Efficiency of air powered water pump

  1. Hi,
    I'm thinking of a water pump with only the valves as moving parts - the compressed air simply displaces water from a cylinder. I understand it's a solution for where it's easier to deliver compressed air than electricity to the pump site.

    My question is: where are the energy losses in the compressed air power transmission? The context is in comparison to turning shaft of a water pump by coupling to an electric motor.

    I worried for a bit that the heat at head of air compressor is all wasted energy, but is that relevant if most of energy was latent heat from the air rather than from motor that drives the compressor?

    I figure it must be an inefficient system otherwise I'd see a lot of these pumps around.
  2. jcsd
  3. Anything you do will have losses. The main losses are in the compressor itself, some losses in getting the air from point A to point B. So, of course it is less efficient than running a pump directly from a motor.
  4. Yes. And if I were shifting energy from electricity to say turning a drill bit in a pneumatic drill the losses of pneumatic drill compared to an electric drill would be huge: there's friction in the compressor, the air hose, and at the drill itself, and then there's exhaust air which still has some pressure.

    But in the water pump there isn't moving parts (except the valves). So it's way in front of the pneumatic drill. And maybe the friction in the air compressor would be similar to that in a conventional water pump and thus negligible net efficiency loss from the compressor.

    I'm pretty sure I'm missing something though. Is it the exhaust air? When the water pump cylinder has to refill with water, compressed air has to be dumped from the cylinder. Yeah, that might be it.
  5. What do you mean by *conventional*? A piston type air compressor will have considerable more friction loss compared to a centrifugal pump made for pumping water for instance.
  6. Ok "conventional" is vague. So a pump that shifts 1litre/second air has more friction inefficiency than pump shifting 1litre/second water, same pressures? Does it depend on the pressure and type of pump required for the that pressure?

    At 1m head both pumps could be centrifugal and maybe the friction losses are similar. At 10m head the water pump could still be centrifugal but the air pump is piston and the centrifugal pump is more efficient. Is that likely?
  7. there ARE lots of these pumps around. Go et an honest job on a ship, or in a mine.... you will be surrounded by these "sandpipers".

    efficiency calculation? Output pumping power (relatively easy to calculate) divided by input energy (easy to calculate, but you gotta make sure you're actually calculating a number which correctly measures THE EXTRA COSTS YOU INCURRED to run the pump. Typically very difficult to isolate those!!)
  8. I checked out the Sandpiper - I see that's a diaphragm pump and not what I started the post with, but near enough. I don't reckon I can easily find the data for the easy calculation of efficiency.

    I'm only after some theory but the more I think about it, the more I realise there's a lot going on.

    E.g. how does efficiency of pumping a volume of air compare to same volume in water (same pressures)? If the volume and pressure were measured at a nozzle then the energy there is the same, right? (The air is lighter but passing through a more choked nozzle and therefore going much faster: 1/2 mv^2.)

    I don't mind giving up on this post. I think it's too broad a subject and I probably need to go back to school if I want to answer all my questions. Not sure whether I'll meanwhile install an air operated pump. If I do I can at least measure its efficiency like suggested.
  9. it's worse than you imagine. In the real world of diesel engines, a power plant opeerating below (let's say typically) 80% of rated, incurs SIZEABLE extra long-term costs for wear-and-tear. Much more than any fuel savings. In many cases, the electricity is - in effect - free. If you were not powering an air compressor with it, you'd PAY to have a large resistive bank to dump it into.
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