Newbie Question - SH Pressure

[keepmecool]

Hi, I am new in the field of refridgeration (domestic) and battling a bit with the fundamentals. So here is my first question to the forum: why does the pressure not rise in an evaporator after all the refidgerant has evaporated and becomes superheated? more heat applied to a gas causes a pressure rise for a set volume was my understanding?
Please explain in '101' language.
Thanks, Michael

[keepmecool]

I am sorry for posting here, so I have reposted the thread in the 'New to RE' forum.
Sorry all.
Michael

[US Iceman]

The 'New to RE' area is where you can introduce yourself when you join. Placing a question as you did is OK as you selected the proper forum and asked nicely.

When the refrigerant is superheated in the last few passes of a direct expansion evaporator you can assume the superheating process occurs at a constant pressure. You need to find a written description of a simple refrigeration cycle which is plotted on a pressure-enthalpy diagram (also called a Mollier diagram).

The pressure in the evaporator actually decreases a slight amount because of pressure loss due to the refrigerant flowing through the tubes.

Welcome to the forums.

[keepmecool]

US Iceman
Thanks for the reply. I have the necessary theory at hand, but sometimes it does'nt provide the 'why'.
You say, there is an insignificant pressure vs. temperature effect during the superheating. So my question is then, is this because the suction effect of the compressor determines what the pressure will be. I can assume it is constant, I just wanted to know why I am making that assumption. Probably it makes no difference , just curious!
Michael

[US Iceman]

As the liquid boils to a vapor flowing through the tubes the vaporizing liquid absorbs heat from the tubes. We know there will be some pressure loss, because if there is no pressure loss, there is no flow. The pressure loss is typically a very small value, so we consider this to be a constant pressure process.

After all of the liquid is boiled off you usually have the last few passes of the evaporator providing the superheat. Remember once all of the vapor is boiled off, it is still saturated. Once the vapor temperature begins to increases above the evaporating temperature (the saturation temperature of the boiling liquid) you are adding superheat. Therefore what you have at this point is a constant pressure process with an increase in temperature.

When the superheating begins the gas density goes down, while the vapor volume increases.

It is assumed the compressor displacement is constant also. That is what keeps the pressure relatively constant in the evaporator.

Curiosity is a good thing. That's how we learn.

[keepmecool]

Great reply! Thanks a stack. I will be sure to keep regular posting and someday give back what I learn...

[Gary]

You may find this thread helpful:

http://www.refrigeration-engineer.co...ad.php?t=19701