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ps.com...
The wiki article
http://wiki.diyfaq.org.uk/index.php?...and_Heat_Banks
says:
Assume (for the sake of illustration) that
the water in the store reaches 70C when fully heated
the incoming cold water is at 10C
the hot water must be at least 40C to be acceptable
the heat exchanger is perfect, i.e. there is no temperature drop across
it.
As DHW is heated it will rise to 70C (when the store is fully heated)
and will cool the water in the store to 10C. If the cooled water is
thoroughly mixed with the hot water the whole store will progressively
cool until it reaches 40C after which the temperature of the DHW
becomes unacceptable. However if the water cooled to 10C is kept
separate and only the water at 70C is fed to the heat exchanger then
the DHW will continue to produce acceptable water until the whole store
contains only water at 10C. In the first scenario the amount of heat
available by cooling the store from 70C to 40C is available for heating
DHW, while in the second the amount of heat available is that of
cooling the store from 70C to 10C. This gives much more DHW for a given
volume of store at a given temperature. Stratification, where the cool
water at the bottom of the store remains relatively separate from the
hotter water further up, provides a means of approaching this goal.
========
Comments here concern the thermal store.
I'd like to suggest that with stratified water, at end of useful hot
water output one has a situation where the water at the top of the heat
exchanger is 40C, and water below graduates down to 10C, rather than
all water being at 10C. There isnt any way you could get hot water at
40C from a tank thats all at 10C, or 20 or 30. This still gives much
more output than with a no-stratified tank.
Second I have a question related to the thermal store diagram and how
it might perhaps operate more effectively. I think we can accept that
the water in teh cylinder will vary from bottom to top in a continous
temperature progresion, ie 10,11,12,13,14 etc all the way to perhaps
65C or thereabouts. And we're assuming the water enters e heat
exchanger at 10C for this example.
I only have perssonal experience of the Heatbank installed in my house
(others may vary) but IMHO your suggestion doesn't track with actuality ....
I cannot "accept that the water in teh cylinder will vary from bottom to
top in a continous temperature progresion, ie 10,11,12,13,14 etc all the way
to perhaps 65C or thereabouts"
Experience, and judicous prodding of the pipes with a digital thermomer,
leads me to conclude that hot water 'floats' on top of colder water. {BTW; I
have a _personal_ dislike of the term 'water' used when describing Heatbank
operation - I prefer to use the term _fluid_ (albeit it water) to
differentiate between 'water' that becomes DHW and 'fluid' that lives within
the boiler, heatbank and radiator circuits ~ I trust you'll bear with this
foilble}.
On the heatbank I've installed 'fluid' is drawn off the top of the cylinder
(where it's hottest} and enters the Heat Plate Exchanger{HPE) surrendering
some of its heat. This fluid is injected back into the cylinder at the
bottom. Thus the hottest fluid is always at the top. {This situation
continues until there's no more 70deg 'fluid' at the upper surface. IMHO,
the continuos gradient you/'ve surmised doesn't arise.
The thermal store diagram is shown with the exchanger in the top half
of the cylinder only. The water here would mostly be well above 10C.
My actual installation has the HPE more-or-less in the middle BUT it's fed
off two aperture locateted 'very close' to the top of the cyclinder (Hot)
and 'very close' to the bottom of the cylinder for 'cooled' return.
Now, if the exchanger is in the top part of the cylinder, the mains
pressure cold water comes into the bottom of the heat exchanger, which
is halfway up the tank, and will meet tank water temp of somewhere
between 10 and 65C, lets say maybe 35-40C. What this means is at end of
useful hot output, the water below the heat exchnager will be about the
same temp as it was before HW was drawn off, namely a gradient from 10C
to 40C.
Dubious conslusion: the HPE accepts fluid from the very top of the
cylinder -
it's physical location is irrelevant. For practical purposes it's apposite
to install it onto the cylinder removed from the penetrations -hence 'mine'
is mounted about 'half-way-up' ... but it's fed off apertures at the top and
bottom. Your other surmises are premised on a dodgy conclusion.
OTOH if one were to use a full height heat exchnager,
the 'height ' of a HPE is once again irrelevant ... the area of the HPE is
scaled to transfer heat from the fluid to the rising main transforming it
from water @ 10 deg C to water @ 40 deg C [your figures re-quoted). The
fuild is circulated through the HPE by a pump while cold mains water is
blended with the heated water emenating from the HPE to maintain a
demanded temperature [ ... my setting is to provide Hot water (at mains
pressure) @ 55 deg C ... ]
with the cold fed
to the heat exchange pipe at the bottom of the tank, not half way up,
that bottom half of tank water prewarms the HW before it reaches the
half way up the tank level in the heat exchange pipe. This means the
top half of tank water has to do less work, and the heat, or just
warmth, is being drawn out of the colder water in the bottom half. IOW
you'll get more total volume of HW out, and the tank will end up with
colder water in the bottom half than if it used a top section only heat
exhanger.
I'm not sure if I've explained it too good!
NT
Me too; I hope I've explained it as well.
--
Brian