View Full Version : Freezer Room Defrost

Samarjit Sen
06-06-2007, 02:48 PM
We have installed three sets of refrigeration plant using semi hermetic compressor. There are three rooms. One is for - 40 Deg. C, the other for - 35 Deg. C and the third is for 2 Deg. C. In fact this is a Blood Bank.

The problem is that for the - 40 Deg. C room which is very small. At -35 Deg. C there is considerable amount of frosting on the coil. These have electric defrost system. It takes about 15 mnts. to complete clear the Coil, but by then the Room Temperature rises to + 2 Deg. C. We have tried to have the defrost timing set at 1.5 hrs and shorten the defrost time to 10 mnts. With this the temperature rises to 0 Deg. C.

The compressor capacity is .8 kw at -45 Deg. C Te and 52 Deg. C Tc. The Coil has been selected accordingly.

I have tried to put the electric defrost heaters in series, but then the coil does not clear off completely.

The size of the room is very small and is meant to store Blood Plasma. Could it be possible that once there is a product load, the rise in temperature during the defrost cycle will not be so high. Another thing is that if there is a slight rise with the product load inside, will the temperature of the product too will rise.

Advice and remedy is earnestly requested.

US Iceman
06-06-2007, 03:36 PM
It sounds like your rooms are empty. An empty room will of course change temperatures very quickly as there is no mass in the room. Any mass added will help to stabilize the room temperature during defrost, or when the door opens briefly.

The stabilization of temperature is better when the room is full. This is one of the old problems of cold storage temperature control.

Are we controlling air temperature or product temperature? If you have to meet a performance guarantee you want to do it with the room full of product.

Another thing that may help you is to use the plastic strip curtains on the door entrance. These help to reduce the air infiltration into the storage space.

If you reduce the infiltration air, you also reduce the total moisture content entering the space.

Renato RR
06-06-2007, 04:01 PM
Do you have Carel or something like that for temp. control?


Samarjit Sen
06-06-2007, 04:53 PM
The entrance to the Room of - 40 Deg. C is through the Room at -35 Deg. C. Yes we need to put PVC curtains.

Yes you are also right that this is an empty room. Since Blood Plasma are going to be stored in this room, I want to be sure before I proceed. That is why I approached the forum for guidance.

Yes for the temperature controller we are using of Sub Zero, Italy make.

Please give me some more suggestions.

old gas bottle
06-06-2007, 08:37 PM
i dont like the way the coil is icing up so quickly,i,am going to suggest checking the basics, is there a pressure relief valve fitted to the room, inspect the joints on the construction for gaps and lack of sealant, check the door seal and heater,has the floor heater been installed under the insulation and is there indeed a insulated floor ,min 100mm,if all thats o/k it may have too long on defrost misting off into the room, add that on to whats all ready been sugested and let us know.

06-06-2007, 10:39 PM
Is it possible to install some temporary load in the room such as bottle/cans of water or similar to enable a proper load test?

US Iceman
07-06-2007, 01:29 AM
That's a good point Brian. I have heard of some using electric heaters and regulating them to produce a constant load by measuring current and voltage.

Samarjit Sen
07-06-2007, 03:04 AM
Yes initially there were lot of leakages on the walls, floor and ceiling. The party who constructed the puf insulated panels made a terrible job. We had to get all the leakages sealed. The walls, floor and ceiling are 150 mm thick puf panels. No there are no heaters on the floor.

Yes artificial loads can be created by putting some glycol pads inside the room, or some sealed water bottles. I was thinking of doing this.

We have

old gas bottle
07-06-2007, 07:56 AM
thought so, thing is running around with a silocon gun wont be the answer, i still ask is there a PRV valve fitted, this can make a differance especiely after a defrost with the air pressure differance, in a small room it will suck air in through the door like the clappers and can blow them open asswell.

07-06-2007, 09:29 AM
Hi there,

I have been involved in design and commissioning of 12 cold stores for plasma storage. We did not have any problem with defrost.(inside temp -38C)
As mentioned by the other experts, the empty room does not hold the temperature and it is very obvious.
My suggestion is this : if the refrigeration system has no problem then don't worry and put the plasma bags inside the room.

Good luck

Samarjit Sen
07-06-2007, 11:23 AM
Hi Lana,

Since you have done similar projects, you think that once the room is loaded, the rise in temperature during the defrost will not be so high. As earlier suggested by US Iceman, I have asked my people to put 10 bottles of 20 ltrs each water bottle inside the room and then to operate the same. But again at this temperature the water will become ice and the bottles will burst. I guess I shall have brine put in the bottle.

Samarjit Sen
07-06-2007, 11:26 AM
Hello Old Gas Bottle,

No we have not put any PRV in the room. Usually the room temperature at the time defrost does not go up so high. Anyway I shall have a PRV put as per your suggestion which I feel should have been done by me earlier.

07-06-2007, 01:16 PM
Hi Samarjit Sen,

We also had a thermo grapher which registered the temperatures all day (which included the defrost time). These thermo graphs showed that the room temperature increased during defrost but only up to -20C. Of course this temperature rise depends on defrost time.
Another point is the thermometer prob is located at the back of the evaporators (air inlet) and because it is near the defrost area then the recorded temperature is higher but the rest of the room will be more than safe.


07-06-2007, 03:01 PM
Does your temperature controller comes with a defrost Stop temperature. Defrost stop temperature (defined by the evaporator probe)? Is this function being use?
Fan delay time function. Delay time in activating fans after a defrost operation.

Samarjit Sen
07-06-2007, 05:56 PM
Since the room is very small in comparison to other rooms and the size of the coil as per selection is pretty large, half the ceiling surface is taken up by the coil. The probe therefore is near to the defrost coil.

The defrost is being controlled by a separate timer. Yes there is a delay after the defrost is over.

old gas bottle
08-06-2007, 08:03 AM
sounds like your ready to take the plunge !, on jobs like this its all about where you monitor your readings, you need your room and evap probes in the right place,but also the product needs monitoring seperatly,i would make up a dummy load, ,similar to the real thing with a data logger "inside" the packing,so it only reads the product temperature and not the changing air temp,record this and hand it over when your happy,you will struggle to get a stable reading of a empty box with such a large evap.

08-06-2007, 11:18 AM
Our experince with Elec Defrost is not good compared to Hot gas defrost.

The coil bends is the area which does not have heaters ( typically) & that area never gets defrosted. But with Hot gas, the defrost cycle is good enough.
Also check out on the drain from the Evaporator,. Have you provided the U loop with water column?

That issue is always missed out & means lots of frosting all the time

Samarjit Sen
08-06-2007, 11:30 AM
What I plan to do now is put dummy loads inside the room and operate the plant. As Old Gas Bottle says, I shall monitor the temperature of the load.

With the electric heaters we never had problems. Rather with hot gas defrost system , the defrosting time was longer and the compressor never got any rest.

US Iceman
08-06-2007, 04:38 PM
Have you provided the U loop with water column?

This is a good point to make. If the drain lines from the evaporator are not trapped outside the cold room, the moist air will travel back through the drain line and allow frost to collect on the evaporatro coil.

The difference in vapor pressure from -40C to outdoor ambient is very high.

09-06-2007, 04:32 PM
Our experience with Hot gas is very very good. Our system has a specifically designed Orifice to control the gas qty going to the evaporator & the system works in the Hot temp zone only The liquid does not come back to the compressor. The system has a thiben timer whch limits the defrost cycle to 30 min max. Otherwise, it stops on suction temp reaching 20 deg c. ( for -25 deg c units). We have over 300 units all over India mostly on sea coast areas with hot gas defrost.

09-06-2007, 09:12 PM
please help to make an industrial ice machine
1 ton ,where i can fine the information about industrial ice machine ,how we can make the evaporator for ice machine,

please help

Samarjit Sen
10-06-2007, 10:46 AM
Hi smpsmp45,

I am interested to know the system which you are adopting for the Hot Gas Defrost. Could you please be more specific so that we all can learn from it. This system will definately help us in defrosting the coil for our low temperature projects.

Samarjit Sen
10-06-2007, 10:48 AM
Hello Rajeevcoldtech,

Could you please let me know as to what type of Ice you propose to make. Is it Block Ice, Tube Ice, Flake Ice or any other form of Ice.

21-06-2007, 06:16 AM
I am thinking that you may have better success at stable room temp, if you use 2 smaller evaporators, with different defrost times, instead of one big one.

Also am thinking that you might want to use different defrost system...not sure what it is called, have only seen it one time about 20 years ago, on ice cream storage wharehouse. They reversed the duty of the evaporator during defrost. The defrosting evaporator became the condensor for the duration of the defrost time. This method cleared the coil of all ice, very fast.