Modelling Airflow Rate through Perforated Benches in Greenhouses

L. Jacobsen, O. F Nielsen


The purpose of the perforated bench is to reduce the air humidity beneath the plant canopy without
excessive energy consumption. A bench heating system with heating pipes under a perforated bench
with skirts creates an airflow through the bench openings due to buoyancy forces. Because the
heated air is relatively dry compared to the surrounding air, it has a moisture absorbing and
transporting capacity. The moisture transporting capacity will be decrease, thereby reducing the
advantages of the perforated bench, if airflow rate is impeded by the bench structure, plant shape or
leaf area index. Thus the airflow rate is of importance for the performance of a perforated bench.
In an experimental setup the airflow rate was determined for different perforated bench designs
with and without plants. Measurements were carried out with five different plant types on four
bench designs. It was found that the airflow rate was effected by the design of the perforated bench,
but on the same type of bench the effect of plant type and leaf area index was insignificant.
A model to predict the airflow rate for each bench design as a function of temperature difference
created by the bench heating system for the investigated bench designs is presented.

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