Greenhouse Ventilation and Cooling
Fan and Pad Evaporative Cooling Systems
During periods of extreme heat the temperature inside the greenhouse can exceed that outside by 10 to 20 degrees F (5.6 to 11.1 °C) even with a well-designed forced-air ventilation system. This puts a stress on plants, reducing the quality and growth. Evaporative cooling is one way to reduce temperatures inside the greenhouse. Evaporative cooling is a process that reduces the temperature of air by the evaporation of water into the airstream. As water is evaporated, heat energy is lost from the air, which reduces its temperature. The most common way of accomplishing evaporative cooling in a greenhouse is with a fan and pad system. With this type of system, exhaust fans are placed in one wall of the greenhouse and pads are in the opposite wall. Water is pumped to the top of the pad and released through small openings along the entire length of the supply pipe. These openings are typically pointed upward to prevent clogging by any debris that might be pumped through the system (installing a filter system is recommended). A cover is used to channel the water downwards onto the top of the pads after it is released from the openings. The opening spacing is designed so that the entire pad area wets evenly without allowing patches to remain dry. At the bottom of the pad, excess water is collected and returned to a sump tank so it can be reused. The fans exhaust air from the building and draw in fresh air through the wet porous pads. The major components of the cooling system are: pad media, water supply, pump, distribution pipe, gutter, sump, and bleed-off line. Water is continuously circulated over and through the pad cells during operation. A pump transports water from a sump through a filter and to a distribution pipe along the top of each pad. A gutter collects unevaporated water that drains from the bottom of each pad. As air flows past the moist pad surfaces, some of the moisture evaporates into the air stream. Heat is withdrawn from the air during this process and the air leaves the pad at a lower temperature with higher moisture content. A typical fan and pad installation for a greenhouse is shown in Figure 3.7.
The fans exhaust the air and develop a slight vacuum or negative pressure throughout the entire greenhouse because it is substantially airtight. This slight vacuum draws air in through the cooling pad system and causes cooled air to move smoothly through the growing region of the crops absorbing heat from solar radiation, plants, and soil. The resulting temperature increase as air moves down the greenhouse produces a temperature gradient across the length of the greenhouse, with the pad side being coolest and the fan side warmest. The warmed, air is then expelled by the exhaust fans in the opposite wall.
Evaporative cooling is a process that reduces air temperature by evaporation of water into the airstream. As water evaporates, energy is lost from the air causing its temperature 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 temperature indicated by a moistened thermometer bulb exposed to air. A wet bulb thermometer measures the extent of cooling that happens as moisture dries from a surface (evaporative cooling). The wet bulb temperature is always lower than the dry bulb temperature except when there is 100 percent relative humidity.
Determining Greenhouse Ventilation Requirements
To determine the rate of air removal required for a greenhouse under standard conditions use the equations below, where L and W represent the greenhouse length and width, respectively. This equation calls for the removal of 8 cfm/ft2 (2.5 cmm/m2) of floor area, which is acceptable in cold climates.
Exchange Rate Adjustments
In warm climates and greenhouses with tall gutters (>12 feet), constants between 11 to 17 cfm/ft2 (3.4 to 5.2 cmm/m2) of floor area are advisable. This basic airflow rate is then adjusted for elevations over 1,000 feet above sea level, the expected interior light intensity, the allowable greenhouse temperature increase, and the distance from the pad to the fan.
Total Air Flow Required
The correct factor FVel is ignored for pad-to-fan distances of 100 feet or greater. For pad-to-fan distances less than 100 feet, calculate BOTH FHouse and FVel and use the LARGER of the two to complete the total air flow requirement where,
Evaporative Cooling Pads
Cooling Pad Types
The most widely used type of pad material is corrugated cellulose that has been impregnated with wetting agents and insoluble salts to help resist rot. These pads are expensive but, when properly maintained, do an excellent job of cooling air. With proper maintenance, corrugated pads should have a lifetime of ten years. A cellulose pad typically needs more air and water flow than does an aspen pad.
Determining Cooling Pad Size
The total area of pad required is determined by dividing the volume of air that must be removed from the greenhouse in 1 minute by the volume of air that can be moved through a square foot of pad in 1 minute. The cooling pad should extend over the entire length of the wall of the greenhouse to ensure that all plants receive cooled air. Pads are most often placed immediately inside the side or end wall. The pad wall should be equipped with ventilators exterior to the pad to permit air entry during hot weather and for sealing off the outside air during cooler spring and fall nights. In this case, the ventilator arms and gears are located exterior to the greenhouse (See Figure 3.10).
Pump and Sump Tank Operation
Determining Pump Capacity for Pads
Water must be delivered to the top of a 4-inch (10 cm) thick pad at the rate of 0.5 gpm per linear foot of pad (6.2 L/min/m of pad). If in doubt about the correct quantity of water flow, check with the pad manufacturer. For pad lengths of 30 to 50 feet (9.1 to 15.2 m), a 1.25-inch (32-mm) water distribution pipe is required, while for lengths of 50 to 60 feet (15.2 to 18.3 m) a 1.5 inch (38 mm) pipe is needed. Sixty feet (18.3 m) is the longest recommended pipe length. A 120-foot (36.6 m) pad length could be serviced from a water supply at the midpoint supplying two 60 foot (18.3 m) distribution pipes. At every 3 inches (7.6 cm), 1/8-inch (3 mm) holes should be made in the pipe.
Determining Sump Tank Volume
The sump tank volume should be at least 0.75 gal/ft2 (30.5 L/m2) of 4-inch thick pad and 1.0 gal/ft2 of 6-inch thick pad. These sump volumes are designed to operate at half the depth of the tank and will provide room to accommodate water returning from the pad when the system is turned off.
Location of Fans and Cooling Pads
The best distance between the pad and exhaust fans is a tradeoff between the optimum dimensions of the greenhouse (based on efficiency, function, and operation) and the tolerance of the crop to higher temperatures. The greater the range of the crop’s temperature tolerance, the greater the distance between pad and fans can be. It is not practical to separate the pad and exhaust fans by more than 200 feet. A distance of 150 feet or less is preferred. Location of pads and fans will be influenced by several factors. Keep in mind:
The evaporative pad cooling system must have adequate controls for the operator to be able to adjust the house environment to provide the best growing conditions for plants and a comfortable environment for workers. Thermostats are usually used to turn fans and pumps on and off as required to optimize response to outdoor climate changes and maintain more uniform greenhouse temperatures with lower operating costs. Thermostats should be checked each spring and fall against an accurate thermometer to insure proper operation.
Fan and Pad Evaporative Cooling System Maintenance
Evaporative cooling pads lose efficiency due to clogging from impurities in the water, algae growth and decay. If the pad material is clogged or decomposed its ability to function as designed is impaired. Air exhausted by the fans will enter the building at the point(s) of least resistance. If a pad area is totally or partially clogged, very little if any air will pass through that portion of the pad. If the pad has holes, the air will move directly through them. This means less contact between air and water and much less cooling. When a pad has decayed, the only alternative is to install a new pad.
Salt concentration in the recirculating water should be below 50,000 ppm and in the make-up water it should be below 40,000 ppm. The pH level of water in the system should be between 6 and 9. Outside this range, protective chemicals are leached from the pad. This allows for quick breakdown of the pad.
Pumps and sumps should be cleaned several times during the months of summer operation to prevent algae and sludge or sediment clogging that causes reduced water pressure across the distribution header and poor, non-uniform pad wetting.
Keeping water filters cleaned can be one of the most time consuming problems, especially for growers with poor water quality. Filters do put a load on recirculating pumps, and when they start getting clogged they reduce water pressure in the distribution pipe and cause under wetted pads, so they must be regularly cleaned out or replaced.
Maintaining fully functioning distribution headers, with water coming out evenly along the entire length of the pipe, is also essential for good cooling. It is easy to spot clogged holes in the pipe because there will be a dry column underneath the clogged section.
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