Measurement of air exchange time in a mushroom tunnel

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. Th e objective of this study was to determine the air exchange time in a stand ard mushroom tunnel as a function of the flap position. In combination with an understanding of tunnel air exchange efficiency, knowledge of the relat ion between air exchange time and flap position facilitates control of the tunnel climate, through the use of predictive control strategies. Measureme nt of the air exchange time was achieved by marking the air in the tunnel w ith a tracer gas (CO2) and then recording its rate of decay in the tunnel. A range of mixing flap settings, from 0 to 90 degrees, were used. Results s howed that the effect of the flap angle on air exchange time may be charact erised by a logarithmic relation. A wide variation in air exchange time exi sts for any one setting of the flap, especially at high flap angles when th e 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 th at is controlled by flap angle position. Both of these mechanisms are weath er dependent. Back-how of conditioned air through the mixing box and out th rough the fresh air inlet was identified as a preventable ventilation short -circuit. The potential increase in air exchange efficiency that results fr om the increase in understanding of the air conditioning system can be tran slated into direct energy cost savings to the grower.