no superheat on AC

[speedy17]

Hi all,

I have installed an AC split unit in a supermarket, 7kW.
The system is working well, my suction pressure is about 6 bar, -4 degrees celsius. It's R410a.
But the superheat is about 1 K. system is cooling ok, but after several hours I get some ice on the evaporator. How is this possible. I already checked the tubes and connections on restrictions, but everything is OK.
I think the capilar on the outdoor unit is not ok, and getting to much liquid in my evaporator.
Can you let me now your vision on this issue.

Thanks and regards,

speedy

[Brian_UK]

Hi Speedy and welcome.

-4°C is too cold for air conditioning. If the temperature is above 0°C then ice should not form.

Is the pipe length within the allowed limits and was any extra refrigerant required ?

[speedy17]

Hi Brian,

The length of the pipes are less than 3m and no additional refrigirant was added.

regards,

speedy

[Brian_UK]

Sorry Speedy, forgot to ask, which make/model is it ?

[The Viking]

Speedy,

What room is it actually cooling?

Office?
Cashier's?
IT?
General store?

[Thermatech]

Some manufacturers precharge for much longer pipe length but do not have suction accumulator to store excess refrigerant when very short pipe length.
So when units are installed back to back you can remove some refrigerant otherwise the system is overcharged.
This all depends on the exact model & the type of application.

Assuming the system has low ambient control which is operating correctly & the indoor air on coil temperature is above 15 degC wet bulb then -4 deg C evaporating & 1 deg C SH makes the system look overcharged.

Suggest
1/ check indoor room humidity % RH & WB are ok for manufacturers specification.
2/ Check outdoor unit low ambient control working correctly.
3/ Try remove small refrigerant volume & monitor system operation.

[temperzone]

I'd agree it's overcharged, 410a should be running around 7-8 bar. every system i've dealt with is pre-charged for 7mtrs, and you need to add/remove 20g of refrigerant per metre.

[nike123]

I read that your piping is very short which makes my magic thinking that your pressures are going to be slightly higher. But as you are telling us that it is 6 bar pressure then I think you have a gas leak on the system.

I respect all the answers from my colleagues but the actual running pressures are making the a/c freeze up.
The only other problem for the freezing could be very low humidity.

If he got leak than superheat would be high, not low!

[nike123]

Hi Brian,

The length of the pipes are less than 3m and no additional refrigirant was added.

regards,

speedy

Some manufacturers state in his instalation manuals their minimum pipe lengths (mainly because of noise transfered thru the pipes) and for 7 kW unit that could be 5m!
Remove some refrigerant (about 60g) to see change in superheat.

What is your unit make and model?

[speedy17]

Hi all,

The brand is TCL Aircon. I have installed several of these machines, 7Kw with the same length, even shorter pipes. And it works perfect with an superheat between 7 and 12k
The evaporator is installed in a room where also several cooling machines, for products are installed. These machines have an integrated aggregate, which produce a lot of heat in the room, that's the reason why this AC is installed in this room. To get the temp down, otherwise these machine will not cool enough.
The temperature in this room was 29 degrees and within 1 hour it was 22 degrees, so the AC is working well. It also produces a lot of condens water.
The oudoor unit has an accumulator where the refrigerant can be stored.
I already removed 500grams of refrigerant, system has 1900 grams. The suction pressure stays the same and also the superheat.

Regards,

Speedy

[The Viking]

Speedy,

Under those circumstances, my suspicion is that the humidity in the room gets too low for the A/C to operate properly.

Check the humidity, if it's 35% or less, you have found your problem.

[nike123]

Hi all,

The brand is TCL Aircon. I have installed several of these machines, 7Kw with the same length, even shorter pipes. And it works perfect with an superheat between 7 and 12k
The evaporator is installed in a room where also several cooling machines, for products are installed. These machines have an integrated aggregate, which produce a lot of heat in the room, that's the reason why this AC is installed in this room. To get the temp down, otherwise these machine will not cool enough.
The temperature in this room was 29 degrees and within 1 hour it was 22 degrees, so the AC is working well. It also produces a lot of condens water.
The oudoor unit has an accumulator where the refrigerant can be stored.
I already removed 500grams of refrigerant, system has 1900 grams. The suction pressure stays the same and also the superheat.

Regards,

Speedy
Are you sure in your measuring results. Also, recover all refrigerant and see if it was factory overcharged.

[tedre]

Hi all,
, my suction pressure is about 6 bar, -4 degrees celsius. It's R410a.
Too low for AC.
But the superheat is about 1 K. system is cooling ok,
Too low for any DX coil.
speedy

I don't deal with any AC here. We are still using heaters.

This problem is sound like air handler problem to me.
Low air flow over the coil makes low suction pressure and low superheat same time. Overcharge the system (only 60g) dose not make much difference. Unless you have high outdoor temperature.

tedre

[speedy17]

Hi Nike123,

I already removed the complete charge, it was 1900 grams, and refilled with new clean refrigerant.

Regards,

Speedy17

[speedy17]

Hi Viking,

If the humidity is to low, I don't think so but I will measure this, what can I do to solve this?

Regards,

Speedy17

[speedy17]

Hi Tedre,

I agree with your comments. But I put the fan speed on manually on high. I also checked the evaporator if nothing is blocking airflow, but it's completely clean without any blockage.

Regards,

Speedy17

[sinewave]

Fit a Humidifier! :D

[nike123]

Is that inverter unit? If it is, could be false reading of sensor who then false controls EEV.
If that and air flow is OK (check with anemometer), than you have low humidity problem?

That space you are cooling must be of very low humidity because air is cooled with AC and then heated by condensers of other equipment constantly. This works exactly as dehumidifier works. You need to keep humidity up with humidifier, or install some unit with 100% SHR (sensible heat ratio) capability (it mean low delta T and high air flow) like this one (http://www.directaircon.co.uk/shop.html) BERGDORF 24K COMPUTER SERVER ROOM *HIGH EFFICIENCY*!
Are the peoples work/live in that space? If answer is yes, then you leaved with only options to install humidifier or increase air change rate (and loose a lot of energy) to keep humidity up.

Introduce moisture in space and then check your superheat.

[Thermatech]

I have had to trouble shoot this type of problem in small supermarket shops a number of times over the years.
The chilled display cabinetts have intergral condensers which discharge 100% sensible heat & the a/c is constantly dehumidifying. The only latent heat load comes from the 3 or 4 staff & customers & fresh air from the front door which is shut most of the time.
Staff & most customers spend most time at the front of the shop at the check out tills next to the front door.
So the humidity level will tend to be better at the front of the shop but lower at the back of shop in this case.
The heat is being generated by the integral condensers which tend to be located in the middle to rear of the shop so the a/c contractor installs split system in the location of lowest humidity.
With Low wet bulb temp / low RH some split systems will operate with exactly the problems you have described. The compressor will have much reduced life due to liquid flood back caused by frosted indoor coil.

Ideally the customer needs to replace the integral condensers with remote condensers located outside the building. This will save energy as the condensers will will for large part of the year operate with lower ambient temperature & the a/c heat load in the store will be much reduced.

Heat recovery ventilation will help by introducing some needed humidity & free cooling for a large portion of the year.