Refrigeration System for Agricultural Produce

[Samarjit Sen]

I have been working in the field of Precooling and High Humidity Cold Stores for various Fruits and Vegetables. Studying various books and seeing the installed plants, I find that the system where the air in the Cold Rooms are being sucked in and blown over bare tube coils, and water in being sprayed or spilled over the coils and the air containing the water droplets are being supplied to the room are being considered as more efficient. This system is being termed as Forced Air Cooling.

The reason for the efficiency is being claimed is that as the air is being sucked through the boxes, the cooling of the produce is better.

The air being sucked are usually propellor fans. Now what would be the static pressure so as to have a high efficiency for such aplication. Further the water moisture which is being supplied to the room are larger in size due to which they tend to settle on the card board boxes and also on the fruits and floor of the room. This tends to damage the fruit. What could be the highest humidity achieved.

I have installed plants with DX ceiling hanging . They have pull through fans and as such the velocity is higher. The surface area is much higher with more air supply. The td of the coil has been reduced and the controls have been selected properly. I have not used any additional humidifiers. The coil itself is generating the required humidity due to which the moisture articles are not only very small, but they do not tend to settle on the boxes, fruits and the floor.

The results have been good as there has been no rejection of the fruits exported so far.

Under the circumstances as stated above, which system would it be considered better.

I am asking this, as there are a large number of people in the forum who are executing similar projects . What is their experience and opinion.

[hendry]

Samarjit Sen,
i find u has been a regular in this forum. good!

about your Q ...
i install mainly the later type. we do not practice formal b'coz the setback on big droplets.
the later, yes, produces better RH ctrl.
but, one must sequence the defrost procedures right otherwise condensate water will still fly everywhere upon fan operations.

[Samarjit Sen]

Hello Hendry,

We put electric defrost and do not have any problem. The condensate water does not carryover through the fan.

What is the Rm. Temp. and the RH being maintained in your plant.

[Brian_UK]

one must sequence the defrost procedures right otherwise condensate water will still fly everywhere upon fan operations.

I'm sure that most are aware of that.

[hendry]

Brian, how much do you know about the individual steps within defrost mechanism? How should be the sequence for each step to take place? How long should that step to be?
>> mismatched! costly plant to operate & bad pull down.

Samarjit, we have had experiences with different type.
1] 65%, +18degC;
2] 95%, -20degC

Ammonia & HFC too!

[Samarjit Sen]

The refrigeration systems that are being installed by us are generally for Fruit & Vegetable Storage with a temperature of -1 oC and a RH of 95 to 97% and Ice Cream Hardening and Storage with a Temperature range of - 25 oC to - 40 oC. These plants are being operated with R 22.

The defrost is important and so is the sequence and I am sure that the people involved in such projects are very well aware of the same.

[hendry]

samarjit,

agreed.
when i write 'sequence', it means the tiny little details of the mechanism. It is far more than just defrost in every 4 hours only.
i hope you are clear & so do Brian.

[Samarjit Sen]

hendry,

I am sorry, but to be honest I am yet to understand what yiou said . I shall appreciate if you could please do clarify your comments.

[US Iceman]

Samarjit,

He is talking about the sequence of operation during a defrost cycle.

1. De-energize liquid line solenoid
2. Pump down evaporator
3. Stop fans
4. Close suction valve
5. Energize hot gas solenoid
6. After defrost is complete...
7. De-energize hot gas solenoid
8. Equalize evaporator pressure with suction pressure
9. Open suction valve
10. Energize liquid line solenoid and let coil re-freeze water droplets
11. Turn on fans

Those are the major items and you could possibly have a few more depending on circumstances.

[Samarjit Sen]

Hi Mike,

Thank you for stating the explanations. I am sure that any one involved in defrosting of the coil would be aware of the sequences. Hiowever, we are doing the the defrosting with electrical heaters which we find much more efficient and effective. Earlier we were defrosting the coil with hot gas, but found that at a temperature of -35 oC or below, the process does not work as the defrost is not proper.

My initial question is still unanswered. What is your opinion on the same.

[hendry]

hendry,

I am sorry, but to be honest I am yet to understand what yiou said . I shall appreciate if you could please do clarify your comments.

samarjit,
i think i could not provide much of my expertise to others from now onwards ...

Because i've been informed to tune down my pace.

may be ... some other time.

[hendry]

Samarjit,

He is talking about the sequence of operation during a defrost cycle.

1. De-energize liquid line solenoid
2. Pump down evaporator
3. Stop fans
4. Close suction valve
5. Energize hot gas solenoid
6. After defrost is complete...
7. De-energize hot gas solenoid
8. Equalize evaporator pressure with suction pressure
9. Open suction valve
10. Energize liquid line solenoid and let coil re-freeze water droplets
11. Turn on fans

Those are the major items and you could possibly have a few more depending on circumstances.

thank you, Iceman. at least i know i join the right group of people to further enhance my knowledge.

[Samarjit Sen]

I was browsing through the website when I came accross the undermentioned link. Maybe this would be of interest to some.

http://postharvest.tfrec.wsu.edu/pgD...?article=PC97U

[hendry]

Hi, M/S samarjit.

i've just been released from "quarantine".

here i'm again sharing my view/s with you on an public channel/s.

yes, i ran thru you recommended webpage/s.

i also use some of those modern tech. to enhance our design & pursue better efficiency in total response/s.

i hope your quest on unit cooler's design has ended with fruitful info after so many parties have answered with much enthusiasm.

i also take this opportunity to apologize to you in case i might have offend your feeling/s in one way or the other.

see you in my thread/s [i've started 1 but no response as of to-date] ....