Today I took more measurements after my cooling tower had stabilized. The ambient conditions were 33.4C ( 94.6F) and 41.4% RH. My psyrometric chart indicates the wet bulb temperature should be 22.8C (73F). Measurement of the exit air showed it was 30.35C (86F), substantially higher than the wet bulb temperature. The temperature of the (recirculated) water was about 74.5F so it IS close to the wet bulb temperature.
The reason the exit air is much warmer than wet bulb may be due to excessively high air flow through the evaporation pad; or perhaps the air never is going to get all that close to wet bulb. After all, the wet bulb measurement doesn't measure the temperature of the air after evaporation, it basically measures the residual water surrounding the thermometer bulb. Videos done by Desertsun02 suggest that the exit air temperature should be much lower, but his setup isn't exactly like mine.
I'd like to get the temperature of the exit air lower, because I can use it (via a second heat exchanger) to cool the return water from the interior heat exchanger. That would increase the system efficiency, perhaps by a significant amount. To work on this, I bought a couple of PWM motor speed controllers to experiment with air flows through the evaporation tower and inside heat exchanger.
I did try throttling down the water pump to see how that would affect the system, but reducing the flow rate by about 50% didn't have a noticeable impact on the measurements.
Examination of my psyrometric chart did suggest a way to improve the system performance by a small amount. Although it sounds counterintuitive, if the air entering the cooling tower is pre-cooled by passing it through a heat exchanger, the wet bulb temperature of the cooler-but-more-humid air is lower. To check this out, you need to look at what happens when the air is cooled by a heat exchanger. It doesn't pick up any more moisture so the "humidity ratio", which is the ratio of dissolved water vs air masses, remains constant. So the heat exchanger just moves the air straight to the left (i.e., it just moves along a constant humidity ratio line). Then you look at what the resultant wet bulb temperature would be. Here's an example. Looking at the ambient conditions I get a humidity ratio of 14. Cooling the air down to 25C before it enters the tower should produce a wet bulb temperature of 21.5C, which is about 1C lower. Not a huge improvement, so I don't think it's worth the added cost and complexity. It's more worthwhile to get the exit air temperature closer to wet bulb so I can reduce the heat load on the recirculated-water loop. This will ONLY work if the exit air temperature is lower than the return water from the interior heat exchanger....which, in turn, can't be any higher than the ambient temperature in the house. Clearly, we want to cool the house down so ideally there will be a substantial change in coolant temperature from inlet to outlet.
I have to say that my measurements don't look all that promising for cooling our house with this setup. There is a final way to (potentially) greatly improve the performance of an evaporative cooler, but it comes with quite an increased bit of complexity. It would reduce the humidity of the air going into the cooling tower using something called "liquid desiccant", which will improve operation of the system in regions where the relative humidity is moderately high to high (and our region seems to fall into that category) . The liquid desiccant absorbs moisture but must be regenerated on a continuous basis in order to keep working. This requires some sort of heat source and another "tower" to help extract the absorbed water from the desiccant. Sounds complicated? Yes, but fortunately it appears that it can be done using a fairly low-tech approach. I will leave it there for now.
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