# Hydraulic motor driven by steam/compressed air

1. Nov 14, 2011

### mellotango

Hi, can anyone let me know whether it is possible to use non-liquid fluids such as steam or compressed air to drive a hydraulic motor? We want to consider doing this for a new application. If the answer is yes, that's great. If the answer is no, how can one create a mechanism such that a supply of steam or compressed air can be used to drive a hydraulic motor?

2. Nov 14, 2011

### rsk2mc

What type of motor? Radial, axial what type is it

3. Nov 15, 2011

### mellotango

At the moment, we are still not sure which types of hydraulic motor would be suitable. Without stating the obvious, radial or axial piston types would be the ideal choice as they are meant to be the most resilient, compared to vane or gear types. But whether that fact applies for steam or compressed air still remains to be seen. So we would like some feedback on whether compressed air/steam can be used to drive a hydraulic motor, and which types of motors would be suitable for this types of working fluid. What's your opinion on this, R2skmc?

4. Nov 15, 2011

### Q_Goest

An http://www.ingersollrandproducts.com/IS/Category.aspx-am_en-4256" [Broken] is a motor driven by compressed air, typically around 100 psi. The air is vented to atmosphere when it comes out of the motor and it turns a shaft. You could then connect the shaft to a hydraulic pump if that's what you're after. The isentropic efficiency of these motors is poor however, typically 50% or less, sometimes much less. Using an electric motor is much more efficient.

Not sure about steam, but I suspect there are small, low pressure steam turbines available also, or you could try driving the air motor using steam.

Why do you want to use air or steam anyway?

Last edited by a moderator: May 5, 2017
5. Nov 15, 2011

### mellotango

My idea is to attach a generator to the hydraulic motor shaft for small scale power generation. I'm trying to see whether its possible to use off-the-shelf commercially available components that can handle up to 300 psi of compressed gas as working fluid. You mentioned air motors, which according to my research handles up to a maximum of 150 psi. And there are air leakages involved. So its probably not an ideal candidate. The reason I'm considering hydraulic motors is because they are designed to handles enormous amounts of pressure, and they are probably better sealed since they cannot allow hydraulic oil to leak. So question still remains if highly compressed gas is possible to directly drive a hydraulic motor. Thx

6. Nov 15, 2011

### Q_Goest

I misunderstood you then. I thought you were trying to drive a hydraulic system using air as your energy source.

I suspect if you put high pressure air into a hydrauilc motor, it would turn very inefficiently. The hydraulic oil is relatively incompressible but air has to be expanded to extract work, so the chamber through which the hydraulic oil would pass will stay at a fairly constant pressure as the volume changes whereas a chamber through which air passes has a large change in pressure as it increases in volume. For example, imagine a cylinder with a piston in it. If you put hydraulic oil in at high pressure, you would continue to feed that oil in till the piston had gone through pretty much the entire stroke, all at fairly constant pressure. You would then stop the admission of oil and the very small amount of decompression you would get from the oil wouldn't move the piston significantly. The piston would then come forward to displace the oil at low pressure in preparation for a new charge of oil. In comparison, if air were used to expand in a cylinder, only a small amount of movement from the piston would be needed while air was admitted under high pressure. The air would then be shut off and the air allowed to expand and do work against the piston untill the cylinder was full and the air pressure had dropped to the outlet pressure. The cycle would then start over.

This same cylinder/piston thought experiment is true for other types of arrangements such as gear or vane motors. Hydraulic oil can't expand like air, so they will necessarily be built differently.

Another problem is that hydraulic oil is intended to lubricate, so air will be problematic in a hydraulic motor. The motor will likely have insufficient lubricaiton.

Sealing is also an issue. Seals in a hydraulic motor don't have to be as tight as an air motor. And on the discharge, either of these devices will only have atmospheric pressure unless they are specifically designed to handle pressure on the outlet, which is not very common.

I guess the question then, is what pressures and flow rates are you looking for? Air motors might be found with higher pressure capacity and there might be other options.

7. Nov 15, 2011

### mellotango

Thanks for your detailed explanation. I suspect there are inherent problems in attempting this, and you have helped to elaborate on them. It's interesting you said there might be air motors to handle higher pressure. I'm looking for something that can handle up to 300 psi, and as I mentioned before the ones I come across are usually rated max pressure of 150 psi or less. If you or anyone know of any vendors that sells such high pressure air motors, I'd be glad to check it out.

8. Nov 15, 2011

### Q_Goest

What flow rate or power output are you looking for?

9. Nov 15, 2011

### rsk2mc

You could take a hydrolic radial pump and convert it to air. Let the air into the motor however you still need to spin the shaft. You could house a turbine , like get a turbo from a car and build a make shift housing. Shoot the air to spin the turbine and connect the shaft to your generator.

10. Nov 15, 2011

### mellotango

According to my calculations, the required flow rate is 15.2 CFM of compressed air at 300 psi. If using 150 psi, then flow rate is 30.5 CFM. I based my calculation on a 20 hp shaft power or 10 kw net generator output. Rskmc, do you have a google image URL of the hydraulic radial pump? The search terms i did came up with radial piston pump pics and centrifugal pump pics, so not sure which ones you might be referrring to.

11. Nov 16, 2011

### Q_Goest

Hi mellotango,
First off, please don't use CFM. It's confusing and almost never used in industry. Use either mass flow rate (lbm/s or similar) or standard flow rate (SCFM or similar) so there's no confusion over what you mean. CFM must be associated with a temperature and pressure to mean anything so it is only used by compressor manufacturers to indicate displacement and used to "cover their azz" in case their customer uses the machine under conditions other than standard. So I assume you mean 326 SCFM or 0.407 lbm/s or there about. Given that, your isentropic efficiency is just under 50%. That's all good to hear because that puts it in the ball park of air motors. You won't be able to use a hydraulic motor or turbine from a car because they simply aren't made for it for the reason I provided above.

You'll need to do a bit of leg work. Here's a list of air motor manufacturers/suppliers on Thomasnet:
http://www.thomasnet.com/products/air-motors-52370202-1.html
Check the manufacturer's web site, but you'll need to call someone and ask specifics. Don't do what engineers straight out of college do and just do web searches. You need to contact people on the phone and talk to them. If they don't make what you want, ask them if they know who does. Or ask them if they can modify something they already have to meet your criteria.

Good luck!

12. Nov 17, 2011

### mellotango

Thx for the list of air motor vendors. And I agree it is best to contact them to see if they are able to come up with something suitable. I'll def explore this avenue. Another avenue might be potentially looking a centrifugal pumps made to run in reverse as rskwc suggested. There are quite a few success stories with this too. My search will continue to look for the best option and it could be weeks or months to come. I'll try to enjoy the time searching and learning at the same time.. :-)

13. Dec 6, 2011

### Q_Goest

Hi mellotango. I guess I've been assuming this is strictly a college related course. That's most of what PF is used for, college students. You indicated this is a real project so can you be a bit more specific on your process? What are your conditions?
- Inlet pressure (300 psig?)
- Inlet temperature
- Flow rate (lbm/s or SCFM/SCFH)
- Discharge pressure
- Media (air, steam or mixture?)

Also, what country are you in?

I'm assuming this project has a reasonable budget. If you have a potnetial of 10 kW of power and assuming $0.10 per kW hr and a payback of 5 years, that's around$45k, which probably isn't enough to install a typical expander package. But if the payback is longer and/or the power output is higher it becomes a real possibility. Just off the cuff, I'd think you need to show a power savings of at least \$100k over the payback period to make this viable, preferably much more. Do you know what kind of payback period your company would find acceptable?

There are plenty of manufacturers that make expanders and turbines for things like this but the specific conditions (most preferred conditions) you have are critical in getting you started in the right direciton and to see if there's any possibility that the project cost will make it commercially viable.

14. Dec 6, 2011

### mellotango

Hi Q_Goest, I sent you a private message. Only realised they had this function in PF.. :-) thx