Humidification Basics Part 8: Adiabatic Humidification

By Chad Edmondson (JMP) and Norman Hall (RLD)

In the previous blog we discussed isothermal humidification.  Today, we’re going to talk about the other method for humidification, which is called “Adiabatic Humidification.”

Adiabatic humidification injects water (not steam) directly into the air using a spray mechanism or wetted medium, and the heat from the surrounding air causes the water to evaporate. So instead of sliding up the vertical dry bulb temperature line on the psychrometric chart, we move diagonally up the wet bulb line.

Here’s what that looks like:

This chart actually shows a few different humidification scenarios; but no worries, we’re going to break it down for you, starting with the black dot which is our outdoor supply air condition of 55°F at 30% RH.

Using either type of adiabatic humidifier, we put moisture into the air, increasing the humidity from approximately 30% to about 85%. Note that the black like moves up and to the left in parallel with the wet bulb lines on the chart.  At the same time, natural evaporation drops the air temperature to about 45 °F.  This is too cold to put directly into the space, so we have to add heat to get the temperature back up to 55°F, which causes the RH to drop from 85% to approximately 50%.  If we put this air into our 72°F space, we end up with an RH of approximately 30%.

But let’s say we want to take the space up to 45% RH. To do that, we would have to add more moisture and more heat, basically zigzagging up the psychrometric chart.  At this point, you would actually need both pre-heat and re-heat to offset the cooling effect of the evaporation before the air is put into the space.

You may wonder, given these particular conditions, why you don’t want to add the 55°F at 30% RH directly into the space. Again, you can’t change one value without changing another. If we were to put the 55°F/30% RH air directly into a 72°F space (thus changing the dry bulb), we’d simultaneously lower the RH to 15% as we move horizontally to right on the psychrometric chart. That is far too dry for human comfort.  Therefore, adiabatic humidification typically involves some degree of reheat.

So when is adiabatic a good solution for humidification?  The best applications are for when you have a warm and dry supply air and the absorption distance is not critical.  In these cases, you can take advantage of the sensible heat in the supply air and in the right situations take advantage of the free cooling that the adiabatic process can provide.   They can also be utilized with RO or DI water to reduce maintenance and provide pure, particulate free humidity.

Some typical Adiabatic technologies include:

·         Pressurized water humidifiers

·         Wetted media humidifiers

·         Ultrasonic humidifiers

·         Pressured air and water humidifiers

·         Centrifugal humidifiers

Pressurized water humidifier when used with DI or RO water can provide a high level of capacity and control while having a low maintenance requirement.  They also have the highest ability to stay online if being used in a critical process where space conditions have to always be maintained.

In both adiabatic and isothermal humidification processes, approximate 1000 BTU’s per pound of water are required to change the water from a liquid to a vapor. However, in isothermal humidification, that energy is taken from boiling water. In adiabatic humidification, that energy comes from the surrounding air.