Heat in hydraulic systems 5

[danhelgerson]

Heat is generated in a hydraulic system whenever the fluid moves from high pressure to low pressure without doing work. It is a measure of the efficiency of the system.

For example, if a pump is directing fluid across a relief valve, all the fluid is going from high pressure upsteam from the relief valve to low pressure downstream from the relief valve with no useful work being done. All the energy put into the fluid through the pump is turned directly into btu's and warms the fluid.

If you have excessive heat in your hydraulic system, look for places where high pressure fluid is escaping to the low pressure side; across a relief valve, internal leakage in the pump, leakage within the directional valve, flow control valves, etc.

 

[tc7]

Hi Dan-
I have been looking for a technical explanation of why so much heat is generated during throttling. Usually the Thermo and Fluids texts only describe the heat generated as "losses" while some refernce books may convert the fluid flow and pressure drop into work or HP, and then proceed to convert into BTU's per time. It is left to the imagination WHY heat is produced. Is the heat due to fluid friction alone or is there another phenomenon at work? I'd like to see a text or an article which can explain this. Thanks if you can help.
Tom

 

[danhelgerson]

In a fixed displacement hydraulic system, throttling will always introduce heat. The result of friction is a factor but not he major one. Fluid friction is simply a way of describing what is going on at the molecular level as the liquid passes through the connectors. Hydraulic fluid is formulated with a relatively long molecular chain which gives it its viscosity and lubricity. As the fluid stream passes through the hose, the molecules at the edges of the stream tend to adhere to the inner walls of the hose. This slows down the flow at the outer edges of the fluid stream and causes a stress and tearing of the molecular chains. This stress draws energy from the fluid and is the reason for the addition of BTU's to any liquid on motion. This is the reason for sizing the connectors in a way that will reduce the velocity of the fluid; the lower the velocity, the smaller the ratio of stressed molecular chains and so the ammount of heat build up in the fluid.

However, in a throttling situation, there is something else going on. Energy from the prime mover has been transfered into the fluid by the pump. This energy must either do work or be turned into heat. The molecules of the fluid have been compressed, slowing the molecular activity. When we throttle a system, we allow some of the compressed fluid to escape to the low pressure stream. This portion of the fluid decompresses, increasing the molecular activity (endothermic?) and releases the energy as heat.

I hope this is useful information.

 

[RBedi]

Even if you go by Bernaullis equation, you will find while throtlling, just at throttle point or at orifice point whole pressure drop or pressure energy will convert into kenematic energy means velocity of fluid at that point will be very high as proportion to pressure drop. BUT when this velocity & small area flow comes into contact with full diameter or full surface of downtream pipe, then this velocity will reduce to very less, so whole kinematic enregy will reduce, so where this lost energy will go, this will be converted into heat due to reducing of speed of fast moving particles to almost zero, acting like a braking efect. Speed of these fast moving particles reduced by striking on surface of full diameter & due to sudden stoppage & friction, it converted into heat.
I think I explained or let me know.

 

[danhelgerson]

Thank you for your comment. If I understand you correctly, you are saying that it is primarily the change in velocity that produces heat, is that correct?

I always thought of Bernoulli's Principle as relating to pressure and velocity as opposed to the addition of heat to the fluid. I agree that the fluid that is passing across an orifice will increase velocity at that point with a consequent drop in pressure. Are you saying that the high speed, low pressure particles that now enter the low speed return line stream release their energy in heat? If this true, the heat is actually generated sometime after it passes across the orifice.

 

[vettedriver]

Is there a listing of fluid power formulas that can calculate the amount of heat generated by a given hydraulic fluid through a valve, an orifice, or even a given pressure drop through a fitting?

 

[danhelgerson]

There are a couple of simple formulas and concepts that may be helpful. The first is the hp formula: HP = (GPM x psi) / 1714. Or KW = (LPM x bar)/ 600. This is useful when using a flow divider or a bleed-off orifice and a fixed displacement pump. For example, if a pump is producing a flow of 10 gpm with a resistive load of 1714 psi, 10 hp will be generated through the fluid. If you were to bleed off 5 gpm through a flow divider or an orifice, then 5 hp would be wasted and turned into btu's.
If the same 10 gpm is divided into one stream of 5 gpm at 1714 psi and another stream of 5 gpm at 857 psi, the result would be a waste of 2.5 hp of heat.

Another formula is for the pressure drop across an orifice. Delta P=(GPM /(18.5*d^2))^2 where delta P is the change in pressure across the orifice and d is the diameter of the orifice in inches. To try out the formula, 10 gpm across an .25" orifice would result in a pressure drop of about 74.8 psi.

If you combine the hp and delta P formulas, you would find the heat added as a result of the orifice to be (10 gpm x 74.8)/1714 or .44 hp.

I hope this is helpful.

 

[RBedi]

Dear danhelgerson

yes, this high velocity particles when enter into return lines having very less pressure & at the same time sudden low velocity, causes loss of pressure energy, see, you have high pressure & low velocity before orifice, now just at orifice, velocity is very high & pressure is very less, but as these enter into more dia pipe having already low pressure(return line), then you cannot get back your lost high pr. so to make energy balance, lost energy is the converted HEAT.

Actually, this change in velocity after orifice will produce heat. If you keep the dia of downstream pipe very - very less & pressure is also less, you should find less heat as compared to more dia of downstream pipe.Pl let me know if you are agree with this.

Rajan

 

[danhelgerson]

I agree. However, your last statement about the small diameter of the downstream pipe could be confusing to some readers. I am sure you will agree but I want to state it to prevent any misunderstanding.

If the downstream pipe diameter is very small and yet still ultimately leads to the reservoir, we have simply exchanged our sharp-edged throttle to a throttling pipe. The fluid will ultimately go from high pressure to low presssure without doing useful work. The energy loss will still be in terms of heat. The difference will be that the fluid will have to travel further to loose the energy and a greater portion of the energy will be lost do to the frictional losses through the small pipe.

If we could put a series of teperature gauges along our small diameter pipe and a temperature gauge just downstrem from out throttling orifice, in the first case we would see the gradual increase in temperature as the fluid moves toward the reservoir; in the second case we would see the immediate increase as the fluid enters the low pressure stream, but the last gauge on the small pipe and the gauge at the orifice would have the same reading.

 

[sailoday28]

I believe that heat is generated solely due to the friction generated by the flow of the liquid-whether from high to low pressure or vice versa.
If the pipe were perfectly insulated and friction negligible, no heat would be generated. Temperature would change to due the work of compression or expansion.

 

[danhelgerson]

Thank you for your comment. While it is true that there will always be flow when a liquid is moving from high pressure to low pressure, friction alone does not seem to account for the heat generation in a system. For example, using a cylinder to push a load at some speed will require some amount of power, but will generate very little heat because all of the energy will be transferred into the load. However, if you were to use the same power to push the liquid across a relief valve, almost all of the energy would be turned into heat because no usable work is being performed. The same flow at the same pressure is being moved from high pressure to low pressure. The difference is the work accomplished.

 

[sailoday28]

Fluid can flow from low pressure to higher pressure-as from the throat of a venturi to its outlet.
If the venturi were frictionless, kinetic, potential and internal energy would be conserved and no heat generated.
With flow resistance, the work to overcome friction is converted to heat.

 

[tc7]

Well, we can agree that there is an energy exchange between the dynamic and static pressures as fluid passes through the valve orifice, but the end result is always a drop in pressure and rise in temperature. We have not yet explained why the temperature rises except to say the various forms of energy remain in balance. I agree with Dan that friction alone does not seem to account for the heat generation in the throttling process. Sure, there is friction and viscous shear which must produce some of the temperature rise but I am beginning to think that cavitation may help explain it, based on all I have ever read on this question for several years: the idea is that bubbles form due to static pressure drop in the fluid, then temp of the bubbles rise (as a product of heat of compression) when they pass to a higher pressure in the down stream pressure recovery zone. This in turn results in heating the surrounding fluid.
I realize this is no more quantitative than blaming all the heat on friction, but maybe it adds an idea that someone else can take further.
Good discussions!
Tom

 

[sailoday28]

After much typing I lost my new post. This will be much shorter.
For processes that are throttling --and where Kinetic energy and PE are neglected, the change in enthalpy is 0.
The Joule Thompson (J-T)coefficent is a property of all fluids (such as specific heat, etc)
It is defined as a differential dT/dP at constant enthalpy. When J-T =0 the point is the inversion point.
Either side of the inversion point, the slope is either + or -.
From all your experience with liquids, the coefficient is negative.

 

[bvuk]

Question for danhelgerson:
What book can you recommend or is there anything on internet about detailed calculation of generated heat through orifices?

 

[hydromech]

Excellent thread guy's...

Having read it all...I would like to throw my hat into the ring and say...

As some clever chap once said or rather found out,"Energy cannot be created or destroyed, its form can be only be transformed from one state to another".

If you have a 100 HP motor on a system, it will put 100 HP of energy into the oil. If the oil cannot get rid of the energy by doing work it will, no matter what you do, transfer its energy to somewhere else.

The only form of energy oil can store is potential energy. Oil will emit energy very freely. If it cannot get rid of its energy by transfering it to kinetic, sound, force or via an accumulator into compressing gas, it will always realease it in the form of heat.

Simply put, if the oil is getting TOO hot, there is something wrong with the hydraulic system.

Hydromech
Hydraulic Systems Engineer

 

[budt]

Here's another Hat in the Ring.

In my experience a hydraulic system will not run hot unless there is wasted energy or on rare ocasions when they are located in a hot environment.

To see examples of energy wasters see my article on "Cool Hydraulics" at

http://www.fluidpower1.us

When I wrote this I did not include Proportional or Servo valve circuits that always require cooling. To get the control required for these circuits maximum energy must always be available regardless of how little work is required at any point in the cycle. This means the system is operating at low efficiency a large portion of a cycle.


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

http://www.fluidpower1.us

 

[danhelgerson]

Response to bvuk.

You asked if there is a book that I would recommend for calculating the generation of heat in hydraulic systems. My first choice would be to have you get a copy of the “Lightning Reference Handbook” which is available through the Fluid Power Society. You can find it at

https://www.ifps.org/Store/ord_form.htm#books

and it is item #18. The price is $15.50. Once you have access to the book, look on page 119. There is a lot of information there regarding heat in hydraulic systems.


Dan Helgerson CFPS, AFPI, AJPP

http://www.cfpsos.com

 

[frans]

If I were to stir a tin of cold syrup with a spoon, then over time the syrup will flow easier - heat is generated in a constant pressure environment. This heat is due to the interaction of the syrup molecules being pushed around by my labour. My labour also goes into kinetic energy, but when the syrup is cold and the molecular bonds strong I can hardly move the spoon, so I will argue the heat in this case is almost all generated by energy released when the molecular bonds are broken and re-formed.

Logically I would think the sam thing happens in hydraulic oil, hence I have to say that dan's argument makes a lot of sense.