hydraulic cooler design concept

[subsearobot]

Hi
I am designing a fluid power manifold that will hopefully shed some heat. It's a long story. the short of it is that I need to shed about 7 HP of heat energy, keeping the working fluid around 150 degrees. Did i mention that this is a rush job (prototype deadline has come and past), and my specialty is not heat transfer.

the manifold will be sit in water (90degF max), and I am hoping to maximize convection currents. It will be oriented as shown in the upload picture.

I am hoping you folks can give me a good rule of thumb. We are debating whether better heat shedding will take place with the 18 1/4" holes that are shown, or if a smaller number of larger holes would increase convection.

My thought is that smaller holes= greater surface area. Greater surface area will increase heat shedding. convection currents will be ample.

My antagonist argues that small holes will lead to decreased fluid velocity due to skin friction. therefore larger holes would benefit heat transfer.

I am beginning to dig back into my heat transfer book, but it's been years... any input would be very helpful. We are on a tight deadline. that said, I am not looking for perfection here. this is a prototype, so we are not budgeted for several days worth of proper heat transfer engineering (insert expletive toward accountants here).

thanks!

 

[subsearobot]

forgot to mention, the manifold is 7" tall.

 

[ione]

Assuming a given temperature difference, both the parameters you’ve mentioned play a role here, that is area and flow velocity.
The faster the stream flows inside your channels (holes) the higher the heat transfer coefficient. Forced convection is the mechanism involved and the heat transfer coefficient as velocity increase. Now the velocity of your fluid inside your channels is strictly related to the area of the channels and to the flow rate. For a given flow rate the smaller the channel the higher the fluid velocity.
If you are pumping your cooling fluid inside the channel the problem is to size the pump such a way to provide enough head to counterbalance pressure loss of your circuit (friction losses are just a part of them) for the desired flow rate. In conclusion I will definitely go for an increased number of smaller holes as this will produce a double beneficial effect: velocity increase (that means higher heat transfer coefficient) and increased area.

 

[ivymike]

um, a 5kW toaster, the size of a kleenex box? that baby is going to get HOT.

 

[ivymike]

...okay, so it's not a resistive heater and won't get any hotter than the working fluid, but without getting very hot I don't see it moving 5kW into the water (finite eyeball analysis). Maybe if you took your design details and plugged them into an online heat exchanger sizing calculator you would find a bit of info on your required sizing. "Immersion" exchangers sound like what you're talking about at the moment.

 

[StrykerTECH]

If you could give me a couple data points I can run a quick calculation for you.

Flow rate of Hydraulic fluid - GPM
Internal surface area of the where the Hyraulic fluid contacts in^2
External surface area of where the water contacts. in^2
Volume of Water bath - Gallons

Assuming 150 degree F Target for Hydraulic fluid temp
And 90 degree F max for water.

Essentially I will be able to tell you approximately where your fluid temp will be, then we can play with the surface areas and water temp to get the desired result.

StrykerTECH Engineering Staff
Milwaukee, WI

http://www.stryker-tech.com/

 

[subsearobot]

ione, I am looking at free convection for the external holes, there is no set flow rate. that said, my feeling that free convection currents in water will not be fast enough to be largely affected by skin friction in a 14" diameter hole. what do you think?

ivymike, it is essentially a heater. I am trying to keep the fluid at 150-175. I am setting hydraulic system pressure here, which when no work is being done, converts the full power (3.5 GPM @3300 psi)into heat. without good heat shedding, we will cook the hydraulic oil in short order. that said, we have a limited amount of space on the vehicle. If we can not shed the required heat, we will use an electromechanical valve to divert flow before the pressure is generated.

Stryker,
-internal flow will be approximately 3.5 GPM (~22 ft/sec).
-internal surface area~ 37 in^2
(approx 60" of 1/4" diameter cross ports)
-external surface area =~300 in^2 (including the 18 thru holes)
or ~200 in^2 if you neglect the 18 thru holes.

the water bath volume is infinite (the ocean)

Our machine shop has started cutting the manifold. the only variable that I have at this point is to change the size of the 18 1/4" thru holes.

thanks all!