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A compressor based dehumidifier uses a compressed refrigerant to cool metal coils (evaporator coils). The hot humid air that enters the dehumidifier condenses on these cool coils. This leaves the air that exhausts out of the dehumidifier still warm but drier than when it entered. Desiccant dehumidifiers work quite differently. Instead of using condensation (on cold evaporator coils) to remove humidity from air, desiccant dehumidifiers use a chemical (called a desiccant) to do so. We’ll discuss the ins and outs of exactly how a desiccant dehumidifier works in much greater detail later on in this guide. For now, let’s take a look at the four different types of desiccant dehumidifiers and the strengths and weaknesses of each.
1. Disposable desiccant dehumidifiers – also called “moisture absorbers”, the two most prominent features of this type of desiccant dehumidifier are their price and the fact that their disposable. These products are very inexpensive but cannot be reused. They also remove very little moisture per day and cannot be used for large spaces. That’s not to say that they don’t have any use at all. Moisture absorbers are perfect for the right situation. To learn more, see our reviews for two such products below.
2. Rechargeable desiccant dehumidifiers – like disposable moisture absorbers these dehumidifiers are very small in price, very small in size, and unfortunately very small in moisture removal rate. They’re also “wireless” – meaning that they don’t need to be plugged in to a wall outlet for them to actively dehumidify (they only need to be plugged in to recharge, which only needs to be done once every several weeks). The caveat with these units, as it is with disposable desiccants, is that they cannot be used in any sizable space or any space with anything more than only very mild humidity levels (because of their low moisture removal rate). We’ve ranked those rechargeable desiccant dehumidifiers we’ve reviewed from best to worst below. Note that any one of these five units is recommended over disposable desiccants for most applications.
3. Full size consumer grade desiccant dehumidifiers – this is the type of desiccant dehumidifier we’re referring to when we talk about “desiccant dehumidifiers” in general in other parts of our website. These are also the dehumidifiers which we describe in detail in the “how it works” and “advantages and disadvantages of using a desiccant dehumidifier” sections further below in this guide. This type of dehumidifier retails for between $150 and $350 and is the only type of desiccant dehumidifier that is comparable to a compressor based dehumidifier, at least in terms of size and moisture removal rate. That being said, they are still smaller, lighter, and remove less moisture per day than comparable compressor based dehumidifiers in most scenarios. For much more information on this type of dehumidifier and how it compares to a compressor based dehumidifier see the rest of our guide below.
Here are the four full size consumer grade desiccant dehumidifiers we’ve reviewed thus far, ranked from best to worst. For specific reasons as to why we rank these dehumidifiers in this order see the end of our EcoSeb DD122EA-Classic review.
4. Commercial desiccant dehumidifiers – these dehumidifiers are normally rented by contractors involved in construction. Because of their unique ability to dry already dry air and extremely difficult to dry materials even at extremely cold or extremely warm temperatures these dehumidifiers are often used instead of compressor based commercial dehumidifiers in commercial settings.
What follows is a detailed analysis of how a consumer grade full size desiccant dehumidifier works. Note that many of the same principles of moisture adsorption and the liberation of that moisture which apply to full size consumer grade units also apply to other types of desiccant dehumidifiers, including disposable and rechargeable desiccant dehumidifiers. You can also find specific detailed descriptions of how moisture absorbers and rechargeable desiccant dehumidifiers work in our individual reviews for those products. With those disclaimers out of the way, let’s begin our analysis of how desiccant dehumidifiers work, at least those desiccant dehumidifiers that are most representative of the term “desiccant dehumidifier” – i.e. full size consumer grade desiccant dehumidifiers.
In trying to describe how a consumer grade desiccant dehumidifier works, we’ll first evaluate all of the critical components that make up the dehumidifier. We’ll then look at how all of these parts work together to remove moisture from the warm humid air that enters the dehumidifier. We’ll finish our discussion by noting the differences between consumer grade desiccant dehumidifiers and commercial desiccant dehumidifiers and the differences between desiccant dehumidifiers and compressor based dehumidifiers.
A consumer grade desiccant dehumidifier consists of several critical components. They are:
1. The desiccant. Perhaps the most important component of a desiccant dehumidifier is the desiccant itself. A desiccant is quite simply a chemical that adsorbs (not absorbs) moisture. Moreover, the desiccants used in desiccant dehumidifiers are able release the same adsorbed moisture when heated. This ability to not only adsorb moisture but also to release it when heated is absolutely critical to the proper functioning of a desiccant dehumidifier. Thus, in the same way in which the chemical and physical properties of a chemical refrigerant allows for the proper functioning of compressor based dehumidifiers, the chemical and physical properties of desiccants allow for the proper functioning of desiccant dehumidifiers.
2. A large rotating drum. This drum is about as wide as the dehumidifier itself and an inch or two thick (depending on manufacturer and model). It’s made up of alternate layers of flat and corrugated sheets which are impregnated with the desiccant. These sheets are arranged so that they allow for air flow perpendicular to the drum. This drum or “desiccant rotor” functions to remove moisture from incoming room air.
3. A condenser. Made of plastic it’s about as long and wide as the dehumidifier itself. It functions to condense warm moist air.
4. A heater. Its sole function is to reheat air.
5. One or several blowers. These function to move air throughout the system. Most commercial grade desiccant dehumidifiers have one big blower on the back of the dehumidifier. It pulls air in through the front of the dehumidifier and exhausts it out of the top of the dehumidifier. Many units also come equipped with a secondary smaller blower that circulates the air that is moved through the recharge zone of the desiccant drum.
The desiccant rotor (the large rotating drum at the heart of the dehumidifier) has two different zones – a “process zone” which makes up about 75% of the area of the rotor and a “recharge zone” which makes up the remaining 25% of the rotor. The warm humid air that enters the dehumidifier is pulled through the process zone. Here, moisture is adsorbed by the desiccant material. The incoming humid air goes through the process zone and immediately exhausts out of the dehumidifier.
Covering the recharge zone of the rotor is a heater. It warms circulated humid air which is pulled back through the desiccant drum in the opposite direction of incoming humid air. This warm air (heated by the heater) liberates moisture from the desiccant. Thus, moisture is transferred from the desiccant to the air. The warm humid air that leaves the recharge zone enters the condenser on the front of the dehumidifier. Here warm humid air condenses at room temperature. The condensate drips down into a condensate collection bucket, much the same way as it does in a compressor based dehumidifier.
Let’s summarize the movement of air and the transfer of moisture within the system”
1. Incoming humid room air is pulled first through the condenser on the front of the dehumidifier and second through the process zone of the desiccant drum. It exhausts as warm dry air out of the dehumidifier. Moisture is transferred once – on the desiccant rotor – moisture is transferred from the incoming humid air to the desiccant by adsorption.
2. Circulating humid air within the dehumidifier is pulled first through the recharge zone of the desiccant rotor and second through the condenser on the front of the dehumidifier. Moisture is transferred twice – first its transferred on the desiccant rotor – moisture is transferred from the desiccant to the hot air (which is warmed by the heater first) moving through the rotor. Second, moisture is transferred on the condenser – moisture is transferred from the hot humid air to the condenser by condensation.
Air is moving in one direction through the process zone – in through the front of the dehumidifier, through the condenser, through the desiccant rotor, and out through the top of the dehumidifier.
Air is moving in the opposite direction through the recharge zone – through the desiccant rotor and into the condenser.
The room temperature air that is being pulled into the dehumidifier goes through the condenser first. This is so that it can “cool” the condenser. The air that is returning to the condenser from the recharge zone is very warm in comparison. It is because of this temperature difference that the returning air, which just picked up moisture from the desiccant in the recharge zone, is able to condense in the cool (by comparison) condenser. This temperature difference is crucial as without it moisture would not be able to be removed from the system by the condenser.
At this point in our analysis we have to make a very important distinction. Commercial desiccant dehumidifiers do not work the same way that consumer grade desiccant dehumidifiers work. They share many of the same parts (although the parts used on commercial units are much more heavy duty) and work very similarly, but they are not one and the same.
The primary difference between the two has to do with the warm humid air that leaves the recharge zone of the desiccant rotor. As we discussed above, this air is immediately funneled to a condenser on a consumer grade desiccant dehumidifier. Here, the warm humid air condenses and the condensate drips down into condensate collection bucket that can easily be removed from the dehumidifier.
Commercial units do not have a condenser. Instead, the warm moist air that leaves the recharge zone of the desiccant drum is exhausted out of the building that is to dehumidified through ductwork. Thus, there is no condensate that forms anywhere in the system. This allows for commercial units to dehumidify at even colder temperatures and even lower humidity than residential consumer grade units.
In comparing these two very different types of dehumidifiers let’s take a look at the advantages and disadvantages of desiccant dehumidifiers. In doing so, we’ll contrast them with the advantages and disadvantages of compressor based dehumidifiers.
As we discussed above, in a desiccant dehumidifier moisture is removed from incoming humid room air by the process of adsorption. This process does not involve or require condensation. In other words, this process does not require that water vapor in the air change state from vapor to liquid. The moisture is transferred between the incoming humid air and the desiccant in its vapor form. Yes, the same moisture eventually finds its way back to the dehumidifier’s condenser, but the actual process or means by which moisture is initially removed from the air does not require condensation.
This stands in stark contrast to the process by which moisture is removed from air in a traditional compressor based dehumidifier. Remember, as we discuss, that compressor based dehumidifiers remove moisture by cooling air. The air is cooled until it reaches 100% relative humidity at which point condensate forms. This condensate forms over the dehumidifier’s very cold evaporator coils. Thus, frost build up even at temperatures as high as 65° F is a real problem in compressor based dehumidifiers. Because condensate is not formed by the actual dehumidification process in a desiccant dehumidifier there is no risk of frost build up.
Remember also that there is a heater over 25% of the desiccant dehumidifier’s desiccant rotor, constantly warming the air going back through the desiccant material in the recharge zone. This indirectly heats the desiccant that will eventually rotate back into the process zone. This further reduces risk of frost and further enables a desiccant dehumidifier to operate efficiently at colder temperatures. Compressor based units also warm air before it exhausts out of the dehumidifier, but this doesn’t affect “process air” and the actual dehumidification of the “process air” the same way that it does in a desiccant dehumidifier.
In summary, desiccants have superior low temperature capability because
1. The process by which they initially remove humidity from the air does not require that condensate forms. As we all know, condensate is a liquid and liquids tend to freeze at cold temperatures. Thus, compressor based dehumidifiers are much more prone to frost build up, even at moderately cool temperatures.
2. A compressor based dehumidifier requires that air be cooled to remove moisture from it. Cooling already cool air further exacerbates frost buildup issues. Desiccant dehumidifiers do not require that air be cooled to enable dehumidification.
3. A heater is present within desiccant dehumidifiers which constantly (albeit indirectly) heats the desiccant through which incoming humid air is processed. This means that air close to freezing temperatures can still be processed for dehumidification in desiccant dehumidifiers.
Your typical compressor based dehumidifier can be set to as low as approximately 35% relative humidity. This is already below what we’d consider a comfortable humidity level and represents the absolute lowest humidity level most consumers are ever going to want to achieve in their homes. The reason why desiccant dehumidifiers are used in industry is because they can achieve humidity levels much lower than 35% or even 30%. They can do so (achieve lower humidity levels) because the process by which air is dehumidified in a desiccant dehumidifier (adsorption) is much less limiting (we’ll expound upon this idea later) than the process by which air is dehumidified in a traditional compressor based dehumidifier.
The process by which desiccant dehumidifiers remove moisture from ambient air also allows for consistent dehumidifying efficiency independent of the temperature and humidity level of the room/space that is to be dehumidified. A 15 pint per day consumer grade desiccant dehumidifier will remove just as much moisture in a room with high temperatures and high humidity levels (15 pints per day) as it does in a room with low temperatures and low humidity levels (15 pints per day). In contrast, the efficiency of compressor based dehumidifiers varies depending on temperature and humidity level. Compressor based units are much more efficient at higher temperatures and higher humidity levels than they are at lower temperatures and lower humidity levels. The commercially available (approx.) $3100 Dri-Eaz LGR 3500i, for example, removes an amazing 240 pints of moisture per day in an environment at 90° F and 90% RH. At reduced temperatures and humidity levels its efficiency is drastically reduced. In an environment at 80° F and 20% RH it removes only 22 pints of moisture per day. This same variance in moisture removal rate is also present in consumer grade compressor based dehumidifiers. 70 pint compressor based dehumidifiers remove 70 pints of moisture at 80° F and 60% RH. At lower temperatures and/or lower humidity levels they remove a much lower quantity of moisture per day. Again, this occurs because of the method or process by which a compressor based dehumidifier removes moisture from the air.
To remove moisture from the ambient air a compressor based dehumidifier cools air to a localized dew point within the dehumidifier. As we discuss, temperature and relative humidity are inherently related. As temperature decreases, humidity increases. Warm humid air is easy to dehumidify because it only needs to be cooled even a little bit before it reaches 100% relative humidity and condensate forms. As the humidity of the air goes down, it needs to be cooled more and more to reach that same 100% relative humidity to form condensate. In other words, as the humidity of the air within the particular room that is to be dehumidified is reduced it becomes increasingly difficult to remove more moisture from it with traditional compressor based dehumidifiers. The dehumidifier’s efficiency is progressively reduced as the relative humidity in the room is lowered. Eventually you reach a point where the air simply cannot be cooled to a low enough temperature to remove any more humidity from it. This is why these units are limited to achieving about 30 to 40% relative humidity.
Desiccant dehumidifiers, on the other hand, do not require that a localized dew point be reached over the air that is to be dehumidified. The desiccant can just as easily remove moisture from incoming highly humid air as it can from incoming mildly humid air or even close to dry air. The desiccant is “hungry” for any moisture at all. It will adsorb moisture until its saturated and because its constantly recharged (dried), saturation is never reached. The implications of this process (by which a desiccant dehumidifier dehumidifies humid air) are twofold. The first implication is that, because of the nature of this process, a desiccant dehumidifier can continue to dehumidify air even at humidity levels far below what is comfortable for the average human being. Humidity levels as low as 20%, 10%, etc. are possible with a desiccant dehumidifier. In industry, these dehumidifiers are also used to dehumidify materials that are difficult to dry – especially dense materials such as plaster and hardwood flooring – because of their ability to dry or dehumidify even in extremely dry environments. The second implication of the method by which desiccant dehumidifier dehumidify is that desiccant dehumidifiers are consistently efficient regardless of temperature and humidity levels. The desiccant is just as “hungry” for moisture at high temperatures and high humidity levels as it is at lower temperatures and lower humidity levels. Thus a desiccant dehumidifier is consistently efficient throughout its manufacturer specified operating temperature range and at any humidity level.
Desiccant dehumidifiers have a very slowly rotating desiccant drum and one or two blowers that pull or push air through the dehumidifier. Other than the fan noise produced by their blowers, desiccant units are exceptionally quiet.
Compressor based dehumidifiers, on the other hand, have a compressor in addition to a high CFM fan pulling air through them. The compressor makes noise and the fan on these units is also generally louder than the blowers on desiccant dehumidifiers. Thus, compressor based units are inherently more noisy than their desiccant counterparts.
The parts constituting a desiccant dehumidifier are much lighter than the parts that make up a compressor based dehumidifier. A compressor is a heavy part. An evaporator is heavy. And the condenser on a compressor based unit is metal (heavier) unlike the light plastic condenser on a desiccant unit. Your full size consumer grade desiccant dehumidifier weighs between 12 and 18 pounds. Even the smallest compressor based units (30 pint) weigh in at over 30 pounds with larger units (70 pint) weighing in at upwards of 50 pounds.
Compressor based units also have a charged refrigerant running through them. If you move these units you need to be careful to keep them upright. Otherwise, you’ll need to leave the unit off for a period of time before powering it back on to ensure that the refrigerant has “settled” properly within the system. In contrast, desiccant units can be oriented any which way you want to as you move them from one location to another.
While commercial units can remove an almost boundless amount of moisture per day, consumer grade desiccant dehumidifiers are surprisingly limited. Most of the units we’ve reviewed can only remove a maximum of about 20 pints of moisture per day.
This is where compressor based dehumidifiers truly shine. They are the workhorse of the consumer grade dehumidifier family. Even the least heavy duty compressor based dehumidifier can remove 30 pints of moisture per day. The #1 rated Frigidaire FFAD7033R1 can remove a whopping 70 pints of moisture per day. That’s almost 400% more moisture removal per day than comparably priced desiccant units.
Note that this disadvantage applies to most temperatures and humidity levels (that you’re likely to see in most homes) but not all of them. At certain temperatures and certain humidity levels low moisture removal rate desiccant units can remove more moisture (per day) than even the highest capacity compressor based units. We make the cut-off at 50° F. Only at temperatures below 50° F and especially 40° F is a compressor based dehumidifier’s efficiency so dramatically reduced that it removes less moisture per day than comparable desiccant units. For more information on exactly why we make this cut-off at 50° F please see our basement dehumidifier guide.
This disadvantage ties into the first disadvantage we covered above. The price to moisture removal rate ratio for a desiccant dehumidifier pales in comparison to what you get with a compressor based unit (at most temperatures and humidity levels). The EcoSeb DD322EA-SIMPLE, for example, retails for around $280 and only removes 21 pints of moisture per day. The top rated compressor based Frigidaire FFAD7033R1 normally retails for about $30 less and removes over 3 times as much moisture per day. Of course, the caveat once again is that these ratios are only true at higher temperatures and higher humidity levels. Should you be buying a dehumidifier for an environment that sees extremely low temperatures and/or humidity levels a desiccant dehumidifier can actually be a better value (in terms of moisture removal rate per dollar spent) than a comparable compressor based dehumidifier.
Desiccant dehumidifiers can potentially give off a pungent odor. This occurs because the desiccant material, along with the moisture in the air its dehumidifying, can also absorb odors contained within the same air. It will often absorb odors you may not even be aware of but it will especially absorb odors from items such as cigarettes, cosmetics, building materials, and new furniture (as per the EcoSeb 122EA-SIMPLE manual). In any case, the smell coming from the dehumidifier is not due to the absorption of that odor, but rather the heating up of that odor when the desiccant material is reheated (in order to liberate moisture from it). Note that this smell isn’t harmful and should only be temporary. However, we still consider it to be a disadvantage of using a desiccant dehumidifier as it may preclude the use of this type of dehumidifier under certain circumstances – in a business setting, for example.
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