New compressor high head pressure

[Crypton]

Hi, I installed a new compressor on a serveover today. The old compressor was an Aspera compressor on a condensing unit packed underneath the cabinet. The compressor was gone. I changed the compressor to a hermatic. I crossed referenced it in a fridge suppliers. Anyway I put in the new compressor, changed drier, pressure tested, vac out. There was no badge on the serveover to say what charge was required so I weighed in 1kg of R404a. I estimaed the charge having changed a compressor on a cabinet last week weighing in 550g. This serveover was a lot bigger. I changed the electrics and ran up the unit. She pulled down temp no problem. Cutting out at 2c and cutting back in at 4c. It was evaporating at about -10 which is perfect but the head perssure was 21bar which seemed to high. usually you should get a head pressure between 16 to 18 bar. I cleaned out the condenser coil but it was extremly warm underneath the cabinet. There is no grilles or air vent on the back of the condenser coil it simply sits up close to a wodden decorative panel. . I was suggesting putting a mesh grill into the decorative panel to allow the unit to breath. with no sight glass on the system it is hard to judge the right charge. Am I going down the wrong road with the air flow or do you think it is charge related. Would be grateful for any help.

[Brian_UK]

Airflow will be a problem but perhaps you should check the subcooling to see if you are overcharged as well.

What sort of expansion device is fitted?

[Latte]

Well i think you have answered youre own question - AIRFLOW. You say its hot underneath, how hot, anything above 28-29 and the HP on 404 is going to go divvy, I am assuming the old compressor was on 404.

Its a common problem especially in pubs/resturants where they have these lovely wooden salad carts in the middle of the pub for people to help themselves. As brian says is it cap or TEV, normally they are TEC so you could fit a sightglass but i think you may be onto a loosing battle if youre goint to try and fight the HP in a high ambient at the condenser

[monkey spanners]

Another helpful clue it how much refrigerant you took out to change the compressor, allowing for a bit staying in the reco rig etc :)

[Crypton]

Thanks lads I never got the subcooling today I will in the morning. The old one was on 404. It is a TEV as well. I never weighed the charge out stupidly. I just reclaimed it into a bottle. I always like to put a fresh charge with a new compressor you never know who was at the system before you. The underneath was about 28-29c really warm no air flow. Its a killer the way it pulled the temp so well in the cabinet but the head pressure just went so high. I just could not trust the head pressure so I left it off until I come up with a solution. So the only way out of this is to get the air flow better and make sure the charge is correct? would you be prepared to let the unit run away with the pressure that high?

[monkey spanners]

Has the system got a reciever? if so how big does it look in litres? Can you pump it down without head pressure rising much? A very rough guide is a kilo a litre for max charge, if you don't go over this then liquid should not be backing up into the condenser and reducing useful heat transfer area and thus raising the head pressure.
If its really hot under there i expect the pressures are normal. Many of the dx milk tanks i work on will run up to 24 bar (R22) in the summer and they are outside, but see what others more familiar with this type of kit think.

Jon :)

[mad fridgie]

sort out the air flow, thats always a good thing, but saying that your head pressure is well within the normal working envelope of the compressor.
Check subcooling to see if over charged.
Whats the weather like, is it hot, if so this will also increase head pressure

[Crypton]

There is a reciever. It's small roughly about a liter in size. I'll pump it down tomorrow and see if head pressure rises. It's in an enclosed space and it is warm out side a good few other fridges around it which adds to the heat. Thanks for your help. I'll check it out 2moro and let u know. Cheers

[Crypton]

Went back this morning to check the SC. Temp of pipe at condenser outlet 37c. Head pressure 310psi on a slide ruler that converts to 47.5c. So with a difference of 10deg between the two would I be right in saying it is over charged? I usually dont check the SC have I done the proceedure properly? I noticed something funny when I was trying to get the SC, the temp at the condenser outlet was 37c on the same pipe 150mm away heading towards the reciever the pipe was 40c, the reciever was 43c and the liquid line leaving the reciever was 37c. Is that strange? I am going to buy a steel air vent for the far side so the condenser is no longer rammed up against a timber decorative panel this should allow it to breath and obviously if it is over charged I will reclaim some of the refrigerant. If you can put me straight on the SC I would appreciate it.