# heat generated in a hydraulic system question

• posted

Hello,

How can I calculate / estimate the heat generated in a hydraulic circuit? I was able to estimate the pressure drop (PSI) thru the system lines due to frictional loss but not too sure how to estimate the heat generated by the hydraulic motor. Motor rating at 2000 PSI: input flow of 435 LPM output power of 81 kW

2000rpm speed.

Hydraulic system pump output flow is 466 LPM at 2600PSI.

The system will be running for 2 hours continuosly. Can anyone help me estimate / cvalculate how much heat is generated and the temperature rise of the hydraulic fluid after 2 hrs of operation? Ambient temp is around 40degC. How can I select the required hydraulic cooler to suit the requirements?

I appreciate anybody's help.

Ed

• posted

Dear edp:

Heat rise = work applied - work removed

If you work it as an instantaneous rate, then you have: heat_removal_rate = electric_motor_P - hydraulic_motor_P

You haven't provided enough information to fill this out. The pump, the lines, and the motor will all have leakage (and a return line filter, I hope), so will all have loss terms.

David A. Smith

• posted

Any oil at pressure that is allowed to go to atmospheric pressure without doing work will produce heat. A simple formula for how much heat is produced is to figure Horsepower and then convert it to BTU's HP= PSI*GPM*.000583 and each HP los is equal to 2544 BTU/Hr.

Pressure drop such as line losses, component bypass pump inefficiency are constants and are part of all circuits. These losses can be reduced but not eliminated in a typical hydraulic circuit.

I have put an article on my web site that explains hydraulic circuit heating to some degree and points out some of the major heat producers in common circuits. Remove the "Dashes" to go to the site.

BudT

"N:dlzc D:aol T:com (dlzc)" wrote in message news:F1F0f.2714\$lq6.2163@fed1read01...

• posted

Motors running at this speed can be 85% efficient - sometimes higher. That 15% wasted from 81KW represents 12 KW of heating. How much the oil temp rises depends on the amount of oil in the system. But a cooler rated for 12KW at the flow in question, at an acceptable pressure drop would stabilize the temperature for sure in my opinion....

Brian W

• posted

#1) depends on the in-place efficiency of the motor, which can be anywhere from 50-85% depending on the age, quality, type, which end of the mfg curve it hit, coupler alignment, and system design (especially input head). In other words, a lot of range of efficiency.

#2) depends on the amount of hydraulic fluid and the type in the system (specific heat and volume of the fluid), ambient temperature, the length and type of lines (they dissipate a lot of heat), if you have a proper reservoir (non-settled air-entrained above-ambient fluid runs hotter than reservoir-settled near-ambient fluid), the kinds and differential settings of the safeties vs the system pressure valve (cracking leakage), and the surface area of reservoir.

Ambient temp is around 40degC.

if you use a simple reservoir for all the other reasons one uses a reservoir besides heat, you probably won't need a cooler - the lines and a rule-of-thumb reservoir will handle the heat - especially for a two hour run-time.

• posted

I should have added something for you to gage the scale of cooling you might need:

12 kW is less than a lawn tractor radiator puts out. But I should mention that the compressor also has its inefficiencies which ups the heating loss.

Brian Whatcott Altus OK

• posted

I've got an installation with a 70gpm v(3000psi) ariable displacement hydraulic pump.

It has a water/oil heat exchanger................ to keep the thing from an "over temp" shutdown I need to supply ~10 to 15gpm water at ~70F, the water comes out of the heat exchanger a nice warm whaoer temp I'm guessing about 100 to 105-ish

Run the number & you'll get an idea of the heat you need to kill.

In another sit tuaion I've got a 40hp constant displacemnt pump at

3000psi, intermittant duty. I use a coil in an ice bath to kill the waate heat.

Hope these numbers help

cheers Bob

• posted

Hello everyone, Thanks for your help. It gave me an idea and direction and a confirmation on how I should approach the problem. It turned out that we designed an air cooler to handle around 18kW of heat which turned out to be a single pass air cooler. Not too much heat to dissipate. We believe that a hydraulic cooler is not required but because the previous job we had ( done by different people who did not leave any documentation), we seem to be obligated to do the same to avoid further blame if in any case something goes wrong, or just to save our butts.

Motor is brand new. I calculated the heat gerated by hydraulic motor HP=Input HP - Output Hp. Where: Input Hp=input flow (GPM) x 2000psi / 1714. Output HP = from motor catalog. The efficiency is 80%. This is the same method as suggested by Brians - thanks. I also aded the heat generated by the system line based on Darcy-Weisbach equation in the total heat load.

The solution of having a sufficient reservoir won't apply to us as we are utilising the hydraulics system of a caterpillar dump truck 777D. We don't have the luxury of space. We are tapping into the hoist hydraulics system and divering the flow to run our motor to run our water pump. We are actually altering the truck hydraulics design so we need to make sure that we are not putting additional heat into the system which might affect the truck's overall performance.

Brian, could you please elaborate furthet what you mean by: "at an acceptable pressure drop"

Thanks, Edgar

• posted

• posted

This suggestion is not at all profound - but in hydraulic circuits as in electrical circuits, current times pressure drop is a measure of power dissipated.

Your cooler might be an in-line add on. If that is so, too much flow resistance [which causes pressure drop] increases the heat to be dissipated, at the cost of useful power.

Nobody ever went bust by increasing losses by 10% though.....

Brian Whatcott

• posted

Dear edp:

Things to watch out for are:

- mobile hydraulics are notorious for dumping excess fluid across the system relief valve. This is pure waste heat, and typically runs straight to the reservoir.

- there is very little "separation volume" in mobile hydraulics for any entrained gasses to separate from the fluid. Bypassing over the relief valve for long periods will turn your hydraulc fluid milky, as will also happen if the fluid does manage to get hot.

Be sure you take all the flow you can through the heat exchanger you are adding. Just modify the truck hydraulics to provide "power take off"; and you handle the system relief, speed control, and cooling "off line".

David A. Smith

• posted

Thanks to all of you who have responded! Cheers

Edgar P

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