Understanding Condensation & Vacuum in Steam Engines

[jamesd2008]

Hello everyone,

Could someone help me with the following question:

I'm trying to find out the equations that govern this process; why if you condense steam in a cylinder is a partial vacuum created (such as in early steam engines)? The pressure difference then causes the atmosphere to push down a cylinder?

Any suggestions much appreciated

James

 

[jbriggs444]

Google "vapor pressure".

Left alone in sealed box, a cup of water will evaporate until the partial pressure of water vapor in the air is high enough that the rate of evaporation equals the rate of condensation, resulting in an equilibrium. This partial pressure will depend on the temperature of the box and is called the "vapor pressure" of water at that temperature.

Boiling occurs when the temperature of the water is high enough so that its vapor pressure is equal to atmospheric pressure. At this point an equilibrium cannot be attained within a leaky container. Water will turn into steam faster than it can recondense and steam will come out through the leaks.

Significantly, in a chamber that is almost sealed this eventually drives all of the air out of the chamber leaving nothing but water and water vapor.

If you seal the chamber and reduce the temperature then the vapor will condense until the pressure in the chamber is equal to the vapor pressure of water at the reduced temperature.

You can google for a table of water vapor pressure as a function of temperature. I believe that there are also formulas that can predict this pressure, but my chemistry education does not reach that far.

 

[rollingstein]

Google "vapor pressure".

You can google for a table of water vapor pressure as a function of temperature. I believe that there are also formulas that can predict this pressure, but my chemistry education does not reach that far.

The Antoine equation:

For Water the eq. is:

 

[Chestermiller]

Google "vapor pressure".

Left alone in sealed box, a cup of water will evaporate until the partial pressure of water vapor in the air is high enough that the rate of evaporation equals the rate of condensation, resulting in an equilibrium. This partial pressure will depend on the temperature of the box and is called the "vapor pressure" of water at that temperature.

Boiling occurs when the temperature of the water is high enough so that its vapor pressure is equal to atmospheric pressure. At this point an equilibrium cannot be attained within a leaky container. Water will turn into steam faster than it can recondense and steam will come out through the leaks.

Significantly, in a chamber that is almost sealed this eventually drives all of the air out of the chamber leaving nothing but water and water vapor.

If you seal the chamber and reduce the temperature then the vapor will condense until the pressure in the chamber is equal to the vapor pressure of water at the reduced temperature.

You can google for a table of water vapor pressure as a function of temperature. I believe that there are also formulas that can predict this pressure, but my chemistry education does not reach that far.

This happens when you sterilize and can foods at home in mason jars. You get a partial vacuum in the jar as a result of the water vapor condensation from the head space. It is also a characteristic of industrial food canning operations (vacuum sealing). If you don't get a hissing when you open the can, don't eat the food.

 

[jamesd2008]

Thanks for your replies, you give me a good start at reading further on this. Thanks

 

[jamesd2008]

I was trying to apply the gas laws, but vapour pressure is something I'm not familiar with.

 

[jamesd2008]

Also what is the situation if the cylinder is completely sealed?

 

[Chestermiller]

I was trying to apply the gas laws, but vapour pressure is something I'm not familiar with.

Vapor pressure is an extremely important concept in vapor-liquid equilibrium behavior.

 

[rollingstein]

Also what is the situation if the cylinder is completely sealed?

If you fully seal the cylinder before heating then no vacuum is created.

 

[Chestermiller]

If you fully seal the cylinder before heating then no vacuum is created.

However, we should also mention that during heating of a fully sealed cylinder, the pressure inside the cylinder gets much higher.

 

[sanka]

If you fully seal the cylinder before heating then no vacuum is created.

If the cylinder is fully sealed and you condense steam within it a vacuum will be created. The temperature of the steam-water mixture will drop to the surrounding ambient temp and the vapor pressure will correspondingly drop also. Air in the cylinder will contribute to a partial pressure but the total pressure will still be less than atmosphere

 

[Chestermiller]

If you fully seal the cylinder before heating then no vacuum is created.

I think what rollingstein was saying was that, if you seal the can before heating it, the pressure within the can will start out at atmospheric pressure. When you heat the can and then return it to its original temperature, the state of the liquid and gas in the can will be the same as when you started. In this case, no vacuum will be created.

 

[sanka]

My bad, misinterpreted the question