Greenhouse Ventilation and Cooling
Greenhouse Humidity Control
Humidity control is most difficult during the fall and spring seasons when the outside temperature and humidity are like those inside the greenhouse. High humidity is not likely to occur during freezing weather since the relative humidity of the outside air is very low. Humidity in greenhouses is controlled to minimize the spread of fungal pathogens such as Botrytis and powdery mildew and to regulate transpiration. Moisture buildup can be a major problem in greenhouses used for plant production. At high levels of relative humidity, the risk for condensation on leaves is high (especially at night), and thus, the risk of Botrytis and other fungal diseases increases. There are few fungi that thrive under low relative humidity. The amount of moisture in the air (aka humidity) affects the transpiration rate of plants, which is responsible for moving water and nutrients from the root zone to other parts of the plant. When the humidity levels are too high or too low, transpiration will slow, inhibiting plant health, growth, and development. As a general guide, it is often recommended that greenhouse relative humidity be maintained between 65 to 75 percent during the night and 80 to 90 percent during the day for healthy plant growth. High humidity also causes condensate droplets on glazing, reducing the light transmission into the greenhouse for plant growth.
Relationship between Humidity and Temperature
The amount of moisture in the air is generally expressed as relative humidity (RH), which is the ratio between the weight of moisture present in the air and the total moisture-holding capacity of a unit volume of air at a specific temperature and pressure. This term can sometimes be misleading because it is temperature-dependent. Warm air has a higher moisture-holding capacity than cooler air; therefore, as the temperature of air increases, the relative humidity decreases even though the amount of water remains constant.
What is a Desirable Humidity Level?
The desirable humidity varies with temperature. Plants in warmer environments can tolerate higher relative humidity. The chart below provides corresponding temperature and relative humidity set points for disease prevention.
Dew Point Temperature
Dewpoint temperature indicates the temperature at which water will begin to condense out of moist air. At any given temperature and pressure, there is a maximum amount of water that can be held in the air. This is known as the saturation point. At any time when the air is nearly saturated with water vapor, all it takes is a slight drop in temperature to reach the dewpoint. The higher the moisture content of the air, the higher the dewpoint temperature.
Humidity-Measuring Instruments
Sling Psychrometer
A sling psychrometer can be used to determine the relative humidity. A sling psychrometer is mounted on a swiveled handle and whirled rapidly. The sling psychrometer consists of two thermometers exposed to the same airstream. The end of one thermometer is covered by a wet wick. As the water in the wick evaporates, the temperature of the thermometer decreases to the wet bulb temperature. The difference in temperature between wet-bulb and dry-bulb sensors is known as the wet-bulb depression, which can be used to determine the relative humidity (Table 6.3) and dew point temperature (Table 6.4)
Hand Held Humidity Meters
Hand-held electronic humidity meters that display relative humidity are more convenient than wet bulb psychrometers. However, wet bulb temperature is a more useful measurement for evaporative cooling systems because it directly determines the temperature to which air can be cooled by evaporative cooling alone. Some electronic humidity meters do have an option to display wet bulb temperatures.
Wireless Temperature and Humidity Sensors
Wireless temperature and humidity sensors, wireless sensor networks, are another option and are more suited to larger greenhouse operations. The wireless greenhouse temperature monitoring system is composed of wireless temperature and humidity sensors, a data collector, and a managing server. The sensors are installed inside a greenhouse for collecting environmental data. The collected data are transmitted wirelessly to the data collector, which transmits the information to a central database via the Internet.
Reducing Greenhouse Humidity
Greenhouse growers usually try to avoid humidity levels near the dew point since free water condensing onto plant surfaces can promote the growth of disease organisms. Excess humidity is usually more problematic in the spring and fall seasons when the weather is cool and moist. High humidity is not likely to occur during freezing weather since the relative humidity of the outside air is very low. The common strategy used to reduce greenhouse humidity involves the following methods.
Cultural Practices
Proper planting dates, adequate spacing, and morning watering (so that foliage can dry prior to lower night temperatures) are good cultural practices for managing relative humidity and controlling plant diseases. Closely spaced plants and overlapping canopies can create microclimates different from the rest of the structure.
Ventilation and Heating
A common dehumidification practice is simply to open the windows, allowing moist greenhouse air to be replaced by relatively dry outside air. Venting for humidity control is most effective when outside air is significantly cooler and drier than that inside the greenhouse. As cool, dry air heats up in the greenhouse, it absorbs moisture and lowers the humidity. Humidity reduction by bringing in outside air can be somewhat effective even if the outside air is very humid, as long as it is significantly cooler than the inside air. In practical terms, however, outside air should be significantly cooler and drier to justify the cost of ventilation.
Air Circulation
Air movement is another important consideration when managing diseases in the greenhouse. Moving air is continually mixed, resulting in very small temperature differences. Adequate air movement around the plant occurs when the leaves move slightly. The moisture does not get a chance to condense on the leaf surfaces because the mixing action caused by the movement prevents the air along the surface from cooling to below the dew point, resulting in less Botrytis.
Greenhouse Dehumidifiers
The most common dehumidifiers are based on refrigeration. Refrigerant dehumidifiers operate by pulling moist air over a cold coil, causing the moisture to condense into water droplets that are collected and removed. The dry air is reheated and discharged back into the greenhouse. These dehumidifiers are extremely effective in warm, humid environments and are often used in greenhouses where excessive humidity levels are a typical concern. Desiccant dehumidifiers work in a different way to remove moisture from the air. They use a desiccant substance, such as silica gel, to absorb moisture from the air.
Bottom Heat
Bottom heat will improve air circulation inside plant canopies and will help to prevent condensation on leaf surfaces. The warm air that rises creates air movement around the plants. Bottom heat also keeps the plant surfaces warm, preventing condensation on the plants.
Greenhouse Design
A sloped roof (rise to run of 1:2) will encourage moisture to move toward the gutter and collect without dripping, compared with a roof with a shallow slope. A double-layer glazing will have a warmer interior-layer surface temperature because of the air-gap insulation between the layers, and thus less condensation.
Anti-Drip Plastic
The use of a wetting agent either sprayed on the interior surface or as part of the formulation of the glazing on poly covered greenhouses can also help to reduce the humidity level.
Vapor Pressure Deficit
Instead of relative humidity, the more accurate way to express the driving force of water loss from a leaf is vapor pressure deficit (VPD). Its value is independent of temperature. Vapor pressure deficit is the difference between the amount of moisture in the air and how much moisture the air could potentially hold when it’s saturated. It’s often measured in pounds per square inch (PSI) or kilopascal (kPa).
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