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SDHW Pump Selection - Page 2

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Posted by Brian Graham on February 4, 2005, 3:12 pm
 
Shouldn't the pump speed be variable as a function of the
collector temp -- faster when the collector is hot, slower
when it's cooler but above ambient, and stopped when it's
the same as or less than ambient?

A PV powered DC pump would mimic that somewhat. In low-lighting conditions it
doesn't get much power and pumps more slowly. This is good as the collector
isn't as hot. At mid-day it is getting full sunlight and delivering max power.
But is it the optimal flowrate??

But a PV powered DC pump is no good for pumping water over a wood stove
throughout the night.

Posted by DJ on February 4, 2005, 3:31 pm
 
Absolutely correct. For a wood stove loop, you'd be advised to use a
temperature differential controller and an independant power supply.

DJ


Posted by DJ on February 4, 2005, 2:55 pm
 Nick,

We disagree on that. You need heat transfer, and that requires
residence time.Single straight tube heat exchangers are the least
efficient of them all (compared to shell and multitube, plate, spiral
tube), so you have to keep the fluid in the hotter/cooler region as
long as possible. Of course, it depends, too, on what you want to heat,
and what the transfer fluid is; with water, sure, err on the side of
caution, and go faster. Glycol, however, with a higher boiling point,
takes more heat.
 If you're heating a pool, sure, you don't want it hot. If you want to
heat domestic hot water to the other side of 50 C, that means what's
coming down from those plates on the roof has to be a fair bit hotter
than that for normal consumption purposes.

Now, myself, with over a decade as an industrial mechanic/millwright in
a chemical plant, I've worked on damn near every heat exchanger style,
size, and purpose out there. Add to that the fact that Thermo-Dynamics
is one of the biggest companies in the business doing this work
professionally... and their biggest recirc pump is, I think, a 3 GPM.

My two cents, anyway.

DJ


Posted by nicksanspam on February 4, 2005, 9:39 pm
 

Heat transfer is more efficient with faster flow, which
raises thermal conductance through the transfer surface.

More mass flow also lowers the collector temp, for a given insolation,
which makes the collector more efficient, since it loses less heat
to the outdoors.

Perhaps we need a few numbers here.

Nick


Posted by daestrom on February 4, 2005, 10:38 pm
 

I have to go along with Nick on this one.  Of course there are limits, but a
higher velocity in the tube makes for a thinner film boundary and better
heat transfer.

A lot of folks are tossing around 'efficiency' and how 'fast' or 'slow' is
more 'efficient'.  This gets a little messy because not everyone thinks of
'efficiency' in the same way.  For traditional heat exchangers between two
fluids, efficiency is how close the cold fluid is warmed to the hot fluids
temperature and/or how cool the hot fluid is cooled down.

For example, with 80F water entering one side, and 190F water entering on
the other side of a counter-flow, shell and tube heat exchanger (same flow
rate on both sides), a 'perfect' heat exchanger would heat the 80F water up
to 190F while the 190F water would be cooled down to 80F.  Of course in a
'real' hx, this isn't possible, but efficiencies of 90% are possible.  So
the 80F water would be heated to about 80F + 0.90*(190-80) = 179F and the
190F water would be cooled to 190F - 0.90*(190-80) = 91F.

But a solar collector isn't strictly a HX.  There is only one fluid flowing.
The energy input from the sun is fixed, regardless of the temperature of the
fluid.  The ambient losses from the collector are a function of the
collector's temperature.  Also, the temperature of the fluid *entering* the
HX is not completely fixed.

If you run the system 'fast', the incoming water will be not be any cooler
than the tank.  While the fluid is not in the collector for very long, it
still picks up heat from the collector.  So it returns to the tank warmer
than when it left.  With high velocity through the tank coil, there is good
heat transfer, and heat is effectively being moved from the collector to the
tank.

If you run the system 'slow', the incoming water will be the same
temperature as before.  The fluid will be in the collector longer and pick
up more heat per pound of fluid.  So the average fluid temperature and
collector temperature will be higher.  That's bad as it means more ambient
losses from the hot collector.  Some may argue that this hotter fluid heats
up the tank 'better'.  But it does not.  While each pound of fluid carries
more energy, there are fewer pounds of fluid entering the tank.  In fact an
energy analysis will show that the rise in temperature is a direct result of
the drop in flow rate.  Neglecting ambient losses, the energy transfered
would be exactly the same.  But neglecting losses is a bad thing, it will
get you in trouble.

If the sun energy input to the collector is the same, then the energy that
leaves the collector *must* be the same.  It leaves by two methods, ambient
losses because it's warmer than the environment, and convection of
circulating fluid.  If the ambient losses are higher because the average
temperature is higher, then the net energy delivered to the tank (solar
input - losses) *must* be less.

Of course, 'fast' requires more pumping power, and there are practical
limits to this.  Many commercial HX strive for velocities in 3/4 to 1 inch
tubes of about 7 ft/sec.  Any faster and the pumping power losses become
severe without improving heat transfer much further.  Slower than about 4
and the fluid film thickens hampering heat transfer.

daestrom



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