Ice on Strip of Evaporator

[georgedvf]

Hello. I have a problem with a room with two evaporators (air defrost) using R-404A, the room is supposed to be at 2°C, but I can't get to lower it more than 8°C because there has been ice building up just in a horizontal strip (all long of the back of the evaporator), the distributor and the pulp tubes are also frozen, but at the suction side only the 3 tubes of the circuits on the strip where there's ice are frozen.

Suction pressure: 56psi
Suction temp: 0°C
Superheat: 6°C

This room is part of a system supplied from a Parallel Compresors Rack, there are another 4 rooms working fine.

Any help is appreciated!

[aramis]

How's your defrost cycle programmed.

At what distance is the evaporator from the back wall. Is there any product piled up underneath the evap that may be blocking the airflow?

What's the fan diameter.

Anything blocking the air in front of the fans that may be causing cold air recirculation?

[georgedvf]

THank you for your quick response aramis. Here are the answers to your questions:

How's your defrost cycle programmed.
4 times a day - 30 minutes


At what distance is the evaporator from the back wall. Is there any product piled up underneath the evap that may be blocking the airflow?
Aprox. 1.5mt from the wall, the evaporator is on a aisle so there no product near it.


What's the fan diameter.
24 inches


Anything blocking the air in front of the fans that may be causing cold air recirculation?
Nop, there's nothing on the back neither on the front of the evaporator.

Hope you can help.

[georgedvf]

Here are some pictures, visual reference always help.
87508751

[aramis]

Strange, I can't say I've seen this before.

The pictures were a good idea.

Is the frosted section only one circuit? If so I would go first for possibility 2 below.

My best guess is that you either have something in the coil that is blocking air flow on those rows or this row is not being fed the same amount of refrigerant than the others.

1) You should check with a lantern and compare the light pattern directly above and below with the frosted section, preferably after defrost.

2) For mal distribution you should check that there are no bumped curves or anything strange in the copper tubes. This includes the distribution tubes, see that no one has been squeezed. The distributor would be my next check, impossible to do on the outside.

I hope this helps.

[aramis]

Oh and ... ¡Viva México Lindo!

[mikeref]

I think this room is the last one to be fed from your system and not getting the benefit of 100% liquid to expansion valve.

[georgedvf]

This is not the last room to be fed, in fact is the second of four, but this system has Electronic EPR valves, I noticed this morning there was an erratic functioning on this particular EPR and the opening percentage was at 25%, when I correct it the opening percentage went up to 100%.

Does this erratic opening of the EPR has something to do with the ice problem?

I will be monitoring the function of the system to see if it gets better.

[aramis]

Does this erratic opening of the EPR has something to do with the ice problem?


The immediate cause of the strip is poor distribution or refrigerant.

Starving the evaporator causes low flow which causes low pressure drop in the distributor and it doesn't feed the circuits correctly.

You need a pressure drop of (about) 15 psi through the distributor for it to work correctly.

You may well see the problem solved tomorrow (crossing my fingers).

[mikeref]

If you know you have a solid liquid supply to expansion valve, and it is feeding correctly through the distributor, than there will be an even feed through all evap circuits. EPR is there to restrict vapour return to compressor, (to maintain a desired pressure in evaporator that is higher than the fluctuation suction pressure in multi room systems) I see one circuit is feeding far more than the others in that evaporator.
That is why i was looking at liquid supply. Might be worth your time to pull apart expansion valve and check the orifice..Mike.

[aramis]

Mikeref is right you should check your expansion valve.

The EPR could also be the problem because if it closes too much increases the pressure in the evaporator, this decreases the pressure differential in the expansion valve and you get low refrigerant flow which also causes maldistribution.

If the EPR is working erratic, you will have this problem at random, probably when you are not there to measure but the ice lasts longer and you see it.

Not to mention that you could have dirt, humidity and many other problems that could cause this.

Still have my fingers crossed!

[georgedvf]

This morning the ice is thiner and seems to be melting and the room temperature is lower, I will force a manual defrost for 1hr and see if this takes away all the ice. I'll keep you posted.Thanks.

[georgedvf]

I checked this afternoon and the ice is gone. The room is at desired temperature and everything seems ok.

I didn't uninstall the TEV, just the adjustment on EPR....:eek:

[aramis]

See, crossing fingers does work!

[georgedvf]

Yeah! I will cross my fingers from now on!!!!

Thank you aramis and mikeref.

Regards.

[Peter_1]

I think the problem will come back very soon :-(

[georgedvf]

Why is that Peter???

[aramis]

I think the problem will come back very soon :-(

Right! The reason for maldistribution under low flow conditions is still present.

But you can get by avoiding low flow.

[mad fridgie]

EPR incorrectly set, first problem solved.
Looks like a forced draft evap, you may find that it is not a refrigeration distribution problem, but an energy distribution problem. Less air flow through the center of the coil, so compared to the outer circuits. hence more frost leaving the middle circuits! Hope that makes sense.

[Magoo]

Looking at pics, there is heavy ice build up on distributor tails for +2'C room. They should be reasonably clear of ice if feeding correctly and distributor orifice and tail length and diameter are sized correctly. Suggest that the selection of total pressure drop across TXV distrib orifice and tails is too low and TXV will be hunting. The EPR is masking the real problem.

[aramis]

EPR incorrectly set, first problem solved.
Looks like a forced draft evap, you may find that it is not a refrigeration distribution problem, but an energy distribution problem. Less air flow through the center of the coil, so compared to the outer circuits. hence more frost leaving the middle circuits! Hope that makes sense.

It does make sense, was my first choice too. Georgedvf should make sure the air flow through this circuit is not obstructed.

But you see that the ice follows a circuit perfectly up to the collector, besides these are 24” fans.

I think this circuit was getting more liquid than the others so it is the only one that worked ok while the evaporator was starved.


Looking at pics, there is heavy ice build up on distributor tails for +2'C room. They should be reasonably clear of ice if feeding correctly and distributor orifice and tail length and diameter are sized correctly. Suggest that the selection of total pressure drop across TXV distrib orifice and tails is too low and TXV will be hunting. The EPR is masking the real problem.

Good point Magoo. There could be more causes of low flow than just the EPR. This should also be checked.

Also the tubes or this circuit should be inspected and the distributor too.

[aramis]

... or just keep fingers crossed.

[mikeref]

... or just keep fingers crossed. Sorry aramis. Can't let that one go as if it was my system, i'd pick it apart until i found out why the problem happened in the first place ;). Sooner or later, this problem will come back to bite you in the butt. Been there, done that and bought the shirt as well. lesson learnt a lifetime ago. Mike.

[Magoo]

Sporlan have and excellent software program for selection of TX, orifice and distrib tail length and sizing.

Hi Mikeref, I totally agree, I have always wanted to understand why some thing is not working in balance in a system. Rules of thumb don't work for me and certainly not crossing thumbs. Things have got simpler with the net and software programs, saves alot of time reading manuals.

[mad fridgie]

What is the pack saturated suction temp?

[mikeref]

Sporlan have and excellent software program for selection of TX, orifice and distrib tail length and sizing.

Hi Mikeref, I totally agree, I have always wanted to understand why some thing is not working in balance in a system. Rules of thumb don't work for me and certainly not crossing thumbs. Things have got simpler with the net and software programs, saves alot of time reading manuals. Evening Magoo.Hope your Frydee nite dinner hit the spot along with the odd ale ;) ...:off topic:

[aramis]

Sorry aramis. Can't let that one go as if it was my system, i'd pick it apart until i found out why the problem happened in the first place ;). Sooner or later, this problem will come back to bite you in the butt. Been there, done that and bought the shirt as well. lesson learnt a lifetime ago. Mike.

Me too! .

[Peter_1]

I'm happy others are thinking just as I did that the problem isn't solved yet.
Possible causes for me are
1. partially blocked distributor or cap lines between coil and distributor blocked
2. Wrong orifice plate in the distributor or tilted in the distributor.
3. No airflow over the coil where it is frosting up.
4. No feeding at all or severe blocked refrigerant feeding to the rest of the coils
5. Too low DP over the distributor /TEV

The EPR is for me not the cause.

[Peter_1]

Perhaps I missed something but did it work properly in the past or is this a new install?
I think that this circuit is responsible for the SH the TEV/EEV is regulating. The TEV controls this one circuit with its lower energy load, resulting in blocking off all the others. If you could make a thermal scan, you should 'see' the problem.
It this is a new evaporator, then there's a fault in it. can it be water or an oil/water mixture frosting up in the cap tubes? What happens if you de-ice them completely and start up again?

[georgedvf]

Wow, lot of answers.

What is the pack saturated suction temp?
It is R404A - 56psig - 0°C



1. partially blocked distributor or cap lines between coil and distributor blocked
2. Wrong orifice plate in the distributor or tilted in the distributor.
3. No airflow over the coil where it is frosting up.
4. No feeding at all or severe blocked refrigerant feeding to the rest of the coils
5. Too low DP over the distributor /TEV

I haven't checked No.1,2,4 and 5, I can't shut off the room the client has product on it; No. 3 the airflow is even on the evaporator.




Perhaps I missed something but did it work properly in the past or is this a new install?


It worked propoerly for 2 months, and then the client calls me because the temperature rised and will not drop to the set point again.

Today the evaporator is running normally, no ice on it.

[Peter_1]

No excessive moisture in the system?

[georgedvf]

No, tested the oil and result = ok, also no visual sign on the sight glass.

[mad fridgie]

I am struggling here a Bit, if the rack SST is 0C how can ice be forming. I believe it has to be much lower.
I suggest that your EPR )(hunting) was fighting the PID loop on the EEV, and that irregular flow occurred, along with vapour entrapment with in the evap circuitry. (showing as mal-distribution). This could also cause slight oil logging in the distributor legs. With a more continuous flow, the distribution becomes more even.
I have to say from experience that it rare to see 100% perfection in distribution, and this normally masked by the superheat requirement.

[georgedvf]

Sorry, 0°C is the thermometer lecture at the copper tube... SST is around -6°C according to the Sporlan P-T chart...

[mikeref]

Ah, rereading origional post, you said this room has 2 evaporators. What was the other evap doing while this one had that problem?

[georgedvf]

Copying the other one ja!

It had the same problem as the other, just that the ice was on the bottom.

On this picture you can see both:
8753

Sorry I did not mentioned it before...

[Magoo]

As per my earlier comment, the combination pressure drop across valve orifice and distrib tails is too low, the liquid is flashing off in the tails giving uneven distribution to the coil. If both evaps has similar frost build then the problem is factory fitted. Or else an undersized TX vav.
Can you indicate the TXValve rated duty,[ name and model/size ] the length and diameter and number of tails on distributor, and the rated duty of evaporator. SST and liquid entering temp at TXValve.

[mad fridgie]

As per my earlier comment, the combination pressure drop across valve orifice and distrib tails is too low, the liquid is flashing off in the tails giving uneven distribution to the coil. If both evaps has similar frost build then the problem is factory fitted. Or else an undersized TX vav.
Can you indicate the TXValve rated duty,[ name and model/size ] the length and diameter and number of tails on distributor, and the rated duty of evaporator. SST and liquid entering temp at TXValve.
I am going to disagree, only because it is now working. Yes poor distribution, but i would say because of irregular liquid feed. EEV and EPR fighting.
If you are not getting a consistent regular feed to the distributor, you will not get a regular flow pattern.
The different distributors are more than likely sitting in a slightly different position, hence the different circuits showing frost. But I do not mind being wrong!

[Magoo]

Hi MF,
looking at pics way back in post, the TXV looks to be a sporlan externally equalized, possibly a balanced port type, the EPR and balanced port vav would be fighting each other.
Plus a +2'C room and SST at -6'C is a evap system TD of 8 degrees, too wide. The TXV will be hunting big time with a balanced port vav.
It would be interesting to have a sightglass in liquid line right at the TXV. Possible liquid line losses with length and diameter and vertical separation is creating flashing in liquid line, more complications
I still reacon there is a factory fitted problem with the ice build on distrib tails that will complicate everything if delta P is below 35psig across orifice and tails for 404a. Have a look at Sporlan cattledog [ nz slang for catalogue.]

[georgedvf]

That is right, TEV is Sporlan externally equalized type S, 7 TONS, not balanced port.

Distributor has 14 tails, 3/16 inches diameter and 25 inches lenght.

Evaporator is rated 76,000BTUH (6.3 TONS aprox.), I didn't get liquid temperatures readings when there was the problem.

[mikeref]

So... when you say, the coil is clear of ice and the room is down to temperature, is the other evaporator also free of ice?
One EPR for each evap right?

[georgedvf]

That's correct mikeref, the other evap is also free of ice.

There's just one EEPR for both evaps.

[mikeref]

I was looking for an edge on this problem. One EPR.. One leg of distributor feeds on each evap and does so enough to fool the expansion valve element into throttling back.
Pressure in evap should have been high enough to prevent that amount of ice (in your pic) from happening.
Flow through expansion valves?

[mad fridgie]

**** up by "Mad" not EEV but TEV. SORRY.

[georgedvf]

Flow through expansion valves?

Sorry mikefer I got lost with that...:confused:, what do you mean?

[mikeref]

Sorry mikefer I got lost with that...:confused:, what do you mean?Oh, i was thinking out out loud.. O.K, so the last time you checked this room, both evaps were free of ice and there was no overfeed from Tx valves? No problem from one or more distributor feeds, as in all circuits were feeding fine and no ice on suction lines?

[georgedvf]

Yep, just as you described it, everything is working properly...:eek:

[mikeref]

O.K, had a thought about how this effect might have come about. (Ice buildup on part of coils).
EPR was regulating the pressure on the two evaporators. The pressure was low enough to allow a restricted flow through both TX valves, however, since the room thermostat was calling for cooling, and the valves could only deliver a fraction of their ability, the result is the loss of average or normal defrost cycles when the room was to reach it's set point.
With only your forced defrost cycles, the room never achieved temperature, and the refrigerant took the path of least resistance for the available "on cycle" time.
Now, the EPR allows more vapour flow and the evaporators can do their job and cycle at the thermostat setting, masking an unbalanced refrigeration flow through distributor.
Reach set point on a coldroom, and evidence disappears with the defrost cycle.;)

[georgedvf]

Ok, I like that explanation. So if there's no vapor flow restrictions in the future there shouldn't be ice formation problems.

There's still the distribution problem, wich has no other repercussions if the room temperature is at desired and superheat is ok, right?

[Magoo]

Hi George, I ran your info through the spolan distrib select pogram and everything is pretty much on the button. One thing that influences the orifice size is the entering liquid temp. [ my earlier question ]
For a +2'C room, sst -8'C and correct 35 psi delta P there should be zero ice build up on distrib tails.
Suggest that the orifice has been left out and feed from TXV direct, so tails are at evap pressure and giving erratic feed pattons. Your pics show ice from body of valve right to evap., thick ice for 3/16 " tails.

The prgm also had 100% plus rating, which could mean the TX is undersized for application, creating same symptons as no orifice and what you have.
The liquid temp at TXV and condensing temp/ pressure would allow me to check further. TXV 's are effectively variable flow control devises, that respond differently at different delta pressures

[sterl]

Is there some sort of subcooler that actually reduces liquid temperature below the evap temperature on this thing? Economized screw?

That is a pretty descriptive frost pattern on the suction header in one of those photos...Appears like three tubes are getting all the liquid. In the meantime the frost is all the way back to the outlet stub on the TXV...

Somebody mentioned orifice plate...I've seen TXV's this way just a few times. One TXV was receiving Min 15 deg upstream liquid and trying to maintain a 9-deg room on a 0-deg EPR setting. TXV head was much colder than the suction and all the charge was lying right on top of the diaphragm. No Feed.

Other one had no orifice plate in the distributor but that did not cause funny distribution....just a lot of frost on the cap tubes.

Third one had a collection of tubes crushed due to ice bridging at the pan; that never cleared during the defrost. Didn't pass much gas during the defrost either.

This unit have a hot gas header? Does it use hot gas in its pan?

[georgedvf]

Hello Ster, sorry for delay. There is no Hot Gas on this system, and neither there is subcooling/economizer.