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Flash gas

Member Posts: 2,343
I'll take a stab at it. The term "Flash Gas" is used to indicate that portion of the refrigerant which evaporates instantly, (flashes) and turns into a vapor as it passes through the refrigerant control orifice. The instant vaporizing of some of the liquid refrigerant, (flash gas) cools the rest of the liquid to the evaporating temperature. Eugene Silberstein just held a very informative class on this very topic which I think you would be very interested in. Latent heat of fusion and Latent heat of evaporation are 2 terms that all Chill heads should know well. It can solve all of your questions when applying it to a Pressure Enthalpy applications. If I remember correctly, a cooler liquid line increases your net refrigeration. It's been a long time. That's why I am ordering Eugene's book.

In your second Q: If I had 1 lb. of water @ 211*f and wanted to raise it to 212*f, it would only take 1 BTU/HR to accomplish this. Now take that 212*f water and convert that same 1 lb. of water into steam, it would take 977 BTU/HR. Thus "Latent heat of vaporization". Alot of work is being done here.

I hope this helped a little bit. Mr Silberstein will chime in soon to clarify more.

Mike T.

• Member Posts: 9
Flash Gas

I really want to know how flash gas really works. I know that when you run a fluid through an orfice, the fluid has a lower temperature due to evaporation. How and why does that fluid evaporate when it goes through a pressure drop. I really don't see also how water evaporates at other temperatures than 212 deg. F.
thanks
Jesse
• Member Posts: 1,380
Here Goes

The metering device is, in effect, a restriction in the liquid line that causes refrigerant to back up on the high side of the system. On the other side of the "restriction" refrigerant experiences a sudden decrease in pressure as the volume that the refrigerant can occupy has increased a great deal. Consider the metering device as being an accident on a three lane higheay that has blocked two of the lanes. Before the accident there are many, many cars backed up for miles and miles. Now, what happens when you pass the accident? You now have your three lanes wide open and the traffic jam has disappeared.

Now, consider the same thing with refrigerant. As the liquid refrigerant flows through the metering device, it suddenly experiences this larger volume and the pressure drops. This reduction in pressure causes a portion of the liquid refrigerant to boil off into a vapor. Consider this:

In Denver Colorado, water boil at a temperature below 212 degrees because of the high altitude. The pressure in Denver is lower than the pressure at sea level. Using a pressure cooker in Denver is common practice buit how dies it work? THe tight lid on the pressure cooker causes vapor pressure to build up over the liquid, preventing the liquid from boiling until the temperature of the water has increased. The same holds true for our refrigeration example.

If the pressure above the liquid is reduced, the boiling temperature will drop as well. So, when refrigerant leaves the metering device and experiences the sudden decrease in pressure, a portion of the liquid vaporizes. Now, a vaporizing liquid absorbs heat. In this case, the vaporizing liquid absorbs heat from the remaining liquid entering the evaporator. This increases the cooling effect as the temperature of the refrigerant in the evaporator is now cooler.

Now here's a cool thought. Since increased subcooling is an indication of increased system efficiency and flash gas is a good thing, we must make a fine balance between the two. Here's why. An increase in subcooling decreases the amount of flash gas and an increase in flash gas decreases the amount of condenser subcooling. This is why systems typically operate with 15 to 20 degrees of subcooling and have about 20 to 25% flash gas in the evaporator.

And yes, Mike, plotting this puppy out on a pressure enthalpy chart will make this all crystal clear.

It looks like the next stop on the PRESSURE ENTHALPY WITHOUT TEARS tour will be in April in sunny Atlanta Georgia. Stay tuned!
• Member Posts: 67
Now thats

just not right Professor.I guess you been in class to long and you forgot how to map your course to squeez in as many
calls as possible.

With that being said...VirginiaBeach Virginia should be your first stop before Atlanta Georgia.

Dont you think?

• Member Posts: 1,380
Atlanta it is....

I will be in northern Virginia at an educators conference from April 5 to7th though. That's probably about 4 hours from you, right?
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