# Friction Loss in very small (~5 mm) tubing

• cwrm4
In summary, you are questioning the way in which the friction factor should be calculated for a nozzle circuit with a relatively low flow rate. Your approach will work, but the calculation appears to be incorrect.f

#### cwrm4

Hello - just discovered this forum, which has been a big help since I graduated ME 15 years ago, but never had an opportunity to practice "real" engineering...until now.

I am trying to develop a spreadsheet that models friction loss, using the derivation of Bernoulli's equation, over a nozzle circuit. Given the size of the tubing used, and a relatively low flow rate, I am questioning the way in which the friction factor should be calculated.

The circuit is constructed of smooth nylon tubing with an ID of .18 in (4.572 mm).

The average flow rate of a nozzle is .75 mL/s (varying of course with pressure).

Let's say I have ten nozzles at the end of a 100 m run of tubing. The velocity of the fluid in the tube would be very low, .46 m/s.

Assuming water at room temp, by my calculations the Reynolds number would be approximately 2 in this example. This Reynolds number would then give me a friction factor (64/Re) = 32.

For input to the pipe loss equation, can I correctly model the friction factor this way, with such a small Re? What if I only had 3 nozzles, and the Re was less than 1? Or should I be modeling this problem in a different fashion entirely?

Hello - just discovered this forum, which has been a big help since I graduated ME 15 years ago, but never had an opportunity to practice "real" engineering...until now.

I am trying to develop a spreadsheet that models friction loss, using the derivation of Bernoulli's equation, over a nozzle circuit. Given the size of the tubing used, and a relatively low flow rate, I am questioning the way in which the friction factor should be calculated.

The circuit is constructed of smooth nylon tubing with an ID of .18 in (4.572 mm).

The average flow rate of a nozzle is .75 mL/s (varying of course with pressure).

Let's say I have ten nozzles at the end of a 100 m run of tubing. The velocity of the fluid in the tube would be very low, .46 m/s.

Assuming water at room temp, by my calculations the Reynolds number would be approximately 2 in this example. This Reynolds number would then give me a friction factor (64/Re) = 32.

For input to the pipe loss equation, can I correctly model the friction factor this way, with such a small Re? What if I only had 3 nozzles, and the Re was less than 1? Or should I be modeling this problem in a different fashion entirely?

Your approach will work...however, your calculation appears to be incorrect. Check your numbers for velocity and Reynold's number.

With a quick hand calc I get an Re of about 457 and a friction factor of about 0.14 for water at 60 deg F.

CS