little air is discharged. All this actually does is aggravate the problem as less static pressure is built up in the air duct so that even less air is expelled by the fan. As a guideline to ensure good air distribution along the length of the room, the surface area of all the openings in the air duct must never exceed half the surface of the air duct in total. When air liners are used, which correct the air discharge, a slightly larger margin can be used.


Air movement


Certain calculation formulas can be applied for lateral air distribution in the room, however this does not apply to the sideways movement of air across the beds. In this case, growers have to rely on their experience and smoke measurements. Apply the following principles: the size of the openings depends on the width of the (side) aisle, the height of the growing room and the desired mutual distance between the openings. Where possible, opt for openings without liners, as this allows more indirect air movement between the beds to be achieved. Another option is to use airliners. Airliners give less indirect air movement (induction), but do have a narrower airflow cone, which is necessary with high- ceilinged rooms and narrow side aisles, such as those on farms that harvest mechanically. Airliners can also correct the spread of air along the length of the room if lack of physical space means air ducts with a smaller diameter have to be used. The discharge velocity (throw) depends on the height of the room, the bed width, the space below the lower bed, between the beds and between the upper bed and the ceiling. Generally speaking, a slightly lower air velocity in this case will spread the air more easily over the beds in the middle. A high air velocity entails the risk of the beds in the middle being left out. In princi- ple, openings should only be made in the ducting immediately above the shelving. Only make openings in the ducting before the shelving begins if the climate unit is positioned at the front of the room, and ordinary holes are used, to compensate for the diagonal angle of the air discharge by ordinary openings (with airliners this effect is minimal, and openings can be made just before the point where the shelving starts.) If the correct hole pattern is used, the openings can be positioned straight downwards in practically all cases. If the openings are inclined too steeply towards the wall, the jets of air risk being bounced off the wall onto the beds.


Correction


Perforated plastic is often used along the lower beds as a corrective measure. This prevents air


from flowing too fast over the lower beds, but it can also cause new problems as it blocks the main flow of air and displaces the problem to the bed above so that perforated plastic has to be suspended here too (and sometimes right up to the third bed). Often the only way to achieve any air movement at all is to work with a too-high fan speed. A better solution is to give the air some way of moving without it flowing directly over the cropping surface. This can be done by making openings in the perforated plastic just below the bed above. The space below the lower bed should also be unobstructed so that some of the air can flow here as well.


Correct installation


Irrespective of the type of air duct, correct installation is also vital. In practice this is often done very carelessly, with all the inherent consequences for air movement and quality. The ducting should obviously be hung horizontally without any sagging and spaced equidistantly from the beds. Ducts suspended in the outer aisle must also be hung sufficiently far from the side wall, If not, at high fan speeds there will be greater pressure in the duct and it will twist towards the beds, with all the consequences this involves. Another important aspect is correctly aligning the air discharge direction of the openings. And finally, it is essential to ensure the ducts cannot twist laterally. This frequently happens, and depends on the manufacture and whether the rooms are cooked out or not. Suspension eyes at the ends of the duct often fail to correct this problem properly. The best solution is to fix an end bracket (and maybe a central bracket) to the wall or ceiling, to anchor the duct in place and in the right position.


Air discharge distributor on a farm with four tiers of shelves.


“Air ducts with


an insufficiently large diameter cause most problems.”


Ñ MUSHROOM BUSINESS 15


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