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System Pressure and Expansion Vessels - Page 2

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Posted by nicksanspam on January 21, 2007, 10:12 am

The vapor pressure of water at temp T (F) is roughly e^(17.863-9621/(460+T)
"Hg vs one atmosphere at 29.921 "Hg, ie 14.696 psi, and a foot of water
weighs 0.433 psi, so it will boil (cavitate) in a long enough (14.696/0.433
= 34') pipe with a vacuum at the top. Sven Tjernagel stopped installing
tanks far below collectors after too many complaints of "rats in the walls."

Since this is more than 34', it seems the pump still has to do lifting.


How can we move water up and down a great height without much work?

A 5-story Archimedes spiral with two pipes? :-) A UK shower pump with
two pumps on the same shaft and one working as a microturbine? Some
sort of multiple syphon arrangement? A DC pump with an open reservoir
upstairs that works first as a pump, and then as a turbine, recharging
a battery, or an AC version charging the grid? I have a nice little
book called "Pumps as Turbines."

Solar water heating is still a good energy bargain, but...


Posted by nicksanspam on January 21, 2007, 9:00 pm

Something like this:

20 K=(LOG(14.696)-LOG(.36328))/(1/(460+70)-1/(460+212))
30 C=K/672+LOG(14.696)'C-C vapor pressure constants
40 WPSIb.33/144'water weight per foot (psi)
50 FOR Tp TO 210 STEP 10'downgoing water temp (F)
60 P=EXP(C-K/(T+460))'water vapor pressure (psi)
70 DP.696-P'atmos-water vapor pressure (psi)
80 H=DP/WPSI'max unpressurized downpipe height (ft)

Temp (F)   Max height (ft)

 70        33.11265
 80        32.79119
 90        32.36542
100        31.80754
110        31.08396
120        30.15451
130        28.97144
140        27.47859
150        25.61028
160        23.2903
170        20.43085
180        16.93133
190        12.67727
200         7.53915
210         1.371191

C          K  

16.49802   9280.618


Posted by daestrom on January 20, 2007, 9:17 pm

Well, a slightly different way to look at it...

In a closed system, the fluid must be kept pressurized at least enough to
maintain pressure at the high point.  If the pressure is not at least high
enough to force fluid up to the high point, any leakage there would let in
air.  Also, when starting up the system, the circulating pump would have to
overcome the head needed to 'push' the water up to the high point.  If it
can't develop that much head, then no circulation occurs.

An easy way to do this is to provide a relatively 'large' volume that is
only about half full.  The other half is kept pressurized with a gas.  A gas
is preferred since a small change in volume (as during temperature changes
in the liquid) will not cause a large change in pressure.  If the system
didn't have some sort of 'accumulator' or 'expansion tank' with an air
bladder, then the pressure in the closed system would change greatly with
just a small change in temperature.


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