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Why Greenhouse Dehumidifiers Aren’t Effective When Using Wet Pad and Fan Systems

Evaporative cooling, such as wet pad and fan systems, is common in greenhouses in warm regions. Growers often ask if, and how, dehumidifiers can work in conjunction with wet pads. The short answer is – they shouldn’t run simultaneously.

Pad and fan systems and dehumidifiers have opposite functions, making them negate each other’s operation:

  • Dehumidifiers remove water from the air – wet pad and fan systems add water.

  • Dehumidifiers require a closed greenhouse – pad and fan systems rely on air exchange with the outdoors.

How Wet Pad and Fan Cooling Systems Work

Pad and fan systems cool down greenhouse temperatures, using evaporative cooling. This is an effective way to relieve the heat during hot summer days and provide plants with the best climate conditions.

Fan and pad cooling systems use exhaust fans to suck air from outdoors into the greenhouse. The air is then blown through the pad, which constantly evaporates water, creating a cooling effect. The fans blow the cool air into the greenhouse, leading to an overall drop in temperature.

Providing heat relief means pad and fan systems are mostly necessary during hot days with high radiation. However, some greenhouses do require cooling at night, mostly in tropical regions. But there’s a problem – in these regions, humidity is often high during the night. So wet pad and fan systems may not provide the desired results.

When conditions outside are humid or cold, pad and fan systems lose effectivity and provide a much lower cooling capacity. This is because evaporative cooling relies on the difference between outdoor and indoor conditions.

Wet Pad Cooling Capacity

When the air is dry, water evaporates rapidly, providing a more intense cooling effect. When the air is humid, less evaporation can occur, making the system less effective. So using a wet pad and fan, during humid or cold periods, will give less cooling and add water to the air. This not only makes the system ineffective, but also dangerous. High humidity may lead to dew point condensation in the greenhouse and increases the risk of disease outbreaks.

In the table below, you can see that at 100% relative humidity, pad and fan systems provide no temperature reduction at all. But at lower relative humidity levels, the cooling capacity increases. It’s also apparent that the amount of cooling wet pads provide increases at higher temperatures. So the colder it is, the less cooling you’ll get.

 Wet Pad and Fan Cooling Capacity

This doesn’t mean wet pad and fan systems aren’t effective. During the day, if relative humidity is lower outside, growers can use a wet pad to reduce temperatures, while ventilating to regulate humidity.
For greenhouses facing high temperatures in humid regions, it’s best to look for dry cooling solutions.

How Dehumidifiers Work

Dehumidifiers have one task – to remove water vapor from the air and reduce relative humidity.
Refrigeration-based dehumidifiers, like DryGair, remove water from the air using condensation. The unit sucks in humid air, runs it through a cold coil, which causes water to condense, and releases dry air back into the greenhouse.

Traditionally, growers would control humidity by opening greenhouse vents or windows, to let the humid air out. This is an effective method when outdoor conditions are favorable, meaning comfortable temperatures and relatively dry air, which isn’t always the case.

This is where dehumidifiers come in. Greenhouse dehumidifiers are designed to reduce humidity when ventilation isn’t an option – overnight, or during cold, cloudy, rainy, or humid days. During these periods, opening the greenhouse to reduce humidity is inefficient and often won’t result in a reduction at all, leaving the plants wet and susceptible to diseases.

Many greenhouse operators use blackout screens to control day length or reduce light pollution. Blackout screens make it much harder to ventilate effectively. They also insulate the greenhouse, causing temperatures to rise. These situations may require both dehumidification and cooling simultaneously, so wet pad and fans aren’t a viable option.

Dehumidifiers and Pad and Fan Systems Perform Opposite Tasks

To provide optimal results, dehumidifiers should operate in closed spaces. In a controlled environment, continuously removing water vapor from the air will reduce humidity levels.

But in an open greenhouse, which exchanges air with the outdoors, dehumidifiers are limited. Treated air constantly escapes the greenhouse and is replaced with untreated air from outside. In these situations, dehumidifiers are inefficient and often ineffective.

It’s important to understand that the pad and fan system creates an exchange of air, meaning the greenhouse isn’t closed.

But it’s not just about the air exchange. By using evaporation to cool the air, wet pad and fan systems also introduce water vapor to the greenhouse, making them raise humidity and completely undermine the dehumidifier’s operation.

It’s Still Possible to Integrate Greenhouse Dehumidification with Wet Pad and Fan Cooling

Dehumidifiers and wet pad and fan systems negate each other and shouldn’t operate at the same time. But having a pad and fan system doesn’t mean you shouldn’t introduce a dehumidifier for humidity control.

Fortunately, the two systems often operate at different times. This means it’s completely possible to use the pad and fans to reduce temperatures during hot days, and dehumidifiers to control humidity during the night or on cold, humid, rainy or cloudy days.

Greenhouses in tropical regions, that require both dehumidification and cooling simultaneously, need a system that provides dry cooling.

DryGair combined with cooling is our answer to these situations. These units are standard DryGair dehumidifiers, but with an added system for dry cooling. Simply connect the unit to a cold-water source and it will chill the coils inside the unit, causing it to expel the treated, dry air, at a lower temperature. This can provide a significant temperature drop, without adding moisture, letting you regulate both temperature and humidity on warm nights.

For more information on DryGair combined with cooling or integrating dehumidifiers in greenhouses with wet pad and fan cooling systems, please contact us.

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