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Thread: Coil Technology

  1. #1
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    Coil Technology



    I would like to know if it is possible for an Evaporator to produce a Relative Humidity of 95% without any additional humidifier. The room is air tight and as such there is no leakage of outside air. The product is grapes or other fruit. The Evaporator is a DX one.

    Kindly let me know the information as early as possible.

    Thank you,



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    Re: Coil Technology

    Hi Samarjit

    Select the evaporating temperature less than the required space temperature by 5°C (8F) i.e. if the required space temperature is -5°C the evaporating temperature should be -10°C. The smaller difference between the evaporator and the design space temperature the higher relative humidity exit from the evaporator. If u want to know the explanation of that u will find it out in the attached file. I wish it could help

    Cheers
    Attached Files Attached Files

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    Re: Coil Technology

    Dear Mohamed Khamis,

    The room temperature is to be maintained at - 1 oC and the Rh between 95 to 98%. By reducing the Td, I have not been able to attain such a high RH. At the most I acjeve 75% RH. The rooms are air tight and are kept closed for the 6 hrs the period it requires to pre cool.

    What I do is install an adiabatic humidifier with the moisture discharge particles at 2 to 3 microns. This gives the result.

    Now some people are claiming that only with the coil they can achieve the required RH. I has consulted the Coil manufacturers and there contention is to achieve such a high RH, one would require additional Humidifier or fogging equipment.

    I was hoping that either you or Lana would be able to provide me some datas to substantiate my comments. I shall appreciate if you could just clarify this a bit in detail with practical application, as I have a meeting to attend tomorrow

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    Re: Coil Technology

    Quote Originally Posted by Samarjit Sen View Post
    Dear Mohamed Khamis,

    The room temperature is to be maintained at - 1 oC and the Rh between 95 to 98%. By reducing the Td, I have not been able to attain such a high RH. At the most I acjeve 75% RH. The rooms are air tight and are kept closed for the 6 hrs the period it requires to pre cool.
    Dear Samarjit

    As i mentioned to you the smaller TD of 5 K will give RH of 95% and this is mentioned in textbook "Principles of Refrigeration" authorized by Roy J. Dossat. Regarding to the giving off 75% RH, it sounds that this selected coil has low air circulation or u have big infiltration rate. Or perhaps the TXV is under sizing or the differential range for thermostat is quite big. Give more details about the system which gave u 75% Rh with coil specifications to make more analysis

    Quote Originally Posted by Samarjit Sen View Post
    What I do is install an adiabatic humidifier with the moisture discharge particles at 2 to 3 microns. This gives the result.
    Yes it is correct this is called mechanical humidifcations but has some problems of biological growth of bacteria and fungi.

    Quote Originally Posted by Samarjit Sen View Post
    Now some people are claiming that only with the coil they can achieve the required RH. I has consulted the Coil manufacturers and there contention is to achieve such a high RH, one would require additional Humidifier or fogging equipment.
    Normally the off coil RH is of 90 to 95% for an efficient coil (with higher effectiveness of 0.8 to 0.85) and this is very common in AC and this also is found in Ref. application though number of fins is quite smaller (3 or 4 fpi) for low temp applications. So I got surprised when u said they told the coil does not fulfill this requirement without additional humidifier. As a general rule if the exit RH of coil less than 90% this coil has a lack of efficiency (high bypass factor) or drastically fouled. Again give me more details to understand why the coil gives these conditions.


    Cheers

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    Re: Coil Technology

    The coil cfm is 15100, velocity 600 fpm, fin spacing is 6 fpi. The room size is 25 ft. x 15 ft. x 12 ft. ht. The room conditions are -1oC temp. and 95% RH. The total load is 7 MT of Grapes.

    As per theory the 95% RH should be achieved by the coil, but in the practical, it is very difficult to attain. This is only one example I have stated. Those who had earlier stated that they are attaining the conditions, later on accepted the fact that they could never get the conditions.

    I had spoken to a number of renowned coil manufacturers and they too have said that theoritically it should produce, but in fact to attain such high RH it is always better to provide additional humidifiers.

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    Re: Coil Technology

    Quote Originally Posted by Samarjit Sen View Post
    The coil cfm is 15100, velocity 600 fpm, fin spacing is 6 fpi. The room size is 25 ft. x 15 ft. x 12 ft. ht. The room conditions are -1oC temp. and 95% RH. The total load is 7 MT of Grapes.
    Dear Samarjit

    I need the frontal area of the coil (length and width) and number of rows, number of tubes per row, tube diameter. What does mean of MT? i want to cite the cooling load in k W involving all load items. In theoretical and practical considerations, the off air RH from the cooling coil is above 90% and i tested many coils for air conditioning purposes and it is conformed to what i said to u. Anyhow may be in cold rooms it is different, what is the design entering conditions for the coil, usually it is supposed as the design cold room condition, is it right or wrong?. waiting for ur feed backing.

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    Smile Re: Coil Technology

    Hi Samarjit.

    Is it not an impossible task to hold this RH when you
    work below zero oC? I am not an expert, but been into some testchambers/climat chambers over the Years. At first to avoid to much condensation on the evaporator, the all brand I know of use a glykol or similar liquid over an evaporator to get a heat transfer as near as the actual and an evaporator who who has the same temperature all the way due to their liquid circulation. They all use a humidifyer of differernt style, but all types seen make a humidifier
    shut down at +5c due to the migration to the cold
    evaporator. In your case, the humidity is supportet by the products and how long can they keep it high?
    The evaporator has to draw a lot from your products.
    May be I am wrong, but just an input this one.
    PS: Any humidifier will just send what you want straight to the evapoartors cold surface. Any eva. running at -5C are dehumidifiers arent they? Highest RH I have seen was a chamber who by liquid in all walls, bottom and top made the temperature. The surface is then so big so the difference to hold the
    wanted is minor. Expensive piece of instrument.
    Per

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    Re: Coil Technology

    Dear Mohamed Khamis,

    I shall be able to provide the details of the coils after a couple of days. At present I am going out of station.

    AQs per Principle of Refrigeration by Roy Dossat, he has very clearly stated that a 5K Td the humidity is high. He does not say as to how much. Further at another chapter, for pre cooling of fruits and vegetables he has said that for attaining a high humidity, either a spray of water on the evaporator or some sort of additional humidity by spray of salt water is required. Due to high volume of air passing over the produces, chances of dehydration is there.

    In some book, I had read that additional humidification by way of either pouring water on the floor or spraying water on the evaporator coils are necessary for pre cooling of fruits and vegetables.

    As you rightly said that Air Conditioning Application and Cold Storage Application are quite different. By MT I meant Metric Ton ( The total product load in tons )

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    Re: Coil Technology

    Hello Norseman,

    In fruit pre cooling, the process of cooling has to be fast so as to remove the field heat. To attain this we have to have an evaporator coil with an oversized face area and a high volume of air. This can dehydrate the product and spoil the same. To avoid dehydration, a RH of about 90% to 95 % depending on the product is maintained inside the room. The Room temperature is maintained at -1oC, so that in 6 hrs. the core temperature of the product is brought down to 0oC.

    For attaining the RH as stated above there are different ways. Some people pour a brine solution over the bare tube coil and blow air through it. This results in higher size water particles which settle on the produce and damages the fruit.

    I am putting an Adiabatic Humidifier so as not to add additional heat. This humidifier sprays water particles of the size 2 microns to the air, which mixes with the air blast from the cooling coil and creates a fog with a RH of 95%. The products retain their colour and texture.

    I had contacted the Cil manufacturers on this point and they have very clearly stated that a coil by itself can not attain 95% RH. It will require an additional support. I want to get a clarification on this point.

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    Re: Coil Technology

    This seems as if you are fighting two problems. Cooling the product after harvest and then maintaining the desired storage conditions.

    Post harvest cooling is normally done with hydro-cooling to get the product temperature down to the desired level.

    Using a lot of forced convection air for pull down also adds to the problem of maintaining the product quality as you have noted.

    In the old days, the fruit, etc. was stored and cooled by using static evaporators that were mounted on the ceiling. No fans, only natural convection. These rooms stay at very high humidity and did not require all of the items you have mentioned.

    Perhaps there might be a different method of performing what you want to accomplish.
    If all else fails, ask for help.


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    Re: Coil Technology

    After harvesting Pre Cooling are carried out, where in the field heat are to be removed. Hydrocooling is not applicable to all the produces. Generally hydrocooling is applicable to leafy vegetables. There are different systems for different products. What we are basically dealing in is the pre cooling of grapes, lemon, pomogrenates, apples etc. In such cases Forced Air Cooling is the better option.

    What I want to know, is it possible for the DX Coil to maintain a RH of 95% by itself at a Room Temperature of -1oC. As far as I know it is not. The same thing the coil manufacturers or also confirms.

    There are some refrigeration engineers, who claim that this RH can be maintained by the coils. The coils for such application must have high volume of air discharge. I say that additional humdifiers must be provided to attain the conditions. In all the plants that I have installed so far, I have provided additional humidifiers and they are able to maintain the RH. I am yet to see the same RH being maintained by a DX coil.

    I wanted to have the opinion from you all

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    Re: Coil Technology

    Hi Samarjit

    There was misunderstood i have cought it at this moment. Firstly the evaporator can discharge air at RH 95% but the problem u want maintain the space RH at 95%. there is abig differnce between the discharge air and the space air. The discharge air carries over the cooling load (sensible and latent) and changes to the space conditions i.e. assume the discharge or supply air at -6°C and 95% Rh and it carries the product load to make the space at -1°C and 75%Rh. So the people who are claiming that the evaportaor can handle that (space RH 95% not the discharge air), yes it is correct in only case the latent load (respiration load) of the product is high compared to the sensible, let us assume the SHF is0.4. however, if the sensible heat is more signifcant than the latent load (SHF >=0.7) the evaporator can not maintain by itself the space at 95% Rh and it needs supplementry humidifer as u did. So there ara cases the evaporator can do and other does not according to the sHF (sensible heat factor) to the cold room and as u mentioned ur applications is precooling to papare the harvest to staorge that means the sensible load is quite high due to the product enters at the ambient conditions and has fresh heat content. Hope that i find out the answer to ur question

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    Re: Coil Technology

    OK, the basic premise of a DX coil is that you need a certain temperature difference to achieve the coil capacity. At the same time, this temperature difference also helps to provide the superheat for the TXV to control the superheat.

    If you have a small temperature difference, you will also have a low superheat, which the TSV cannot control well. Perhaps an electronic expansion valve could??

    If you are using a liquid overfeed or gravity flooded coil you are not limited to some minimum superheat. It's already zero, so the coil temperature difference can be very small.

    This reduces the difference in vapor pressure between the room and the evaporator coil. The lower the vapor pressure difference the higher the room RH will be.
    If all else fails, ask for help.


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    Re: Coil Technology

    Hi samarjit

    this attached file demonstartes more description for what i posted earily.

    Cheers
    Attached Files Attached Files

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    Re: Coil Technology

    Hi US iceman
    Quote Originally Posted by US Iceman View Post
    OK, the basic premise of a DX coil is that you need a certain temperature difference to achieve the coil capacity. At the same time, this temperature difference also helps to provide the superheat for the TXV to control the superheat.
    The temperature difference between the cold space and evaporator has no absolutely relationship with the value of superheat. The bigger difference the smaller surface area for the same cooling capacity and vice versa.

    If the cooling load is increased (represented by an increase in evaporator inlet air temperature) the evaporating temperature is also increased to sustain approximately the temperature difference. While at this moment the superheat is increased to respond with the load augmentation. The reason of superheat increase is as the evaporating temperature increases the latent heat of the refrigernat decreases and leaves opportunity for refrigerant sensible load (in superheating zone) to increase to maintain the same refrigerating effect. The increase in superheat causes more opening to the TXV and the cooling capacity is increased complying with that.

    Quote Originally Posted by US Iceman View Post
    If you have a small temperature difference, you will also have a low superheat, which the TSV cannot control well. Perhaps an electronic expansion valve could??
    I think u mean if u have small pressure difference (condensing pressure – evaporating pressure) u will need electronic expansion valve to overcome the problem of low pressure difference.

    Quote Originally Posted by US Iceman View Post
    If you are using a liquid overfeed or gravity flooded coil you are not limited to some minimum superheat. It's already zero, so the coil temperature difference can be very small.
    Yes a liquid overfeed or gravity flooded coil has low superheat not due to a small temperature difference but due to the high value of refrigerant latent heat. In these systems the refrigerant enters the evaporator coil as saturated liquid (no vapor content) so the latent heat for this condition is high and no chance for the sensible load to take it place. Therefore, the superheat is quite small. And it may need boosting superheat from liquid-suction-line heat exchanger to compensate that and protect the compressor.


    cheers

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    Re: Coil Technology

    Hi,

    The evaporator capacity is based on capacity (btuh or equivalent) per degree of temperature difference (TD). The TD is based on room temp. minus evap. temp. If the room temp is 4.4°C (40°F) with an evaporating temperature of -1.1°C (30°F), then the coil TD is only 3.3°K (or 10°F).

    With a saturated evaporating temperature of -1.1°C, you can only obtain a superheat of 3.3°k maximum. There is no other heat to use to increase the gas temperature in the superheated region. In fact, I doubt you will be able to obtain that much.

    In other words, if the room temperature is only 4.4°C (40°F), you can't raise the superheat any higher.

    Quote Originally Posted by mohamed khamis
    I think u mean if u have small pressure difference...
    No, what I meant was the you can only use an electronic expansion valve to control very low superheat with any degree of success. A normal TXV will present more difficulty in controlling a low superheat.

    Quote Originally Posted by mohamed khamis
    Yes a liquid overfeed or gravity flooded coil has low superheat not due to a small temperature difference but due to the high value of refrigerant latent heat.
    No, the superheat is zero when 100% of the internal coil surface is fully wetted. This has nothing to do with latent heat. And, the temperature difference has nothing to do with the superheat on a flooded or liquid overfeed evaporator. If the coil surface is fully wetted, the superheat should be zero.
    If all else fails, ask for help.


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    Re: Coil Technology

    Hi everybody,

    This thread is very interesting.

    Actually I agree with Mohamed's explanations about the sensible and latent heat gains.

    I give you an actual experience.
    Kiwi fruits are grown in the Caspian Sea area. Notice that this area is moist therefore, high relative humidity, specially during Autumn time when fruits are harvested and stored.
    This rooms must be ventilated because of high ethylene production of Kiwi fruit. For this purpose they install exhaust fans on the walls. When fans are turned on they leave the door ajar. Now you can see that moist air enters the room during ventilation.
    In these rooms I used evaporators with TD=5°C and I got 95%-97% relative humidity without any humidifiers. I must say that these rooms are for storage and not for pre-cooling.

    What Mohamed explained is true and this depends on the conditions and the fruits to be stored.

    Hope this helps.
    Cheers
    Even Einstein Asked Questions

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    Re: Coil Technology

    Quote Originally Posted by Samarjit Sen View Post
    I would like to know if it is possible for an Evaporator to produce a Relative Humidity of 95% without any additional humidifier. The room is air tight and as such there is no leakage of outside air. The product is grapes or other fruit. The Evaporator is a DX one.

    Kindly let me know the information as early as possible.

    Thank you,
    Dear Mr. Sen
    Yes, it can be done.
    Please give me the room size, insulation details, outside ambient conditions, quantity of grapes.

    But if the room is air-tight, how will you drive the Carbon-di-oxide out?
    Regards

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    Re: Coil Technology

    Quote Originally Posted by US Iceman View Post
    The Evaporator capacity is based on capacity (btuh or equivalent) per degree of Temperature Difference (TD). The TD is based on room temp. minus evap. temp. If the room temp is 4.4°C (40°F) with an evaporating temperature of -1.1°C (30°F), then the coil TD is only 3.3°K (or 10°F).

    With a saturated evaporating temperature of -1.1°C, you can only obtain a superheat of 3.3°k maximum. There is no other heat to use to increase the gas temperature in the superheated region. In fact, I doubt you will be able to obtain that much.

    In other words, if the room temperature is only 4.4°C (40°F), you can't raise the superheat any higher.
    Ok I got what u mean. Firstly the TD in example is 5.5K but ok I consider it as u wrote 3.3 K, from these figures u mean that the degree of superheat is mainly depends on the TD. Ok I agree with u but this is only in case the air is passed across the evaporator in counter flow with the refrigerant. However, if the air in parallel flow with the refrigerant the superheat is maximum of 1.1 K.

    I want to say it is logic for the degree of superheat value to be in the range of the evaporating temperature and the evaporating air inlet temperature. This does not mean the TD which identifies the degree of superheat. According to many designs I have done the superheating zone takes place around 5% to 10% of the total cooling capacity and as it is well known the vapor has low heat transfer coefficient so it is expected it is impossible for the superheating temperature to equal to the evaporator air inlet temperature and as my knowledge from the many designs it equals the inlet air temperature minus 5 or 6.


    Quote Originally Posted by US Iceman View Post
    If you are using a liquid overfeed or gravity flooded coil you are not limited to some minimum superheat. It's already zero, so the coil Temperature Difference can be very small.

    Ok yes it is correct the overfeed evaporator has zero superheat not from refrigerant latent heat but it is nature from overfeed evaporator. However, the temperature difference is small here because the refrigerant enters the evaporator fully liquid 100% and this enhances the overall heat transfer coefficient significantly and thus in turn to a decrease in TD for the same cooling capacity and the same surface area of DX evaporator.

    Q = U A TD

    Therefore this type of evaporator is widely used in the low-temperature applications

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    Re: Coil Technology

    After having going through the various posts of Moh. Khamis, US Iceman and Lana, I feel the following datas may help you in giving some suggestions.

    The products are grapes. They are put in perforated plastic boxes, which in turn is put in Cardboard boxes with the top open and are palletised. These are put inside the pre cooling chambers which are 25 ft. x 15 ft. x 12 ft. high. The rooms are air tight. The grapes do not generate ethylene. The doors are opened only at the time of loading and unloading. The room temperature are to be maintained at -1oC with a RH of 95%, so that the grape core temperature is 0oC. The volume of air discharge from the evaporator is 15600 cfm.

    The room door opens to an Ante Room where the temperature is 10oC and the RH is approximately 60%.

    We can not change the Room Temperature as it would effect the quality of grapes.

    Kindly send your suggestion as early as possible. The grapes are inside the pre cooling chamber are for a time period of 6 hrs. They are entering at a temperature of 30oC.

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    Re: Coil Technology

    well samarjit

    God willing i will prepare to you my detailed suggestion when i go to my office tomorrow. Now the matter becomes more clear. I wish i could help u.

    Cheers

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    Re: Coil Technology

    Dear Moh. Khamis,

    I will appreciate if you could please let me know your views and suggestions by 9.30 AM Indian Standard Time, as I have a meeting with the party and his consultant at about 10.00AM IST and I want to be ready with the answers.

    Sorry to push you,

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    Re: Coil Technology

    Quote Originally Posted by mohamed khamis
    OK I agree with u but this is only in case the air is passed across the evaporator in counter flow with the refrigerant. However, if the air in parallel flow with the refrigerant the superheat is maximum of 1.1 K.
    I agree with these points. Parallel air flow increases the problem of low superheat control for the TXV's. Thank you for finding my mistake on the TD. It was getting late last night.

    Quote Originally Posted by mohamed khamis
    This does not mean the TD which identifies the degree of superheat. According to many designs I have done the superheating zone takes place around 5% to 10% of the total cooling capacity and as it is well known the vapor has low heat transfer coefficient so it is expected it is impossible for the superheating temperature to equal to the evaporator air inlet temperature and as my knowledge from the many designs it equals the inlet air temperature minus 5 or 6.
    The TD identifies the maximum value of superheat that is available, but as you say, you are not able to utilize all of this because of other conditions. I did mention the available superheat would be less than the TD.

    I agree the coil surface requires about 5-15% of the total surface to provide the superheat because of the low heat transfer coefficient of vapor.

    Quote Originally Posted by mohamed khamis
    However, the temperature difference is small here because the refrigerant enters the evaporator fully liquid 100%...
    The temperature difference is what the coil is selected for to provide the required capacity. If a low TD is used to create a higher room RH, a larger coil will result. This is in accordance with Q=U A TD in this case.

    Good discussion.

    Welcome back lana. Nice to see you here again.

    Quote Originally Posted by lana
    In these rooms I used evaporators with TD=5°C and I got 95%-97% relative humidity without any humidifiers. I must say that these rooms are for storage and not for pre-cooling.
    Here is an example of what I was trying to describe. A low TD provides a lower vapor pressure difference between the coil temperature and the air. This helps to minimize the humidity removal and maintain a higher room RH.

    Samarjit, one way you might look at this situation is to size the evaporators for two operating conditions. One where the pull-down load is considered with a slightly higher TD to gain evaporator capacity, and then reset the suction pressure control to allow for a higher evaporating temperature (which results in a lower evaporator TD) for the normal storage conditions.

    With a liquid overfeed system this would not be a problem as you do not have to be concerned with superheat control.
    If all else fails, ask for help.


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    Re: Coil Technology

    Hi samarjit

    Never mind i'd like to be in ur service....Just i want to know the mass of the grapes to calculate the respiration load which will tell us the right direction for using humdifier or not. Waiting for ur response ASAP.

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    Re: Coil Technology

    The quantity of grapes that are pre cooled at a time varies from 4 Tons to 5 Tons.

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    Re: Coil Technology

    Dear Samarjit

    Ok i made roughly estimation according to ur posted data,and i found out the latent load is very weak compared with the sensible load.

    The sources of the latent loads are the respiration load, infiltration, and air change due to opening the door. The respiration load in this case is estimated to not be exceeded 0.16 kW (from total cooling capacity of 24.5 kW).

    The infiltration is negligible due to the room is tight

    The air change is estimated to be as a maximum of 1.859 kW. Therefore the sum of the total latent loads is 2.019 kW leading to a sensible heat factor of 0.92. Thus without drawing it is expected that the evaporator can not achieve the room required and humidifier is required to boost up the room humidity, I will send to you afterwards the drawing and expected the amount of brine to be added into the humidifier. I wish it could help u in ur meeting
    Last edited by mohamed khamis; 30-07-2007 at 04:37 AM.

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    Re: Coil Technology

    Hello US Iceman,

    You mention about liquid overfeed system. Do you mean that we go in for a flooded system with R 22.
    Two evaporators means the cost goes up. This the customer will not agree. We have to install one evaporator. What if the Evaporator is suffeciently oversized. Please give me your suggestion and opinion as early as possible as I shall be meeting the party along with his consultant.

    Heatcraft were kind enough to speak to me a while ago. They too are sending some informations.

    It is very nice to recieve assistance when you need it.

    With best wishes,

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    Re: Coil Technology

    Quote Originally Posted by Samarjit Sen View Post
    Hello US Iceman,

    You mention about liquid overfeed system. Do you mean that we go in for a flooded system with R 22.
    Two evaporators means the cost goes up. This the customer will not agree. We have to install one evaporator.

    With best wishes,
    I am not expert in either overfeed or flooding evaporator but i will participate as my knowledge about them. Samajit I think u mixed between the two types, flooded system has usually has single evaporator to be controlled conveniently but the overfeed which usually has multiple evaporators for part-load control.

    The overfeed system has of course a higher capital cost than DX coil and most frequently the circulating pump is subjected to cavitaion problems due to a high negtive pressure results in a vapor formulation inside the suction side in the pump unless the pump is centrifugal and gas type and it is expensive.

    The flooded system has problem of oil return back to the compressor especially at low temperature due to the gummy form of the oil in the low-pressure receiver unless there is oil skimmers. in addition to the high initial cost.

    Quote Originally Posted by Samarjit Sen View Post
    What if the Evaporator is suffeciently oversized.
    In ur case it is favorable because u will make the coil effectiveness is enhanced and therefore reduces the bypass factor and the relative humidity of the discharge air from the evaporator but not to excessive limit of 10% of the design area. Because this will push u to use more moisture added to mitigate the excessive drop in discharge air temperature plus lengthen the time span of thermostat and more compressor energy consumption

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    Re: Coil Technology

    Dear Moh. Khamis,

    Thank you for your your help and the suggestions. I have now got a general idea and shall be able to discuss with the client and his consultant.

    Now there is another point which may come up. The water in these area has a high calcium deposits and they are pretty hard. In case we are using the coils with humidifiers, then we have to use water softening plants. Now these water softening plants will reduce the ppm to the desired level, but in turn will produce Sodium. Now this sodium is harmful to grapes. Now what could be the alternative to counteract the sodium.

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    Re: Coil Technology

    Dear Samarjit

    I m so sorry I don not have significant information about the water harding and its treatment. Perhaps some of members are experts in this area.

    Best wishes

  31. #31
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    Re: Coil Technology

    Quote Originally Posted by Samarjit Sen
    What if the Evaporator is sufficiently oversized.
    That is exactly what I meant to a certain extent. Use a single coil, but use your ingenuity when selecting it.

    During the pull-down the loads are much greater. If you select a coil capacity with a slightly greater TD, your capacity will increase during this time to help pull-down the room load.

    As the room temperature achieves the desired temperature, then reset the suction pressure control for a slightly higher suction pressure. This will increase the evaporating pressure and decrease the TD.

    A room thermostat would still control the liquid line solenoid valve, but the suction pressure control gives you another aspect of controlling the system operation.

    I was not really suggesting a flooded evaporator especially with the new refrigerants. If this is only a single room or several rooms a liquid overfeed system would be too expensive to install.

    However, if you have many rooms (say 5-10 or more), this may help you.

    Quote Originally Posted by mohamed khamis
    ...most frequently the circulating pump is subjected to cavitation problems...
    Refrigerant pumps cavitate because the system was not designed or operated properly. Although this is a common problem, it is not a fault of the system or pump. It is a fault of the system designer!

    Samarjit, on the water problem...

    Can yo use a water softener and then use a reverse osmosis (RO) system to remove the other chemicals from the water. The RO system should produce very clean water.
    If all else fails, ask for help.


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    Re: Coil Technology

    Hello fellows!Grapes and lemon are normally stored
    from 13-15 degrees C,so your temperature are decay
    controlling by storage at 0 degrees C.Your evaporation temp is below 0 degrees so its diffucult to avoid dehumidifying the air when you are below the dewpoint.
    Maybe a dewpoint controller and and frequence converter for the compressor could do the job keeping
    evaporation temp above the dewpoint.(normally I use 86-88 RH) for grape and 84-88 for lemon)
    As the US Iceman mentioned there is still hope for a good solution.Have a nice day.
    norcool

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    Re: Coil Technology

    An RO system is pretty xpensive. Is there any other alternative to remove the sodium from the water, as I understand that sodium is harmful to grapes.

    There are 4 to 6 rooms. Each room has to have its independent refrigeration system.

    The informations provided by all of you has really helped me and I am very much grateful for the same.

    Just for my education purpose I would like to know a bit more about Td control. Heatcraft has provided me with some documents. In case any of you have some thing, please do put it on the forum so that I may learn a bit more.

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    Re: Coil Technology

    Hi Samarjit,

    How was your meeting?
    If all else fails, ask for help.


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    Re: Coil Technology

    Thanks to you and all the forum freinds, not only did I learn a lot, but the meeting went off very well.

    I hope I will get this project.

    Thanks again to all the dear RE Forum members.

  36. #36
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    Re: Coil Technology

    Quote Originally Posted by Samarjit Sen
    Just for my education purpose I would like to know a bit more about Td Control. Heatcraft has provided me with some documents.
    Why not start a new thread on this and provide a link to the documents if possible?
    If all else fails, ask for help.


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    Re: Coil Technology

    Quote Originally Posted by Samarjit Sen View Post
    Thanks to you and all the forum freinds, not only did I learn a lot, but the meeting went off very well.

    I hope I will get this project.

    Thanks again to all the dear RE Forum members.
    I am so gald to hear that u did well in ur Project and hopefully the best in all ur life

    Cheers

  38. #38
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    Re: Coil Technology

    Mike, it is not a bad idea. I shall put the documents on the forum and start a new thread. Let us learn more.

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    Re: Coil Technology

    Hi Mohd. Khamis, thanks to you and all the members of the RE Forum. There is so much to learn in refrigeration. I do not feel that age restrictes education. As Mike suggested, let us go deep into it with a fresh thread.

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    Re: Coil Technology

    Can some one guide me to a software where coil selections could be made based on the parameters as required for the appropriate coils. By coils I mean DX evaporator coils.

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    Re: Coil Technology

    The reason you cannot keep the humidity up is that the coil does not have an even surface temp. I.e. some of the coil is colder than other, thereby producing a lower dewpoint, whereto moisture condenses. This can occur even though your airflow TD is very low. A coil designed for very low refrigerant TD and dP can help You can also use an EPR to maximize the boiling surface area.

  42. #42
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    Re: Coil Technology

    Thank you Javabones, I propose to use a lower TD and DP Coil with an EEV and a EPR.

  43. #43
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    Re: Coil Technology

    While browsing came across this link for on line calculations. Maybe of use to some members.

    http://fridgetech.com/calculators/

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    Re: Coil Technology

    http://www.fruitcontrol.it/

    More information on this site. This company's products are being used in India now & are found to be satisfactory in performance.

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    Re: Coil Technology

    Hi smpsmp45,

    Thank you for the link. This firm is mainly dealing with CA equipments for the Fruit Storage. They are not providing any refrigeration system. However it is very useful link. Thanks again

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    Unhappy Re: Coil Technology

    Hi Guys,

    I know this is out of our technical headlines.Any have can you pls help me.

    I have to know how to post a new thread ?

    Expecting your replies that help me much more.

    Thanking you,

    tjk2007


    Quote Originally Posted by Samarjit Sen View Post
    Dear Mohamed Khamis,

    The room temperature is to be maintained at - 1 oC and the Rh between 95 to 98%. By reducing the Td, I have not been able to attain such a high RH. At the most I acjeve 75% RH. The rooms are air tight and are kept closed for the 6 hrs the period it requires to pre cool.

    What I do is install an adiabatic humidifier with the moisture discharge particles at 2 to 3 microns. This gives the result.

    Now some people are claiming that only with the coil they can achieve the required RH. I has consulted the Coil manufacturers and there contention is to achieve such a high RH, one would require additional Humidifier or fogging equipment.

    I was hoping that either you or Lana would be able to provide me some datas to substantiate my comments. I shall appreciate if you could just clarify this a bit in detail with practical application, as I have a meeting to attend tomorrow

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    Re: Coil Technology

    Quote Originally Posted by tjk2007
    I have to know how to post a new thread ?
    Go to any forum (commercial, Industrial, Air COnditioning, etc) that is similar to the nature of your question. Then click the New Thread Button at the top left of the thread listings.

    That's it. Just follow the directions.
    If all else fails, ask for help.


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    Re: Coil Technology

    hi samarjeet,

    What i understood is, you want to precool grapes which are in the cardboard carton.
    For grapes storage temperature is 0-5 degree C, I dont know why you considered -10degree C. This high TD will definitely reduce the rh% in the room. And second this is precooling duration for grapes is is usually 4-8 hrs, you wont loose much produce moisture in this short peroid of time, when compared to storage in cold room for day and even for months. Still if you want to increase humidity try using cooling pads.
    For forced air precooling you require much more CFM and face velocity, to certain extent this will also help in reduced frost formation on the coil and getting high humidity level.

  49. #49
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    Re: Coil Technology

    Quote Originally Posted by Samarjit Sen View Post
    I would like to know if it is possible for an Evaporator to produce a Relative Humidity of 95% without any additional humidifier. The room is air tight and as such there is no leakage of outside air. The product is grapes or other fruit. The Evaporator is a DX one.

    Kindly let me know the information as early as possible.

    Thank you,
    Can what is the temerature and capacity of compartment

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    Re: Coil Technology

    Hi Samarjit,I know this is a bit late as I have not been on the web for a while.Have you considered using a hot gas injection valve with an EEV which will inject hot gas into the coil controlled by the air off coil temp. As the load drops the valve will increase hot gas injection into the coil giving you a consistent low TD at design conditions.You should eventually get an air off temp close to your design temp.I have done this with export apples at -0.5C room temp using a siemans electronic hot gas injection valve take your pick on the EEV.

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