How Dehumidifiers Work


Dehumidifiers have been used in water restoration for decades to remove vapor from the air.


How Refrigerant Dehumidifiers Work


Evaporation causes cold to happen where the evaporation takes place. This is how sweat works.

Inside a refrigerant dehumidifier is the “evaporative coil.” As the name indicates, this is where a liquid inside the coil is turned to vapor. At the point of evaporation, it's cold. We now have cold coils, on the back of the unit. Warm, wet, air from the flooded area of the building, is passed over these cold coils, lowering the temperature, of the air, to the “dew point”. The “dew point” happens when air is cooled and some of the vapor can no longer stay as vapor. It takes a certain amount of heat to remain as vapor. This point , where it has lost enough heat energy to go back to water, is called condensation. (water) It's what you see on a cold can of soda. The surface of the soda can is at or below the "dew point." 


How Conventional Refrigerant Dehumidifiers Work

When using conventional units, air is passed over the coils and some of the vapor gets condensed, while some passes right on through and out the exhaust, because the vapor didn’t make it to the “dew point”. It went through too fast to have its temperature lowered enough to cause condensation.


How Low Grain Refrigerant Dehumidifiers Work

The basic difference in the operation of  “low grain refrigerants” and “conventional” refrigerants is that low grain refrigerants (LGRs) circulate the air, entering the unit, before sending it to the cooling coils. This cools the air so it’s closer to the “dew point” – the point where condensation forms. Because the air is cooler when it hits the coils, more of the vapor makes it down to the dew point and is condensed.

Not only does an LGR dehumidifier produce dryer air, it works in hotter rooms. Some LGRs can operate in up to 110 degrees.

For a conventional, that sends the air through one quick time, without pre cooling, it’s just too long a trip to get the air down, from a high room temperature to the, lower, dew point. In fact, above ninety degrees, they start to have problems condensing vapor.



A desiccant material absorbs things. Obviously, during a water restoration job, we want to attract moisture. Warm, moist air goes into the desiccant material and drier air goes out the front. Moisture is evaporated off the desiccant material and usually vented out a window.

Desiccants receive moisture two ways: 1. absorbent – moisture goes all the way through, like the color in a carrot and 2. adsorbent – moisture stays on the surface of the material, like the color on a radish. Most desiccants are “adsorbent”. Moisture goes into an outside layer and is evaporated off. Desiccants produce drier are than refrigerants. They are  more expensive than refrigerants and are usually only used for “tough to dry” class 4 floods.  


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           Dennis Klager

           IICRC Instructor 


Copyright 2012 by Dennis Klager