Articles
FAN VENTILATED GREENHOUSE COOLING: SOME CONSIDERATIONS FOR DESIGN
Article number
719_7
Pages
83 – 96
Language
English
Abstract
Fan-ventilated greenhouse cooling is sufficiently complex that simple design models are not always easy to use and understand.
For example, the ASAE design equation requires choosing a value for the evapotranspiration coefficient, E, a critical parameter for which there is little guidance and perhaps even less understanding.
Another potential problem area for designers is the lack of information on thermal stratification.
The common perception is that stratification is not a factor in fan-ventilated greenhouses; however, evidence exists that it may be important and that it can have detrimental consequences for cooling.
This paper considers the effect of airflow rate, outside relative humidity, and canopy size on the behavior of E and upon thermal stratification.
The value of E was found to vary with outside relative humidity and airflow rate, and to a lesser extent with canopy size.
Coefficients larger than 1.0 did not indicate maximum cooling, as some have suggested.
When evaporation from the pad is included, values of E exceeding 4.5 were predicted for low outside relative humidities (8.6%). When outside relative humidities were high (75%), values of E of 1.5 were still found to be possible.
Predictions of thermal stratification for the non-evaporative pad cooling case suggested that it might be problematic with large canopies where the quantity of air above the canopy is relatively small compared to that within the canopy.
A smaller, but significant, degree of stratification was also suggested for small canopies when outside relative humidities are low.
In those cases, mixing of the layers within the greenhouse would seem to be adverse to good cooling design.
For example, the ASAE design equation requires choosing a value for the evapotranspiration coefficient, E, a critical parameter for which there is little guidance and perhaps even less understanding.
Another potential problem area for designers is the lack of information on thermal stratification.
The common perception is that stratification is not a factor in fan-ventilated greenhouses; however, evidence exists that it may be important and that it can have detrimental consequences for cooling.
This paper considers the effect of airflow rate, outside relative humidity, and canopy size on the behavior of E and upon thermal stratification.
The value of E was found to vary with outside relative humidity and airflow rate, and to a lesser extent with canopy size.
Coefficients larger than 1.0 did not indicate maximum cooling, as some have suggested.
When evaporation from the pad is included, values of E exceeding 4.5 were predicted for low outside relative humidities (8.6%). When outside relative humidities were high (75%), values of E of 1.5 were still found to be possible.
Predictions of thermal stratification for the non-evaporative pad cooling case suggested that it might be problematic with large canopies where the quantity of air above the canopy is relatively small compared to that within the canopy.
A smaller, but significant, degree of stratification was also suggested for small canopies when outside relative humidities are low.
In those cases, mixing of the layers within the greenhouse would seem to be adverse to good cooling design.
Publication
Authors
D.H. Willits
Keywords
Greenhouses, fan ventilation, greenhouse cooling
Online Articles (73)