Greenhouse Ventilation and Cooling
Fan and Pad Evaporative Cooling Systems
The most commonly used evaporative cooling system used in greenhouses is the fan and pad evaporative cooling system (Figures 6.9). With this type of system, exhaust fans are placed in one wall of the greenhouse and pads are in the opposite wall. The fans exhaust air from the greenhouse and draw in fresh air through the pads. Water is continuously circulated over and through the pad cells during operation. 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 Heat is withdrawn from the air during this process and the air leaves the pads at a lower temperature with higher moisture content. The drop-in temperature depends on how much water the air can absorb (a function of the relative humidity), how evenly the pad media is wetted, and how long the air is exposed to the pad (a factor of turbulence, wetness, and speed of air movement).
Operating Fan and Pad Evaporative Cooling System
With a fan and pad cooling system, water is pumped to the top of the horizontal pads and released through small openings along the entire length of the supply pipe. The opening spacing is designed so that the entire pad area wets evenly without allowing patches to remain dry. The fans exhaust air from the building and draw in fresh air through the wet porous pads. At the bottom of the pads, excess water is collected and returned to a sump tank so it can be reused.
Greenhouse Temperature Gradient
As air moves down the greenhouse it produces a temperature gradient across the length of the greenhouse, with the pad side being coolest and the fan side warmest. The temperature gradient should be minimal to provide all plants with similar conditions, though a gradient of 7 to 10 degrees F (3.9–5.6°C) is common.
Evaporative Cooling Pads
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. Other pad materials are also on the market, but none have seen wide acceptance. Among these are pads fabricated from aluminum and from plastic fibers. Both these pads types are expensive and show no advantages over corrugated cellulose.
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 one minute by the volume of air that can be moved through a square foot of pad in one 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 5.8).
Determining Pump Capacity for Pads
To maximize operating efficiency, you must have adequate pad surface area and an adequate water supply and distribution system. The amount of water needed will vary with the type of system used. 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 Exhaust Fans and Cooling Pads
When possible, locate pads on the prevailing summer wind side and locate the fans on the downwind side of the greenhouse. If the pads are sheltered by another house within 25 feet (7.6 m), the wind effect is negligible and can be ignored. If it is necessary to face exhaust fans into the prevailing winds, increase exhaust fan CFM capacity 10 to 15 percent and correspondingly increase fan motor horse power and add shutters or back draft dampers. Exhaust fans from one greenhouse should not exhaust warm moist air toward the pads of an adjacent greenhouse if the adjacent greenhouse is within 50 feet (15.2m) of the fans.
Thermostats
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. The fans may be wired so that the thermostats will turn on alternate banks of fans in sequence as the temperature demands. A thermostat should be used as the main pump control. The thermostat should be set to stop the pump before all the fans go off so that the pad can dry out.
Evaporative Cooling System Maintenance
Pads
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.
Recirculation Pumps
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. Extreme clogging can cause the entire system to fail and recirculation pumps to burn out. Reliable recirculation pumping is so important some growers use two pumps on each system.
Distribution Headers
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. This is easily corrected by unclogging the hole with a screwdriver or nail small enough to fit into the hole.
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