To low condensing temperature

[crocens]

Hi again :)

Just a small question this time :)

We have a DX system with TXV and compressor without capacity controll, there is liquid reciver without sightglass (sightglass is after reciver on pipe).
Lets say it works good on -10/+45*C.
But when we get condensing temp lower i get "bubbling" in sightglass.
I understand, that evaporating temp will go lower and the capacity of the system will all in all getsmaller, so i do not understand why i have bubbling in sightglass...

Thanks for replays, Marcin

[install monkey]

over condensing,either block of some of the condenser or fit a pressure switch to stop the fan in low ambient

[nike123]

What refrigerant is in system?
You need to measure subcooling (and superheat) in order to get correct answer.
Bubbles in sight glass could be result of sight glass change in shape where liquid refrigerant flows and sudden change in pressure drop occur across that shape change. With multi component refrigerants, that could develop bubbles of more volatile component of refrigerant composition in glass which later disappear when refrigerant get to pipe after sight glass, and subcooling is not substantial to prevent that.
Therefore, you need to have enough subcooling to prevent that.

Normal behaviour is when cooling starts and load is heavy that sight glass is full of bubbles and then when temperature is near set temperature that sight glass is clear.

Measure your subcooling at condenser outlet when condensation pressure is low and post here.

http://www.refrigeration-engineer.com/forums/showthread.php?21057-Over-condensing-under-condensing
http://www.refrigeration-engineer.com/forums/showthread.php?13857-What-is-overcondensing

http://www.refrigeration-engineer.com/forums/showthread.php?27543-Over-Condensing/page2

[Peter_1]

Good post Nike! INdeed , as Nike said, you need to measure your SC.
It's all because your liquid is near saturated condition and the smallest pressure drop you then have will create flashgas.
Are your fans on the condenser switching off/and on or are the proportionally controlled?
The moment you see flashgas, block of once for 30 to 45 seconds your condenser airflow with a cardboard. You will see the bubbles will disappear and come back once you remove the cardboard again

[crocens]

R-404a, r-134a and other simillar.
i have seen that few times, currently i do not have such a problem anywhere :) i have had such problems in winter on units. The fans where on/off. When they turned on and pressure droped, bubbles occured, when they turned off and pressure rised everything was good.

I am just curious what is happening inside condenser (and whole unit) when pressure get to low. I mean i know what is happening but i can not see how can that influence my problem. I have tryed to put some excamples on log(p-h) charts using Coolpack and everytime i get to the point that "bubbleing" should not occur...

Edit:
I want to do some system optimalizations (hope to reduce electrical costs) so i have to understand all the problems i can have :)

[Peter_1]

If you want to do optimization, go as low as you can on your HP. But reducing HP will reduce also your LP (increased COP, lower DP over compressor, increasing volumetric efficiency) so destroying again somehow the benefits. If you reduce HP, then you must slow down the speed of the compressor (VFD) or shutting down other compressors on a rack to maintain a stable LP with decreasing HP.

The phenomena with the bubbles appearing when fans switching can't be plot in Coolpack but it can be explained.

[crocens]

yes, that is what i want to do.
Lets say we are building a new system. We have stady load all around the year.
I want to force compressor to draw less power in winter. So i put larger condenser and compressor with capacity control (speed control) to get evaporating pressure at constant level. In winter compressor have more capacity (due to lower HP) so i put alsow larger evaporators (at summer they will work on lower TD then "standard" evaporators). Oh, and i will put EXV.

Is it a good idea? how do you think?


"The phenomena with the bubbles appearing when fans switching can't be plot in Coolpack but it can be explained."
So maybe, if you have time, you will explain that? :)

[Peter_1]

Perfect solution / setup, especially in your country when it gets cold in winter.

If you use Danfoss valves, you even don't need an EXV valve because a DP f 4 bar is acceptable.

The bubbles phenomena... I demonstrate this always in my classes, attaching a manifold and a temperature meter on the outlet of the condenser and teh outlet of the liquid receiver.

The system is starting up at relative cold ambients (you will see the flashgas more in cold ambient) Condenser pressure rises and also the associated liquid temperature. There's a relation between liquid and pressure because we have saturated liquid leaving the condenser, mostly very slight saturated liquid. Pressure rises further till HP engages the fans. What then happens is a very sudden drop of the pressure (mostly within seconds) but the mass of liquid and the temperature of the liquid receiver can't follow fast enough in temperature drop due to the thermal inertia of the liquid remaining in the receiver and the steel of the receiver. So, you were just on the left line of the saturated curve of the log p/h and pressure drops very fast but temperature remains temporarily the same. So you're going straight down, vertically in the log p/h where you arrive in the mixed zone of the log p/h. Fans remain on for some time and temperature of liquid and receiver decreases more and more to a saturated condition because there's colder liquid flowing continuously. So the point on the log p/h which was at the right side of the saturated line slowly moves now to the left horizontally now (same pressure but decreasing temperature) You will notice a decreasing number of bubbles. Fans are stopping due HP lower setpoint reached and pressure increases again very fast but now with a cold liquid remaining in the receiver and a cold receiver itself, again with thermal inertia working. So, our working point in the log p/h is moving now vertically upwards, moving very fast outside the mixed zone to a saturated and subcooled condition and bubbles all disappear within seconds. We're back at the beginning of the cycle.

We measure this in classes and you really can see very good temperature an pressure not following each other.

Where will you see this phenomena? Small units with almost no subcooling, cold ambient around the condenser, fans switching on and off and low loads on the evaporator.

Hopes it's cleared a bit for you. I can draw on a log p/h what happens when fans ware switching on and/off if you like. The working point is making a small square shaped movement just around the saturated curve (from left to right of it and vice-versa)

[mad fridgie]

When a system is designed, a selection is normally made at a steady condition (normally max load and high ambient)
Dropping the head pressure will reduce power draw, and increase refrigeration capacity. Your evap was designed for the original set of conditions. The suction pressure must drop to meet the new equalibrium conditions. You then have to consider, what effects this has on the working envelope of the system components, and on the chilled product (lower SST will dry a product out) and is cheaper to run with a lower SST and SCT, compared to an intermediated set of conditions. What you really need to do is to reduce the system mass flow at the lower ambient conditions, either by speed control of the compressor, multiple compressors or unloading of the mechanics of the compressor.
The bubbles as explained by Peter are just rapid changes in pressure, you can also get liquid "hang up" where flow through the condensor is poor and liquid just sits in the condensor. (ofter seen on cond coils which are in a horizontal position)

[crocens]

mad fridge, thank you for your opinion. Now i know another problem that can i have.
Peter_1, i understand it very good and i see that on log(p)-h chart clearly. Thank you very much for the time you spend to explain that to me.

Thanks for rest of you for advices you gave me!

And sorry for my english, i know it is horrible :)
Peter_1, here in Poland 95% of cooling units (generally whole refrigeration) is designed like it was done in early 90' of 20 century... We have problem becouse there are only 3 or 4 schools where you can learn refrigeration... so we have no good specialists. That why I am here and asking those, sometimes, silly questions :)

But i will change that :) wish me luck, this year i'm building testing chambers to test my energy saving systems (and see what other problem may i have). Next year i will try to get clients who want to save energy :)

[Peter_1]

Hey Crocens, i bet you speak better English than most English peoples speaks Polish.I'm sure some also have more than a smile on their face when thy read my phrases.

Don't be afraid to ask questions, we're a technical community where techs help each other without a language barrier. Silly questions don't exist well...almost. Some are sometimes really stupid questions.

Go on with your ideas because they will improve your machines and save the environment.

[tbirdtbird]

also, Tecumseh says "To avoid overcharging do NOT charge HFC blends to a clear sight glass." pg 6
http://www.tecumsehcoolproducts.com/inside/File_attachments/Service%20Guidelines%20HCFC%20R22%20to%20HFC%20Blends-RD-0003-E.pdf

[mmack1993]

Dropping the head pressure will reduce power draw, and increase refrigeration capacity. Your evap was designed for the original set of conditions.
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