How to dump moist air without heat backflow in evaporative cooling system?

Question

I'm looking to design or purchase an evaporative cooling system for a small 70ft^2 California dorm room without air conditioning. This room has a single window and door to a large common space with a freezer. The common space and room currentl reach temperatures of 80+, while outside the room reaches temperatures of 90+.

An evaporative cooling system cools air by using heat to vaporize water. This can only go so far, as a closed room will eventually become too humid and water will cease to evaporate. For this to work in steady-state, then, moisture or moist air must continuously be sucked away from the room. Practically, I can only think of doing this by having the moist air leave the room. Mass balance implies that new (unfortunately hot) air must enter the room.

So our system is now dry hot air entering through one window, passing through a fan and over a basin of water to cool it, and leaving through my door towards warm common space.

My questions:

1) Would simply having an ultrasonic humidifier blow water into a continuously flowing air stream be sufficient to cool the air, or otherwise be more efficient than just running the air over a water bucket? If the heat is truly just going into enthalpy of vaporization, this seems like it'd be more efficient.

2) This design relies on the air cooling process being sufficiently "fast" relative to the air's flow rate. Otherwise the hot air will enter but only be slightly cooled by the system before leaving our cooling system, entering the main room and discomforting the occupants. Everyday experience suggests that opening a window, even with an ice bucket in front of the window, won't absorb heat quickly enough when it's 90 degrees outside and 80 inside. At the same time, evaporative coolers have found commercial success, so is this simply not true?

3) How can I effectively have moist air "leave" a room? Is simply opening a door enough such that the cool, moist air will diffuse out on its own? If I leave the door closed and turn on a fan and ice water in front of an open window, can I expect warm dry air to come in through the bottom of the window and by mass balance necessarily leave as cool, moist air (unintuitively) through the top?

4) I'm hoping that my moisture sink will be the common space outside my door, but given that this is almost, if not just-as warm as my room, it seems that would let heat back in from that end as well. Does this backflow doom our system?

I understand that, in more industrial contexts, these kinds of questions require simulation tools to answer. I just wonder if there's back-of-the-hand heuristics (e.g., convection is faster than conduction) that I can use to do this without needing to break out Ansys or fuss around too much with system configurations.

If cooling my room like this is, indeed, hopeless, how does a $40 evaporative cooling system manage to circumvent these problems from a heat/ mass balance perspective? Where does it dump the moisture, and how does it prevent heat backflow? Is there a better system design I'm missing?

Answer 1

I'm not an expert on swamp coolers, but I would try to keep all the evaporation outside of your dorm room.

Here's an example: You build a heat exchanger between inside and outside, then you mist the exterior with water. You'd put fans on both sides preferably. If you mist enough, the exterior will reach the dew point temperature of the exterior environment. There are charts you can use to calculate this temperature. Assuming it's dry outside, the temperature will probably be quite low. Heat leaves your room through the heat exchanger (as long as your room is warmer than the outside dew point). And the humidity in your room does not increase.

Answer 2

Evaporative cooling system's efficiency is greatly influenced by the ambient moisture, and is not even advisable for high humidity zones. And it needs to be installed in a shaded area or have a vented enclosure.

The way they operate is by blowing dry air through filters kept wet by a small pump circulating water from a reservoir on the floor of the cooler box refilled by a flush ball valve.

The optimal setting for releasing the moist air is to maintain a small opening somewhere in the room, small enough to keep the air pressure in the room fractionally higher than outside preventing the hot air to circulate back to the room.

The methods you suggest are not going to be practical.

Answer 3

You are right, evaporative cooling systems require a continuous flow of large amounts of air through the space being cooled in order to be effective. These work well in places where the ambient humidity is very low (Arizona deserts or the central valley in California). Bowls of water and fans are not going to make a difference.

You will find useful rules of thumb for evaporative air conditioning in heating/ventilating/air conditioning (HVAC) handbooks published by the makers of air conditioning equipment, like Trane.

But... if you are in a dorm, this means you are not paying for electricity, yes? Then the smart thing to do is buy a small window-mount refrigeration-style air conditioning unit that plugs into the wall, and build a wooden mount for it to fit the window you have. These sell for $100 new or $50 used. As a recovering ex-engineer, I would do this and declare VICTORY!