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  1. 1. Greenhouse Cooling Concepts
  2. 2. Why Cool Greenhouses? <ul><li>Most places have a summer climate that requires greenhouse cooling – even Vermont </li></ul><ul><li>A greenhouse must be capable of cooling in the winter and summer </li></ul><ul><li>With passive cooling greenhouses can reach temperatures of 20°F greater (or more) than the air temperature </li></ul>
  3. 3. Too HOT!! <ul><li>Loss of stem strength </li></ul><ul><li>Reduction of flower size </li></ul><ul><li>Delay of flowering </li></ul><ul><li>Bud abortion </li></ul>
  4. 5. SUMMER COOLING <ul><li>Active </li></ul><ul><ul><li>Fan-and-pad cooling </li></ul></ul><ul><ul><li>Fog </li></ul></ul><ul><li>Passive </li></ul><ul><ul><li>ventilators </li></ul></ul><ul><li>HAF fans </li></ul><ul><li>Convection tube cooling </li></ul>WINTER COOLING
  5. 6. Passive Cooling <ul><li>Percent of roof space ventilated has increased over time with design improvements </li></ul><ul><li>Success is very crop specific </li></ul><ul><li>Even fully retractable roof designs are now available </li></ul><ul><li>Cheaper to operate than active cooling systems, but construction costs aren’t less expensive </li></ul>
  6. 7. Acta Hortic. 443: 31-38
  7. 10. Effectiveness of Summer Cooling <ul><li>Fan and pad cooling can lower to 80% of the difference between the wet and dry bulb temperatures </li></ul><ul><li>Fog cooling can lower the temperature by nearly all of the difference </li></ul><ul><li>Both of these systems are most effective at low humidity </li></ul>
  8. 11. Evaporative Cooling <ul><li>Works well in most climates; where might it not function effectively? </li></ul><ul><li>Based on heat absorption during the evaporation of water </li></ul><ul><li>Relatively inexpensive compared to other types of cooling </li></ul>
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  10. 14. Pad and Fan Cooling <ul><li>Available for almost 50 years </li></ul><ul><li>Most common system for summer cooling </li></ul><ul><li>Originally the pad was composed of wood shreds </li></ul><ul><li>Today it is composed of cellulose </li></ul><ul><li>Exhaust fans are placed on the opposite wall </li></ul>
  11. 17. Active Summer Cooling System Calculations (Basics) <ul><li>Fan-and-Pad system </li></ul><ul><li>Rate at which warm air must be removed from the greenhouse </li></ul><ul><ul><li>Types of pads used </li></ul></ul><ul><ul><li>Fan placement </li></ul></ul><ul><ul><li>Path of the airstream </li></ul></ul>
  12. 18. Pad Types and Specifications <ul><li>Excelsior pads (wood fiber) had to be framed in wire mesh for support; required annual replacement </li></ul><ul><li>Cross-fluted cellulose is the most popular today, can last up to 10 years </li></ul><ul><ul><li>Should be kept from heavy rains </li></ul></ul><ul><ul><li>Only move if dry </li></ul></ul><ul><li>Other types of pads include aluminum fiber, glass fiber, and plastic fiber </li></ul><ul><li>Why are pads thick? and why do they have a cross fluted design? </li></ul>
  13. 20. Cross-fluted cellulose pads <ul><li>Come in height increments of ft </li></ul><ul><li>Available in 2, 4, 6, and 12 inches thick </li></ul><ul><li>A 4-inch-thick pad will handle an air intake of 250 cfm/ft 2 ; a six inch 350 cfm/ft 2 </li></ul><ul><li>By way of comparison excelsior pads can only support an airflow rate of 150 cfm/ft 2 </li></ul><ul><li>You want vents over the exterior of the pads to seal the external air source off when active cooling isn’t needed </li></ul>
  14. 22. More Details <ul><li>Water must be delivered to a 4-inch pad at the rate of 0.5 gpm per linear foot of pad </li></ul><ul><li>For a 6-inch thick cellulose pad a 0.75 gpm per linear foot is required </li></ul><ul><li>Longest recommended delivery pipe is 60ft for the 4 inch system and 50 ft for the 6 inch system1/8 inch holes every three inches are required for both systems </li></ul><ul><li>Holes point upward and release water into an impingement cover – water drips down onto a distribution pad </li></ul>
  15. 23. Rate of Air Exchange <ul><li>Measured in cfm (cubic feet per minute) </li></ul><ul><li>NGMA uses 8 cfm/ft2 of floor space as a standard </li></ul><ul><li>In warmer climates 1 volume per minute recommended roughly 11-17 cfm/ft2 </li></ul><ul><li>As elevation increases so must the rate of air removal.  Why? </li></ul>
  16. 24. Other factors: <ul><li>Light Intensity </li></ul><ul><li>Temperature rise across the greenhouse </li></ul><ul><li>Pad-to-fan distance </li></ul>
  17. 25. Calculating Air Removal Rate <ul><li>Calculate the standard cfm = Greenhouse area X 8 cfm/ft </li></ul><ul><li>Correct for the standard rate of air removal using the larger of F house or (F vel ) </li></ul><ul><li>F house = F elev X F light X F temp </li></ul><ul><li>Total cfm = standard cfm X (F house or F vel ) </li></ul><ul><li>Select the fans to install </li></ul>
  18. 27. Exhaust Fan Placement Rules <ul><li>Should not be more than 25 ft apart </li></ul><ul><li>If the end of the greenhouse is 60 ft wide you will need at least 3 fans </li></ul><ul><li>Fans should be evenly spaced at plant height </li></ul><ul><li>Place fans on leeward side of the greenhouse </li></ul><ul><li>Rules change with multiple houses </li></ul><ul><li>Protect fans from weather and provide screening on both sides to protect workers, visitors, and wildlife </li></ul><ul><li>Air movement can cause special problems in larger houses </li></ul>
  19. 31. Fog Cooling <ul><li>20-year-old technology </li></ul><ul><li>High pressure water delivery system generates a fog of very fine water particles (<10 microns) </li></ul><ul><li>Drops evaporate in the air </li></ul><ul><li>Even dispersal of the particles means cooling of throughout the greenhouse </li></ul>
  20. 32. Fog Cooling <ul><li>Initial cost usually close to that of fan and pad cooling systems (water quality determining factor) </li></ul><ul><li>Operating cost less than fan-and-pad cooling </li></ul><ul><li>Dispersion of water particles in the greenhouse air where they extract heat from the air as they evaporate. </li></ul><ul><li>Rate of cooling increases proportionately as water droplet size decreases. </li></ul><ul><li>Systems allow near 100 percent cooling efficiency and wet bulb temperatures can essentially be obtained </li></ul>
  21. 35. Fog Cooling II <ul><li>Exhaust fans still used </li></ul><ul><li>Fog nozzles installed just inside the inlet ventilators </li></ul><ul><li>Roughly half the exhaust fan capacity of fan- and-pad cooling systems is necessary </li></ul><ul><li>High water quality is critical </li></ul><ul><li>Can also be used with plant propagation systems </li></ul><ul><li>– Disease occurrence much lest than with a mist system. Why? </li></ul>
  22. 36. Fog Cooling Advantages <ul><li>There is less electrical consumption </li></ul><ul><li>Heat rise across the greenhouse is controlled </li></ul><ul><li>Cooler average temperatures can be achieved across the greenhouse </li></ul><ul><li>System is good substitute for mist systems on propagation benches. </li></ul>
  23. 37. Effectiveness of Winter Cooling <ul><li>Ventilators ‘used’ to be the only way to winter cool – problems </li></ul><ul><li>Convection-tube and HAF eliminate horizontal temperature gradient problems </li></ul><ul><li>Both modern systems circulate air in the greenhouse </li></ul>
  24. 39. Active Winter Cooling <ul><li>Convection tube cooling </li></ul><ul><ul><li>Exhaust fan turned on </li></ul></ul><ul><ul><li>A louver opens in the gable </li></ul></ul><ul><ul><li>A pressurizing fan in the end of the polyethylene tubes turns on </li></ul></ul><ul><ul><li>Cool air mixes with greenhouse warm air and galls to the floor cooling the plant growing area </li></ul></ul><ul><li>Pressurizing fan must move as much air as the exhaust fan. </li></ul><ul><li>2 cfm required </li></ul>
  25. 43. HAF fans <ul><li>Similar to convection-tube-system </li></ul><ul><li>Requires HAF fans in the place of convection tubes </li></ul><ul><li>HAF fans can be used for air circulation when neither heating nor cooling is in operation </li></ul>
  26. 46. Integrating of Heating and Cooling Systems <ul><li>Remember: there are some spring and fall days when you may have to use, summer heating, winter cooling and summer cooling systems all on the same day </li></ul>
  27. 47. BRING CALCULATORS and TEXTBOOK on Thursday !!!!