quote:

A*dx = v*dm

The left hand side as units of length^3 or volume. The right hand side has units of volume*dm? dm better be dimensionless or the units don't match. Try again.

quote:

The net flow work is the difference between the flow work entering and leaving. Equation (3.51) becomes

It is hard to see what you are getting at since you don't define all your variables. I suppose that dQ/t is heat instead of flow in this case. Then it makes sense. BTW, what does this have to do with work is the integral of force over distance? That equation simply tells how much work can possibly be done using a energy balance. It is not the definition for work.

How is your BP doing?

quote:

work is the integral of force over distance

But here's another definition...
"Work can be defined as transfer of energy.
In physics we say that work is done on an object when you transfer energy to that object. If one object transfers (gives) energy to a second object, then the first object does work on the second object."

A volume of oil ( mass ) moves from the high pressure side oil volume into the low pressure side oil volume as it attempts to equalise the pressure difference....So a mass has moved a distance (internally) is this not work?

Hmmmmm...

There seems however difficulty in defining the "object" ...( unless I borrow J.A's barrier )... so it would seem that the diffuse nature of this mass transfer should only be considered in energy terms?

http://www.askdrthermo.com/ask...energy_equation.html

Does it matter in this instance ...other than academically ...wether the energy transfer is defined as work or some other energy form?


Regards Woody

This message has been edited. Last edited by: Woodygb,

quote:

Originally posted by Woodygb:

quote:

work is the integral of force over distance

But here's another definition...
"Work can be defined as transfer of energy.

Are you an JA going to make all the websites change the definition of work?
I agree that work does transfer energy but work is just one of many ways of transferring energy. Work is specifically the integral of force over distance.

quote:

In physics we say that work is done on an object when you transfer energy to that object.

Can an object do work on itself? Sounds kinky. Can a cross section of a spring do work on the next cross section of the spring? Or is work done only on the spring itself by external forces?

quote:

If one object transfers (gives) energy to a second object, then the first object does work on the second object."

So if I place my hot stone next to your cold stone does my hot stone is do work on your cold stone? This is radiation.

quote:

A volume of oil ( mass ) moves from the high pressure side oil volume into the low pressure side oil volume as it attempts to equalise the pressure difference....So a mass has moved a distance (internally) is this not work?

This is a better argument than JA made. I wouldn't have bought this if JA was smart enough to come up this answer. I don't even think it requires the "invisible barrier". Before issue is whether work is done within the same object like the spring or the equalizing oil pressure. Now the center of mass as changed.

quote:

Hmmmmm...

There seems however difficulty in defining the "object" ...( unless I borrow J.A's barrier )... so it would seem that the diffuse nature of this mass transfer should only be considered in energy terms?

Yes. Actually I presented the 'barrier' on Posted 09-10-03, 20:08. I don't think JA could have thought of that on his own. I don't like the invisible barrier idea. I like the center of mass changing as the oil pressure equalizes.

quote:

http://www.askdrthermo.com/ask...energy_equation.html

This is a better description of what JA was trying to say. At least the units are right. I book marked this. It is too bad the links don't work at the bottom.

quote:

Does it matter in this instance ...other than academically ...wether the energy transfer is defined as work or some other energy form?

JA and I agreed that the energy is still the same. It is just a matter of definitions. I am just giving JA grief over his terminology. Too many use the term work when they really means something else. JA isn't as good as your are at presenting a valid argument for his case. I haven't seen anybody calling equalizing pressure as work. I also said I have not seen an alternative term for equalizing pressure but I found a link that best described what JA was talking about. Your shifting mass idea adds a new wrinkle. Note, if the fluid is incompressible then the center of mass will not move and no work will be done. Therefore equalizing pressure in itself is not doing work.

You are going to ruin my fun. Hopefully you didn't spend too many hours finding the answer to this. I intended JA to waste the hours finding an answer to this if there is one.

quote:

Originally posted by Peter Nachtwey:


It is too bad the links don't work at the bottom.

Worked for me....Links at bottom

Edit:
Here is a link to main entrance of AskDrThermo

quote:

Note, if the fluid is incompressible then the center of mass will not move and no work will be done

You'd also never be able to add pressure energy to the fluid if it was incompressible and pressure equalisation would become irrelevant.

Cheers Woody

I had invited Peter to come up with a name for the process known as flow work some time ago hoping he would deliver another of his comical descriptions, but he hasn’t. Perhaps he can come up with a new term for pressure. Perhaps you want to call pressure hydra-energy, then whenever someone says something about pressure you can jump on them and tell them they don’t know the difference between pressure and energy. That sounds like an excellent plan to continue to confuse and mislead with useless drivel. No need to let science get in the way.

Variables for the equation in my previous post.

v – specific volume
m – mass
Ecv – Total energy of the fluid in the control volume
e – specific energy of the fluid
Q – heat energy being added to the control volume
Wm – mechanical work being done by the fluid
Wflow – flow work being done on or by the control volume

The equation is simply stating the rate energy is changing in the control volume is equal to the rate of heat addition, minus the rate mechanical work is being extracted, plus the rate fluid energy is entering the volume, (e*dm/dt)in, plus the rate flow work is being done on the volume, (p*v*dm/dt)in, minus the rate fluid energy is leaving the volume, (e*dm/dt)out, minus the rate flow work is being done by the volume, (p*v*dm/dt)out. If you are unable to understand

δ (Wflow)=p*A*dx=p*v*dm

I don’t know what to say to you. Not only are you ignorant of basic fluid dynamics but you can’t even recognize facts placed in front of you. Let’s see, p*A, isn’t that force? So isn’t δW=F*dx? Keep youreyes closed and misunderstand all you see Peter.

Peter’s ignorance of fluid remains astounding. All of science and engineering uses the proper definition of work but Peter insists on remaining ignorant.

Peter Nachtwey quote October 23, 2009, 7:05pm
“Are you an JA going to make all the websites change the definition of work?
I agree that work does transfer energy but work is just one of many ways of transferring energy. Work is specifically the integral of force over distance.”

Don’t need to Peter, the definitions are consistent with flow work. Are you going to change all the physics, fluid dynamics and thermodynamic texts to support your ignorance?

Peter Nachtwey quote October 23, 2009, 7:05pm
“Can an object do work on itself? Sounds kinky. Can a cross section of a spring do work on the next cross section of the spring? Or is work done only on the spring itself by external forces?”

Of course it does Peter. Take a free-body diagram of a single coil of in the middle the spring. When the spring is compressing there is an input force and input velocity to the coil and an output force and output velocity at the other end. The input end is the rate work is done on the coil and the output end is the rate the coil is doing work. The difference is the rate the coil energy is increasing, distortion and kinetic energy. You are so hapless Peter. Again, this is covered in freshman physics.

Woodygb quote 02 October, 2009
“And finally 3 answers to the potential energy equations

Yours using the final volume.
1250 in/lbf
Yours using a derived initial volume of 81 cubic inches.
1265.63 in/lbf
My calc using the webpage formula and the same derived initial volume.
1265.625 in/lbf
and using Peters with a derived initial volume.
1255.181 in/lbf”

I assume these are in-lbf in 40 in^3 of fluid at 2500 psi or about 30 psi in energy per unit volume. Flow work is not energy in the fluid. The Pressure Energy definition you provided is not dealing with energy in the fluid. It describes the work done by the fluid or the energy transferred by the fluid. This energy is transmitted through the fluid, from the source, say a pump to the load, less frictional losses that convert to thermal energy in the fluid, less increases in kinetic energy of the fluid, less increases in compression energy of the fluid, less any of other potential energy in the fluid, such a gravitational potential. P*V is not energy in the fluid. As you have shown, the compression energy in the fluid due to pressure is about 30 psi, not 2500 psi. You could state both in BTU/gallon and still the pressure would be 80 times greater than the compression energy in the fluid. The work done by the fluid is not in the fluid at any time any more than the work transmitted by the piston rod, from the piston to the load, is in the piston rod. As work is done on the piston, the piston rod instantaneously is doing work on the load, less any distortion or kinetic energy in the rod. The same as with the fluid column. The energy in the transmission medium is small compared to the work done. This is not a heat cycle or compressed gas where considerable energy is put in the fluid. Conservation of energy is an important priciple in engineering and science and saying the energy density of the fluid is 2500psi simply is wrong.

quote:

Originally posted by John Andderson:
I had invited Peter to come up with a name for the process known as flow work some time ago hoping he would deliver another of his comical descriptions, but he hasn’t.

NO YOU DIDN'T LIAR. You recently introduced the topic of work flow but that still isn't the definition of work. It just tells how much work can be done or at what rate.

quote:

Perhaps he can come up with a new term for pressure. Perhaps you want to call pressure hydra-energy, then whenever someone says something about pressure you can jump on them and tell them they don’t know the difference between pressure and energy. That sounds like an excellent plan to continue to confuse and mislead with useless drivel. No need to let science get in the way.

This really bugs you doesn't it. I am not misleading anybody. You are. Just tell me what force over what distance then you can call it work. Otherwise start re writing all the text books and changing all the websites.

quote:

Variables for the equation in my previous post.

v – specific volume
m – mass
Ecv – Total energy of the fluid in the control volume
e – specific energy of the fluid
Q – heat energy being added to the control volume
Wm – mechanical work being done by the fluid
Wflow – flow work being done on or by the control volume

Finally, it is hard to read the a mind that is intentionally left blank.

quote:

The equation is simply stating the rate energy is changing in the control volume is equal to the rate of heat addition, minus the rate mechanical work is being extracted, plus the rate fluid energy is entering the volume, (e*dm/dt)in, plus the rate flow work is being done on the volume, (p*v*dm/dt)in, minus the rate fluid energy is leaving the volume, (e*dm/dt)out, minus the rate flow work is being done by the volume, (p*v*dm/dt)out. If you are unable to understand

Yeah, yeah. Why bother? Woodygb beat you to it. You couldn't get even your units right.
The result of your energy balance is just how much energy has been converted to work but that isn't the definition of work itself.

quote:

δ (Wflow)=p*A*dx=p*v*dm

What is wrong with you? Are you trying to change the subject because you are stumped? What is with this little delta stuff? Can't you answer the simple question. What force over what distance? You haven't answered that. The best argument requires invisible barriers so the high pressure part pushes on the low pressure part. I had to come up with that argument but there are no invisible barriers in reality. Does the math work out the same? Yes we agree on that but change the definition of work on Wikipedia from something other than the integral of force over distance. I dare you. DO IT OR GET LOST.

BTW, what have you done again? Nothing we know of.

Peter seems to be completely delsuional.
Posted by Peter Nachtwey October 27, 2009 12:55pm

"quote:
Originally posted by John Andderson:
I had invited Peter to come up with a name for the process known as flow work some time ago hoping he would deliver another of his comical descriptions, but he hasn’t.


NO YOU DIDN'T LIAR. You recently introduced the topic of work flow but that still isn't the definition of work. It just tells how much work can be done or at what rate."

Post by John Andderson September 24, 2009 1:07pm
"Peter,
I’m surprised, after having discovered the harm done by an innocuous statement like “Flow makes it go”, what sort of trouble will be caused by misunderstanding the process of transferring energy to fluid in the cylinder? Isn’t this the crux of it all Peter? How does energy get from the pump shaft to the rod end? If we don’t know how the energy is transferred we are bound to make errors. You’ve agreed that fluid flowing into the cylinder is increasing the energy of the fluid in the cylinder. You also made a strong and harsh statement that I don’t know the difference between work and energy so you must have a pretty clear idea how the energy is transferred. Only an “ignorant troll” would make such a statement when they didn’t know. Energy conversion and utilization is a fairly important cross discipline area of engineering that relies on precise definitions and equations so why don’t you just tell us what the energy transfer process is so we can move along and not spend 10 years on this."

I said flow work is flow work, what funny name do you want to give it so you can continue your idiotic quest to misinform and mislead. Amazing that you cant understand the simplist equations

quote:

Originally posted by John Andderson:
Peter seems to be completely delsuional.

No, I am just screwing with your mind but it appears to be stripped.
JA still hasn't figured out quotes, cuts and paste.

quote:

I had invited Peter to come up with a name for the process known as flow work some time ago hoping he would deliver another of his comical descriptions, but he hasn’t.

You did finally found the right term, flow work. You must of recently discovered this term. You use of the term 'work' has been wrong.

quote:

You also made a strong and harsh statement that I don’t know the difference between work and energy so you must have a pretty clear idea how the energy is transferred. Only an “ignorant troll” would make such a statement when they didn’t know.

It it hard to know when you use terms like work incorrectly and can't get your units right. You have been so sloppy.

quote:

Energy conversion and utilization is a fairly important cross discipline area of engineering that relies on precise definitions and equations so why don’t you just tell us what the energy transfer process is so we can move along and not spend 10 years on this."

I accept the term flow work or flow energy but that isn't what you said when you were correcting Doug Hanson and giving me grief not knowing the correct term for flow work. You used the term work which is wrong as work is the integral of force over distance. Admit you were sloppy and in error and used the term work incorrectly instead of flow work.

I had to give you grief for getting your units wrong and for posting all that junk about mass when a simple link would do like I posted below. It looked to me like you were shot gunning the answer instead of giving a simple precise answer or perhaps you don't understand what part of all those equations is flow work. I know picky picky but I wouldn't want to be "sloppy"

I admitted that I didn't know the proper name for flow work but I knew it that it isn't the same as work. I took the safe route and didn't go out a limb like you did. I did know that pressure is an an energy density which is more than what you knew.

http://en.wikipedia.org/wiki/Enthalpy

There is a paragraph that starts
"The form pV (sometimes called "flow work")"
You can see that pressure must be a measure of energy density.

So we all learned a new term
and you have been wrong twice.
1 not knowing the definition of work and using it in a "sloppy" manner.
2 not knowing that pressure is a measure of energy density. Sure there are other forms of energy. Perhaps I would be more correct to say pressure is a measure of flow energy density.

Perhaps if you did more research before being so critical you wouldn't find yourself in these embarrassing situations.

Pay backs are a bitch aren't they.
So what have you done? Anything?
You can apologize at any time for all those insults you hurled at me or just go away.

Hi Peter, I realize one can never know everything there is in the fluid power world but with your obvious elevated knowledge,(Im not trying to offend!)can you troubleshoot fluid power problems in the field? Book knowledge is great if used in a practical way that serves the greater good, however if used to intimidate the less knowledgable or qualified, what else is it being used for? I mean are you qualified or can you troubleshoot on your feet? Your posts sound as though you are trying to impress or intimidate us "knuckle draggers",of which I do consider myself,(qualified to troubleshoot on my feet).Lets hear about your "in the trenchs story". I will admit, you do have some interesting posts! Thanks Much! Brettl3

quote:

Originally posted by brettl3:
Hi Peter, I realize one can never know everything there is in the fluid power world but with your obvious elevated knowledge,(Im not trying to offend!)can you troubleshoot fluid power problems in the field?

Yes, but I have it easy. The motion controller can record what is happening in great detail.
This makes diagnosing problems mush easier. Often I can tell what is wrong by looking at the data. This data can be sent to me in a e-mail from around the world.

The knowlege of how the system should work helps too. I have experience on many different types of machines.

quote:

Book knowledge is great if used in a practical way that serves the greater good, however if used to intimidate the less knowledgable or qualified, what else is it being used for?

My company makes hydraulic motion controllers. How can we control what we don't understand? Believe me we apply what we know and we are limited only by the processing power available and how mush people are willing to pay for a controller.

quote:

I mean are you qualified or can you troubleshoot on your feet?

How many people do you know that have been paid to go around the world to trouble shoot and install systems? I/we get paid the big bucks to go into the field.

quote:

Your posts sound as though you are trying to impress or intimidate us "knuckle draggers",of which I do consider myself,(qualified to troubleshoot on my feet).

I am not trying to intimidate anybody but John Andderson and he asked for it long ago at the beginning of this thread. I stay out of most thread that deal with parts, log splitters, skid steers etc. There is nothing to be gain intiminating maintenance people and technician. I am mostly interested in the designers that can't design a servo system. Then we get the call when the hydraulic people blame the motion controller.

quote:

Lets hear about your "in the trenchs story". I will admit, you do have some interesting posts! Thanks Much! Brettl3

What are you interested in? A press, flying shear, injection modling machine, powdered metal press, steel mill, synchronizing axes on a 4 knee carriage, actuall there are many saw mill applications. There are many more where I could trouble shoot from the office.
Pick one. I have many. I will post one in a different thread. There is one that I talked about at a seminar at a IFPE show.

I have been at this for over 25 years. My first control system was in 1983. I was thrown to the wolves and I survived. I was ignorant then, I just did better than others. I have made no secret about it. I learned a lot from the school of hard knocks.

I find this interesting. I am usually the one that asks "what have you done?".

Are you aware that my company makes hydraulic motion controllers? See
http://www.deltamotion.com/
You can ask for this
http://www.deltamotion.com/com...news/pr_20090714.php
It is a collection of articles I have written for H&P magazine, the same guys that sponsor this forum. It is a very pretty booklet but it is light weight. There are others that monitor this forum that have seen me "geek out" with differential equations and simulation.
Here is a fraction of the applications that we have been involved in.
http://www.deltamotion.com/applications/index.php

We also make hydraulic motion controllers for Rockwell or Allen Bradley. We designed and manufacture the 1746-QS, 1756-HYD02 and the 1756-M02AS.

Hi Peter, Thanks for the quick reply-I believe we have your companies controllers here at our plant. They were recently istalled on six of our TMP Refiners. Anti-plate clash is what they were installed for. Problems with our operators not paying attention to the equipment and hence the anti-plate clash istall. I also have been hydraulic troubleshooting for 25 years and have a long way to go- A "so called fluid power expert" got me interested-(by the time he was finished with the hydraulic problem, believe me, we were way worse than when he started!) No one else in the plant knew anything about fluid power,(including myself) so that got me started in an extremely interesting field and I never looked back. I cant get enough troubleshooting to suit me but thats OK for now. Anyhow I believe you have the smarts for the design end and now I know who is a "Go To Guy" when Im stumped. By the way, when a mechanic or technician blames the controller or the pump,99% of the time you can bet he dont know what he is doing. haha- Thanks, Brett

Peter remians the laughable fool. Like a 5 year old with frosting on his face he denies taking the cupcake. Yeah, right Peter. You want to reinvent yourself and pretend you were just trying to be dishonest with people but the truth is you just aren't educated nor honest. You still don't know the difference between work and energy. What sort of ignorant saps believe you? Work is work, whether it is done by mechanical elements or by fluid elements. Yes, pv is the work done by a fluid delivered by a hydraulic source at constant pressure, it is not the energy in the fluid. In science and engineeging, energy density is a property of a substance and mulitplying the energy density by the volume of the substance gives the total for that type of energy in the substance. The compression energy density in a fluid is roughly proportional to the square of the pressure, the pressure is not the energy density.

Just for laughs Peter, lets say we have two identical coil springs and two identical cylinders. The spring rates are 1000lbf/inch and the cylinders are single ended and have 5square inch drive areas. Both are pressurized to 1000psi to compress the springs 5inches. Cylinder A is filled with a massless, frictionless fluid with a constant bulk modulus of 100,000 psi. Cylinder B is filled with a massless, frictionless, incompressible fluid. How much energy is in cylinder A and how much energy is in cylinder B? Both cylinders are perfectly rigid. You've been given the answer to this several times so why can't you figure it out? Perhaps your lack of education makes it hopeless explaining things to you.

Also, the units are not wrong, you just are not able to understand that the specific energy times mass flow rate is the rate energy is beign brought into the volume by the fluid mass and the specific volume times the mass flow rate is the volumetric flow rate. You need to take freshman physics before you can begin to intelligently discuss fluids.

quote:

Originally posted by Peter Nachtwey:

quote:

Originally posted by John Andderson:
Peter seems to be completely delsuional.

No, I am just screwing with your mind but it appears to be stripped.
JA still hasn't figured out quotes, cuts and paste.

quote:

I had invited Peter to come up with a name for the process known as flow work some time ago hoping he would deliver another of his comical descriptions, but he hasn’t.

You did finally found the right term, flow work. You must of recently discovered this term. You use of the term 'work' has been wrong.

quote:

You also made a strong and harsh statement that I don’t know the difference between work and energy so you must have a pretty clear idea how the energy is transferred. Only an “ignorant troll” would make such a statement when they didn’t know.

It it hard to know when you use terms like work incorrectly and can't get your units right. You have been so sloppy.

quote:

Energy conversion and utilization is a fairly important cross discipline area of engineering that relies on precise definitions and equations so why don’t you just tell us what the energy transfer process is so we can move along and not spend 10 years on this."

I accept the term flow work or flow energy but that isn't what you said when you were correcting Doug Hanson and giving me grief not knowing the correct term for flow work. You used the term work which is wrong as work is the integral of force over distance. Admit you were sloppy and in error and used the term work incorrectly instead of flow work.

I had to give you grief for getting your units wrong and for posting all that junk about mass when a simple link would do like I posted below. It looked to me like you were shot gunning the answer instead of giving a simple precise answer or perhaps you don't understand what part of all those equations is flow work. I know picky picky but I wouldn't want to be "sloppy"

I admitted that I didn't know the proper name for flow work but I knew it that it isn't the same as work. I took the safe route and didn't go out a limb like you did. I did know that pressure is an an energy density which is more than what you knew.

http://en.wikipedia.org/wiki/Enthalpy

There is a paragraph that starts
"The form pV (sometimes called "flow work")"
You can see that pressure must be a measure of energy density.

So we all learned a new term
and you have been wrong twice.
1 not knowing the definition of work and using it in a "sloppy" manner.
2 not knowing that pressure is a measure of energy density. Sure there are other forms of energy. Perhaps I would be more correct to say pressure is a measure of flow energy density.

Perhaps if you did more research before being so critical you wouldn't find yourself in these embarrassing situations.

Pay backs are a bitch aren't they.
So what have you done? Anything?
You can apologize at any time for all those insults you hurled at me or just go away.

JA is still trolling. I have posted the formula for calculating the energy due to compression. What part don't you understand? You seem to be thinking that I said the 1 cubic of oil at 1000 psi has 1000 in-lbs of energy but I never did. You simply can't read. I wonder if you can even derive the formula.

As far as the units in your formula way back. They don't match. How simple can it be. Perhaps you should have defined the units so they wouldn't be misinterpreted. I have trouble reading minds that are intentionally left blank.

You used the wrong term (work) in your explanation of oil compressing to Doug. I at least knew that work was the wrong term. Now get over it and go away.

BTW, what have you done? Nothing that any of us can see.

JA: with all of the interesting posts that have graced the pages of this forum lately, I come across yours. Very dissapointing. You open with a personal attack against an individual and then a blanket insult towards everyone on the forum that may disagree with your opinion. Please stop posting and go away, or contribute, but drop the insults and agree to disagree when required. You are the first person to join my 'ignore list'.

quote:

Originally posted by John Andderson:
Peter remians the laughable fool. Like a 5 year old with frosting on his face he denies taking the cupcake. Yeah, right Peter. You want to reinvent yourself and pretend you were just trying to be dishonest with people but the truth is you just aren't educated nor honest. You still don't know the difference between work and energy. What sort of ignorant saps believe you? Work is work, whether it is done by mechanical elements or by fluid elements. Yes, pv is the work done by a fluid delivered by a hydraulic source at constant pressure, it is not the energy in the fluid. In science and engineeging, energy density is a property of a substance and mulitplying the energy density by the volume of the substance gives the total for that type of energy in the substance. The compression energy density in a fluid is roughly proportional to the square of the pressure, the pressure is not the energy density.

Just for laughs Peter, lets say we have two identical coil springs and two identical cylinders. The spring rates are 1000lbf/inch and the cylinders are single ended and have 5square inch drive areas. Both are pressurized to 1000psi to compress the springs 5inches. Cylinder A is filled with a massless, frictionless fluid with a constant bulk modulus of 100,000 psi. Cylinder B is filled with a massless, frictionless, incompressible fluid. How much energy is in cylinder A and how much energy is in cylinder B? Both cylinders are perfectly rigid. You've been given the answer to this several times so why can't you figure it out? Perhaps your lack of education makes it hopeless explaining things to you.

Also, the units are not wrong, you just are not able to understand that the specific energy times mass flow rate is the rate energy is beign brought into the volume by the fluid mass and the specific volume times the mass flow rate is the volumetric flow rate. You need to take freshman physics before you can begin to intelligently discuss fluids.