The concept of horizontal airflow, or HAF, is based on the principle that air moving in a coherent horizontal pattern in a building, such as a greenhouse, needs only enough energy to overcome the loss. turbulence and friction and keep it moving. In other words, just “kick”. The fans must be correctly sized and placed correctly to do so.
The air is heavy: The air above each square foot of floor area in a typical greenhouse weighs about one pound. A 30 ‘x 100’ greenhouse holds approximately 1.5 tonnes of air. Once the air is moving, it rolls like a train. That’s why HAF is so effective. Only four small fans are required to continue operating at a rate of 50 to 100 feet per minute (ppm) in the greenhouse above.
More uniform temperature: When air flows horizontally from side to side in an insulated greenhouse or bay and back into an adjacent bay of a house connected to a gutter, a mixture occurs from side to side and from floor to ceiling. We have instrumented a number of houses and rarely see more than 2 degrees F. difference between two points. Due to the constant movement of air, the heat supplied at one end is quickly conveyed to all parts of the greenhouse. Lamination is also eliminated.
Fewer problems of illness: Research has also shown that air movements of 50 to 100 feet per minute help maintain the night-time temperature of the leaves almost the same as that of the surrounding air. When the temperature of the leaves is allowed to cool much below the temperature of the air, the dew point is reached and condensation occurs, harboring pathogenic organisms. Radiant cooling on clear nights, especially in houses with non-infrared polyethylene, will cool the leaves of plants several degrees below the air temperature. HAF will reduce this difference.
Higher levels of carbon dioxide: During the day, photosynthesis depletes carbon dioxide in the air boundary layer next to the leaf. Moving air will replace this with fresh air with a higher carbon dioxide content. If additional carbon dioxide is added, a lower level is usually sufficient to achieve the same plant responses, for example, 1000 ppm instead of 1200 ppm.
Refreshing effect: During hot spring and fall days, solar radiation warms exposed leaf surfaces to 15 ° F above air temperature. This can cause burning of leaves, flowers or fruits. HAF will remove this excess heat and increase plant growth.
Provide adequate ventilation capacity:
For an efficient system, the total capacity of the fan should be about twice the floor area. For example, in a 30 ‘x 100’ greenhouse the fan capacity should be 30 ‘x 100’ x 2 = 6,000 cfm. If a large crop such as tomatoes is grown or there are hanging baskets, a slightly larger capacity is needed to overcome the extra turbulence created.
Low power fans reduce operating costs:
Small circulating fans from 1/10 to 1/15 HP work well to provide the necessary air movement. These are designed to move air with little resistance compared to exhaust fans that have blades designed to overcome the resistance of air moving through louvers or vents. The use of a permanent permanent capacitor motor saves up to a third of the electricity of the more common shaded pole motor.
If you have a hot air heating system, it is possible to use the heaters, if properly positioned, to replace one or more HAF fans. This could reduce installation costs, but in general, the heater blower motor is 1/3 or 1/2 hp and operating costs are significantly higher.
HAF fans are available with 12, 16, 18 and 20 inch diameter blades. Fan power and efficiency increase with increasing blade diameter. The design of the guard also affects the output.
The number of fans required can be calculated by dividing the required capacity by the power of a fan. In the above example of a 30 ‘x 100’ house, the required ventilation capacity is 6000 cfm. If you plan to use fans with a capacity of 1675 cm, 6000 cm divided by 1675 cfm / fan = 3.6 fans. To obtain the required capacity, use 4 fans.
The location of the fans is also important:
The first fan should be placed about 10 ‘to 15’ from an end wall to draw in the air that goes to the corner on the other side. The following fans should be located at a distance of 40 ‘to 50’ in order to keep the air mass moving. If the fans are too far apart, the air mass will be short-circuited to an adjacent airflow and create an airless cold spot.
In an autonomous greenhouse, the air is moved from one side to the other. This creates a horizontal and circular air pattern. In areas connected by gutters, it is more efficient to move air into a bay and return to an adjacent bay. This eliminates friction between the two opposing moving air masses.
Fans must be located near the center of the air mass to which they add energy. For greenhouses with crops on the ground or benchtop, a correct location at 7 or 8 ‘above the ground is good. In many greenhouses, a lattice or collar tie can be used as a support. For a greenhouse with hanging baskets, a location below or above the basket level provides the least energy. It also eliminates the drying of foliage with a direct flow of air. The fans must be placed under the energy curtain so that air can circulate at night so that the temperature is uniform.
The horizontal airflow fans in a tomato greenhouse are mounted above the braces so that air can still be moved once the crop has reached its maximum height. Photo: Vern Grubinger, Extension of the University of Vermont
The HAF system must operate 24 hours a day, except when the exhaust fans are running or the vents are open. A relay can be added to the circuit to turn off the fans when the ventilation system is activated. The operating costs of one of these small fans are about $ 0.25 / day at an electricity rate of 10 cents per kilowatt.
Frequent maintenance keeps the air moving:
Maintenance is also important to achieve high efficiency. The fans are frequently moved. They must be restrained by a support or chains to prevent them from moving. They should also be positioned to point directly to the opposite end wall to transfer as much energy as possible to the air mass.
The fan blade guard and motor housing absorb a considerable amount of dust in the air passing in front of them. It is important to wipe them several times a year.
The movement of the air mass can be observed by lighting a nontoxic smoke bomb behind one of the fans. Be sure to let the fans run for a few minutes before doing so to establish the horizontal pattern. Observe the movement of the smoke to make sure there is no short circuit. Also time the movement to get the flow. An incense stick is also a good tool to observe the movement of air in corners and around obstacles.
The benefits of HAF have existed for many years. Fine tuning your system will improve its performance and increase its efficiency.