words, words, words...
Lighting Why blow into the light when you could blow out? Makes no sense to me
- Thread starter Groff
- Start date
I apologise but you really lost me @GroffYou are talking a lot about delta T's etc which makes sense but the overall scheme of it i got completely lost in haha...I have put some of my thoughts in red below...apologies if anything comes across rude its not meant like that at all
I see what you mean. Let me try and simplify it.
Imagine (for the sake of simplicity), ambient temps are 25ºC and the LED/heatsink interface is at 45ºC
Ultimately what I'm trying to demonstrate is that blowing into an enclosed case will create a pressure build-up. This pressure build up will lessen the cooling capacity. I theorise this pressure build-up plus added turbulence plus lower fan efficiency (= lower CFM) will result in more added heat than external ambient temp… all this will lessen the capacity to transfer heat out the unit. The heatsink will definitely be at a more homogenous temperature, but the overall mean average heat will be higher. Lets say the internal temperature is 35ºC all round, making the delta-T = 10ºC (45-35=10).
If one blows in, being negative pressure with path of least resistance, the heatsink will be colder on the outside and warmer on the inside. Average temps may be the same 35ºC all round, but the outer portion of the aluminium heatsink will be say 30ºC and the inner 45ºC. In this scenario detla-T will be 15ºC, so a higher cooling potential by 5ºC. Add to this higher CFM, lower pressure & turbulence and this delta can be even higher.
And since aluminium is way way waaay denser than air, it's heat transfer capacity is much higher. So even if there are zones where the air is hotter, the heatsink is what is actually doing most of the work and will effectively be more efficient to dissipate heat from the LED chip/aluminium interface.
Make sense?
I believe conduction heat transference to be much higher than convection heat transfer, otherwise we wouldn't need heatsink and just use air. So what matters is the thermal difference of the heatsink, not the average internal air temperature. If we blow into the unit, we're getting closer to thermal equilibrium.
Thank you. Very insightful. Really adds to the discussion
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answered below in red again...
I see what you mean. Let me try and simplify it.
Imagine (for the sake of simplicity), ambient temps are 25ºC and the LED/heatsink interface is at 45ºC
Ultimately what I'm trying to demonstrate is that blowing into an enclosed case will create a pressure build-up. This pressure build up will lessen the cooling capacity. I theorise this pressure build-up plus added turbulence plus lower fan efficiency (= lower CFM) will result in more added heat than external ambient temp… all this will lessen the capacity to transfer heat out the unit. The heatsink will definitely be at a more homogenous temperature, but the overall mean average heat will be higher. Lets say the internal temperature is 35ºC all round, making the delta-T = 10ºC (45-35=10).
There is no pressure buildup though...the fan creates a negative pressure inside the space it as it moves the air from the grow space and pushes it into the casing which would as you said create a positive pressure....the casing has holes in the sides to allow this air to leave and return into the negatively pressurised space...basic fluid dynamics, which is why natural ventilation works using large atriums or houses etc using the 'stack effect'...as the warm air rises it creates negative pressures at low level which then draws air in from the positively pressurised adjacent rooms and then from openings that draw air in from the atmosphere outside...yes it may create a slightly higher resistance as its oging to find its way into all the veins of the heat sinks but thats not an issue, we want turbulent flow as it allows for a higher level of heat transfer between the 2 mediums....
If one blows in, being negative pressure with path of least resistance, the heatsink will be colder on the outside and warmer on the inside. Average temps may be the same 35ºC all round, but the outer portion of the aluminium heatsink will be say 30ºC and the inner 45ºC. In this scenario detla-T will be 15ºC, so a higher cooling potential by 5ºC. Add to this higher CFM, lower pressure & turbulence and this delta can be even higher.
And since aluminium is way way waaay denser than air, it's heat transfer capacity is much higher. So even if there are zones where the air is hotter, the heatsink is what is actually doing most of the work and will effectively be more efficient to dissipate heat from the LED chip/aluminium interface.
yes i get that heat transfer through aluminium is higher than air so it 'should' evenly distribute but it wont...if heat has travelled up a fin on a heat sink why would it then travel back down the fin, then across the heat sink, then back up a fin located on the edge where the laminar air path is travelling? it would but its not the most efficient way of removing heat from the heat sink...
Also going back to your tables of materials, yes every material/medium has a specific heat capacity but as air is so crap you need as much air to come in contact with the heat sink as possible so every molecule of air an absorb the heat energy and dissipate it to the grow space, going back to the original point of turbulent vs laminar flow for heat transfer...i mean why would you want to move say 50 litres of air and only allow for 10% of the air to come in contact with the outer edge of the heat sink to absorb heat...you want all 50 litres to come in contact with the heat sink and absorb heat to maximise heat transfer....
if you look at water cooling you can see the veins run all over the chips not just on the outer edges
Make sense?
I believe conduction heat transference to be much higher than convection heat transfer, otherwise we wouldn't need heatsink and just use air. So what matters is the thermal difference of the heatsink, not the average internal air temperature. If we blow into the unit, we're getting closer to thermal equilibrium.
The reason for the heatsink is that it adds surface area to the heat source allowing for a greater amount of heat transfer...the heat transfer equations are something like below...
Q (heat) = Hc (Heat transfer coefficient) x Area x Delta T
Hc varies depending on how its transferred, i believe conduction has a Hc of 10 where as forced convection has a Hc of like 50-100....im pulling these numbers out of my head from 2/3 years ago at uni though so dont quote me...
So the larger the area the greater the heat output...so you dont just cool the back of the COB which will have a tiny surface area, the heatsink probably increases the surface area by 20 times....
Dont take this the wrong way but i think thats where your getting a bit muddled up...you want the heat sink to be as close to the thermal equilibrium as possible...with a heat sink you want the delta T to be zero or as close to zero as possible...if you can get this temperature difference to zero that means your cooling is more than adequate....if your heatsinks are at 100°C and your air is at 20°C, i completely agree that yes because of the greater temperature difference you would get more heat transfer (to the point of air not being able to actually absorb any more heat)....but who wants their cobs and heat sinks to run at 100°C? the aim of cooling is to keep the heatsink temperature as low as possible...so yes the lower it gets the less heat is transferred but thats what your goal is....if you were sizing a heating system such as radiators etc then yes a higher delta T means you get a larger heat output from the emitter but we dont want that
Thank you. Very insightful. Really adds to the discussion
Agree been good to have a good sit down and think about this and why its done in the way it is...always good to question these things and hear other opinions!
Great stuff, great discussion.....
https://en.m.wikipedia.org/wiki/Newton's_law_of_cooling
Why don't we call in the special operatives...That's why they're here or wot
Guys, @AMARE-TECH-Vic @PlatinumLEDTom @Spectrum King LED @Tasty @PhotonLabs @MarshydroTina @GrowNorthern @BigSm0
Why is the airflow into your lights from the top down and not from down up?
Apologies to any operatives to was too deep undercover to be discovered!
https://en.m.wikipedia.org/wiki/Newton's_law_of_cooling
Why don't we call in the special operatives...That's why they're here or wot
Guys, @AMARE-TECH-Vic @PlatinumLEDTom @Spectrum King LED @Tasty @PhotonLabs @MarshydroTina @GrowNorthern @BigSm0
Why is the airflow into your lights from the top down and not from down up?
Apologies to any operatives to was too deep undercover to be discovered!
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@Corgy
forced air is better to push into the heastink. That way with force and direction it will reach the hottest part of the heatsink for max effectiveness
If we were you pull air out then it will just suck air from around. It may not suck up air from the hottest part of the heatsink, there by reducing cooling effectiveness.,
forced air is better to push into the heastink. That way with force and direction it will reach the hottest part of the heatsink for max effectiveness
If we were you pull air out then it will just suck air from around. It may not suck up air from the hottest part of the heatsink, there by reducing cooling effectiveness.,
The prime reason for blowing in is the increase in turbulence as the air is forced down through the fins at the correct positions causing more gas molecules to contact the aluminum surface. It can matter more or less depending on the design of the lamp. My larger bar lamps have two fans, one blowing in and one blowing out so they're not working against one another. Surface area, base plate thickness, airflow pattern, volume and pressure all go into the final result. If you have a thermocouple you can secure to the test point on a cob, or even a heat gun, you can try the fans both ways and see how much difference it makes. In most cases it won't make a lot of difference, but a small difference is still a difference.
Tasty
TastyLED
Tasty
TastyLED
Absolutely, I understand the logic. But what makes my head scratch though, is exactly that. It's a small difference, but a huge difference at the same time. For example, these computer-syle fans and not very efficient working against pressure. So while added turbulence promotes more molecular heat exchange, maybe the total number of molecules is lower due the lower CFM capacity while working agains pressure.
Also, since it's a small difference, added turbulence will in itself increase temperature due to friction.
Ultimately, do we want a heatsink with a more homogenous temperature reading, or a heatsink with a higher temperature difference from COB to external parts?
What I'm trying to get at is that since Alu is much denser, the thermal capacity is much higher than air. So the limiting factor is NOT air movement, it's the aluminium delta-T differential. Air in this situation is used to remove temperature build-up, not necessarily cooling.
That is why we see more and more DIY passive COBs with large heatsinks. The heatsink alone is more than capable if it's big enough. By being big, it's not necessarily the surface area for air cooling per se(remember this is not active cooling), but the bigger the heatsink the higher the thermal difference from the extremities (secondary to heat exchange with air), so the higher the heat transfer.
woody
Cultivators Club
There may be a good reason not to do this but its the first thing that popped into my head.
Why not have the fans mounted vertically on the side casing and blowing air across the tops of the heatsink and out the other side?
Why not have the fans mounted vertically on the side casing and blowing air across the tops of the heatsink and out the other side?
Some designs have incorporated the fans perpendicular to the sinks. As long as the enclosure channels the air properly it will work well.
If a fan lowers the thermal impedance of a sink there should be a corresponding temp drop at the test point. This goes for pin fin sinks as well. How that relationship scales I'm not sure but there is likely a small discrepancy involved.
If a fan lowers the thermal impedance of a sink there should be a corresponding temp drop at the test point. This goes for pin fin sinks as well. How that relationship scales I'm not sure but there is likely a small discrepancy involved.
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