Need Help On experimental System

[manuelcm]

Hi everyone.

I've been working on a experimetal refrigeration vapor compression system on r134a.

The idea is to simulate a system that for some strange reason (may sound silly, but still I have to do it) has a variable speed compressor on the roof of a house (10 meters of height).

The pipes are considerably long, 10 metres up, and 10 metres down.

I had the system charged by a tecnician, but something went wrong, because the pressure in the low pressure side, goes down as soon as the system starts. At first, the system appeared to work well...........................but now, the pressure is negative at the evaporator outlet. I thought that maybe there was no evaporation, and the compressor was trying to pull liquid, and therefore the low pressure..but today I superheated the outlet of the evaporator to guarantee a full change of phase, but still I had the same problem.

I´m posting the data I logged today. The working time is short, because after a few minutes, I saw no temperature drop in the cold room, and the pressure was going negative, so the compressor might be suffering.

image ->

wwwest.uniandes.edu.co/~ma-calda/graph.JPG



Leaks are discarded as the pressure when idle, has been the same for days.

Even though I'm superheating inside the refrigerated space, the pipe that runs out of the evaporator is freezing. I've read that this could mean low charge.......but, how do I find the optimal charge for a experimental system??

Any help would be GREATLY appreciated.

Thanks in advance.

MANUEL

[Brian_UK]

Check your sub-cooling at the condenser and your superheat at the evaporator and compressor inlet. Getting these correct will establish whether you have the correct charge or not.

How did you decide on the initial refrigerant charge weight?

The low pressure will ALWAYS go down when the compressor starts, it is generating a reduced pressure to induce the vapour towards the compressor inlet.

[Abby Normal]

restriction

[Peter_1]

The link you provided isn't working.

What's the type and brand of: compressor, evaporator, condensor and expansion and orifice?

Somethings seems wrong selected or there's indeed a restriction.

We need more information. The fact that there's a VFD has nothing to do with the low suction pressure.

How you can know the correct charge? Measure SH and SC and watch also the sightglass and you must measure the right figures

[Brian_UK]

Here is the link Peter
http://wwwest.uniandes.edu.co/~ma-calda/graph.JPG

[manuelcm]

Thanks for the replies.

Today I decided not to operate the system, given that I suspect there's a lack of charge. I didn't want to damage the compressor. I'll have the system charged angain because apparently the tecnician I hired was sort of new to the business and may have misjudjed the amount needed. From what I've read, R134a is not as forgiving as other refrigerants with excess/lack.

But as I have the system instrumentated, and I'll keep an eye on the things you all mentioned.

Brian, Is it normal indeed for the pressure to go that low? I undestand the concept, but that shoudn't last long, I guess it lasts until it reaches its working pressure maybe? what would be a typical time?

I know that when a restricion exists, the time it takes for the system to equalise the pressure is quite large, (like 5 minutes or more) isn't it? Here it takes roughly a minute and a half.

Thanks for your interest. I´ll do some tests tomorrow and the next monday and we'll see how it evolves.

Manuel

[Peter_1]

ManuelCm, what about the answer on my other questions? They're needed also for the others to help you further.
With what program have you logged all the pressures and temperatures? (Picolog?)

[manuelcm]

Peter:

The compressor I'm working with, is a sanden sd5H09 Wobble Plate type, 87cc/rev aprox.

The condenser is of the parallel flow fin&tube type, rated 1/2HP

The evaporator is a U bent evaporator plate like the one you would find on domestic fridges.

The Expansion device is a capillary tube that came with the evaporator, I don't have the exact dimensions at hand right now, but very roughly it has a lenght of 1.5m and an internal diameter of 1mm, but I'll have to check in the lab.

The data was acquired using DAQFactory.

[Pooh]

Manuel
may sound like a silly question but is it a vertical 10m pipe run as part of your problem may be that you have filled the evaporator with oil from the comp which would explain why the plant run initially.
May be worth checking.

Ian

[manuelcm]

Ian

What you think is by no means silly. Indeed is a big problem, so when I designed the system, I put an oil trap in the discharge, and some valves, so eventually, when I see some oil acccumulated, I can take it back to the compressor. The worst case scenario would be the compressor running out of oil and subsequently failing, but nonetheless, the circuit is always circulating oil-free refrigerant as I could check in the sightglasses I have installed.

[Pooh]

Manuel
it would be better if you fitted at least 3 traps in the vertical suction line to aid the return of any oil which will definatelly be moving around the system.

[Abby Normal]

Peter:

The compressor I'm working with, is a sanden sd5H09 Wobble Plate type, 87cc/rev aprox.

The condenser is of the parallel flow fin&tube type, rated 1/2HP

The evaporator is a U bent evaporator plate like the one you would find on domestic fridges.

The Expansion device is a capillary tube that came with the evaporator, I don't have the exact dimensions at hand right now, but very roughly it has a lenght of 1.5m and an internal diameter of 1mm, but I'll have to check in the lab.

The data was acquired using DAQFactory.

Cut 2.54 CM off of the inlet end of the captube and try again

[swiftkool]

Hi there check for blockage in system and install u traps every 3 metres vertical rise of suction pipe. suction pipe may be logged with oil at lowest point as cant work its way back to comp on 10m rise.

[manuelcm]

I checked for blockage and apparently there is any. Since the beginning I have suspected that there's low charge. I've taken into account all that you have mentioned. I know this may sound silly, but, is there any other rule of thumb that should be followed to choose the right amount of r134a needed? I know one has to pay attention to the sightglass when charging, which by the way has to be clear. When the system is running, at some point I cannot tell if there's liquid flowing, because the sightglass looks like it was "dimmed". even though the system is free of humidity. By the way, I don't know If I already mentioned it, but the line that runs out of the evaporator is freezing.

[Renato RR]

Compressors smoler then 5kW of refrigeration can do this hight.Also did you calculate pressure drop of pipe when you pick pipes.Maybe you need double risers.Can we se your calculation?

Renato

[Larry2]

You say you're not sure if the guy charged it enough. Did he draw a good vacuum and rid all the water? Restrictions will show up various spots where moisture freezes. If there's water in there, all bets are off. Maybe it's time to empty the system nice and clean and measure in a known charge.

[Actinide]

The line you say that is freezing at the evaporator somewhere, where exactly is it occouring. Usually that is a indication there is a blockage as being the most restricted part of the system.. If it is part of the suction line after the evaporator, I would be looking more into that part of the system...

And yes most definately I would be checking for oil accumulation and would be installing some P-traps. Is there anyway of checking your oil level at the compressor? Check your superheat at the evaporator and you condensor subcooling aswell...

Being a capillary system if it were short of gas the capillary would be frosting back from where it enters the evaporator piping, and it needs to be spot-on with the charge to get correct pressures..