Greenhouse Ventilation and Cooling
Forced-Air Ventilation in Greenhouses
Many greenhouse operations rely on forced-air ventilation fans to move air into and out of the greenhouse. When inside temperatures exceed the desired level, a thermostat opens the shutters and starts the exhaust fan(s). As the fans exhaust the heated air, a slight vacuum is created that draws in cooler outside air through vents or other openings. When the desired temperature has been reestablished, the thermostat shuts off the exhaust fans and closes the motorized shutters. The principle of forced-air ventilation is to create air flow through the greenhouse. As air flows through the plants in the greenhouse, it absorbs heat and increase in both temperature and humidity before exiting the greenhouse through the exhaust fans. Alternatively, fans can pull fresh air into a greenhouse. Although there is no special preference, most commercial greenhouse operations use fans to exhaust air with replacement air pulled through vents from the outside. Forced-air ventilation includes exhaust fans on one end of the greenhouse (Figure 6.6) and motorized louvered air inlet shutters (Figure 6.7), or vents, at the opposite end for air exchange.
Ventilation System Performance
Efficiency and performance are key to successfully ventilating greenhouses. A mechanical ventilation system that is not properly maintained performs poorly and costs more to operate as it consumes power inefficiently. Even though fans appear to be running correctly, the greenhouse may be subject to common environmental complaints such as cold drafts and stale air zones.
Static Pressure Difference
Static pressure difference is the driving force for air movement in a ventilation system. Air moves from regions of higher to lower static pressure so air enters or leaves the building because the interior static pressure is different from the outside static pressure. Likewise, air moves from room to room within a structure because of static pressure differences between the rooms. Static pressure is measured with a manometer and is indicated by changes in fluid level. In greenhouses, the static pressure difference between the ventilated space and the outside conditions is measured in inches of water. By maintaining a relatively constant static pressure difference between the inside and outside conditions, the speed of the air entering through the ventilation inlet will also be relatively constant.
Cooling Effectiveness of Forced-Air Ventilation
Cooling effectiveness of the air flow is reduced as it travels across a greenhouse, resulting in a temperature gradient between the air inlet and exhaust. Increasing the air flow rate or limiting the length of the greenhouse can control the rate of temperature and humidity rise. The distance from the inlet to the exhaust fan should not exceed 150 feet (46m) for single bay or 200 feet (60m) for multi-span greenhouses, unless a large air temperature rise (> 10–15°F, 5–8°C) is tolerable between the inlets and exhaust fans.
Greenhouse Ventilation System Capacity
The ventilation system capacity equals the sum of all individual fan capacities. For each type of fan in a ventilation system, one set of representative data may be used if the fans are all the same model. When there are differences in fan types due to manufacturer, motor, blades, maintenance, or suspected reliability, air speed measurements will need to be taken for each. Fans in locations where obstructions or wind effects are dominant features also will need to be evaluated separately. There is no need to measure airflow at each and every fan unless an unusual airflow imbalance is suspected.
Determining Ventilation Rate for a Greenhouse
To determine the ventilation requirement for a greenhouse under standard conditions use the equations below, where L and W represent the greenhouse length and width, respectively.
Exhaust Fans
Exhaust Fans Air Exchange Rates
Fan systems are designed to provide approximately one air change per minute for the growing area. Recommendations vary but generally 7 to 10 cfm per square foot is used as a design parameter. If thermal screens are used for summer shading, 7 cfm/sq ft is the preferable design parameter. The National Greenhouse Manufacturers Association (NGMA) recommendation of 8 cfm has an origin at the time when greenhouse eaves and gutters were typically 8 feet (2.4m) high.
Selecting Exhaust Fans for Forced-Air Ventilation
The capacity of the exhaust fan needs to be correlated and matched to the intake of air through louvers or vents Fan capacity is measured as the volume of air (cubic feet) moved per unit of time (minute). It is usually expressed as cubic feet per minute (cfm). The amount of air a fan moves depends on the blade diameter, blade shape, fan speed (revolutions per minute, rpm), motor horsepower, and the shape of the housing (See Table 6.1).
Exhast Fans Static Pressure Rating
Airflow rate (cfm) and static pressure are closely related for fans and ventilation systems. The air moving capacity of a fan (cfm) is directly affected by the system static pressure. As the resistance to airflow (static pressure) increases, the delivered airflow capacity decreases. Hence, a fan delivers more air against a low static pressure than against a high static pressure.
Exhaust Fans Static Pressure Rating
Air flow rate (cfm) and static pressure are closely related for fans and ventilation systems. The air moving capacity of a fan (cfm) is directly affected by the system static pressure. As the resistance to air flow (static pressure) increases, the delivered air flow capacity decreases. Hence, a fan delivers more air against a low static pressure than against a high static pressure. The fan output is recorded and indicated as cfm under no air resistance (free air) and as one or several static pressure values measured in inches of water column.
Determining the Number and Size of Exhaust Fans
Once the ventilation requirement of the greenhouse has been calculated (cfm) the next step is to select the number and size of fans that collectively equal or exceed the rate of air movement required based on the static pressure. The collective capacity of the fans should be at least equal to the rate of air removal required and should be rated to do so at a static pressure of at least 0.1 inch (30Pa).
Exhaust Fan Optimization
There are many types of exhaust fans on the market, so selection to achieve maximum efficiency at the lowest cost is important. The following are some important points to consider when selecting an exhaust fan. A larger-diameter fan with a smaller motor horsepower is more efficient. For example, a 48-inch diameter, 0.5 horsepower (HP) fan has an output of 12,983 cfm and uses 662 watts/hour. A 36-inch diameter, 1 HP fan has an output of 12,168 cfm and uses 1,193 watts/hour. If three phase electric power is available in the greenhouse, three-phase motors should be used because they’re cheaper and require less maintenance than single-phase motors.
Exhaust Fan Staging
Because ventilation rates will be much less during cool days than during warm days, ventilation systems must be operated in steps or stages to provide the proper amount of incoming air for good temperature control. There should be sufficient ventilation stages so that transitions from stage to stage do not result in large indoor temperature swings. The first stage of ventilation can be provided by a single fan and the second stage by another fan or a two-speed fan with thermostat control to provide the low-high ventilation rates. Either of these ventilation systems will provide the levels of ventilation needed to maintain the optimum growing environment inside the greenhouse when properly sized and installed.
Air Intake Shutters
When using exhaust fans, air intake vents are needed for air intake (Figure 6.8). Regardless the equipment of choice, the vent(s) on the opposing end of the greenhouse must work in conjunction with the exhaust fans. Comparable air inlet shutters are required at the opposite end of the greenhouse. Small exhaust fans may push open the shutters by the fan-generated air pressure. With larger shutters the shutters are motorized and synchronized to the exhaust fans through a control system. Air flows easily through motorized systems since the moving air does not have to hold the shutters open.
Ventilation Placement
Most ventilation systems are designed to release warm air at the top of the greenhouse with the cool air intake placed closer to ground level. In colder climates, this strategy may not work because cool air at crop height may damage or even freeze the plants.
Determing Vent Inlet Size
To provide adequate airflow and ventilation, the surface area of the vent openings should be at least 1.25 to 1.5 times the area of the fans or sized to provide an apparent velocity of 700 feet per minute (fpm). The cross-sectional area can be determined by dividing the air capacity of the fan in cfm by the inlet design velocity in fpm, which gives excellent mixing.
Thermostat Selection and Placement
Exhaust fans and vent motors are usually controlled by thermostats or preferably by a computerized climate control system. The control range of a thermostat should be from 45 to 90 degrees F (7 to 32°C). A smaller control range is not recommended because the fans will cycle on and off more often than is necessary. Select accurate thermostats that will withstand the greenhouse environment and maintain their calibration. Often these have a wide differential between the off and on position, sometimes as much as 6 to 8 degrees F (3.3 to 4.4°C).
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