Official URL: http://www.jstor.org/stable/25562350

The rate at which fresh air is introduced to a mushroom tunnel has a major impact on the climate in the tunnel. Control of this rate is by adjustment of the ventilation mixing flap setting within the air conditioning unit. The objective of this study was to determine the air exchange time in a standard mushroom tunnel as a function of the flap position. In combination with an understanding of tunnel air exchange efficiency, knowledge of the relation between air exchange time and flap position facilitates control of the tunnel climate, through the use of predictive control strategies. Measurement of the air exchange time was achieved by marking the air in the tunnel with a tracer gas (C02) and then recording its rate of decay in the tunnel. A range of mixing flap settings, from 0 to 90°, were used. Results showed that the effect of the flap angle on air exchange time may be characterised by a logarithmic relation. A wide variation in air exchange time exists for any one setting of the flap, especially at high flap angles when the bulk of the air is being re-circulated. This is due to the dependence of air exchange time on both uncontrolled infiltration and fresh air inflow that is controlled by flap angle position. Both of these mechanisms are weather dependent. Back-flow of conditioned air through the mixing box and out through the fresh air inlet was identified as a preventable ventilation short-circuit. The potential increase in air exchange efficiency that results from the increase in understanding of the air conditioning.