What is the RH of blown air effect?

Question

From xto 2 is a dehumidification process to an air inside an air conditioner. This process will sure give you air with low moisture content which will let you feel dry air. But also this air holds a 100% RH won't this Rh let you also feel warmer and sweater?

Answer 1

The overall goal of dehumidification is clearly not to cool the air but to reduce relative humidity. For that you reheat the dehumidified air to the original temperature of 30 °C. So what is missing from the process diagram is a second horizontal line from 2 to say point 3, which would be at 30 °C, but a lot lower than point 1, vertically, that is, at a much lower relative humidity (e.g. 40%, just a guess). What matters to you feeling heavy and sweaty is only relative humidity.

Now you may say, oh why not just leave the air at 14°C as a nice side-effect of dehumidification. This is the wrong approach for two reasons:

1. as you already correctly noticed, relative humidity at 14°C would be 100%, and that won't feel nice although the temperature is lower; you might still be sweating, and the sweat wouldn't dry because it can't evaporate into the saturated air
2. cooling air is a lot more energy-consuming than just dehumidification; in the dehumidifier one can guide the dried air from the cold side right back to the hot side of the chiller in order to heat it up again (or use a passive heat exchanger for that); while this might seem waste of energy at first sight, it actually increases energy efficiency of the dehumidifier because the chiller does not need to fight against the already hot side to make it hotter still, in order to cool down the cold side (which tends to get warmer due to condensation heat); as you might know, the efficiency of a chiller decreases if the temperature difference between its plates rises; by lowering the temperature on the hot side, we ease the process

By such a clever construction, the chiller/heat pump need only be supplied the energy for pumping the condensation heat away from the air to be dried, while it does not need the energy for also cooling the air.