Chapter 5

Greenhouse Heating

Air Distribution in Greenhouses

Air distribution within the greenhouse is important if near-optimal levels of temperature and humidity and normal or above-normal levels of carbon dioxide are to be maintained near the growing plants. Since there is very little natural air movement in a closed greenhouse, temperature variations between the roof and plant surfaces will exist as warm, lighter air rises toward the roof while cool, heavier air is pushed toward the growing plants. Moving or stirring the air with fans reduces these temperature variations by distributing the heat more uniformly throughout the greenhouse. Uniform heat distribution causes the temperature near the growing plants to be more stable, which reduces growing stress. Uniform distribution also reduces heating costs, because warm air is placed near the plants where it is needed and not along the greenhouse roof. Moving air through the plant canopy also keeps the leaf surfaces dryer, reducing the risk of disease. The two most commonly used methods for circulating air in a greenhouse are horizontal air flow fans and polytube systems.

Horizontal Air Flow Fans

HHorizontal air flow (HAF) fans, also called mixing fans or circulation fans, are used to mix the air mass inside greenhouses to provide a more uniform environment (Figure 5.9). For example, radiant cooling on clear nights, especially in non-infrared poly covered houses, will cool plant leaves several degrees below air temperature. Horizontal air flow fans will reduce this difference. Besides the obvious advantage of more uniform temperature within the greenhouse, HAF systems can reduce the incidence of foliar diseases. The moving air removes moisture from the plant canopy, resulting in a drier microclimate.

Placement of Horizontal Air Flow Fans

A well-designed HAF system consists of fans moving air the length of each greenhouse (Figure 5.10). In a free-standing greenhouse, the fans should be arranged in such a way that air flows are directed down one bay parallel to the ground and aligned with similar fans pushing air the opposite direction down an adjacent bay. The first fan should be placed about 10 to 15 feet (3–4.6m) from one end wall to pick up the air that is coming around the corner from the other side.

Shrouded versus Unshrouded Fans

One key HAF feature is fan shrouding which is nothing more than simple as a sheet metal ring, or cylinder, around the fan blades. The closer the shroud is to the blade tips, the more effective and efficient the fan is at moving air along the axis of the fan. If only short crops such as bedding plants and plugs are being grown, the unshrouded fans can work quite well.

Horizontal Air Flow Fans Capacity

Keeping the air mass moving at a velocity of 50 to 100 feet per minute requires enough energy to overcome turbulence and friction losses. A rule of thumb based on greenhouse trials is one-quarter of the greenhouse volume per minute of fan capacity for each cubic foot of greenhouse volume.

Poly Convection Tubbing

The main function of the perforated polytube is to convey and distribute air from a high-velocity fan at one end of the heated or ventilated space through the pressurized tube and into the house as uniformly as possible. The polyethylene tube is installed along the length of the greenhouse above plant height or below the benches and is sealed at the distant end (Figure 5.11). This system ensures that heat is distributed from one end of the greenhouse to the other as well as moving warm air from the greenhouse peak down to plants. These same convection tubes can be used for forced-air ventilation and cooling in summer, eliminating the need for manual side and top vents. When neither heating nor cooling is required, many growers keep the fan in the unit heater running without heat so that air from the greenhouse is continuously circulated through the tube.

Heating with Polytubes

Heated air can be injected into the polytube for mixing and distribution. However, connecting a polytube directly to any unit heater can result in a burned-out heater or a melted polytube due to restricted air flow. One approach is a combination fan and heater unit that is located between the inlet shutter and polytube.

Calculating Size and Number of Holes for Polytubes

Proper performance in a greenhouse requires polytubes sized and punched (perforated) for a given greenhouse length and fan capacity. Shortened tubes or tubes with too small or too few holes restrict air flow which may overload and damage certain type fans or cause inadequate air movement in the house. Conversely, excessively large holes or long tubes on undersized fans will not give satisfactory air flow. This poor match is sometimes shown by the tube contracting and flapping near the fan.

Tube Suspension

For optimum air distribution the tube should be suspended about 7 to 9 feet (2.1–2.6m) above the floor with the. The tube must be hung straight without bends or twists. Recommended tube suspension with hoops is approximately 8 feet (2.4m) apart, which will be adequate for polytube weight and sufficient to dampen-out snapping action when the tube is initially inflated.

Vertical Air Flow Fans

The vertical air flow (VAF) fan, manufactured by Vostermans, produces an air current that is forced outward and downward along the roof and walls of the greenhouse, and then is pulled upward through the crop (Figure 5.12). Using this type of fan can lead to a better and more uniform climate and it can also lead to energy savings. The V-Flo Fan offers growers the opportunity to reduce the negative impacts of humidity in a simple and energy efficient way, and it is also easy to mount in a greenhouse and easy to maintain.

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