oil cooler challenges 1

[chumet1]

our mill gearboxes are cooled by a circulating oil cooling system comprising of pump, filter, cooler and gearbox. there are differences in component sizes on different which can be combined in 3 sets of systems, A, B & C as follows:
pump A = Pump B = pump C
filter A = filter B = filter C
gearbox A = gearbox B > gearbox C
cooler A > cooler B = cooler C
system B has a small cooler working on a big gearbox and having a pressure drop of around 600kPa causing transmitted pressure to function close to the system alarm value thus any small drop of pressure causes the alarm to go off. but the same size cooler on a smaller gearbox gives a pressure drop of 400kPa and also the big cooler on big gearbox gives a pressure drop of 400kPa.
my question is does the pressure drop around the cooler dependent on the final delivery destination of the oil and would replacing small cooler with bigger cooler assist me

 

[hydtools]

Cooler B may need to be cleaned. Something inside is causing higher flow resistance.

Ted

 

[LittleInch]

Yes I think it would (bigger cooler for system B). If you assume oil flow is proportional to gearbox size, then you have a higher flow going through cooler B than cooler C, hence a higher DP for the same size cooler, plus possibly higher DP in the tubing / pipes connecting cooler to gearbox.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[hydtools]

Are the pumps positive displacement pumps?

Ted

 

[Dougt115]

You need to balance your system components.

Mixing the sizes of the cooler and the gear box is resulting in your problem. The flow is being restricted around your PD meaning it is the dominant resistance in system. IF you balance your system the PD will be distributed resulting in a lower measured PD at the one point.

 

[tbuelna]

If pumps A,B,C are identical positive displacement pumps and operate at similar conditions with the same oil, their output volume should be similar. The volume flow thru the rest of the circuit (ie thru the filter, heat exchanger, etc) should also be similar, unless some flow is diverted by a pressure relief valve at some point. Since the oil is delivered by a positive displacement pump, the only thing that will be affected by the heat exchanger restriction is the pressure between the pump and heat exchanger inlet.

When using oil to cool the components of a gearbox, what matters is ensuring the correct mass flow of oil at each cooling jet orifice. The cooling jet orifice is designed to provide a specific oil mass flow at a certain delivery pressure and oil temperature/viscosity. So the pressure drop across the heat exchanger core by itself is not a problem as long as it is accounted for, and does not create an inadequate mass flow at the cooling jet locations.

On the other hand, a 600kPa pressure drop (or even a 400kPa pressure drop) across an oil heat exchanger core is quite substantial. Is that PD just across the heat exchanger core, or does it include pressure losses from filters and other things upstream of the heat exchanger core? A PD of 400kPa across your heat exchanger core would indicate to me that the core flow area is way undersized for the application. Gearboxes that I have worked on usually are designed for an oil circuit pressure of around 60-80psig at the jet orifice. At operating temps, the combined PD from the filter, heat exchanger core, and piping is usually nowhere close to the numbers you describe.

If you are getting adequate heat rejection from the existing "small" cooler, then what you probably need is just a core with larger cross section tubes for the oil flow circuit to reduce the PD. If you want to make sure your oil cooling system is adequate for all combinations of parts you described, then you need to do a thorough analysis of mechanical losses in the gearbox, heat transfer and oil mass flows for each configuration.

 

[LittleInch]

chumet1,

Although I read the OP several times I still got it wrong.

based on your post (equal size pumps),I think hydpost has it right - there seems to be something wrong in your B system.

A sketch of your system would be good to see where your 6 bar losses are occurring.

Are all the filters really the same? Are they dirty?, Is the oil the same?

SO apologies for incorrect post earlier, but on the descriptions you've given there is something wrong in system B not connected to the size of the cooler.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Dougt115]

Assuming size is directly related to flow rate, big = large flow rate, smaller = lower flow rate. And Flow rate is directly related to pressure drop, flow rate increases as the pressure drop increases.

For the same pump and filter system
flow rate through gear box = flow rate through cooler (conservation of material)

(PD of gear box)*(Gear box size) = (PD of cooler) * (Cooler size)

From this one can see that increasing the gear box size unbalances the equation. If the cooler is unchanged then the PD must increase at the cooler to rebalance the equation. Likewise if the gearbox and cooler are changed, both smaller or both bigger, then the PD will not need to change.

(This can be extended to the filter and the pump if desired.)