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. Mechanical ventilation uses fans to exchange air with the outside. These fans are typically sized based on maximum cooling requirements and minimum ventilation requirements (for CO2 replenishment and/or moisture removal). Unlike natural ventilation, fans used for forced-air ventilation provide predictable quantities of air exchange and consistent directions of air flow. Fans can either push air into the greenhouse via positive displacement or they can pull air out of the greenhouse via negative displacement.
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. The two main functions of a ventilation system are to provide air exchange, which is simply fresh air in and stale air out, and air distribution, which is providing air movement throughout the entire structure. Sidewall fans provide air exchange, while air distribution is most influenced by the ventilation inlets. Static pressure difference links the fan and inlet performance.
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.
Cooling Effectiveness of Forced-Air Ventilation
The cooling effectiveness of the airflow 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.
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, airspeed measurements will need to be taken for each.
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, seven /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 or 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 (Table 6.1). The two most common measurements used to describe the characteristics of a fan are blade diameter and motor horsepower. These are useful measurements, but without performance characteristics (airflow and static pressure), these are only general indicators of fan capacity.
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.
Exhaust Fan Staging
Because ventilation rates will be much lower 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 fans can be provided by a single fan, the second stage by another fan, and the third stage by another fan as the temperature rises and shut off in reverse sequence as the temperature falls.
Air Intake Shutters
When using exhaust fans, air intake vents are needed for air intake (Figure 6.9). Regardless of 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. The air intake needs to be well above the height of the crops during spring, fall, and winter conditions to allow the cold, fresh air to mix with warmer greenhouse air before coming in contact with 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 per 1,000 CFM (0.14 m2 per 0.47 m3/sec) 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. Following is an example of a suggested procedure for determining the appropriate size of a ventilation inlet:
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 (7–32°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. Usually these have a wide differential between the off and on position, sometimes as much as 6 to 8 degrees F (3.3–4.4°C).
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