Excessive superheat overheating compressor

[Contactor]

Hello

Small bespoke domestic fridge system, approx 15m run to condensing unit along warm external wall. R407C -7 / 3.5 bar LP (Approx) set point 3c

Compressor cycling on thermal protection.

I've opened the expansion valve to 7k superheat measuring the suction pipe temperature AT THE COMPRESSOR, this has kept the compressor cold and all running well.

I am unable to take a pressure at the evaporator and I'm worried by opening the TEV (internally equalised) the compressor will flood in very cold weather.

All thoughts welcome

Thanks

[Contactor]

Prior to opening the valve superheat at the compressor inlet was approx. 23k with pipe temp of about 16c

[TRASH101]

Hello contactor

You say

opened the expansion valve to 7k superheat

what was it before and what temp was your fridge?

AND what is the superheat when it gets down to setpoint?

[Contactor]

So if for example, the outdoor temperature fell to -10, the compressor could presumably be flooded by a 2 phase mixture reduced to liquid sub cooled by 3k ?

[Contactor]

thanks for your reply trash, i think my edits crossed with your post -

fridge was up to 17c temp while cycling on thermal protection

you make a good point about measuring the sh at set point, i'm not sure about that one

[TRASH101]

So if for example, the outdoor temperature fell to -10, the compressor would presumably be flooded by a 2 phase mixture sub cooled by at least 3k 07-07-2010 12:08 AM

So if for example, the outdoor temperature fell to -10, the compressor would presumably be flooded by a 2 phase mixture sub cooled by at least 3k

Not neccessarily and maybe not at all. That wouuld depend on what load you got and how your TXV copes and of course you will have the condensor to consider.

[lowcool]

wouldnt measuring superheat temp at evap with thermometers be more accurate and is the suction line well insulated?i dont know about using 407c for this application either?

[Contactor]

system was commissioned to 7k superheat at evaporator but this wasn't enough to keep the compressor running. The suction line is insulated in 3/8'' black armaflex, but I wondered if this is only making matters worse in the blazing sun as it seems to get very hot.

[TRASH101]

wouldnt measuring superheat temp at evap with thermometers be more accurate and is the suction line well insulated?

Valid points

dont know about using 407c for this application either?

I must admit I'm more AC than fridge but evaporating at -7c doesn't sound bad for 407c or is there something I'm not getting?

[TRASH101]

system was commissioned to 7k superheat at evaporator but this wasn't enough to keep the compressor running.

Under what load?

16K superheat just from the suction line

[lowcool]

i was of the belief that 407c was high temp refrigerant,dunno havent played with the stuff yet.are the current draws in range if not maybe check for non condesables,condenser air flows,run capacitor etc.
3/8 wall does sound to thin i now use 19mm wall on medium temp stuff and have done so since the tirade of new refrigerants.

[lowcool]

you say it was commissioned to 7 kelvin was this on 407c or 22 if the later then your condenser size is probably to small,was always under the impression that 407 ran a higher head.
might pay to fit schraeder and reprocess system.

[TRASH101]

You have got to find out what is going on with the suction line. Even uninsulated it shouldn't gain 16k over 14m of pipe.

[Contactor]

SORRY IT'S 404A !!!!!! my typo, sorry!!

maybe my temps are out slightly, also I only measured them twice and at the same time in the cycle, but generally the SH was high.

This fridge is made of bricks, it's an old archway in a cellar in a big old house with a made to measure steel framed double glazed fridge door on it, it was empty so very low load