R22 Suction freezing

[Kirky]

Hi all hope you are well

Got called out to a Split system in a server room. The unit had a system pressure of 0 bar so there was a leak. Customer tells me this unit hasnt ran for a long while and its sister unit ( a seperate split system) has managed by itself. I have found the leak and repaired it and recharged the system with R22 (FACTORY CHARGE). Started the unit and it runs however the system pressures were only 7 bar discharge 3 bar suction. The suction pipe started to freeze up so i thought the system was still short on gas and added an additional 2kg bringing the suction pressure to 4 bar however the suction pipe is now frosting . I have checked the indoor unit plenty of air flow across coil. Whats puzzling is that the pipe run is no more than 15m having worked mostly on new installs i wouldnt add any charge to the base charge for that size pipe run. The unit was installed in 1997 were base charges less then i.e 5m. Also if the other system in the server room has coped for such a while by itself could it be that the together they are over sized i.e very little load on the unit.

[james10]

i would switch the sister unit off and only run the suspect unit, if your getting a decent temp drop accross the evap around 10-12 degrees (when the room is near setpoint) then i wouldnt worry you could always check the operating pressures against the good unit

[mad fridgie]

Must of been a cold day

[chillerman2006]

Hi all hope you are well

Got called out to a Split system in a server room. The unit had a system pressure of 0 bar so there was a leak. Customer tells me this unit hasnt ran for a long while and its sister unit ( a seperate split system) has managed by itself. I have found the leak and repaired it and recharged the system with R22 (FACTORY CHARGE). Started the unit and it runs however the system pressures were only 7 bar discharge 3 bar suction. The suction pipe started to freeze up so i thought the system was still short on gas and added an additional 2kg bringing the suction pressure to 4 bar however the suction pipe is now frosting . I have checked the indoor unit plenty of air flow across coil. Whats puzzling is that the pipe run is no more than 15m having worked mostly on new installs i wouldnt add any charge to the base charge for that size pipe run. The unit was installed in 1997 were base charges less then i.e 5m. Also if the other system in the server room has coped for such a while by itself could it be that the together they are over sized i.e very little load on the unit.

Hi Kirky

From your pressures we have your saturated temps but missing liquid line and suction temps, so we can not work out your subcooling and superheat
without which its guess work

if this is capillary fed evap - overcharge
if its txv - overcharge or txv is stuck open or incorrectly set

i would be inclined to decant and start again
charging to subcooling and superheat

[Kirky]

Ok thanks guys it wasn't a overly cold day ambient of about 13 degC I checked it's sister unit suction 3 bar discharge 7 bar however only had a 7 deg drop across evap coil. With one unit off the room temp rarely ventured about 20 degC I wil work out superheat and subcooling. As for over charge I ran the unit on it's base charge and freezing was worse at base charge the pipe frosted a lot further down the vapour when I added 2 kg it only frosted inside the outdoor unit could that still mean over charge it's a capillary tube by the way

[r.bartlett]

Depending on size and manufacturer most will be precharged to 30m Therefore this system is now well overcharged. look elsewhere. low load and poor airflow are two. However 13c outdoor in a server room application calls for fan speed control of the outdoor unit. Over condensing as I call it :-)

[hyperion]

Dependant on the make of unit, as some, particualrly Fujitsu are much more critical on gas charge. Get the server room temperature up to 22 or 23degC to give a good load to the system. As other have suggested decant and start again.
Try to get the discharge pressure up to 15-16bar, block off part of the condenser coil if necessary. Make sure that your gauges are registering the true discharge pressure and not connected to a gauge point after the expansion device.
With this discharge pressure and a full load you should probably be getting a suction pressure of 3.5-4bar and the suction line should only be sweating back to the compressor. There should not be any frosting.
Adjust the gas charge if required to obtain the best subcooling and superheat.

[Kirky]

Ok Thanks ill do that. I do think that the unit is over condensing as there is very little load on the system and the condenser fans run at a constant high speed. There is no sign of a fan speed controller. I turned the unit off and got the room to 23 degC re started and same thing low suction and discharge however the frosting wasnt as bad. Just cant get the pressures up. I tried blocking a section of the condenser however due to the wind and rain the cardboard didnt last long. By over charging the system wont I reduce condensing capacity as the liquid will sit in the condenser or is this not the way to do it. Im only looking for a short term fix as the unit is clearly over sized. I would switch one off as they coped on one unit for a long while through much warmer weather than this however the condensates are linked and if i switch one unit of the other pumps condensate back into the indoor causing it to fill and leak. Also they have decided to install server stacks directly under the indoor unit meaning that a water leak is very bad. Maybe Im thinking to much about it my job description was to pressure test locate and repair leak and re charge. Which ive done, the fact that the unit isnt working correctly due to low load etc should be additional work shouldnt it

[tradybrad]

hi kirky
where on system were you putting high side gauge line on?

[nike123]

Server room mean low humidity, if there is no humidifier installed.
Low humidity mean low wet bulb temperature.
Since normal air conditioner has around 15K TD (or even lot more when not clean or on low speed) and you cold have 20°C DB and 10°C WB (30%Rh) temperature of air inside server room, your evaporating temperature could easily be 10°C-15K=-5°C
That converted to gauge pressure of R22 mean 3,2Barg which is what you actually getting. And (if overcharged and low load) you probably don't have any superheat, and that is why your suction line is freezing.
Therefore, your air conditioner has worked perfectly OK after initial charging.
Now you overcharged it by 2kg. Stop it immediately, since it is going to kill your compressor.
I don't know what is original charge, but that sound to me as minimum 50% overcharge. In critically charged systems (capillary tube) 5% overcharge is huge.
Recover refrigerant, charge by weight on nameplate, add (if necessary) charge for long pipe run by charging required amount per m of excessive length , and do not touch charge any more.

Tell your customers that they need to control humidity in their server rooms (45%Rh ±5%Rh), and eventually, to buy dedicated equipment for that job like close-control air-conditioners .

Measure DB and WB temperature (or %Rh) in that room and you will see!

[Gary]

At 3 bar for R134a, the SST is above freezing temp... so how can there be frost?

Oops... my bad... it's R22... got this confused with another thread.

[nike123]

Gary, did you read a title of this post?:D

[Gary]

Ok thanks guys it wasn't a overly cold day ambient of about 13 degC I checked it's sister unit suction 3 bar discharge 7 bar however only had a 7 deg drop across evap coil.

Seems to be my day for confusion. Is that 7 deg dT across the problem unit coil or the sister unit coil?

[chillerman2006]

Seems to be my day for confusion. Is that 7 deg dT across the problem unit coil or the sister unit coil?

Your not confused, your on the ball this time

I made the mistake of presuming

And we know how the saying goes

"presumption is the mother of all f*kups" :D

[Kirky]

Hi guys

Cheers for all your help. Ive recovered the gas and given it its factory charge and left it at that its a old unit with no fan speed control so would suspect the unit is simply over condensing. I have advised the customer to up the setpoint in the comms room from 18 to 21 degC and that the indoor unit requires moving as someone has decided to plonk a new server stack directly under the indoor cassette leaving a 5 inch gap from the top of the stack to the unit facia. What a clever person obviously got a lot of faith in the units condensate pump.

[still learning]

coms rooms and splits . nike 123 is right they can suffer from low humidity, and something to do with latent heat , and sensible heat, of which i am unqualified to talk about .
some units have a setting , which is for use when the unit is in a comms room , dakin do , cant say about others .
ive measured some 38 degrees c of the back of some racks , in a small, say single garage sized sealed room.
the only good thing about a split in a comms room is it can be thrashed into the ground , and a new outdoor unit fitted quickly when it goes bang in the end .
after testing the oil for acid , and applying a hope for the best anti dote , which up untill now has been a good way to go .
i also often wonder why the it department want to keep penguins in the room , and wack the unit down to flat out and never stop, and sneak more kit in with out checking the heat load in the room first.