On Wed, 22 Sep 2004 14:00:09 +0100, Andy Hall
wrote:

With an alpha, as TRVs close the flow falls but the head does not rise
as in normal pumps.

Not exactly.

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.

OK. Nothing is perfect but the TRVs will close enough to reduce the
flow otherwise what are they doing, nothing. The flow has to reduce
to lower the average radiator temperature and so lower its output. So
at the bit of the alpha curve where the head rises to keep the flow up
then all that will happen is the TRVs close more. They must reduce
the flow.

With a normal pump you need a bypass. So normal
pump+bypass gives constant-ish flow rate and return temperature rises
as TRVs close. Alpha gives reducing flow and a lower return
temperature.

Only close to the point of total closure.

The heat flow from the boiler is the flow, kg/s, and the difference
across the boiler. The heat output from the rads is the temperature
difference. Since the input water temperature is fixed then TRVs must
slow the flow down so the output temperature is lower and thus the
average water temperature lower. Ergo TRVs must reduce the flow AT
any time they are having an effect. NOT just when they are about to
shut the rad off completely.

Hence Alpha's are a good idea if you have a condensing boiler since
those are more efficient with lower return temperature. But normal +
bypass is probably better for a standard boiler since they do NOT want
a lower return temperature due to the risk of condensing causing
boiler corrosion.

it's suitable for a normal boiler as well, since if the bypass opens,
the flow increases anyway.

So what is the point of an alpha? If the bypass is kicking in before
the alpha "effect" why not use a normal cheaper pump! If the alpha
"effect" kicks in the bypass won't open. Both effects are driven by
pressure, ones got to happen first then the other won't.

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).




--

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
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