Pipe sizing problem

[habs73]

What i am given: There is a 75kw heater that circulates fluid to two heat exchangers. Fluid enters each heat exchanger at 200 Celsius and exits at 150 Celsius with a flow rate of 0.00197m^3/s. The Length of the pipe to each heat exchanger is 10 m with 5 elbows. The specific heat, density, and viscosity are given also and are 1.95 kj/kg K, 900kg/m^3, and 0.38E-3 N-s/m^2 respectively.

How can i use this information to determine the Diameter of the pipe necessary for this system. Any guidance is appreciated.

[admin]

You need either pressure drop in pipe or velocity.
Now, if you want to have 1 m/s velocity than you can calculate diameter for given flow rate and that velocity.
http://www.pipeflowcalculations.com/flowrate/index.htm
I get 50 mm internal diameter tube.

After that you should check pressure drop, to be sure that you have sufficient pressure on the start of pipe.
http://www.pipeflowcalculations.com/pre ... /index.htm

OR,

If you want to have certain pressure drop, than based on it you should calculate diameter for given flow rate.

Local resistance factor K should be calculated also at:
http://www.pipeflowcalculations.com/res ... /index.htm
I get for 2" tube and R/D=1.5 K=0.266 for one elbow, and for 5 five it is K=1.33

For your values I get pressure drop of 21 mbar, which is I think small pressure drop, so maybe you can go with 2 m/s velocity and get smaller pipe.

[habs73]

Thanks for the guidance, i just have one more question:

How do i know that the pressure drop is significant, for example what makes using a velocity of 2m/s better than 1 m/s in order to get a larger pressure drop.

[admin]

with 2 m/s you will get smaller pipe - which is cheaper, but you have to check pressure drop not to be too high, because if the pressure drop is to high you will have no flow at all.