Help with a drying problem
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It feels like everyone is very willing to help, so please share any questions you have since I was restored, so we can learn together. Title: The following three types of air are used as drying media. Which type of air results in a faster drying speed? Why? (1) t=60℃, H=0.01 kg of water per kg of dry air; (2) t=70℃, H=0.036 kg of water per kg of dry air; (3) t=80℃, H=0.045 kg of water per kg of dry air. The order of drying for these three cases is (3) > (1) > (2). The answers are as follows: (1) Using the enthalpy-humidity diagram, we find that Tw=28℃, Φ=7%, and Hw=0.052 kg of water per kg of dry air. Here, t – Tw=32℃ and Hw – H=0.042 kg of water per kg of dry air. (2) Tw=40℃, φ=17%, and H=0.049. In this case, t – Tw=30℃ and Hw – H=0.013 kg of water per kg of dry air. (3) Tw=44℃, φ=15%, and Hw=0.062. Here, t – Tw=80–44=36℃ and Hw – H=0.017 kg of water per kg of dry air. I’ve tried to understand this for a long time but still can’t figure it out Why? Could that person explain it to me? Why, for relative humidity φ, is (2) > (3) > (1), but the drying rate is still (3) > (1) > (2)? Is it by following some kind of sequential pattern for comparison?
The drying rate formula can be estimated using the following equation: Uc = a(t – tw) / r, where a is the convective heat transfer coefficient between the air and the material, which can be considered constant under the given conditions； r is the heat of vaporization of water at temperature tw; the higher the temperature, the lower the heat of vaporization of water, hence the drying rate is inversely proportional to tw ； So, based on the answer given in your question, the drying rate is proportional to t-tw. It seems that the factor of r, which represents the heat of vaporization of water at tw, isn’t taken into account. If you’re interested, you can check the steam tables found at the end of textbooks on chemical engineering, and see what happens when the heat of vaporization of water at different tw temperatures is used – whether the same results are obtained. As for the meaning of the value hw-h, I’m not sure how it’s used; those who know might want to discuss it as well