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bandit
23-10-2002, 07:09 AM
i'm totally confused with what is flash gas i've been told that flash gas happens when a fridge is short of refrigerant and you see the liquid "flashing through the sight glass.

i am then told this is wrong and that flash gas is something occuring at the tev when something else is happening

please whats the correct defanition of flash gas

Argus
23-10-2002, 09:51 AM
The term "Flash Gas" is another expression for evaporation, hence the term "flash point" to denote a spontaneous evaporation temperature as applied to solvents for example. For any volatile fluid to flash there must be an exchange of heat or energy.

Bubbles in a sight glass are just that - bubbles caused by a partially full pipe. There will be vapour present. Less commonly, flash gas will occur also as a result of excessive pressure loss in a liquid line, typically through the weight of a column of liquid refrigerant in a liquid line. The head of a column of liquid will result in a pressure loss over that pipe. Usually this is taken into account in normal system design losses. A practical example of this can be seen in excessively high liquid line risers where the evaporator is above the receiver. Usually this is in the order of tens of metres so you won't come across it often but when it happens a sight glass at the top will sometimes show signs of bubbles, or flashing, while one at the bottom would be clear. This is bad news for the TEV because it will be starved at the evaporator entry point. You also get flashing where a drier is partially blocked and restricting the flow. Another cause can be excessively long pipe runs. The pressure loss causes evaporation to occur. These are all abnormal.

Flash gas in a TEV is a different thing altogether and a little more difficult to explain. It's a normal occurrence and is part of the function of the TEV. Liquid refrigerant enters the TEV at a high pressure and compared to the evaporation condition, high temperature.
We all understand the pressure loss through a TEV but typically, as in an A/C application, the temperature difference can be 30 deg C or more. Whatever the application, there will be a TD in the refrigerant across the valve. At the point of exit from the TEV orifice we have to drop this temperature at the same time as the pressure drops, to the equivalent of the evaporation temperature. This is done through the spontaneous 'sacrifice' of a small portion of the mass of liquid that evaporates (or "Flashes"). To do this it takes heat from the surrounding body of liquid and drops its temperature. These two things happen simulataneously and together, they cannot be separated.

Hope this explains it?..

Good luck
________
Honda Pilot (ATV) specifications (http://www.honda-wiki.org/wiki/Honda_Pilot_(ATV))

Dan
23-10-2002, 11:35 PM
That was a pretty good explanation, Argus.

Bandit's advisor who says that flash gas is something happening at the TEV, more than likely is referring to what Argus is describing as the necessary flashing or evaporating of the liquid just after the TEV. This evaporation is, in effect, simply refrigerating the remaining portion of the liquid passing through TEV until it reaches the design temperature of the evaporator. After that, the evaporating liquid is doing useful work removing heat from the product being cooled.

This is why cooling of the liquid provides energy savings; the cooler the liquid is, the less the refrigerant has to evaporate to bring the rest of the liquid down to temperature, and the more the refrigerant is available for useful work on the product being cooled.

Flash gas in the liquid line before the TEV is problematical. If you are providing a vapor/liquid refrigerant mix to the valve, it will lose capacity dramatically because so much volume has to pass through the valve compared to a full liquid feed.

One other thought: In instances where you are concerned about the state of the gas feeding the TEV, just because you have a bubbling sightglass at the compressor unit, it doesn't necessarily follow that you have the same condition at the TEV, nor does having a full sightglass at the compressor unit insure that you will have the same condition at the TEV.

Gary
24-10-2002, 05:52 AM
It might help to think of flashing as very rapid evaporation, near instantaneous.

Dan
25-10-2002, 04:32 AM
It might help to think of flashing as very rapid evaporation, near instantaneous.

Gary's comment reminds me of my puzzlements regarding the differences between evaporation, boiling, flashing, and sublimation. The commonality among those ways of turning a liquid or solid into a vapor or gas is that the latent properties of the material all are consistent with thermodynamic tables.

For example, if you watch an ice puddle or the cubes in your refrigerator shrink in size, the heat transfer that is happening includes the latent heat of melting and evaporation, no different than if you watched the ice melt into water and then boil into vapor.

I would like to think that the key term is "evaporation." Words such as "sublimate" , "flash", "boil", etc are discriptors of how we get there.

Most interesting and puzzling to me is the reversal of the process. Condensation. Surely, if something can turn from solid to vapor without bothering to become a liquid, the reverse can happen.

But more to the point regarding refrigeration fluids, you can have incomplete condensation and what we call flash gas leaving the condenser.

The puzzle for me has always been reconciling that this means that condensing and evaporating are the same thing. Preborn flashing right out of the condenser?

It's a word thing. So here is how I see the words:

Evaporating: Changing from liquid to vapor

Boiling: Rapidly changing from liquid to vapor

Flashing: Really, really rapidly changing from liquid to vapor.

Sublimating: Ice turing into vapor without visibly becoming liquid.

Condensing: Changing from vapor to liquid.

What is the opposite of boiling?,...I get stuck here. Is it the portion of a condenser where all the action is?

What is the opposite of sublimating? Making snow? Frost forming on round ice cream containers in a reach-in freezer?

Same goes for the reverse of flashing:

Really, really rapidly changing from vapor to liquid? Where does that happen in a normal refrigeration process?

LOL. The science appears to be consistent, but the words are lacking.

Pardon the ramble.:)

Prof Sporlan
03-11-2002, 07:06 PM
The expression “flash gas” is a colloquialism in our industry. The Prof would think it best if it were simply looked at as another term for two-phase (liquid and vapor) flow. The problem with this approach is we think of “flash gas” more as a symptom of a problem identified at the sightglass instead of flow. We don’t normally think of the two-phase refrigerant flow thru a condenser of evaporator as “flash glass”.

Nico
06-11-2002, 09:13 AM
I would think of "flash gas" as an evaporation proces that is initiated by a drop in pressure. Typical is that evaporation starts immidiate and all trough the liquid. Only when you have a large vessel (tank) the hight seems to have influence on the rate of evaporation. The flashing evaporation is due to the huge surface area of the heat exchanger because practical all the liquid is acting as a heat exchanger for the leaving (evaporating) molecules.

Boiling however is typical initiated by a rize in temperature. The rate of boiling is in practice limited by the heat added and thus by the heat exchanger. Only where heat is added you will find boiling so boiling occurs typical on the heat exchanger surface. For us boiling is most interesting because that proces limmitates the refrigerant site of the heat exchanger (evaporator).

After all, all are evaporation processes following the rules of physics as we know from the Moulier diagram.

Oposit of sublimation is seen all the time in the coldstore as (white) frost or hoarfrost (got that from the dictionary). Snow is a typical form as well.

Nico.

herefishy
10-11-2002, 05:32 PM
i've been told that flash gas happens when a fridge is short of refrigerant and you see the liquid "flashing through the sight glass.

I think that observation of "bubbles" or vapor in the sightglass is NOT flashing gas, unless there is some restriction at the inlet of the sightglass. I am guilty of referring to the observation of intermittent and possibly rapidly occurring bubbles in the sightglass as "flashing", but consider my use of the term in that instance (relative to the physics of the refrigerant), incorrect. However, I am only describing the visual effect of what I am seeing.



the term "flash point" to denote a spontaneous evaporation

... and such spontaneous evaporation is not occuring in the sightglass, unless of course there is a restriction and pressure drop at the inlet of the sightglass causing liquid to flash, so...


Bubbles in a sight glass are just that - bubbles caused by a partially full pipe

.... and only that. the fact is, that the bubbles you see in the sightglass were never condensed into a liquid after having entered the condenser (unless of course we have a restriction and pressure drop at the inlet of the sightglass)...


the spontaneous 'sacrifice' of a small portion of the mass of liquid that evaporates (or "Flashes") takes heat from the surrounding body of liquid and drops (the remaining liquid's) temperature

... therefore .....


This evaporation is, in effect, simply refrigerating the remaining portion of the liquid passing through TEV until it reaches the design temperature of the evaporator. After that, the evaporating liquid is doing useful work removing heat from the product being cooled.

...so...


think of flashing as very rapid evaporation, near instantaneous.


"Flash gas" occurs normally due to the physical state of the universe. It's usual derogatory interpretation in regard to the operation of a refrigeration system is typically in reference to, "the only thing that is happenning at the TEV (outlet) is Flashing, and the evaporator is being starved.

.... I just thought I'd put everything together :)

Frosty
15-11-2002, 10:46 PM
Hi Bandit

Don't know whether you are familiar with Mollier diagrams (PH charts) If not, get someone 'in the know' to drop a system design onto one (preferably with no liquid sub-cooling before the expansion device.) Look what happens to the percentage of refrigerant immediately available for latent heat exchange in the evaporator, a percentage of refrigerant has already been flashed off - reducing the efficiency of the evaporator! After you've done that, do another one, this time with 10 deg C liquid sub-cooling. Notice the difference? The percentage of refrigerant flashed of before entering the evaporator has reduced! The ratio of gas to liquid entering the evaporator is known as refrigerant 'quality'.

Hope this helps you????

Frosty

ragu
16-09-2010, 03:51 PM
i have little more doubt, if some of the refrigerant is lost by evaporation every time , then we need to charge the system very often, which is not the case in refrigerator. i dont think its lost, since its a closed system, being condensed again is a condenser. also whether there is any significance for Joule thompson effect here in the exp valve???

ragu
16-09-2010, 03:55 PM
Hi Argus,
i have little more doubt, if some of the refrigerant is lost by evaporation every time , then we need to charge the system very often, which is not the case in refrigerator. i dont think its lost, since its a closed system, being condensed again is a condenser. also whether there is any significance for Joule thompson effect here in the exp valve???

james10
16-09-2010, 05:11 PM
Flashgas=Saturated two phase mixture 20% vapour 80% liquid

shieldcracker
18-09-2010, 03:09 AM
I would like to think that the key term is "evaporation." Words such as "sublimate" , "flash", "boil", etc are discriptors of how we get there.

Most interesting and puzzling to me is the reversal of the process. Condensation. Surely, if something can turn from solid to vapor without bothering to become a liquid, the reverse can happen.

But more to the point regarding refrigeration fluids, you can have incomplete condensation and what we call flash gas leaving the condenser.

The puzzle for me has always been reconciling that this means that condensing and evaporating are the same thing. Preborn flashing right out of the condenser?

It's a word thing. So here is how I see the words:

Evaporating: Changing from liquid to vapor

Boiling: Rapidly changing from liquid to vapor

Flashing: Really, really rapidly changing from liquid to vapor.

Sublimating: Ice turing into vapor without visibly becoming liquid.

Condensing: Changing from vapor to liquid.

What is the opposite of boiling?,...I get stuck here. Is it the portion of a condenser where all the action is?

What is the opposite of sublimating? Making snow? Frost forming on round ice cream containers in a reach-in freezer?

Same goes for the reverse of flashing:

Really, really rapidly changing from vapor to liquid? Where does that happen in a normal refrigeration process?

LOL. The science appears to be consistent, but the words are lacking.

Pardon the ramble.:)
Maybe I can clarify first the change of states...
solid to liquid= melting
liquid to gas= vaporization
gas to liquid=condensation
liquid to solid=freezing
solid to gas= sublimation
gas to solid= deposition
In a phase change both phases are present. For example in a condenser and in a evaporator coil both gas and liquid refrigerant are present; the difference lies in the direction of heat flow (pressure being constant), heat is absorbed/rejected to vaporize/condense refrigerant in the evaporator/condenser.
Now your definitions for boiling and flashing simply don't exist in the language of science because they have no physical meaning and this might be the reason you cant find the right word for the reverse associations.

Peter_1
19-09-2010, 06:21 PM
i have little more doubt, if some of the refrigerant is lost by evaporation every time , then we need to charge the system very often, w...
Where did you read Argus said it's lost?
And please, don't double post