5 Gallon Air Conditioner - Does it work? To Insulate or Not Insulate? That is the Question.

5 Gallon Air Conditioner - Does it work? To Insulate or Not Insulate? That is the Question.

So, here at 5gallonairconditioner.com, we continue to get asked about why I don't insulate the Ice Model air conditioners. So, in this short blog post, I will explain the reasoning and the physics behind that decision.

By definition, the purpose of insulation is to "reduce the heat transfer between the outside environment and the inside environment."

So, that is exactly the opposite of what we are trying to do with the ice model units.

Ok, so here is a quick thermodynamics lesson, in easy to understand language.  First off, you need to think in terms of heat, not cold.  So, when an item is cold, like an ice cube, in layman terms that means that it is "giving off" cold, but in thermodynamics, it is "bringing in" heat. So, remember, think in terms of heat, not cold.

So, when something "brings in heat" that means, by definition, that it is "giving off" cold.

So, now, in our ice model units, when the user puts a block of ice inside the bucket, what is the ice going to immediately start doing?  It's going to immediately start melting right?  When ice is melting, thinking in terms of thermodynamics, what is happening?  It is absorbing the surrounding heat, right?  That's what we want.

We want "the space" that we are trying to cool, to be devoid of heat, so therefore, we want as much coolness as possible.

So, how do we go about doing that?

By design, the ice model AC unit fan sucks in hot/warm air in the top, and blows that hot air straight down into the bucket cylinder.  There is ice inside that cylinder, the ice absorbs the heat (melts) and as the ice absorbs the heat, it gives off cool, and then that cool air is forced out of the ports on the side of the cylinder.  Pretty basic and simple.

But, what would happen if the cylinder was insulated? That is the question.

Well, first off, there would be less space available for ice, so there would be less "cooling capacity" just simply due to volumetrics. But, in reality, would the ice last longer, that's what everyone wants to know, right?

That's where the concept of air flow comes into play.

Let's just use the example of a 10'x10'x8' room.  Let's use the example of that room, which is a pretty small bedroom, or approximately a 6-8 man tent. So, the cubic volume of that room is 800 cubic feet, right? 10*10 = 100, and then 100*8=800. So, there is 800 cubic feet of air inside that room, if the room was completely sealed, w no air leaks etc, and let's also assume that the room is perfectly insulated, and no heat escaped our enclosure.

Now, let's assume that the air inside that room is 90 degF, and for simplicity, lets assume 0% relative humidity.

So, here is the formula:

BTU=1.08 * dT * CFM

1.08 = BTU multiplier at sea level. Well, what does that mean, right?

That calculates out as this: Air weighs 0.075lbs per cubic foot multiplied by the specific heat of air (0.24) multiplied by 60 minutes in an hour. When you multiply that out it comes to 1.08.

dT= the change in temperature of input air to output air.

CFM= the cubic feet per minute of circulating air flow.

We also know that ice has a BTU content of 144 BTUs/lb.

We also know that our fan on these units flows 250 cfm.

Ok, so now we have plenty to work with. Let's assume that we have a 10lb block of ice in our ice model AC unit, and there are no people etc, inside this perfectly insulated enclosure.

What would be our lowest temperature we would be able to obtain?

In this case, we are solving for dT.

dT=BTU/ (1.08XCFM) 

dT=1440 (10lbs of ice) / (1.08 X 250) = 5.3 degF.

But, what does that really mean?

That means, that the air inside our test chamber will be 84.7 degF at the instant the last chunk of ice melts and turns to 32 degF water.

But what about insulation, right? That's what we're trying to figure out. If the AC unit is "more efficient" when it's insulated.

When you look at these calculations, you can see that there isn't "a factor" involved inside the test chamber.  Now, you definitely want the OUTSIDE of the test chamber to be insulated, but INSIDE, it doesn't matter. The physics and the calculations are the same.

From a practical standpoint, in our 800 cubic foot (cf) test chamber, and a 250 cfm fan, the air inside the chamber is exchanging approximately every 3 minutes.

So, now, from a physical perspective, 3 exchanges per minute, that's a lot.  That is "noticeable" airflow, that's almost considered "breezy."

So, if air is blowing by a 5 gallon bucket that has ice inside it, that means that the outside surface of the bucket is cool to the touch as well, and this warm "breezy" air is blowing by the cool outside surface of the bucket, then the bucket will absorb more heat, and cool the air faster. 

So, if the bucket absorbs more heat, that means that the ice is melting faster. If the ice is melting faster, then that means that the ice won't last as long.

BUT...

It also means that the ice absorbed the heat faster, and therefore the space cooled down faster.

So, the bottom line in all of this discussion is, YES insulating the bucket will make the ice last a little longer. BUT, that also means that if the bucket is insulated, then it will take a little longer to cool down the space.

So, it's a trade-off, with no practical difference, and virtually no difference when the amount of reduced ice/cooling capacity is factored in.

So, then when looking at things practically, the foam liners are very thin, don't offer much if any real insulation value, they take up space which could be used for more ice,  the foam is very "brittle" which means that it leaves those little pellet sized foam "nuggets" everywhere and is always breaking and cracking. Also, even if the sides of the bucket were insulated, there is still a gaping hole in the top of the bucket where the fan attaches and the fan is blowing hot air inside the unit, which is what the insulation is trying to protect against, in addition to the 3 gaping outlet holes.

So, for all of those reasons, the ICE model 5 Gallon Air Conditioners are un-insulated.