Heat Balance Characteristics in a Fan-ventilated Plastichouse

Abstract

Continual measurement of energy fluxes in a fan-ventilated plastichouse had been carried out during the growing season of melon crops. The equations to estimate the ventilation rate were derived from the experimental results.The experimental plastichouse had a floor area of 200m2. A fan with a capacity of 320m3 min-1 at the gauge pressure of 1.5mmAq was used to ventilate the plastichouse. Two different mulch conditions were examined; one without mulch and the other with 80% of the floor area covered. The experiment was conducted from April to June, 1977.Inside net radiation and soil heat flux were measured with a net radiometer and heat flux plates, respectively. Both sensible and latent heat flux due to ventilation were calculated from the ventilation rate and the dry and the wet bulb temperatures measured inside and outside the plastichouse. Heat transmission through glazing was estimated from the inside-outside temperature difference and the heat transmission coefficient.1. The ratio of latent heat flux to inside net radiation increased with the increase in LAI. The mean value of the ratio for LAI more than 1 was 55.1% when the floor was not covered with mulch, while 52.6% when 80% covered.2. Sensible heat flux due to ventilation gradually decreased with the increase in LAI. The mean ratio to net radiation for LAI more than 1 was 32% when not covered, while 36% when 80% covered To maintain the same inside air temperature, therefore, it is estimated that more ventilation rate is required when the floor is covered than the case when not covered.3. When LAI was small, sensible heat flux due to ventilation exceeded latent heat flux, while it was reverse when LAI was large. This change occurred at LAI equale to 0.4 when the floor was not covered, but at LAI of 0.7 when 80% covered.4. Heat transmission through glazing was also affected by the floor cover. Its ratio to the inside net radiation was averagely 7% when not covered, while 8.9% when 80% covered.5. Soil heat flux was less dependent on the floor cover, but more on LAI. Namely, the flux decreased as LAI increased. The average ratio during the growing season was 6%.6. Bowen ratio in the ventilation heat fluxes at LAI of 0.08 was 1.44 without floor cover and 2.77 with mulch. These ratios decreased with the increase in LAI.7. The following equations to estimate the ventilation rate was derived experimentally from the energy balance measurement.In case without mulch, q=1/cpρ(0.437aSi/ti-to-wk)In case 80% of the floor covered, q=1/cpρ(0.481aSi/ti-to-wk)where, q; ventilation rate, cp; specific heat of air, ρ; air density, a; ratio of sunlit area to total floor area, Si; inside net radiation, ti; inside air temperature, to; outside air temperature, w; floor to glazing area ratio, k; heat transmission coefficient of glazing.