Handpump limitations

Hi Guest,

The maximum discharge pressure would depend on (1) what type of pump it is, (2) how hard you are willing to crank the pump, and (3) how robust the pump is. (I think that these cover the "limiting factors" as well)

If it is a PD (positive displacement) pump, the work needed to operate the pump (and from this the maximum discharge pressure) can be calculated from various physical properties of the pump and the drive connection.

If you want a more detailed answer, please give more detail!

Mike

I am interested in whether it would be possible to operate an RO desalination system of this capacity using hand-crank power exclusively. Some of the newer RO systems are said to operate at much lower pressures than the older systems.

Yes, it is. Hand-pump RO is an option for cross-Atlantic rowing expeditions, for example.

Seawater RO requires around 5kWh per tonne. So, say 0.01 tons drinking water per day per person @ 5kWh per tonne comes out at 2.1W continuous rating, which is well within the capacity of individual human effort.

Practically explained, the power required to drive a positive displacement pump (efficiencies excluded) is

HP = (Q x p) / 1714

Where Q = flow rate, pump output in gal/min; p = pressure in lbs/sq.in.; 1714 as conversion factor.

Your flow rate for the size displacement of the pump will be derived from:

Q = (n x d) / 231

Where n = speed in rpm; d = pump displacement in cubic inches; 231 as a conversion factor for gal to cu.in./gal

And, the speed at which you have to turn the pump is:

n = (231 x Q) / d

So, you can see that given a desired flow of 2-3 gpm, the governing factors are how fast (or slow) do you want to crank the pump based on the displacement and how strong are you to develop a desired pressure.

Now it's up to you as to what you can produce by hand; i.e., a conversion of horsepower to handpower.

The limitation is the power an operator can bring. Which ever the pump type is it is assumed it was designed to accept the maximal pressure it will have to deliver.

If it is a lever pump than you should consider that the maximal power is for a short time up to 90..100 W and it will go down to 60..70 W for a longer time.

The lower the delivery the higher the pressure and vice-versa.

In fact the available power depends on the muscles used for actuating the pump.

Above values are for a lever pump actuated with one arm. If one will use a pump actuated a bicycle than the available power can get as high as 400..500W for a "normal" person and up to 800w for champion.

Comparing our puny muscle power against modern motors is always sort of a fun thing to do. Here's some interesting comparisons:

A world class cyclist can put out about 450-500W on a somewhat continuous basis.

A very fit recreational rider is good for about 1/2 that.

A person of average cardisvascular shape is probably good for about 1/2 to 2/3 of the fit recreational rider, or ~125 W.

If these numbers seem low, ask how many riders can accomplish 2500' elevation gain in one hour on a mountain where the grade is sufficiently steep that wind and rolling resistance are minimal (ie, all the energy is going into elevation gain). For a rider plus bicycle weighing 200 pounds, that amounts to 0.3 HP, or 225 W.

Another way to quickly evaluate human power is to contemplate climbing flights of stairs in tall buildings. Our 175# hero producng 0.3 HP can climb a nominal 100 story building in 20 minutes.

1 HP = 745W = 550 ft#/sec

Now realize that a bicycle is efficiently using the largest muscles on the human body.

I don't have a good basis to say what a person can do with one arm, but I would guess 30 watts is about as good as one can expect and I doubt anyones muscles are conditioned like a bike riders legs to do it continuously (there are paraplegics riding hand bikes who do amazing things with their arms). To the extent that's correct, your theoretical hand pump delivering 3 GPM with 80% efficiency might be able to supply 20 psi differential pressure. Thats the equivalent of lifting 25 pounds of water 40' in the air every mnute. That sounds like a lot of work to me. I wouldn't want to weary my bones doing it very long.

As previously pointed out man-power is the key, e.g.

Semi-rotary hand pump..............OK for small amounts of liquid only............at maximum pumping rate, average is twenty minutes duration...............I used one of these pumps to keep sea water out of a motor cutter for 4 hours.............'tis truly marvelous the effect of adrenaline. I can recommend this type of pump as to being one of the best manual pumps.

The motor cutter was from the HMAS "Melbourne" and we were looking for survivors from the HMAS "Voyager" that the"Melbourne" had just cut in half.

Not a pleasant night.