Evaluation of Tube Diameter and Fan Induced Swirl in Polyethylene Ventilation Tubes
ABSTRACT THE influence of flow swirl and tube diameter reduction have been examined for polyethylene ventilation tubes suitable for greenhouse applications. It was found that a conventional unmodified ventilation fan provided nearly identical flow rates from a 610 mm or 457 mm perforated ventilation tube, provided that swirl control was employed at the tube inlets. A six vane antiswirl device was installed that removed swirl for free fan exhaust. The antiswirl device can be made by a greenhouse operator or tradesman. Operation of the 457 mm tube without swirl control was found to be unacceptable, since the required contraction piece increased the fan induced swirl, causing tube vibration and poor flow distribution. Measurements of discharge from the ventilation holes were obtained along both tubes for flow with and without swirl control. The total discharge was found to be in good agreement with the measured flow into the fan in all cases. Friction factors were obtained for a nonperforated 610 mm tube. A cosine correction for swirl was included in the analysis to account for the actual length of flow path. The resulting friction factors were slightly lower than smooth pipe values in all cases. Polyethylene tubes can be reduced in size for a number of applications if flow swirl is controlled. Smaller tube sizes for overhead ventilation are of interest to greenhouse operators, because size reduction leads to less light blockage and hence to greater crop productivity.