Chapter 2

Greenhouse Heating

Horizontal Air Flow (HAF) Fans for Greenhouses

Similarly to the poly tubes, the horizontal airflow fans (HAF) can be used to distribute heat in the greenhouse. HAF systems can be used to bring warm air down from the upper level of the gable and to provide uniform temperature in the plant zone in addition to reducing carbon dioxide stratification and reducing condensation formation on the glazing. HAF systems can also reduce the incidence of foliar diseases by removing moisture from the plant canopy, resulting in a drier microclimate. HAF systems can have a cooling effect on plants on hot summer days, resulting in lower air temperatures on the exposed leaf surfaces. The air movement created by HAF systems gives more uniform pesticide coverage and better penetration into the foliage.

Horizontal Air Flow (HAF) Fans Capacity

A rule of thumb based on greenhouse trials is 2 cubic feet/minute (cfm) of fan capacity for each square foot of floor area. For example, in a 30 x 100 foot greenhouse the total cfm fan capacity needed is 6,000 cfm (30 x 100 x 2 cfm/sq ft). If you installed fans having 1,600 cfm output, four fans would be needed (6000 cfm ÷ 1600 cfm/fan = 4 fans).

Types of Horizontal Air Flow (HAF) Fans

HAF fans are available with 12, 16, 18, and 20 inch diameter blades. Fan output and efficiency increase with increase in blade diameter. HAF systems normally push the air about 30 times their diameter. A 20 inch fan will move the air about 50 feet. Resistance from hanging baskets or tall plants will reduce airflow some. Installing fans with shrouds will extend the distance the air will be pushed. The shroud can be a sheet metal band or closely spaced wires on the fan guard in the area of the fan blades. This reduces the spread of the air stream as it leaves the blades.

Placement of Horizontal Air Flow (HAF) Fans

In a freestanding greenhouse, the fans should be arranged in such a way that air flows are directed along the length of the greenhouse and parallel to the ground (See Figure 2.13).  They are arranged such that the airflow is directed by one row of the fans along the length of the greenhouse down one side to the opposite end and then back along the other side by another row of fans. This creates a circular horizontal air pattern. In gutter-connected greenhouses, it is more efficient to move the air down one bay and back in an adjacent bay. This eliminates the friction between the two, opposite moving air masses. In each greenhouse section or bay, fans point in one (longitudinal) direction on one side and in the opposite direction on the other.

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