if we have a constant rate of suction in a house, will air be drawn faster through a smaller opening or?

Question

a larger one?

in my house with 3 box fans sucking out... if we have one window open my roomate says it will cool off faster than having a more than one open...

Answer 1

The air will be drawn faster through the smaller opening but the total air flow wouldn't be any more or less regardless of how many openings you had.

Whether or not it will cool it faster is a bit more difficult to answer because the airflow itself is a result of pressure changes. When the air comes rushing in through the window it is moving from a state of higher pressure to a state of lower pressure in the house. (the difference in pressure is what actual draws the air in). When pressure decreases so does temperature. (This why a breeze feels cool) So it is possible that the air rushing through one window may be cooler than the air rushing through more windows (less of a pressure difference, the more openings you have,but still same air flow).

So yes, I would have to agree with your roommate.

Answer 2

The first two answers are correct. The velocities are inversely proportional to window size, but the volumes should be the same (within reasonable limits).

Don't ventilate the house if outside temperatures are higher.

Also, if all your windows face one direction and the wind is from the opposite direction, it's more beneficial to have 1-2 fans sucking IN and 1-2 fans sucking out, rather than all 3 sucking out.

Edit: Unless the rooms are very small, the velocity of incoming air should make little difference to how fast the heat is vented.

Answer 3

mass flow rate (m-dot) = rho * A * v
where rho density of air
A = area of cross section of open window
v = speed of air flow

when suction is constant due to fixed pressure diff then (m-dot) is constant or room will have input air at constant mass flow rate

m-dot (in) = m-dot (out)
Rho = const
Av = const
v = K (1/A)
smaller the (A) faster will be flow.
larger the (A) slower will be air swept out of room. Heat content of room will leave the room much slowly from larger window
*** heat content of the room is what walls/roof throw inside at variable rates (depending upon the the material used in these, lati/longi location of house, climatic zone, etc). Remember, in olden times, and even still today mass flow rate based cooling is widely used. Even design of chimneys - used to throw out exhaust gases uses this mechnism effectively

your friend is right.

Answer 4

If the volume of air moved is constant, the air will move with a higher velocity through a smaller opening. But that has very little to do with 'cooling'.

Doug

Answer 5

smaller/no brainer.IF YOU WANT COOL TRY A/C, NOT THIS.