freezer R404a too much ice on evap

[The captain]

Found a freezer at -18'C or 0'F running on R404a SCT 27'C or 81'F 1200 kpa or 175 psi with subcooling at 8'K or 15'K Ambient temp today 21'C or 69'F with a clear liquid sightglass to the TXV. acapac brand distributed by Actrol parts here in Oz, rated at 46'C or 114'F. Problem is electric element defrost at 40 min every 6 hrs time terminated is not clearing the evap of ice and coil is frezzing over causing no air flow. Second condenser fan is set on a switch with 1400 kpa or 203 psi range and 200 kpa or 29 psi differential. Lead condenser fan is switched with the compressor. SST is around -32'C or -25'F 100 kpa or 14 psi R 404a. Does 27'C or 81'F SCT seem a bit low or is it ok with 21'C or 69'F ambient temp?

[AUcooler]

The moisture freezing on the coil is introduced from somewhere. I would be checking for air leaks, via door gaskets, and pipe penetrations if you havnt checked already.

Also do you know for sure that your defrost is time only terminated, and there is no other thermistor or clicson terminating the defrost prematurely?

I would expect your head pressure to be around 15degC above ambient.

When the evaporator is iced up, is the ice build unform over the whole coil, or favoring one end?

What type of freezer is it? and its contents?

[The captain]

The freezer is 2m x 4m walk in freezer room. Yes i will check the pipework penetration as some ice is present on the pipework now i think about it. Actrol gear with a carell master cellar PLC, new stuff. Whole coil icing over have tested defrost via manual operation and found Elements working ok. I think the PLC may be at fault intermitantly not stopping the compressor during defrost mode. Evap fans stop and elements power up ok. Not happy with SCT at 27'C would like to see 35'C ish like you said. Thanks for the help out. my background is mainly Air conditioning.

[AUcooler]

Carel generally have a lot of configuration parameters. If you dont have all the setup options you can download a manual here- http://www.carel.com/carelcom/web/en...5&id_mercato=3

I would also check the coil probe is not mount to close to any heater element causing premature defrost termination.

[tonyhavcr]

Found a freezer at -18'C or 0'F running on R404a SCT 27'C or 81'F 1200 kpa or 175 psi with subcooling at 8'K or 15'K Ambient temp today 21'C or 69'F with a clear liquid sightglass to the TXV. acapac brand distributed by Actrol parts here in Oz, rated at 46'C or 114'F. Problem is electric element defrost at 40 min every 6 hrs time terminated is not clearing the evap of ice and coil is frezzing over causing no air flow. Second condenser fan is set on a switch with 1400 kpa or 203 psi range and 200 kpa or 29 psi differential. Lead condenser fan is switched with the compressor. SST is around -32'C or -25'F 100 kpa or 14 psi R 404a. Does 27'C or 81'F SCT seem a bit low or is it ok with 21'C or 69'F ambient temp?

Is every 6 hr of defrost normal for you?
We use 3 hr @ 30 min.

I'm with ( AUcooler moisture freezing on the coil is introduced from somewhere. I would be checking for air leaks, via door gaskets or customer is keeping the door open when loading box.
(The captain)
compressor not stopping during defrost mode
should be pumping down quick test.

[minus1]

heaters must ov gone dont pump down no need

[The captain]

No pump down solenoid on this system. Is 6 hr defrost intervalls normal to me? Well the answer is no its not! thats why i changed it to 4hr with 30 min termination and 5 min coil dripping time, Carell feature!.

[tonyhavcr]

Why no pump down?
if no pump down
When the unit goes into defrost any refrigerant in the evap may migrate into the compressor, and if controlled by a presser stat will keep your compressor running in defrost this will make your ice on the coil add a pump down

[Rigbee]

Sounds like your freezer is set up fairly well. Don't run at -35sst, will throw your superheat out of whack. -27 to -30 should hold your 6k. 4 defrosts at 30 mins should work ok in Oz. Go with checking for vapour leaks and the defrost sensor. Carel low temp stuff run defrost probes. very common problem is the location. also run the termination temperature higher than the old school 8 degrees they always seem to need to be well above 10.

[tonyhavcr]

Hi all still would love to know why there is no pump down on a system noted by
the captain w/ electric defrost.
When the unit goes into defrost any refrigerant in the evap may migrate into the compressor?


When you put that with Whole coil icing all over, the compressor not stopping during defrost mode. Ice over the hole coil would be what you get would it not?

[Rigbee]

All low temp ACPAC units are fitted with sump heaters, this will prevent the oil vapour pressure dropping below the refrigerant vapour pressure therefore preventing liquid migration. Liquid line sol valve for pump down is a good practice to get into, definatly required for low ambient conditions but not a necessity in OZ.

[lawrence1]

You need to check the defrost termination sensor setting,,,factory setting of 8c is way too low,,,,i set mine to 25c with the probe taped to teh suction line 300mm above the bottom of the evap and insulate it.
LJ

[sork]

the problem is with the defrost duration, when in ref cycle the hot liquid floods the cooler , so there must be no frost left and when fan cuts in clears it.
corell busy controller.

[Gary]

Also do you know for sure that your defrost is time only terminated, and there is no other thermistor or clicson terminating the defrost prematurely?

You have it backwards. When there is temperature or pressure termination installed it is the primary and time termination is the secondary. IOW, time termination is premature termination on such systems.

[simon@parker]

hi guys very interesting personally would like to see this pumpdown i put probe at highest point of evap or on a return bend furthest away from txv elec defrost would set to 14deg c cut out duration 25min with 4 defrosts per day has it got door flaps or a curtain if not and door open alot go for 6 or 8 defrosts per day and increase defrost duration you gonna either put a probe in evap and run it on cool and defrost to observe wot goin on or stand in room (wear a hat lol) you can check evap probe at same time yr testing seems fairly simple problem but it will take time you need to observe the people using it to quote a funny man i can fix broken but i cant fix stupid its possibly customer havin door open for hr loading each day or not shutting it good luck let me no how it goes hope this helps

[simon@parker]

quick thought theres not one of those crappy overheat cut outs on heater feed ? could drop out heater during defrost ?

[digerious]

if freezer coldroom is tev with receiver like you would expect, then no pump down on defrost is odd. on the other hand if this sys has worked for years without this fault occurring, why now? sounds like controller fault.change controller and probes.

[tonyelian]

hi
did u check fin spacing
goods entering this room are fresh or frozen
high turn over
those are other causes to build frost

[Scottie]

Things to consider, heat exchanger in evap, do you have frost on the outgoing suction line? If so you may have to cut it out! Is there a fan delay after defrost? Does the coil clear entirely during the defrost period and how long before it ices over?

[Magoo]

Something does not stack up with the original post details, for a -18'C room. You have condensing at +27'C and evaporating at -32'C. The whole system is out of balance.
The compressor conditions should be based around +35'C condensing, and - 22'C evaporating at evaporator with a system TD of 4'C.
The lower condensing pressure and sub cooling will totally screw the TX vav, they are rated on PD across valve to give designed duty, so with super low evap temp in coil due to lack of liquid you will freeze up coil real quick.
magoo

[D.D.KORANNE]

Is heater of sufficient capacity rating ?