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  1. #1
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    Speed in evaporator tube



    Someone able to calculate these for me please?
    I personally know who can do this - I can't - but i want to make this a general question.

    What will be the theoretical speed through an 1/2" line - length 15, 30 and 70 m - of an evaporator, 2 kW, evaporating at 0°C for R410a and R407c, condensing at 35°C and 40°C and a SC of 10K and second case 20K, Sh of 5K?

    We start with a saturated gas/liquid mixture in the beginning and ending with a superheated gas.
    Last edited by Peter_1; 31-12-2010 at 08:13 AM. Reason: Forgot most important data to add


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    Re: Speed in evaporator tube

    Keeping with the theme of the question, we are missing some data, to calculate the next stage!
    Data can be supplied in a number of ways!!!!!!!

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    Re: Speed in evaporator tube

    Edited, indeed, forgot some data, in fact most important data. Sorry
    Reason for my question: a colleague/friend is installing DX heatpumps and he's using for this copper in lengths of 70m. I was suggesting that this is far too long and should be something in the range of 30 m. I say he could have pressure drops of more than 1 bar and he didn't believe this.
    Last edited by Peter_1; 31-12-2010 at 08:16 AM.
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    Re: Speed in evaporator tube

    Quote Originally Posted by Peter_1 View Post
    Someone able to calculate these for me please?
    I personally know who can do this - I can't - but i want to make this a general question.

    What will be the theoretical speed through an 1/2" line - length 15, 30 and 70 m - of an evaporator, 2 kW, evaporating at 0°C for R410a and R407c, condensing at 35°C and 40°C and a SC of 10K and second case 20K, Sh of 5K?

    We start with a saturated gas/liquid mixture in the beginning and ending with a superheated gas.
    To try the simple approach, first.

    The evaporator & condenser designers will probably use something along these lines:
    1. Liquid : v,max ~ 1.5 m/s
    2. Vapour : v,max ~ 15-25 m/s.

    The major constraint is allowable pressure drop, with vapour erosion the next possibility.
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    Re: Speed in evaporator tube

    Quote Originally Posted by Peter_1 View Post
    Edited, indeed, forgot some data
    Reason for my question: a colleague/friend is installing DX heatpumps and he's using for this copper in lengths of 70m. I was suggesting that this is far too long and should be something in the range of 30 m. I say he could have pressure drops of more than 1 bar.
    Still a bit more data required, if you want to know the velocity, your need to area, density and something else?

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    Re: Speed in evaporator tube

    m' = r*Ac*v

    Where :
    m' = mass flowrate [kg/s]
    r = density (vapour, or liquid, or 2-phase) [kg/m3]
    Ac = cross-sectional area of pipe (ID-based) [m]
    v = velocity [m/s]
    You can work backwards to find 'v', if you have all the other data.
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    Re: Speed in evaporator tube

    Carrying on now we have more data,
    your evaporating pressure, is this your outlet pressure leaving the pipe.

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    Re: Speed in evaporator tube

    Peter_1, do you need to know results or theoretical calculations?

    Quote Originally Posted by mad fridgie View Post
    Still a bit more data required, if you want to know the velocity, your need to area, density and something else?
    Also pipe's location
    Attached Images Attached Images
    In some places will have to think ...

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    Re: Speed in evaporator tube

    Quote Originally Posted by mad fridgie View Post
    Carrying on now we have more data,
    your evaporating pressure, is this your outlet pressure leaving the pipe.
    Indeed MadFridgie. I know that a phenomena will happen in the tubes where liquid will be trapped between gas and vice-versa.
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

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    Re: Speed in evaporator tube

    Quote Originally Posted by Peter_1 View Post
    Indeed MadFridgie. I know that a phenomena will happen in the tubes where liquid will be trapped between gas and vice-versa.
    So you now need to work out the mass flow of each refrigerant/condition to meet your 2 KW, use PH chart, whilst looking at the chart look up the density of the refrigerant at the pipe exit conditions, then calculate as per DesA formular (maximum velocity/speed)
    Or Just use Coolpack !!!!!!!
    Last edited by mad fridgie; 31-12-2010 at 09:12 AM.

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    Re: Speed in evaporator tube

    Quote Originally Posted by Aik View Post
    Peter_1, do you need to know results or theoretical calculations?


    Also pipe's location
    Only 1 straight pipe Aik where the give evaporating pressure is the outlet pressure.

    My colleague is now measuring 0°C (converted from pressure of 3.5 bar) for R407c (!) at the compressor inlet and he's thinking that the whole circuit of 14 kW (7 circuits of 2 kW) is evaporating at 0°C.
    If there should be no pressure drop, then the whole circuit starts at -6°C (+/-)
    Due to a possible pressure drop of 1 bar for R407c, the whole circuit stays at 0°C.

    I advised him to reduce his lines to 30 m and that this should benefit the COP of the compressor due to the lower DP over the copper.

    If I calculate Dp for a 1/2" line of 70m , R407c for the given conditions, then this gives me 1.2 bar DP (8.7K) for a gas line and a neglectable DP for the 1/2" liquid line.
    Last edited by Peter_1; 31-12-2010 at 09:18 AM.
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    Re: Speed in evaporator tube

    Quote Originally Posted by mad fridgie View Post
    So you now need to work out the mass flow of each refrigerant/condition to meet your 2 KW, use PH chart, whilst looking at the chart look up the density of the refrigerant at the pipe exit conditions, then calculate as per DesA formular (maximum velocity/speed)
    Or Just use Coolpack !!!!!!!
    The condition of the gas constantly changes while the refrigerant flows from the beginning to the end of the pipe. So I think we just can't assume outlet nor inlet conditions to calculate speed in this line. It's somehow an mathematical expression that needs to be used.
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

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    Re: Speed in evaporator tube

    Quote Originally Posted by Peter_1 View Post
    The condition of the gas constantly changes while the refrigerant flows from the beginning to the end of the pipe. So I think we just can't assume outlet nor inlet conditions to calculate speed in this line. It's somehow an mathematical expression that needs to be used.
    At those conditions the speed at the exit will remain constant (for each refrigerant and set of conditions)
    Your speed will increase from the start to the end, due to change in density.
    Have you worked out the amount of of surface area you require,
    So if I understand it correctly, you have 7 circuits at 70 meters, you want lets say 14 circuits at 35 meters (same surface area, reduced pressure drop) but could reduced refrigerant heat transfer co-efficient

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    Re: Speed in evaporator tube

    Q=m'*dH - capacity [kW]
    m' - mass flowrate [kg/s]
    dH - difference of enthalpies of evaporating and suction refrigerant [kJ/kg]
    Acording to this formula you can calculate m'. After with help DesA's formula you can calculate velocity. Entalpy you can find using CoolPack.

    dP=lamda*(l/d)*(r*v^2)/2 - pressure drop on straight tube [Pa]
    lamda - friction coefficient, for copper tubes lamda=0.03
    l - tube's length [m]
    d - inner diameter of tube [m]
    r - density (vapour, or liquid, or 2-phase) [kg/m3]
    v - velocity [m/s]

    P.S. for more accuracy for 10^5< Re < 10^8
    lamda=0.0032+(0.221/(Re^0.237))
    Re - Reynolds number
    Last edited by Aik; 31-12-2010 at 10:40 AM. Reason: correction spelling errors
    In some places will have to think ...

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    Re: Speed in evaporator tube

    See Spread Sheet attached in message further down the thread....
    Last edited by DTLarca; 31-12-2010 at 11:21 PM.
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    Re: Speed in evaporator tube

    Quote Originally Posted by mad fridgie View Post
    At those conditions the speed at the exit will remain constant (for each refrigerant and set of conditions)
    Your speed will increase from the start to the end, due to change in density.
    Have you worked out the amount of of surface area you require,
    So if I understand it correctly, you have 7 circuits at 70 meters, you want lets say 14 circuits at 35 meters (same surface area, reduced pressure drop) but could reduced refrigerant heat transfer co-efficient
    @Mad Fridgie, indeed, the speed will increase but I can imagine this will be not a linear function. So, that becomes a little bit, in fact a lot of a gray zone for me.

    What I was saying was twofold:
    1. sending 14 kW through 7 lines of 70 m gives a pressure drop of +/- 1 bar and reduces COP
    2. Spreading the 490 m over 14 coils of 35 m will increase system performance.
    But this is something I feel on my elbow but I can't prove t with figures.
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    Thumbs up Re: Speed in evaporator tube

    Quote Originally Posted by mad fridgie View Post
    So you now need to work out the mass flow of each refrigerant/condition to meet your 2 KW, use PH chart, whilst looking at the chart look up the density of the refrigerant at the pipe exit conditions, then calculate as per DesA formular (maximum velocity/speed)
    Or Just use Coolpack !!!!!!!
    I can calculate it manually with the refrigerant properties out of saturated tables or superheated valeus via the log p/h but then I'm starting with conditions which doesn't change that much while transporting the gas or liquid along the lines.
    In the evaporator tubes, we have a very rapid and intense phase change an a huge expanding of the liquid to its gaseous state. So the characteristics changes continuously along the line of 70 m.
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

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    Re: Speed in evaporator tube

    A gift for Peter. Happy New Year for 2011.



    'Natural' circuit. 3c.



    7c. 2 unlinked tubes.



    14c. 2 unlinked tubes.



    'Natural' circuit. 19c.
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    Re: Speed in evaporator tube

    desA, first of all, thanks for your help - as usual. For you and all the other colleagues, a Happy NewYear (in some parts of the world, it's already Newyear I guess)

    desA, Let's take your coil consisting of 7 tubes, where the total coil DP is 7.66 kPa (or 0.076 bar...I'm used to work with bar)
    Because the total pressure = 1/partial pressure losses (like a parallel connection of electrical resistances), can I then say that the DP over 1 tube of the coil is then 7 times 7.66 kPa or 53.62kPa (= 0.53 bar)?
    Splitting the same capacity over 14 circuits reduces the DP with a factor of 7.
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

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    Re: Speed in evaporator tube

    Quote Originally Posted by DTLarca View Post
    Here's a spread sheet I produced, I'm thinking about 6 years ago maybe, for another discussion we had here.

    You could just change the numbers for your conditions.
    Thanks M..., DTLarca, I will try to figure out how this works. As usual if you made something, seems very impressing and professionally made. Something for tomorrow or Sunday.
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    Re: Speed in evaporator tube

    Quote Originally Posted by Peter_1 View Post
    desA, first of all, thanks for your help - as usual. For you and all the other colleagues, a Happy NewYear (in some parts of the world, it's already Newyear I guess)

    desA, Let's take your coil consisting of 7 tubes, where the total coil DP is 7.66 kPa (or 0.076 bar...I'm used to work with bar)
    Because the total pressure = 1/partial pressure losses (like a parallel connection of electrical resistances), can I then say that the DP over 1 tube of the coil is then 7 times 7.66 kPa or 53.62kPa (= 0.53 bar)?
    Splitting the same capacity over 14 circuits reduces the DP with a factor of 7.
    As a first approximation, we could use the Bernoulli equation across each circuit.

    dP,14/dP,7 ~ (Ac,7/Ac,14)^2 = (Ac,7/(2*Ac,7))^2 = 1/4

    Backcheck:
    dP,14/dP,7 = 0.0696/0.248 = 0.2806 ~ 0.25 = 1/4

    The difference probably comes from his bend calculations, or the missing tubes. Each parallel pass in the evap same will have 'roughly' the same dP, since the system splits & meets again. Practically though, the end manifolds will really determine the true velocity distribution across each circuit. Also probably have to factor in the 2x length of the 7c versus the 14c - haven't gone through the hand calcs fully. You could plot the data I provided with circuits on horizontal axis & dP on vertical axis. Then curve-fit & estimate from there...
    Last edited by desA; 31-12-2010 at 03:52 PM.
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    Re: Speed in evaporator tube

    DTLArca, Where can I find different viscosities of refrigerant at different conditions?

    In attachment what I calculated but don't know if it's correct 85 kPa pressure drop.
    Attached Files Attached Files
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    Re: Speed in evaporator tube

    Happy New Year.
    Where can I find different viscosities of refrigerant at different conditions?
    Solkane will help you

    http://www.solvay-fluor.com/docroot/...s/download.htm
    In some places will have to think ...

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    Re: Speed in evaporator tube

    Quote Originally Posted by Aik View Post
    Q=m'*dH - capacity [kW]
    m' - mass flowrate [kg/s]
    dH - difference of enthalpies of evaporating and suction refrigerant [kJ/kg]
    Acording to this formula you can calculate m'. After with help DesA's formula you can calculate velocity. Entalpy you can find using CoolPack.

    dP=lamda*(l/d)*(r*v^2)/2 - pressure drop on straight tube [Pa]
    lamda - friction coefficient, for copper tubes lamda=0.03
    l - tube's length [m]
    d - inner diameter of tube [m]
    r - density (vapour, or liquid, or 2-phase) [kg/m3]
    v - velocity [m/s]

    P.S. for more accuracy for 10^5< Re < 10^8
    lamda=0.0032+(0.221/(Re^0.237))
    Re - Reynolds number
    Sorry, it's for single-phase substance. For two-phase states it must be integration is over velocity and density...
    In some places will have to think ...

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    Re: Speed in evaporator tube

    Quote Originally Posted by Peter_1 View Post
    DTLArca, Where can I find different viscosities of refrigerant at different conditions?

    In attachment what I calculated but don't know if it's correct 85 kPa pressure drop.
    I'll modify the spread sheet a little later so that it can be used for this particular purpose.

    I think I generated all my refrigerant data using coolpack or Klea Calc - I forget - will have a look later.
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    Re: Speed in evaporator tube

    Quote Originally Posted by desA View Post
    As a first approximation, we could use the Bernoulli equation across each circuit.

    dP,14/dP,7 ~ (Ac,7/Ac,14)^2 = (Ac,7/(2*Ac,7))^2 = 1/4

    Backcheck:
    dP,14/dP,7 = 0.0696/0.248 = 0.2806 ~ 0.25 = 1/4

    The difference probably comes from his bend calculations, or the missing tubes. Each parallel pass in the evap same will have 'roughly' the same dP, since the system splits & meets again. Practically though, the end manifolds will really determine the true velocity distribution across each circuit. Also probably have to factor in the 2x length of the 7c versus the 14c - haven't gone through the hand calcs fully. You could plot the data I provided with circuits on horizontal axis & dP on vertical axis. Then curve-fit & estimate from there...
    desA, was my assumption then correct about the dp over 1 circuit (7 times total DP over the coil)?
    I think your explanation was about the change from a 7 to 14 circuit evaporator.
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

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    Re: Speed in evaporator tube

    Quote Originally Posted by Peter_1 View Post
    DTLArca, Where can I find different viscosities of refrigerant at different conditions?

    In attachment what I calculated but don't know if it's correct 85 kPa pressure drop.
    I think you have used a vapour density 1/10th of what it actually will be, so, your vapour velocity is of course 10 times faster than it actually will be. I get a total pressure drop of 0.2Bar along the 70m. Your liquid calcs look okay though. Have a look at the modified spread sheet attached...




    There are 4 sheets now:
    1. Copper Tube Dimension Choices
    2. Refrigerant and tube values converted to velocity if all liquid and then again velocity if all vapour and thereafter, based on average X (kg/kg) values for each 5th pipe length, a total pressure drop is calculated based on the implied proportions of liquid versus vapour.
    3. R410A values
    4. R407C values.
    But these calcs will be, seriously, estimates only. Very simple assumptions are being made.
    Attached Files Attached Files
    Last edited by DTLarca; 31-12-2010 at 11:17 PM. Reason: Edited mm to m in spread sheet then re-uploaded spread sheet.
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    Re: Speed in evaporator tube

    What is the heat source for the evap?

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    Re: Speed in evaporator tube



    Single circuit. 1c. dP/L=8.77 kPa/m ; dP=1896 kPa = 18.96 bar



    7c. 2 unlinked tubes. dP/L=0.248 kPa/m ; dP=7.66 kPa = 0.0766 bar



    14c. 2 unlinked tubes. dP/L=0.0696 kPa/m ; dP=1.07 kPa = 0.0107 bar

    ------------

    The useful feature here is to compare the dP/L values i.e. the pressure loss per meter of pipe run, in either the 1c, 7c, or 14c configurations.

    dP = (dP/L)*L ; where L=pipe length.
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    Re: Speed in evaporator tube

    Some good software:
    http://www.nist.gov/el/building_envi...d_software.cfm

    I think it wont tell you speed, but it will tell you pressure drop.

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    Re: Speed in evaporator tube

    Quote Originally Posted by mad fridgie View Post
    What is the heat source for the evap?
    The ground MadFridgie
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    Re: Speed in evaporator tube

    Your load is not steady state, so massive differences in working pressures occur, I now understand the need to know your velocity through out the evap, I am not sure what velocities you require to ensure that oil logging/filming is limited (when you are changing phase), maybe some others have this required data

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    Re: Speed in evaporator tube

    Quote Originally Posted by DTLarca View Post
    I think you have used a vapour density 1/10th of what it actually will be, so, your vapour velocity is of course 10 times faster than it actually will be. I get a total pressure drop of 0.2Bar along the 70m. Your liquid calcs look okay though. Have a look at the modified spread sheet attached...

    There are 4 sheets now:
    1. Copper Tube Dimension Choices
    2. Refrigerant and tube values converted to velocity if all liquid and then again velocity if all vapour and thereafter, based on average X (kg/kg) values for each 5th pipe length, a total pressure drop is calculated based on the implied proportions of liquid versus vapour.
    3. R410A values
    4. R407C values.
    But these calcs will be, seriously, estimates only. Very simple assumptions are being made.
    I think DTLarca enlightened a little bit my confusion. The entire length of 70 m was divided in 5 different sections where in each section we have each time a new 'mixture' due to the ever increasing gas phase (and decreasing liquid phase) of the refrigerant along the copper tube.
    But, I see a big difference between DTLarca and desA. I think I will make a practical setup for this and test this once.
    Last edited by Peter_1; 01-01-2011 at 11:13 PM.
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    Re: Speed in evaporator tube

    The info I quoted comes from a commercial evap coil design program. Their pressure drops seem credible.
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    Re: Speed in evaporator tube

    Quote Originally Posted by desA View Post
    The info I quoted comes from a commercial evap coil design program. Their pressure drops seem credible.
    To be not misunderstood, I don't doubt any of the posts here, this is all something which can't be found in common refrigeration books. This is very learning for me.
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    Re: Speed in evaporator tube

    With R410a the pressure drop on either case is pretty minimal, less than 1C on your sst, I would be more worried about oil logging, at these lower velocities and pipe volume (will the compressor run out of oil over time)

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    Re: Speed in evaporator tube

    Well, for the moment, these systems are running on R407c but we want to change this to R410a. We then can install smaller copper, 3/8" will then be our choice.
    But the R407c is a special case, especially if you should encounter a pressure drop of around 1 bar along the line . You then should measure the same temperature (not same pressure) at the end and beginning of the copper.
    My colleague is now measuring only at the outlet and he's measuring 4.5 bar ( equals 0°C ) and he's thinking that this is the same all over the line, from beginning till the end.
    I know think he's right and I'm wrong. I thought he should face much larger DP's over 70 m copper.
    To be 100% sure and to convince myself, I will make a practical setup in my evening classes and connected different lengths of copper to a speed regulated compressor.
    It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.

  38. #38
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    Re: Speed in evaporator tube

    You will have a bigger tempertaure difference with R407C, approx 2.5C.
    +2.5Cevap at inlet and 0C sst at outlet.

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    Re: Speed in evaporator tube

    Testing testing testing testing...
    Only the dogmatist says he will never change his mind. We all know that some of our opinions are wrong but none of us know which they are for if we did then they just wouldn't be our opinions. - JS Mill.

  40. #40
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    Re: Speed in evaporator tube

    Testing testing testing testing - am I being limited to the number of words I can post?
    Only the dogmatist says he will never change his mind. We all know that some of our opinions are wrong but none of us know which they are for if we did then they just wouldn't be our opinions. - JS Mill.

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    Re: Speed in evaporator tube

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    Only the dogmatist says he will never change his mind. We all know that some of our opinions are wrong but none of us know which they are for if we did then they just wouldn't be our opinions. - JS Mill.

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    Re: Speed in evaporator tube

    Thus - if you had 1bar pressure drop along a single length (70m + lots of bends) then you would have 0.25Bar pressure drop along two parallel 70m lengths. But then you would also have twice as much pipe surface with two 70m lengths. So you half the lengths each to 35m. Now your pressure drop will be 0.125Bar.
    Only the dogmatist says he will never change his mind. We all know that some of our opinions are wrong but none of us know which they are for if we did then they just wouldn't be our opinions. - JS Mill.

  43. #43
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    Re: Speed in evaporator tube

    Testing testing testing ...

    Testing new line

    Testing fractions 1/4 and 1/2 and 1/3

    Testing brackets ()

    Testing editing...

    Testing editing again...

    Cool, editing worked twice
    Last edited by DTLarca; 02-01-2011 at 01:07 PM.
    Only the dogmatist says he will never change his mind. We all know that some of our opinions are wrong but none of us know which they are for if we did then they just wouldn't be our opinions. - JS Mill.

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    Re: Speed in evaporator tube

    Testing testing testing...

    Testing quotes and moderators names...

    Quote Originally Posted by Peter_1 View Post
    I think DTLarca enlightened a little bit my confusion. The entire length of 70 m was divided in 5 different sections where in each section we have each time a new 'mixture' due to the ever increasing gas phase (and decreasing liquid phase) of the refrigerant along the copper tube.
    But, I see a big difference between DTLarca and desA. I think I will make a practical setup for this and test this once.
    Testing other users names...

    DesA

    Testing reply...

    DesA's coil calcs have bends - I assume?

    My spreadsheet can be altered with the addition of a 6th section of pipe length for superheating but then the superheated vapour's density and viscosity are also required.

    Testing...

    So far able to post some stuff but not able to go back and edit those posts.
    Only the dogmatist says he will never change his mind. We all know that some of our opinions are wrong but none of us know which they are for if we did then they just wouldn't be our opinions. - JS Mill.

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    Re: Speed in evaporator tube

    For 1/2" pipe add about 0.75 meters for every 180° U-Bend per attached image.
    Attached Images Attached Images
    Only the dogmatist says he will never change his mind. We all know that some of our opinions are wrong but none of us know which they are for if we did then they just wouldn't be our opinions. - JS Mill.

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