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.
     
  10. Resurrecting an old thread: I think this works best in an environment where compressed air is a "free" or is a more efficient storage system than electric or mechanical pumps may be able to provide. These are primarily pragmatic questions rather than efficiency questions - one may find it easier to store a large amount of compressed air in old propane tanks, where a battery bank for electric would be expensive or a diesel engine is a maintenance chore. The efficiency of the actual pumps is kind of irrelevant when compared with how you'd have to waste compressed air in your storage tanks if you're working with "no pressure" input. In other words, if your water source is at zero PSI or roughly zero, then you'll need to have a two-tank system for constant pressure. One tank will be your "working tank" where you have water and a compressed air head that pushes the water to your consumers. The other tank will have to have zero pressure in it, so the input water can enter the tank. After a level is reached, the water input is closed, and air pressure is applied to the top-of-tank head so that it can be used as a source. Repeat this cycle between the two tanks to maintain at least one (but possibly two) tanks providing water under pressure to your consumers. But the trick is that you need to release ALL the pressure from whatever tank is being filled before you refill it with water because you can't add zero-pressure water to a tank with remaining positive pressure. This is a big waste of energy, and you already identified it in the comments above. However, if you have "free" compressed air then maybe this isn't a big deal. If you have a wind-powered compressor, or you have a compressor that acts as an energy dump on some larger system, AND/OR you have suitable storage facilities to keep a large amount of this low-cost air at pressure then these problems don't effect your design. Maybe if you can't get a diaphragm powered pump this would be a fun project, but seems like a lot of work compared to just installing an air-powered diaphragm pump from Sandpiper or Warren-Rupp or one of those vendors.
     
  11. am i right with the drawing this is what your trying to go for?[​IMG]
     
  12. Looks correct, except missing the valves. There also would be pressure blow-off pipes/valves on each water tank. Also, why three water tanks instead of just two?

    Doing some quick looking at a two-tank system, it would require 8 valves. Probably more.
     
  13. you can use float switches to start and stop the pressure with anti back-flow valves on the feed water pipes (yes i didn't add the relief pipes to bleed off the pressure after tank is mostly empty)
    three tanks is for constant user pressure. with two you may be still refilling a tank when the next one runs dry where as a third means its being filled while the other two are transitioning and should be around half full when one is starting to empty and the other is empty.

    it would take a lot of valves most of them one way flow a few run by the float switches to allow pressure in and bleed off the pressure and to stop the water going out when the tank is almost empty to not allow air into the feed pipes.
    you'd also need a bleed off filter container to extract humidity in the air lines leading to the water tanks so you don't contaminate the water.
     
  14. I didn't think about oil or water in the air lines. Ugh - that adds a serious snag if this is for potable water. If it's for some other purpose, it's not an issue (or less of an issue.)

    I thought about this problem a bit more, and I think that there is a more elegant way to solve it that uses the same physics for storage of energy (potential vs. chemical with batteries.) Use the compressed air at the time of creation (little or no storage) and fill a large water bag (like one of these: http://www.water-storage-containers.com/waterstoragebladders.html) and put a steel plate the same size as the bag on the top of the bag whose weight will vary depending on what pressure you want. Then the bag is filled with an air-powered water pump, which lifts the plate. This gets a large amount of storage capacity and pressure that is captured at the time when the compressed air supply is available. This could also be done with a cylinder with a large weighted piston, but that would require a watertight fit between the piston and the sleeve - it seems a bag is easier to manage/build/maintain. There is also no waste like in the air tank method, which has LARGE waste issues if the pressure desired at the consumption point is relatively high.
     
  15. hmm i dont think the oil water issue is as bad as you think. a simple filter at the compressor would remove most of the oils and another past the air tank would remove the humidity from the lines.
    the constant pressure issue should be pretty stable with a large air tank instead of direct feed from the compressor as long as you don't expect a PSI close to the systems limit.
    you could remove the compressed air from the water with a bag inside a tank so the air pressure collapses the bag just not sure how you could refill it with water using a non-pressurized source.

    wasted pressure is irrelevant if you use a wind powered source to run the compressor either as a direct drive or an electrical source to run a compressor as long as you have a fairly stable wind source.
     
    Last edited: Jun 15, 2015
  16. russ_watters

    Staff: Mentor

    So, what exactly is the goal of this project? Or, rather, why would you choose this type of pump over a normal electric pump for a particular application?
     
  17. Hello Charlie,

    Not only where there are no electricity supply available: certain site do not allow electricity due to safety issues.

    There are plenty solution in the market to solve this problem: if you need water with no contaminants (oil, etc.), you will need an air operated diaphragm pump. In this case you wont have contact between the water and the air compressed.

    There are also several strategies which allow to you to reduce the presence of contaminants in the compressed air: condensed water, oil, steel wool produced by compressed air friction against the pipe, rust, etc.)

    Consider that the condensation, in a compressed air flow, is a chemical- physical phenomena, and, depends on the environment temperature, mainly (this is fluid mechanics issue which could be explained by many people here, better than me) : if you see an air pipe line compressed at 5:00 a.m., in winter time, you will see condensation even outside of the pipe, and , if you open a line valve be ready to receive an awful shower of dirty, frost and pressured water...

    My final suggestion is:

    Define the application of the installation: human consume, industrial use, etc.
    Define how many liters per hour you will need.

    With these input you will be able to define a strategy because an installation for one use could be really different than other, so the dollars for each installation are pretty different.

    There are several proved devices for make an air compressed flow cleaner even pure, all depends on the use and the budget, and the regulations and requirements.

    Good luck.
     
  18. Charlie,
    Please pay attention to Russ question.
     
Know someone interested in this topic? Share this thead via email, Google+, Twitter, or Facebook

Have something to add?

0
Draft saved Draft deleted