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    which evaporators work better ?



    Hi guys,

    Suppose that we have 2 air-cooled evaporators with for example 200 square meter heat transfer area.
    And this evaporator has fans with air volume for example 50 m3/hr.
    We can choose for example 50 cm diameter fans with 900 rpm and we can choose 40 cm diameter fans with 1440 rpm.
    In both cases above both of fans have 50 m3/hr but of evaporators has bigger diameter of fan with lower rpm and other one has smaller diameter of fan and higher rpm.

    You think, Which evaporator will work better ?

    Sincerely yours.
    Last edited by A.Mortezania; 02-02-2017 at 07:18 PM.



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    Re: which evaporators work better ?

    Hi, A.Mortezania

    good question ....

    Quote Originally Posted by A.Mortezania View Post
    Hi guys,

    Suppose that we have 2 air-cooled evaporators with for example 200 square meter heat transfer area.
    And this evaporator has fans with air volume for example 50 m3/hr.
    We can choose for example 50 cm diameter fans with 900 rpm and we can choose 40 cm diameter fans with 1440 rpm.
    In both cases above both of fans have 50 m3/hr but of evaporators has bigger diameter of fan with lower rpm and other one has smaller diameter of fan and higher rpm.

    You think, Which evaporator will work better ?

    Sincerely yours.

    What is your opinion which one will work better and why?

    Best regards, Josip

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    Re: which evaporators work better ?

    Quote Originally Posted by Josip View Post
    Hi, A.Mortezania

    good question ....




    What is your opinion which one will work better and why?

    Best regards, Josip
    Hi Josip,

    I think evaporator with higher diameter of fan and with lower rpm is much more better than evaporators with higher rpm of fan.
    Because fan speed in some cases prevents evaporating of liquid refrigerant inside of evaporator.
    For example when we want to go to -50 C, We use very low rpm of fan for evaporator because fan do not let us to reach this temperature.

    Sincerely yours.

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    Re: which evaporators work better ?

    Given that you have the identical evaporator size and identical airflows it makes no difference apart from electrical running costs of the fans and increased noise from the higher rpm.
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    Re: which evaporators work better ?

    Quote Originally Posted by Brian_UK View Post
    Given that you have the identical evaporator size and identical airflows it makes no difference apart from electrical running costs of the fans and increased noise from the higher rpm.
    Dear Brain,

    Thanks for your reply.

    With respect, But I think Fan speed is very important. Air flow is constant but I think fan speed is very important on evaporator and condensers.
    Fan with high speed sometimes prevents heat exchanging on evaporator and condenser.
    Some of US evaporators and condensers use bigger fan with lower rpm and European evaporators and condensers use smaller diameter in case of fan with higher rpm.
    And when we compare, US evaporators and condenser with same heat transfer area, Have better efficiency.

    Sincerely yours.

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    Re: which evaporators work better ?

    You are talking about different subjects.

    Your first premise was an evaporator of a set size with a choice of two different fan sizes/speeds. However you state that the airflow rate is the same; therefore the air flow through the evaporator will be the same.

    The size and speed of the fans is irrelevant if they are producing the same airflow.

    If you start changing the evaporator size then you will begin to have different effects occuring.
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    Re: which evaporators work better ?

    Quote Originally Posted by Brian_UK View Post
    You are talking about different subjects.

    Your first premise was an evaporator of a set size with a choice of two different fan sizes/speeds. However you state that the airflow rate is the same; therefore the air flow through the evaporator will be the same.

    The size and speed of the fans is irrelevant if they are producing the same airflow.

    If you start changing the evaporator size then you will begin to have different effects occuring.

    Dear Brian,

    Thanks for your reply.

    There is misunderstanding. Sorry if I couldn't be able to explain well.
    My mean is just on case of fan, I know air flow in both cases are same. But with respect I can not accept that two evaporators (One with bigger fan diameter and lower rpm and second one with smaller fan diameter and higher rpm) have same efficiency.
    I graduated mechanic of fluids form university and I know about rules of passing air form surface of evaporator fin. Also on all of software, Just air flow is important to calculate evaporator's capacity, But when we uses these 2 kinds of fans on evaporators we realize that bigger diameter with lower rpm is much better and it causes reaching to our demand room temperature faster.

    Sincerely yours.
    Last edited by A.Mortezania; 04-02-2017 at 06:16 AM.

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    Re: which evaporators work better ?

    Now you are talking about an air distribution. It has nothing to do with the cooling capacity.
    Please see the file attached .
    Attached Files Attached Files

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    Re: which evaporators work better ?

    Quote Originally Posted by narkom View Post
    Now you are talking about an air distribution. It has nothing to do with the cooling capacity.
    Please see the file attached .
    Dear Narkon,

    Thanks for your reply.

    Air distribution is other material, We are talking just specially on evaporators and condensers. Air flow of fan is important subject on determining capacity of evaporator and condensers. You made mistake, Capacity of evaporator and condenser as result of fan air flow and fan air speed is other thing.

    Twice I type my question :

    We have 2 evaporators with same air flow fan , one of them has 50 cm diameter fan with 900 rpm and for example other one has 40 cm diameter fan with 1440 rpm. Twice I emphasize, in both air flow is same.
    Which evaporator has better efficiency ?


    Sincerely yours.
    Last edited by A.Mortezania; 04-02-2017 at 10:36 AM.

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    Re: which evaporators work better ?

    You need to be more specific in your question.

    No matter where you have studied the laws of physics remain the same.

    Start here - what is the face area of your evaporator?

    Let us say it is 2sq.m.

    Your fan handles 50 cu.m/s.

    What is the velocity of the air through the evaporator?

    Now change the fan size and speed but maintain 50 cu.m/s airflow.

    Once again - What is the velocity of the air through the evaporator? If you say it has changed please explain how.
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    Re: which evaporators work better ?

    It depends of what do you mean by efficiency. If you talk about energy efficiency, we don't have info about power of evaporator fans. I agree with Brian UK that no difference how you produce air flow. If air flow is equal for two different types of fans(the same evaporator frame and surface), energy use of the fans can be different and this is determine energy efficiency of the evaporator.

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    Re: which evaporators work better ?

    Quote Originally Posted by Segei View Post
    It depends of what do you mean by efficiency. If you talk about energy efficiency, we don't have info about power of evaporator fans. I agree with Brian UK that no difference how you produce air flow. If air flow is equal for two different types of fans(the same evaporator frame and surface), energy use of the fans can be different and this is determine energy efficiency of the evaporator.
    Dear Segei,

    OK. I correct my sentence. "Which evaporator will reach to our demand temperature faster" ?
    Evaporator with lower rpm and bigger diameter of fan ? Or evaporator with higher rpm and smaller diameter of fan ? Or you tell both are same ?

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    Re: which evaporators work better ?

    Quote Originally Posted by Brian_UK View Post
    You need to be more specific in your question.

    No matter where you have studied the laws of physics remain the same.

    Start here - what is the face area of your evaporator?

    Let us say it is 2sq.m.

    Your fan handles 50 cu.m/s.

    What is the velocity of the air through the evaporator?

    Now change the fan size and speed but maintain 50 cu.m/s airflow.

    Once again - What is the velocity of the air through the evaporator? If you say it has changed please explain how.
    Dear Brian,
    OK. I say this to explain it better :
    Evaporator No 1
    Heat transfer area = 2 sq.m
    Fan air flow = 50 cu.m/s
    Fan diameter = 50 cm
    Fan rpm = 900
    Fan air velocity = V1

    Evaporator No 2
    Heat transfer area = 2 sq.m
    Fan air flow = 50 cu.m/s
    Fan diameter = 40 cm
    Fan rpm = 1440
    Fan air velocity = V2

    And V2 > V1 because rpm of Fan 2 > rpm of fan 1.

    Now, You think Evaporator 2 cool room faster than Evaporator 1 ?
    Why ? As result of higher fan air flow velocity ?

    Sincerely yours.

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    Re: which evaporators work better ?

    You talk about capacity not efficiency. Capacity depends of 3 factors. Temperature difference between air temperature and evaporator temperature(suction temperature(pressure)) is the same. Heat transfer surface of evaporator is the same. Heat transfer coefficient depends of air flow. If air flow is equal, heat transfer coefficients will be that same. All 3 factors are equal and capacities will be equal as well.

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    Re: which evaporators work better ?

    For heat transfer coefficient important velocity of air going in evaporator not velocity going out of fans. I assume that air first go to evaporator than to the fan and than to the refrigerated room. Fan air velocity important to provide required air throw. We need this throw to avoid "hot spots" in the refrigerated room.
    Last edited by Segei; 04-02-2017 at 09:15 PM.

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    Re: which evaporators work better ?

    Quote Originally Posted by A.Mortezania View Post
    Dear Brian,
    OK. I say this to explain it better :
    Evaporator No 1
    Heat transfer area = 2 sq.m
    Fan air flow = 50 cu.m/s
    Fan diameter = 50 cm
    Fan rpm = 900
    Fan air velocity = V1

    Evaporator No 2
    Heat transfer area = 2 sq.m
    Fan air flow = 50 cu.m/s
    Fan diameter = 40 cm
    Fan rpm = 1440
    Fan air velocity = V2

    And V2 > V1 because rpm of Fan 2 > rpm of fan 1.

    Now, You think Evaporator 2 cool room faster than Evaporator 1 ?
    Why ? As result of higher fan air flow velocity ?

    Sincerely yours.
    Correct, fan velocity is different but airflow through evaporator remains constant. You seem unable to understand basic air flow dynamics.

    50 cubes of air is still 50 cubes of air regardless of what is moving it.

    Until you can grasp this fact there is no point in continuing.
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    Re: which evaporators work better ?

    Quote Originally Posted by Brian_UK View Post
    Correct, fan velocity is different but airflow through evaporator remains constant. You seem unable to understand basic air flow dynamics.

    50 cubes of air is still 50 cubes of air regardless of what is moving it.

    Until you can grasp this fact there is no point in continuing.
    Dear All of friend,
    Thanks for your opinions and your helps regarding my question.

    Please twice check below about fan :

    You say, Air flow is constant and by this both of evaporators have same capacity. OK, I can understand about dynamics of air because I graduated in this filed, OK, I'm surrendered.

    But in the other hand I have another question, Suppose that we have 8 cu.m fridge and we want to decrease temperature to for example -55 C.
    In sources and heat-transfer books when we increase air flow of air we increase coefficient of heat transfer, OK ?
    But practically we see that if we want to use fan in this kind of evaporators, Fan prevent reach to this temperature fast and by using fan we will have floodback, By this we use evaporators include just fin and copper tube without fan for ultra low temperature.
    Because fan prevent good het-exchanging between fin of evaporator with air of fridge. Do you accept this ?

    Sincerely yours.
    Last edited by A.Mortezania; 05-02-2017 at 09:05 AM.

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    Re: which evaporators work better ?

    Now you are talking about the Natural Convection type evaporator coils.
    Please follow the link
    http://www.nptel.ac.in/courses/11210...cture%2023.pdf

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    Re: which evaporators work better ?

    In sources and heat-transfer books when we increase air flow of air we increase coefficient of heat transfer


    Incorrect. The coefficient of heat transfer of the air remains constant. If you increase the air mass flow then you allow more heat to be absorbed due to the increased mass flow.

    Q = m c dt
    I'm back on the Pale

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    Re: which evaporators work better ?

    Quote Originally Posted by A.Mortezania View Post
    Dear All of friend,
    Thanks for your opinions and your helps regarding my question.

    Please twice check below about fan :

    You say, Air flow is constant and by this both of evaporators have same capacity. OK, I can understand about dynamics of air because I graduated in this filed, OK, I'm surrendered.

    But in the other hand I have another question, Suppose that we have 8 cu.m fridge and we want to decrease temperature to for example -55 C.
    In sources and heat-transfer books when we increase air flow of air we increase coefficient of heat transfer, OK ?
    But practically we see that if we want to use fan in this kind of evaporators, Fan prevent reach to this temperature fast and by using fan we will have floodback, By this we use evaporators include just fin and copper tube without fan for ultra low temperature.
    Because fan prevent good het-exchanging between fin of evaporator with air of fridge. Do you accept this ?

    Sincerely yours.
    Move
    your temperature probe from under the static coil to above the static coil. Se how results now change.
    A static coil air mass flow is reduce compared to a forced/induced air coil, so the air off a static coil generally is colder.

    PS your not talking about a freeze drier by any chance?
    Last edited by mad fridgie; 06-02-2017 at 03:26 AM.

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