Greenhouse Ventilation in part

Farming is a lot of hard work. The results are borne by the effort put into farming. For many areas in Canada and certainly in Northwestern Ontario, the weather does not give the growing results needed. To solve that problem, an investment often looked into is a greenhouse. Since a greenhouse can take many forms, one needs to know what type of structure to get into. A cold frame is a good way to start or even an open ended shelter allows you more growing opportunity. Perhaps a double poly greenhouse with an air buffer between the poly layers, a hothouse, or even a year-round greenhouse would be the ticket.

No matter what type is used, the advantage is to gain a bit more heat by trapping it in the structure. The disadvantage is the necessary requirement to control the air temperature from getting too hot. There are several ways to accomplish cooling on hot, sunny days. Doors and windows on smaller greenhouses will often allow a breeze to cool it down. Larger greenhouses including some commercial models have roll-up side panels that keep the lower zone of the greenhouse cooler. Yet another method is to use fans and vents, forcibly exchanging the air in the greenhouse.

Our greenhouse has all three of the aforementioned methods available for cooling. Depending on the weather, we can chose the method that works best. When the weather is hot and sunny and there is no breeze, the fan and vent method is the only way to keep it cooler in the greenhouse. Over the last few weeks, we have been designing a more efficient fan and vent control system. Our greenhouse has two 42" cabinet fans that can be run at low or high speed. We also have two powered vents at the opposite end of the greenhouse. Typically, the fans would start together, the vents would open, and the greenhouse would start to cool. On days where the weather is cool but the sun is hot, full fan and vent is not required, but rather maybe one fan running slowly would be enough. An average day with sun and cloud can become problematic, too. Full speed fan operation might create too much change too quickly. So now for the technical side of things!

The decision to design the new control system comes from the desire to keep the plants in the greenhouse from being shocked by a sudden or rapid temperature change. The engineering behind the function is to run the fan and vent system in four logical steps. When the temperature gets too warm in the greenhouse, one vent opens and one fan on low speed starts up. If that doesn't satisfy the control parameters, the second fan starts on low speed with the second vent opening. Should the parameters still not be met, one of the fans goes to high speed and finally the second fan would go to high speed if required. Essentially, this is a cascade control for ventilation.

In the construction of the system controls, each stage is operated by individual temperature sensors and adjustable controls. This will allow for customizing the ventilation where one side of the greenhouse might get hot more quickly than the other side, or on the cool down, one side cools before the other. A combination of NTC sensors and bimetallic sensors should allow for a smooth operation. Sensor locations can be adjusted to meet the canopy level of the plants and are generally placed mid-greenhouse to minimize airflow disturbances. Further to that, the NTC sensors are placed inside perforated pipe (Buffer Tubes) to keep the temperature sensing from becoming erratic. Most of the equipment used we have been able to recycle, reuse or at least, use part of. Two of the controllers we are using are new and extremely accurate. Items like the controller enclosure were salvaged from an industrial site demolition. Finally, we have independent wireless temperature/humidity sensors that we can monitor via a smart phone app to keep a watchful eye on the system. Below are some photos of the ventilation equipment.

Hopefully you can make sense of what I am writing about in this blog update from the farm. If you want to check it out in person, a tour would definitely be in order!