared radiation from the sun.

## 4. How does the 2nd Law of Thermodynamics apply to everyday life?

The 2nd Law of Thermodynamics has many real-world applications, such as the efficiency of engines and power plants, the direction of chemical reactions, and the behavior of weather systems. It also explains why it is impossible to create a perpetual motion machine, as energy will always be lost due to entropy.

## 5. Can the 2nd Law of Thermodynamics be violated?

No, the 2nd Law of Thermodynamics is a fundamental law of nature and has been observed to hold true in all physical processes. While it may seem like certain systems or processes go against this law, they do not violate it, but rather follow it in a more complex manner.

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