Help required - bad ice build up on freezer rack compressors

[cool me]

Gday
Ive got a Rack of Compressors running Freezers for a Supermarket.
The set point is -25 degree Celsius
Running on 404a Refrigerant
Has an Enviroguard system installed and running
The issue we are having is the cabinets are taking a long time to reach set point, therefore the
Suction pressure never gets below 68KPA, to then be able to start cycling all 3 compressors.
The compressors are constantly on and therefore causing our ice build up the the compressors but that opinion is open for debate.
One of my questions is how can you ensure correct gas charge for a system that has the
Enviroguard system set up?

Thanks guys
Any Help or knowledge would be appreciated.

[taz24]

.

With that amount of flood back I would say you have more than a high suction problem.

I would look at all the expansion devices and make sure they are set correctly (and opperational)
and I would look to see if you have any heat exchangers between the liquid and suction pipes
inside the cases.

From the photos I would say you have a serious flood back problem and it
appears so bad that it is effecting the suction pressure.

All the best

taz
..

[aramis]

You probably have a "hunting" problem, how does superheat at the compressors suction vary?

[Coorsman777]

It takes a lot of liquid to get that much ice build up. I would say it has absolutely nothing to do with the Environguard portion of the system. If it is a hot gas defrost is there a check valve stuck open? Its either that or an expansion device way out of adjustment or sized wrong.

The Environguard is charged by subcooling. There should be all the info and setpoints in the operation manual.

[monkey spanners]

Looks like those comps are chuggin' on liquid!

[newbie]

suction accumulators not on this set up no?

[RANGER1]

If this was ammomnia ice build up would be normal!

Here is a few thoughts!

Compressors have oil failure switches, do they work, are they set correctly?

I would have thought with flooback it would continually wash oil out, sump would be cool or cold, oil would be foaming, discharge temp would be very low if not cold.

Know nothing about envirogaurd but measuring subcooling in liquid line would be a start.

Confirm compressors do cycle on pressure by shutting some cabinets off etc to lower load.
Check suction compressor control for correct setting with reliable gauge, digital would be most accurate.

Maybe check compressor valves are OK by closing suction, bridge out LP.
Close discharge, see if pressure drops quickly after compressor is turned off.

[cool me]

Gday Again
Thanks for the responces
To Taz - With a long pipe run like i have, would a 3K Superheat be acceptable for the TX valves?. There are no heat exchanges installed for each evaporator.
To RANGER1 - We have confirmed that the compressors do cycle correctly when load is taken off. In reguards to the oil in the compressors, each compressor is fitted with a trax oil system along with susction accumilator and oil seperator however i do agree with what your saying but no oil issues yet.

Thanks again

[Magoo]

Definitly a lack of suction superheat, the enthalpy of vapour will not form that sort of ice buildup. The evaporator superheats is a good place to start at and test and reset. Average compressor superheat stated by semi-hermetic manufacturers in the range of 15'C

[aramis]

15 °C at the compresor suction!

3K superheat is too low for mechanic expansion valves, the minimum would be 5K. For electronic it may be 3K or lower. These values have to be minimum settings. If it reaches 3K it could well be saturated refrigerant due to measurement errors.

Too low superheat causes unstable control of the TXV or "Hunting" which causes high average suction pressure because of all the liquid in the suction line and does not comply with set temperatures as it periodically starves the evaporator.

I'd say these compressors will need inspection soon, they may have lubrication problems.

The accumulator could be too small for the application or it may contain oil. Did you have to add oil periodically?

Do you have the compressors models and the accumulator model?

[RONB]

I may be stating the obvious but are all of your evaps ice free,are they getting a good defrost etc. How are your evap fans controlled. I had a similar problem a while back and it was the evap fan control strategy on the cabinet controllers which had gone to ****. It was starting and stopping the evap fans when it wanted to,it also sent the defrost control dodgy aswell. I had a faulty case controller which was sending the network to pot. It was a bit of a swine to find.

[newbie]

i think somebody already said this but expansion valve wide open?
this happened to me,4 a.m call out compressors not running,pure liquid getting back to compressors even with accumulators on the line,turned out the expansion valves on 3 fridges were wide open.wrecked the oil pump on one compressor

[750 Valve]

Suction superheat is an issue here - or lack of it.

Firstly what is the model of the freezer case? Most require -30 SST setpoint.

Secondly once running at design SST, aim for a 10K minimum suction superheat at the stubs entering the header, you may have a few stubs coming back hard causing the issue. The rack is designed at max 20K suction superheat.

Thirdly make sure the enviroguard is set up right. Getting such low ambients you can't go too low in condensing or the TXVs lose capacity, I wouldn't go past a 10 bar pressure drop across the valves as a rule of thumb, you could find out their exact capacity quite easily - i think they are Sporlan SBF balanced port valves.

If the rack is at -25 this could be part of your problem, the valves will be oversized and control may be a little hard hence the low superheats.

[750 Valve]

Also bear in mind that with a -30 setpoint and 20K superheat that -10 vapour will still cause the bellhousings to form ice. Depending on the conditions of the plantroom the ice might just be a small frost or could form large balls over time.