Changes in the greenhouse industry are occurring with increased production of vegetables, small fruit and cannabis. This makes sense in that facilities have more year-round utilization, thereby spreading the cost, improving labor use and giving a larger total return on investment.
Growing crops during the summer is a challenge in terms of keeping the temperature down. Fan ventilation, in addition to being expensive to operate, provides limited flexibility in control. Although some staging can be obtained with two-speed motors or multiple fans in larger greenhouses, fans are either on or off. Low Press Centrifugal Fan
Natural ventilation through vents or roll-up sidewalls not only provides more uniform cooling throughout the growing area, but also eliminates most of the electricity cost. Vents can be fitted to most existing greenhouses. In some states, the National Resources Conservation Service’s Environmental Quality Incentives Program provides payments for part of the cost.
Natural-ventilation systems operate on a principle of removing heat by a pressure difference created by wind and temperature gradients. Wind plays a major role in the process. For a well-designed greenhouse, wind speeds of 1 mile per hour are adequate to keep the inside temperature within two degrees of outdoor ambient heat. Weather records show that there are very few days that the wind is less than 1 mph, especially if the outdoor temperature is above 80° F.
Buoyancy, the effect from air getting lighter as it is heated, also aids ventilation. The greater the temperature difference between inside and outside, the greater is the air movement. Also, taller greenhouses help by providing more distance between the cooler air on the floor and the roof.
Besides the typical hinged roof and sidewall vents, roll-up sidewalls, endwall shutters and open-roof designs provide natural ventilation.
The basic control system for vents includes a sensor, environment control, contactor panel, vent controller and gearmotor that moves the vent arms. As ventilation can be used to control both temperature and humidity, both types of sensors should be included. A thermostat is used with a simple system and electric-resistance type sensors — such as thermistors — are used with electronic controls. A humidistat or thin-film-capacitive sensor handles humidity. A weather station is frequently added to provide current outdoor conditions that help to anticipate necessary changes.
Based on sensor information, the environment control determines what equipment needs to be activated to maintain the setpoint. It tells the vent control how much the vent should be open. It can also provide space for relays or contactor switches that activate the gearmotor, or in larger installations, a separate contactor panel may be necessary. Relays are used for small motors and motor starters, and contactors for larger ones (usually more than 1/6 horsepower [hp]). For a single freestanding greenhouse or hoophouse, a lower cost, stand-alone unit that includes a sensor, transformer and relays can provide simple fully-pen or fully-closed operation.
The vent controller includes a transformer that changes the voltage to 24 volts for the gearmotors. Some units will handle several roof and sidewall vents. The controller also includes relays and toggle switches that allow for manual operation, and electronic timers that tell the motors how long to operate for each stage. The typical control operates incrementally in four stages: closed, one-quarter open, one-half open and fully open. When connected to an electronic environment control, vents can be operated proportionally to as many as eight stages. Safety protection that may be included are electronic and thermal overload protection, motor-reversal delay and limit switches.
The gearmotor unit rotates the shaft with the pinion gears attached and can be located on one end of the shaft for short greenhouses or in the center for long ones. The gearbox has a large ratio so that a small horsepower motor (1/20 to 1/4 hp) will handle the torque needed in a long shaft. Ratios can be as large as 1900:1(1900 motor shaft revolutions provide one shaft revolution). As the shaft turns at less than five revolutions per minute, the vent opens slowly. Depending on weight and design, vents as long as 450 feet can be moved by one gearmotor. Safety torque and limit switches protect the motor.
A rack that is either straight or curved attaches to the vent. The teeth on the pinion power the rack that adjusts the vent position. Aluminum is the common rack material, with steel being used for long greenhouse lengths or heavy open-roof sections. Racks are usually spaced less than 10 feet apart. For roll-up sidewalls, the gearmotor is usually attached directly to the shaft and rides up and down on a guidebar at the end of the greenhouse.
John is an agricultural engineer, an emeritus extension professor at the University of Connecticut and a regular contributor to Greenhouse Management. He is an author, consultant and certified technical service provider doing greenhouse energy audits for USDA grant programs in New England. jbartok@rcn.com
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