Managing the Greenhouse Environment for 
Ventilation and Cooling

William J. Roberts
Director CCEA, Rutgers University
New Brunswick, NJ 

            Many of the principles for  air movement for heating systems also apply to ventilation systems. The National Greenhouse Manufacturing Association has developed excellent standards for ventilating and cooling greenhouses.¹  These include recommendations and designs affecting site elevation, sunlight intensity, orientation and shape of the greenhouse and crops being grown. The following is a discussion of systems and requirements.

            Greenhouse ventilation is required to control temperature and moisture levels and provide CO₂ for good crop production.  There are two basic ventilation systems used in greenhouse production systems, natural and mechanical ventilation systems. Natural ventilation depends upon normal air movement created by wind pressures or by gradients induced by differences in air temperatures between the growing area and outside the greenhouse. Mechanical ventilation is air movement created by fans that bring air into the growing area through controlled entrances built into the greenhouse area and exhaust it through the fan assembly. The ability to change the size of inlets is important to the design of good mechanical ventilation systems. Fan ventilation is normally controlled by thermostats and in some cases by humidity-sensing devices when relative humidity is the controlling factor for disease control.

NATURAL VENTILATION

            Natural ventilation is driven by temperature differences or wind conditions. Natural ventilation occurs when there is a temperature difference between the inside and the outside of the greenhouse and a vent is opened to allow the warmer air to leave and cooler replacement air to enter. The greatest potential for natural ventilation is during the winter, when the temperature difference between inside the greenhouse and outside is the greatest. Unfortunately, this occurs when the need for ventilation is the least. On excessively hot summer days, the outside temperature may be only slightly cooler than the inside temperature. The ventilation potential is practically nonexistent when the need is the greatest. Adequate ventilation during warm and hot summer periods must be wind-driven and is often site-specific. Areas of naturally occurring breezes provide the best opportunities for warm weather ventilation.

            Naturally occurring breezes with proper greenhouse orientation can provide excellent ventilation at some sites. The wind in some areas is often unpredictable, however, and adequate temperature control is very difficult to achieve. Knowing the meteorological information about the proposed site is essential in designing a natural ventilation system.

            Natural ventilation system designs include roll-up sides, either hand or automatically operated, and ridge vents constructed as an integral part of the greenhouse structure. Although difficult to install, ridge vents in polyethylene-glazed structures can provide good options for natural ventilation. In gutter-connected or ridge-and-furrow greenhouses, ridge vents perform better than the vents that open at the gutter. Although the gutter units are easier to recover and construct, they do not perform as well as ridge systems. Time required for attention each day and the loss of control, particularly during cold weather, are the most often mentioned complaints of natural ventilation systems using roll-up sides. Most glass greenhouses are ventilated naturally using ridge and side ventilators. These are usually automated systems, but are still limited by the factors listed above.
New Jersey Agricultural Experiment Station Paper # P03130-07-99

            Several newer greenhouse designs for warmer climates feature greenhouse structures with no glazing. These are designed with retractable thermal screens and provide opportunity for excellent environmental control during warmer weather. Site selection is important for heating considerations when growing throughout the year. Some growers use them for increasing growing space and hardening off areas in the spring.  Another design features articulating roof sections where the entire roof opens giving more than 90% opening.  One design hinges at the gutter and opens at the ridge.  The other design hinges at one gutter and the ridge and opens at the gutter.  Figure 1 illustrates these two types of structures.  These are superior to the sliding thermal screen type because they have all the attributes of conventional greenhouses.  When the roof is closed sunlight enters as it would in a normal greenhouse.  When the roof is open natural light reaches the plants without having to pass through the glazing.  Orientation of North South is important to ensure that the shadow pattern moves throughout the day.

 MECHANICAL VENTILATION

             Fan ventilation systems with properly designed inlets can provide excellent temperature control in all seasons. The most desirable feature is the ability to easily automate the entire system. This has become increasingly true with the use of computer based control. This feature is especially useful to growers with other responsibilities who may be away from the greenhouse during the day and who have difficulty obtaining labor on the weekends. The negative aspects of mechanical ventilation systems are the higher installation and operating costs.

           Fan systems are designed to provide approximately one air change per minute for the growing area. Recommendations vary but generally 7-10 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. It is generally desirable to provide this ventilation capacity with two fans, unless the greenhouse is very small and costs for installation would become too high. The use of multiple fans provides an easy opportunity for using more than one ventilation stage, a feature very desirable in cooler times of the growing season.

 The design for a 30-x 96-ft greenhouse would be as follows. 

             In any ventilation system the size and location of the inlets are the most important design consideration. Air entering the greenhouse is always cooler than the inside temperature during colder weather. It is important to obtain proper mixing of the inlet air with the ambient greenhouse air, so that local cold spots or unequal temperatures are not experienced throughout the growing area.  Figure 2 illustrates the action of air moving through a restricted opening and the resultant distribution pattern. The high-velocity air moving through the opening causes significant mixing of the cold incoming air with the ambient greenhouse air. It is similar to using a jet of water coming from a hose to mix the liquid in a barrel. Another similarity is the human nose. We exhale CO₂ from our lungs and inhale O₂. The reason we do not inhale the breath we just exhaled is because of the mixing action of the tiny jets of air created by our lungs when we exhale. The action of these jets mixes the CO₂ with the ambient air so that when we inhale we get a proper mixture of air.

            In ventilation systems the location of the inlets is of paramount importance. It is desirable to keep the length of air travel to approximately 100 feet in free-standing houses. The upper limit for gutter-connected greenhouses appears to be 200 feet. Fans are usually mounted in one end of the house and air inlets on the other end.

Fans should be provided with gravity shutters and safety wire screens and have the fan motors protected locally with proper electrical protection and an on-off switch to protect workers when servicing the fans. Inlet shutters should be motorized. Gravity-type shutters have been used, but are subject to wind action in adverse weather and are not suitable for winter operation. 

        Inlets should be sized to provide an apparent velocity of 700 feet per minute or 1.4 square feet of inlet per 1000 CFM of installed fan capacity. The cross-sectional area can be determined by dividing the air capacity of the fan in cfm by the inlet pre-determined 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

Figure 2.  Diagram illustrates the mixing  action which occurs
when cold incoming air enters a greenhouse inlet through a 
controlled window vent opening.

            The example cited earlier, a 30-x 96- foot greenhouse with two 15,000 cfm fans would require the following inlet area. Area = cfm/velocity Area = 30,000/700 = 43 square feet Area = 20,000/700 = 29 square feet Two 48-by 48-inch and one 42-by 42-inch motorized shutter would provide 44 square feet of opening

            Motorized shutters can be a problem during the colder part of the year. The inlets direct a large volume of air to the crop directly in front of the opening and can cause reduced temperatures at that location. If the velocity of air moving through the shutter is low, then the cold air tends to settle without mixing and move across the greenhouse to the fan and be exhausted, having had no impact on the control thermostat located usually at the 6-ft level. The fan continues  to operate because the thermostat

cannot sense the cold temperatures at the floor level. It would be desirable to open the shutters in stages to match the number of fans operating. Because of this, continuous window vents with openings that can be regulated are very popular. The manufacturer often provides continuous aluminum extrusions that serve as hinges, making the windows essentially maintenance free. They are often glazed with acrylic or polycarbonate panels. 

            In the example, the design calls for six fans. A suggested control strategy would be to use three stages. If the fans were aligned along one wall, fan number 3 could operate as stage number 1. Fans 1 and 6 could be turned on for the second stage, and fans 2, 4 and 5 could be turned on for the final stage of ventilation. The table indicates the three fan stages, the ventilation volume being delivered and the window opening required to provide a velocity of 700 fpm through the opening and good mixing of the incoming air. Computer-based systems provide excellent control by staging the inlet window opening depending upon the number of fans operating, based on desired temperature settings recorded in the computer program. 

COMBINATION HEATING AND PARTIAL VENTILATION SYSTEM
Some growers have had good success with a design that provides both heating and partial ventilation maximizing the use of the system, Figure 3.

Figure 3. A system designed many years ago and used successfully by both flower and vegetable growers who were trying to eliminate the pollution effects caused by heating units located within the greenhouse.  

 The purpose of this type of system is to minimize the effect of the fans operating at the same time as the hot air heaters. Separated combustion units have eliminated the problem of working the heating system and fan system simultaneously because of the separate air paths for the combustion air and the greenhouse air being heated by passing through the heat exchanger.

A horizontally fired unit is used, which is connected directly to the polyethylene heating tube located along the exterior wall of the greenhouse.  Directly above the furnace is a plywood chamber, approximately the same size as the furnace.  This chamber has one inlet from the greenhouse and one from the outside, each controlled by a motorized shutter. 

            A horizontally fired unit is used, which is connected directly to the polyethylene heating tube located along the exterior wall of the greenhouse.  Directly above the furnace is a plywood chamber, approximately the same size as the furnace.  This chamber has one inlet from the greenhouse and one from the outside, each controlled by a motorized shutter.

 ENVIRONMENTAL CONTROL
            Controls are an important part of any heating and ventilating system. Capillary bulb-type thermostats are the most durable for greenhouse use. Home-use thermostats are usually more accurate but are also more subject to deterioration and malfunctioning caused by the greenhouse atmosphere. Mercury-type thermostats are often affected by vibration, which occurs in strong winds, and should not be attached to the greenhouse structure. Aspiration, or passing air over thermostats or computer  sensors, is mandatory for good environmental control. Figure 4 indicates a method to construct an aspirated chamber containing a small blower or fan that draws air over the thermostat sensing units of the thermostats. Figure 5 indicates test results obtained by simply blowing air on a traditional capillary bulb thermostat.  This can produce increased productivity by allowing the grower to more closely mange the crop based on the best known day and night growing temperatures.  Totaling all the time that the air temperature is over the set point will give an idea of the energy required for heating being wasted with a thermostat that is not aspirated.  The benefit to the plants is also important.

 Figure 4.  Wooden housing for aspirated thermostats with a small fan mounted in one end drawing air through a screened inlet causing it to pass over the capillary bulb of  the thermostats.

Figure 5 illustrates the effect of air moving swiftly over a conventional capillary bulb thermostat.

            The diagram indicates the variability of about 8^oF between the on-off cycle of the heater.  At the point the thermostat was aspirated this variability decreased to about 2^oF.  After the daytime hours the fan was disconnected and the 8^oF temperature difference between on and off was again observed.

            Sophisticated environmental control units now being marketed have the distinct advantage of providing temperature information throughout the day.  The data-acquisition feature of these computer-based systems is the most attractive aspect for the grower.  They provide various stages of heating and ventilation control for time of day applications, can integrate light sensing equipment into the system and control the operation of the thermal screen.

            Computer-based systems should be used only to control a well-engineered heating and ventilating system.  A heating or ventilation system that is poorly designed cannot be improved simply by installing a better control system.  The control system works best with a properly engineered heating and ventilation system.

 SCREENING
            Screens have been used for insect control in dwellings and the work place for many years. Insect screens limit air movement and provide an engineering challenge to exclude insects and not decrease the efficacy of the installed mechanical ventilation system.  Fans used for greenhouse cooling are typically of low pressure design with a normal operating range up to 0.125 inches of water static pressure. These fans are designed for moving up to 20,000 CFM per horsepower at low static pressures but are unable to move air at higher pressure differences. Air tends to move off the blades at the tip of the blade. As the static pressure between inside and outside the greenhouse increases the air has a tendency to return to the greenhouse through the center of the fan. This is why higher pressure fans have the characteristic large hub to eliminate this condition and are able to perform at higher static pressures.

             Dr. James Bethke of the University of California and Dr. James Baker of North Carolina State University have determined the size of screening which various insects can penetrate.  Several publications describe their research and are listed in the bibliography. Thrips and white fly are common insects which greenhouse vegetable growers would like to exclude.

             Table 1 is a summary of some research work and lists size of aperture of screening and the insects which can be excluded. 

 TABLE 1

            Insect Pests                              microns                        inches               mesh

           ____________________________________________________________

             leafminers                                  640                            0.025                 40

             whiteflies                                   462                            0.018                 52

             aphids                                       340                            0.013                 78

             flower thrips                              192                            0.0075              132

Unfortunately screening is currently limited to mechanically ventilated greenhouses at this house.  Naturally ventilated greenhouses present special problems for screening. Results of tests conducted in the Netherlands on two glasshouses growing greenhouse vegetables illustrated the difficulty of screening naturally ventilated greenhouses. Both of these houses were 40 feet wide with large 5' wide ridge vents. One greenhouse was equipped with a screen material to limit pests entering the greenhouse through the vents and the other was not screened. The temperature on clear days in the screened house was on the average 9^oF above the unscreened greenhouse. To overcome this problem Dutch engineers have designed accordion type units to fit into the vents to increase the screen surface area. These increase the screen area without limiting ventilation but tend to be expensive and difficult to maintain.

             Air flow characteristics of fans are determined by their design  Propeller fans used for ventilating greenhouses have low pressure characteristics and move large quantities of air at low static pressures of approximately 0.1" to 0.15" inches of water. The design static pressure used for most systems is 0.10".

             Using this criteria the following design procedure seems appropriate. Approximately 30% to 50% of the total pressure drop allowable which the fan will experience should be attributed to the screening. This leaves the remaining 50% to 70% available for the normal pressure losses in the total ventilation system including, automatic fan shutters and the window vent openings.  This allowance also provides for insect and debris buildup on the screening before cleaning is required. In practice this design procedure has proven to be effective and efficient with no adverse affect on the ventilation system. As indicated earlier, the normal design calls for 8 - 10 cfm of ventilation for each square foot of greenhouse area.

             Dr. James Baker and Mr. Ed Shearin of North Carolina State University have developed a computer model, programmed in QBasic to help designers calculate the area of screening required for a particular screening material being selected by the grower to exclude pests from their operation.  This model requires for input parameters, the size of the greenhouse, the number, type and manufacturer of the fans being used, the static pressure of the building when all the fans are operating and all the vents are open, the physical characteristics of the window vents and the screening being selected. The program's output is the required area of screen for several different materials.

             The design procedure using the computer program for a vegetable production greenhouse which is 30' by 84' feet, equipped with insect screening would be as follows. Greenhouse data, the type and number of fans and area of inlet would be entered into the program. Table 2 indicates the area of screening required for four types of screening compared to the pressure drop through the screening.

 TABLE 2 

*Reference to commercial products or trade names is made with the understanding that no discrimination or endorsement is intended or implied.

             For instance, the no thrips material would require 254 square feet of screening if an allowable pressure drop of 0.04" was the design parameter for the screen. This would leave .06" pressure drop available for the rest of the ventilation system. By allowing a pressure drop of .06" through the screen only 181 square feet of screening is required with only 0.04" available for the rest of the system including insect and debris buildup on the screening.

             By selecting another material such as Econet T, 103 square feet of screening and 76 square feet of screening would be required under the same conditions as stated above.       

            This information can be used in several ways.  For a typical free-standing greenhouse which uses fans on one end and motorized shutters on the other Figure 6 indicates a means of calculating the area needed for enclosures to be built over the motorized shutter ventilation inlets. This design leaves the integrity of the motorized shutter in tact so that no changes are required in the operation of the ventilation system throughout the year. If screening is installed as a substitute for the glazing in the end of the greenhouse it is easy to have large areas of screening and a minimum of effect on the ventilation system. However, provisions for closing the end of the greenhouse by either covering over the screen or reglazing with the covering is required for the greater part of the operating season. 

            A  30'  by 84' greenhouse equipped with a continuous vent as indicated in Figure 3 for the same  conditions  would  require  a similar screened inlet area.  This  type  of  ventilation  window permits a grower to design for a lower pressure drop through the screening because the ventilation inlet  area  of  the  continuous vent window is usually much larger than  for  a  similar  greenhouse equipped with two motorized shutters.  If allowing a pressure drop of .04”  then 103 square  feet  of screening  is required. The greenhouse is 30 feet wide so the area of screening required per foot of width of the greenhouse is  approximately 40 inches.   If the screening material is available  in  48  inch widths  then  the  pressure drop through the system would be about  0.03"  which  is predicted for a 48 inch wide screen. 

        Growers who have installed screening are excited and optimistic about its performance. We have been testing several screens for many years.  One  grower reduced total sprays for white fly protection from 13 to 3 over a two year period and used only 8 spot sprays on the locations indicated by the yellow sticky indicator cards. Although there are certainly yearly differences, these data  indicate the effectiveness of the screening. 

REFERENCES

1."Standard  Design Loads in Greenhouse Structures, Ventilation and Cooling Greenhouses  Greenhouse Heat Loss”   National  Greenhouse Manufacturing Association,  1981.

2. "The  Greenhouse  Climate Control Handbook, " 1993 Acme  Engineering  and Manufacturing Corp., Muskogee, OK., St.   Joseph, MI.

3. Roberts, W. J. " Environmental Control of Greenhouses, E213, 1997.  Extension publication of Rutgers University

*Reference to commercial products or trade names is made with the understanding that no discrimination or endorsement is intended or implied

Proceedings - Table of Contents