Packaged unit

[marc5180]

I used my digital gauge's today for the first time since buying them last friday on a packaged AC unit that hadn't run for a while.
Firstly i turned the controller on to cooling and checked the supply grills to see what temperature i was getting off, which i found to be 15C with a room temp of 22C.
I checked the coil was clear and the fan was rotating and also that the compressor was running- which they all were.

Next i connected the gauges and found the Suction pressure at 3.2 bar -7C and the discharge at 9 bar or 38C, it was on R22 by the way, i though this was pretty low and normally i would add some refrigerant but i thought i would check the subcooling first which turned out to be 7K. then i checked the superheat and found it to be 48K.

The evaporator was obviously being starved so i looked for anything that could indicate a blockage i.e a filter drier or expansion device.
I noticed that the sensing phial wasnt strapped properly to the suction pipe and that there was no insulation around the phial.
I strapped the phial back to the pipe and insulated it then ran the unit back up and checked the superheat... this was now 3K. I know this isn't really enough but i thought i would then check the supply air off in the room which was 9C.
Although the pressures are still low and the SC and SH are a little bit low i did'nt add any additional refrigerant as the supply air was as per spec, what do you guys think?

It just goes to show that by using superheat and subcooling you can find the cause of the problem a lot easier in my opinion, instead of just guessing.

[Brian_UK]

Well done Marc, yes you've proved that taking the right set of readings helps no end with analysis of a system. Also a good visual inspection always pays off.

A lesser man would have bumped in some more gas hoping to get both the pressure and the superheat that he wanted.

[frank]

With a unit running in cooling at these ambient temperatures, you can't expect it to be performing anywhere near it's design, and pressures will seem low compared to peak design performance.

What you really should do is ignore 'what pressure you think it should be' and stick to temperature measurements.

Well done Marc

[marc5180]

Thanks guys, i think i'm getting the hang of this A/C melarchy:D

[Gary]

Ahhh... I see you've been doing your homework.

Good job. :cool:

[marc5180]

Ahhh... I see you've been doing your homework.

Good job. :cool:
I sure have:D

[Springbok]

Hey Marc5180.You would be even more impressed with yourself that not just doing the essentials and proper checks and locating the problem...your new digital gauges actually delivered!!:D Good money well spent...:p

[Gary]

I strapped the phial back to the pipe and insulated it then ran the unit back up and checked the superheat... this was now 3K. I know this isn't really enough but i thought i would then check the supply air off in the room which was 9C.

Although the pressures are still low and the SC and SH are a little bit low i did'nt add any additional refrigerant as the supply air was as per spec, what do you guys think?


Under low load and low ambient conditions, we might expect the superheat to be on the low side. The low superheat means the coil is fully flooded, thus working efficiently and effectively. The only drawback is possible flooding of the compressor, and low load/low ambient conditions is where floodback is most likely.

Liquid droplets disappear at about 10F/5.5K SH. Most compressor manufacturers recommend a minimum of 15-20F/7.5-11K SH at the compressor inlet.

If the compressor inlet SH is less than 15F/7.5K, then you may want to tweak the TXV in just a little in order to safeguard the compressor.

[marc5180]

Under low load and low ambient conditions, we might expect the superheat to be on the low side. The low superheat means the coil is fully flooded, thus working efficiently and effectively. The only drawback is possible flooding of the compressor, and low load/low ambient conditions is where floodback is most likely.

Liquid droplets disappear at about 10F/5.5K SH. Most compressor manufacturers recommend a minimum of 15-20F/7.5-11K SH at the compressor inlet.

If the compressor inlet SH is less than 15F/7.5K, then you may want to tweak the TXV in just a little in order to safeguard the compressor. So you don't think it low superheat due to a lack of refrigerant?. I didn't want to touch the TXV because i always thought it should never need adjusting unless you are first commissioning the system.

[nike123]

So you don't think it low superheat due to a lack of refrigerant?. I didn't want to touch the TXV because i always thought it should never need adjusting unless you are first commissioning the system.

Low superheat could not be due lack of refrigerant. It could only be higher, and no lower, superheat when system is low on refrigerant.

[Gary]

: Originally Posted by: marc5180

So you don't think it low superheat due to a lack of refrigerant?.

If there is enough refrigerant to completely flood the coil (normal to low superheat), and a little subcooling for reserve, then there is plenty of refrigerant in the system.

It is when a system has low to no subcooling (no reserve) and high superheat (not enough to flood the coil) that we know it needs refrigerant.


: Originally Posted by: marc5180
I didn't want to touch the TXV because i always thought it should never need adjusting unless you are first commissioning the system. ... or if the guy before you didn't do it right. TXV adjustment should be done only after it is proven to be at fault and only when the refrigerated space is down to temp. It should be done reluctantly and carefully....

[frank]

... or if the guy before you didn't do it right. TXV adjustment should be done only after it is proven to be at fault and only when the refrigerated space is down to temp. It should be done reluctantly and carefully....
And remember to check any adjustments you made during low ambients when you re-visit the site and the ambient temperatures are higher

[marc5180]

Sorry about the late reply, i've been on holiday and just arrived back:(.
Thanks for all the replies