My Evaporative Cooling Test Bed, A Review

While working on my previous post regarding the usefulness of the Psychrometric Chart, I had a thought regarding the test bed I built last season.  I noticed large differences in the exit air temperature between my setup and a similar one built by Desertsun02 -- his system was outputting colder air than mine.

In retrospect, this probably is due to different cooling pads.  Doing some online searching revealed that different pads have higher efficiency compared to some of the synthetic ones, like the ones I'm using.  The old-fashioned shredded Aspen pads apparently are pretty good, as well as paper ones with a honeycomb pattern.  One big difference may be that the synthetic pad type I'm using isn't very thick so the air doesn't have as long a "dwell time" in the pad compared to thicker ones.

I'd like to get the exit air temperature lower because I think I can use it to pre-cool the air flowing _into_ the evaporative cooler.  That could get me closer to performance like the Maisotsenko Cycle, which theoretically can output air that is very close to the dew point. So it looks like I need to experiment with a different pad, along with everything else.

Air Conditioning With A "Swamp Cooler": the Psychrometric Chart

 As the climate heats up and energy resources become more and more stressed, interest in alternative approaches to compressor-based A/C has increased.  A lot.  The basis of many alternatives circulates around evaporative cooling technology, most commonly referred to as the swamp cooler.

As a kid I remember swamp coolers in two different situations.  For some time we lived in the Four Corners area, where Colorado, Utah, Arizona and New Mexico meet at one point.  The area is high desert, hot and dry in the summer; and that's perfect for the old-style swamp cooler.  The version we had in our house took hot and dry air from the outside and blew it through a water-soaked membrane.  Evaporation occurred, which cooled the air and also added some welcome humidity to the air, which usually had a very low relative humidity.  The cooled and wetter air was blown into the house using the same heater ducts that were used to heat the house in the wintertime.

The other situation was during summertime visits to relatives in Oklahoma.  At that time compressor-type A/C was expensive, so they couldn't afford to get that.  So they used swamp coolers there, too.  But in that case, while the air temperature was comparable to what we got in the Four Corners area, the humidity was much higher.  In that case, the swamp cooler was less effective for two reasons.  First, the high humidity reduced the amount of evaporation that could occur in the swamp cooler.  The second has to do with our perception of comfort.  When in a high-humidity environment WE also are less able to cool ourselves, because our sweat is less able to evaporate.  The end result was that the swamp cooler in Oklahoma really didn't make me feel any cooler than just staying outside in the shade, hoping for a bit of wind to come by.

This is where the Psychrometric Chart comes in, to help us understand what's happening.  I'm working on an upcoming post where I (hopefully) explain how a relatively new evaporative cooling technology based on something called the "Maisotsenko Cycle" works, and how a version of it can be relatively-easily added to your basic evaporative cooler, to significantly improve its performance; and the explanation heavily depends on use of the Psychrometric Chart.

Anyway, back to our simple swamp cooler.  Today I measured the exterior peak temperature and ambient relative humidity and got 32.2C (just shy of 90F) and about 37% relative humidity.  I plotted that point on a copy of my Psychrometric Chart and it looks like this:

The vertical axis is the temperature, but the relative humidity curves are the upward-trending ones as you look from left to right.  The dark point shows the conditions at our house.  Another set of curves are straight lines that go down from left to right, not quite at a 45 degree slope.  Those are lines of constant wet-bulb temperature, and give the temperature of the water in the swamp cooler membrane.  In this case we get about 70 degrees Fahrenheit.  That sounds pretty good, dropping the exterior air temperature down to 70F:  but that's the temperature that the WATER gets to.  My previous experiments with a home-brew swamp cooler show that the exit air temperature can be ten degrees higher than that, perhaps more if the air flow is excessive.  This means that the air temperature coming of out my swamp cooler might be about 80F.  Better than 90, but that wet-bulb temperature sure sounds better.

Some may wonder why the air and water temperatures aren't the same.  I think that's because the air carries off the heat extracted from the evaporating water.  Also based on my experiments, excess air flow also can be a factor.

The difference between the wet-bulb temperature of 70F and the exit air temperature makes the use of a slightly-more complicated system attractive.  That's an indirect evaporative cooler, where the chilled water is piped into the interior space and passed through an air-water heat exchanger.  In this case, the water also is continuously circulated through the evaporative cooler because we want to use the chilled water to cool the house.  A level sensor detects when the water level in the chiller falls to the point where it needs to be topped-up.

In either case, once the exterior relative humidity rises above about 50% they become pretty ineffective as an A/C system.  But there is a way to get the water in the chiller colder, approaching the dew point temperature (rather than the higher wet-bulb temperature).  In the case of my example, the difference is about 5 degrees C,  which would produce water at about 60F, ten degrees colder yet.  More on that in another post, which includes a more in-depth exploration of the Psychometric Chart.

Some may wonder why the wet-bulb temperature is higher than the dew point.  I think that is because there are two effects that are in equilibrium at the wet bulb temperature.  The first is the heat extracted from water by evaporation.  The second is the heat contained in the air being transferred to the wet bulb.

Check Valve Redux

 I had an opportunity to test my bearing-ball check valve idea, at least in terms of how it works in a pneumatic sense.  I discovered that my idea was flawed by the need to incorporate two incompatible requirements.  The first:  the ball has to fit easily into the end of the tube to seal.  The second:  the force pulling the ball into the tube has to be small so the check valve works with a very small pressure differential.

The practical result of these requirements was that the hole in the piston where the ball goes has to be very nearly the same diameter as the ball.  But this means that the air has to flow through a pretty small constriction around the ball. The end result was that there wasn't much of a difference in the flow rates.  I thought of some ways to get around this problem but they all had their own complications -- including noticeably more machining work.

So instead of that approach I went with a much simpler flap valve arrangement, made with a square piece of acrylic film.  The film is placed over a hole drilled in the base, which serves as the air inlet.  To promote free flow of the air, I also milled a shallow slot from the end of the base up to the hole.  I cut a square piece of the plastic film and then made three cuts in the form of a U, to free that portion of the plastic, enabling it to move up (away) from the hole, or down toward it to form a seal when the piston is being pushed into the cylinder.  That worked OK, but it worked even better when I made more cuts to widen the gaps between the flap and the rest of the plastic sheet.  I think the narrow slits didn't allow the flap to completely seal.

Putting a little lubricating oil in between the plastic and base would probably work even better -- for awhile, but the oil could attract dust and then the seal would likely fail.  In this case a little less (of a seal) is more, in terms of longevity of the damper.  That said, I also made the damper so it can be taken apart and cleaned if that becomes necessary.

A Check Valve For My Soft-Close Drawer Project

 I had originally intended to use an off-the-shelf check valve to use in my pneumatic soft-close mechanism, but the more I played around with the overall idea, the less suitable it appeared to be.  It would have to hang off the side of the cylinder.  The piston needed to be "special" to accommodate the need to put a hole in the cylinder for the air to pass through.  And so on.

So I was thinking about integrating my own check valve into the piston itself.  The original design used the aluminum stem for the air flow (it is a tube), with a ball bearing to act as the seal.  The bearing would be pushed against the end of the tube with a small spring.  The tube, bearing and spring would be installed in the piston.

But my piston is only .75" long, and it was hard to find the right itty-bitty spring so I decided to replace the spring with yet another magnet that would pull the bearing onto the end of the tube.  The magnet would be a ring magnet that the aluminum tube slides through, and would be glued to the top (or outer) side of the piston.  I liked this approach because it's based on a simple physical mechanism (magnetism) and should work OK for a very long time.  The only thing that might mess it up is dust and dirt between the ball and end of the tube; and if that becomes a problem I could glue a small air filter to the bottom of the piston to junk out.  Maybe I'll be pro-active and just do that up front :).

But the question arose:  would there be enough force acting on the bearing ball to pull it into the end of the tube?   Or would the force be too great so the check valve wouldn't permit the damper to easily open (and therefore cause the drawer to not easily open)?

This looked like another simulation to perform using FEMM, Finite Element Modelling, so I could answer these questions.  Long story short:  it looks good.  The attractive force is small, on the order of 3.9 grams; but since the check valve will be on the horizontal plane it won't take much force to pull the ball toward the end of the tube.  And 3.9 grams over an area of .049 square inches (the cross-sectional area of the tube) indicates that the check valve should open with a pressure differential of just .17 pounds per square inch.

Here's a screen shot of the simulation:

I included a steel plate that will be used to mount my damper on the back of the base unit where the drawer goes, just to make sure it wouldn't cause problems in the operation of the check valve.  And it doesn't alter the results to any great extent.  It does increase the attractive force between the magnet and baseplate, but it still is pretty low, about 14 grams.  That's less than one ounce.  At that point in the operation of the system -- damper plus long-distance magnetic latch -- the attractive force of the mag-latch is MUCH higher so it won't materially alter how the overall system behaves.

Forever Products -- Why Not?

 A significant part of the waste we all send to the landfill are products that fail due to some proprietary component,  or because they weren't designed to be repairable.  Often the bad component can't be replaced because the manufacturer either (A) doesn't offer it  (B) they did but only for a short while; or (C) it isn't possible to replace the failed part because the product wasn't designed to permit that.

All these issues are things that can be addressed in a variety of ways.  While the "right to repair" movement has gained some traction, my examples in the previous paragraph show that it can only go so far -- unless the design process used to make our "stuff" includes the requirement that the item can be repaired for a very long time, even long after the original manufacturer has gone out of business.

Our military has some similar requirements, considering how large their inventory of materiel can be, and concerns regarding the availability of replacements in a wartime situation.  But, considering the concurrent issues of waste reduction and the reduction of resource depletion -- both mineral and energy resources -- and the rather large multiplier of a consumer economy at work -  that also has large implications, given current trends.

So, what's in the way of making products that use off-the-shelf components as much as possible, so they can be replaced long after the manufacturer has declared the product obsolete?  What's in the way of requiring manufacturers to provide design data on their proprietary components for those same products so they can be made with 3D printers?  Many companies these days employ designed-in obsolescence as a part of their business model, so they can sell new stuff.  But that tactic has become a larger and larger problem, given the issues of waste and all the resource-consuming aspects of making new items that, in many cases, aren't any better (often less) than what they replaced.

This is where government has a role to play, basically drawing a line and saying that youse-guys have to clean up your act.  Of course, manufacturers should be able to offer new products with new features:  but they need to both design their products so they can be repaired; or if some unique parts are in there, once they have come out with a new model they have to make the design information available for the older item so replacement parts can be fabricated by a third party, or, if possible, with a 3D printer.

The overall impact of this would be multifold.  For starters, manufacturers that simply "churn" their products so older, but equivalent, products become obsolete, will have a greatly reduced incentive to do so.  To appeal to consumers, new products would only succeed if they offered better functionality, or added functions.  This would promote innovation rather than just putting a different color of lipstick on the same pig.  Of course, the new products would have to be Forever Products too, so the pattern of innovation would continue.  Or maybe some vendors would just offer Forever Products and tap into the demand for something that can be repaired until long into the foreseeable future.

Some manufacturers are very good about providing replacement components for the products they sell, but they seem to be in the minority.  That has to change.