AC man becomes fridge man and is out of his depth (Walk in fridge frosting)

[marcymarc]

Hi, I have been fitting and servicing air con units for the last 15yrs but now I have been asked to look after catering fridges and if I'm honest I'm out of my depth, but hopefully never too old to learn

It is at this point in time I wish I paid more attention to the TXV lessons when carrying out my Nvq, hindsight is a wonderful thing


Equipment that I am currently working on is a walk in fridge with a set point of 4'c, customer complains it takes longer than normal to cool and the left hand side (TXV end) freezes up

* R404a stamped onto the TXV and condensor
* Suction pressure 20psig Suction line temp 18.7'c
* Liquid line pressure 180 psig Liquid line temp 26.2'c
* Standing pressure at 20'c 90 psig
* Sight glass full of bubbles

* Indoor coil fully stripped and internally cleaned by myself as that was my initial diagnosis of insufficient airflow

Before I go ahead I would appreciate your experience on what the possible causes could be, I don't want to just put extra gas in as it seems to exhibit hallmarks of being low without knowing for sure and why etc

[Brian_UK]

Yes, initial indications would be short of gas(sog).

Your suction temperature is about -25°C but with +18°C pipe temperature giving 43°K superheat, way too large.

Start thinking in temperatures not pressures as that is what the end result should be - the correct temperature. Just like AC the pressures/temperatures will not show as 'ideal' until the space is at set-point temperature.

A bubbling sight glass could indicate it being SOG but until the space temperature has been pulled down to temperature you can't be 100% certain. The TXV could be over feeding to reduce the superheat.

You now need to go over the system with a leak detector, bubble liquid, and see if you can find any leaks. A torch and eyeballs also work at this task.

Also check the temperatures across thing like driers and solenoid valve to see if there any blockages.

[marcymarc]

Am I right in thinking that achieving a superheat of 4-7'c is optimum to maximise efficiency and not allow liquid to flood back

Would you be able to recommend a suitable sub cooling temperature range

What should the discharge and suction temperatures be on a capillary fridge with R134a and R404a rerigerants

Starting to build a notebook on these systems and refrigerants

[Tayters]

Superheat measured is to avoid liquid getting back to the compressor but ensuring the coil gets a good whack of refrigerant.
Use the temperature of the refrigerant to figure out what the pressure will be. As a rule of thumb the suction pressure (evaporating temp) is 10k below room setpoint and discharge (or rather condensing temp) around 40°C in UK.
For a freezer you'll be aiming to evaporate around -30°C, for a fridge -10. Superheat measured when you are a few degrees from setpoint, 4-7 ideally, subcooling generally not above 5k or a full sight glass, whatever comes first.
In a capillary system the charge plate should be right so weigh it in first. I've measured subcooling 2-5k on working systems, 8k with blocked cap tubes.
There's a set of books, Tech Method which is pretty good.
Common place for slow leaks on a chiller room is around the TEV.

Cheers,
Andy.