High RH Cold rooms

[tadipatrir]

Hi,

Can anyone suggest me the methods of increasing the RH in cold rooms to the extent of 90 to 95%

Thanks in advance.

TR

[lana]

Hi there,

You have to select the evaporator for low temperature difference.

TDe = tai - te
tai = room temperature
te = evaporating temperature

Fruits and vegetables need high relative humidity.

When the evaporator is selected for low temperature difference (5 or 6°C) then high relative humidity will be maintained in the room.
Also in this case the physical size of the evaporator will be bigger.

Cheers

[tadipatrir]

Thanks for information.

But with lower TD whether the performance will be satisfactory?
Or it may mean higher fan motor CFm output?

Would you clarify.

Regards,

[lana]

If manufacturer' data are reliable then there will be no problem for the performance.
When the evaporator is selected for lower TD then the size of the evaporator will be bigger (relative to the higher TD). So may be you get more fans on it.

Cheers

[Samarjit Sen]

Hi Lana, I would not agree with you this time. This is feild where I have done a bit of work. None of the Evaporators can maintain the RH of 90 to 95% by its own self. As per the Evaporator manufacturers dats the humidity of 95% can be attained if the air inlet to the coil is at 80 to 85% RH. A number of people in our country has tried but had failed.

I put an additional adiabatic humidifier suitable for the capacity of the load, room and air volume discharge. The results have been very good.

A lot of such projects are coming up in our country and the refrigeration contractors are a bit confused. I have installed a couple of plants with my own designed system and are maintaining a temperature of - 1 Deg. C and a RH of 97% inside the room.

These are for application for Precooling of Grapes and Pomogrenate. Now more fruits are being added

[Samarjit Sen]

Hi Lana, I would not agree with you this time. This is a field where I have done a bit of work. None of the Evaporators can maintain the RH of 90 to 95% by its own self. As per the Evaporator manufacturers dats the humidity of 95% can be attained if the air inlet to the coil is at 80 to 85% RH. A number of people in our country has tried but had failed.

I put an additional adiabatic humidifier suitable for the capacity of the load, room and air volume discharge. The results have been very good.

A lot of such projects are coming up in our country and the refrigeration contractors are a bit confused. I have installed a couple of plants with my own designed system and are maintaining a temperature of - 1 Deg. C and a RH of 97% inside the room.

These are for application for Precooling of Grapes and Pomogrenate. Now more fruits are being added

[lana]

Hi Samarjit Sen,
I am not sure which part you don't agree with me.

As you mentioned, also I have designed and installed a system for Kiwi fruit. There I selected evaporators for TDe = 5°C. I personally measured the relative humidity in the room which was RH=97% without any water on the floor.
I explain "water on the floor". Because of incorrect evaporator selection most of the cold stores for Kiwi , lack for humidity. Therefore, the owner physically put water on the floor of the cold room to compensate for the humidity. This also brings defrost problem with it.
This plant which I mentioned has become a famous one among the cold store owners (for Kiwi fruit).
It is like we did a miracle or something. It is a science and nothing else.

Cheers

[US Iceman]

I going to take a different approach and say I agree with both lana and Samarjit Sen.

I think lana is correct as the lower TD will promote a "relatively" higher RH in the room. The question is how much can you expect? In relative terms, the RH will be higher with a low TD rather than a higher TD. In addition, if the room is closed you will not dilute the inside air.

At 97% RH you are removing very little water from the space as the majority of the cooling is sensible. However, some will still leave the room as liquid down the drain. Over a long period of time, the RH would decrease unless additional water is added.

If the doors were opened and low RH air is allowed to enter, the space RH would change also. Again more water would have to be added to maintain a constant RH in the storage area.

Samarjit Sen's idea is to add post-humidification after the coil, which would allow more moisture to be added after cooling to increase the RH of the air. This may be easier to control and to achieve a stable RH.

One other thing to consider is the use of low TD's also limits how much superheat you have for control of the TXV.

This is a fascinating area of discussion as all too often little attention is given to constant RH conditions.

[Samarjit Sen]

These Cold Storages are in existince in the Central and Western part of our country. Till now system that was being adopted and is still being adopted are installing a floor mounted cooling unit with bare copper tube coils and spraying water over the same. The result was that humidity maintained was not upto the required level nor the temperature.

When I was brought into the picture for one of the projects by the owners, by that time the equipments were already there. I told the contractors to change the coil and suggested the make and model of DX Coil with low Td, high volume of air and high velocity. With great reluctance this was done. The refrigeration controls were changed also. On commissioning the units, they could not believe what they saw. In all the rooms I had installed adiabatic humidifiers which were local made but similar and better in performance. The size of the moisture were 2 to 3 microns. So there were no droplets.

This plant became a show piece . On the basis of this plant I got more orders but the customers insisted that I also execute the same. All the plants installed on the same fashion by us are working fine and the quality og grapes being exported are excellent. Now this units are being shown to the new clients by the Government autorities to the prospective clients as a model and asking them to install similar plants.

There is no water on the floor. All the doors are air tight and once the rooms are loaded the frequency of opening the doors are negligible or nill.

I had a long discussions about the Coils maintaining the high RH with reuted Coil manufacturers from USA and Europe and they agreed with my views.

I wish you could see one of the projects and will appreciate the same.

US Iceman has judged the concept correctly. In my installations I ensure that the doors and rooms are air tight, as the clients in future may go in for CA storage. All the chambers of puf panels imported from out side India. The conditions required for Grapes precooling and storage is -1 Deg. C with a RH of 90 to 97 %. This enables the core temperature to go down to 0 Deg C of the grapes

[lana]

Hi Samarjit Sen,

I wish you could see one of the projects and will appreciate the same.

I know and appreciate.

I completely agree with you and US Iceman. As you know most of fruits like Kiwi, produce ethylene and are sensitive to this gas. In order to reduce the amount of ethylene, the cold room air must be ventilated, i.e some air must be exhausted and some fresh air must come in. Here there is no CA and therefore, exhaust fans are installed for ventilation. On the other hand the area which these cold stores are located is near sea side which has high humidity.
When exhaust fan is switched on, the door of the cold store is opened a fraction to allow air movement.
You can see the picture here. With this system ethylene is discharged from the room and fresh humid air is going in and relative humidity is maintained.
But if the evaporators are selected with high TD then this fresh air (moist) will not be enough to maintain the humidity level and they need to add water physically.

Also I had experience with grapes. Grapes do not produce ethylene and therefore your method is the solution.

I think there is no absolute answer for this because there are lots of different fruits and vegetables with different conditions and behavior. Experience proves the best method for each product.

Cheers

[Lc_shi]

It makes sense if the evaopator work at above dew point,there's no moisture removed and only sensible cooling. But in reality, it's impossible to prevent air leak and water evaporated from fruit. Install a humidifier is a better choice.
Lana,could u give an overall description of your project and share with us

best regards
LC

[Peter_1]

I only can add on this issue that we have some fruit cold stores where the evepoartors were selected with a DT of 5K and a low air volicty and that the RH is 92 to 95 %.

[Samarjit Sen]

Hi Peter 1,

A fruit Cold Storage and a precooling rooms are two different things. In the case of Fruit or for that sake any other post harvest produce after harvesting has to be pre cooled for at least 6 hrs to remove the field heat. The required conditions are different for different produces. But in the precooling room you require high velocity and high volume of air resulting in the surface area of the evaporator being large. After the products are pre cooled they are stored in the Cold Storage to maintain the temperature. Here we do not require high velocity or high air volume. In a Cold Storage there is a lot of door usage, due to which the RH rises.

What you are saying is correct. But to ensure proper RH in Cold Stores, we prefer to provide a separate adiabatic humidifier.

[Samarjit Sen]

Hello Lana,

As far as I know and have read, all fruits generate ethylene. It is how much do they do. One of the reasons of ripening is ethylene. Grapes also do generate ethylene but in small quantity.

I am very much interested in knowing more about the technical details of the Kiwi project that you have done.

Charcoal filters are very good absorbers for ethylene gas. I am planning to use these for the Cold Storages.

I shall be looking forward to your discussions, and also what refrigeration compressors and evaporators you are using.

[Peter_1]

The original question of Tadipatrir was as far as I've read and understood it for a simple cold room, not a precooling room.
I think we also must consider the fact that the cold rooms are located in different parts of the world where the outside conditions vary within very wide margins which influence also the indoor conditions.

[Samarjit Sen]

Hi Peter,

Tadipatrir is from India. There are a large number of Fruit Export projects in existence and are coming up here. I had done a survey before I got involved. All the Cold Storages of this type has to have at least one precooling room and two storage. Generally there are two precooling and four Cold Storages. These Cold Storages are unable to maintain the desired conditions such as the RH and the Temperature. I am sure that a lot of Forum members who are in this field are keenly studying this thread.

I wish Tadipatrir should come into this discussions with his problems and comments.

What I have been stating is from experience as I am very much involved in this area.

[lana]

Hi everybody,

First about the grapes and ethylene. I have a book called " Guide to food transport- Fruit and vegetables " Mercatila Publishers Copenhagen. This book is a good reference for fruits. In this book it is stated that grapes has no ethylene production and also it is not sensitive to that gas.
Also in order to extent shelf life, sulphur dioxide is injected in the cold stores (this is illegal in some countries).
Cold stores for grapes which I have worked do not have any additional humidifiers or any water on the floor. The humidity is maintained by proper evaporator selection. But I know a lot of cold stores for grapes which require humidifiers (because of incorrect evaporators). The problem they have is, this water makes the grape go black when they come out of the cold rooms.
These cold rooms have an exhaust fan just to discharge additional sulfur dioxide and the fan is switched on once a week.

I am not saying this is the correct method but this is how it is done here.

About Kiwi fruit : This fruit is grown in areas with high humidity (near Caspian Sea). Kiwi is harvested when unripe and then stored in the cold room (0°C - 95%RH). Kiwi produces a lot of ethylene gas and it is very sensitive to it. Therefore this gas must be removed from the cold room. As I explained before, there are fans installed which discharge this gas and fresh air goes into the room.
One of my client imported a machine which absorbers ethylene chemically but I have no info about it. They used it recently and I have no feed back about the efficiency of this machine.

Peter mentioned a good point about air velocity. If air velocity over the product is too high then this can cause dehydration of the product even there is a high RH in the room.

About the evaporators and equipment I have to say that I use the evaporators and air cooled condensers which are locally made (my own company), and compressors I have used Copeland and Bitzer semi hermetic, and they both work fine.

Hope this helps.

Cheers

[Samarjit Sen]

Hi Lana,

About the ethylene generation, I was wrong as I have found out from the books and literatures that grapes do not generate ethylene nor are they sensitive to ethylene. Thank you for correcting me and giving me a valuable information.

Sulphur Dioxide is not injected into the room. At the time when the grapes are sorted, graded and packed, a pad containing Sulphur Dioxide is placed within the individual cartons. This is essential for extending the shelf life while in transit.

Initially at the time of precooling a large volume of air is required. This does not dehydrate the grapes, as additional humidifiers are in the storage for maintaining the humidity. We do not put water on the floor . The stacking of the pallets should be such so that they are at least 750 mm below the Evaporators bottom level. If you refer to any article on pre cooling, you will find that air plays an important role both in pre cooling as well as storage. While in preecooling it should be on the higher side and in the Cold Stores it should be reduced just to maintain the temperature.

The Evaporators that I am using is either Guntner or Heatcraft Larkin. The refrigeration Compressors are Gelfred semi hermetic with Swep PHE as Condenser.
Earlier I had used air cooled condenser, but the results were not good. With water cooled condensers the consumed power is low and the performance is better.

For preecooling there are different methods and systems.

I design my system on DX system with forced air cooling the room. I have seen many a Fruit preecooling systems in our country and finally I arrived at this system, which I do not know how has been very successful.

There are refrigeration engineers here who had criticized the system adopted by us, but then I am able to maintain the desired humidity and temperature. Further the grapes exported this year from the units where we had installed the plant has been 100% accepted

[US Iceman]

I don't mean to discourage a really good discussion...

But there is a lot of good information being added here that I think could be categorized.

1. Precooling
2. Cold storage (short term and long term)
3. location of facility
4. required operating conditions
5. and many more

In general, I think it is safe to say:

• A low TD for unit cooler selections is used to promote a higher relative humidity.
• High velocity air is useful for precooling, but not for long term storage as this tends to dry the product being stored (unless the product is stored in hermetically sealed packaging).
• Ambient conditions and operation of the facility can have a huge impact on the operation of the system.

I believe the one thing we have to be careful of is using the rules-of thumb like; "A low TD for unit cooler selections is used to promote a higher relative humidity" are general statements.

These seem to work in a majority of applications and conditions, but there is always a possibility of some unknown factor that could prevent the desired results.

Refrigeration is still a science tempered by prior experience and some luck.

[lana]

Hi Samarjit Sen,

Sulphur Dioxide is not injected into the room. At the time when the grapes are sorted, graded and packed, a pad containing Sulphur Dioxide is placed within the individual cartons. This is essential for extending the shelf life while in transit.

Unfortunately they do inject sulphur dioxide into the room here and this destroys the evaporator fins after a few years. I suggested that they turn off the fans when injecting to minimize the destruction .

The stacking of the pallets should be such so that they are at least 750 mm below the Evaporators bottom level. If you refer to any article on pre cooling, you will find that air plays an important role both in pre cooling as well as storage. While in preecooling it should be on the higher side and in the Cold Stores it should be reduced just to maintain the temperature.

I could not agree more.
BUT , here they want to use the space "efficiently" and they pack the room with the product. Maybe you don't believe me but most of the time there is no room for air movement. Also they stack the product high up to the evaporator which blows the air directly on to the product.
If everything is done according to good practice and standard then life would be much easier for us.

I design my system on DC system with forced air cooling the room. I have seen many a Fruit preecooling systems in our country and finally I arrived at this system, which I do not know how has been very successful.

There are refrigeration engineers here who had criticized the system adopted by us, but then I am able to maintain the desired humidity and temperature. Further the grapes exported this year from the units where we had installed the plant has been 100% accepted

If your design works then why change it?
Continue the good work.
Cheers

[lana]

Hi US ICeman,

You never discourage anybody, I think you summarized it beautifully.

Cheers

[Samarjit Sen]

Dear US Iceman,

You have very correctly stated the basic facts of Fruit Pre Cooling and Storage. Your last sentence is very appropriate.

There are some members who can further educate us have not put any posts in this thread. I presume they must be pre occupied.

[Samarjit Sen]

Lana, I very much appreciate your views. Before I get involved with the projects, I tell my clients that they shall have to operate as per my instructions. In a couple places they did what you stated ie packed the room. That spoiled the products.

Another thing I just noticed that I have stated DC system, please read it as DX system.

I am not changing my system nor can I. The authorities concerned with the export of grapes have appreciated our system .

Lana you had stated in your earlier post about the book Guide to Food Transport - Fruit & Vegetable. I had tried the Google to find its avaibility but could not. In case you are aware could you please let me know as to where to get it.

[lana]

Lana you had stated in your earlier post about the book Guide to Food Transport - Fruit & Vegetable. I had tried the Google to find its avaibility but could not. In case you are aware could you please let me know as to where to get it.

I bought it in 1995 from Denmark.
Try http://www.mercantila.dk/GTFT/

Cheers

[US Iceman]

Thank you for posting the link to the books lana. It appears I now have to add more content to my library.

[Peter_1]

We've done some CA rooms and they (Van Amerongen) told me that they 'wash-out the ethylene via a scrubber.
http://www.van-amerongen.nl/default.htm

[Peter_1]

After visiting the site, I think they wash out the CO2 and the O2.
The cold rooms we did were for apples and pears.

[lana]

Thank you for posting the link to the books lana. It appears I now have to add more content to my library.

They are really good reference books to have.

Regards LANA

[Samarjit Sen]

Thank you Lana for the Link. I have ordered for all the three books.

[tadipatrir]

Dear Lana, Samarjit, peter & US Iceman,

I was away on personal work for last 5 days and hence couldn't reply.

Thank you very much for one and all for your valuable inputs and suggestions.

My requirement is 1deg C temp. and 95 % Rh for potatoe storage.

Anything more to be considered..?

[Samarjit Sen]

Hi Tadipatrir,

For storage of potatoes you require a temperature of 4 to 6 Deg. C with a RH of 95 to 98%. The storage period is for 56 to 140 days

[sharshal]

Hi all, I am a little new to this discussion, but why are we not looking at the 'generic' kind of pre-cooling units which actually use chilled water (brine, rather) to cool the air, at the same time increase the humidity ? These units will have the chilled brine generated using DX coils and air coming in direct contact with the chilled brine by means of a counter flow direct heat exchange arrangement. Such units are installed in many numbers and working perfectly for 95% + applications, primarily grapes ..

Of course, if the ethylene generation concept is coming into picture and 'fresh' air is to be added, then its a different ball game altogether !

Though the topic was more-or-less closed by lana, I still felt some more discussion can do ..

[tadipatrir]

Hi Samarjit,

Can you give me the sources of Adiabatic humidifiers in India and abroad?

Thanks,
Tadipatrir

[Samarjit Sen]

Hello Sharshal,

The type of cooling you have stated is very much common in India for precooling and storage of grapes. There are certain facts you have to consider. I have visited a number of units and had discussions with the various authorities and the owners also. The general complain is that these units are unable to maintain the required RH and the Temperature. Further the moisture particles are more than 20 microns resulting in their deposit over the cartons and the floor.

I had designed the system using Guntner coils in one project and Larkin coils in another project. Additional adiabatic humidifiers were installed. Fortunately these units are maintaining a perfect conditions and the clients are also very happy. Further the quality of grapes exported from these units have maintained a very high quality even after shipping in containers.

Further as the Evaporators are ceiling hung, a lot os space are available in the room.

[Samarjit Sen]

The Adiabatic Humidifiers are available from Carell, Italy, Goughi, Italy and from Amoto Engineers in Mumbai. We are using Amoto adiabatic humidifiers as the moisture molecule size is much lower than the imported ones. Further they are lower in price. you may contact them at amoto@vsnl.com and their web site is www.amotoindia.com .

[tadipatrir]

Dear Samarjit,

I am very thankful to you for your information. This forum indeed been faster than Google in terms of getting appropriate information. I am proud to be a member of it.

Best regards,

Tadipatrir

[smpsmp45]

For Straberrries we had used Pizoelectric based water atomisers to increase the humidity & those had worked well.

[Samarjit Sen]

Hello Smpsmp45,

Could you please provide more information on the Piezoelectric based water atomiser and also the name of the manufacturers of the same from whom it could be made available. I am interested in incorporating this for the humidification if this does not increase the heat load and the moisture particles are less than 2 microns.