Zebra Coil

[PaulL]

Hi there,

I’d like to ask your opinion concerning this issue: I have seen an heat pump not getting required capacity, therefore evaporating temperature is lower than design point.
In particular, is lower than 0°C while air dew point is around 4°C. I would exclude an oversizing of the coil.
Concerning temperature readings, superheating and inlet valve temperature are as design conditions requires.
I would – and indeed there is – dehumidification.
What really surprises me is that if I look at the finned pack, it is similar to a Zebra !
There are parts that are frozen, other not. But this difference is quite evident.
If I look at the feedings, some of them are frozen, some others not. The same observation is made for the circuits.
I checked air distribution, but it seems to be regular within the whole pack
Concerning capillars and feeding length, they all are identical.
I won’t suppose that the dimension of capillars would be the cause of this effect.

My diagnosis is that there is a refrigerant mal distribution.

Have any of you had similar experiences ?
Which is your opinion ?


Many thanks for your kind help.

[taz24]

Hi there,

I’d like to ask your opinion concerning this issue: I have seen an heat pump not getting required capacity, therefore evaporating temperature is lower than design point.
In particular, is lower than 0°C while air dew point is around 4°C. I would exclude an oversizing of the coil.
Concerning temperature readings, superheating and inlet valve temperature are as design conditions requires.
I would – and indeed there is – dehumidification.
What really surprises me is that if I look at the finned pack, it is similar to a Zebra !
There are parts that are frozen, other not. But this difference is quite evident.
If I look at the feedings, some of them are frozen, some others not. The same observation is made for the circuits.
I checked air distribution, but it seems to be regular within the whole pack
Concerning capillars and feeding length, they all are identical.
I won’t suppose that the dimension of capillars would be the cause of this effect.

My diagnosis is that there is a refrigerant mal distribution.

Have any of you had similar experiences ?
Which is your opinion ?


Many thanks for your kind help.

It may be one of the capillaries is blocked or not getting it's fair share of refrigerant. did it ever work or is it a new instalation?

Is the system charged correctley?

Cheers taz.

.

[PaulL]

Many thanks Mr. Taz
Indeed it is a new installation, but what puzzles me is that in condensing mode worked fine - capacity was got this summer -!
I suppose the system is charged correctly. I do suppose that it may be even over-charged to increase evaporating pressure.

Many thanks

PL

[NoNickName]

Are the zebra stripes vertical or horizontal?

[PaulL]

Hello Mr. NoNickName.

indeed the stripes was on tubes length direction, therefore horizontal and not vertical.
Moreover, frosting effect was more present near coil inlet.

OT:
Please, what would happen if stripes would be vertical and not horizontal ?
End OT

Please, Mr. Taz and Mr. NoNickName, do you think that feeding tube selection may affect this problem ?
Please, have you got any suggestion for selecting tube feeding ?

many thanks

[NoNickName]

Definitely seems a maldistribution or clogging of some sort.
Not that I ever saw a vertical striping, that's what I was just making sure of.

[taz24]

Hello Mr. NoNickName.

indeed the stripes was on tubes length direction, therefore horizontal and not vertical.
Moreover, frosting effect was more present near coil inlet.

OT:
Please, what would happen if stripes would be vertical and not horizontal ?
End OT

Please, Mr. Taz and Mr. NoNickName, do you think that feeding tube selection may affect this problem ?
Please, have you got any suggestion for selecting tube feeding ?

many thanks

If it was undercharged or the refrigerant flow was restricted and if the distributor was not facing in the correct direction, then that could cause the problem.

I would also look at the distributor and if nessesary remove the refrigerant, seperate the TEV from the evap and blow OFN back down the suction and see if it clears the distributor.

Cheers taz.

.

[desA]

Take a picture of the coil - clearly marking air inlet, showing inlet/outlet manifolds, distributor, capillary tubes. We need to understand the coil layout - this is important.

Also, please measure the operating parameters - temperatures, pressures, sub-cooling, superheat, ambient temp, RH%.

After this, we can assist further.

I suspect that your coil may have an odd configuration, causing frosting on the entry face. The alternative may be incorrect gas charge.

[PaulL]

Many thanks to all for your kind answers.

The system has been stopped owing to season holidays and it has been decided to replace the zebra coil with a new one.
Therefore it is not possible to get pictures of coil in working conditions.
New coil will be installed in second week of January.

What has been changed in the new coil is the size of distributor tubing - in has been chosen of higher dimension than old one -

To Mr. desA

Basically the systems works with coil in parallel heat exchange configuration when working as heat pump.
The coil presents an even number of tubes for each feeding.
Superheating, subcooling are as design conditions - please remember there is a TXV - while evaporating temperature is 2K lower than design conditions.

I won't suppose it is lack of air or something related to air distribution
After seeing the zebra coil, I would suspect that indeed the problem is something related to blocked distributor tubes or a faulty distributor.

Please, following question arose after the change of coil.
Subject are distributor tubing.

Question 1: may the dimension of the distributor tubing affect the distribution ?
Question 2: may the distributor tubing affect the distribution ?
Question 3: Would it be possible for you to send a reference where to verify the dimension and length of distributor tubing when utilizing a system with TXV valve ?

Many thanks

Please let me send you my Best Wishes for a Wonderful 2010.

[desA]

Hi PaulL,

Good to see that you are sorting things out. Take a look at the following link, to Sporlan refrigerant distributor information. It will help you to check what kinds of distributor pressure drop is considered 'normal', & also the usual piping allowance.

http://sporlan.jandrewschoen.com/refrigerants.shtm

It will be useful to see a picture of the ice buildup if this occurs on the new coil. Let us know how it goes.

[PaulL]

Hello there,

the problem seem to be solved for this system.

Nevertheless, a new one - in a different system - has arisen.

Following is the issue.
I am dealing with a V shaped equipment, composed by two coils, that is evaporating @ -4.5°C. The system utilizes a TXV valve.
Fans are on the top part of the equipment
Air is around 8°C with 80%
In this condition, in one of the two coils, It seems that there is a huge non - uniformity of distribution. Nevertheless no zebra effect is present, only a non uniformity of frozen parts on the sides of the coil.

Following your advices I have measured temperatures and I found a huge non-uniformity of the outlet temperatures of the various feedings.
On the top part, outlet temperatures are higher so that I can estimate a superheating of 10K. On the bottom part, I am finding also negatives temperatures so that I can estimate a limited subcooling - let's say 2K.
By the way subcooling is fine, so is evaporating temperature

I would suppose that I could get a better performance of the equipment if I could reduce this non - uniformity.
My question is: is there a possibility to improve this situation ?

Has anybody of you faced to this situation ?
Could someone kindly send me some advices to improve distribution uniformity ?

I do apologize but I have the feeling that solving this issues seems a trial and error method

Many thanks