No one likes humid conditions. As the air becomes saturated with water vapor, it can feel oppressive, make it difficult to keep cool and make it tough to breathe. Humidity is particularly troublesome for people with respiratory conditions, and it can cause fragile items to deteriorate faster. In short, humidity is a nuisance, and both home and business owners are combatting it with dehumidifying technology. But how do dehumidifiers work, and what is the science behind their operation?
Dehumidifiers come in two primary varieties. Thermal condensers are common, and include standard air conditioners, which are found everywhere. Desiccant, or absorptive, dehumidifiers are an emerging technology which can provide relief in settings where thermal condensers aren’t as effective. Let’s go into how each works, and what they can offer a home or business.
Relative Humidity and Dew Point
Thermal condensers operate by manipulating dew point and relative humidity. What, though, are dew point and relative humidity?
Dew point is actually what most people think of when they consider humidity. Simply put, it’s the temperature at which water vapor will begin condensing and form dew. The dew point is expressed as a temperature, and usually ranges between 50 degrees Fahrenheit and 80 degrees Fahrenheit. So, for example, if the dew point is at 70 degrees Fahrenheit, then as soon as the ambient air temperature drops to 70 degrees, water in the air will condense at the same rate it evaporates. If the ambient air temperature drops under 70 degrees, water in the air will condense at a faster rate than it evaporates, forming an excess of dew. This is why there is usually a lot of dew on the ground in the morning, when it’s the coolest.
Put another way, the dew point refers to the temperature that would result in maximum water vapor saturation. So, if the dew point is 70 degrees Fahrenheit, then at 70 degrees, the air can no longer hold any more water vapor.
The dew point is measured by meteorologists using hygrometers. Hygrometers include a metal mirror that cools as air passes over it. Eventually, it cools to the point where water vapor begins condensing on it, and this temperature is the dew point. Using hygrometers, meteorologists can calibrate their humidity instruments and make precise estimates regarding the amount of humidity in the air.
Relative humidity is different. Unlike dew point, relative humidity is a ratio, and refers to the amount of water in the air, compared to the amount of water it could hold if the air was fully saturated with water vapor. So, for instance, if the relative humidity is 30 percent, then the air is only holding 30 percent of the water vapor it could hold. Relative humidity is a poor metric to use when determining comfort, as it does not address the actual amount of vapor in the air, and is greatly affected by increases or decreases in temperature.
Relative humidity rises quickly as the temperature drops because air becomes denser as it cools. As the air increases in density, there is less room to accommodate water molecules floating in the air, and eventually they have to condense into liquid water once there is no longer any room to fit the water vapor. So, it’s possible for the relative humidity to be high without adversely affecting comfort if the temperature is cool enough outside. At colder temperatures, there just isn’t enough room for water vapor to reach high enough concentrations outside for it to be noticeable.
This last concept is the guiding principle behind thermal condensers.
How Thermal Condensers Work
Thermal condensers are the go to technology in most homes, and even conventional air conditioners act like thermal condenser dehumidifiers to an extent, though they are less efficient in this regard. Both operate using refrigerant that is manipulated physically to alter its temperature and the amount of water it can hold. Here’s a snapshot of how a system operates:
1. A fan pulls in humid ambient air of moderate temperature, and directs it over a series of evaporator coils.
2. The coils, filled with expanded, cold refrigerant, drop the air’s temperature to below its dew point, raising relative humidity beyond its saturation point and causing air vapor to condense rapidly.
3. The condensed vapor is pumped to a drain outside the home or is collected for later removal.
4. The refrigerant is channeled to a compressor that ramps up the pressure and increases it temperature as a result. This refrigerant is pumped to a set of condenser coils.
5. The cold, dry air is then directed over the hot condenser coils, warming it up and increasing its capacity for picking up humidity. This air is then pushed back in to the room to repeat the cycle again.
Although the dehumidifier’s components are each a feat of engineering, the overall system is easy to understand and is extremely reliable. Systems that use refrigerant are most efficient in settings where temperature and relative humidity are both high, and in these settings, thermal condensers can quickly remove humidity out of the air.
However, at lower relative humidity (usually around 45 percent and below), thermal condensers have to be cooled to such a degree to maintain below dew point temperatures that they can began icing up. In these situations, a desiccant dehumidifier makes more sense, so let’s address how those work next.
How Desiccant Dehumidifiers Work
Desiccant dehumidifiers ignore complicating factors like dew point temperature because they operate using a completely different principle. Specifically, desiccants attract water on a chemical level, and can do so through various means. However, the vast majority of desiccant dehumidifiers use silica gel, which cranks up the rate of condensation by greatly increasing the available surface area of water to adhere to and condense. Here’s a quick look at how a desiccant dehumidifier works:
1. Humid area is drawn into the desiccant dehumidifier, and pushed across a rotating disc impregnated with silica gel. The disc resembles a bundle of straws through which air can flow, and the silica gel is lined along the inside of these tubes.
2. The silica gel is filled with nano-sized pores, increasing the surface area of the gel exponentially. In fact, a single teaspoon of silica gel has so many pores that its surface area is similar to the surface area of a football field.
3. With so much area to condense on, the kinetic energy threshold separating vapor molecules and condensed water molecules dips greatly, meaning vapor molecules exhibiting less energy will be much more likely to condense.
4. As water collects on the disc, a portion of the air drawn into the dehumidifier is heated and used to recharge the condensation so that it vaporizes again. This air is ducted to the exterior to deposit the water outside and is returned to the building. The remaining dry air is shunted back into the room.
Because desiccant dehumidifiers work using chemical properties, they are unaffected by low dew points and relative humidity. This makes them much more efficient in drier settings and they can be used to drop the relative humidity of a space to nearly one percent. However, they aren’t as energy efficient and fast as thermal condensers when the temperature and relative humidity is higher, so many locations use both to complement each other’s strengths and shortcomings.
That’s a quick look at the science behind the most common dehumidifiers on the market. Of course, it gets more complicated than that, and the physics involved are fit for a college level class, but what’s important is that both technologies are reliable, easy to control and are extremely effective at doing their jobs.