Improve HAF System Efficiency

The HAF (Horizontal Air Flow) concept utilizes the principle that moving air in a coherent horizontal pattern needs only enough energy to overcome turbulence and friction loss to keep it moving. In other words: You just have to “kick it along.”

Besides the obvious advantage of more uniform temperature within the greenhouse, HAF systems reduce the incidence of foliar diseases. The moving air removes moisture from the plant canopy resulting in a drier microclimate. HAF also replaces air with a higher carbon dioxide level at the leaf surface.

As I visit growers, I frequently see systems that could be improved. Common installation errors include too little fan capacity, poor spacing and location of the fans and poor maintenance. Many systems could be improved with just a little work.

Provide adequate fan capacity

For an efficient system, the total fan capacity should be about two times the floor area. For example, in a 30-foot-by-100-foot greenhouse, the fan capacity should be 30 feet x 100 feet x 2 = 6000 cfm. If a tall crop such as tomatoes are grown or if there are hanging baskets, a slightly greater capacity is needed to overcome the additional turbulence created.

Low horsepower fans keep operating cost down

Small, 1/10 to 1/15 horsepower circulating fans work well in providing the air movement needed. These are made to move air with little resistance as compared to exhaust fans that have blades designed to overcome the resistance of air moving through louvers or vents. Select fans that have a high Ventilating Efficiency Ratio (VER) measured as cfm of output per watt of electricity input.

HAF fans are available with 12-inch, 16-inch, 18-inch and 20-inch diameter blades. Fan output and efficiency increase with an increase in blade diameter. Guard design also affects output. Fans with shrouds can provide more directional flow.

Fan location is also important

In a freestanding greenhouse, the air is moved down one side and back the other. This creates a circular horizontal air pattern. In gutter-connected ranges, it is more efficient to move the air down one bay and back in an adjacent bay. This reduces the friction between the two, opposite moving air masses.

The first fan should be placed about 10 to 15 feet from one endwall to pick up the air that is coming around the corner from the other side. Subsequent fans should be located 40 to 50 feet apart to keep the air mass moving. If fans are spaced too far apart, the air mass will short circuit to an adjacent air stream and create a dead air cold spot. You can check airflow with a smoke bomb. An incense stick is also a good tool that can be used to observe air movement in corners and around obstructions.

The fans work best when located near the center of the air mass to which they are adding energy. For greenhouses with floor or bench crops, a location 7 or 8 feet above the floor is good. For a greenhouse with hanging baskets, a location below or above the basket level provides the energy with least resistance. It also eliminates drying of the foliage from a direct air stream. Fans should be located below the energy curtain so that air movement can be provided at night to keep temperature uniform.

The HAF system should be operated 24 hours a day except when the exhaust fans are on or when the vents are open. A relay can be added to the circuit to switch the fans off when the ventilation system activates. Operating cost for one of these small fans is about $0.25 per day at 10 cent/kilowatt electricity rate.

Frequent maintenance keeps the air moving

Maintenance is important in achieving high efficiency. Fans frequently get moved out of position. They should be restrained by a bracket or chains to keep them from moving. Locate them to point horizontally towards the opposite endwall to transfer maximum energy to the air mass.

Fan blade guards and motor casings pick up considerable dust from the air moving past them. It is important to clean these several times a year.

The benefits of HAF have been around for many years. Fine-tuning your system will improve its performance and increase its efficiency.

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