Airedale CUS3.5 unit R22 to R407C change out

[ianparker0801]

Hi
I've been in the aircon trade for a few years now and finally got a chance at a proper repair job, just after some advice to but my mind at rest. Details are as follows;

The unit is:- Airedale, CUS3.5 HI

When I came to it it had blown it main fuses, this traced back to the compressor which was down to earth. So I recovered the gas and found it to be well bellow its correct charge, 4.7Kg of R22 recovered should have been 7.5kg. Suggests a leak.
Now as R22 is being got rid of slowly I thought it was a pervect opertunity to change the gas, after speaking to airedale I found out the system would run fine with R407c. Got a new compressor, TEV, service port valves, high and low pressure switches and went about installing it all.
After alot of swearing and head scratching I got it all installed (remember this is my first repair job and I'm on my own). Strength tested the system with OFN to 20bar and used bubble spray to check for leaks, all was ok, dropped the pressure to 14bar which held for 48hrs.
Triple evacuated the system, final evacuation down to 0.5 torr. Managed to charge with 6kg of R407c, airdale suggest you will need roughly 10% less R407c to R22, it is .75kg away from this. I ran the system with the following results,

Comp. Amps. 6.06, 6.13, 5.62

discharge temp. 48.5 C
Liquid line pressure. 15 bar
Liquid line temp. 27.1
Subcooling. 8.9
Suction pressure. 4.2 bar
Suction Temp. 7.7 C
Super heat. 3.7
Air off condensor 34C/33.6C
ambient 7 C
Air off evaporator 12.3 C
Air on evaporator 17 C

I'm worried that my superheat is a little low, airedale recommend it to be around 10. it is giving me more cooling than it use to when running on R22 but I know this doesn't mean it is running correctly. Any advice anyone one can give me would be appreciated and if any of my figures look wrong or out of place please let me know. I feel I should but more gas in it but from what I can work out this will not help my superheat or subcooling. Oh the unit also has a hot gas bypass fitted to lower the units capacity, whilst I understand the theory behind it I not certain how this would effect the systems behaviour.
I am on my own with this and have no other refrigeration engineers to turn to, please take pitty on me even if is only to mock me.

Ian

[stufus]

Your superheat is a bit on the low side , but not critically low .Is there a load in the room you have listed air on to the evap at 17c.Is there a sightglass on the system and if so what is it showing.
On a side note what type of oil is in the compressor ,i hope its not mineral oil.
Cheers
Stu

[ianparker0801]

No it the compressor runs on POE oil, the old one did to, hence why the swap over isn't to complicated (well according to airedale anyway).

There isn't a huge load on the system, there is a UPS in one of the rooms it serves but even with the cooling off it takes a couple of hours before the room gets hot. When I took the readings I was running it in hand, so the liquid line and hot gas bypass solenoids were opened and a run signal is sent to the condensing unit. At the time I thought the site glass was running clear bar a couple of bubbles. This morning I let the room warm up for a while then switched the repaired unit on, although I didn't take any new readings (its raining) it brought the room temp down well but I noticed the site glass was flashing every min or so (there are two site glasses on the system, one by the filter dryer just before the condenser and one just after the liquid line solenoid valve in the evaporator unit, both flash the same). The other three identical airedale units have always done the same and I've just assumed it is to do with the hot gas bypass.

[ianparker0801]

When it isn't flashing the site glasses run completely clear.

[slingblade]

Why not screw the txv in a bit to lift the superheat. also check the evap coil/filters are clean. Did you set the superheat up in the first place after new txv fitting?

[ianparker0801]

Both coils have been well cleaned and new filters put in.
I'm a little weary of messing with the TEV, I've always been told that they come preset when brought from the units manufacturer. Airedale were aware of the situation, although that means nothing really. I might be completely wrong, it is just what I have been told on various training courses.
The other problem being the load on the system, it is still cold outside so the system isn't having to work very hard to keep the area cool.

[slingblade]

A couple of things bother me about your reply. There is no such thing as a preset TXV unless it is part of a full system pre run, tested and comissioned prior to installation on site. As your unit was existing and you retrofitted it, there will be some setting up involved. The TXV controls the superheat, but can be affected by other factors. Not having done any Airedale plant for a while im not sure what is fitted to a CU 3.5, but it sounds quite small. Does it have a fan speed control for the condenser fan? 15 bar liquid line pressure sounds a bit low, i would expect nearer 19 bar. If no FSC is fitted stick a piece of cardboard over 1/2 of the condenser to lift the discharge to 45 deg c or there abouts. After the heat rejection has been confirmed to be ok at the condensing unit, then adjust the TXV to give the correct superheat ( anywhere between 7 and 15 c IMO ). if you cant get it that high then there is either an airflow or coil fouling problem on the evap, or the TXV orafice is too big.

[mad fridgie]

TEVs have a pre set spring tension (at some conditions can be assumed to be preset superheat setting)
Is the superheat relatively constant, then the valve is OK, also check the compressor suction inlet temp, This should be a few degrees higher than your evap temp (this will confirm no liquid in the vapour steam) If the same slightly close TEV, let settle and re-check. When dealing with temps close to saturation, accuracy of your measurements is crucial, for this reason many advocate high superheat to avoid liquid flood back.
Your load does seem low, your air on to the cond is also likely to be low, you would expect lower suction pressures

[hyperion]

With the hot gas bypass valve energised, it is going to be difficult to get a clear sight glass. If the room load is relatively low and the discharge pressure is also slightly low as indicated, this will also provide lower than normal suction pressures and por=tentially temperatures. Try putting a false load in the room like a 2 or 3kw electric heater, this may help you to get a good test run.

[ianparker0801]

ok, that is alot of information, thanks for all the responses.
Right the system does have a FSC.
The evaporator coil has a three stage electric heater built in, by raising the temp set point and running the condensor in hand (thus putting a load on the system) the supper heat creeps up to 5. Even when the room has been brought right down in temperature (i.e. no load) my superheat doesn't drop below 2.
My suction inlet temp is always a few degrees higher than the evap temp (assueming the evap temp is worked out from the pressure at the suction service valve), is this not what the superheat is?
I would pressume the hot gas bypass, when active, will bring about slightly lower liquid line pressures than expected. as it takes hot gas from before the liquid line service valve. (again I could be very wrong)
The general feeling seems to be that I need to adjust the TEV, I'm assuming this is best done with the false load on the system (i.e. electric heaters on). Just to double check I'm on the same lines as everyone, I am taking my super heat readings from the suction line services port just before the condenser unit (this is what I have been taught). I have found some suggestion that this could be taken around the evaporator. I worry that if I adjust the super heat up to 10 now that when the summer comes it will be way to high, will this be a problem or just lower the efficiency of the system.
Sorry if none of this makes sense, I'm desperatly trying to learn everything I need to but finding information or people who are willing to part with it seems a bit of a nightmare. Thanks for all the info so far, this forum has been a life saver as my company don't seem to care what I do or how I do it.

[a/c.king]

dear ianparker
it sems to me from your discription that your system is runing ok ut you paniking. if you operating the system on low load so you will have low superheat, from my expiernce if your suction line is not iceing up and you have 7c to 13c suction line. then you dont have to worry.spacialy if you have suction line accumilator on thesystem.
rgards
as king

[Tradewinds]

Hi Ian,
I could be barking up the wrong tree here but is this the unit you are working on?

http://www.modine.com/web_server_con...M_CU1-4_UK.pdf

It says the CUS3.5 has a gas charge of 1.5kg of R407c. With your high subcooling and low superheat it could simply be overcharged. I would probably take all the gas out and weight it in slowly. Put 1kg in then take all your readings. Then put another kg in and do it again. Do this all the way up to 6kg and watch what happens to your figures. You should be able to see them come into line so you get between 4-8k on both superheat and subcooling. You should be getting 10k air temp across your evaporator

Ambient +15'c = SCT (Probably set to about 43'c with FSC)
Cond TD = 12-20K
Subcooling 4-8K

Evap TD = 10K (Air On/Air Off)
Air off - 10k = SST

Discharge and Suction Temps- Is that saturated or pipe temps
Is your Cond fan running flat out?