Evaporative Cooling Systems with Fans and Pads
Evaporative cooling Systems.
This evaporative cooling system uses the heat in the air to evaporate. Water from plants and other wet surfaces can reduce the greenhouse by 10 to 20 degrees Fahrenheit below the ambient temperature. Evaporative cooling is more successful in hotter areas. such as the southwest, but it can be use anywhere in the United States.
Changing the state of water from liquid to vapor absorbs a lot of heat. One Btu of heat is require to raise the temperature of one pound of water by one degree Fahrenheit, whereas 1,060 Btu of heat is require to convert the same amount of water into vapor. In other words, each gallon of water evaporated absorbs around 8700 Btu of heat. Evaporative cooling systems are provided by exhausting this warm air from the greenhouse.
The graph depicts the impact of outside air relative humidity on the temperature of the air leaving the evaporative cooling systems. If the relative humidity is at 30%, the air will be cool to roughly 67 degrees Fahrenheit. but only to 82 degrees Fahrenheit if the relative humidity is 70%. Greater greenhouse cooling will come from outside air with lower relative humidity.
For more than 50 years, the fan and pad system has been the industry standard for evaporative cooling. Aspen or cellulose pads are install on one end wall or sidewall of the greenhouse with this method. They get water from a pipe above the pads, and any extra water drains into a gutter at the bottom. Fans positioned on the opposite end wall or sidewall suck air thru the wet pads. which saturate and cool the greenhouse. The pads are made.
For cooling a greenhouse using fans, the standard design airflow rate is eight cubic feet per minute (cfm) per square foot of floor surface. If the greenhouse is not shade and the summer light levels inside are above 5,000 foot-candles. if the greenhouse is located at a high altitude above 1000 feet, or if the distance between the pad and the fan is less than 100 feet, this must be change. The total fan capacity required is the airflow rate. which should be delivere at 0.1″ of water static pressure in evaporative cooling systems.
For aspen pads, the overall pad area should be design to support 150 cfm per square foot of pad area, and for cellulose pads, 250 cfm and 350 cfm per square foot of pad area. Divide the airflow rate by one of the above figures to get the total pad area (square feet).
The pads require an appropriate flow of water to keep them moist. but not so much that they form a solid water curtain. For aspen pads, this is generally 1/3 gallon per minute (GPM) per foot of pad length, 12 GPM for 4″ thick cellulose pads, and 34 GPM for 6″ thick cellulose pads. Based on the square feet of pad area, the pump must be large enough to give the above rates. Several pumps may be required in bigger systems. It’s also necessary to have a supply of makeup water. It should be size to handle one gallon of water per minute per 100 square feet of the pad surface.
The water collection sump might be a tank or a segment of high diameter PVC pipe in certain modular devices. It must be big enough to power the pump. 12 gallon per square foot is advise for 2″ thick pads, 34 gallons for 4″ thick pads, and 1 gallon for 6″ thick pads.
A thermostat, controller, or computer can all be use to regulate it. These are the recommended controls since the controller and computer enable numerous cooling phases. A humidistat might be use to control the pumps of the evaporative cooling system.
Assume a 30′ x 100′ greenhouse with the pads at one end and the fans. at the opposite end of the wall. base on 8 cfm per square foot of floor space. the total fan capacity is 24,000 cfm (30′ x 100′ x 8 cfm/sq ft = 24,000 cfm). This might be provided by two people – 42 “12 horsepower fans, etc. Using a four-inch square with a 250 cm/minute airflow over . the pad, the total pad area required is 96 sq ft (24,000 cfm x 250 cfm/sq foot = 96 sq ft). This criterion might be met with a pad that is 4′ high by 24′ long.
The system’s minimum sump capacity is 72 gallons (96 sq ft x 34 gallons/sq ft = 72 gallons). 12 GPM is the recommended pump size (24 ft x 12 GPM/ft = 12 GPM). The size of the supply pipe above the pad will be determine by the installation’s design. The water pipe must be at least 1-1/4″ in diameter if it is delivered from one end. It can be as little as 1″ if it is supplied from the middle. A minimum of 1 GPM of water should be available to compensate for the evaporated water.
Fan and pad systems are made up of various components that may be customized to meet the specific greenhouse. Pad height should be no less than 2′ and no more than 8′ to provide uniform pad soaking. There are modular units that come with all of the components installed and ready to attach to the wall. It’s possible that many units will be require to achieve the desired pad size.
Some farmers have been shown to be effective for some farmers. These are wheels and require only a hose and an electricity hookup. They are located at a door in the greenhouse’s end zone, opposite the fans. Due of the restricted amount of pad surface, they have a limited capacity in hot temperatures. Many units may be require to provide appropriate cooling.
Spray-on evaporative cooling systems spray water onto the pads from nozzles positioned outside the greenhouse. Because the extra water flows away from the greenhouse, these are non-recycling systems.