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Originally Posted by hvacrmedic

Yes, that's correct. There's no area of the evaporator coil containing pure liquid. From the metering device outlet to the evaporator coil outlet the percentage of liquid in the mixture continuously diminishes until finally there is only a pure vapor left.

You might find this interesting.

http://www.cibse.org/pdfs/twophase%2...in%20horiz.pdf

Thanks, good read. Yeah I knew that, just tried to build off of been there's post. The only way we'd have pure liquid is if we subcooled the liquid to the evaporator saturation temperature prior to the txv.

Originally Posted by git-r-dun

Agreed. Although I stated otherwise above.

Yes, that's correct. There's no area of the evaporator coil containing pure liquid. From the metering device outlet to the evaporator coil outlet the percentage of liquid in the mixture continuously diminishes until finally there is only a pure vapor left.

You might find this interesting.

http://www.cibse.org/pdfs/twophase%2...in%20horiz.pdf

Agreed. Although I stated otherwise above.

First part of the coil is not liquid in a DX system. Liquid enters the metering device and the subsequent pressure drop causes flash gas. Refrigerant exiting the metering device will be saturated. This can be seen right on any pressure enthalpy chart.

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Originally Posted by beenthere

Its not that cut and dry. You won't have 50/50 liquid and vapor traveling the length of the coil. Normal operating system. Part of the coil will always have liquid, part will have liquid and vapor, and the other part will have vapor only. And you will have SH.

Actually it really is. The refrigerant relies on the conditioned air to give it enough heat as much as the air relies on the refrigerant to "cool" it ( heat exchange). You will have liquid in the first part of the coil and you will have a mix further down. For the end it will vary. If there was not enough BTUs in the air then you will never become a vapor only, thus no SH. If there was enough heat you'll become a full vapor and gain some heat, thus SH. If there was too much heat, then it became a vapor too early and gained a lot more heat, thus high SH.

Its not that cut and dry. You won't have 50/50 liquid and vapor traveling the length of the coil. Normal operating system. Part of the coil will always have liquid, part will have liquid and vapor, and the other part will have vapor only. And you will have SH.

Originally Posted by beenthere

Basically 100 vapor at the last section. Which still means that if the coil holds 2 pounds of liquid, not all of the 2 pounds had to boil/vaporize to give a SH. Which is basically what your in a post.

If you have a 2 pound capacity and 1 of it changes state it becomes a mixture of both throughout the circuit until additional heat can be absorbed to boil of the remaining half. When that occurs you can then as a vapor gain sensible heat, thus superheat. The vapor of a half boiled refrigerant does not separate from the liquid at any point. Sounds like you're thinking the vapor leaves the liquid behind and picks up heat (literally ). The only time it can leave the liquid behind is when it no longer is a liquid because it consumed the amount of BTUs to do so. If a pound of R-22 has a latent heat capacity of let's say 80 btus and you were to heat it with 40 btus then half of it would become a vapor and the other half would remain liquid. Both would be at the same temperature (saturation). Vapor would not pick up sensible heat. If it "tries" the liquid will "steel" it and apply it to its phase change. So if there was not enough of a load to complete the change,some heat was still absorbed, but the SH would be 0 as liquid and vapor mix will exit the evaporator.

Basically 100 vapor at the last section. Which still means that if the coil holds 2 pounds of liquid, not all of the 2 pounds had to boil/vaporize to give a SH. Which is basically what your in a post.

When there is no superheat measured that doesn't mean that there was no change of state. If your coil holds 2 pounds of refrigerant and one pound changes state (from liquid to gas) what do you suppose your SH is gonna be? It will be 0 but you still had heat exchange. It's not all or nothing. Superheat will occur when the entire liquid has "boiled" into a vapor and then picks up additional heat. Think of flash gas.

Originally Posted by beenthere

That would be a 100% flooded evap, so no its not possible. However if all but the last 2 runs of the circuit/circuits had 36 degree liquid, the last 2 runs of the circuits could have 40 degree vapor.

So that being said, when we are measuring superheat, at the outlet of the evaporator, that 36 degree liquid would of have to become a 40 degree vapor giving a 4 degree SH and 100% vapor.

If no, what happen to the liquid at the last runs?

Originally Posted by git-r-dun

It was, thank you.

So is it possible at that last portion, lets just say the outlet of the coil, to have 36 degree liquid with a vapor of 40 degrees?

That would be a 100% flooded evap, so no its not possible. However if all but the last 2 runs of the circuit/circuits had 36 degree liquid, the last 2 runs of the circuits could have 40 degree vapor.

It was, thank you.

So is it possible at that last portion, lets just say the outlet of the coil, to have 36 degree liquid with a vapor of 40 degrees?

Originally Posted by beenthere

Yes. The temp across an evap coil is not the same from one end to the other. The beginning would have the 36 degree liquid, and the end would have the 40 degree vapor.

The same with a condenser. The condenser only has liquid in the lower part of it. The top part has the hot gas from the compressor, and the lower part has the sub cooled liquid. With only the middle section having refrigerant at sat temp and pressure.

I believe he meant 36° liquid and 40° vapor "at the same point" in the coil.

Originally Posted by git-r-dun

So are you saying: A 63 psi R-22 coil with a saturation temperature of 36 degrees can have a mixture of 36 degrees liquid and 40 degrees vapor?

Yes. The temp across an evap coil is not the same from one end to the other. The beginning would have the 36 degree liquid, and the end would have the 40 degree vapor.

The same with a condenser. The condenser only has liquid in the lower part of it. The top part has the hot gas from the compressor, and the lower part has the sub cooled liquid. With only the middle section having refrigerant at sat temp and pressure.

Originally Posted by git-r-dun

I'll have to start adding salt to my R-22

LOL..

Originally Posted by git-r-dun

Good point, but I think what we see there is the heat from the warmer sub cooled liquid out of the condenser being absorbed from the cooler, saturated liquid in the evaporator. The btus required for it to match the evaporators saturation temp is given up from the refrigerant itself, which causes it to flash.

Had to correct the above statement.

While we got the water nice and muddy, let me throw in another variable. The compressor is constantly pulsating refer threw a system and as we know vibration can cause vapor. Such as an ultra sonic humidifier. These vibrations could atomize molecules of refrigerant thus isolating them from there subcooled reservoir such as a receiver. Allowing superheated vapor and and subcooled liquid to coexist.

Originally Posted by Core_d

I think that answers your question really. It does explain the flashing at the txv, after pushed into the lower pressure "vacum" some molecules become isolated from the liquid allowing them to vaporize and are quickly returned to a liquid when contact is made. All the while the vast portion of refer maintains a subcooled state.

Good point, but I think what we see there is the warmer sub cooled liquid out of the condenser absorbing heat from the cooler, saturated liquid in the evaporator. The btus required for it to match the evaporators saturation temp is given up from the refrigerant itself, which causes it to flash.

Originally Posted by beenthere

I know. My point is. Your are incorrect when you say that you wouldn't have SH if only 1 pound out of 2 pounds of liquid in the evap boiled off.

So are you saying: A 63 psi R-22 coil with a saturation temperature of 36 degrees can have a mixture of 36 degrees liquid and 40 degrees vapor?

Originally Posted by shaka

Yes in some cases I guess you would see it in the sight glass.
Here's a couple of examples.
Where 10 or more of SC and vapor exist.

1) when unit runs in low ambient condition,
Where hot gas subcools real fast,
And volume of refrigerant is not enough to completely fill
the condenser coil.( vapor and liquid present)
Liquid line is probably the same temp as ambient.

2) it happens in the receiver in commercial refrigeration
Where condenser outlet can have a15f subcool entering receiver
And receiver has both vapor and liquid at the same time.

The reason I asked was, my school book says a sight glass provides a slight pressure drop and a vapor pocket or "lazy stream" is not uncommon under low load conditions. But I dont know, Ive been on both sides of the fence more than once since ive joined this discussion.

Originally Posted by beenthere

If I add salt to it I can.

I'll have to start adding salt to my R-22