This article throws light upon the three ways for cooling of green house. The ways are: 1. Fan Pad Cooling System 2. Pad Size 3. Fog Cooling System.
Way # 1. Fan Pad Cooling System:
This system is available since 1954 and still today it is most ideal for summer cooling. In this system on one wall of green-house water is passed through a pad which is placed vertically in wall. On the opposite wall exhaust fans are fixed.
Warm air from outside of the green house is passed through wet pad and pushed outside from the other side. During this process water absorbs heat from surroundings and evaporates thus reducing the temperature of green house.
This reduction in temperature depends upon:
(1) The rate at which warm air is being removed from the green house,
(2) The type of pads used for evaporation,
(3) The size and placement of fans,
(4) The path of air stream.
The normal rate of exchange of air is 2.5 cubic meter per meter which has been found satisfactory.
Way # 2. Pad Size:
The pads of different types and sizes i.e. thickness, length and width are used, keeping various factors in view. Earlier wood fiber (shredding’s) type pads were used which were 2.5 to 4 cm thick and called is excelsior type.
These are to be replaced annually. Now-a-days, pads used are made up of wax impregnated cellulose material available in the market of great thickness varying from 5, 10, 15 and 30 cm. Similarly these are available in different heights are: 30, 60, 90, 120, 180 cm etc.
Holes of pads are in cross fluted design. The height of pad is determined by dividing total area of pad by length of the pad. The pad should be placed inside at the end wall of entire width. The pad wall should be equipped with ventilators exterior to the pad to permit the air entry during hot weather and for sealing off the outside air during colder period. Since the fan and pad are on opposite wall and should be at plant height to keep the cooling air on the plants.
To keep the pad wet for efficient cooling, water should be delivered at the top of 10 cm thick pad at the rate of 6.2 L/m/min of pad (it is 0.5 gm.). For pad length of 9 tol5 m, 32 mm water pipe is required while for the length of 15 to 18 m, 38 mm pipe is used. For more longer pad length, water should be supplied from the mid-point. Pipes should have holes at every 7-8 cm of 3 mm size for efficient wetting of pad.
Way # 3. Fog Cooling System:
The rate of cooling of air increases proportionately as water drop- let size decrease. The droplet size of mist is 1000 micron whereas the size of droplet of fog is 40 micron. The droplets of this size remain suspended in air while they evaporate to cool the air which occurs without water condensation at surface of leaves.
The fog cooling system is used green houses to be cooled by ventilators only. Fog nozzles are spaced above plants throughout the green house. Fog emits intermittently through nozzles to cool the air which has entered green-house through ventilators. The cooled air warms up and leaves the green house through ventilators. More outside air is drawn in and is cooled by subsequent fog.
Green house fog cooling systems are available that can convert 95% of water to 40 micron or small with an average droplet size lever than 10 micron. This droplet evaporates 40,000 times faster than water evaporates from crop.
Such a fast evaporation results cooling nearly to 100 per cent. For such system water should be of good quality. Generally clogging of foggers is a problem, and hence multiple filters capable of screening down to particle up to 5 micron are used.
Different timers can help to regulate the fogging system. Fogging system can also be used for seed germination and propagation of plants through cuttings by replacing misting. Foliar fertilization can also be done efficiently through fogging system.