HVAC guru's.

[T77911S]

i need some help on a r22 5 ton unit here at work.
we have 2 roof top units that are not very old. one has never worked since it was installed. ( dont even go there. it was not my doing and i had no say so about it) BTW, TRANE installed the system.
here is what is going on. the unit is freezing up. but, it freezes on the low side from the comp to the service port just outside the unit. it looks like it starts at the comp, and works its way to the port, but once it starts to freeze, the whole line kinda freezes at once. the port itsefl does not freeze and it does not quite freeze all the way to the port unless it runs long enough. low side is about 48-50psi and hi is about 190. it has a site glass and looking at the glass it looks low. i did charge it some but i did not see any improvement, plus, trane has been back out here and i am sure they would have added r22 if needed. i dont know that if the compressor was bad, would it make it look low according to the site glass?
my thought is a compressor, i just dont have that 100% sure feeling and i dont want to commit to it just yet.
i know the trane guy well and he is a good guy. if all it is is that it is low, he would have fixed it. when trane installed it, they worked on it for almost a week i think, but it has never worked.

[myamoto1]

Not an HVAC guru, but I've purchased one in the past I think if the condensor is freezing it's often due to the condesor fan not working properly or low refrigerant (possible leak). My brand new condensor came from the factory w/ a leak, but the install tech found it right away and repaired it. One last issue may be that the condesor is too small and is being overworked, causing it to freeze. HTH

[rick-l]

I am not an HVAC guy but I did take a thermodynamics elective 35 years ago.

If you are getting frost starting at the service port there is a restriction there that allows the coolant to expand and suck in heat.

[T77911S]

yes, if it was freezing at the service port first, then going towards the comp, i would suspect the port, BTW, there is a shutoff valve at the service port. but it seems like it is freezing at the comp first.
usually when the low side is clogged, the hi side should be much higher.

[Jagshund]

Shouldn't there be some sort of safeguard to shut off the compressor if the temp drops too low?

[billybek]

There could be several issues at play here.
If it is a TX (thermostatic expansion) valve system and the liquid line (the small one exiting the condenser) has a sight glass and you can see bubbles in it all the time, it is probably low on refrigerant.
Trane employed a layered condensing coil on a lot of their roof top units. If you live somewhere that there is a lot of pollen/poplar fluff/seed blowing around, the coil can look clean from the outside and inside, but split it apart and there could be a blanket layer in between blocking air flow. This should cause unit cycling on high pressure and will not keep up with the load.
Possible that you have too high a load (too much air flow or too high of indoor air temp or both) and that would cause a TX valve to go wide open. This would result in bubbles in the liquid line sight glass.
I worked for Trane for many years and started with there roof top equipment and have seen many quirks right at start up with some units. We sold a ton (forgive the pun) of these units so finding a few quirks is not unusual.
A good mechanic should be able to find what is going on pretty quickly.
Don't let them run away from a problem that isn't yours.

[T77911S]

Originally Posted by billybek

There could be several issues at play here.
If it is a TX (thermostatic expansion) valve system and the liquid line (the small one exiting the condenser) has a sight glass and you can see bubbles in it all the time, it is probably low on refrigerant.
Trane employed a layered condensing coil on a lot of their roof top units. If you live somewhere that there is a lot of pollen/poplar fluff/seed blowing around, the coil can look clean from the outside and inside, but split it apart and there could be a blanket layer in between blocking air flow. This should cause unit cycling on high pressure and will not keep up with the load.
Possible that you have too high a load (too much air flow or too high of indoor air temp or both) and that would cause a TX valve to go wide open. This would result in bubbles in the liquid line sight glass.
I worked for Trane for many years and started with there roof top equipment and have seen many quirks right at start up with some units. We sold a ton (forgive the pun) of these units so finding a few quirks is not unusual.
A good mechanic should be able to find what is going on pretty quickly.
Don't let them run away from a problem that isn't yours.

thanks for the input bill. it does have a TX valve. but here is another kicker. the evap is waay too small for the condensing unit, plus, the unit (carrier?) came fully charged.

some more details.
we have a small chiller unit that cools the whole tower. (i work for the FAA and this is for our control tower). when the military had the building, they had 2 air handlers for the chiller that were used to cool the tower cab itself. when the chiller broke (this was back in the 90's), we did not have a backup unit to cool the cab, so we, the FAA, put 2 condensor units (5 ton ) on the roof and converted one of the chiller air handlers over for the condensor units. the air handler had 2 seperate coils so TRANE used each coil for each condensor unit. IE, the coils were not changed plus, they may be too small. whatever they are, we could only run one condensor unit at a time or they would both freeze up because we were trying to put 10 tons of cooling into a 5 ton air handler. anyhow, trane never got the one unit to work from the time it was put in. BTW, the water pump broke on the chiller last week so the chiler was OTS for several days. trane screwed us on that deal too. but thats another story. (we keep going back to trane because they are better than the others we have to choose from, plus the chiller is a trane).

now that we have done this, the chiller can not cool the cab because it is too small, so we have to use one of the condensor units to cool in the summer. problem is, we dont have a backup other than the chiller.

i was not responsibe for the HVAC back then and was not really involved in this addition.


what would the indications be if the low side of the comp had a small blockage or something was wrong with the comp?

the site glass did show it was low and i put in a good bit of r22, but never saw any change in pressures or the site glass. but why would it freeze from the comp back to the service port.

[billybek]

They (the contractor) may have been running the system short of gas due to the mismatch in pumping capacity.
The condenser will show a high pressure if it is logging up too much refrigerant because of a too small expansion valve. The refrigerant leaving the condensing coil should be sub cooled 10 deg F from saturation. For R-22 typically you see 200 psi = 100 deg F at saturation. Subcooled 10 deg F the liquid line temp should be about 90 deg F.
On the evap side, typically for air conditioning duty you see suction pressures somewhere north of 60 psi at the lowest. Why? 60 psi = 33 deg F. Any lower and you start seeing frost and ice on the coil and suction line. You usually see the TX valve set for about 15 deg superheat for air conditioning duty so that would make the suction line temperature 48 deg F at 60 psi suction.
Now that you have a mismatched system, short charged perhaps (too keep head pressure down) with bubbles in the liquid line will cause the TX valve to pretty much go wide open and not control.
At this point all bets are off, but it sounds like they trim charged the machine to provide you with some service.
The coils should be matched in capacity or this system will limp along like a dog with two dicks.
Hope this helps.

[futuresoptions]

I have seen something as odd as a liquid line filter fail and begin to clog the TXV and I have seen in situations like yours where the TXV has failed or to where the fixed orifice was sized incorrectly to the condensor (especially in a mis-matched system)... alot of factors go into a system that has never worked correctly... one has to be there to look at the piping etc... to find clues to what is really going on.. IMHO there should never be bubbles in a sight glass if everything is sized correctly... with the pressures you gave I would think of low charge, but I have seen units with bad valves show the same thing... if the TXV is adjustable, it could use some tweeking, if it never worked correctly, you could have had defective parts from the very beginning.... just hard to trouble shoot a never worked situation from cyber space...

[turbo6bar]

It's no surprise your line is freezing with those suction pressures. In this heat, I'd expect to see much higher pressures on both sides.

I agree with the other comments, particularly billybek. They may be running a low charge to account for the smaller evaporator coil and airflow.

Are you saying the chiller air handler is only 5 tons, with each coil being only 2.5 tons?
jurgen

[futuresoptions]

after reading again, sounds like thats what he's saying... geez, that's sound waaaay too much.... sounds like he has a 5 ton split evaporator.... he would only need two 2ton condensors outside or a two stage single with a solenoind coil that would feed the 2nd stage when called for....

[T77911S]

i will check with trane again on the size of the evap. i cant remember for sure what robert said as far as the evap size to condensor size. but i do know both condensor units together are too big for the air handler unit. the liquid line is about 80-90 degrees. i dont remember what superheat was or if i even got to check it before due to the line freezing. i did not even try to check it the last ime i went up there.

the other condensor unit cools fine. there are air bubble in that units site glass too, but not nearly as bad as the bad unit.

i dont know if the air handler will ever be changed. the space it is in if VERY tight. it is up in the ceiling (drop ceiling) with water pipes, electrical pips and stuff all around, so it may be a matter of trying to get it to work the best it can.

originally there were 2 seperate air handlers for the chiller up on the 7th floor. one is in a closet, the other in the ceiling. i think the one in the ceiling provides about 5 ton of air, the one in the closet is bigger, but i dont know what size it is. the AH in the ceiiling was converted to a condensor system as a backup to the chiller, but in reality, now the condensor units are the main units in the summer, but only one works.

the one condensor unit works, i would like to get the other working and alternate units each month.

the last time i was up there, i pumped all the r22 into the condensor becuase i was going to open the lines to the evap and blow all that out to make sure it was not clogged. i originally thought it was freezing at the evap. before i opened it, i thught i would take a better look at where it was freezing, so i let the r22 back into the rest of the system, fired it up and that is when i saw it freeze at the comp. which is not what i thought it originally did.
when i let the charge back into the evap side, could it have knocked lose what was clogging the line in the evap?

it is a splitter evap. but they are 2 seperate evaps in the same air handler.

with the low pressures, shouldnt the evap freeze? not the comp?

[billybek]

There might just be enough heat at the evap to keep it from freezing. There is also a pressure drop through most evaporators that is equivalent to 2 F degrees.
It is one of those things that a guy would need to see (and feel) to get a real sense of what is going on.
Ask the service company for the guy that will be able to figure this out. Keep in mind it will take some time to do this and at this stage of the game, I doubt that they will eat any of their time.

[T77911S]

i put the gages back on this morning.
83 outside
50 lo side
190 hi
0 degrees super heat.- 27 degrees using a thermometer inserted into the line.
it froze from the lo service valve to the compressor.

lots of bubbles in the site glass.

i am thinking about removing some r22 to see if the super heat will come up, but if it is overcharged, the lo side should be higher.

i pumped all the r22 into the condensor, then opened the lo side service valve to see if i could dislodge whatever could be clogging it, but the charge would not go that way. the txv should not be blocking it should it? it would only go back out the liquid line.