Pressure and flow changes 22mm to 15mm and back

In article , Richard writes

If you want to keep it in the O-level physics domain then think of resistors and you have a direct parallel (or series :-). Twice the length of 15mm will have twice the resistance so if you're worried about flow restriction then the shorter the length of 15mm the better.

What's the application?

Thanks.... Just to feed garden hose etc. So flow rate is directly proportional to the length of the reduced cross section and hence running in increased cross section, even post

Hi,

Using the table at the end of this document:

it looks like 1m of 15mm is equivalent to 1.63m of 22mm

There's also tables of equivalent pipe lengths for different fittings.

cheers, Pete.

Sorry that's a broken link for me at least.

I would say that 5m of 22mm pipe offers the same restriction as 1m of

So much depends on the supply pressure as to waht you can get out of the mains.

Resistance is certainly proportional to the length of the reduced cross section so reducing that length should be of benefit over doing it all at the reduced diameter. Watch out for the proviso that Ed mentioned, that you may be fighting against the resistance in your water company supply pipe.

I certainly believe that keeping the hose short and maximising the length of full bore fixed piping is a good idea. Make sure the pipe is below the freezing depth or insulate it well.

or

Where abouts? I couldn't see anything that gave a straightforward comparison between pipe sizes, I saw those rather scary low resolution graphs but didn't get a lot from them.

I don't think 1:1.63 sounds right, the area ratio between 15 and 22mm is

Yep, got that, Appendix H, I reckon that gives the extra equivalent length for a given fitting in the same pipe size but can't see anything to give equivalence between pipe sizes.

I was looking at the flow velocity and not volumetric flow

So 1m of 15mm gives the same pressure drop as 1.63m of 22mm at the same _flow velocity_.

(The water in the 22mm pipe flowing just over two times as slowly and the hard to read chart being linear)

Very true indeed.

cheers, Pete.

The message

from Pete C contains these words:

There is a table in my idiots (beginners) guide to central heating that tabulates heat capacity, flow and head loss for 15mm, 22mm and 28mm pipes. Seeing this is aimed at central heating use the limit is the maximum recommended flow rate of 3 feet/sec.

At the maximum flow of 105 gallons/hour for 15mm the head loss is 0.90 inches water gauge/foot run while for 22mm it is only 0.13 for the same flow rate. But the relationship is not linear.

Flow rate head loss 15 22

=A0 =A0 =A0 22

True, had another look last night and found pressure drop is proportional to (L/D) x V**2

where L is length, D diameter and V velocity.

This gives about 7:1 for 15mm vs 22mm, which agrees with 9:1.3

That said there is a change between turbulent and laminar flow, but the latter only makes a large difference at low flow velocity.

cheers, Pete.

There is something wrong with this simple analogy surely?

Consider an installation with 1 metre of 22mm pipe. If you add a section of 15mm pipe, say 1mm long (e.g. in the form of a 15mm bore washer at the end of the 22mm pipe) then using this analogy the total flow resistance only goes up by the same amount as increasing the original 22mm pipe by 5mm! Now think about flow restrictors in taps.

CRB

So you also need to consider the internal diameter and length of the hose, if it's flow out of the hose that is important to you.

MBQ

I'll be amazed if you can get laminar flow in your central heating pipes. Too fast, too rough, and too long.

Andy

No mention in the o/p's post of inserting 15mm i/d washers in his pipes. For any sensible pipe length of restricted diameter the resistor is a very good analogy.

Yes, of course - but you have missed the point. Part of the o/p's question was "Or does the fact that it has to pass through a constriction regardless of for how long, dominate the effect on flow/ pressure". I am merely pointing out that a pedantic application of your simple analogy would lead to the wrong conclusion. I would be interested to know how to work out the flow rate in the example I quoted - and I don't think it could be achieved by your simple analogy.

CRB

Yikes! Never thought it would be so complicated! Thanks for all the answers, most useful. WRT the last post re the washer, I was indeed curious as to what would happen with a 'pinch' to a pipe that reduced the i.d. but confess that it isnt really why I asked the question. Bottom line is use 22mm wherever you can to squeeze the best pressure and flow rate out of what youve got I guess, but expect the results to be great if you have rubbish pressure/flow to start with - its just damage limitation! I was surprised that pressure was so affected - thought only flow took a hit with smaller pipes?? Cheers.

Agreed, and the same for supply pipes too.

I disagree with the any tables that suggest pressure drop is proportional to flow rate. Hence why I suggest that 22mm is five time less restriction (than 15mm) for the same flow rate.

I didn't say you could, did I ? :)

Just that the formula I quoted won't be accurate at low flow velocity.

cheers, Pete.