# Thermodynamics: water in a piston-cylinder device

• fishingspree2
In summary: So, the pressure stays constant at 3.5 MPa while the temperature decreases. In summary, a piston-cylinder device initially contains superheated steam at 3.5 MPa and 5°C. As it loses heat, the piston moves down and the cylinder eventually contains saturated liquid water at 200°C. To find the initial temperature, you can look up the saturation temperature at 3.5 MPa and add 5°C. To determine the enthalpy change per unit mass, you need to find the difference between the specific enthalpies at 3.5 MPa and the corresponding saturation temperatures. For the final pressure and quality, you can assume that the pressure remains constant at 3.5 MPa
fishingspree2

## Homework Statement

A piston–cylinder device initially contains steam at
3.5 MPa, superheated by 5°C. Now, steam loses heat to the
surroundings and the piston moves down hitting a set of stops
at which point the cylinder contains saturated liquid water.
The cooling continues until the cylinder contains water at
200°C. Determine (a) the initial temperature, (b) the enthalpy
change per unit mass of the steam by the time the piston first
hits the stops, and (c) the final pressure and the quality (if
mixture).

2. The attempt at a solution

a. is easy... I lookup Tsat@3.5MPA in a table and add 5°C to it.

b) we are looking for h2-h1 where h2 is hf@3.5MPA,Tsat and h1 is hg@3.5MPA,Tsat+5

c) This is where I am stuck. I only have one intensive property, which is temperature=200°C. I need to find another one to find the state of the final water. Since it is impossible to tell whether or not we have a mixture with the temperature and the pressure only, I am guessing the other intensive property I need to find is the specifc volume. However I have no idea how to do that.

After the piston hits the stops, the volume is constant and the pressure changes. Thus, if at 200°C we still have some gas, then obviously the final pressure is Psat@200°C. However, I can't tell if we still have some gas.

Thank you

I think what is happening here is that they expect you to assume that the pressure is constant through the changes at 3.5 MPa.

## What is thermodynamics?

Thermodynamics is the study of the relationships between heat, energy, and work in a system. It focuses on the behavior and changes of energy in various forms, particularly in relation to temperature and thermal equilibrium.

## What is a piston-cylinder device?

A piston-cylinder device is a simple apparatus consisting of a cylinder with a piston that can move up and down inside it. It is used to study the behavior of gases under different pressures, temperatures, and volumes.

## How does water behave in a piston-cylinder device?

In a piston-cylinder device, water behaves as a liquid. It will fill the bottom of the cylinder and exert a pressure on the piston. As the piston moves up, the volume of water decreases, causing an increase in pressure. Similarly, as the piston moves down, the volume of water increases, causing a decrease in pressure.

## What is the relationship between pressure and volume in a piston-cylinder device?

In a piston-cylinder device, pressure and volume have an inverse relationship. This means that as the volume of the water decreases, the pressure increases, and vice versa. This relationship is described by Boyle's Law, which states that at a constant temperature, the product of pressure and volume is constant.

## How does the temperature of the water affect its behavior in a piston-cylinder device?

The temperature of the water in a piston-cylinder device affects its behavior by changing its volume and pressure. As the temperature of the water increases, its volume also increases, causing a decrease in pressure. This is described by Charles's Law, which states that at a constant pressure, the volume of a gas is directly proportional to its temperature.

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