Question for all the engineers amongst you: I was at work today and an engineering instructor tells me that to cool a room you should point your fan out the window as the pressure drop from air sucked in under the closed door will lower the temperature most efficiently.
What do you reckon? Fan blowing on person (evaporative cooling) or fan out window pressure drop cooling, which is better/more efficient?

(Obviously all things being equal ie. Ducted fan or obscured window etc etc.)

I mean I'm curious because think on a small sailboat with summer coming...

Cheers k.

Is it cooler on the other side of the door?

Thats what I asked! For the sake of the question please assume the temperature on both sides of the door are equal.

Btw I noticed there have been heaps of people viewing the question and you are the first to reply (thank you) I was half scared I was asking a obvious question.

I call bull**** on your lecturer. The fan doesn't cool the room down (it introduces heat from friction) and its not creating a vacuum that is going to cause a temperature differential- if this worked you would get a significant cooling effect through a venturi even when the air was hot.

A fan works by moving air which makes it easier to cool by convection and radiation, which is much more efficient. This is pretty easy to prove, sit in the room with the fan pointing out, then after 20 minutes compare with putting your head in front of it.

maybe his point is that fans don't cool a room, which is true- the moving air cools you.

Fan out the window. It works even better if you have a second window to let cool air in. Yes in summer the air is usually cooler outside the house than in. The theory of heat reduction with expanding air is correct but you are never going to create the situation just by pointing a house fan out a window.

Hardly any air would come in under the door even without a door snake. Any air moving out the window will be replaced by air coming back in around the sides of the same window. That would change if the door was open to allow air in, but the drag caused by the narrowness of the gap under the door is much greater than the drag around the window frame.
PS You can make a fan into an evaporative air cooler in hot dry weather by hanging a wet wicking rag over it. When I tried the system I had ambient 40C air coming down to 24C. But you have to leave a way for the moist air to escape the room to keep evaporation doing its work

My 2c back to the boat thing to its esp noticeable all u need is a hatch at the rear of an aft cabin. Because the bow normally points into
the breeze at anchor eben the slightest draw or cross ventiation makes a massive difference. If u try and fan into it you are stopping the natural airflow. Fan out the window (or oppposite end to hatch) towards hatch helps the flow. Like increasing the draw on a fireplace chimney

I like the wicking water thing. On a boat you have an infinite supply of water. Hang the wick over the side of the boat and drap it over the windward opening. If there no wind, use the fan. Vary size of wick to control temperature.

^^^Problem is that the air is already close to saturated so there's not a lot of cooling to be had

Select to expand quote

decrepit said..
I like the wicking water thing. On a boat you have an infinite supply of water. Hang the wick over the side of the boat and drap it over the windward opening. If there no wind, use the fan. Vary size of wick to control temperature.

I would have thought this would be as bad as trying to use an evaporative cooler in Sydney...you end up sweating in a high humidity environment which doesn't feel any better even though it might be a few degrees cooler.

To the OP....the pressure drop argument is BS - you're not going to maintain any discernible difference in air pressure between inside and out. I guess blowing air out the window has the advantage that any heat produced by the fan motor is also going out. But what are you trying to do....cool the space in the room, or increase the ability of the body to cool itself by sweating?

If the situation you are trying to address is a hot room where there is cool air outside, definitely do the ducted fan to outside. If temperature is same inside and out, you want a fan blowing on you with some ventilation to stop a build up of humidity.

Thank you everyone for your input. The subject came up again today and old mate was (light heartedly) mocking me for not believing him. I also do not think the pressure difference would be enough to make a difference to cooling the room.
I guess the next step if a couple of thermometeres, a shoe box and computer fan.

I'll post picks if I do it after the weekend (going sailing).

(And yes I know its a lot of effort for a simple opinion but its also a chance to score a point for the techo's against the big dog at his own game )

Cheers K.

Rails for the win! That was pretty well done by that guy, it also shows something that I did not know: a desk fan collects air from all directions and pushes it out in a narrow beam or cone. Not how I thought which was cone in from back and cone out from front.
Cheers K.

This is really a different thing though, its ventilation rather than cooling. Is the question where is the best place to put a fan to maximise airflow, or what is the best way to cool a person in a room with a fan?

Q1 is answered in that video, Q2 is the same if the ambient air is significantly cooler outside than in, if the outside air is warmer than inside, your best option is to point the fan at you and use evaporative and convection cooling.

Engineering 101, "define the problem"!

I think there's another aspect to that YouTube. With the fan close up, you are trying to push all the air through a small area of the fly screen. Move the fan back and the stream of air will be wider and slower, spread over a larger area of fly screen, offering less resistance to the air flow. I'd like to see the experiment repeated with the fly screen removed.

.and now you've got flies in the house and its still
hot. Time to invest in airconditioning.

True, the youtube clip does not answer the original question re pressure drop that I started with- I was just excited by the data.

I only just came across this.The issue is that you have put forward two different mechanisms and are trying to equate them.
To cool an entire room, fan out (or in) the window is the only real option and anything else just circulates the air internally. Fan out the window is definately more efficient than fan in, which is why whirlybirds draw out, not push in. However if your goal is to try to cool a person in a warm room, and it is just as warm outside, then airflow over that person will be most effective due to evaporative cooling. It will do nothing for the temperature of the room though.

That said on a small boat, the skin temp of the boat will be high due to the sun and that will warm the cabin. Best solution is an extractive fan to cool the room and a personal fan to induce evaporative cooling.

Engineering is mostly about defining the problem.