Temperatures can easily exceed 100 °F in Florida green-houses during the summer if they are not equipped with cooling systems. Such high temperatures reduce crop quality and worker productivity. Evaporative cooling is the most common method for reducing the temperature inside a greenhouse. Air conditioning or refrigeration systems can be used, but their installation and operating costs are usually prohibitive.
EVAPORATIVE COOLING
Evaporative cooling is a process that reduces air tempera-ture by evaporation of water into the airstream. As water evaporates, energy is lost from the air causing its tempera-ture to drop. Two temperatures are important when dealing with evaporative cooling systems – dry bulb temperature and wet bulb temperature. Dry bulb temperature is the temperature that we usually think of as air temperature. It is the temperature measured by a regular thermometer exposed to the airstream. Wet bulb temperature is the lowest temperature that can be reached by the evaporation
of water only. It is the temperature you feel when your skin is wet and is exposed to moving air. Unlike dry bulb temperature, wet bulb temperature is an indication of the amount of moisture in the air.Wet bulb temperatures can be determined by checking with your local weather station or by investing in an aspirated psychrometer, a sling psychrometer, or an electronic humidity meter. Wet bulb psychrometers consist of two thermometers exposed to the same airstream. The end of
one thermometer is covered by a wetted wick. As the water in the wick evaporates, the temperature of the thermometer decreases to the wet bulb temperature. The other ther-mometer is exposed directly to the airstream and measures
the dry bulb temperature. The difference between a sling psychrometer and an aspirated psychrometer is the way the
airstream is provided. A sling psychrometer is mounted on a swiveled handle and whirled rapidly, while an aspirated
psychrometer uses a small fan to provide air movement.A more common term for measuring the amount of moisture in the air is relative humidity. Hand held elec-tronic 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. Even if a humidity
meter does not display wet bulb temperature, tables and graphs known as psychrometric charts are available to
calculate wet bulb temperature based on the dry bulb temperature and relative humidity. Software packages are
also available to calculate wet bulb temperatures. It should be noted that electronic humidity meters frequently drift
out of calibration and they should be checked against standards on a regular basis.
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