Cool Room Dehumidification Techniques

There are 2 main types of Dehumidifier, Refrigerant and Desiccant. While initially it looks like the refrigerant units are superior, things change at lower temperatures like in a cool room. This is because of how the dew point, condensation and relative humidity (RH) work. To get a better understanding, let’s relate it to something familiar. You know when you come outside on a cold morning and see your car covered with speckles of water. This condensation happens because the surface of the car is below the dew point in the air.

Relative humidity is a measurement of how much moisture the air can hold. The problem with this is the amount of moisture that air can hold changes with temperature. Roughly every 11c the air can hold double the water volume. This means that air at 21c 60% RH is holding double that of air at 10c 60% RH. As the air hits a cold surface it cools rapidly and hits 100% RH, therefor turning in to condensation.

Refrigerant Dehumidifier

A refrigerant dehumidifier works by chilling the grill down far below the dew point and even below freezing. The grill builds up condensation over time, also freezing it due to sub zero temperatures. To drain the frozen water the dehumidifier reverses the cycle of the gas the same way a split system does switching from cooling to heating.

In an environment like a cool room that’s at 2-5c, the dew point is quite low. Therefor a refrigerant dehumidifier starts to really struggle pulling moisture out of the air. Less condensation forms on the grill and the unit will only perform at a fraction of the rated L/day.

Refrigerant Internals

Desiccant Dehumidifier

A desiccant dehumidifier works on a totally different principle. These units contain a desiccant rotor filled with tiny straws. Each straw has some silica gel inside, the same stuff in the desiccant packets in vitamin bottles and electronics. I think it goes without saying but you probably don’t want to eat a desiccant dehumidifier either.

Silica gel absorbs water like a sponge pulling moisture from the air. The principle is the same as drying a wet phone in a container filled with rice. Another benefit is, due to no reliance on dew point, it works much better at low temperatures than refrigerant models. The rotor rotates very slowly with a slice on a separate air channel. Hot air is blown through the slice to dry the rotor and exhausted outside. This design also means the heat leaves the cool room. This means there is less temperature increase in your cool room due to the dehumidifier.

Desiccant Rotor

What Is Right For Your Cool Room

A Desiccant unit seems expensive and inefficient compared to a refrigerant if you look only at the performance above 10c. Below about 8c it starts to make a lot more sense to consider a desiccant model. Especially so if you don’t have a powerful refrigeration unit and can’t withstand the heat load of a refrigerant model.

As an example of the performance gap at 5c 60% RH between our SP1500C 150L/day refrigerant and FD-D600K 72L/day desiccant. The desiccant unit will remove 38.4L/day while the refrigerant unit will remove 34L/day at 5c 60% RH. Here the desiccant comes out victorious despite having a rating under half the L/day of the refrigerant. A desiccant unit can also get humidity lower than a refrigerant. You may get the RH as low as 10-15% compared to a refrigerant only going down to 25%.

The refrigerant may still be an acceptable choice. Even at the 5c range due to similar price and no requirement of ducting installation. You only require a drain hose. Wall mounting the refrigerant unit is another advantage. You will however need a powerful refrigeration unit to handle the heat load.

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