The best way to circulate air in a glasshouse
On Tuesday 09 October, a group of around 20 growers attended a GrowSave event on Air Movement, hosted by W. D. Smith and Son near Battlesbridge, Essex. The day was split into a morning session focusing on the theory of air movement and why it is important, while the afternoon comprised of a series of practical demonstrations carried out in one of the glasshouses.
Event Notes
The theory and scientific background were explained by Peter van Weel, a Dutch expert on air movement, who spent his career as a researcher at Wageningen University and Research in the Netherlands. His wealth of experience in optimising airflow around greenhouses came across in his presentation, which focused on how to improve crop health and resilience while also saving energy. His knowledge was invaluable in helping delegates understand the influence of the myriad of factors which can lead to temperature variations in and around the crop. An important point was how leaf/flower temperature can be very different from the ambient air temperature, and this is where problems can arise. To help combat this, an effective air movement solution is required.
In the afternoon, the group headed over to one of the nursery’s glasshouses, where bedding plants were being grown, to see some demonstrations of air movement using smoke. The smoke tests were designed to help visualise where the air actually ends up. Five scenarios were carried out:
- No fans; screens and vents closed
- Horizontal fans; screens and vents closed
- Horizontal fans; small screen gap and 10% vent
- Nivolator; screens and vents closed
- Nivolator; small screen gap and 10% vent
The aim of the first test was to show the general state of the glasshouse air when no action is taken. It quickly became clear that the air was mostly stationary, especially close to the floor at crop level.
Many growers are familiar with traditional horizontal fan setups, which help to move and mix the large volume of air within the structure. Typically these fans are placed high up and are set-up to work in unison, pushing air in opposing directions. However, depending on the type of crop, the movement of air at crop level could be very different from that at the fan level, and may not be what is necessary to maintain an optimum climate. This was seen in tests 2 and 3. Although some movement was generated within the crop, there was no clear pattern of airflow. The nature of air being blown in opposite directions leads to a lot of turbulence and can result in the air not going where intended, i.e. through the crop.
An alternative to horizontal fans is vertical fans. A representative from Nivola was on-hand to demonstrate the Nivolator, a vertical fan which pushes air downwards in a conical shape. The design ensures penetration of the crop canopy, which could be seen in the smoke tests. This appeared to be a more effective solution for plants at ground level, although whether the benefit would be the same in a taller or leafier crop remains to be seen.
It was also interesting to see the effects of using the fans with a small gap in the screen and a small vent opening. The lower pressure outside the glasshouse meant air moved upwards, through the gaps in the screen and out through the vents. While this may be beneficial for reducing glasshouse humidity, there was no obvious benefit to the amount of airflow at crop level; if anything, it was detrimental and the advice would be to keep screens and vents fully closed when trying to create a homogenous climate.
Having heard the theory and seen the demonstrations, one of the big take-home messages was that the two do not always match-up in reality. External factors, such as wind direction, can affect the internal atmosphere, while turbulent mixing of air can make it difficult to accurately predict flow patterns.
GrowSave hopes to build on the knowledge learned and develop further content, including technical features and future events.
