Alternative Pipe Diameter Calculation (?)

[ice_cold]

Hi there, the newbie has got a question again!

I am trying to determine the way to calculate the right size (diameter) for the pipes to be used
in the refrigeration system.

What i do is:
1- Draw the cycle on Danfoss Coolpack within defined parameters (Tevap , Tcond, SH, SC
Isentropic eff.) for a selected refrigerant.
2- From the cycle i pick the density value of the refrigerant at the start of suction line,
discharge line and liquid line.
3- Define a flow speed for each line mentioned above in "2". (How fast the refrigerant should go at each line? Please give your opinions)
4- Get the compressor mass flow rate from the manufacturer catalogue / software
5- Do a basic calculation for the pipe diameter that can handle the mass flow rate:

Mass Flow Rate = (Pipe_dia^2 * Pi * Flow speed) * (Density of the refrigerant at related point)


I am expecting your quotes and recommendations on this.


There are various documents that i found online, but they don't seem to follow a similar way.
Isn't there a fundamental way of doing this calculation?


Cheers

[samiam]

Hi Ice Cold,

To be honest - I utilsie the Ashrae tables for the regrigerants. Has work well for me.

Regards,

[US Iceman]

Reynolds number
friction factor from Moody diagram
Darcy-Weisbach for pressure loss


Velocity is not the only thing you need to know to select pipe diameters. You have two issues. oil return & pressure loss.

You need a minimum velocity to return oil (pay attention to either full load or part operation).

If the pressure loss is too high (high velocity) you can change the saturation temperatures of the refrigerant.

If it's liquid you can cause flashing without sufficient subcooing to offset the pressure loss and reduction in saturation temp's.

[desA]

I am trying to determine the way to calculate the right size (diameter) for the pipes to be used in the refrigeration system.

What i do is:
1- Draw the cycle on Danfoss Coolpack within defined parameters (Tevap , Tcond, SH, SC
Isentropic eff.) for a selected refrigerant.
2- From the cycle i pick the density value of the refrigerant at the start of suction line,
discharge line and liquid line.
3- Define a flow speed for each line mentioned above in "2". (How fast the refrigerant should go at each line? Please give your opinions)
4- Get the compressor mass flow rate from the manufacturer catalogue / software
5- Do a basic calculation for the pipe diameter that can handle the mass flow rate:

...
There are various documents that i found online, but they don't seem to follow a similar way.
Isn't there a fundamental way of doing this calculation?


Questions:
How will the 'system' operate?
How do you know the compressor mass flow-rate? (Remember, you have made assumptions on Te,at & Tc,sat).

[ice_cold]

Thank you everybody for your replies.
BTW i would like to make a little correction
Mass Flow Rate = (Pipe_dia^2 * Pi * Flow speed) * (Density of the refrigerant at related point) ---> should be Pipe_radius^2 instead of Pipe_dia^2

And desa i don't understand your first question
For your second question: The commercial calculation software of the compressor manufacturer gives the mass flow rate in kg/h

[shaman]

Nice in depth info

[Peter_1]

I use 5 to 8 m/s for suction, 10 m/s to 12m/s for vertical suctions, 0,5 m/s to 0,8 m/s liquid and 8 to 10 m/s for discharge.

Then calculate/ckeck afterwards if DP isn't too big for a certain line. If DP is to high, then some lines must be taken wider.
For suction and discharge, you can take lower speeds if your lines are flowing downwards.

On the other hand, you many times can use smaller lines because DP is far below max 1K rule (which I use and is also a German recommendation)

Look also once here for a discussion I started some time ago http://www.refrigeration-engineer.com/forums/showthread.php?t=18375

[ice_cold]

Thanks Peter_1 for your quote!