On Thu, 23 Sep 2004 09:33:06 +0100, Andy Hall
wrote:

On Thu, 23 Sep 2004 09:18:51 +0100, Malcolm Reeves
wrote:



Between a certain range of flow/pressure, the Alpha behaves
conventionally. When the flow falls below a certain point, that is
detected and the power is reduced to reduce the head.

Not according to the alpha data sheet which says:

Electronically controlled, the pump can, by means of a
selector switch located on the terminal box, be set to:
• 2 constant pressure curves
• 2 proportional pressure curves
• 3 fixed speed curves.

So either constant head (I think I said that), proportional head (head
goes down with lower flow), or 3 normal pump modes.

You need to look at the technical manual for the Alpha.

I am looking at the pdf from the grundfos website
alpha_data_booklet.pdf, 16 pages with graphs etc. from which the quote
above is taken - page 6.

Protection of standard pump nuts can be achieved by a bypass.

Possibly, but there's no point.

You need auto bypass for standard pump - I was being humorous
referring to your comment it would scream its nuts off.

The heat flow from the boiler is the flow, kg/s, and the difference
across the boiler.

Only while the boiler is firing. At the point where the heat used is
less than that produced it will begin to cycle.

Of course but what is the relevance of that. It will always cycle
when Watts in Watts out. We are discussing TRVs and pumps.

I know. The point was that the behaviour is not simple because the
burner is being fired on and off. This has a substantial effect on
the temperatures.

If the boiler is short cycling yes but if it is doing that you need to
fix it since that is very wasteful. If it is on a long cycle then you
can consider it as being in almost steady state. There will be a
steady climb in temperatures but relative temperatures will be similar
during this time.



From the perspective of heat output to the room, it is approximately
proportional to the Mean Water to Air Temperature.

From the perspective of heat delivery to the radiator (which will be
the same since none of it disappears) it will be proportional to the
flow and temperature difference across the radiator.

Agreed (I think I said that).

You can't treat them in isolation though. If the boiler is cycling,
the output temperature is far from being fixed,

Short cycling is a different problem and needs to be fixed. Normal
long cycling is relatively stable temperature difference on an overall
rising base line.

Both effects are driven by
pressure, ones got to happen first then the other won't.

You're missing the point. It is far better to have the pump reduce
output as the heat flow reduces rather than shortcircuiting it back to
the boiler with the pump on full power.

Why? Assuming you have a normal pump set to the right setting. In
fact since there is a recommend minimum speed to stop sludging it
could be argued that a bypass, which keeps the boiler flow constant is
better as it stops the sludge settling in the boiler. Of course if
you correctly treat the water then sludge is not an issue, hopefully.

An alpha would suit something like a coal fired room heater where the
best way to run it is slow continuous release of heat, not
super-burn/off/super-burn/off as you get with normal pump + stat (no
bypass). Also a condensing modulating gas boiler (as long as it could
cope with low flow (coal fires can).

This misses the point completely.

Why? The alpha's modes of constant and proportional head are designed
for system that work best with a variable flow. That has to be
boilers that modulate their output. Also, those boilers that are
happy with a low return temperature, which is another effect of low
flow.

Set it any differently and it's just a fine adjustment standard pump.

Even a fixed boiler has varying output temperature as the burner
cycles. There isn't going to be an issue with the return
temperature being low at low flow rates anyway.

I would tend to agree with you as it would seem reasonable that a low
flow low temperature return is going to quickly warm up in the heat
exchange. However, I'd want to check with the boiler manufacturer
that he was happy with that. I suspect most might prefer the constant
flow arrangement so as to avoid even the only perceived risk of
condensation. So I'd reckon most would recommend normal + bypass
(that's the result I had when talking to HRM). Of course, assuming
this is a standard boiler, not a condensing.

--

Malcolm

Malcolm Reeves BSc CEng MIEE MIRSE, Full Circuit Ltd, Chippenham, UK
, or ).
Design Service for Analogue/Digital H/W & S/W Railway Signalling and Power
electronics. More details plus freeware, Win95/98 DUN and Pspice tips, see:

http://www.fullcircuit.com or http://www.fullcircuit.co.uk

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