Frost

[R717]

Hi everyone,

has anyone come across a instance where you have normal to more than normal evaporator superheat but have liquid returning to the comp in the suction line ??

Many thanks

[The Viking]

Well, by definition that situation would not be possible on a correctly designed system.

Superheat = temperature of the refrigerant above the temperature at which the refrigerant evaporated.

Now, the only reason for liquid to be carried over even when the superheat is there would be if the velocity of the refrigerant is too high in the suction line (for example if the suction line is to short or too narrow).

:cool:

.

[monkey spanners]

Wrong refrigerant in system, poorly designed pipework, had system with air in that would slug liquid back now and again once.

[R717]

Well, by definition that situation would not be possible on a correctly designed system.

Superheat = temperature of the refrigerant above the temperature at which the refrigerant evaporated.

Now, the only reason for liquid to be carried over even when the superheat is there would be if the velocity of the refrigerant is too high in the suction line (for example if the suction line is to short or too narrow).

:cool:

.

I must say i agree with your definition, this system seems to be defying logic.

It is a old R12 system running on R134a. It has tube and fin condenser and evaporator. The evaporator is face split, divided into 2 sections. Liquid flows to single txv and then the line is divided to feed the upper and lower sections of the evaporator.

I do suspect uneven distribution to the evaporator sections as the lower section has frost on every tube and the upper section has frost only on the lowest tubes, the upper tubes only show a little condensation.

Suction pressure is 15psig, and suction temp is 5 degrees C. Superheat= 14.4 degrees C, but there is frost on the compressor suction line fitting, it is at -2 degrees C.

Temp is dropping 7 degrees C over the length of the suction line. Which makes me think there must be liquid in it evaporating, but with 14.4 degrees superheat that seems not possible ???

The suction line is short, only 20 inches. It's diameter is 3/4 inch, which seems to be the standard suction line size for these compressors. System design is definately out of whack with R134a in a R12 system.

Any more thoughts anyone ??

[Brian_UK]

Seems that the lower section of the coil has liquid flowing through it and vapour going through the upper section.

Are the two tubes from the TXV shaped in any way to encourage equal flow in the tubes or are they simple straight runs into the coil?

Also, is the sensing bulb of the TXV in a good location?

A second also, is the TXV externally equalised?
[edit]
This is something that Sporlan pick up on in their literature, see The Distribution Problem on page 2 of this...
http://www.sporlanonline.com/20-10_022011.pdf

[monkey spanners]

As Brian said, have seen tev outlet split in two where liquid ran through lower tube and vapour in the other, this resulted in low duty, flooding back and in this case lower half of coil frosted over.

Turned the distributor through 90' so both tubes where horizontal and sid by side.

[R717]

Seems that the lower section of the coil has liquid flowing through it and vapour going through the upper section.

Are the two tubes from the TXV shaped in any way to encourage equal flow in the tubes or are they simple straight runs into the coil?

Also, is the sensing bulb of the TXV in a good location?

A second also, is the TXV externally equalised?
[edit]
This is something that Sporlan pick up on in their literature, see The Distribution Problem on page 2 of this...
http://www.sporlanonline.com/20-10_022011.pdf

I feel the distributor tubes do not really encourage equal flow. I'll post a picture.

The txv is externally equalized and i think the bulb location on the suction line just downstream from the evaporator is pretty good.

9980

[Brian_UK]

Yep, looks like you need to rotate that distributor by 90°.

For a video of the effects in a similar scenario, Daikin Refnet, see here....
http://www.youtube.com/watch?v=HcyfweCquR0

[R717]

Yes, rotating by 90 is great idea and i definately like to try it, but unfortunately it is not possible to get to it as it is buried deep inside a car's dashboard (the photo is of a spare) and i'm not keen on dismembering the car.

I redirected the evaporator fan's airflow to force more air over the lower section, this helped, but given that reorienting the distributor is not so easy to do i wonder if there might be other solutions ??

[MikeHolm]

Yep, looks like you need to rotate that distributor by 90°.

For a video of the effects in a similar scenario, Daikin Refnet, see here....
http://www.youtube.com/watch?v=HcyfweCquR0

That picture is worth a thousand words

[R717]

That picture is worth a thousand words

Yes, a excellent video.

It really makes me wonder how these things can be manufactured with such obvious defects ?

Surely it costs nothing to orient parts in the correct way ?

A big thankyou for every ones help *