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Thread: Condenser of an air conditioner
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20-04-2012, 05:42 AM #1
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.
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20-04-2012, 07:04 PM #2
Re: Condenser of an air conditioner
http://www.google.co.uk/products/cat...d=0CIUBEPMCMAE#
or alternativly read the nameplate for the motor.
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20-04-2012, 07:45 PM #3
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.
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20-04-2012, 11:05 PM #4
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
PSrpm = 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 - Newton Abbot, Devon, UK
Retired March 2015
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21-04-2012, 07:23 AM #5
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.
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23-04-2012, 03:44 AM #6
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.
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23-04-2012, 09:31 PM #7
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.
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24-04-2012, 08:55 AM #8
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 70°C -> 15°C 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 80°C->25°C 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?
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24-04-2012, 07:55 PM #9
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 70°C -> 15°C 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 80°C->25°C 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.
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 fan´s 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:
70°C 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.
80°C 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?
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.
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25-04-2012, 03:53 AM #10
Re: Condenser of an air conditioner
In the example you give using R22 and supposing your pressures are gage:
70°C 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.
80°C 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.
rho=p/(R*T)
rho=density [kg/m³]
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, 70°C and 15bar, 80°C) 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?
Another comment
Qingdao? No, I'm not. I'm a german Student and make a Project in a chinese University in Hangzhou.
Too bad! I’ve been told they make the best beer in the world there!
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25-04-2012, 05:50 AM #11
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/(m²*K)]
A=surface of the condenser [m²]
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.
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28-04-2012, 12:07 AM #12
Re: Condenser of an air conditioner
How do you get the 17.27L/kg?
rho=p/(R*T)
rho=density [kg/m³]
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, 70°C and 15bar, 80°C) 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).
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?
But it is right, that it is only cooled so much, that everything is condensed?
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?
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?
also without undercooling. And if I want to cool it more down, I need a second heat exchanger?
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.Last edited by aramis; 28-04-2012 at 12:11 AM. Reason: Spelling, spelling and more spelling. Who invented spelling?
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02-05-2012, 06:07 AM #13
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.
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06-05-2012, 09:19 PM #14
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!