Autec vacuum cooler low suction

We have recently picked up the service on 3 Autec vacuum coolers at a mushroom farm. One system has been giving grief and not performing and is running a lower suction than the others at 1.5 bar instead of around 3 bar on r404a. Motor amps are lower at 20A instead of 25A yet the superheat is low and obviously the coil is filling but it seems as if there’s a restriction in the liquid supply into the evaporator. New drier, no suction filter, short pipe run, static coil, new TEV, no MOP function, Alco replaced with Danfoss TES5 & #2, but no improvement.
It’s a basic fridge circuit with no clever valves or bypass line though the electrical controls side is extensive but when it calls to run it’s just not doing the duty it used to.

I’m open to any suggestions...

Bitzer 4G-30.2-40P, air cooled condenser. Vacuum side works as it always has.

Hi

check your evaporator fans ( rotation ) and inside of that should be clean to get good air flow

keep your discharge pressure above 200 psi for R 404a

When you say your vacuum cycle works as it should!
What is lowest vacuum pressure achieved? You need to be at 5 Torr to achieve 2 DegC of cooling of Shrooms.
Even at 1.5 bar suction you should still be condensing sufficiently in chamber for correct operation.
Is chamber draining after each cycle and coil clearing of ice?
is there a sight glass on liquid line?
What are you compressor high & low pressures?

Sorry more questions than answers but need to 'feel' the system from a long way away :)

Thanks guys

No Evap fans, static coil.

6.5 - 7 mbar vacuum, any lower and we risk freezing the product according to manufacturers manual (Autec, Southport, Lancs, now dissolved). Haven’t checked that this is an accurate reading but the sensors are calibrated yearly so should be ok

Sight glass is mostly clear except towards the end of the cycle (30mins).

LP starts at around 3bar but quickly drops to around 1.5bar while the other systems stay around 3bar. HP is around 16bar. I’ll try to get this higher, others run at 18-20bar.

Super heat is hunting from 0-15K which suggests TEV too big or too open but if I close down to stabilise LP drops further along with the suction temperature. There is a suction temp safety t/stat that was set at -10C (same as others) that shuts the liquid solenoid as the cycle progresses then opens as it warms up again. I have had to drop this to -30 to keep production running and get at least some performance. At -10 the product only gets to around +8, should be at 2.5 - 3.5 to pass.

I was starting to suspect a blockage in some of the distributor leads (though not sure if it has one as it’s all behind stainless panels) to account for the low LP but still feeding liquid but can’t see how this could happen. System appears to be dry and no signs of wax.

Appreciate your time and assistance

Hi Kiwi

did your system lost gas before or compressor lost oil

you know R 404a:

It is a mixture of HFC-125, HFC-143a, and HFC-134a, and is a pseudo-azeotropic refrigerant

if your answer is yes for gas

you have to collect gas of system and charged it again with new 404a gas

Had a similar problem years ago on a lt coldroom, eventually found inline heat exchanger was restricting, replaced it, job done. Like you went thru everything else first, including replacing distributer and exp valve, frost on evap but no bite to it. Only came across this the one time. Restriction was in suction side of heat exchanger. Can you fit a gauge at evap outlet as well as at comp?

Sight glass mostly clear until end of cycle, low high pressure side

These hint at a shortage of refrigerant.

Hi Gary
What valves and components have you between the condenser and evaporator and in what order ? and are the other systems installed in the same configuration.?

Hi Gary
Your orifice selection is slightly oversized for the compressor duty, I would reduce to #1 Orifice which should give a rated duty of 15Kw. I would then set TEV back to neutral position(factory set).

I would also check your vacuum reading with an absolute gauge, preferably on chamber side not at vacuum pump as this will be lower than chamber pressure.
Your internal coil is running too low and thereby effectively reducing its capacity to reduce chamber vapour (vapour - condenser),

Safety t/stat needs to go back to -10 after setting up.

Heading back to site this morn (Saturday 😬). Will get HP up and see if if can verify the vacuum reading.

Not sure if there is a suction to liquid HX but will have to get the panels off around the Evap coil sooner or later and inspect.

I hear what you’re saying saying about the 400 series refrigerants but with the volume of new 404a that’s gone in the ratio of existing to new should be at least diluting the fractionated leak effect but always a good one to remember. On a smaller system I can’t see that being a factor here. Have seen it on larger multi room systems though.

System is a factory built , skid mounted package so short pipe runs and is as basic as it gets. Compressor, discharge, AC condenser, receiver, shell drier, LLSV, s/g, TEV - TES5+#2, Evap inside vacuum chamber behind panels, horizontal accumulator, compressor (Bitzer 4G-30). It’s that simple!

The original call was plant not performing duty and low suction was noted by the first serviceman. That situation has not changed. I am starting to lean towards the vacuum reading being a lot lower than the actual vacuum therefore much less water vapour to recover giving lower load on the evaporator. Having said that there’s an almighty roar when the chamber vacuum is vented at the end of the cycle and it equalises to atmospheric pressure so maybe not?

Nice way to spend your Saturday!:mad:
We have a lot of the Autec vac coolers on various sites and they all pretty basic systems, they have reincarnated a few times under various names but seem to get into financial troubles on all guises of company.

Major problem with vacuum cooling is always vacuum pressure, 5 Torr / 7 mbar standard target pressure on most products.
Even if refrigerant system fails you can still cool and item that suffers is the vacuum chamber pressure due to steam expansion.

Good Luck