Evaporator Td Correction Factor

[Samarjit Sen]

Most of the Evaporators are calculated at 6oC Td. When the Relative Humidity is to be raised the Td is below 6oC. Has any one got the correctyion factors for assesing the Evaporators at the various Td with R 22 refrigerant.

I shall be grateful if some one can help me.

[John Hunter]

Hi Man . There is no direct rule of thumb, but fof all conditions above freezing the performance is based on the LMTD ,(Log mean temperature difference) (T1-T2)/ Loge(Ti,T2), and the surface transfer coeficient . This can vary from below 19 watts/m2/LMTD, to >34 w/m2/LMTD. The evaporators basic design is important,air flow rate, between 1.5 to 3 meters. sec and the refrigerant supply and distribution has to be able to cope.Best to check manufactures data if you can.

[US Iceman]

Samarjit,

If the manufacturers rating is based on TD (entering air temperature minus evaporating temperature) then you can divide the published rating by the rated TD. This gives you the basic unit rating of kW per °K of TD. Then you can simply rate the coil for other conditions by multiplying this value by the desired TD.

You have to read the rating basis used by the manufacturer to be accurate.

If the evaporators are rated with a refrigerant other than R-22, look at the different rating factors used for air-cooled condensers. These usually have various ratings for different refrigerants. It may give you an indication of the magnitude of the change.

Or, ask the manufacturer for the specific rating.

[Samarjit Sen]

Thank you Mike,

What you have stated is more on the same line as Heatcraft. I am interested in installing a plant with RH 97% without using the additional humidifiers. For this I require about 3oC Td. All the manufacturers have stated in their literatures a minimum of 6oC and was not very clear about the selection procedures.

I hope you are back now and slightly free.

[nike123]

I think that you have great tool for your calculations.
Just type "coolpack software" in Google.

[Peter_1]

Samarjit,
Is this what you're looking for on page 21, right side of the page http://www.heatcraftrpd.com/resource...H-ENGM0806.pdf

[Samarjit Sen]

Hi Peter 1,

Thanks for your help. I have the manual of Heatcrafts both the soft copy as well as the hard copy. We are using only Heatcrafts Larkin Evaporators for all our projects.

What I am interested to know as to how do I find the capacity of the Evaporator at a Td other than what is specified in the Catalogs, I guess that Mike has provided some answers. I am interested in knowing more. Maybe it is my curosity to learn at this age.

[US Iceman]

Samarjit,

As an example: if you take an evaporator sized for a 6°K TD and operate it with a 3°K TD, then the unit capacity will be 1/2 of the capacity at the 6°K TD rating.

To obtain the required refrigeration capacity with the 3°K TD the evaporator capacity will have to increase by a factor of 2.

This raises the evaporating temperature assuming the air temperature is constant. By increasing the evaporator temperature, the vapor pressure difference between the coil and inside room air is also decreased. This results in higher room RH.

Does that help?

The one thing I think you need to be careful of is the TXV's. If a standard TXV is used for low superheat control it will control erratically if the load changes from a steady state operation.

A balanced port TXV or EXV may provide better control.

Also, when you operate with low TD's the amount of superheat you have for controlling the TXV is also much lower so you may need accumulators to provide protection for the compressor.

If this was an ammonia system you would not have these problems if a flooded or liquid overfeed evaporator were used. An ammonia TXV would work as described above.

[nike123]

What I am interested to know as to how do I find the capacity of the Evaporator at a Td other than what is specified in the Catalogs, I guess that Mike has provided some answers. I am interested in knowing more. Maybe it is my curosity to learn at this age.

REMOVEhttp:REMOVE/REMOVE/tinyurl.REMOVEcom/38pu2tREMOVE

Just use this software and insert your data, and software will calculate catalog data for you. Its free and easy to work.

[Samarjit Sen]

Hello Nike 123,

Which is the link to the software you are talking about.

[Samarjit Sen]

Hi Mike,

Thank you for the information. This is more helpful. I propose to use EEVs for such applications.

[Josip]

Hi, Samarjit Sen

Thank you Mike,

What you have stated is more on the same line as Heatcraft. I am interested in installing a plant with RH 97% without using the additional humidifiers. For this I require about 3oC Td. All the manufacturers have stated in their literatures a minimum of 6oC and was not very clear about the selection procedures.

I hope you are back now and slightly free.

Part of answer is given by US Iceman -bigger evaporator then problems with TXV, but I am afraid it will not be possible to come to such high RH without humidifiers....

Practically it can be very expensive....and final results not good...

Best regards, Josip

[Samarjit Sen]

Hi Josip,

I have been able to achieve upto 90% without the humidifiers. The humidifiers that I am using is adiabatic humidifiers. I have to try to get 97% without the humidifiers, but shall retain ultrasonic humidifiers for emergency and which will only start if the humidity does not increase beyond 90%.

What is your opinion?

[Josip]

Hi, Samarjit Sen

Hi Josip,

I have been able to achieve upto 90% without the humidifiers. The humidifiers that I am using is adiabatic humidifiers. I have to try to get 97% without the humidifiers, but shall retain ultrasonic humidifiers for emergency and which will only start if the humidity does not increase beyond 90%.

What is your opinion?

My opinion...I have to admit I am not an expert for food preservation in cold stores i.e. to prescribe recommended temperature and RH), but working with people doing just that I've learned that we need to have and use humidifiers to reduce product weight/moisture loss...

Idea to have ultrasonic humidifiers is ok(at least you can spray water on the floor of cold room in case you need more moisture), but you can try without and see what will happen...of course, please, come back with final results

Best regards, Josip

[Samarjit Sen]

Dear Josip,

Incrasing the humidity by spraying water in the floor is a process which is being followed by a lot of people. This creates hindrances in working. Further spraying water molecules by means of humidifiers makes the cardboard cartons containing the fruits etc wet and spoils the packages.

As it is the treated water contains Sodium Chloride, which is again harmful to health. Now creating a high humidity with humidifiers the possibilities are damaging the packages and also providing sodium chloride.

Now as far as I could think, there are two possibilities. One is to have ultrasonic humidifiers by which the sizes of the water molecules are 0.8 to 1 microns and as such instead of depositing on the floor or the product boxes they shall remain suspended in the air, In this case the water supply has to be through a RO system. The second option is to have a low Evapotaor Td and raise the RH level to 97%. This will no doubt be an expensive process, but the result will definately be better. This will involve the use of some special controls to attain a very accurate Te, uniform flow of refrigerant and a constant Evaporative Pressure.

I will keep you informed as to the progress as I make.

With best wishes,

[nike123]

Hello Nike 123,

Which is the link to the software you are talking about.

I can't post link because I don't have 15 posts yet.
Just type "coolpack software" in Google and first result is link to download site.

[Samarjit Sen]

I have the CoolPack. Unfortunately it does not have the datas as required by me or what I am looking for. However thank you very much.

[lana]

Hi everybody,

The relationship of TD and evaporator capacity is not absolutely linear. The correct procedure is : you have to find the new capacity at new evaporating temperature and new TD. When you find the correction factors and plot them you will see that the graph is not a line (is not linear).
These correction factors can be calculated by basic refrigeration analysis taking into account the refrigerant densities at new conditions.
By knowing the capacity at a given conditions then you can find the new capacities and then divide this new capacity to the known one to get the correction factors. You may notice in some manufacturers catalog the correction factors are greater than 1 but in some, the figures are less than one. This is why in some catalogs you have to divide the figure and in some you have to multiply the correction factors.

I have done this for our evaporator catalogs many years ago. It is not difficult if you know what you are doing.

Using an evaporator with 3°C TD is little bit scary. Some of the reasons are given by the others and also it would be so huge and also would cost a fortune.

I will be in a site with four cold stores for Kiwi fruit during next week for commissioning. These are designed with 5°C TD. I will post the results here for discussion.

Hope you get your answer.
Cheers

[Samarjit Sen]

Hi Lana,

Thank you for your opinion and suggestion. I shall be keenly awaiting to hear from you regarding your proposed visit to the sites for the commissioning of the plants.

I shall be happy if you could please provide me with the details.

[lana]

Hi Lana,

Thank you for your opinion and suggestion. I shall be keenly awaiting to hear from you regarding your proposed visit to the sites for the commissioning of the plants.

I shall be happy if you could please provide me with the details.

Hi Samarjit Sen,

By all means. I will post the results on the forums.
Hope it goes well .

Cheers

[nike123]

I have the CoolPack. Unfortunately it does not have the datas as required by me or what I am looking for. However thank you very much.

If you are not familiar with CoolPack, could you, please, post here project data? Maybe I could make calculations with CoolPack tools for you, if you send here all needed data.
These are green numbers in input rectangles (see picture on this address: hZZZttpZ:Z/Z/tinyZurl.cZom/2ZZen9bm ).

Remove Z from address!

[Samarjit Sen]

Thank you nike123,

I have got what I wanted. I am now awaiting to hear about the results from Lana.

[lana]

Hi Everybody,

I have just came back from the site. This site has 4 cold rooms for Kiwi fruit. Each room has two compressors but the number of evaporators varies.
The biggest room has one 40HP compressor with three evaporators, also one 35 HP compressor with two evaporators. The 40HP system is designed with TDe=5°C but the 35HP system is designed with TDe=8°C (because of economic reasons the owner asked me to do this). Anyway, I did not have enough time to adjust the TEVs so I have to visit the site again. I measured -0.5°C room temperature with 92%RH. We have 45m of pipe run on this system but there is no oil return problem. There are no products in these rooms so we have to wait and see what happens when they are loaded.
There are some installation problems because the contractor did not follow my instructions but now I can do nothing because it is done.

Commissioning a system is relatively easy if you have designed the system because you are sure about the components and their design data.

So far I am happy with these systems but the real result is when they are loaded.
I will keep you informed.

I have attached some photos for you.

Cheers

[Samarjit Sen]

Hello Lana,

It appears that you have done a good job. The actual status will only be available with the rooms being loaded.

In the projects that we do the rooms are as large as the one done by you. The refrigeration loads are at the maximum 38 kw. Further what we are doing that we are having individual Evaporators with each Condensing Units.

I would be very keen to learn more after your next visit to the site.

[US Iceman]

Commissioning a system is relatively easy if you have designed the system because you are sure about the components and their design data.

That is absolutely true. I just started up a highly modified ammonia liquid overfeed system last week. Start up only took two days. The biggest problem found was the controls contractor still needs to do some more work. Otherwise the system ran great.

Nice job lana!

[lana]

Hi Samarjit Sen and US Iceman,

Thank you. I will keep you informed when the rooms are loaded. Hope it goes well.

Cheers

[lana]

Hi everybody,

As promised I would like to inform you how the project went. I just came back from the site and they started to load the Kiwi into the cold rooms.
First let me explain something which I did for the first time. I ordered a system to an electronic company to monitor temperature and relative humidity inside the cold room. The difference is this system can accommodate up to 32 sensors in a room. This system works like this : you choose how many sensors you want to put in the room, then you input their relative co-ordinates (relative to the door) into the panel, then on the screen you can see the sensors as a dot. The system reads the temp. and RH for each sensor and shows them on another panel. (the picture is attached).
We installed this system in one room with 8 sensors. These sensors cover the perimeter of the room. With this system you can monitor the temp and RH differences in one room which would be caused be incorrect loading.
I got between 90 and 98% RH with 0°C all around the room. I must mention that there is no humidifier or wet floor.

As you can see from the pictures, the loading is not finished yet but so far everything goes as designed.

So as I explained many times in this forums, high RH can be achieved by correct evaporator design.

Cheers

[Peter_1]

Indeed, VERY nice job Lana.
Are these Profroid evaporators?
They look anyhow like Profroid.

[lana]

Hi Peter,
Thanks a lot.

No, they are manufactured by my own company.
They are home made.

Cheers

[Peter_1]

Wawwwwwwwwwwwwww

[US Iceman]

The 40HP system is designed with TDe=5°C but the 35HP system is designed with TDe=8°C (because of economic reasons the owner asked me to do this).

Hi lana, which system is holding the RH?

I got between 90 and 98% RH with 0°C all around the room. I must mention that there is no humidifier or wet floor.

Also, was the variation in RH due to any particular reason? The reason I am asking is I'm trying to define the actual RH range for the specific TDe and if this was constant during any time or associated operation.

Thanks.

[lana]

Hi US Iceman,

The design of two systems for this room is done according to the kg/day loading which was assumed initially. Now they can not load that much product into the room therefore, only 40HP system is working and it is enough to pull-down the temp. (the reason they can not load that much is because of shortage of fork lift and personnel).
So up till now only 40HP system with TD=5°C holds the RH. When the room is fully loaded, I will visit the site and will bring the 35HP system into operation and see what happens.

The variation of RH is normal except for one sensor which shows the highest of all (98%). This sensor is the nearest to the door and product was loaded directly beneath it. I assume that these are the reasons for the high reading. I might change this sensor next time to see what happens (of course if I can go into the room after the loading).

The other sensors were showing from 90 to 93% which is acceptable. First I suspected the readings and I put my Testo hand held Hygrometer into the room but it registered the same value for RH.

I hope I explained it well.
Cheers

[Samarjit Sen]

Hello Everyone,

I was away for quite some time and visited one of my sites. The Pre Cooling Rooms and the Cold Storages are being loaded with Pomegranates, where the required Room Temp. is 4oC and a RH of 90%. I found that without using the humidifiers these conditions were being maintained. As in within a few months the party would be loading Grapes, just out of curosity, I had the temperature brought down to -1oC Room Temperature and found the RH has gone upto 98% and was being maintained.

I have disconnected the humdifiers as they are not required.

As Lana says that we can attain the high humidity with the Evaporators and without any additional humidifiers if the system design is correct.

The temperature and the RH is uniform throughout the Room and I did not install any controls such as EEV or CPR.

[US Iceman]

I'm seeing a consensus starting to form!

The interesting point brought up by all of this is; if the system is designed properly, it should produce the desired results.