[realpeopleio]

My 4rd grade teacher was convinced of the science of using two fans to cool off the classroom: one blowing cool air into the classroom from the hallway and one fan blowing hot air out of the classroom through a window. The kids that sat near the fan blowing in the room from the hallway were quite content, but the kids by the window were always complaining and they would secretly turn the fan in the window around so it was also blowing in. This would infuriate our teacher when he would discover it and he would claim that the reason the classroom is hot is because the fans were no longer arranged to create air movement through the classroom. But the window kids weren't buying it.

So if sweaty 4th graders are any indication, have the fans blow on you regardless of whatever air movement is supposed to be made.

[tomohawk]

If the goal is to make the room cooler, your teacher is right. If the goal is to make the students cooler, then the 4th graders are right. 4th graders sweat, and the fan will cause convective cooling of the 4th graders.

[lathiat]

This is a concept often poorly understood in basic cooling design. Even without sweat, if your AC vent is blowing directly on you the temperature it is at needs to be vastly different than that of air being using to more actively cool the entire room.

This is often a key difference between Car and Room AC systems.

There’s also a surprising gradient based on how consistent the temperature is top to bottom (and you get into trouble with this on reverse cycle AC systems)

And lastly it’s easy to ignore radiant heat from both walls/ceilings as well as even large monitors.

It all plays in together.

[masklinn]

> This is a concept often poorly understood in basic cooling design.

Really? You'd think windchill is a well-understood concept.

[lathiat]

When it comes to basic price central ducted HVAC system installation you'd be surprised! At least in Australia.

[zAy0LfpBZLC8mAC]

s/convective/evaporative/

[pbhjpbhj]

Also those by the window were maybe in direct sun.

[realpeopleio]

I should add that my teacher would think we couldn't understand the science or concept behind airflow going through the classroom to cool us off. He'd explain it over and over and get frustrated that so many students didn't seem to "get it." But really they did get it, it just wasn't working for them. I wonder if the arguments about scientific things nowadays aren't similar. It may not be that someone is stupid or doesn't get it, or even that they disagree, just that it's not working for them. The window kids understood that the room was supposed to be getting cooler, but it wasn't for them. They weren't against air flow, but it didn't seem to be working for them. Perhaps in theory the 2 fans in the classroom should have worked, but maybe the fans weren't strong enough, there were other places for the air to go (like the ceiling), etc. so that in practice it didn't actually work like the teacher claimed it would. Maybe there's a lesson there for public debates.

[raldi]

We used to make paper fans and the teachers insisted "they just make you hotter" (due to energy expended waving them) but it sure didn't feel like it.

[heavenlyblue]

Sounds like a bunch of anti-vaxxers coming together and saying vaxxing doesn't work for them. This is the same reason why pilots are being trained not to listen to their own senses when flying: because those senses work against their reason.

I can see how a single fan blowing right into someone's face could be more efficient at cooling that person than the same fan working for the whole room; but frankly you also forget about all of the other pupils who aren't under the direct "sunlight".

[neltnerb]

Sounds like a perfect opportunity for the teacher to demonstrate the scientific method by setting up thermometers throughout the room and recording them throughout the day.

[latchkey]

The issue is that the teacher needs to actually change the pressure in the room in order to create enough airflow. The fan in the hall needs to create a greater pressure (push air into the room) and the fan in the window needs to remove the pressure. If the fan in the window is stronger than the fan in the hall, then there will be a negative pressure (suction) to pull air into the room.

The problem is that the room probably has leaks such that if the fan in the hall is creating pressure, then the air doesn't all escape through the window. It'll escape through the path of least resistance or maybe even just spin inside the room (hot air rises too). If there is two doors for the room, some of it may be escaping under the other door. In that case, then air is just circulating back into the hallway. People also forget to take into consideration the expansion of hot air, which I've seen quotes for being up to 6x the volume. This complicates the pressures in the room.

As mentioned elsewhere in the discussion, the kids probably wanted to feel cool via evaporation since neither of the fans was moving nearly enough air to cool the entire room enough.

I run a couple large crypto mining farms in a pretty hot country. The 'kids' generate the heat and we have to move large amounts of air through them in order to keep them relatively cool. I've had to learn a lot about airflow design and it sounds very similar to what your teacher was trying to achieve.

[mrhappyunhappy]

Speaking of school, and forgive me for not adding anything to discussion at hand, my biggest distraction was always sweat. I’d get so hot and sweaty most of the times that my butt would leave a sweat stain on the seat. I’d say I spent a good portion of total learning time just worrying about the embarrassment that would result from having a sweaty butt and leaving a stain. Mind you I was not overweight, just a very sweaty kid. I don’t know this for certain but if my school had ac I may have actually learned better by paying attention in class instead of worrying about my butt. Educators take notice.

[raldi]

If all the other teachers were doing this too, the hall might have been negative-pressure enough that the air blowing in was circulating right back out before reaching the rest of the room.

That would mean the kids near the outlet fan were getting air that was just pulled in from outside via the window seams.