Large Superhating temperature variation

[MrReds]

Hello Everybody

I am writing to this forum to ask you an opinion concerning this odd situation I am facing to.
I hope to post in the correct section.

I have a slanted finned coil, 1300mm height, working as evaporator in an equipment with temperatures 80.6F/50 RH.

The coil utilizes a sole distributor and it is regulated by an electronic valve.

The equipment works evaporates @ positive temperatures (48 F)

What happens is this:
The evaporating pressure is constant and it is close to design point.
The capacity requirement is respected, too

Nonetheless these is a huge excursion in the outlet temperatures of the gas.

The variation is about 4K in some minutes in the same circuit, but in every time measurements show that there is a minimum super heating of 4.5 K to a max of 8.5 K

The same excursion is verified in all the circuits, with no difference with respect of the position.
This has been measured by some temperature proofs and can be easily verified by putting an hand on the outlet header
What more puzzles me is this:

There seems to be liquid slugging in the suction header of the compressor - sometimes it seems to vibrate -, but temperature measurements do not confirm this impression, since they are always higher than suction evaporating temperature.

Please, can anybody of you give me some advice concerning this case ?

One possible idea is that the temperature proofs are not measuring the temperature correctly, but I made realize two different measurements that confirm the temperature variation explained above.

Any hint, any suggestion ?
Thanks for your help

[joe-ice]

What is controlling the electronic valve ,is pressure transmitter and superheat sensor or just sensor.What refrigerant is used is it a blend.

[MrReds]

Hello
thanks for your answer

1 - Only temperature
2 - the refrigerant is R410A

Thanks

[RusBuka]

Watch for condensation pressure. The consumption of liquid refrigerant directly depends on it.
Than the condensing pressure is regulated?

[joe-ice]

A lot of possibilities for this ,sounds like the valve is hunting but have also come across this with just bad distributor and also distributors with their inserts left out.Hunting almost always caused by too big valve orifice or bad thermal contact of superheat probe.first i would do is validate superheat probe and if ok check its position on suction pipe ,far enough away to pick up a settled flow also at the right angle to pick up liquid in the pipe

[RANGER1]

Watch for condensation pressure. The consumption of liquid refrigerant directly depends on it.
Than the condensing pressure is regulated?

This comment is extremely relavant as it makes superheat swing as condensing pressure changes.
Lack of refrigerant is also similar result causing hunting.
Need liquid line 100% full at all times.
Do the basics first

[Tesla]

Search this forum for superheat measurement / calculation as there are many who only measure the outside of the pipe when it should be measured on the inside with a superheat calculation tool. This directly measures the temperature of the refrigerant. Some gauges can record temp and press to download to a pc where a graph can show a lot more than static measurements. Valve control on EEVs can be tricky sometimes.

[Mick13]

does the condenser fan cycle? would explain it....

[MrReds]

Hello I'd like to thank everybody for kind help.
I think that there are two points where I shall focus.
To Mr Joe-Ice
Is there a suggested position of the temperature probe on the pipe ?
Which is the right angle to pick up liquid ?

To Mr. Tesla
I am afraid it is not possible to go into the pipe to measure directly the superheating.
Anyhow, I have the opinion that the temperature difference is very big so that even measuring into the pipe - with direct contact with refrigerant - would not justify this difference.

To Mr. RusBuka
Please, could you be more specific about influence of condensing pressure ?

To Mr. Mick13
Please, could you tell me what you mean for condenser fan cycle ?
The condenser is water cooled.

Thanks a lot for your help

[nike123]

Depending on type of temperature probe and its thermal contact and thermal conductivity of material/s between probe and refrigerant, you need some time in order that probe reach temperature of measured refrigerant, or, if changes are quick enough, it will never show actual refrigerant temperature. It looks like your system is hunting and that mean , for electronic expansion valve equipped system, that your control algorithm is probably inadequate for current application, probe is badly selected and/or placed, valve is somehow too big, etc.... I am not good (yet ;) ) in setting of electronic expansion valve but some other members of this forum could probably help you.

[MrReds]

Hello there,
I am reporting some new pieces of information.
I have been on site, and I have checked all the equipment.
Following some hints.
1 – condensation is quite stable, and we have a subcooling of 8K, approx
2 – temperature at the valve inlet is constant.
3 – temperature on the different circuits of the coil is even for the all coil length
4 – all measurements done demonstrates that evaporation suction temperature is lower than circuits temperature. That is : there is superheating, and the minimum value is 4K, and then increases to even 8K

Nonetheless, it happens that



when putting the hand on the suction .header, there is an evident excursion of temperature
the discharge temperature of the compressor is very low (10 K). I deem this evidences that there is liquid going to the compressor.
When the temperature in the suction header is low, there is an evident increase of Amperes of the compressor.
This happens in a cyclic way, every 30 seconds.


I even tried to stop the EEV, but the same phenomena was happening.
In addition to what I have described to you, I tried to lower evaporating temperaure, so that I could verify any zebra effect, with no success.
In normal conditions I would have diagnosed that there was a problem of the evaporator, such as mal distribution.
Nonetheless, all temperature measurements demonstrate that there is superheating, and there should not be liquid going to the compressor.
Now I kindly ask you if you could help me in diagnosing this problem.
I am really in a dilemma !
Can anybody give me an idea of what is happening ?
Thanks in advance !!

[MrReds]

Hello,

please may I ask you if I have been unclear ?
Please, can anybody give me a piece of advice, or telling me if he has never faced a similar situation ?

Thanks

[joe-ice]

What is the make of the valve and its controller.A good position for superheat sensing probe is 4 o clock on horizontal pipe after cooler.Also a good idea to insulate these probes

[Royal241]

is this a hot gas defrost? (bad solenoid valve) just thinking, but nothing happens... external equalizing valve I assume...(bad valve bypassing evap.) well, I'm shooting blanks but that might be what you are looking for

[MrReds]

Hello

To Mr Joe-Ice
Yes, the valve sensing probe is installed as suggested by manufacturer, and properly insulated.

To Mr. Royal241
No, this is not a hot gas defrost. There is no frost formation at all, we are working at positive temperatures.

Thanks

[PaulL]

Hello there

after a long time I would propose you an idea I have about this phenomena, after reading

MrReds, could it be that suction pipe is dimensioned not correctly ?

To other folks, to be more specific:
what happens if suction pipe is under dimensioned for required capacity ?
what happens if suction pipe is over dimensioned for required capacity ?

thanks