Sight glass , Flashing ?

[kfjoe]

Could anyone explain why under normal running there would be Bubbles in the sight glass(short of charge?).
But on increasing the head pressure(switching off the condesor fans) the sight glass will clear.

I would have thought that this would reduce the condesing and cause bubbles/less liquid?

Also when a system comes down to Temp I notice Bubbles in the sight glass as if it is short of charge, but there ic surely less load and liquid being used in the evaporator?

normal dx system , tev, no head pressure control etc.

I apologise if I am missing something obvious, but seem to have confused myself

I would appreciate any help, thankyou.

[shogun7]

If you have a pressure drop in the liquid line say because of a long run and or vertical lift it, happens because the temp of the liquiq is exceedind the pressure it is under. if you go to the PT chart for a refrigerant and pick out any pressure you will see that a temp corresponds to that particular pressure if you lower the pressue the temp of the liquid must follow in order to stay saturated ...it will do this by evaporating it's own liquid (FLASH GAS) to get itself down to the new pressure.
amicus humani generis

[Peter_1]

in a lo-P chart, if you lower the upper condensing line vertically, then the left point (end of condesation) can come with these low condensing pressures in the liquid/gas area. When you increase the HP, the horizontal condensing line is going up vertically and the same point comes now in the liquid area.
Thats how I explain it.

[Prof Sporlan]

But on increasing the head pressure(switching off the condesor fans) the sight glass will clear.

Correct.

I would have thought that this would reduce the condesing and cause bubbles/less liquid?

Consider the temperature of the refrigerant in the receiver (or the liquid refrigerant in the condenser for that matter if the unit doesn't have a receiver) cannot change as quickly the pressure changes from a condenser fan that cycles. As Emeril Lagasse might explain: "Condenser fan cycles off. Bam! Condensing pressure goes up pushing the liquid in the receiver into its subcooled region. Condenser fan cycles on. Bam! Condensing pressure drops causing liquid in the receiver to evaporate, causing flash gas in the liquid line."

[kfjoe]

Excellent Thankyou guys
And a very nice flash on your website marc.
The pressue/ enthalpy chart has helped a lot.

But If a system is running at a Low Head pressure due to a Low ambient, and Flashing(liquid/vapour) is seen in the sight glass then this doesnt necessarly mean it is short on charge ?
Would charging more help any ( more liquid reducing vapour space ?)
Also on a system system that is down to temp and has very low suction pressure and lowish head pressure sometimes seem to show flashing in the sight glass.

I am basically asking how you determine weather it is a low charge or just due to low pressure and therefore the sight glass area is now in the Vapour/liquid area on the chart ( wouldnt a good design prevent this ?)

Thanks again
I very much appreciate your help.

[Peter_1]

Originally posted by kfjoe
[B]
But If a system is running at a Low Head pressure due to a Low ambient, and Flashing(liquid/vapor) is seen in the sight glass then this doesn't necessarily mean it is short on charge ?

No, you're right.

Would charging more help any ( more liquid reducing vapor space ?)

Yes, but you also increase the subcooling of the liquid (this point in the P-T is going far more to the right in the upper horizontal condensing line, thus more in the safe side of the 100% liquid region.
You reduce also condenser capacity and are able to maintain a higher condensing pressure

Also on a system system that is down to temp and has very low suction pressure and lowish head pressure sometimes seem to show flashing in the sight glass.

Mostly, very low pressure are associated with low condensing pressures (lower suction pressures then normal conditions is smaller capacity then for what the condenser is made and therefore lower condensing pressures)

I am basically asking how you determine weather it is a low charge or just due to low pressure and therefore the sight glass area is now in the Vapor/liquid area on the chart ( wouldn't a good design prevent this ?)

If I'm not sure, especially when it's cold, I place a cardboard or paper before the condenser to cover it partially (+/- 2/3) If flash gas is going away and it stays away, then system is correct charged. If flash comes back again with cardboard in place, then you can think that there is lack of gas in the system.

Same phenomena when charging an airco with low ambient temperatures (if you don't know the exact weight to fill in the system)

[Peter_1]

Originally posted by Marc O'Brien
I tackle all of this in the recent components of my ACR News series of articles.

Is this their website Marc? http://www.achrnews.com/

[Peter_1]

But if it's a good magazine in your opinion, I will make a subscription for it (if possible for Belgian citizens)

Peter

[chemi-cool]

hi peter,

its expensive, too many advertisments and mainly printed for the american market.

maybe now with the crazy euro rising all the time it will be cheaper for you.

try to subscribe for the minimum tim so you can see if you want it before you loos a lot of money.

chemi

[Peter_1]

Ithought it was a UK magazine.
Any member who can recommend a good magazine?
In Germany, you had Kalte Klima Aktuell, in The Netherlands Koude & Klimaat.
RAC of the Uk, is this good?

Anyone ideas?

[shogun7]

kfjoe: All refrigeration systems are designed to operate within certain perameters in order to obtain the best efficiency. therefor if you do not maintain a correct head pressure even though your system is full you can experience flashing (bubbles) in the sight. Why? for various reasons like not enough pressure in liquid line to overcome the pressure drop caused by of the length,weight, or vertical lift in the liquid line.
I once had a 120 ton evaporative condenser that used to flash every time the fan would cycle on and the sight glass would clear up when the fan would shut off. I had a vertical lift of about 20 ft.
(?) meters. the only solution was to install a heat exchanger in the liquid line to sub cool the liquid. this condenser didn't have a sub cooler built in (cheap) . So what's the lesson here? ANS. if there's even a possibility that you will have flash gas due to pressure drop then always consider a way to sub cool the liquid.
aut disce aut discede
Roger the dodger

[RogGoetsch]

Originally posted by shogun7

aut disce aut discede
Roger the dodger

"Either learn or leave"...? Nice.

Harsh... but nice.

[kfjoe]

Thanks for the replys all
Has helped greatly.

docendo discimus

[shogun7]

docendo discimus teach in order to learn (we learn by teaching) very true!!!
Hay maybe we should learn latin while we discuss problems on this forum? whata think?

[Prof Sporlan]

A "quid pro quo" perhaps...

[shogun7]

Why Not ?