Evaporator iceing up

[240dae]

Hi
Having trouble with an air con unit.
Evaporator is iceing up within two days.
The fins are clean and free from fouling, the air flow is excellent.

Compressor is unloading at 4 bar and suction pressure remains between 3.8 and 4 bar.
Refrigerant is R 407C, so my evaporating temperature is between 1° & 2°c, so how come ice is forming?

System is recirc type, so very little warm or humid air from outside enters the space, except when people open doors to pass through the space.

Liquid line sight glass is full, no bubbles.
Water cooled condenser is clean and constant flow of cooling water at 32°c.

Really appreciate any help and ideas.
Many thanks

[nike123]

3,8 bar (gauge) suction pressure is -5°C bubbling temperature and 1,5°C dew temperature for R407C and in evaporator you have all these temperatures in between these two values and you also have superheated gas at exit of evaporator (if expansion valve is correct).
Either you are running unit with too low temperature of entering air, or your air flow is not so "excellent", or you have high superheat.
So, question is what is temperature of evaporator entering and exiting air and what is superheat value?

[Sandro Baptista]

Hi,

First can you tell me if the frost formation is homogeneus along the refrigerant circuit? Can you tell me where are you measure the suction pressure?

[desA]

A few thoughts:

Refrigerant charge - correct amount?
TXV - ok?
Liquid line filter-drier - blocked, water?
Insulation - ok?

[240dae]

Hello everyone.
Many thanks for your replys.

My current air inlet temp is 26°c.
Suction pressure 4.8 bar.
Phail temp 8°c.
Compressor unloaded, that is running on 50%.

Ice starts to form on the lower section (inlet) of the evaporator after about two days, resulting in the suction pressure dropping.

Evaporator has recently been cleaned with 'air cooler cleaner' chemical.

There is no temperature change in the pipework across the soleniod valve or the liquid line filter.

TXV appears to be regulating as the phail temp is fluctuationg by 0.8° in use.

I believe the charge is sufficent as there are no bubbles visable in the liquid line sight glass.

The compressor, condensor and evaporator are all together so any pressure losses in pipework are minimal.

Once again many thanks, and any advice would be well received.

[nike123]

What is evaporator air off temperature?
Is this 4,8 bar gauge or absolute, since you stated earlier 3,8 bar, or you are giving us new conditions?

If this is new reading and pressure is in bar gauge, than you don't have correct superheat (evaporation temperature is 7,2°C and phial temp. is 8°C therefore superheat is 0,8K), which points to expansion valve problem!
Since "hunting" is only 0,8K, it appears that it has some control, but either is someone changed superheat or bulb is in wrong place or valve charge is leaking.
Check valve for proper function and then adjust if necessary.
http://www.refrigeration-engineer.com/forums/showthread.php?t=11903&highlight=adjust
And please, next time give us full readings.

http://farm4.static.flickr.com/3213/2900913505_64df73c316_o_d.jpg

[desA]

Phail temp 8°c.

Can you measure actual evap tube temp within the evap itself? This will give you an idea of the superheat you're getting, prior to compressor suction.

Two thoughts here:
1. If your superheat is say 6-8'C, then you are in the range to make ice on the evap (8-6=2'C).
2. Is there any remote chance that you could have reversed the evap piping in/out connections?

The reason for (2) is that many evaps are designed to take in the hot air across the superheating section of the evap i.e. the last refrigerant tube. If these are reversed, you could have some issues here.

Also, are you able to alter the TXV setting, to raise the superheat a little?

---------
Added later: Didn't see Nike123's post. I agree with his thoughts. :)

[240dae]

Hello, thanks for your continued advice.

The changes in suction pressure (gauge) are due to current changes in enviromental conditions.
I'm currently on a ship in the Gulf of Mexico, therefore our weather (temp and humidity) changes on a daily basis.
The unit cools an adjacent space with vent ducting recircing the air, however the compartment that the unit is in also doubles as a passage way between other compartments, thus there is doors and people passing through all day.

The phail is mounted corrently (on a horizontal section of the evaporator outlet) and is insulated.
I'm measuring using a digital temp probe inserted into a hole in the insulation.

I have just adjusted the TXV 1/2 and noted the following changes.

Phail temp now fluctuating between 8.4°c and 7.1°c.
Suction pressure fluctuating between 4.8bar and 4.6 bar.
These changes take about one minute to cycle.

There is no facility to change the evaporator piping.
The evaporator ducting is heavily insulated however my infa red probe indicates 7° at a part where the insulation does not fit correctly.

Many thanks once again

[nike123]

Take time after Txv adjustment (20min) before pipe measuring temperature at bulb location.
Also, you need good thermal contact for correct measurement of pipe temperature so it should be measured at clean portion of pipe and wrapped with piece of metal strip (i use self-adhesive aluminum tape for ducting jobs). If you don't have good thermal contact than your readings could be pretty much off. That mean that you think that you have superheat, but actually you don't, and pipe is at saturation temperature.

For now, I would say that TXV is flooding.
Or maybe, with compressor unloaded at 50%, TXV is too big!

[desA]

I notice you're on a ship.

A few more thoughts:
1. Is this a new vessel?
2. If not, has it recently been refurbished? (the evap?)
3. Has this problem just started, or it a legacy problem?

[Gary]

In order to judge the airflow we need the temperature of the air entering and leaving the coil, measured close to the coil.

My guess at this point is that your airflow is far from "excellent" or is perhaps bypassing the coil. Even with the entering air at 26C and the compressor unloaded, the suction pressure is low. It is not picking up sufficient heat load in the evaporator... not even enough heat load for the TXV, much less the compressor.

[Gary]

The evaporator ducting is heavily insulated however my infa red probe indicates 7° at a part where the insulation does not fit correctly.

Are you saying that the air leaving the coil is 7C? If so, the airflow through the coil is grossly insufficient.

26C/78F - 7C/45F = 19K/33F dT (That's way too high)

The TXV is not the problem. Return it to it's original setting.

[Gary]

Evaporator has recently been cleaned with 'air cooler cleaner' chemical.


Has the blower been cleaned? Is it turning in the right direction? Belts/pulleys/bearings worn out?

[Sandro Baptista]

Hi,

Can you please measure the liquid temperature at inlet of the liquid receiver? Is it very different from the corresponding discharge pressure? I'm asking this because it could have air or N2 in the system (insufficient vacuum before you charge the system). This would effect in a partial pressure of ***** which when you think that would evaporate at 0ºC, for instead, it would evaporate at -10ºC, for instead (depending on the fraction of air/N2 in the system). These would explain why you have the inlet with very different ice formation compared with the outlet even when you have positive saturation evaporation temperatures.

Make a good the vacuum to the sistem and recharged and the tell me something.

[240dae]

Hello everyone
Many thanks for all your advice.

My unit has stopped iceing up now, the slight change I made to the TXV seems to have done the trick.

I've worked on this ship for five years so I know the history and status of all the equipment.
The air flow was excellent and not compromised in any way.
I checked the data I'd colleced with the commissioning report.

Its always difficult to asses these problems with our enviroment changing on a daily basis.

Thanks again for all your help.

Regards

[nike123]

Hello everyone
Many thanks for all your advice.

My unit has stopped iceing up now, the slight change I made to the TXV seems to have done the trick.

I've worked on this ship for five years so I know the history and status of all the equipment.
The air flow was excellent and not compromised in any way.
I checked the data I'd colleced with the commissioning report.

Its always difficult to asses these problems with our enviroment changing on a daily basis.

Thanks again for all your help.

Regards

Well, if unit previously worked OK and you, or anyone else, did not make any change in superheat adjustment before this symptom, than you have faulty valve since change in superheat adjustment to regain correct functioning AGAIN mean that your charge in valve power head leaks, or equalization line is partially clogged, or superheat adjusting spring in valve body looses its elastic force for some reason.
Therefore, if above is correct, you could expect same symptoms soon.