Heat in hydraulic systems
Heat in hydraulic systems
(OP)
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.
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.





RE: Heat in hydraulic systems
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
RE: Heat in hydraulic systems
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.
RE: Heat in hydraulic systems
I think I explained or let me know.
RE: Heat in hydraulic systems
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.
RE: Heat in hydraulic systems
RE: Heat in hydraulic systems
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.
RE: Heat in hydraulic systems
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
RE: Heat in hydraulic systems
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.
RE: Heat in hydraulic systems
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.
RE: Heat in hydraulic systems
RE: Heat in hydraulic systems
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.
RE: Heat in hydraulic systems
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
RE: Heat in hydraulic systems
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.
RE: Heat in hydraulic systems
Question for danhelgerson:
What book can you recommend or is there anything on internet about detailed calculation of generated heat through orifices?
RE: Heat in hydraulic systems
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
RE: Heat in hydraulic systems
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 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
RE: Heat in hydraulic systems
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
www.cfpsos.com
RE: Heat in hydraulic systems
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.