PDA

View Full Version : Frost Formation in Cold Rooms







Samarjit Sen
31-07-2007, 02:39 PM
We have installed a Cold Room with an inside temperature of -35 o C and another adjoining it is -40 o C. The door of - 40oC opens into - 35oC and the door of -35 o C opens into an Ante Room which is connected to a Room at 4 o C. We find that the -35 oC accumulates a lot of frost, whereas - 40 o C has less frost. The walls are air tight. What could be the reason and is there any remedy for the same so that we have less frost in -35 o C Room.

US Iceman
31-07-2007, 02:56 PM
Hi Samarjit,

The moisture in the air is being removed sequentially by each room. In effect, you are using the -35C room to dehumidify your -40C room.

In the Ante room; what is the temperature there? And, how large is this room and are there evaporators installed in the Ante room?

In the +4C room, what is the evaporating temperature of the coils used in this room? These are your primary sources of dehumidifying the air before the air flows into the Ante room and subsequently the freezers.

Do you notice a lot of frost also around the doors going into the -35C freezer and on product, or the racks in this area?

Samarjit Sen
31-07-2007, 04:37 PM
The Ante Room between the +4oC and the -35oC is about 6ft. x 6 ft. x9 ft. ht. The Ante Room Coil Te is at 0oC.

There are no ice formation on the doors. Yes on the racks inside the - 35oC there is frost formation.

old gas bottle
31-07-2007, 05:49 PM
sounds like the moisture travels from the 4 deg c room intio the -35,gets caught and then with there not being much temperature differance into the next one ,that ones clear,just as iceman says,one thing though, have there been any pressure relief valves fitted and in what location !

samiam
31-07-2007, 06:24 PM
Hi,

For concerns/problems - I now incorporate a dehumifier for most of the -30C Freezer designs. Especially for vey large coldrooms.

Samarjit Sen
31-07-2007, 06:43 PM
Hi Old Gas Bottle. After the 4oC room there is an ante chamber after which is the -35oC Room. No there are no pressure relief valve.

Samarjit Sen
31-07-2007, 06:45 PM
Hello Samiam,

Could you please ellaborate a bit more about providing dehumidifiers in the -30oC Room. What type of dehumidifiers are you putting. Will it not have an effect on the Cooling Load ?

US Iceman
31-07-2007, 06:58 PM
Consider this problem as similar to refrigerant migrating to the coldest location in the system. Water wants to do this also.

The Ante room is a good idea as it is an attempt to isolate the moisture from the warmer room. The problem is this room does not provide sufficient dehumidifying for the air before it reaches the -35C room. Does the Ante room only have one evaporator, or does it have any at all?

Some moisture is inevitable, but the frost on the racks is due to the moist air freezing out as it enters the space. I normally try to have the evaporator fans blowing towards the doors as this provides a "barrier" of sorts. This directed air flow helps to absorb the entering moisture before it freezes on products, door frames, or racks.

Desiccant system work fine, but you also have to re-cool the discharge air off of the unit. While this is only a sensible load you should compare this freezing with the capacity to freeze the same amount of water removed by the desiccant system.

samiam
31-07-2007, 07:24 PM
Similar to what has been stated above and a warehouse layout that I have currently worked on - a freezer of -30C, would have different temperature such a s Receing/Despatch (at -15C) and Ante Chamber (0C).

The air in the rooms are processed and the mositure removed - by use of a dessicant dehumifier and also resulting in fewer defrost.

Basically air is taken form the required area, passed through a dehimifier (with rotating Selicant Roter), mosture is removed form the air, i.e. processed air and then the dry air is returned to the room. (this porcess continues). That it in a nutshell.

The dehumidifier chosen depends on the locations climatic conditiions, layout, room temperature and the usage of doors, etc....

I suggets you have a look at the :
DST-Sebu Giken website
Bry Air website
(or possibly Munter website)
And I'm sure theres more

Samarjit Sen
01-08-2007, 01:31 AM
Mike, the Ante Room does not have an Evaporator. Further the evaporators located inside the room do not face the doors.

US Iceman
01-08-2007, 02:15 AM
If the air flow is directed towards the door opening from above, this helps to create a barrier across the door. This helps to absorb the moisture flowing in and prevent it the frost from collecting on cold surfaces such as racks and product.

Air is going to flow out anyway, since it is heavier than the warm air flowing in at the top of the door opening.

If the doors to the Ante room and freezer are open at the same time, there is nothing preventing the moisture from entering the -25C freezer.

Samarjit Sen
01-08-2007, 04:46 AM
If the evaporators are facing the door, don't you think that the cold air will flow out and there would be a loss of temperature. But then your suggestion is very much valid. I feel if an additional coil is provided facing the door, things would be better.

old gas bottle
01-08-2007, 08:23 AM
while you chaps are sorting out the technical details,let me go back to the problems of not having pressure relief valves on freezers of which you state there are none on this installation.

after a defrost or after the door as been open /shut there will be a massive pressure differance causing a rush in of air through somewhere and not allways the door,and it will get in somehow,this results in frost accumilation in certain parts of the room not allways on the evaporator,by fitting a prv in the cieling above the door frame or ajacient to the door this allows a ballance of the air pressure and in most cases stops this.

after all why is it industry standard to fit them.

Samarjit Sen
01-08-2007, 11:07 AM
Thank you Old Gas Bottle. I will arrange to fit one prv immediately.

US Iceman
01-08-2007, 03:33 PM
If the evaporators are facing the door, don't you think that the cold Air (http://www.refrigeration-engineer.com/forums/glossary.php?do=viewglossary&term=17) will flow out and there would be a loss of temperature.


Cold air will flow out anyway. When an unprotected door is open cold air flows out the lower 50% of the door. During this same time warm air flows into the upper 50% of the door opening.

The percentages are approximate to illustrate the situation, and an unprotected door is defined as a door that has no barrier to prevent free flowing air.

The air is not directly discharge into the door opening. The coil is mounted on the ceiling and blows towards the WALL where the door is. The majority of the air hits the wall and spreads out and down, flowing across the door opening, and then sweeps back into the room.

I've seen evaporators installed directly over the entrance doors thinking this will help. It actually makes it worse, because the warm moist air directly impinges on the evaporator and causes frost problems severely.

old gas bottle has an excellent point he is making on the relief ports. If the pressure can't equalize through the port, it then tries to do this when the doors are open. This is just like a home refrigerator. If you open the door and then close it, and then try to re-open it you may notice the door is hard to pull back open.

Samarjit Sen
01-08-2007, 05:46 PM
I fully understand the implications. Today I was at the site where I found that there were leakages at the joints of the panels. I have asked my people to fill it up by injecting puf. There is one more thing that I thought may help. If a pvc strip curtains are put on the door, will it help in any way ? As per the advice of Old Gas Bottle, I shall arrange to fix a prf above the door.

US Iceman
01-08-2007, 06:36 PM
In my opinion th estrip curtains do a good job IF they are maintained and replaced when they get torn. They are simply a barrier to prevent the air flow in or out of the freezer.

The problem is people do not like them as the curtains are in the way of transporting product, or getting slapped in the face with one of the strips.:D

They reduce some of the air infiltration and are a cheap alternative to other methods.

Samarjit Sen
01-08-2007, 06:59 PM
We are expecting another similar contract. The basic problem was created by the contractor whom we awarded the puf panelling. He made a total mess and we had to rectify/modify/improve etc... and at least bring it to this stage. Our clients are aware of this. In our next similar project we shall be importing the panels, and am sure that things would be better.

US Iceman
01-08-2007, 08:22 PM
Air infiltration problems are hard to solve if the panels leak!:o

One of the best ideas is to try to reduce the infiltration wherever possible by static means. The strip curtains are one idea. Another may be the use of good air curtains. I posted a thread on this several weeks ago.

Dock doors are the enemy along with door operation in general. Some people have had success with fast operating doors which are used during daytime production and shipping. Then the permanent doors are closed during off hours.

Using a desiccant system works, but there is a cost for operating this also.

I recently had the opportunity to review this same problem on a large freezer that used rooftop mounted penthouse units. The air flow was just directed in no particular fashion and ice is all over the inside door frames, above the doors on the walls, and on the product and racks.

That's why I suggested the use of directional air flow towards the door.

In the ASHARE Refrigeration handbook I believe there is a discussion on air infiltration through doors you may want to review.

old gas bottle
01-08-2007, 08:30 PM
yup,thoses curtains can be a pain,what we do is use the kits where you get two red strips for the sides so they dont bump into the door frame and then use a light weight strip and just butt them up to each other instead of using the overlap way, makes it easier to fight your way through,dont seal the room untill the PRV is fitted as it will break through somwhere else,not uncommon to see the doorframes breaking away from the panel.:eek:

Samarjit Sen
04-08-2007, 04:40 PM
I have provided a PRV in the room for -35oC above the door and have also provided PVC strip Curtains. The result has been good. The ice formation has considerably reduced.

I agree to Mikes suggestion that the Air Cooler should have been directed towards the door as it will prevent infiltration. In my future project for such low temperature application I am going to do installation as per Mikes and Old Gas Bottles suggestions.

We have executed so many low temperature Cold Rooms, but never had such frosting problems. Yes there are frostings, but not to the extend that we had a few days ago prior to the modifications.

US Iceman
04-08-2007, 05:19 PM
The result.... Another satisified customer!;)