Hi All,

I was trying to figure out how big of an intake port I would need to ensure the cross sectional area is the same as that of the exhaust port. (laymens terms.. how big does the cooling hole have to be to be the same size as the hole(s) at the back of the projector with the fan(s))

Came up with this nifty calculator so I thought I would share it with you...

http://insomnyak.centelia.net/box_calc.htm

You simply enter the number and size of your fans, enter a width or a length for your intake and the calculator will work out how big the intake has to be to match the exhaust.

Feedback and criticism welcomed!

yaK.

The intake area does not need to match the exhaust area. It only needs to match the cross sectional area between the lcd and fresnel. The airflow is determined by the smallest 'hole' which the air flows through.

My $.02

Thanks for your comments MrBill. Surely there is an 'ideal' ratio between intake and exhaust cross sectional area. (however, I don't think it is 1:1)

My calculator needs to be modified to allow different ratios.

yaK

I was looking at your calculator. I'm thinking the cross sectional area of your fans should be calculated as a circle not a square. So for a 120mm diameter fan the area wouldn't be 120mmx120mm but it would be 3.14x(60^2). This will end up decreasing the size of your intake slot. Not that all this really matters but just so ya know.

I have a 13" long slot and 2 80-90mm fans...I put in the specs I already had and it said my intake needs to be 837mm wide???? that's many times wider than my box.

I get about 39mm wide. That's with 2 exhaust fans with a diameter of 80mm and length of 330mm. Did you change 13" to mm?

very nice tool.
What does everyone think the optimum in/out ratio is?
More than likely it's dependant on a million other things unique to each projector. Being more optomistic, I would say that the intake should be slightly smaller than the exhaust hole(s), so that air is sucked faster over the lcd. Maybe the gap between the fresnel and LCD would also affect this. The smaller the gap, the faster air will move through the gap. Would it be a good idea to make a seperate intake tunnel that releases the incoming air onto the bulb?

here's my take on this:
the best way to cool a system is more complicated than a few equations. Ideally, to maxmize airflow (cm^3 / sec) you should have as big of a cross-sectional area inside as possible - this decreases resistance. However as you can see this does not prove too adequate as only the air closest to the object that that needs cooling (LCD) effectively absorbs heat. by making the distance between the LCD and the collimator fresnel very small more air gets closer to the LCD and is able to absorb heat better. However making the distance (and thus the cross-section) too small will bring about greater resistance in the enclosure and the small fan will be put under some pressure.
So in essence, you have to find the best ratio between too small a gap and too large meaning too much air resistance and too ineffective cooling. My guess is that somewhere between 1cm and 2cm is good.

Ideally you should also think about the speed of the air in various sections of the projector. Ie between the fresnel and LCD the crosectional airflow area should be larger than your intake, to slow the air down (low pressure air is slow), so it remains in contact with the hot LCD for longer, picking up more heat. It can then speed up in other areas by increasing the pressure by narrowing the crossection. For example, I want fast air past my reflector, as most of the light engine cooling (I hope) will come from conduction and radiation into the large low pressure area behind it.

Of course, as zyxstand says, you need to consider restrictions on overall efficiency of the system too. I haven't done any calculations with mine - just guess work based on dear old Bernoulli.

But my exhaust size was determined by the 2x120mm fans, and the intake size was largely determined by the gap between the mounting frames for the LCD and fresnel, so the rest of my air system (from a practical point of view) is just rough science and idle speculation

I guess I'd better buy a pyrometer before I leave it switched on for too long!