A special humidity-absorbing material “desiccant” absorbs moisture but gives out this moisture when it is heated. The desiccant material is usually made into a wheel that allows air to flow through it. It resembles a cross section of a cardboard with flutes in it rolled and sliced into a wheel. Moist air from the room is drawn through a section of the wheel by way of a fan. The air that passes through is therefore dry and its moisture given up to the wheel. The wheel is continuously turning. and a section of the wheel is then exposed to a different air flow that is this time heated air. This heated airflow dries the wheel and now carries the moisture away so it can be vented or the water can be condensed & poured away.
Because of the lack of compressor parts desiccant dehumidifiers are often lighter and quieter than compressor dehumidifiers. Desiccant dehumidifiers can also operate at lower temperatures than compressor dehumidifiers more effectively below 17°C.
A range of materials are commonly used to remove moisture from a confined space or from an air stream. These are commonly known as desiccants. A desiccant is a substance with very hygroscopic properties. The essential characteristic of a desiccant is low surface vapor pressure. If the desiccant is cool and dry its surface vapour pressure is low and it can attract moisture from the air which has a high vapour pressure when it is moist. After the desiccant becomes wet and hot its surface vapour pressure is high and it will give off water vapour to the surrounding air. Vapour moves from the air to the desiccant and back again depending on the vapour pressure difference. Desiccants remove moisture in one of three ways; absorption, adsorption or through a chemical reaction or change.
“…Ecor Pro & Toyotomi have never had a dehumidifier that has burst into flames because of our safety measures & the quality of our products…”
Absorption is when a substance is chemically integrated into another such as salt dissolving in water and becoming salt water. Adsorption is the physical attraction and adherence of gas or liquid molecules to the surface of a solid.
Desiccants are of four types, silica gel, montmorillonite clay, molecular sieve (zeolites), and calcium oxide. Some of these materials can be regenerated (purged of contaminants and reused) and some cannot. It depends on the change in the physical structure of the desiccant and the deleterious effects of regeneration on the material.
When you place a properly prepared desiccant into an air stream with a humidity level higher than that of the desiccant the desiccant will remove moisture up to it’s saturation point or equilibrium capacity. Saturation is when the desiccant is full and even if there are moisture molecules present it will not absorb or adsorb any more moisture. Equilibrium capacity is when the desiccant has pulled so much moisture out of the air that the air retains a stronger hold on the moisture molecules than the desiccant can exert. At equilibrium capacity adding more desiccant will not bring the Relative humidity lower.
Zeolite In Use
Zeolites have a high affinity for water and have the capability of adsorbing and absorbing it without damage to the crystal structure. This property makes them useful as desiccants. When the zeolite is placed in an air stream it will adsorb moisture up to it’s saturation point (as discussed above) and will not, at that point, adsorb any more moisture.
When the level of humidity in the air stream falls below the saturation point of the zeolite it will begin to release moisture back into the air stream. Therefore, it is not “eliminating” moisture, but rather merely holding or releasing it depending on the relative level of humidity of the air stream and the saturation point of the zeolite. To increase the ability of the zeolite to actually remove moisture the desiccant must be regenerated and the extracted moisture must be vented away from the conditioned space. Regeneration of the zeolite is both simple and complex. Approximately 70-80% of the moisture can generally be removed by “pulling a vacuum”, i.e.; lowering the humidity level. This fraction is known as bulk water. The amount of water removed from the zeolite is dependent on how low the humidity is lowered but, without a method to exhaust this extracted water, the moisture merely leaves the zeolite and re-enters the air stream. This again lowers the saturation point of the zeolite. Therefore, the zeolite adsorbs moisture until it cannot adsorb more. If the humidity level falls the zeolite deadsorbs moisture until it is in “balance” with the lower humidity level. At that point it begins to adsorb moisture and the cycle is repeated.
The remaining water (which cannot be removed by pulling a vacuum) is very difficult to remove and may require prolonged heating at moderate (150C or 212F, the boiling point of water) to very high (600-800C, 1112-1472F) temperatures.
Zeolites are commonly used in dehumidification devices known generally as desiccant wheels. These devices lower humidity levels and provide many benefits over the sustainable long term. However, it is very important to realize that these devices continually regenerate the zeolite or other desiccant by passing the saturated desiccant through a heated air stream and venting the moisture laden exhaust away from the air stream and conditioned space.
“…Ecor Pro & Toyotomi have triple safety cut outs rather than double as with most others…”
This is necessary to prevent the captured moisture from being reintroduced into the conditioned space. In practice, this is the only way zeolites are used to control humidity levels in conditioned spaces. The zeolite adsorbs water at or near to the saturation point and then is rotated out of the air stream and into a heated exhaust stream. This regenerates the zeolite on a continuing basis, exhausts the moisture away from the conditioned space, and effectively eliminates moisture from the area being treated. While this is a fairly simple description of the desiccant wheels they are actually quite complex and expensive to produce still.
Practical Considerations of Home Desiccant Dehumidifiers
Essentially a desiccant dehumidifier is a desiccant wheel, a slow stepper motor to turn the wheel, a heater to regenerate the wheel and a fan to move the air. In some cases there will be another stepper motor for the louvres to help circulate the air in the room.
The critical parts in a home desiccant dehumidifier are the stepper motor and how it turns the wheel as this can sometimes can wear or be knocked off its cogged system with jolts to the casing and most importantly the heater.
The internal heater is not heating the room as such and its not to be considered as a room heater. The heater is to regenerate the zeolite wheel. This on Ecor Pro & Toyotomi units is PTC heater. Other systems may use a simple wire wound heater. PTC is self regulating in terms of energy it consumes & should not over heat.
In addition Ecor Pro & Toyotomi have triple safety cut outs rather than double as with most others who sell desiccant dehumidifiers in the market. This is because in time there can be scenarios where the wheel motor can stop turning the wheel & the heater keeps heating. It should be noted that simple wire resistance heaters (found in other manufacturer;s models) are much worse as they will keep heating until over heat.
Ecor Pro & Toyotomi have never had a dehumidifier that has burst into flames because of our safety measures and the quality of our products. However, we still recommend that if the application is all day every day unattended dehumidification then a metal unit should be considered from our industrial range. Smaller capacity metal units will soon be available and they are a must for laundry rooms, crawl spaces, out buldings, boats, garages, holiday homes etc.