Pressure drop and dirty tubing
 

A quote from the study page,

"If all other variables are held constant, the pressure drop is proportional to 1/D5. This means that the
pressure drop at a given flow rate is very sensitive to the diameter. A diameter chance in our tube of 1/100
in. (at the limit of our measurability) would increase the pressure drop by about 14%." end quote

read the pdf here

This should be a factor in keeping a rad as clean as possible. Any more comments, I thought this could make an interesting discussion.

(re-post)

The surface roughness adds to the flow restriction, as per this formula:

f=0.2083 (100 / C) ^ 1.812 * (Q^1.852 / d^4.8655)

Where
f = friction loss per 100 feet in feet of water
C=roughness coefficient (Hazen-Williams factor)
Q=flow rate (gpm)
d=inside diameter of pipe (inches)

Source: http://www.ppfahome.org/pdf/pvcpipewaterspec.pdf

For PVC piping, a value of 150 is used for C. A steel pipe would have a C value of about 100, and a rusty one about 80.

[addition]
Smooth copper piping is calculated with a value of 140.

Well, I didn't mean to stop the conversation!

Dirty tubing will reduce flow. We could compare say, a 120 gph rig, with 1/2 inch ID tubing, using a factor of 150 (clean) and a factor of 80 (~dirty).