Domestic CH circulator type pumps. The graphs giving flow rate use "head" as the other axis. What head is this? Highest to lowest point in the loop? Pump to lowest, pump to highest? Something completely differnt like flow resistance of the entire loop?
I'd like to have a rough guesstimate as to the flow rate the load controller on the woodburner is producing so I can get an idea of how much energy is being transfered from the woodburners boiler to the thermal store.
Hmm! I'd be interested in what the experts think. Lets hope it does not deteriorate to sniping though!
At first thought I would expect it to be the highest point that water has to be pumped to with respect to the lowest point (rather than anything to do with the pressure in an older vented system- second later thought!)
Unless you have very small bore pipe and looong horizontal runs, I'd expect the work needed to pump top to bottom to be greater than pipe friction.
Does the chart give any other fixed conditions such as pipe bore?
Bob
It is the hydraulic resistance of the system, expressed in metres head of water.
If you have the pump running in a test rig, you'd measure the water flow rate and the water pressure at the pump inlet and outlet. The pressure difference is equal to density x g x h, where h is the equivalent head of water.
A pipe circuit and radiators say, piped in 28mm tube would have one resistance at a certain flow rate. If you pipe the same circuit in
22mm tube, it will have a higher resistance at the same flow rate requiring a bigger pressure difference to shove the water around the loop. You'd typically need a bigger pump or the same pump running at a higher speed.
In a full-of-water circulating system, whether closed or open with an overflow pipe into the header tank, I would expect all the head (that is, the pressure produced by the pump) to be used in overcoming the resistance of the pipework. The height of the system makes no difference, the water on the downward leg exactly balancing that on the upward one.
The pressure required to pump water at a given rate through a pipe is proportional to the length of the pipe (not surprisingly) and inversely proportional to the fourth power of the pipe's diameter so long runs of 15mm pipe produce a lot of resistance, nearly five times that of 22mm if my arithmetic is correct.
Stephen
Flow resistance related.
There is also a static head figure which will be the minimum head needed to avoid cavitation. There is also a maximum head before it breaks, but I don't recall seeing it quoted on any domestic circulator.
You need to workout the flow resistance of all the pipes and joints and then lookup the flow in the table.
Thanks - makes sense
Quite an eye-opener!
Bob
Most solid fuel boilers are, or should be, on a gravity circulation loop in which the differential pressure is produced by the different densities of the flow and return water columns.
Those that are connected to a central heating system are usually connected through an injection tee, so that the circulation route through the boiler is not restricted by the pumps and valves. The pump can become blocked, the valves can be closed.
Yes, indeed - something different as already explained by several people.
If you can somehow measure the differential pressure across the pump and convert that to head of water, you can then look at the graph to determine flow rate at the appropriate pump speed. You might be able to do that with a differential pressure gauge, or two separate gauges - or even a U tube containing mercury (it would have to be quite big if it only had water in it!).
Alternatively, if it's flow you're interested in, why not install a flow meter in the pipe?
Pressure tappings should be some way downstream (10 D) from the pump. The water discharged is swirling and the velocities cause an apparent drop in the pressure as per Bernouilli. The swirling dies away downstream and the pressure recovers. Pressure gauges fitted directly to the pump's inlet & outlet (or flanges on big pumps) won't give you an accurate reading, but are slightly better than nothing.
There is a gravity loop.
This pump is in a load controller that circulates water through it and the boiler until the water gets up to 60C it then has thermostatic valve that closes forcing the circulation via the thermal store. The idea is that a) the cold water in the boiler and loop doesn't reach the store thus cooling it b) once up to temperature the water in the boiler is never below 60C which hopefully reduces condensation (water and tar) on the boiler.
Is there a nice handy look up table for 28mm copper 45 and 90 bends a resistance per unit length of tube?
Ta, I figured that in a closed system all the pump has to do is overcome the resistance but not being familiar with pump specifications got confused by "head" not really being a vertical distance just another way of expressing a pressure.
.
No Harry, it is not. You contradict yourself in your very next post, in which you say that "The purpose of a gravity lop is in case of pump/ electricity failure". It seems that you do agree, in that most solid fuel boilers are, or should be, on a gravity circulation loop.
It is mostly taken from the HETAS recommendations and handbooks. I have got the HETAS handbooks, having done their training course some years ago.
Do you have any particular solid fuel training or experience to support your "drivel" comment?
Will you be retracting your "drivel" comment and apologising for the same?
What don't you understand about "There is a gravity loop."?
I'm still not convinced I understand your set-up. You say there is 'a gravity loop' - is this just a dump into a small radiator, or is this all the heat extraction from the stove ?
I'm interested in that long before we all got together as Usenet - actually before the web and PC's existed in any numbers, I added an oil burner to my wood burner CH system and incorporated a Dunsley Neutraliser to meld the output of the two sources. My understanding is that a thermal store is at the system neutral point and does much the same thing.
What I don't recognise is the term 'load controller' - the concept of a thermostatic valve in a gravity feed circuit seems an anathema too.
Rob
The load controller sits across the flow and return pipes from the woodburner. Feed from bottom of thermal store passes through a one way flap valve in the load controller and then down to the wood burner. Flow back up from woodburner goes to the top of the thermal store but there is a T into the top of the load controller. There is a thermostatic mixer valve that starts to move at 60C (fully moved by
70C), this valve selects where the pump takes its feed water from. Either from the flow from the woodburner (cold state) or pre the one way valve in the feed from the thermal store (hot state). The output from the pump is feed into the return to the woodburner after the one way valve. Without power the one way flap valve is biased open so there is a "normal" open loop between the woodburner and the store. With power the +ve pressure from the pump shuts the flap valve and circulates water to maintain the feed to the woodburner at 60C (or above if the store bottom is hotter than 60C).Simple enough as far as the water side is concerned. The hard bit is the electrical control side. Don't particulary want the thing running when the stove isn't lit but don't want to have to keep remembering to switch it on/off either. Automagic switch on is easy, pipe stat on the flow from woodburner set at 50C. The switch off when the stove has died down and is no longer providing any heat is the important one. By then the bottom of the store is probably above 60C so the load controller circulates from the store to the woodburner and back until the store temperature drops below 60C. And I do mean the whole store as the woodburner goes in at the very top and out at the bottom so store stratification goes out the window. Trouble is the backup oil boiler cuts in when the store gets down to 65C at about 1/3 up. This has given the installers something to think about... it should be getting some form of differential control at some point.
Yes all the flows/returns (woodburner, oil boiler, CH) end up in the thermal store which is the neutral point. Tapping points are different, wood burner is very top and very bottom, oil boiler hot half way and bottom, CH flow just below the oil boiler and bottom. CW feed for DHW goes in just above the oil boiler hot input. There is a solar coil in the bottom as well can't remember the relationship of the bottom tappings and that.
See above its not in the gravity loop as in it doesn't control the gravity flow.
bends a
Thanks for that will have to inwardly digest the do a bit of number crunching.
I once fitted a formula to similar tables (Wednesbury Tube data - it was a long time ago, hence the imperial units) to relate flow velocity, pipe resistance and bore as follows:
R = y * (V ^ x)
where R = Resistance in inches of water per 100 foot pipe run V = Flow rate in pounds per hour x and y vary for different pipe sizes
Size(mm) x y 8 1.7650 1.225 *10^-2 10 1.7430 4.150 *10^-3 15 1.7180 5.836 *10^-4 22 1.7495 7.070 *10^-5 28 1.8913 6.648 *10^-6
For 8mm tube this is assumes 0.6mm wall thickness, for 10mm a 0.7mm wall.
These are empirical formula that I derived from tables. If anyone has an analytic version or a single formula that takes bore into account I would like to have it.
From this a 22mm pipe has about 1/7th the resistance of a 15mm pipe of the same length and flow rate. A 28mm pipe has about 1/30th the resistance of 15mm.
I've got an excel sheet that plots these on a chart if anyone wants it.
-- Phil
Harry has gone very quiet.
I suspect 'Harry' might be my old mate Alhoa/Rhondo.
I said Rhondo was a clueless half-wit bodger on the DIYnot forum about a year ago and has been stalking me ever since. He thought he'd been clever taking a drill and hacksaw to a 2-port Honeywell normally closed zone valve when he wanted a normally open valve for his solid fuel boiler. Rhondo/Alhoa/Harry has a limited vocabulary, 'drivel' being one of the few words in it. I could find the link if anyone wants a laugh.
Having my own personal DIY internet stalker really makes me feel like I've achieved something. :-)
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