Partial Freezing of Evaporator and T.E.V

[PaulYeates]

Doing maintenance on a walk in freezer room, noticed a large ice formation on the T.E.V/left evap coils along with some ice build up on the opposite coils.....the finned section ice free.
After removing all the ice I checked the defrost elements/probe reading, which were ok. Turned on the room chilling cycle againThe system evaporating temperature was -32 degrees celcius with the room at a temperature of -20 deg celcius. Its reaching its set point fine and doesnt seem short of refrigerant, although the partial frosting has me in doubt.
Any opinions???
Is it possible that the TEV is not letting enough refrigerant through to the evaporator??? The bulb is definitely in the correct position.
Refrigerant type is r404a

[frank]

With a room temperature below freezing, you are going to see frosting on any exposed sections of pipe/tev.

-32C for a room at -20 seems a little low

[Grizzly]

Frank's your man, Paul.
The lower you take your suction the more ice build up issues you will have.
Try slowly raising suction pressure setpoint or room thermostat dependant upon your type of control.
The closer you have the SST to the room temp required the less issues you will have.
Grizzly

[Tayters]

Maybe not applicable to your case but some freezer rooms i've dabbled with I've noticed the hole for the pipes hasn't been filled in causing moisture to from around the TEV area.

Cheers,
Andy.

[Magoo]

Ice build up on distributor tubes can indicate incorrectly selected tube diameter and or length in relation to orifice sizing and TEV rated duty selection.

[Gary]

Did you measure the subcooling and superheat?

... and doesnt seem short of refrigerant...

What does that mean?

[jamieson91]

The only time I have had irregular ice formations (besides doors being left open)
in efficient compressor..
short of gas
And faulty txv ..

now I have had my techs insist to me a the unit is full of gas.
but when a system has a blocked condenser and a gas leak the system basically will work fine but at half the capacity... If you reduce the refrigeration effect and don't touch the condenser capacity you have a problem vice versa but if they both come down together over time eg slow gas leak and gradual build up of dirt on the condenser the system will just reduce the efficiency without a dramatic change ...
My first step would be clean the condenser and check pressures ,
perform an efficiency test and be sure to check that the high side doesn't equalise thus your discharge valve is in efficient ... Most techs just shut off the suction rotor lock valve and check the suction but never check the migration from a leaking discharge valve ...

Now the only way I have seen the txv fail (besides loss of bulb charge and blocked strainer etc) is ...
with no heat load in the room and just standard operating conditions I noticed my suction pressure would flutter from 30 psi to 20 psi .... And very unison at that. ...this txv was unplaced for a 5 years no dramas previous...
Changed the valve never bother problem ..

Next... Do you have hot air migrating ? No door curtains air lock room
signs will be frost around the fan shrouds and moisture over the roof .. Similar signs can be a faulty fan delay klixon(thermistor)... S implement way to check is throw your coils on to defrost and measure coil temperature raise to +temperatures and then end your defrost period if the fans start straight away you have a faulty klixon... Have a few more ideas but its way past my bebedtime mil me if I am any help

[bigtwin]

Hi Paul, Where in the world are you? Is the condenser air cooled? Do you have head pressure control? Could be over-condensing. Result would be wide evapoartive delta T and no flash gas prodcution to ensure good distribution after valve. Otherwise check expansion valve filter/drier/thermal phial position etc.

[1mikeefc1]

Check the bulb on the tev is in the correct position and tightened securely also it should be insulated.

[Royal241]

I feel left out if not say my part, lol. I had a very similar problem and turned out to be a solenoid leaking through during off cycle and defrost. defrost termination sensor was on the opposite side and terminated defrost based on that side's temp. heaters don't extend as much to the side where the txv is (all your ice). a leaking solenoid will build ice on the txv during off cycle. it's worth taking a look, maybe I got lucky but I definitively had it leaking and after replacement had good news (over 1 year ago)

[jamieson91]

Royal241 I have changed most of my freezers.. Most we're all pump down but after talking to some mentors a few years ago I was convinced to wire my liquid line solenoid in parallel with my compressors and just use it in he off cycle to prevent migration...
The only reason is if your Evap is full of gas I find the end of the coil ( u bends ) don't ice up.. Wen using electric heater defrost there is a greater heat transfer through the coil if the refrigerant is still in the Evap...

[jamieson91]

Also I personally have too many dramas with defrosts controlled by temperature.
I just program mine by time .. Ensures adequate defrost without product temp rising.
I am in Queensland Australia though where the average humidity is 85-90+% and its all over the place.
I have noticed with small commercial units such as Skope (love there products none the less) that prob allocation is terrible.. They place the heaters in the drain pan 5 inches under the defrost termination probe ... Then the factory setting for the defrost termination temperature is around 5degrees (c) and it never gets rid of all the ice ..

I had techs going to this one place for 2 years defrosting the coil every 6 months and not having an answer ... I saw the location of the heater and the defrost termination probe and changed the defrost duration terminated by time .. Haven't been back for a year and a half ..
checked over it whilst I was working on another system at the same venue and it is all okay