OIl Cooler Design with only a little bit of known information

[Kevin-A]

I have never designed an oil cooler before - just wanted to go ahead and throw that out there. I really don't deal with much in the way of thermodynamics, or fluid dynamics, in my daily engineering role.

We currently have an oil cooler that I have been told is an 11 hp oil cooler. It is truck mounted and operates via pump driven by the pto on the truck's transmission. I have been asked to engineer a new oil cooler, radiator if you will, that is capable of 15 hp. The new radiator will need to be fit inside the same housing that our current radiator is in.

Here are some things that I know about our current oil cooler:
The radiator core size is 11 1/2" wide by 9 3/8" tall - this is only the finned area (I will refer to this as "square inch area" in later information)
The radiator thickness will not change between the new to the old
The fan, for the radiator, is set up in a vacuum format. There is a shroud on the back side of the radiator that encompasses the fan.
The fan is 10" in diameter and uses a 6-blade 37° pitch and is placed 1" from the inside surface of the radiator
The fan has a maximum speed of 6000 rpm
The fan is driven by a hydraulic motor
We test the unit at 1800 psi. At 1800 psi, the fan-motor turns between 3800 and 4200 rpm (4000 is target) [FYI, the normal operating pressure is 2500 psi]
The fan is fed by a 1/4" hydraulic line that has an orifice in the fitting entering the motor. The opening in the orifice is Ø1.4mm

Here is what is requested, or can change, in the new oil cooler:
The radiator core size (square inch area) can change but the thickness must remain the same
The fan will remain in a vacuum orientation, with shroud, distance from the surface may be altered some
I can go to a Ø12" fan, but it will still have a 6000 rpm max speed cap
The hydraulic motor will remain the same but the working pressure will be 3000 psi

The fluid being cooled is hydraulic oil. Nobody has been able to tell me the actual temperature drop across our current oil cooler, but the change must remain the same. Because of this, my boss seems to think there is a linear ratio between hp and square inch area on the exposed core. I disagree.
However, with the temperature change being negligible between the two units (because the delta T must remain the same) and the thickness of the cooler will not change, I think I can make an argument that a ratio derived formula can be calculated based on the cfm of air movement through a square inch of surface area: cfm/in^2

First question: Can I relate cfm to square inch IF the two listed factors remain the same?

Next, I am having trouble calculating a few things here and need your help:
How can I calculate the speed of the hydraulic motor at different pressure levels given that the pressure is effected by the orifice in the line?
How can I use that speed to calculate the cfm of the fan?

Any help is appreciated.

 

[LittleInch]

Well you're both partly correct, but you do need some more details to see how this pans out.

The HP I can only assume relates to heat loss through the radiator?

The fluid being cooled is assumed to be the same inlet and outlet temp, but presumably then increased in flow by 35% (4/11)

Now depending on which direction the flow is in the radiator and how your cooling are changes it could be that an increase in area will be enough.

However this requires an increase in CFM through he radiator of 35%.

How can I calculate the speed of the hydraulic motor at different pressure levels given that the pressure is effected by the orifice in the line?
You need to know the differential pressure and flow rate to do any meaningful calculations. The hydraulic motor also needs to be defined a lot more.

How can I use that speed to calculate the cfm of the fan?
You need the fan data on speed vs flow. Each fan is different - very difficult to calculate based on raw data.

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

 

[romke]

you do not mention the material the cooler should be made of. most times a copper alloy is chosen for obvious reasons. however, with a copper radiator that is subjected to vibration, copper particles will be dissipated into the oil stream to be cooled acting as a kind of catalyst for oxidation of the fluid to be cooled resulting in faster oxidation of the oil at a given temperature resulting in a need for a more frequent oil change. oxidation will occur notably less when a aluminium radiator is used, extending the fluid life.