Articles
EFFECT OF CEILING HEIGHT ON THE POWER REQUIREMENT OF FORCED VENTILATION IN GREENHOUSES
The core of the model consists of two pairs of algebraic equations: One pair for the transfer of heat and water vapor from the canopy to the ventilating air, and one pair for the total temperature and humidity rise from inlet to outlet.
One-dimensional, two-layer duct flow is assumed along the greenhouse.
Bulk transfer coefficients are formulated according to findings in the literature, to include both forced and free convection.
The present version of the model requires as input the total (sensible plus latent) heat load to be removed by ventilation, rather than solar and atmospheric radiation.
The next step would be to incorporate it as a sub-model in a more general framework.
Results for a rose-canopy indicate the following: (1) The ventilation rate and power required to maintain a certain canopy temperature is significantly smaller at lower ceiling heights. (2) The trade-off between power requirement and canopy temperature is such that slight relaxation of the temperature standards may result in very considerable savings in the rated power of the fans. (3) The introduction of evaporative cooling, by wet pad or by wetting the canopy, may reduce significantly the power required for ventilation, despite the additional head loss in the pad.
The results illustrate how a model of this kind can be used to evaluate whether and which evaporative cooling systems are required to obtain desirable inside conditions in a given climate.
It can help in identifying severe combinations of climatic parameters as well as help in the design of appropriate climatic control devices.