Refrigerated display cabinet, low head pressure

[expat]

I got called to a butchers with a problem with his display cabinet. He said it was short of gas as he saw bubbles in the sight glass.

Upon inspection I found high SP, low HP, loads of SH and so concluded that he was right.

Now I could not for the life of me find any leaks, neither with aerosol leak detecter or a sniffer. There were no hidden joints so I was perplexed.

I charged up with 1kg of R22 and got my SP to about 2 bar (29 psi).
This gave me 7K SH, as I measured 7°C (45°F) at the evap exit.
The HP was 11.5 bar (165 psi)
32.5°C (90.5°F) at condenser exit
Condenser air on 19°C (66°F) air off 31°C (88°F)

Now I didn't want to add more gas willy nilly to raise my head pressure for fear of also raising my suction pressure.

Now the evaporator is divided into five sections; two going left fed by one TXV and three going right fed by another TXV. The left evaporator is about 1m10 long and right is 4m60.

Before I left he asked me to quote on dividing the right hand evaporator in two and adding a third TXV to better distribute the *****. I said I would think about it!

Sorry this is so long, nearly finished!

He also told me he often needs a gas recharge when going into summer and again going into winter.

1 How come a butcher thinks he needs another TXV?
2 How come someone has changed the origional TXVs for external pressure equalizing?
3 How come he needs gas recharges in winter and summer but I can't find a leak?

One more thing, the condensing groupe is a maneurope with twin ventilators. After blocking half the condenser I got the head pressure up to 16bar (230psi), the SH remained stable and the sight glass improved but still had bubbles.

I'm thinking of putting a pressostat to cut one or both ventilators when ambient is low, to keep the HP up and charging with a little more gas.

Any comments welcome.

ps I don't think there is a leak, I think he gets a recharge in winter and then a discharge in summer!

[expat]

This gave me 7K SH, as I measured 7°C (45°F) at the evap exit.

I meant
This gave me 7K SH, as I measured -7°C (20°F) at the evap exit.

[chillyblue]

Hi

As far as the leak is concerned you need to reclaim the refrigerant and pressurise the system to about 350Psi with OFN. If the leak is on the low side there may not be enough pressure for you to find it easily.
Be carefull though you may need to disconnect LP switches of other pressur sensitive equipment.

CB

[expat]

I appreciate your advice Chillyblue but I put kg of gas in and it still seems as if it's low. Now if he lost that much gas in say six months we're looking at a 2000 gram per year leak! My sniffer or the soap should have found this. No, I think it's a problem with ambient temps in winter that mean the system runs like it's short of gas. Subsequently it gets a top up but then starts playing up in summer when ambient temps rise hence the visit for a recharge in summer (which is actually a reclaim).

This is why I thought of controlling the air flow over the condenser during low ambient.

[Toosh]

Hi Expat, I think you are right it sounds when the ambient falls it is short, does the unit have a head pressure cotroller it might need one.

Norm

[bernard]

Hi

You need to fit a pressure switch to the second condenser fan as the system is overcondensing.Or disconnect the second motor during the winter.

Regards Bernard

[PaulZ]

Hi Expat
As Toosh said you are correct about the condenser fans. It is advisable to cycle the fans when the ambient is low.
TX valves are sized with a set pressure drop across the valve, discharge to suction. The lower this pressure drop the less capacity you will get out of the valve.
Your condensing temperature should be between 10 to 15 degrees above ambient. If you install a pressure control to cycle the fans you can set it to achieve this.
Paul

[Gary]

One more thing, the condensing groupe is a maneurope with twin ventilators. After blocking half the condenser I got the head pressure up to 16bar (230psi), the SH remained stable and the sight glass improved but still had bubbles.


Did you check the subcooling?


This is why I thought of controlling the air flow over the condenser during low ambient.

Yep.

[expat]

Toosh, Paul, Bernard thanks for those comments they confirm what I had thought.

Gary, I read 32.4°C (90°F) at condenser exit with 11.5 bar (165 psi) head. So I seemed to have no subcool.

At 16bar (230psi) I read 40°C (104°f) which gives me 5K subcool. So even with an unclear sight-glass this feels right to me.

While the superheat did not alter after blocking half the condenser to acheive 16bar head pressure there was a noticeable pulsing from 15.5 bar to 16 bar on the gauges. I thought this may have been due to the fact that the TXV feeding the smaller evap may have been overfeeding and causing hunting.

To check this I think I need to shut the system down and check the orifice sizes but it's difficult to get these butchers to give you the time you need.

Again thanks for the replies.

[Gary]

Here's what I would do:

1. Install a fan control to raise the head pressure... then...

2. Check the TXV superheats. If the superheat is low adjust the TXV, otherwise leave it alone... then...

3. Add refrigerant until the subcooling is 8.5K/15F... then...

4. If/when the box is down to design temp, check and adjust the TXV superheat.

[expat]

Thanks for coming back with that Gary. I will be following your advice and let you know the outcome.