H
Heating-eng
Everybody seems to do there own thing when it comes to central heating lol.
BG training was very good years ago , but I would agree its not anymore.
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BG training was very good years ago , but I would agree its not anymore.
T
tamz
I would if i had the brass neck to tell everyone that the problem with their system is it needs a powerflush and there is more of an incentive for me to sell than the £5 and brownie points the service engineers get for selling the visit.
H
Heating-eng
Years ago if you missold power flushes you would get sacked, not sure it's the same now
P
plumman
If the water has to move at a couple of metres per second, or thereabouts, how much pressure is needed?
It's a simple question, but unfortunately there is no simple answer when the water is moving fast enough to be turbulent, as it is in house plumbing. Each case must be individually calculated. But don't despair - the calculation is very easy.
Experiments have shown that the Reynolds number of the flow is crucial - not surprisingly, the frictional resistance goes up with the speed of flow, and in quite a non-linear way.
Some textbooks tell you to begin by calculating the pipe’s “friction factor”, f. For smooth pipes this is really just another way of expressing the Reynolds number, and to keep things simple I show the relationship between the two in this equation and graph. The equation includes √f on both sides, and looks as though it ought to be impossible to solve. In fact, it's quite straightforward.
The trick is to begin by guessing a value for f (say, 0.01), putting this value (and Re) in the right-hand side, working out the value of the left-hand side, and hence finding f. This new value for f is closer to the actual value than the initial guess, so you plug it back into the right-hand side and do the calculation again . After a couple of iterations the answer is usually close enough to be useful. (By the way, the friction factor used by American engineers is for some reason four times bigger than this. But then, most things in America are bigger than they are in England.)
The graph appears to show that the “friction factor” decreases as the Reynolds number goes up. More speed giving less friction? Hardly likely, is it? In fact, that’s not what the graph is saying. The “friction factor” is purely a measure of how the pipe affects the flow, and as the water becomes more turbulent the pipe itself plays a smaller part in events.
The actual pressure difference P needed to push the water along depends much more on the speed of flow v, as you might expect. This is how to calculate P using the Darcy-Weisbach equation I mentioned earlier:
The pressure difference P needed to achieve a flow velocity v depends on the length L and diameter D of the pipe as well as the density of the fluid (ρ - about 1,000 kg/m[SUP]3[/SUP] for cold water) and, of course, f, the fiddle factor – sorry, “friction factor”.
It's a simple question, but unfortunately there is no simple answer when the water is moving fast enough to be turbulent, as it is in house plumbing. Each case must be individually calculated. But don't despair - the calculation is very easy.
Experiments have shown that the Reynolds number of the flow is crucial - not surprisingly, the frictional resistance goes up with the speed of flow, and in quite a non-linear way.
The trick is to begin by guessing a value for f (say, 0.01), putting this value (and Re) in the right-hand side, working out the value of the left-hand side, and hence finding f. This new value for f is closer to the actual value than the initial guess, so you plug it back into the right-hand side and do the calculation again . After a couple of iterations the answer is usually close enough to be useful. (By the way, the friction factor used by American engineers is for some reason four times bigger than this. But then, most things in America are bigger than they are in England.)
The graph appears to show that the “friction factor” decreases as the Reynolds number goes up. More speed giving less friction? Hardly likely, is it? In fact, that’s not what the graph is saying. The “friction factor” is purely a measure of how the pipe affects the flow, and as the water becomes more turbulent the pipe itself plays a smaller part in events.
The actual pressure difference P needed to push the water along depends much more on the speed of flow v, as you might expect. This is how to calculate P using the Darcy-Weisbach equation I mentioned earlier:
P
plumman
Theory is all very well, but let's see some actual numbers here. The kitchen sink is fed by 15mm feed pipe. How much pressure will it take to get hot water (at about 60[SUP]o[/SUP]C, say) moving out of the tap at 2 metres/second, and is this head achievable?
Start with the Reynolds number:
Re = Speed x Diameter x (Density / Viscosity)
We know that the speed is to be 2 m/sec, and the internal diameter of 15mm pipe is 13.6mm. From Table 1, (ρ/μ) for water at 60[SUP]o[/SUP] is about 3.1 x10[SUP]6[/SUP]. Then the Reynolds number in this case is:
Re = 2 x 13.6 x10[SUP]-3[/SUP] x 3.1 x10[SUP]6[/SUP] = 84,000
near enough. From the graph, this Re has a "friction factor" f of about 0.019. So in the pressure-difference equation
we know f (0.019) and v (2 m/s) and ρ (992.1) and D (13.6 mm). For now, assume that the length L is just 1.0 metre. Then the pressure difference (per metre) needed to get the water flowing is:
P = 0.019 x 2[SUP]2[/SUP] x (992.1 / 2) x (1.0 / 13.6 x10[SUP]-3[/SUP]) = 2,800 N/m[SUP]2[/SUP]
This means that each metre length of the 15 mm pipe must have a pressure difference of 2,800 N/sq.m. between its ends to push hot water though it at 2 m/sec. If the pipe is 10m long, the total pressure difference (that is, the head required) would be 28,000 N/sq.m. Or, to put it another way, the water will flow at 2 metres/second if the head happens to be exactly 28,000 N/sq.m.
If you're more comfortable with pressure expressed as the head in feet, the conversion factor is:
A head of 1 foot of water ≈ 3,000 N/sq.m.
So 28,000 N/sq.m. is about the same as a head of 9 feet (or 3m) of water. But if the head is not exactly this - and in practice, Sod's Law says it won't be - the water will flow at a different speed! More on this later.
Start with the Reynolds number:
Re = Speed x Diameter x (Density / Viscosity)
We know that the speed is to be 2 m/sec, and the internal diameter of 15mm pipe is 13.6mm. From Table 1, (ρ/μ) for water at 60[SUP]o[/SUP] is about 3.1 x10[SUP]6[/SUP]. Then the Reynolds number in this case is:
Re = 2 x 13.6 x10[SUP]-3[/SUP] x 3.1 x10[SUP]6[/SUP] = 84,000
near enough. From the graph, this Re has a "friction factor" f of about 0.019. So in the pressure-difference equation
P = 0.019 x 2[SUP]2[/SUP] x (992.1 / 2) x (1.0 / 13.6 x10[SUP]-3[/SUP]) = 2,800 N/m[SUP]2[/SUP]
This means that each metre length of the 15 mm pipe must have a pressure difference of 2,800 N/sq.m. between its ends to push hot water though it at 2 m/sec. If the pipe is 10m long, the total pressure difference (that is, the head required) would be 28,000 N/sq.m. Or, to put it another way, the water will flow at 2 metres/second if the head happens to be exactly 28,000 N/sq.m.
If you're more comfortable with pressure expressed as the head in feet, the conversion factor is:
A head of 1 foot of water ≈ 3,000 N/sq.m.
So 28,000 N/sq.m. is about the same as a head of 9 feet (or 3m) of water. But if the head is not exactly this - and in practice, Sod's Law says it won't be - the water will flow at a different speed! More on this later.
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