CULTIVATIONTIPS FUNGIPOINT Diameter of air ducts
With air distribution, the general rule is that the larger the diameter of the duct, the more uniformly the air will be distributed along the length of the growing room. This is because the horizontal air velocity decreases as the duct size increases. Consequently, the air at the beginning of the duct (seen from the fan) will be blown out of the openings at a less diagonal angle. The diagonal angle explains why less air is blown out of the ope- nings at the start of the ducts than at the end. If the air ducts are large enough, there will be a minimum difference in air volume between the front and the end.
With a climate unit positioned at the front of the growing room, uniform air distribution can be realised with shelving of up to a length of around 30 m. Despite this, in growing rooms with shelving less than 30 m in length, visibly different mushroom growth patterns are still seen on a regular basis. For example, in so-called “sheds” with two rows of shelving and one large air duct. In this kind of shed, just a single air duct has to provide all the air for both rows. These sheds have semi-circular ceilings, which means there is plenty of space and large ducts that sometimes have a dia- meter of 1 m are used.
Air duct in shed with two rows of shelving. By Jan Gielen
General Manager FungiPoint Climate & Energy Specialist
FungiPoint@outlook.com
However, air velocity, smoke and temperature measurements in these sheds may still reveal that the visible differences in growth are not caused by the air distribution pattern, but by temperature differences between the front and the back of the shed. Temperature differences of up to 2 °C can occur in the ducts. During cooling (often in summer), the air towards the end of the duct is warmer than at the front. The opposite applies during heating (often in winter). Any outside influences on the rear wall of the shed also play a role. In a number of cases, the cause of these temperature differences was an air duct with a too large diameter. In this situation, the air velocity in the duct is too low so it takes too long for the air to reach the end of the duct. The large diameter of the duct also creates too much contact surface. This allows a lot of heat and cold exchange with the air in the growing room, which leads to the temperature differences mentioned above.
Conclusions: smaller air duct diameters take the air to the end of the duct faster and reduce temperature differences but blow out air more diagonally and cause greater differences in air distribution. Larger air duct diameters blow out air less diagonally and create uniform air distribution but take the air to the end of the duct slower which can cause temperature differences. To ensure air reaches the end of the duct faster to reduce temperature dif- ferences and still have air blown out straighter for uniform air distribution, reduce the diameter of the air duct and use air liners to correct the direc- tion of the air.
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