In Irish mushroom-growing tunnels, where the single-layer cropping surface at the floor is provided with a continuous airflow by an air conditioning system and a distribution duct, it was found that thermal buoyancy disrupts the air circulation during the operation of the heating system. Stratification of the flow occurs and the low-speed airflow across the crop (typically 10 to 30 cms~(-1) ceases during heating and does not recover until some time after the heater is turned off. This results in a reduction in the average air speed at the crop that is dependent on heating load. Simple measures can be taken to alleviate but not to overcome the problem. A proposal involving the addition of a second distribution duct, for recirculation alone, was investigated and the system was found to provide recovery of air speed if the output from the second duct was boosted during heating. A retrofit solution was developed which brought a number of advantages in addition to the removal of buoyancy effects.
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