Humidification for Agricultural Refrigeration

For pre cooling of fruits etc, precooling are required immediately after post harvest. During the process of pre cooling other than the temperature, humidificationn is essential, otherwise the fruits shall get dehydrated. To attain the RH which is generally about 95% depending on the fruit, seperate fogging equipment or adiabatic humidifiers are used. Recently i came to know that the treated water from the softening plant contains certain chemicals which are harmful to the fruits and as such alternate methods has to be used. I had referred to the RE forum on this subject and the coil manufacturers. The opinion was reduce the Td. In this connection I got some papers one of which i am attaching. The others I am unable to attach due to its size, but I will upload them and put the link on this thread within a couple of days.

If something more could be added to this thread, as it will change the concept of humidification in agricultaural refrigeration, it would be of great advantage.

Samarjit, the attachment describes what I was trying to explain back in the other thread. Lower TD's help to promote higher room RH.

My guess is the table for the four classes of TD selections are based on DX evaporators. I also suspect using these same TD's on flooded or liquid overfeed coils will produce slightly higher room RH. I can't prove this, but it's just a suspicion.

Secondly, I have been thinking about this in my "spare time" (mainly while I'm trying to sleep!). It was common on older style freezer coils to use water defrost.

Why not use the same principle on the evaporators? Any water that is pulled from the air during the cooling process would be replaced by the water spray over the coils. In essence the coils become air washers!

The spray nozzles or water basin on top of the coils adds a small increase in the height of the coils. If the coil face velocity is low enough the water will not be pulled off of the coil.

The you can filter and treat the water in a location outside of the cold room.

Just an idea for you to consider...;)

Hi Mike,

In the earlier days for pre cooling, air handlers were being used. These were just bare copper tubes in banks inside a chamber with brine solutions being spryed from the top. There was a fan, which used to suck the air from the room and discahrge th same from the top. This used to increase the RH but there would be higher sizes of water particles, which would damage the fruit.

Now this concept of the water containing some chemicals which may harm the fruits, makes one to think of a process to get the water from air which would be in a pure form. But then this too has certain disadvantage. Due to too low Td and a temperature of room at -1oC, the possibility of a RH swing is there. Now how to counteract the same. What comes to my mind is use an adiabatic humidifier again for such short periods. But then the story becomes " there is a hole in the bucket"

Earlier you had statd something about the superheat being lowered. Could you please explain that fact.

The type of brine cooler you are describing is what a company called Niagara Blower makes. To my knowledge these were used mainly to ensure the coil did not frost up. The brine absorbed the moisture from the air stream and then the brine was re-concentrated to get rid of the water.

Somewhere (in Spain I believe) they use something very similar to this commonly, except they use only water (not brine). It's like a hydrocooler except they circulate the air, instead of the water.

The earlier comments I made were meant to describe a possible alternative. That is, by reducing the TD used for selecting the evaporator capacity you are reducing the vapor pressure difference between the air and the coil temperature. This is what that chart with the four cases was trying to show. Lower TD's promote higher room RH.

Once the TD starts to get below 8°F the TXV has a difficult time trying to control the superheat at a constant value without hunting. An electronic expansion valve could probably do this with no problem.

OR, as I was suggesting...

A flooded or liquid overfeed system can do this with no problems either. The problem becomes these systems can be expensive if compared to a single room DX system. For the flooded or liquid overfeed system to be cost effective you need to have a lot of rooms or a big facility.

For single room or two, DX is much cheaper.

Hi Samajit

I have some ideas may help u so i want to discuss them

1- If we can mange to make the evaporator discharge air temperature approaches the value of the space temperature with keeping in mind not short cycling for the unit (more than 3 minutes for running) we can success to attain the space Rh reaches 90 or 95% by following:

induce fresh air from outside in duct and mix it with the returning air from the space in plenum before entering the evaporator, the result is:

0.75 of space air at -1 and 0.25 of fresh air at 30C the inlet air temperature to the evaporator is 6.75 C therfore we can drop it to -3.25C (TD = 10) or less according to a proper time cycling for the thermostat. The air is less dehumidified and on the same time the fresh air can assist the space humidity by introducing excess moisture.

The problem of getting this value of -2.25c from space temperature -1C by TD of 1.25 K is very hard plus the frontal area of the evaporator is huge.

Secondly :

after defrosting cycling the drain of melted frost is collected a way off the evaporator and the heater immersed in it is allowed to work longer to boil off the drained water and converts it to steam to increase the amount of humidity. the reason of keeping it a way off the evaporator is to prevent this steam to condense on the blowers blades and frost during unit operation and makes unbalance operation of the blowers. In addition to relieve the evaporator from heating up during defrost.

Or using Multiple circuit evaporator

And this idea i wanted to tell u in the previous post instead of selecting large TD to shorten pull down time and then reset the suction pressure controller to increase the suction temperature. this need manual intervention and if it is forgotten to do the graps will be dehydrated immediately and i know i will go through again with conflict with US iceman but this is nature of life.

The idea is to use two evaporator circuits (1/3 and 2/3)
both have solenoid valves. Both units are working together by sensing the evaporator inlet air temperature (above 6.75C) and when this temperature drops to level of 6.75C on-third is tripped off and the second is kept operating. With the same amount of CFM, the discharge temperture will increase in this case and less dehumdifcations will be occured.

Hi US iceman

Welcome back ....the evaporator which is installed in refrigeration application is normally operated with relative constant cooling load. so the acting of TXV to adjust superheat is not important particularly if there is accumulator and liquid-suction line heat exchanger and pump down cycle for shutting off.

Cheers

The phenomenon of pump cavitation as u mentioned earlier is very common in liquid overfeed system. Unfortunately , we usually lay the blame over the designer of this system. This is system has this tendency to cause pump cavitaion because the suction pressure of the pump is negative and if the distance between the pump and the evaporator is big (away of designer willing) the situation is exacerbated. the pump suction drop is increased and motivate the liquid refrigerant to bubbly "annular" pattern and pitting on the pump impeller.

Cheers

Quote:

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Originally Posted by mohamed khamis

Unfortunately , we usually lay the blame over the designer of this system.

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And why not, this is usually where the problem starts. Refrigerant pumps cavitate when the suction head required by the pump at a specific operating condition is exceeded.

There are all sorts of conditions where this can happen. Some are caused by the hand expansion valves not being set properly causing the pump to "ride out" to the right on the pump curve.

Other times it is due to the suction pressure of the vessel being reduced too quickly. There are all kinds of associated problems with refrigerant pumps.

If the system designer takes into account the factors that can cause cavitation he can do a great service to the facility owner.

For instance, the designer can provide information on the drawings about how to set the hand expansion valves. He can also specify any data on the drawings that were a part of his selection process.

This provides sufficient information for the commissioning agent to start and balance the system. By providing sufficient information on the drawings the local system operator also has more information to use.

If a refrigerant pump cavitates, where would you place the fault? It is certainly not the pumps fault. The pump is only reacting to the operating conditions it encounters.

Improper selection and commissioning account for the majority of refrigerant pump issues. These are all under the control of people, not the pump.

Saying it is would be like blaming a compressor for being slugged with liquid!

Quote:

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Originally Posted by mohamed khamis

And this idea i wanted to tell u in the previous post instead of selecting large TD to shorten pull down time and then reset the suction pressure controller to increase the suction temperature. this need manual intervention and if it is forgotten to do the grapes will be dehydrated immediately and i know i will go through again with conflict with US iceman but this is nature of life.

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No conflict here. I agree with you on this point, but a control system can do this automatically if programmed to do so. Although, I did not say large TD, I simply said increase the TD slightly during the pull-down condition. The TD will be higher during this period anyway due to the higher product loads.

Quote:

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Originally Posted by mohamed khamis

...the evaporator which is installed in refrigeration application is normally operated with relative constant cooling load.

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That's true, but a pull down condition is not a relative constant load. If the TXV's are selected for the capacity during pulldown, then when the load has decreased to storage only, the valve capacity can be too big.

Sure, a pump down cycle is mandatory. The reason accumulators are used on DX systems is to have some protection for the compressor when the TXV looses control.

TXV's are like refrigerant pumps... you have to select and apply them properly to achieve a trouble-free system.

Quote:

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Originally Posted by mohamed khamis

...so the acting of TXV to adjust superheat is not important particularly...

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I don't think this is a philosophy I'm willing to subscribe too.:(

Khamis, by having a heater in the drain pan and creating steam, we are heating the room as well. This is not possible. Secondly none of the such plants have proper operators or rather operators. Due to which manually setting and resetting any valve is out of question.

Today I had called one of the water softening plant engineer. He accepts the fact that the water after being ionised is not drinkable and as such to have the same supplied to the room for humidification is not justified.

I feel that the only way to increase the RH is by the Coils.

I am expecting some Coil manufacturers from Germany next week and I shall discuss this with them.

Mike, what I am concerned most about is the superheat. In one of your posts you did mentioned that this system may effect the superheat. So could you please explain that bit.

I do not know about how these conditions for post harvest treatment and storage are done in other countries, but if this process of raising the humidity by the coil could be achieved, it would change the complete concept. Please note that I mean a constant 95% RH with -1oC Room Temperature.

Well I can't comment on the specific outcome of their system using hyrdocoolers as air washers, but in concept it should work. As to the -1°C room temperature, that is a completely different story. The adiabatic humidifiers may be your only hope of acheiving the RH in a room at that temperature.

If you use a low TD for the coil selection, you also limit the superheat for the TXV. A normal TXV may hunt if the operating conditions are not perfectly stable with a low evaporator superheat. That's my opinion only.

If you notice in the link you posted the classification for the lowest TD had the highest room RH. The equivalent superheat available will certainly be less than the TD used for the coil selection. That's my concern.

Mike I would not like to use hydrocoolers as an air washer. What I propose to do is use the DX Finned Coils with low Td and create the required RH. For the phase when there may be a swing, we keep the adiabatic humidifiers as a standby.

Hi Winfred Dela,

It is nice to have you here again. Busy with your similar projects.

Quote:

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Originally Posted by US Iceman

That's true, but a pull down condition is not a relative constant load. If the TXV's are selected for the capacity during pulldown, then when the load has decreased to storage only, the valve capacity can be too big.

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Yes it is true and the pull-down duration is called transient condition and although it is quite not constant but the TXV will handle it. By the way the pull-down load is taken under the cooling load calculation by multiplying it in factor of safety of 1.1 to 1.15 and all the system components are selected according this capacity so the TXV will handle this duration also.


the pull-down time is determined by

pull-down time = thermal storage capacity for the cold room / (Cooling capacity-initial cooling load)

So as the difference (Cooling capacity-initial cooling load) is smaller the pull-down time is longer and vice versa. So as i mentioned for Samajit design with small TD but with excess area over the design area corresponding to the small TD u get relatively short pull-down.

Quote:

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Originally Posted by US Iceman

I don't think this is a philosophy I'm willing to subscribe too.:(

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Well ..tell me until now the hand expansion valve and capillry tube are still used in HVAC application although don not pay any attention to superheat degree. I am saying if the system is subjected to relatively constant and has compressor protection (accumlator, heat exchanger in suction line) the function of TXV as to ensure superheated refrigernt is not imporatnt or existed .

cheers

Samarjit

I gave to u two ideas (multiple-circuit system and mix fresh air with rerurn air before entering evaporator) and i did not find any comments from u except one assistant idea in the first idea. Anyhow, the steam or the boiled water does not increase the room temperature as u think because the heater does not produce a large amount of steam with high temperature as in steam power plant. The amount of melted frost is relatively small and the heater will not boil off all the pan water it will boil off to level does not bare the heater coil otherwise the heater will be brunt. And this steam will dissolve homogeneously inside the room air by the blower circuliting and don not warry the temperture is not increased signifcntly. Becuase there is a reltive percentage of steam with mix with the indoor air .

Cheers

Dear Khamis,

Adding air from outside will not be possible. These Precooling Chambers have to abide by certain norms and regulation laid down by the Export authorities in our country as well as the authorities in those countries where these fruits are being exported. There should not be any possibilities of contamination. If infiltration of outside were allowed then all these problems would not have come.

The only solution that comes to my mind which may be wrong and if so you have to correct me is that, we lower the Td and attain as much as RH is possible. The only possibility is that at a temperature of -1oC there is a possibility of a swing in the RH and which may be reduced. To counteract the same, I am thinking of installing a small RO system and supply the required water for raising the RH by the adiabatic humidifier. These will be operating for a very short period.

Carel of Italy has a very good book on Humidification. They are not in the market, but are directly available from Carel.

Hi Samarjit,

Quote:

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Carel of Italy has a very good book on Humidification. They are not in the market, but are directly available from Carel.

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Is there a link to an English language version in PDF?

Sorry Mike,

There is no pdf version. It is a thick book. Rather I myself have not been able to go through the entire book. You must be having a dealer or a distributor of Carel in your City. The Book is in English. Tomorrow when I go to my office I shall send the name of the book. You may try mailing a request to Carel in Italy for the Book. The Carel dealer would be having copies of the Book for distribution.

Mike you may visit the undermentioned link to get the e mail id to get the book.

http://www.carel.com/carelcom/web/en...midif_aria.jsp

Thank you Samarjit. I have requested a copy and catalog and wait to hear a reply.

This should prove to be very helpful.

Here is a link to a good reference manual. This may be the same book you already have Samarjit, but I thought I would provide the link for others.

http://www.heatcraftrpd.com/resource...H-ENGM0806.pdf

This book describes some of the aspects I was mentioning about the coil locations, etc. in one of your other threads.

See this for some additional details.
http://www.heatcraftrpd.com/resource...s/H-IM-64L.pdf

Thanks Mike,

I am using only Heatcrafts Larking peoducts for my projects, specially their Evaporators. They have very good literatures and manuals. I already have them and they keep sending me any thing which is new.

In case you are interested in Post Harvest Refrigeration, which is really very interesting, I shall send you some more details for you to study.

Hi Samarjit,

Yes I would like to see the material you have on post-harvest details. This is an area I have not been involved in directly so your input would be welcome.;)

Quote:

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Originally Posted by Samarjit Sen

Dear Khamis,

Adding air from outside will not be possible. These Precooling Chambers have to abide by certain norms and regulation laid down by the Export authorities in our country as well as the authorities in those countries where these fruits are being exported. There should not be any possibilities of contamination. If infiltration of outside were allowed then all these problems would not have come.

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Dear Samajit

I got it, but u can use high efficient filter on the way of fresh air incoming to confine any contamination which is impossible to be done with the infiltration. And u can make external humidifier for this fresh air before mixing the return this poses u to use less water amount to humidify the air and lessen the effect of sodium on the grapes. Because this moisture will be dehumidified in the cooling process in the evaporator and any salt will be subsided with removal moisture. the exit discharge of the evaporator is expected to be 98% to 100% Rh. Give me ur comment because i want to learn about the fruit storage problems to gain experience in this field where all my concern currently on AC applications.

Quote:

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Originally Posted by Samarjit Sen

The only solution that comes to my mind which may be wrong and if so you have to correct me is that, we lower the Td and attain as much as RH is possible. The only possibility is that at a temperature of -1oC there is a possibility of a swing in the RH and which may be reduced. To counteract the same, I am thinking of installing a small RO system and supply the required water for raising the RH by the adiabatic humidifier. These will be operating for a very short period.

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yes it is correct but there are two things should be taken into account:

1- the swing acting of compressor from cycling on/off, as my knowledge the running time for the compressor is not less than 3 min otherwise the oil can be entrapped inside the system and does not back to it until the crank case. furthermore, the compressor which gives 7 MT cooling capacity i think it is huge and the rapid cycling may cause erratic behavior in its operation and strength of fluctuated torque.

2- I don not know u can find out evaporator design which gives u TD of 2 K or less because this renders a huge evaportor surface area either huge frontal area or increasing in number of rooms. Both of them is not required because ur design is restricted by certain CFM and the increase of number of rows will reduce the refigernat velcoity inside the evaportor and reduce its ability to scrub the oil to the compressor.

The second idea is the best solution small TD of 5 k with RO system to supply water to be humidified.

Us iceman

Thanks for great links


Cheers

Mike, I shall be sending some documents to you through yousendit and put the link on the forum, so that whosoever is interested may also study the same. But please give me some time, as I am at present terribly busy with these project design and meeting the clients for discussions.

Khamis, we can not add special filters. There are filters available but they are costly. This will increase the cost.

Generally for pre cooling of 7 MT of Grapes the refrigeration capacity of the compressors is about 27 kw at -6oC Te and 40oC Tc. We use Gelfred Semi Hermetic Compressors. The Evaporator surface area is pretty large with a total cfm of about 16000 at 600 fpm velocity. We have also standardised Heatcrafts Larkin Evaporators. They are really very good.

For humidification I am thinking of reducing the Td by using Electronic Expansion Valve and an Epr. There is bound to be a swing in both temperature and RH. The temperature will not be difficult to adjust, and for the RH, we are going to install the RO unit for both the Condensers which shall be water cooled as well as for the Humidifier.

I shall send some information on the application of refrigeration in agriculture. There is a lot of information available on UC Davis and Sardi websites.

I am giving the limk to some informations on Post Harvest which I had collected. Kindly download the same within 7 days.
http://www.yousendit.com/download/ZUczRFFYTkFubHcwTVE9PQ

I will send more links and informations as soon as I am a bit free.

Interesting reading.

I like using a bypassable EPR with 2 stages of cooling. Stage 1, you run at the higher EPR setting, for minimum dehydration. Stage 2, which cuts in after maybe 15 minutes of Stage 1 not satisfying, bypasses the EPR, to give you a colder compressor suction.

Big coil surface obviously is the biggest factor.

Also, the best humidifiers are "Ultrasonic", 1 micron droplet size.

NE-Postharvest.com/humidifiers.htm


I've used them for years, never a problem. No compressed air, no needle valves, very lenient water filtration, etc.

For all things post-harvest, UC-Davis is king.

Jim