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Hi guys I am working on a walkin freezer R404 liquid cooled system. I am running 250 psi head and 25 psi suct. Sub cooling temperature is 82 outof the condenser giving me about 3 degrees. I cannot clear the sight glass without the the compressor freezing up. The box temps holds steady at 5 degrees. It has a new evap coil, txv, SV., etc. My question is am I flooding back not sending liquid to the txv or just to picky. again the box holds temp it just the sight glass and frost that bothers me.
Your post is a little confusing. You say it's a walk-in freezer yet then temps you give indicate it's a chiller.
Suction pressure is in the middle for either. If it has a manufacturers plate on it with the "weighted charge" it would go with that in the first instance.
250 psia of R404A is 100°F -82°F at condenser outlet is 18°F subcoolingQuote:
Originally Posted by sbayliss01
250 psig of R404A is 104°F -82°F at condenser outlet is 22°F subcooling
How did you came up with 3°F subcooling.
Take your measurements again and this time include also these:
What is pipe temperature at evaporator outlet and at compressor inlet?
What is pipe temperature at TXV inlet.
What is air in and out temperature and RH (or DB and WB) of evaporator?
Pressures are absolute or gauge?
What is outdoor ambient temperature and condenser air off temperature?
What is discharge pipe temperature.
Could be helpful also:
Compressor model# and compressor current.
Just because you have freezing doesn't mean you haveflood back, if you have superheat/
Hi,
With that sub-cooling you have overcharged the system.
Remember for Zeotropic blends like R404a sight glass should not be full.
Cheers
this is a interesting statement, i work with lots of r404a sytems, and always charge to a full sight glass, only time i see the sight glass bubling is when condenser fans cut off, am i doing this wrong?Quote:
Originally Posted by lana
http://www.real-world-refrigeration....t-glass-6.htmlQuote:
Originally Posted by coolhibby1875
http://www.real-world-refrigeration....t-glass-7.html
Tecumseh advises "To avoid overcharging do NOT charge HFC blends to a clear sight glass."
http://www.tecumsehcoolproducts.com/...-RD-0003-E.pdf
I fail to understand how charging to a full sight glass will result in the system being overcharged , the sight glass is also an indication that you have a full head of liquid in the liquid line, if your sight glass is not full you do not have a full head of liquid to your expansion device and this will affect its performance , even in a system running on r22 with a liquid reciever once the sight glass is full you may still add refrigerant this is obviously dependant on the size of the reciever, with this extra refrigerant there is no effect on the system or any symptons of being overcharged the extra refrigerant is stored in the reciever. if overcharged liquid will back into the condenser reducing the effective coil surface area available and in doing this reducing the capacity of your condenser . i accept that the refrigerants have different densitys and the system may require different weighed charges but should the sight glass still not only be an indication that you have fully condensed refrigerant liquid and no vapour at the outlet of the condenser.
Please do not assume i am saying it is correct to add refrigerant once the sight glass is full i have only said this to demonstrate my understanding of what is occuring in the system.
I agree with coolhibby i have also charged many r404a systems to a full sight glass and have had no symptons that the system has been overcharged.
what i have noticed is that when the sight glass is bubbeling the flow rate through the valve seems high and you have pronounced high loading on your evaporator and in turn on the condenser once the sight glass clears the expansion valve settles and system returns to design conditions.
http://wiki.answers.com/Q/What_do_bu...ght_glass_mean
Quote:
There are several reasons for bubbles in the sight glass. If one of the traditional refrigerants showed vapor in the sight glass it often meant there wasn't enough liquid refrigerant being fed to the valve, and more refrigerant was added to the system.
Blends could show flashing for the same reason, however, they can also flash when there is plenty of liquid in the receiver. Ironically, this liquid in the receiver could be causing the problem, particularly when the equipment is in a hot environment. Blends will come out of the condenser slightly subcooled - at a temperature below the saturated temperature of the blend at the existing high side pressure.
Yet when the blend sits in the receiver, it can "locally fractionate," or change composition slightly by shifting one of the components into the vapor space of the receiver. This will effectively produce a saturated liquid in the receiver, at the same pressure you had before, which flashes when it hits the expanded volume of the sight glass. In most cases these bubbles will collapse when the blend gets back into the tubing which feeds the valve, and the system will operate just fine.
The above quote makes no sense to me the author says that the liquid expands in the increased volume in the sight glass how does expansion take place without pressure or temperature change. then again how is this vapor condensed or what forces cause the bubble to collapse it surely needs to change its state from a vapor to a liquid to be absorbed into the rest of the liquid refrigerant, we are not dealing with magic!! vapor cannot collapse it must change state and this can only happen under certain prescribed conditions. if the refrigerant flashes and expands in the sight glass what happens in a drier with 50 to 100 times the volume of the sight glass. there are other areas of the quote that i do also not understand or agree with, but do not have time to go into now.
I am not a qualified design or mechanical engineer but have been working in the industry for 18 years in various roles, so i may be wrong if so please help me to understand your argument as we all should know in this trade you never know it all and are always learning.
Dirk, I give you, and others, link to original article, so your questions should be addressed to author of article. I simply quoted what I taught it make sense to me. ;)
I thought I would explain a bubbles in a sight glass.
Firstly in my opinion there is very little liquid sub cooling leaving a reciever ( the liquid pressure may be lower than the discharge pressure, thuus giving the apperence that the liquid is sub cooled)
If you can accept this point, then we can say the liquid is at saturation point.
Any increase in volume (sight glass, drier), will cause the need for the refrigerant to expand. This can be achieved in to ways either by adding heat to liquid (change its density) or by flashing off some of the liquid, to make vapour. "the bubble"
With your "R4" refrigerants, you have glide, which can be said to be a slight difference between evaporating and condensing temperature at the same pressure. This difference may cause the bubble to remain.
Suppose the system is running and has been charged to a full sight glass, can there still be an increase in volume if all available space, be it in the sight glass, drier or any other vessel in line is occupied by the liquid refrigerant, would it not be necessary to displace this liquid refrigerant before any expansion could take place. if there was flash gas would the extra space taken up by the vapor displace the remaining liquid causing an increase of pressure in the liquid line.If you put a 50 gallon drum in the line and had a liquid line full with subcooled liquid refrigerant would there be any pressure or temperature difference measured between the line directly entering and leaving the drum.
The sub-cooled bit is the difference. It is the pressure drop that causes the flashing. Remember that you have flow, and when you have flow you always have pressure drop. (very tiny) the pressure, temperture, volume and liquid/vapour mix will always reach equalibriumQuote:
Originally Posted by dirk
oops i always charge txv 404a systems to full sight glass as was once taught never to rely on gauges alone. amps and air on /off temps are important
Thanks madfridgie . Your thoughts on the following. If we agree that at conditions of high load where there is no sub cooling and ambient conditions around the liquid line are above saturation temp we will have flash gas. And we also agree that this flash gas is the resultant increase of volume of the refrigerant and would the result in a higher tempreature or pressure in the liquid line. could we then say that flash gas under these conditions is a phenonomen that feeds on itself.
On a r404a low temp system on site i have done the following, charged the system to a full sight glass allowed the system to achieve as close as possible to design temp, then slowly removed charge until there is vapor in the sight glass.performance was logged with on site danfoss monitoring system. I then added refrigerant to ensure the sight glass was full at load after defrost termination. the most noticable difference in performance was that when the system was charged to a full sight glass the recovery to set point after defrost was achieved in close to half the time compared to when the sight glass showed vapor. there was no indication that the system was overcharged.
Hi Dirk, some what difficult to but into a few words.
the liquid line is flowing so the pressure will not be higher than the upstream.
Vapour flowing down a pipe causes a greater pressure drop comared to that of the same "mass" of liquid (not volume) As the pressure drops more vapour has to be produced to fill the void (if we can call it this) in this case by boiling some of the liquid, thus dropping the liquid temp to saturation point. If we then say that the liquid pipe becomes overcome with vapour then little mass flow (down the liquid line) is occurring at this point then the condensor will start to fill with liquid, driving up the head pressure (thus reciever pressure) to a point where is staturation pressure is above that of what is in the liquid line and a increase of liquid/mass will occur.
I am trying to show snap shots in time but in reality these subtle changes are happening all time. "equalibrium"
Please do not get the idea that I am against a full sight glass, because normally i would charge in a similar manor. I was always taught that a sight glass in an aid to charging (Not the be all and end all of charging)
I was trying to point out that you may have a bubble of vaour in the sight glass, not flowing bubbles, when you are short of refrigerant.
I hope that made sense?????????!!!!!!!
Thanks Mad fridge agree with what you have said. It Is a unique industry we work in and allways challanges us. I can not recall how many times i have had heated arguments with design engineers with them arguing that in theory my findings on site are impossible, only to in the end have them go to site and also after a few days walk away scratching thier heads and cussing.
I think MF has it pretty well right. I believe that these bubbles are caused by pressure drop and lack of subcooling. There is alway a pressure drop in an operating system regardless of refrigerant. I can remember this "problem" being evident in the days of R502. When condenser fans cycle the rise and fall of pressure will cause fluctuating amounts of flash gas as well as expansion and contraction of bubbles due to these pressure fluctuations. Systems are engineered to balance at some arbitrary condition set by the engineer. At other points the system will most likely have some form of system imbalance. The sight glass like other parts of the system will also show this imbalance.