Momentum in Greenhouse Air Movement

Transcription

1 Momentum in Greenhouse Air Movement White Paper Horizontal Air Flow has been in use in greenhouses consistently for over 50 years. In the early stages, air speed was the goal. Unfortunately, speed is difficult to keep consistent and hard to maintain. The true goal of any air distribution system in the greenhouse should be to homogenize the air and its constituents: temperature, humidity, CO2, and even pesticide fogs. Plants transpire more evenly and efficiently when the microclimate around the plant is removed. This paper will discuss the use of momentum-based fan systems to achieve better homogenization and better plant growth /

2 Momentum, not speed Horizontal Air Flow, or HAF, is an important tool for managing the greenhouse environment. Proper air circulation not only helps to even out the temperature throughout the greenhouse, but also is responsible for distributing humidity, aerosols, CO2 and pesticide sprays. Good air movement can help keep the foliage dry, helping to curb disease incidence. There is even data to suggest that proper air movement will speed the growth of plants by appropriately increasing transpiration, which allows the plants to increase their uptake of water and nutrients. Initially, the solution for improving air movement in greenhouses was to place or hang a number of fans throughout the greenhouse to circulate the air. A variety of fans and different spacings were used; from home box fans to unguarded spinning blades with a motor. As HAF gained acceptance, several manufacturers began to standardize the HAF fan and offer better fans with more control and lower electrical consumption. As recently as just a few years ago, HAF was thought of solely in terms of velocity moving a certain amount of air through the greenhouse, usually expressed in cubic feet per minute (cfm). In the past, the Ball RedBook cited the figure 2,000 cfm. Other sources called for movement as high as 100' (30 m) per second across the crop. Air velocity can be an important factor in mechanical ventilation, where the goal is to exchange the air volume in the greenhouse with air from the outside. However, when the air in a greenhouse is being moved internally, velocity is not a valid measure. This notion was often hard to calculate and difficult to measure, especially given the varied sizes and shapes of greenhouses used throughout the world. Different optimum numbers were given based on different crops. Originally, HAF was thought of as a way to mitigate the expense of heating, with these systems focused on temperature distribution. Over time, it was realized that there were other, often more important, benefits to plant growth other than just heat distribution. Today, momentum is replacing velocity as the desired goal in greenhouse HAF systems. The goal of a momentum-based system is to create a stable mass of air that slowly builds momentum as it travels through the greenhouse. The focus is on consistency and stability, rather than air speed. Often, HAF fans spinning at high rpm may move the air too fast, causing turbulence and uneven drying throughout a crop. High air speeds can also be responsible for undesirable stratification of the Momentum in Greenhouse Air Movement Dramm Corporation, pg. 2

3 air in a greenhouse. Fast-moving air can become a buffer that prevents venting through buoyancy (the effect of hot and/or humid air rising) in a gutter- or ridge-vented greenhouse. Fan Design Current systems utilize more aerodynamic fans than in the past. By shrouding the fan in a housing, more of the air is thrown forward and not out the sides or down onto the crop. This helps to build the stability of the air in a forward direction and reduces obstruction created from contrary air patterns. Basket fans, without a shroud, throw air in a much wider pattern, often impacting the crop below with varying direction and speed, reducing evenness. These wider patterns minimize forward momentum, as the air from the first fan in a series often conflicts with the air from the second fan, as the direction is non-linear. This results in unevenness and makes the second fan in series work harder to move air forward. Shrouded fans in series work together to carry the air in a forward direction, reducing the energy required, and creating a more even, stable airflow. Additionally, the use of shrouds creates an intake as well as an exhaust, something that unshrouded fans don t have. This intake can help keep the forward momentum going by pulling air in the desired direction. As a result of the increased distances and lower speed requirements of momentum-based systems, facilities need fewer fans and use less electricity. In addition to aerodynamic design, newer fans also make use of advancements in motor technology. While many greenhouse fans use vented motors, which can fail easily due to moisture and dust build up, some newer fans use completely sealed motors. In an environment where growing media and water are prevalent, sealed motors increase fan life as the bearings are protected from these elements. It is not uncommon for these sealed motors to last years without replacement. Sealed motor design where bearings are protected from elements. Momentum in Greenhouse Air Movement Dramm Corporation, pg. 3

4 Speed Regulation To help achieve better momentum with less gusting and less variability, speed regulation is an integral part of a momentum-based HAF system. High rpm fans often cause uneven drying in the crop due the spiraling of air created when the blades turn at high speed. By reducing the speed of the blades, this spiraling effect is reduced, eliminating its uneven effect on the crop. Speed regulation is easily achieved with the right combination of motor and controller. Controllers will either reduce voltage to the internal fan windings or can alter the frequency to the fan motor. Both methods will have the effect of adjusting the speed of the fan. Many speed controllers can be tied to a climate control computer to allow for climate conditions or time of day to influence the air speed. Speed & Turbulence vs Slow and Steady Just like the tortoise and the hare, slow and steady wins the race. Bursts of erratic speed causes instability in temperature and climate that can affect plant growth. A common conception is that plants need to be moving for the air flow system to be doing its job. While this will certainly help remove the microclimate, high speed, turbulent air flow is often inconsistent. This means that some crops will get more air movement than others, causing uneven transpiration. Since even growth is a goal in most growing facilities, high-speed systems can cause problems. In addition to uneven transpiration and drying, turbulent airflows can slow plant growth or even damage plants. Mechanically-Induced Stress (MIS) can reduce plant growth and flowering in many plants 1. Momentum based systems are gentler. A final issue with turbulence and air speed is that HAF fans can often be a contributor to the diseases that they seek to prevent. Powdery mildew is a common problem in both greenhouses and indoor growing facilities. This pathogen has a very specific life cycle. As powdery mildew grows its thin layers of mycelium on the surface of the affected plant tissue, spores are generated. As these growths dry down, the spores are released just like dry leaves in fall. More turbulent, forceful air movement can carry these spores further through the growing facility, infecting more of the crop. Slower, momentum-based, fans can reduce the spread of mildew spores as they create much less forceful air movement and do not tend to move the plant tissue very much. 1 Biddington, N.L. The Effects of Mechanically-Induced Stress in Plants a Review, Plant Growth Regulation, June 1986, Vol 4, pp Momentum in Greenhouse Air Movement Dramm Corporation, pg. 4

5 Site Specific Engineering Like most other greenhouse systems, HAF can be optimized through proper engineering. Different growing facilities, heating scenarios, cropping systems and locations all have different air movement needs and require proper design to work best. When selecting an HAF system, look for companies that will work with you to engineer the air flow for your growing layout to ensure that you have the right type of fan, the right quantity, the right spacing and the proper placement. By using a fan with a specific distance and width of throw, a grower can properly fit his HAF fans to their facility. A proper fit is important to avoid dead spots, and conflicts between fans resulting in eddies of swirling air, or static pressure from too much air moving in one direction. Once a stable flow of air with good momentum has been achieved, one can easily introduce and blend heat, humidity, and CO2. In a momentum-based system, HAF fans can remain running during venting. This more rapidly equalizes the temperature and humidity changes. Slower, momentum-based systems can even remain running during a ridge or gutter venting situation without fear of stratification, since these systems use slower moving fans that won t reduce or prevent convection. Vertical Fans vs Horizontal Fans Interest in vertical fans (VAF) has been increasing in recent years. In general, a vertical fan is one that moves air up and down rather than horizontally. Vertical fans come in many shapes and types, from standard ceiling fans to highly engineered systems. The main benefit to VAF is that they can homogenize the air vertically, removing temperature stratification from top to bottom. As heat rises, VAF fans work to move this heat from the peaks closer to the crops. Additionally VAF can create some movement at the crop level, helping with transpiration and the elimination of microclimates near the plants. However, the main failing of VAF in plant growth is the same as with speed-based fans. Often, these systems do not homogenize the climate across the entire growing area as well as they do vertically, near the fan. Additionally, conflict or buffers between fan patterns can cause unevenness across the crop. Momentum in Greenhouse Air Movement Dramm Corporation, pg. 5

6 Typically, these fans work well for certain times of the day. When juxtaposed with HAF systems to ensure lateral distribution of air, they can provide benefits. Optimal Climate for Growth The goal of any greenhouse or indoor growing facility, and the systems inside, is to optimize the climate so that the plants grow evenly, quickly and healthily. As one of these systems, horizontal airflow is important in helping to homogenize the environment as well as promote appropriate transpiration. As plants transpire, water vapor is expelled from the plant. Much of this moisture remains near the plant, increasing the relative humidity and vapor pressure near the plant. This vapor pressure reduces transpiration. Momentum-based HAF systems will gently remove the microclimate near the plant, helping decrease the vapor pressure near the plant, allowing more transpiration to occur. Combining even transpiration with even temperatures and even CO2 levels leads to healthier plants and improved growth. Momentum in Greenhouse Air Movement Dramm Corporation, pg. 6