difference between Hydraulics & Pnuematics in applications

[heavenly]

Hi Freinds
I know,in Hydraulics,most of aplication only energy transfer is there,& most of pnuematics is uses wit energy transfer takes place wit mass transfer.Is this correct
if so any other applications to differentiate clearly

thanx in before

John

 

[jsummerfield]

Hydraulic controls are fast and forceful. Pneumatic controls are less forceful and half-fast.

John

 

[israelkk]

I am have to disagree with jsummerfield that pneumatics is half fast then hydraulics. Try to pass an hydralic fluid through a known orifice into a cylinder in order to push a piston compared to pass a gas through the same orifice. It is clear that the gas will fill the cylinder many times faster assumeing the inlet pressure is the same. Hydraulic fluid is incompressible where a gas is compressible therefore, against force without movement hyraulic will be faster because there is no need for a fluid flow but if you want to rotate someting such as a missile fin for let say 20 degrees you have to fill the cylinder with a fluid/gas where pneumatics will beat hydralics.

The decision what type of actuation to use is far more complex and vast to get into it on this forum but there are cases where hydraulics will be the best candidate while some other times pneumatics and sometines electric actuation will be the better.

 

[jsummerfield]

israelkk,
I use electronic and pneumatic controls most of the time with rare hydraulic applications. However, 3000 psig hydraulic fluid can move things with much smaller components than possible using 100 psig air.

John

 

[israelkk]

Sure, because you are using 3000 psi for the hydraulics which means that for the same force you can use smaller pistons. However if you will use 3000 psi pneumatic pressure you will see how much faster it will be. You are talking about commercial produts. In the aerospace industry especially in missiles pneumatic pressure goes up to 10000psi and even higher. You are not looking at the physics of fluid flow use the laws of physics and fluid flow equations and see the difference.

 

[hydromech]

Hydraulic & pneumatic systems are both used to transfer energy. Hydraulic systems transfer the energy directly because oil is not compressible, that is in relation to air/gas that is highly compressible.

On a hydraulic system the force applied by a pump is transmitted directly to a cylinder or motor via the pipework.(without considering losses)

On a pneumatic system, a compressor forces air into storage vessels called air receivers. The pressurised air is stored in these receivers until the air is required to move a cylinder or motor. A valve is operated and the air(energy )is allowed out of the air receivers, through the pipework and into the cylinder or motor.

The forces applied are still the same, Force = Pressure x Area etc...

The factor that dictates that pneumatic valves are so much smaller than hydraulic valves is the physical form of gas...in simple terms it is much easier for air to move through a valve than it is for oil. This allows the pneumatic valves to be much smaller than hydraulic valves.

The only thing that relates hydraulics and pneumatics is pressure...other than that, they operate and behave very differnetly.

 

[meintsi]

Pneumatics is a subset of hydraulics. The only difference is the fluid used for power transfer.

It is because of the compressibility of air, that you get differences in response and the amount of work availiable for equal amounts of pressure and flow. Hydraulic sytems are therefore generally more efficient and also more costly.

The compressibility of air also is the reason that pneumatics are basically restricted to smaller power applications. But when applied correctly, pneumatics can be much faster and much more responsive than its oil-hydraulic equivelent.



Remember...
"If you don't use your head,
your going to have to use your feet."

 

[heavenly]

Hi Freinds,
Firstly,i am very thankful to you all for clearing my doubts.
If i join a school which teaches only PNUEMATICS,is that
useful to make career in FLUID POWER,Bz i dont have schools
to teach FLUID POWER

Thanks in before

John

 

[danhelgerson]

I am a little late getting in to this but here is another important aspect to consider; efficiency. In pneumatics, we waste most of the energy because of the way we handle the gas. The typical pneumatic system is only 6% efficient. A well designed system may be 15% efficient. As a general rule, if you are using a 100mm or larger pneumatic cylinder at 6 bar, it would be less costly to use hydraulics in terms of installed and operating costs.

If you get some good pneumatic training, it will be a good foundation for understanding hydraulics. The International Fluid Power Society,

http://www.ifps.org

is a good place to start looking for good study material.

 

[israelkk]

danhelgerson

What happens if you use the pneumatic cylinder at 700 bars?
In aerospace this is a usual pressure in pneumatic cylinder or spheres.

Can you explain more accurately and hopefully mathmatically your statement "In pneumatics, we waste most of the energy because of the way we handle the gas"?

 

[budt]

israelkk;

In an industrial setting air at 700 bar would be such a hazard that I would not want to be in the same building with it. On top of that imagine someone who has never worked with any kind of air equipment or who may work on a 100 PSI system once a month or less, which is the way most of industry maintains air and hydraulic equipment, and then send them out to work on a 4,350 PSI air system.

That would be similar to sending your 12 year old to work on a 4,400 Volt electrical system and expect them to come back alive.

A local plant has over 2,400 HP of air compressor capacity. They also have a 750 PSI setup to leak test their product but that is a whole different operation in only one area of the plant.

Another customer I worked with used 10,000 PSI air to blast coal in underground coal mines. This was due to the fire hazards from dynamite or other explosives. I never saw it work but they claimed it gave a similar effect to blasting powder.

On air compressing it is very inefficient. I had not come across the figures Dan reported for hydraulics but the air figures are close.

Imagine an air compressor piston with a 10" stroke that takes in atmospheric air at sea level and actually was filled with 14.7 PSIA atmospheric pressure when it passed Bottom Dead Center(BDC). When the piston moves half wau back towards Top Dead Center pressure in the cylinder would be 29.4 PSIA. When it moves to within 2.5" of TDC pressure would be 58.8 PSIA and further movement to 1.25" from TDC would get it up to 117.6 PSIA (103.1 PSIG). From this point on their will be some movement of air into the receiver if it is below 100 PSIG so that means the compressor is not moving much air into the system for each stroke it makes. If the system was operating at 150 PSIG you can see it would even be sending less air into the receiver.

If this same piston was full of Liquid instead of Gas it would move fluid from BDC to TDC and be a lot more efficient.

Does that give you an idea of why compressing air is so inefficient.

The 10,000 PSI air comressor I mentioned before actually had 7 pistons in stages. Air from the first piston was portd to the next smaller one and so on until the final 5/8" diameter piston pushed some air out at 10,000 PSIG.

Air is an expensive way to transmit energy. Notice how many persons have tried to use compressed air to drive automobiles. To date I have not come across any on the highways though it would be a very non-polluting way to get around if you didn't count the amount of coal you had to burn at the power plant to produce the energy at such a low efficiency.


Bud Trinkel CFPE
HYDRA-PNEU CONSULTING, INC.
fluidpower1 @ hotmail.com

http://www.fluidpower1.us

 

[hydromech]

Digressing slightly...

How many of us in working in industry have ever stood in a quite machine shop or workshop...etc...with a fully charged compressed air system and not heared the all too familiar sound of air leaking? I'm betting not many.

In the U.K, the average amount of money wasted on running a compressor to recharge the compressed air system to make up for leaks is approximately £2000.00...nearly $4,000.00.

Imagine if that air leaking was actually oil...who would stand for it then. If you can't see it, it can't be there...can it?

For Sale...Pnuematic Valves...buy 1 get 5 free...They can't give them away around here.

 

[hydromech]

for quite read quiet.

 

[danhelgerson]

Israelkk

When air is compressed it gets hot. In average industrial compressors, the air temperature rises to about 150 C. No one wants to grab on to a hand tool with air that hot driving it. The air is cooled either with an after-cooler or through the walls of the receiver. About 20% of the energy used to produce the compressed air is thrown away in BTU’s. Undersized and poorly designed plumbing adds pressure drop to the loss. The “if in doubt, meter out” mentality causes most actuators to be run at regulated pressure with speed being controlled by throttling the exhaust. It is like driving with one foot hard on the accelerator and controlling speed with the brake. It is often thought that hydraulics is much more expensive. But that is only if it is thought that air is free. When the actual operating cost of a system is considered, hydraulics will very often come out on top. With the proper connectors and conductors, it is not only possible but quite easy to make a leak-free hydraulic system.

I hope this is helpful.


Dan Helgerson CFPS, AFPI, AJPP

http://www.cfpsos.com

 

[israelkk]

danhelgerson

Since your experience is industrial use you are assuming a compressor etc. and you assume the gas is air. In the aerospace area the pneumatic pressure is comming from a pre-compressed high to ultra high pressure vessel and the gas can range from Nitogen to Hellium. Therefore, there is no heat generated and noting will beat the speed and size of pneumatic actuator charged with Hellium. How do you think all those high manuever air to air missiles or air to surface missiles can produce high torque at very short time to the fin actuator.

 

[danhelgerson]

Israelkk

The question I was answering is about the overall efficiency of using a compressed gas as a power transfer device. I am not questioning the fact that there are certainly times when a compressed gas is the optimum choice. This does negate the fact that it is very expensive in terms of efficiency when we consider the cost of producing, storing, and conducting the gas. In your field, weight and exhaust are considerations that would naturally cause you to lean toward a compressed gas as the medium of energy transfer. I do not have the numbers but I am convinced that if you look at the amount of energy used to produce your high-pressure gasses as related to the actual power available (Kw in/Kw out) you will find it extremely inefficient.


Dan Helgerson CFPS, AFPI, AJPP

http://www.cfpsos.com

 

[israelkk]

danhelgerson

This is exactly my point!

General discussions are just a waste of time and misleading. From my experience, for every application a preliminary evaluation should be done to find what will work best for the specific applications. This step is very cheap compared to the whole cost of the project but it is the step that mostly affect the total cost of the project. Bad choice can lead to failure to meet specifications, many faults during product life, re-engineering, customer dissatisfaction etc.

I can't count the cases I have seen where the project/system manager failed to do this evaluation and chose the actuating method based on his "fear" from methods he is not familiar with. For example: most of the project/system manager with a background of electronics or control "afraid" of considering pneumatic actuation systems because in it's nature it is non-linear and they can't use linear control analysis. The result is a non robust inferior system that fail to meet the specifications, resulting in re-engineering of the system, delays etc.