Due to being the author of the IX CHART program, which is used by a considerable number of designers and others, I often receive various types of questions. Recently, I realized how misunderstood air cooling is, especially if the cooling is used not only to lower the air temperature but also to dehumidify it, or "squeeze" water out of the air like a "sponge".
I am not surprised by this because few books say anything on this subject, and the truth is more complex than it seems, which can cost many designers dearly if they are unaware of this relationship and design full HVAC systems, i.e., not only temperature control but also humidity control.
So, what is the issue? Most literature presents the cooling process in this way:
Does this look familiar? What do we see here - a beautiful linear drop in temperature and humidity from the initial parameters "Z" to the final "CH". The problem is that the actual cooling process does not run linearly in the cooler! While this doesn't matter for temperature, it matters HUGE for relative humidity. The actual cooling process depends on the construction of the cooler itself (number of exchange rows, number of fins, their spacing). In the initial cooling phase, there is a significant drop in temperature, but not such a big drop in relative humidity, only later there is significant water droplets formation around the saturation line. As a result, we obtain a completely different relative humidity, which can decide whether the desired parameters are maintained or not during air dehumidification.
In general, the deeper we cool, the closer the actual parameters will resemble the theoretical parameters, as shown in the diagram below. In other words, if we follow the IX chart and our endpoint to which we cool is relatively far from the temperature of the cooler wall, then we will make a large error in terms of the actual reduction in relative humidity. Conversely, the deeper we cool, the closer our theoretical considerations will be to the actual cooler.
OK, let's take a concrete example.
Let's assume that our goal is to maintain the parameters in the room not higher than +24oC, RH <50%. If we rely only on theoretical changes, i.e., the linear version of cooling, we will get this:
As I showed earlier, for "some" real cooler, the endpoint of cooling will not be t=+14°C and RH=80%, but rather t=+14°C and RH=92%. Ultimately, in the room, we will get the parameters:
Finally, in the room, we will get the following parameters: t=+14oC is correct, but the humidity RH=~54% not, so unfortunately it did not work! Therefore, we should ALWAYS select a specific unit and check what we get behind the cooler. We should not rely on relative humidity RH at all, but on the absolute moisture content "X" to which we want to bring the air, which will certainly require deeper cooling and will affect both the temperature of the cooler wall and the required cooling power of the secondary heater!! In summary - Chart IX is the best thing we have to do a decent project analysis without the involvement of the manufacturer, and I cannot imagine working without Chart IX. However, this does not exempt us from the obligation to think and look at the actual parameters we get from the manufacturer. To comfort you, I will add that such problems mainly occur in the cooler, where we have to deal with water condensation. Other changes look the same in theoretical and actual terms :) If the article has brought something positive to your development, and you want the website to continue to grow where I share my knowledge, give it a thumbs up, leave a comment, or share it. And of course, you can help me and yourself by purchasing a license or You can just buy me a coffee :)
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