Growers use several types of fans in greenhouses. The most conventional fan type is the belt driven fan, which has a low first cost, but a high operating cost and short lifespan. Direct drive fans have a higher first cost, lower operating cost and longer lifespan, says Dr. Nadia Sabeh, mechanical and agricultural engineer and founder and president of Dr. Greenhouse. Multiple factors affect when growers should check, repair and upgrade their fans.
If growers have a belt driven fan, they need to replace the belt regularly, Sabeh says. Adjusting the tightness and length of the belt fine-tunes the fan’s flowrate.
Growers with direct drive fans should adjust the current flow going to the fan, Sabeh says. “I think something that people don’t do very often with fans is measure the flow rates to see if the fans are actually pulling the CFM (cubic feet per minute) or the airflow that they were originally designed to,” she says. “So, what can happen is that you can build up resistance in other parts of the greenhouse.”
For growers who have a wet pad wall, the wet pad tends to accumulate calcium salts, which resists airflow and takes away some of the pad’s cooling potential, Sabeh says. As airflow in the greenhouse is reduced, it creates a back pressure on the fan.
As a general rule, growers need to change their belts, check their cooling pads for calcium salt buildup and check vents, louvers and openings for obstructions if the fan is pulling less than 85 percent of the airflow that it was originally designed to, Sabeh says.
If growers upgrade from a belt driven fan to a direct drive fan, they can make use of variable frequency drive (VFD) or electronically commutated motor (ECM) technologies, which growers can adjust the airflow and fan speed of based on how much they need to cool or ventilate, Sabeh says. “These fans are more expensive — I would say probably about 25 percent more expensive — and you need some electronics to control it,” she says. “But in the end, you can save a ton of energy because you’re not just turning these fans on at full speed, even if you didn’t need them to be. You can actually save energy by running them at lower speeds and lower power, ultimately, and get better control.”
Other types of fans include horizontal airflow fans and blowers used as a fan-assist to inflate the double-layer polyethylene, Sabeh says. Horizontal airflow fans can build up dust or pollen, or become rusted. Blowers can stop working, so growers should check on them at least once a year to make sure they are still inflating the greenhouse.
If growers are in a marine climate, their fans are susceptible to rusting, and if growers are in a desert climate with a belt driven fan, the belt is susceptible to breaking. “You can get coatings on the fans and the fan blades and the motors so that they are protected from salts and from moisture from those harsh environments,” she says.