2013



Results 1 to 14 of 14
  1. #1
    Join Date
    Apr 2012
    Location
    China
    Age
    29
    Posts
    6
    Rep Power
    0

    Condenser of an air conditioner


    Hey,
    how can i calculate the rotation speed of a condenser fan?

    My problem is I have to simulate an Condenser. For the Input I have the Inputtemperature of the coolant and air the massflow of the coolant and the pressure of the coolant and air (ambient pressure) also the heat transfer coeffizient is set by myself, also known. For the output I want to have the Temperature of the coolant.

    I know the rotation speed depends on the pressure. And I also know, that the fan blows only so much how to condense the refrigerant is used. But this power doesn't depend on the pressure or? I mean, it is Q=m*dH (Q=heat flow, m= massflow of the coolant, dH=condensatoin enthalpy). And at the beginning the coolant is overheated. So, if the fan only blows the condense energy, the coolant at the output of the condenser is steam and liquid.

    I hope you can understand my problem.
    mfg
    Chandler.



  2. #2
    Join Date
    Oct 2007
    Location
    manchester
    Age
    41
    Posts
    5,476
    Rep Power
    30

    Re: Condenser of an air conditioner

    http://www.google.co.uk/products/cat...d=0CIUBEPMCMAE#
    or alternativly read the nameplate for the motor.

  3. #3
    Join Date
    Feb 2012
    Location
    Bolivia
    Posts
    270
    Rep Power
    4

    Re: Condenser of an air conditioner

    Sorry Chandler I can’t understand your last paragraph and don’t know if the little I understand is correct but:

    The power of the fan depends on condensing pressure. You need information of the power output of your inverter to get a model of how the fan power relates to pressure.

    If you are modeling a condenser with variable speed fan then your heat transmission coefficient (especially the outside film coefficient) depends on the speed of the fan and therefore depends also on the pressure inside the condenser.

    So you need at least 2 heat transfer coefficients one with the lowest pressure (and lowest fan speed) and another with the highest pressure (and highest fan speed). You can assume this function is linear. Only doing this you will reflect the effect of the varying fan speed in your calculations.

    If the condenser is doing its job it should output only liquid, not steam.

    By the way we use the word steam only for water vapor, in refrigerants we call it vapor if it is near saturation or gas otherwise.

  4. #4
    Join Date
    Mar 2001
    Location
    Dorset
    Age
    66
    Posts
    10,108
    Rep Power
    41

    Re: Condenser of an air conditioner

    Calculating Motor Speed:

    A squirrel cage induction motor is a constant speed device. It cannot operate for any length of time at speeds below those shown on the nameplate without danger of burning out.

    To Calculate the speed of a induction motor, apply this formula:

    Srpm = 120 x F
    P
    Srpm = synchronous revolutions per minute.
    120 = constant
    F = supply frequency (in cycles/sec)
    P = number of motor winding poles

    Example: What is the synchronous of a motor having 4 poles connected to a 60 hz power supply?

    Srpm = 120 x F
    P
    Srpm = 120 x 60
    4
    Srpm = 7200
    4
    Srpm = 1800 rpm




    With thanks to http://www.elec-toolbox.com
    Last edited by Brian_UK; 20-04-2012 at 11:07 PM.
    Brian - Torquay, Devon, UK
    I have to stop saying "how stupid can you be?" to my co-workers.
    They're starting to take it as a challenge...

    BASIC MAINTENANCE. If it doesn't move and it should then use WD40. If it moves but it shouldn't then use Duct Tape.

  5. #5
    Join Date
    Feb 2012
    Location
    Bolivia
    Posts
    270
    Rep Power
    4

    Re: Condenser of an air conditioner

    And what is the synchronous speed of a 4 pole motor at 59 58 57 56 ... hz, see the variable speed device here?

    In order not to burn it voltage must change too.

    What year is your bibliography ... or is it black humor?
    Last edited by aramis; 21-04-2012 at 07:29 AM.

  6. #6
    Join Date
    Apr 2012
    Location
    China
    Age
    29
    Posts
    6
    Rep Power
    0

    Re: Condenser of an air conditioner

    At first, thanks for your answers. And sorry, that I said steam and not vapor. (I'm german and maybe my english is not so good)
    But I always thought, that the heat transmission coefficient is konstant? Because it depends on the size and material and so on of the condenser?

    but if I set them, I still don't know how to calculate it. Wherefrom does the fan know how much massflow is needed just because of the pressure. At the beginning the refrigerant is overheated and to bring the refrigerant to the condensing temperature also needs heatflow.

    mfg Chander.

  7. #7
    Join Date
    Feb 2012
    Location
    Bolivia
    Posts
    270
    Rep Power
    4

    Re: Condenser of an air conditioner

    German! … Are you from Qingdao?

    Supposing you don’t vary the conditions inside the condenser nor ambient conditions, and the fan speed is constant then the outer film coefficient is constant you have a constant global heat transfer coefficient.

    But if you vary the fan speed, you are changing the outer film heat transfer coefficient and therefore changing the global heat transfer coefficient, everything else remaining constant.

    The fan controller will tell the fan to increase speed if the pressure increases and slow down if it lowers.

    If the pressure increases inside the condenser means that not enough refrigerant is changing state because of low heat flow. Increasing the fan speed will change the global heat transfer coefficient and increase the condensation inside which will lower the pressure.

  8. #8
    Join Date
    Apr 2012
    Location
    China
    Age
    29
    Posts
    6
    Rep Power
    0

    Re: Condenser of an air conditioner

    Qingdao? No, I'm not. I'm a german Student and make a Project in a chinese University in Hangzhou.

    That the heat transfer coefficient changes, I don't understand completely, but I'll look for more literature to understand that.

    But if it is controlled just with the pressure, how is it ensured, that all the refrigerant is liquid? I mean, there is no jump from vapor to liquid.
    Example:
    Input pressure 15bar; Input temperature 70C -> 15C overheated.
    I need a heatflow of Q = m*cp*15+m*dH (m:massflow, cp:specific heat capacity, dH:condensation enthalpy) = 21000W
    2nd
    Input pressure 15bar; Input temperature 80C->25C overheated.
    pressure=15bar -> the fan has to bring a heatflow of 21000W
    But with just 21kW not all is condensed.
    That is my problem, what I can't understand.

    But it is right, that it is only cooled so much, that everything is condensed? and if I need an undercooling, I also need an additional fan?

  9. #9
    Join Date
    Feb 2012
    Location
    Bolivia
    Posts
    270
    Rep Power
    4

    Re: Condenser of an air conditioner

    Qingdao? No, I'm not. I'm a german Student and make a Project in a chinese University in Hangzhou.
    Too bad! Ive been told they make the best beer in the world there!

    That the heat transfer coefficient changes, I don't understand completely, but I'll look for more literature to understand that.
    Yes you do that and check the relation between the global heat transfer coefficient with the externnal forced convection film coefficient.

    But if it is controlled just with the pressure, how is it ensured, that all the refrigerant is liquid? I mean, there is no jump from vapor to liquid.
    Simple! This control does not ensure that!

    Example:
    Input pressure 15bar; Input temperature 70C -> 15C overheated.
    I need a heatflow of Q = m*cp*15+m*dH (m:massflow, cp:specific heat capacity, dH:condensation enthalpy) = 21000W
    2nd
    Input pressure 15bar; Input temperature 80C->25C overheated.
    pressure=15bar -> the fan has to bring a heatflow of 21000W
    But with just 21kW not all is condensed.
    That is my problem, what I can't understand.
    I cant check your calculations because you did not post the refrigerant used but I think you made a mistake the 21000W should be higher!

    The condenser will only condense if the cooling media temperature is below the saturation temperature of the refrigerant inside. Without this condition it will never condense! This condition must be met even at the fans low RPM and at any input conditions of the refrigerant.

    If the inner volume of the condenser divided by the instantaneous mass in it does not equal the specific volume of the gas you are pumping in, it follows the instantaneous pressure MUST go up! This will happen long before you output gas because of the low inner volume of the condenser.

    If the pressure goes up, the fan will increase RPMs and but if the condenser is not able to condense all in this new situation at maximum RPM then you have a small condenser or a slow fan.

    In the example you give using R22 and supposing your pressures are gage:
    70C 15 barg => 17.27 L/kg, for simplicity suppose 17,27 L is the inner volume so you have 1kg in and it was just giving you saturated refrigerant.

    80C 15 barg => 18,15 L/kg in order to get this inside the condenser you necessarily have to raise your pressure to such that the new specific volume will be 17.27 so the pressure must rise from 15 barg to 15.681 bar. This is so because instantaneously you cannot change the mass of the condenser it must remain at 1kg when you are trying to push the new refrigerant in, that push needs a higher pressure.

    What is physically giving you this pressure raise is the pressure drop of the condenser which must increase if you keep flow steady but with more dense refrigerant going trhough!

    But it is right, that it is only cooled so much, that everything is condensed? and if I need an undercooling, I also need an additional fan?
    Dont understand the first question.

    If your system needs more subcooling than your condenser gives you then: yes you may need another heat exchanger with another fan to subcool or feed your TEV from above or several other solutions.

  10. #10
    Join Date
    Apr 2012
    Location
    China
    Age
    29
    Posts
    6
    Rep Power
    0

    Re: Condenser of an air conditioner

    In the example you give using R22 and supposing your pressures are gage:
    70C 15 barg => 17.27 L/kg, for simplicity suppose 17,27 L is the inner volume so you have 1kg in and it was just giving you saturated refrigerant.

    80C 15 barg => 18,15 L/kg in order to get this inside the condenser you necessarily have to raise your pressure to such that the new specific volume will be 17.27 so the pressure must rise from 15 barg to 15.681 bar. This is so because instantaneously you cannot change the mass of the condenser it must remain at 1kg when you are trying to push the new refrigerant in, that push needs a higher pressure.
    How do you get the 17.27L/kg?
    rho=p/(R*T)
    rho=density [kg/m]
    p=pressure [Pa]
    R=specific gas constant [J/(kg*K)]
    T=temperature [K]

    rho=rho/1000 density [kg/L]
    x=17.27=1/rho [L/kg]
    is this way right?
    So, I can't assume, that the pressure keeps constant (like 15bar, 70C and 15bar, 80C) right? I thought I use the pressure output from the compressor. (But this is not the problem now).

    For my example I used R134a. And I made a mistake. The fan doesn't bring the heatflow of 21kW (just a value) it brings a massflow of x(I really don't know).
    But maybe because of the pressure, I also know the input temperature? and than I know how much Heatflow is needed?


    But it is right, that it is only cooled so much, that everything is condensed? and if I need an undercooling, I also need an additional fan?
    I mean, afte the condenser the refrigerant is on the saturated liquid line? also without undercooling. And if I want to cool it more down, I need a second heat exchanger?


    Another comment
    Qingdao? No, I'm not. I'm a german Student and make a Project in a chinese University in Hangzhou.
    Too bad! Ive been told they make the best beer in the world there!
    I think it is not the best in the world, but the best in China. Because (I won't brag, but my chinese friends told it to me) it is made with german technology. So, it has a little bit more alcohol and tastes better than the others here.

  11. #11
    Join Date
    Apr 2012
    Location
    China
    Age
    29
    Posts
    6
    Rep Power
    0

    Re: Condenser of an air conditioner

    If i understood it right, I could make it this way
    Q_needed=m*(cp_gas*(T_in-T_condensation)+dH) (Refrigerant)
    The T_condensationt do I get from the pressure.

    Now I have the equations
    Q=m*cp*(T_out-T_in)
    cp=specific heat capacity of air [J/(kg*K)]
    T_out=output temperature of the air behind the condenser [K]
    T_in=input temperature of the air infront of the condenser [K]
    But the output temperature of the Air is unknown.
    and

    Q=k*A*dTm=k*A*(dTm1+dTm2)
    k=heat transfer coefficient (changes everytime) [W/(m*K)]
    A=surface of the condenser [m]
    dTm=logarithmic different in temperature

    for dTm I have uploaded a picture.
    I made it in two parts
    dTm1=((Tc-Tw1)-(Tc-Tw2s))/ln((Tc-Tw1)/(Tc-Tw2s))
    dTm2=((Tsup-Tw2)-(Tc-Tw2s))/ln((Tsup-Tw2)/(Tsup-Tw2s))

    Tsup is the input temperature of the refrigerant, Tc is the condensationtemperature, Tw1 input temperature of the air, Tw2s temperature of the air at the beginning of the condensation process, Tw2 output temperature of the air
    But the temperatures Tw2s and Tw2 are both unknown.

  12. #12
    Join Date
    Feb 2012
    Location
    Bolivia
    Posts
    270
    Rep Power
    4

    Re: Condenser of an air conditioner

    How do you get the 17.27L/kg?
    Using NISTs Standard Reference Database 23, Refprop version 9

    rho=p/(R*T)
    rho=density [kg/m]
    p=pressure [Pa]
    R=specific gas constant [J/(kg*K)]
    T=temperature [K]
    rho=rho/1000 density [kg/L]
    x=17.27=1/rho [L/kg]
    is this way right?
    No. You’re too close to saturation for ideal gass law to be valid. It would be better if you use Van Der Vaals but much better to use a superheated table or programs like “Refrigeration Utilities” from the Department of Energy Engineering of the Technical University of Denmark (DTU). Or programs like EES that have refrigerant properties programmed.

    So, I can't assume, that the pressure keeps constant (like 15bar, 70C and 15bar, 80C) right? I thought I use the pressure output from the compressor. (But this is not the problem now).
    Right! Condensing pressure goes up and down depending on compressor discharge conditions (as it affects condenser input conditions), mass flow and cooling fluid temperature or more precisely the rate of condensation inside the condenser, and condenser pressure drop too.

    For my example I used R134a. And I made a mistake. The fan doesn't bring the heatflow of 21kW (just a value) it brings a massflow of x(I really don't know).
    I hope x is not mass flow of R134a but air.

    The problem is more complicated than you think, it’s better if you estimate two heat transfer coefficients for minimum and maximum conditions and suppose a linear correlation (really not linear but a plane or flat surface) between the variables. You will be able to draw some conclusions. You may try to read this:
    http://www.ideals.illinois.edu/bitst...9696/TR007.pdf

    But maybe because of the pressure, I also know the input temperature? and than I know how much Heatflow is needed?
    Input temperature can be estimated calculating discharge temperature of the compressor and estimating losses in the discharge line.

    But it is right, that it is only cooled so much, that everything is condensed?
    Normal working condensers will try to condense everything and they are designed to do so, but manufacturers of coils state a nominal air flow to achieve this goal. It is important to respect the minimum flow. If you are a more sophisticated manufacturer like VRV then you need a maximum and minimum air flow for the condenser to do its job.

    Not respecting the minimum air flow usually causes distribution problems in the condenser and they will not condense all.

    and if I need an undercooling, I also need an additional fan?
    Normally condensers are designed to some degree of subcooling. This varies between manufacturers because the larger the subcooling the larger and more expensive the condenser.

    To make sure you have the subcooling you need at all working conditions it is much better to use a subcooler that may use its own fan, but you can also have a subcooling section within the same condenser that use the same condenser fan.

    I mean, afte the condenser the refrigerant is on the saturated liquid line?
    Yes, the correct point to place a subcooler would be at the exit of the receiver. Condensers with subcooling section exit to the receiver, then from the receiver back to the subcooling section and exit to the rest of the system.

    also without undercooling. And if I want to cool it more down, I need a second heat exchanger?
    Excesive subcooling is also bad for the system it causes unstable control from the TXV. You may use EXV with better control though.


    I think it is not the best in the world, but the best in China. Because (I won't brag, but my chinese friends told it to me) it is made with german technology. So, it has a little bit more alcohol and tastes better than the others here.
    Qingdao is the old Tsingdao which was a German colony until the Japs invaded it during WWI. It was renamed Qingdao when the Chinese recovered it.
    Last edited by aramis; 28-04-2012 at 12:11 AM. Reason: Spelling, spelling and more spelling. Who invented spelling?

  13. #13
    Join Date
    Apr 2012
    Location
    China
    Age
    29
    Posts
    6
    Rep Power
    0

    Re: Condenser of an air conditioner

    I have to write an own program, so I can't use another.
    If I use Van der Waals
    n*R*T=(p+(n^2*a)/V^2)*(V-n*b)
    n: amount of substance
    R: universal gas constant
    T: temperature
    p: pressure
    a/b: coefficients
    V: volume of the condenser

    I don't know the temperature (I want to calculate), but also not the amount of substance. So, I have one equation with two unknown variables.

  14. #14
    Join Date
    Feb 2012
    Location
    Bolivia
    Posts
    270
    Rep Power
    4

    Re: Condenser of an air conditioner

    You do know the average temperature (of condensation) and the average pressure this means you know the specific volume of the refrigerant in the condenser and that is all you need.

    Yo have much better results with Peng Robinson Stryjec and Vera's equation of state. You never mentioned the refrigerant, for example if it were R404a you find it here: http://www2.dupont.com/Refrigerants/...o_prop_eng.pdf

    Happy programming!
    Well, did anybody ever dream of calling Aramis a coward? No, certainly not!

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •