High SH no SC, but not short of gas?

[expat]

Hello everyone. I would like to run this one past the members:

Small walk in freezer not pulling down. When I got to site someone had disconnected the evap fan and told me that freezers only had static evaporators in them and he had disconnected it to make the freezer run better. I will not bother you with the ensuing argument.

As the evap was partially dismantled I took the opportunity to place my thermometer probe by the TXV bulb. Before reinstating the fan I took these readings.

R404A
Suction pressure 1bar/14.5psi
Head pressure 14bar/203psi
Temp at bulb fluctuating -23 to -28°C/-8 to -18.5°f
Amps 3.8A(3.6A on comp plate)

Reinstalled fan and freezer pulled down very slowly. After three hours got to
Ambient in freezer -16°C
Temp at bulb -13.3°C
Air off evap -19.5°C
Suction 1.5bar(-25°C)
Head pressure 17bar(39.5°C)
temp after cond 39°C

The liquid line leaving the accumulator goes up above it and bends in the direction of the evap and this is where the sight glass is fitted, almost on the same level as the TXV inside the box. It remanded clear throughout the three hours. The temps above is where we stagnated. Throughout the pull down air off evap, ever decreasing, remained 5°C lower than the bulb temp. I was expecting the SH to reduce as we approached the set temp but as you can see it remained high and my suction pressure/temp is not sufficiently low either.

Does anyone else feel that this is a worn out comp (13 years old)?

[expat]

I forgot to mention that the comp current went up to 5.6A once the fan had been reconnected and thus the evap under load.

[powell]

expat,

Can you furnish the compressor brand and model number? The evap model would be helpful also and the size of this small walk in freezer.

The next big question concerns the history of this equipment. Did it work before, that is, before the knucklehead removed the fans.

[expat]

Thanks for coming back to me Powell.

Unité Hermetique CAJ2446Z:

3.3A, 2290w @ -10°C, 1110w @ -25°C, 550w @ -35°C.

I'm afraid I have no history of the machine as the shop was bought not so long ago. I do however know that upon buying the shop being as the freezer had been out of commission for some months the owner had it looked at by a tech who advised him to have the fluid removed and the exact 960grams on the label to be recharged in. This is why (with the fact that the sight glass is full) I believe the system is not short of gas.

I found the temp probe wildly out calibrated. Coupled with the fact that someone was fiddling with it the day I turned up I have no way of knowing how many button pushers have been there before me.

I don't know what the evap is but it is a wall mount about 40cm wide by 50cm high at front where the evap sits. Above this it slopes back toward the wall to allow the fan to suck air from the ceiling which is blown out across the afore mentioned evap.

I feel that if it can get the evap down to good temps when the fan is not running then this would indicate that the TXV is doing it's job as I saw with the fluctuations with the temps at the bulb. But, once under load the comp is not able to maintain the head and suction pressures hence the slow pull down and stagnation of said. What do you think?

[Refrigerologist]

Hello everyone. I would like to run this one past the members:

Small walk in freezer not pulling down. When I got to site someone had disconnected the evap fan and told me that freezers only had static evaporators in them and he had disconnected it to make the freezer run better. I will not bother you with the ensuing argument.

As the evap was partially dismantled I took the opportunity to place my thermometer probe by the TXV bulb. Before reinstating the fan I took these readings.

R404A
Suction pressure 1bar/14.5psi
Head pressure 14bar/203psi
Temp at bulb fluctuating -23 to -28°C/-8 to -18.5°f
Amps 3.8A(3.6A on comp plate)

Reinstalled fan and freezer pulled down very slowly. After three hours got to
Ambient in freezer -16°C
Temp at bulb -13.3°C
Air off evap -19.5°C
Suction 1.5bar(-25°C)
Head pressure 17bar(39.5°C)
temp after cond 39°C

The liquid line leaving the accumulator goes up above it and bends in the direction of the evap and this is where the sight glass is fitted, almost on the same level as the TXV inside the box. It remanded clear throughout the three hours. The temps above is where we stagnated. Throughout the pull down air off evap, ever decreasing, remained 5°C lower than the bulb temp. I was expecting the SH to reduce as we approached the set temp but as you can see it remained high and my suction pressure/temp is not sufficiently low either.

Does anyone else feel that this is a worn out comp (13 years old)?

I would suggest you check the superheat setting of the expansion valve. From the readings quoted you hav over 11 degrees of superheat instead of 5 to 7.

[nike123]

How sure are you in these measurements? Evaporator side seems OK. What was outdoor ambient temperature when you take this measurements?

If compressor is inefficient you should have low superheat, low subcooling, and smaller amperage then normal for that evaporation temperature. So, to rule out compressor inefficiency you need to know current for actual evaporation and condensation pressure.
Did you recovered all refrigerant and charged new virgin refrigerant?

[Refrigerologist]

Thanks for coming back to me Powell.

Unité Hermetique CAJ2446Z:

3.3A, 2290w @ -10°C, 1110w @ -25°C, 550w @ -35°C.

I'm afraid I have no history of the machine as the shop was bought not so long ago. I do however know that upon buying the shop being as the freezer had been out of commission for some months the owner had it looked at by a tech who advised him to have the fluid removed and the exact 960grams on the label to be recharged in. This is why (with the fact that the sight glass is full) I believe the system is not short of gas.

I found the temp probe wildly out calibrated. Coupled with the fact that someone was fiddling with it the day I turned up I have no way of knowing how many button pushers have been there before me.

I don't know what the evap is but it is a wall mount about 40cm wide by 50cm high at front where the evap sits. Above this it slopes back toward the wall to allow the fan to suck air from the ceiling which is blown out across the afore mentioned evap.

I feel that if it can get the evap down to good temps when the fan is not running then this would indicate that the TXV is doing it's job as I saw with the fluctuations with the temps at the bulb. But, once under load the comp is not able to maintain the head and suction pressures hence the slow pull down and stagnation of said. What do you think?

From everything you have stated I must conclude that the expansion valve needs opening or replacing and setting up correctly. with a 17bar head and 1.5bar suction you have a compression ratio of 7.2. I think it is unlikely that the compressor valves are worn. If you open the expansion valve to acheive a superheat of between 5 and 7 degrees C you should find the air off temperature will rise but the cooling effect will increase. Hope this helps.

[The Viking]

Is it possible to do a pumpdown test of the compressor?

[nike123]

For that evaporation and condensation temperatures that compressor should draw 4 Amps.

[nike123]

We should adjust TEV only when we sure that is enough liquid refrigerant in front of him.
If he don't have subcooling than there is no only liquid in front of TEv.

[expat]

Refrigerologist, Nike, thanks for the replies.

As the TXV was metering correctly while the evap was not under load I, and then never got the chance to prove it could when the evap was under load I chose to come away and ponder the problem.

Nike, I'm sure of the measurements. And ambient was 25°C and air off condenser 35°C. I have not pulled the system and put in a new charge, no. But I can see it may become necessary.

Refrigerologist, you posted at the same time as I was answering so I will add that your post has been most helpful. Adjusting the TXV was not something I wished to do as it is often a knee jerk action. Also I was thinking of the suction pressure increasing but as you have pointed out it should increase the cooling effect.

[Refrigerologist]

Hello everyone. I would like to run this one past the members:

Small walk in freezer not pulling down. When I got to site someone had disconnected the evap fan and told me that freezers only had static evaporators in them and he had disconnected it to make the freezer run better. I will not bother you with the ensuing argument.

As the evap was partially dismantled I took the opportunity to place my thermometer probe by the TXV bulb. Before reinstating the fan I took these readings.

R404A
Suction pressure 1bar/14.5psi
Head pressure 14bar/203psi
Temp at bulb fluctuating -23 to -28°C/-8 to -18.5°f
Amps 3.8A(3.6A on comp plate)

Reinstalled fan and freezer pulled down very slowly. After three hours got to
Ambient in freezer -16°C
Temp at bulb -13.3°C
Air off evap -19.5°C
Suction 1.5bar(-25°C)
Head pressure 17bar(39.5°C)
temp after cond 39°C

The liquid line leaving the accumulator goes up above it and bends in the direction of the evap and this is where the sight glass is fitted, almost on the same level as the TXV inside the box. It remanded clear throughout the three hours. The temps above is where we stagnated. Throughout the pull down air off evap, ever decreasing, remained 5°C lower than the bulb temp. I was expecting the SH to reduce as we approached the set temp but as you can see it remained high and my suction pressure/temp is not sufficiently low either.

Does anyone else feel that this is a worn out comp (13 years old)?

17 bar discharge & 1.5bar suction pressure equates to a compression ratio of 7.2:1 so I think it is unlikely the compressor is not pumping! Have you tried a pump down test to check the compressor? As I said in another thread try adjusting the expansion valve to flood the evaporator further. You should have a superheat of around 5 to 7 degrees C!

[expat]

Nike, that is why I mentioned that the sight glass is practically on the same level as the TXV and was always clear of bubbles. So the valve must be getting liquid.

[expat]

You guys are posting very quikly, I can't keep up.

No I havn't tried a pump down test. Do you mean to see if the valves are leaking past or to see if it can pull a vac?

[Refrigerologist]

We should adjust TEV only when we sure that is enough liquid refrigerant in front of him.
If he don't have subcooling than there is no only liquid in front of TEv.

I thought he had 0.5 degree of sub cooling, not much but it is liquid!

[Refrigerologist]

You guys are posting very quikly, I can't keep up.

No I havn't tried a pump down test. Do you mean to see if the valves are leaking past or to see if it can pull a vac?

If you front seat the suction service valve link out the LP switch and run the compressor, if it pull belpow 15" of Vac it is good enough for refrigeration. Also switch the compressor off and check that the pressures do not equalize to quickly.

[nike123]

Nike, that is why I mentioned that the sight glass is practically on the same level as the TXV and was always clear of bubbles. So the valve must be getting liquid.
I didn't read carefully your post:o.

[nike123]

I thought he had 0.5 degree of sub cooling, not much but it is liquid!
As I said, I didn't read carefuly, header says "no SC!;)

[Refrigerologist]

One other thing, years ago as a nipper I was working with a really good engineer, and he couldn't sort out an old counter type shop unit. He had tried everything. Then I looked at the condenser and it was choked with dust inside the fins. He felt a right pratt, he had checked, but the shop owner had brushed out the fins and it did look clean, until I shone a toarch through it! After cleaning it worked a treat!

[expat]

Ok, fellas thanks for the replies I'll give the TXV screw a twiddle tommorow and see if it helps.

Interesting comments on the compression ratios as this hadn't occured to me.

I'll let you know.

[expat]

Refrigerologist, I'll start by testing the comp then.

Thanks to both of you.

[Refrigerologist]

Refrigerologist, I'll start by testing the comp then.

Thanks to both of you.

Good luck fella! I'll check back to see what you found. I did have a similar problem a couple of years ago. A new blast freezer wasn't pulling down quickly enough. The young service manager kept teling me he had adjusted the TXV, I kept saying had he taken any measurements. He kept saying that they had wound it open a couple of turns. When I went to check it had a superheat of 19 degrees C. But the air off was pretty cold. I put the gauges on, (something he and another guy hadn't done) and a temp probe and adjsuted the TXV for 6 degrees. It worked great even though the air off temp actually went higher.

[expat]

Sorry for no updates lads, haven't had time to go back yet. I'll keep you informed.

[Gary]

I thought he had 0.5 degree of sub cooling, not much but it is liquid!

I would tend to question where and how he is measuring subcooling. He should be measuring the liquid line temperature near the outlet of the receiver, not the outlet of the condenser.

[Gary]

It is entirely possible that the box is experiencing a heavy latent load and once it dries out the temperatures/pressures may be more to your liking.

[Gary]

You might want to try warming the TXV bulb by holding it in your hand. Since there is solid liquid at the TXV inlet, this should flood the coil.

If it does not flood, then adjusting the superheat isn't going to do anything.

If the coil will not flood when the bulb is warmed and there is solid liquid at the inlet, then the problem must be plugged inlet screen or undersized orifice.

[Refrigerologist]

You might want to try warming the TXV bulb by holding it in your hand. Since there is solid liquid at the TXV inlet, this should flood the coil.

If it does not flood, then adjusting the superheat isn't going to do anything.

If the coil will not flood when the bulb is warmed and there is solid liquid at the inlet, then the problem must be plugged inlet screen or undersized orifice.

If you look at the performancefigures for the condensing unit you will see that it will only provide around 500watts of cooling duty at the figures given. If a danfoss valve is installed then the smallest orifice will provide 400watts nominally. Given the high pressure diferential between the liquid and suction pressures this will actually provide the required duty. I agree that it could be a partially restricted orifice screen, but it would not hurt to open the valve and check if the superheat decreases. How often have I heard the myth that expansion valves are factory set! Yes they are, but only for the nominal set of conditions that are set by the manufacturer. If the operating conditions differ then the valve must be set by the techie on site. If he has 11 to 12 degrees of superheat, then he shold try adjusting the valve setting, if no improvement then pump down and strip the valve to see if filter screen blocked.

[Gary]

If you look at the performancefigures for the condensing unit you will see that it will only provide around 500watts of cooling duty at the figures given. If a danfoss valve is installed then the smallest orifice will provide 400watts nominally. Given the high pressure diferential between the liquid and suction pressures this will actually provide the required duty. I agree that it could be a partially restricted orifice screen, but it would not hurt to open the valve and check if the superheat decreases. How often have I heard the myth that expansion valves are factory set! Yes they are, but only for the nominal set of conditions that are set by the manufacturer. If the operating conditions differ then the valve must be set by the techie on site. If he has 11 to 12 degrees of superheat, then he shold try adjusting the valve setting, if no improvement then pump down and strip the valve to see if filter screen blocked.

If a warm bulb doesn't increase refrigerant flow, then adjusting the stem isn't going to increase the flow either.

[expat]

I'm sorry I havn't been online this week to keep you all updated. It was rude, but some weeks are worse than others and then oher weeks are worse still and then there are weeks you wished...etc

As for restrictions in valves I was unfortunately preoccupied this week with the one in my son's head. Happily, the MRI showed his shunt to be working correctly so,..sigh of relief.

I must say I am as usual astounded with the advice from all, but will have to say that the simplest answer (proposed by Gary) seems to be the winner.

I havn't returned to take new measures, only payment as the client was totally satisfied.

I'm so glad to have this forum for support.

Conclusion: The box was experiencing heavy latent load, and someone had been fiddling around with it. I hope one day to have the experience you gentlemen have to diagnose and rectify in less time. But what the hell it's all good fun and my son (Henry) is well.

Things to remember:
If a warm bulb doesn't increase refrigerant flow, then adjusting the stem isn't going to increase the flow either. Gary

[The MG Pony]

We got a big three door reach in at the shop couple days ago, complaint? Not holding temp and long pull down. The thing was process tube so you can't get tighter then that and every thing far as gas was as it should, clean evap and fans ran fine, problem? dirty condencer!

All ways check the cleanliness of coils befor any thing, saves you a truck load of work. and as gary mentioned holding the bulb is a nice way to get a ruff idea fast where to look!

Glad to hear it was sorted out!

[paul_h]

I would tend to question where and how he is measuring subcooling. He should be measuring the liquid line temperature near the outlet of the receiver, not the outlet of the condenser.
I'm confused with this statement, no one in all my years has ever suggested measuring sub cooling after the reciever, it brings an extra variables into play rather than measuring compressor/condenser dynamics.
(edit: I'm treading on egg shells here with regards to how all other subcooling threads here have blown up out of proportions though. Can you sum up or link to your reason why now you have decided subcooling should be after a reciever? What about systems that don't have a reciever, are they now so much more different in behaviour and how do they subcool? ;) )
When testing subcooling, you want to test whether the condenser is up to the job and to test the 'general health' of the system. ie over/under charge, heat rejection and other things that can cause the compressor to draw extra amps.
Taking subcooling at the reciever ignores all the superheated vapour that may exist between the compressor and reciever. As you know, it's the condensers job to cool the refrigerant from superheated to subcooled. When did this turn into the recievers job? If it's not the recievers job, why measure subcooling after the reciever?
Any contanimation will be ignored by reciever outlet subcooling as the reciever only draws liquid. What effect would this have when diagnosing refrigerant blends? Also any incondensibles causing high head pressure and compressor amps will not be measured in post reciever subcooling. (I'm aware of the current debates, arguments, I may have misread, but I thought you were arguing the opposite there)

[paul_h]

Things to remember:
If a warm bulb doesn't increase refrigerant flow, then adjusting the stem isn't going to increase the flow either. Gary
Agreed.
Warming the bulb is a better method, it proves the valve operates on it's own. Turning the adjustment screw to increase refrigerant flow may work, but every valve I've had to do that to have been stuffed in the end.
That's because it only responds to manual adjustments and nothing else, ie no charge in the bellows or something else wrong with it and I've had a callback the next week and had to replace it. It's better to see them operate themselves than to play with one. Every old valve I've had to adjust ended up being stuffed.

[Refrigerologist]

I agree with Gary and Paul h that usually it would better to warm the TXV phial to see if the valve would open. This is something that I too would recommend. Also if the valve body and head is caked in ice build I would first recommend removing the ice and warming the valve. I should not have assumed that this had been done. My reasoning for adjusting the superheat setting had more to do with years of following behind guys who had commissioned equipment which was left operating reasonably well, but not quite getting down to temperature for no other reason than the superheat values had not been checked and adjusted. I had also assumed that the equipment was almost down to temperature at -16 deg C and so a reasonalbe assessment could be made of the superheat setting. I can see that will have to up my game considerably when posting comment with you guys about!

[Gary]

I'm confused with this statement, no one in all my years has ever suggested measuring sub cooling after the reciever, it brings an extra variables into play rather than measuring compressor/condenser dynamics.


Anyone who has read my books can tell you I do things differently. My books contain no list of acknowledgments because they are based on my own research rather than the work of others. I was looking for better ways to do things. Whether I succeeded in this or not is for others to judge, but for better or worse, my methods are... different.

I recommend measuring subcooling near the receiver outlet for the same reason we would place a sightglass near the receiver outlet. We want to know the condition of the refrigerant before it enters the metering device.

On a system that has no receiver, the bottom of the condenser is acting as a receiver. Either way, we are measuring receiver outlet subcooling.

[paul_h]

I respect your opinion, I'm like you, mainly practical trained.
But what's hapenning at the reciever outlet depends on th TX valve flow with all that static refrigerant there, not how the condenser is dealing with the heat rejection.
You might as well have the sightglass and be measuring subcooling at the TX valve inlet, both very useful values, but neither evaluates what the compressor head and condenser are related, which is the purpose of subcooling, ie to make sure the condenser is condensing.
Ambient temps and overcondensing can be an issue, I agree. But if you're going to go down that path you should take all your measurement at the TX valve inlet, not the reciever outlet because it can still warm up on the way the the TXV.
But the purpose for subcooling measurements for me have always been whether the condenser can do it's job, ie enough charge, enough refrigerant or no flow restriction on the liquid line. And if your are dealing with a situation like this original post with high amps, it's the discharge/vapour side and condenser/subcooling you need to look at

[Gary]

I respect your opinion, I'm like you, mainly practical trained.
But what's hapenning at the reciever outlet depends on th TX valve flow with all that static refrigerant there, not how the condenser is dealing with the heat rejection.
You might as well have the sightglass and be measuring subcooling at the TX valve inlet, both very useful values, but neither evaluates what the compressor head and condenser are related, which is the purpose of subcooling, ie to make sure the condenser is condensing.
Ambient temps and overcondensing can be an issue, I agree. But if you're going to go down that path you should take all your measurement at the TX valve inlet, not the reciever outlet because it can still warm up on the way the the TXV.
But the purpose for subcooling measurements for me have always been whether the condenser can do it's job, ie enough charge, enough refrigerant or no flow restriction on the liquid line. And if your are dealing with a situation like this original post with high amps, it's the discharge/vapour side and condenser/subcooling you need to look at

What I found on a wide variety of systems is that liquid began to back up into the condenser, unnecessarily raising head pressure, slightly above 15F/8.5K subcooling at the receiver outlet. Given standard condenser sizing or even reasonable oversizing, the results of these tests were very consistent. In measuring subcooling, I am primarily concerned with backing up liquid into the condenser (overcharge) and I have found that receiver outlet subcooling is an excellent way to measure this (assuming airflow problems have first been eliminated).

In charging a system, if the superheat is right then there is enough refrigerant to do the job. However, we also want enough surplus to ensure solid liquid to the TXV in order to extend its life. What I recommend is to charge until the sightglass is clear or the receiver outlet subcooling is 15F/8.5K, whichever comes first. The clear sightglass almost always comes first, thus we can have a clear sightglass without backing up liquid into the condenser.