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# Fan CFM Computation

## Fan CFM Computation

(OP)
Guys,

I need some help regarding the Fan size needed for my Sensor Module (unit size of 5x3x1 inch) project
The product is mounted on the Lid of a Oven/BBQ Grill which is place outdoor
The temperature inside the Grill or on the Lid surface is max at 350C and the Ambient/surrounding is at 40C
Then, inside the Sensor module are electronics that need to maintain an internal temperature of 60C so that it will not damage the components
I need some help on how to get the right CFM size in order to control the temperature below 60C

### RE: Fan CFM Computation

This is not really an engineering question, but I doubt there is any way to cool this unit reliably to 60C. Conduction of heat through the metal? body is far more efficient than blowing 40C air at it.

Simply not going to be feasible for such a small device IMHO

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

### RE: Fan CFM Computation

NFW you're going to get the temperature inside that sensor to 60C if the surface it's mounted on is going to be 350C, no matter what size fan you use!

H. Bruce Jackson
ElectroMechanical Product Development
UMD 1984
UCF 1993

### RE: Fan CFM Computation

It depends on how effective and thick the insulation pad between them is.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

### RE: Fan CFM Computation

OP just say "mounted on". Hence why you could blow 100 cfm at it and it will still be rather hot. ....

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

### RE: Fan CFM Computation

At the extreme this is asking how much airflow is required to drop the temperature of the contact patch to 60C, so it first depends on the conducted, radiated, and convected heat flow to that patch and the conductivity of the metal and ceramic coating of the lid. Since one is looking to create a delta T of 290C with a delta T of 20C using a material with low thermal mass, that would indicate a huge volume of air is required.

The usual approach is to look at ways to limit the size of that patch to minimize the heat-flow. This is done with standoffs to limit conducted heat flow and insulation to limit radiated and conducted heat flow.

### RE: Fan CFM Computation

Insulation and careful design of the housing would be helpful.

For cooling, investigate "vortex tube" devices. To my simple mind, they operate on witchcraft. Compressed air in...hot air out one side...cold air out the other...no moving parts.

I cannot take credit for the idea, though. I recall that somebody proposed using this approach to protect accelerometers mounted on hot engine parts.

### RE: Fan CFM Computation

is this work or "work" ?

can you get a surplus space shuttle tile ?

an air gap of 1" isn't probably going to work (with the electronics mounts on ceramic stand-offs).

### RE: Fan CFM Computation

60C junction temperature is very low, can you get components with a higher rating? What is it that needs to stay below 60C?

### RE: Fan CFM Computation

(OP)
Thanks Guys for the replies
These are enlightening.
I'd like to share a similar product in the Market that is currently working.
This is the June Life Oven which has internal camera attached to the wall (internal oven temp reaches 500F)
What we want is to somehow use that approach.
Yes, there will be insulation in between the Lid and the Product (plan to use Fibre glass fabric or embedded with Aerogel) to protect the Product Housing form direct heat.

Regarding the Grill Lid temperature, since this is exposed to surrounding which is 40C, isn't it that the temperature above the Lid will not be that Hot as compared with the internal since it has been dissipated by ambient air (my rough simulation say's 200C)

BTW, there is Lithium Battery inside so 60C is required to maintain.

### RE: Fan CFM Computation

I won't get into how silly I think the product is, but...

#### Quote:

Using deep learning neural networks which run in real time on the 192 CUDA Cores, the camera and NVIDIA GPU work together to recognize over 25 foods and recommend the best Adaptive Cook Preset for them.
Overkill, for sure... just to recognize 25 foods that are more easily recognized by a human and selected from a menu

Corning Museum of Glass created a special ceramic window that could withstand the heat of a molten glass crucible, while allowing a camera to view the pool. They designed/created it for some electric crucibles that were placed on three cruise ships, which gave daily shows on glassblowing (I really loved that cruise... 14 days of geeking out in a glassblowing studio, with awesome food and "other" entertainment). The windows slowly failed over several months and required replacement, to the tune of several thousand \$s per window... but your thermal and size needs are considerably less. Something that may be worth looking into...

Dan - Owner
http://www.Hi-TecDesigns.com

### RE: Fan CFM Computation

Well if you look at the oven camera you will find that it is
a) very small
b) behind a glass screen
c) almost certainly isn't attached to the metal hot part of the oven
d) only has a very small amount of heat entering its domain.

your current design has none of those things, what ever it is.

The electronics of the camera will be mounted on the outer shell well away from the internal hot oven casing and even then will probably get pretty hot unless they blow cooling air around the inner oven which will be completely covered in insulation, unlike your BBQ

you need to sketch your anticipated design out a bit to see what it looks like, but for me and as noted above, you will need a awful lot of air moving over a big heat sink.

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

### RE: Fan CFM Computation

(OP)
Thanks again for the answers guys.

I am now considering the replies you've given and need some help on the computations of Heat transfer.
Idea now is to compute the Air GAP so that the Heat (30watts) or Temperature (60C)entering the Housing is Less (*shown on the Sketch)
I was trying to use and figure out the solution using the formula
dQ/dT = AΔT/(L1/k1+L2/k2+L3/k3) but was stuck

### RE: Fan CFM Computation

without meaning to be rude, but how much training have you had in heat transfer ?

maybe a refresher is needed ? maybe something online ??

### RE: Fan CFM Computation

It's not as simple as that.

1) How is your ABS plastic attached to the metal bit?
2) You will have radiative heat (infra red if you like) from the hot metal surface as well as air convection.
3) What is the air doing? moving, staying still?

If you put some really skinny tubes made of some sort of low heat conductivity material between the steel and your ABS and blew lots of ambient temp air in between the two at a gap of say 60 or 70mm then you might just keep the temperature below 60C, but the mechanics of working it out are complex.

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

### RE: Fan CFM Computation

Heat Transfer: conduction, convection, and radiation
Why not keep your temperature sensor remote from your electronics package that could be mounting at a cool location? Having a fan to cool an electronics package for a grille, even a commercial one, seems like a tough and expensive application!

Walt

### RE: Fan CFM Computation

350C is really hot for a consumer BBQ grill.

Are you sure?

### RE: Fan CFM Computation

#### Quote (Nescius)

For cooling, investigate "vortex tube" devices.

Ha! When I replied, I assumed the application involved protecting instrumentation for studying/testing a consumer product, like in a lab setting, not a permanent part of the product.

In my defense, I'm not sure what's actually more bizarre...a grill that requires 20 CFM of compressed air...or a grill with food ID vision system.

### RE: Fan CFM Computation

To follow on Nescius idea of compressed air to cool the instrument, when compressed air at a certain temperature is to be decompressed thru a nozzle, the air flow at atmospheric pressure will be cooler than its temperature in the compressed state, so may then there could be hope instead of a fan system.

### RE: Fan CFM Computation

Along those lines, there vortex tubes, that can achieve a certain level of passive cooling with only compressed air as an input:
https://www.newmantools.com/vortex.htm

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
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### RE: Fan CFM Computation

What happens if the compressed air source fails?

What happens if the end user says "I don't have compressed air, but I'm hungry, so I'm going to use the barbecue anyway"?

Something needs a fundamental re-think.

- Eliminate whatever the fancy electronic gizmo is that warrants temperature control in the first place. My barbecue sure doesn't have, nor need, any such thing.
- If for whatever reason that's not viable, redesign the gizmo so that it's not attached to the high-temperature part of the barbecue. If there's a sensor that needs to sense something, use one that can handle the heat and mount it remotely.

### RE: Fan CFM Computation

A high-temp glass light pipe might resolve the issue... that way, you can put the electronics as far away from the heat source as you'd like.

Dan - Owner
http://www.Hi-TecDesigns.com

### RE: Fan CFM Computation

From a review article here https://www.fastcodesign.com/3065667/this-1500-toa... :

This salmon had become more distracting to babysit than if I’d just cooked it on my own. This salmon had become a metaphor for Silicon Valley itself. Automated yet distracting. Boastful yet mediocre. Confident yet wrong. Most of all, the June is a product built less for you, the user, and more for its own ever-impending perfection as a platform. When you cook salmon wrong, you learn about cooking it right. When the June cooks salmon wrong, its findings are uploaded, aggregated, and averaged into a June database that you hope will allow all June ovens to get it right the next time. Good thing the firmware updates are installed automatically.

It amuses me that a certain segment of the user base (like, say, me - if somebody bought one for me) is likely to think it funny to, for instance, put yesterday's newspaper in the oven, tell the oven software that it's filet mignon, and proceed to reduce said paper to charcoal...and have the oven update its database accordingly.

Back to YOUR proposed device: why a lithium battery and not something less prone to exploding if it gets a bit hot, and easier for consumers to replace (e.g. standard alkaline cells)? I like Mike's suggestion, and thought myself of a periscope up one side of the chamber, with camera safely ensconced down below the hot grill casing.

### RE: Fan CFM Computation

I'm not sure why a battery is even necessary. The beast is plugged in... if it needs to sustain short power outages for time-keeping purposes, install a cap. It's like a VCR that needs resetting after a power outage, so I see no need for an expensive battery that causes even more headache because you have to design around it.

Dan - Owner
http://www.Hi-TecDesigns.com

### RE: Fan CFM Computation

Another method of getting cool air is the Hilsch tube which would require compressed air for the inlet of the tube. Anyone who took thermodynamics would have been analytically exposed to calculate the temperatures of the hot and cold exhaust air.

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