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Taaz1275
26-08-2008, 08:22 AM
hi thea,

i've been facing this problem with this commercial refrigeration unit that they call rack system...ok, it consists of 2 compressors in parallel to a multiple evaporator coffin freezer, marked as section A,B,C & D..the unit runs off a R404A ***** air cooled condenser , the large coffin freezer is 1 piece divided into 2 along the center lengthwise, each section consists of about 6 to 1o separate evaporator coils branching off a main liquid line with 1 each liquid solenoid valve for each section.Each evaporator has its own internally equalised TX valve..each evap coil has 1 evaporator fan..a defrost heater that is fixed only to the face of the evaporator coil & during defrost the evaporator remains running to circulate the warm air off the heaters to the in part of the coil that is iced up...defrost is set so that they donot go on defrost at any 1 time but at different times.On the evaporator coil is a klixion that terminates defrost if too warm....safety.
Now the problem is this ...icing up of the evaporators every now & then ...then normally a manual defrost will be done ..sparying of water over the iced up coil .Oh ..and lastly the sections are all set to a temp of -26deg C with each thermostat on its own.
My question is ..what could be the main reason that this is happening and what can be done to better the working condition of this coffin freezer without having this constant icing up of the evaporator coil?
Pliz advice....

Taaz1275
26-08-2008, 08:33 AM
IN regards to my own problem ..this was wat I was thinkin...how about if I put in a check valve into the outlet of each section that links up to the main suction, since it is programmed that no section is to go on defrost with any other section at any 1 time...that was what I had it mind to try solve this on going problem ..or otherwise put in the check valve on the return of every suction and re programme so that all section intiate and terminate defrost at the same time...and extending the drip time after defrost....just a suggestion which I need approval or correction.

iansuffield
01-09-2008, 12:41 PM
Taaz,
there could be a plethora of solutions to your problems...
I guess to start with would be to separate the airflow between the modules of case somehow (using some cardboard to test and then if successful some perspex) so that when one section goes onto defrost and the air heats up and gets moist, you are not pulling warm and moist air into the adjoining (and colder) case...
I would also look at stopping the evap fans during the defrost period and holding them out once the refrigeration restarts for a period - perhaps a second coil klixon for the fan delay or an addional LP switch to hold the fans out until the coil is nice and cold...
Next to investigate the retention of refrigerant in the coil during the defrost cycle using a suction line solenoid... This would assist in an even defrost throughout the coil as you would be using the refrigerant to distribute the heat and melt the ice. A pressure relief device around the suction solenoid would not hurt here...

I guess from there would be to also try changing defrost time settings. In Aus, open cases such as yours defrost with varying settings... From 1 x 45min/24hr to 3 x 40min + 2 min drain/24hr - depending on manufacturer. Do you have the manufacturer and model for the case??

Hope this helps.

Gary
01-09-2008, 03:54 PM
What is the termination temperature of the Klixon?

Where is the Klixon located?

Is the defrost currently being terminated by the Klixon or by the timer?

You are thinking of the Klixon as a safety, when it is very likely set up as the normal means of termination, with time termination being the safety in case the Klixon fails.

Taaz1275
03-09-2008, 09:22 AM
Hi Gary,

In regards to our help and advice...the klixon: Im not so sure as to what the factory temperature is set to...but to my source of information ...says that it opens up cotact @ +13degC..imnot so sure of how accurate is that info...any idea as to what temperature should them klixon open up?....
As for the location on the evaporator coil....we have bin relocating the positions from the original point which is the outlet of the TX Valve b4 going into the evaporator coil....we have reposition the klixon to the farthest end of the coil where it is farthest to the defrost heater elements as well...hopingthat it does not open up contacts on temperature till all the ice on the coil defrost to the point where the klixon wil sense and open up on temperature terminating defrost as an indication that all ice have been defrosted off the coil within the defrost time interval,....any particular locations on the evaporator coil should this klixons be positioned at that you know please do advice...
As for your 3rd question ....@ times the klixon plays up and terminates the defrost heaters ..and at times I have seen that it terminates defrost without the full coil being defrosted ...is the location of the klixon wrong in that case..coz at times where the klixon is...there is ice all over but it still terminates defrost....
I was thinking a silly idea of bypassing the klixon and extending the defrost time ,but worried if the timer fails to terminate defrost ....is it safe to go without the klixon, provided the timer is set to a certain length oftime that it is able to defrost all the ice off the coil?....

another question ...??? what is the normal life span of a new klixon once you fit in a brand new one ....?

I hope I have given more details to help you advice me more ofthe need to solve or explain the reasons for this on going problem.

Taaz1275
03-09-2008, 09:33 AM
Iansuffield,

Thanks very much for that idea of yours ....I will try that out, just referring back to one of your suggestion...qoute "Next to investigate the retention of refrigerant in the coil during the defrost cycle using a suction line solenoid... This would assist in an even defrost throughout the coil as you would be using the refrigerant to distribute the heat and melt the ice. A pressure relief device around the suction solenoid would not hurt here..."...a pressure relieve device you say,....please could you explain more on this ...to my understanding...the pressure relieve valve is a check valve?..relieving into the suction line back to the compressor ..or do U mean a special kinda valve that will maintain a certain pressure in the evaporator coil while it is on defrost ...the likes of an EPR...please could you explain more on that particular part.

Taaz

Gary
03-09-2008, 10:37 PM
I would move the Klixon back to where the manufacturer intended it to be and then lengthen the defrost time so that the Klixon terminates the defrost. If the airflow portion of the coil is not fully defrosted at termination, then move the Klixon to a part of the coil that is not fully defrosted, so that the coil will be fully defrosted when the Klixon terminates the defrost cycle.

Taaz1275
04-09-2008, 10:21 AM
Thank you Gary for the advice and tips on the klixon...your techinical information is reasonably sensible and I wil try it out ....