Lazy sightglass/Over condensing...

[monkey spanners]

Took a short vid of a direct expansion milk tank i was checking over, the milk was cold so there was not much load on the evaporator and the ambient temp was around 8C so the condenser was taking too much heat out of the refrigerant.

http://www.youtube.com/watch?v=l28FPhN3yY4

I'm sure many of us have seen systems running like this in winter, and may have fitted fan speed control (or put a sack on it....) to get a more stable flow of liquid to the tev.

But what is really going on? Why the bubbles? I think i had 10k subcooling so its not flash gas.

(Systems on R22 was running around 3bar 12k sh and 10bar)

Jon

[mad fridgie]

How dare swear on this site, you should now better MS (lol)
No such thing as over condensing.
You do not have a liquid seal on the outlet of the receiver.
The liquid density changes, and your cond will reach equilibrium with the ambient (fills with refrigerant.)

[Lodiev]

Sorry Monkey i was to late to warn u to not use the term over condensing on this site.
Some people do not use this term as they are to evolved for the rest of us refrigeration mechanics.

Sorry guys mean no harm with this mail.

[desA]

Nice demonstration MS. Thank you.

[chillyblue]

Hi MS

I had the same thing on a twin fan condenser, when only one fan was running the sight glass was full, when the head pressure went up and the second fan run the sight glass bubbled as soon as the head pressure dropped.
I posted it on this forum and somebody had a great replied (something to do with the pressure drop in the reciever, causing liquid to evaporate to rebalance, in turn reducing liquid level etc etc). i can't remember who cause i don't know how to view my old posts?? if you can let me know how to view them i will repost it.

CB

[mikeref]

I was prodding this post with a 10 ft pole yesterday to see if it was going to bite. As it turns out, "Mad" remembers the discussion from back then . BTW, i see this affect with condenser fans cycling, sudden reduction in discharge pressure, balance between vapour/liquid and so it goes.

[monkey spanners]

I've seen it too with fan cycling, with one fan on the liquid line gets warm, then the second fan cuts in and the condensing temp drops but the warmth in the liquid line causes the refrigerant to evaporate till its cooled it down, then the second fan drops out.... doesn't seem a very efficient way of running stuff but its cheap to set up!

When i first heard the term 'over condensing' (as a arrogant apprentice...) it annoyed me a bit as its not possible, but as a short 'label' for the condition it serves its purpose as we all know what it means even if its not an acurate description.

Jon

[Magoo]

Hi MS
a good demo on yu tube. Plus I have been slammed for using THAT terminology as well.

[chillyblue]

Hope this helps, this is what i was looking for, but i can't take the glory it was posted by steinar.

Lets assume the system is running stable at a pressure slightly less than the cut in pressure for fan no2. Then the ambient increase ever so slightly, enough to cause the cut in of no2. The head pressure decreases imidiately due to increased condencer capacity. The refrigerant in the receiver has a higher temperature (and pressure) than the new lower condencing temperature/pressure and starts to boil off refrigerant gas. The outflow of refrigerant from the condenser is greatly reduced or stopped completely due to the rapid reduction in pressure in the condenser and the slower reduction in presure in the receiver. The result of this is the refrigerant level in the receiver drops to the level of the outlet tube, or even under the level of the outlet tube in some cases. And the sight glas start to bubble, foam, or if the refrigerant level drops enough, the sight glas get clear with only gas in it for a short while.

At the same time the condenser fills up with the refrigerant now missing from the receiver. The last part of the condenser (increasingly larger last part) is now filled with increasingly subcooled liquid. As the liquid refrigerant in the receiver has boiled off enough gas to lower the temperature and pressure of the remaining refrigerant, the flow from the condenser is resumed. At this point the condenser pressure is still higher than what it eventually will be at stable running conditions. This is beceause the size of the effective condenser is at this point reduced due to a large part of the condenser filled with subcooled liquid. As this subcooled liquid start to enter the receiver it cools the refrigerant still in the receiver as well as the receiver itself, it condenses some of the gas in the receiver further lowering the pressure. The pressure in the receiver falls more. The receiver starts to fill up with liquid and the sight glas clears. As the amount of subcooled liquid in the condenser decreases as it flows rapidly out of the condenser and into the receiver, the degree of subcooling decreases also. The condensing pressure decreases slightly more as the effective condenser size increases as the subcooled liquid exit the condenser. In the end balance is restored with a new lower condensing pressure than was the starting point.

Cheers CB