Greenhouse Ventilation System Design

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Excessive greenhouse temperatures cause poor plant development, the need for frequent watering, and fans that appear to run continuously, raising the monthly utility bill. Here are a few ideas for improving your system.
Greenhouse Ventilation Systems Provide Natural Air Circulation
The idea of using thermal buoyancy and wind to cool a greenhouse dates back to the dawn of the controlled environment. Prior to the 1950s, all greenhouses had some type of vents or louvers that were opened to allow excess heat to escape and cooler outside air to enter.
When polyethylene was produced with a single sheet spanning the entire roof, it proved difficult to place vents on the roof. Engineers then devised the idea of using fans to take outside air via louvers in one endwall and exhaust it through the opposite endwall. Thermostats or environmental controllers are the conventional way for cooling many structures where positive air circulation is essential.
Hoophouse growers have discovered that roll-up sides work well for warm-season ventilation. There are both manual and motorized systems available. It is necessary to find a site with pleasant summer breezes and lots of space between houses. To decrease the amount of rain that can leak in, greenhouses with vertical sidewalls up to the height of the attachment rail are beneficial.
Heat is extracted from greenhouses with roof and sidewall vents by a pressure difference caused by wind and temperature gradients. The wind is the most important factor. A wind speed of 2-3 miles per hour provides 80% or more of the ventilation in a well-designed greenhouse. Wind blowing across the roof creates a vacuum, sucking the warm air out of the vent. When the sidewall vents are open, cool replacement air enters and falls to the floor. When the sidewall vents are closed, a limited amount of air exchanges thru the roof vent.
Buoyancy, the action of rising warm, moist air, also aids ventilation. As the air near the floor is heated, it becomes lighter and rises towards the roof. The considerable temperature differential creates efficient air exchange on cool days. On hot days, the temperature difference can be as little as 5 or 10 degrees, and the buoyancy impact is minimal. The development of taller greenhouses has aided in raising the hot air over the plants. To avoid destratifying the warm air, turn off horizontal airflow fans.
Roof and side vents on typical greenhouse ventilation design must be large enough to allow for adequate air movement. The American Society of Agricultural Engineers suggests that the combined sidewall vent area should match the combined ridge vent area, and each should be 15 to 20% of the floor area. The greenhouse should be oriented so that the regular summer wind direction blows over the ridge, creating a vacuum on the leeward ridge vent. The windward side vent opening should equal the leeward ridge vent opening for summer ventilation.
Cooling big gutter-connected structures was challenging prior to the advent of the open-roof greenhouse concept, especially in southern regions. Sidewall vent space is typically restricted, and sending cool outside air and warm inside air through roof vents results in unequal cooling.
Most major manufacturers offer open-roof greenhouses. Some have hinged roof panels that open upward from the gutter. Others have hinged panels at the ridge and one gutter that glides sideways on Teflon bearings. The size of the opening can be adjusted between 0% and 75%. To seal the joints, most designs employ rubber gaskets.
There are various ventilation advantages to open-roof greenhouse ventilation design.
During warm weather, the temperature inside the greenhouse can be kept within a degree or two of the outside temperature using little or no energy. Many growers have discovered that doing so reduces production time and yields a higher-quality plant.
Plants can be hardened off in the spring by opening the roof on nice days. This saves a lot of time and effort in transferring plants outside.
The cost of energy is decreased. Fan ventilation can consume between 0.5 and 1-kilowatt hour per square foot per year.
Depending on the design and direction, crops may receive more light in the middle of the day than in a typical greenhouse, or less light in the early morning or late afternoon due to the additional layers of glazing that must be passed through. More research is required in this area.
Reduced irrigation as a result of more constant temperature and the possibility of natural rainfall.
When severe winds or rain prevent the roof from being opened, adding side vents provides for cooling and air movement. The guillotine vent, which is offered by a few manufacturers, replaces the traditional vent with arms that interfere with the interior or outside work area.
A shade system may be required to provide appropriate cooling on hot, sunny days. It should be porous, allowing heat from below to escape through the shade material. Evaporative cooling, whether through a fog system or portable evaporative coolers, can provide additional cooling. Natural cooling is reduced when there are a lot of hanging baskets.
Continuous advancements in the design of natural ventilation systems provide growers with greater temperature and humidity management at a reduced cost. Control can be improved over fan systems with proper dimensions, orientation, and operation.
Motorized vent controllers should be cleaned and greased on a regular basis. A wind sensor should be utilized to prevent hinge and vent arm damage. As severe winds approach, this will cause the vents to seal.
Before the heating season begins, adjust the vents so that they seal evenly and tightly. During the cold, poorly shutting vents cause significant heat loss.
Ventilation with a fan
Under all weather situations, fan systems may offer good air movement through the greenhouse. A little vacuum is formed as the fans exhaust the hot air, drawing in colder outside air through louvers, open doors, and crevices.
Fan Dimensions
In many greenhouses, improper fan sizing is a key cause of poor ventilation. For summer ventilation, the fan system should be sized to generate one volume of air exchange per minute to a height of 8 feet. This will cause an 8-10°F temperature increase from the intake louver to the fan. For a 25-foot-by-96-foot greenhouse, for example, the fans should have a capacity of 25-foot-by-96-foot-by-8-foot = 19,200 cubic feet per minute. A height of 10 is sometimes employed in southern areas to increase the ventilation rate.
If the greenhouse is not used during the summer, for example, for bedding plant production, the capacity can be reduced to 3/4 volume air change per minute. A capacity of 1/4 volume air change per minute is sufficient for winter ventilation. Because greenhouse ventilation requirements change from season to season, it is best to provide multiple ventilation rates. In small houses, two-speed fans can be used, while multi-fan installations can be used in bigger houses. Using two-stage thermostats, temperature controllers, or a computer system instead of a single-stage thermostat will result in energy savings and improved control.
Location of Greenhouse Fans
When the draw is less than 150 feet, fan systems function well. For most greenhouses, this involves putting the fans on one end wall and the louvers on the other (Figure 3). Longer dwellings should have fans installed along the sidewalls to bring air in through louvers on both ends.
Wherever possible, fans should be placed to function with the prevailing summer wind. When a fan is blowing toward the wind, its production is reduced by 10% or more.
The intake louver area should be at least 1.25 times the fan area to provide adequate air for the fans, especially in poly-covered dwellings. A continuous louver or multiple smaller louvers, though more expensive, will provide more constant temperatures within the greenhouse. If the plastic is being dragged tightly against the greenhouse frame while the fans are running, or if the door is difficult to open, the intake area is insufficient. Motorized dampers or solenoids should control the louvers, which should be linked to the thermostat that controls the fan. In small, tight houses, when the fan starts on, it immediately creates a negative pressure, and the motorized damper does not open. A time delay relay can be used to prevent the fan from energizing until the louver is open.
Position the greenhouse fan so that air travels over and through the plant canopy rather than beneath the benches or under the greenhouse's ridge. The fan or louver's bottom should be around 3 feet above the floor.
Thermostats are typically used to control fan and vent motors. These frequently have a large temperature difference between the off and on positions, sometimes as much as 6-8oF. A high electric cost may arise from using this type of thermostat. For example, if the thermostat is set to 75°F, a +/- 2o thermostat will turn off the fan at 73°F, whereas a +/-5o thermostat will allow the fan to cool the greenhouse to 70°F. A thermostat can be checked by turning the control dial and determining the difference between the on and off positions. At the same time, place an accurate thermometer next to the sensor bulb to evaluate the correctness of the thermostat dial setting.
Thermostats should be placed near the center of the greenhouse at plant height for the most accurate temperature control. Aspirating with a squirrel cage blower or muffin fan at 40-60 cubic feet per minute will offer a more representative sample of air to the thermostat. (Fig. 2)
Choose new fans that have been tested in compliance with Air Movement and Control Association (AMCA) standards. The output of fans varies greatly between manufacturers.
Compare the Ventilating Efficiency Ratio as well (VER). This is the volumetric rate of air movement divided by the rate of energy consumption. This ranges between 10 and 20 cubic feet per minute/watt. Fans with a VER of 15 or greater are preferred.
Using larger fans with smaller motors can also save energy. For example, a 36" diameter fan with a 1/3 horsepower motor will produce the same output as a 30" fan with a 12 horsepower motor while saving 180 watts per hour on electricity.
The fans should be serviced on a regular basis. Cleaning blades, eliminating grass or weeds in front of shutters, and tightening fan belts are all part of the job. Dirty, automated shutters that do not fully open significantly limit airflow and reduce a fan's ventilation efficiency. If these shutters do not close properly when the fan is turned off, they will considerably raise the winter heating expenditure by permitting cold outside air entry.
Plant as many plants as possible in the greenhouse to improve evaporative cooling via plant transpiration.
Reduce summer fan operating time by shielding the greenhouse's outside.
Alarms for high temperatures or power outages should be monitored on a regular basis. Without ventilation, it just takes a few minutes on a hot summer day to reach temperatures in excess of 100°F.
Cooling via Evaporation Greenhouse Ventilation
Even with a well-designed fan greenhouse ventilation system, the temperature within the greenhouse during periods of intense heat may exceed that outside by 10-20°F. This stresses the plants, limiting their performance.
Evaporative cooling, which uses evaporated water from pads or high-pressure pumps, can be utilized to cool the greenhouse by 10-20°F below the outside temperature. This works best when the humidity in the outside air is low. These conditions are most typical in the arid South, but even in the more humid northern parts of the United States, significant cooling can be produced on many summer days.
Fans draw air through wet pads that stretch the length of one endwall or sidewall in the most popular cooling system (fan and pad). Aspen and coated cellulose are two common pad materials with a lifespan of one to three years. One square foot of pad is required for every 20 square feet of floor area.
To minimize clogging and coating of the pads, the water for the pads should be pure and low in mineral content. The water is recirculated using a pump, pipes, and gutters. For proper wetting, a flow rate of 113 gallons per minute per linear foot of the pad system should be given.
For consistent soaking of the pads, it is preferable to add a wetting agent to the water, especially in hard water locations. A commercial substance or liquid household detergent can be used at a rate of 2 tablespoons per 100 gallons.
Algae growth in the pads can be an issue, reducing the efficacy of the system and hastening the degeneration of the pads. An algaecide added to the water supply will aid in management.
A fog or fine mist pumped into the intake air stream is an alternative system. Despite the availability of various commercial systems, growers can manufacture and install their own systems utilizing high-pressure piston pumps and fog nozzles. On extremely hot, sunny days, a two-stage system regulated by a two-stage thermostat enables more water to be administered.
Ventilation in the Winter
Ventilation may be required during bright, sunny days in the winter to keep temperatures at an adequate level for good plant growth. The fan and tube technology, which was established several years ago, has become widely used in the industry for this purpose. Before reaching plant level, it mixes the chilly outside air with the warm greenhouse air. There are two kinds of systems.
The least expensive to install and run is a fan that is normally set to a low speed to exhaust the warm air from the greenhouse. The intake air enters the greenhouse through a perforated plastic tube suspended in the ridge and connected to a motorized louver or huge stove pipe elbow. For equal cooling, two tubes should be utilized in greenhouses wider than 25 feet.
Plastic greenhouses should employ tubes with 2-foot hole spacing. Glass greenhouses can employ tubes with a 4-foot spacing. The tubes should be perforated so that the air enters the home horizontally.
The second technique, known as fan-jet, inflates the attached perforated tube using a greenhouse fan positioned in the ridge of one endwall. Air is drawn in via an adjacent motorized louver. When the louver is open, the fan is programmed to run constantly, providing ventilation air and circulating air throughout the greenhouse. The unit should be sized so that it provides approximately 1/2 cubic foot per minute per square foot of floor surface.
The end of the tube that is not connected to the intake louver or fan should be tied off in both systems. Despite the fact that most greenhouse ventilation suppliers use a standard pre-punched tube, the optimum size and number of holes are critical for proper operation and uniform ventilation. Additional holes should be drilled in the tube if the tube pops open when the fan is turned on or if the greenhouse doors are difficult to open. If the tube does not fully inflate, certain holes should be patched closed with poly tape. Check to ensure that the greenhouse doors are closed and that no short circuiting is occurring.

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