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Cooling
 

Cooling

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Cooling Cooling Presentation Transcript

  • Greenhouse Cooling Concepts
  • Why Cool Greenhouses?
    • Most places have a summer climate that requires greenhouse cooling – even Vermont
    • A greenhouse must be capable of cooling in the winter and summer
    • With passive cooling greenhouses can reach temperatures of 20°F greater (or more) than the air temperature
  • Too HOT!!
    • Loss of stem strength
    • Reduction of flower size
    • Delay of flowering
    • Bud abortion
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  • SUMMER COOLING
    • Active
      • Fan-and-pad cooling
      • Fog
    • Passive
      • ventilators
    • HAF fans
    • Convection tube cooling
    WINTER COOLING
  • Passive Cooling
    • Percent of roof space ventilated has increased over time with design improvements
    • Success is very crop specific
    • Even fully retractable roof designs are now available
    • Cheaper to operate than active cooling systems, but construction costs aren’t less expensive
  • Acta Hortic. 443: 31-38 http://aesop.rutgers.edu/~horteng/OPENROOF3.HTM
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  • Effectiveness of Summer Cooling
    • Fan and pad cooling can lower to 80% of the difference between the wet and dry bulb temperatures
    • Fog cooling can lower the temperature by nearly all of the difference
    • Both of these systems are most effective at low humidity
  • Evaporative Cooling
    • Works well in most climates; where might it not function effectively?
    • Based on heat absorption during the evaporation of water
    • Relatively inexpensive compared to other types of cooling
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  • http://www.munters.com/home.nsf/FS1?ReadForm&content=/products.nsf/ByKey/OHAA-55GSWH http://okfirst.ocs.ou.edu/train/meteorology/HeatTransfer.html
  • Pad and Fan Cooling
    • Available for almost 50 years
    • Most common system for summer cooling
    • Originally the pad was composed of wood shreds
    • Today it is composed of cellulose
    • Exhaust fans are placed on the opposite wall
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  • Active Summer Cooling System Calculations (Basics)
    • Fan-and-Pad system
    • Rate at which warm air must be removed from the greenhouse
      • Types of pads used
      • Fan placement
      • Path of the airstream
  • Pad Types and Specifications
    • Excelsior pads (wood fiber) had to be framed in wire mesh for support; required annual replacement
    • Cross-fluted cellulose is the most popular today, can last up to 10 years
      • Should be kept from heavy rains
      • Only move if dry
    • Other types of pads include aluminum fiber, glass fiber, and plastic fiber
    • Why are pads thick? and why do they have a cross fluted design?
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  • Cross-fluted cellulose pads
    • Come in height increments of ft
    • Available in 2, 4, 6, and 12 inches thick
    • A 4-inch-thick pad will handle an air intake of 250 cfm/ft 2 ; a six inch 350 cfm/ft 2
    • By way of comparison excelsior pads can only support an airflow rate of 150 cfm/ft 2
    • You want vents over the exterior of the pads to seal the external air source off when active cooling isn’t needed
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  • More Details
    • Water must be delivered to a 4-inch pad at the rate of 0.5 gpm per linear foot of pad
    • For a 6-inch thick cellulose pad a 0.75 gpm per linear foot is required
    • 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
    • Holes point upward and release water into an impingement cover – water drips down onto a distribution pad
  • Rate of Air Exchange
    • Measured in cfm (cubic feet per minute)
    • NGMA uses 8 cfm/ft2 of floor space as a standard
    • In warmer climates 1 volume per minute recommended roughly 11-17 cfm/ft2
    • As elevation increases so must the rate of air removal.  Why?
  • Other factors:
    • Light Intensity
    • Temperature rise across the greenhouse
    • Pad-to-fan distance
  • Calculating Air Removal Rate
    • Calculate the standard cfm = Greenhouse area X 8 cfm/ft
    • Correct for the standard rate of air removal using the larger of F house or (F vel )
    • F house = F elev X F light X F temp
    • Total cfm = standard cfm X (F house or F vel )
    • Select the fans to install
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  • Exhaust Fan Placement Rules
    • Should not be more than 25 ft apart
    • If the end of the greenhouse is 60 ft wide you will need at least 3 fans
    • Fans should be evenly spaced at plant height
    • Place fans on leeward side of the greenhouse
    • Rules change with multiple houses
    • Protect fans from weather and provide screening on both sides to protect workers, visitors, and wildlife
    • Air movement can cause special problems in larger houses
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  • Fog Cooling
    • 20-year-old technology
    • High pressure water delivery system generates a fog of very fine water particles (<10 microns)
    • Drops evaporate in the air
    • Even dispersal of the particles means cooling of throughout the greenhouse
    http://www.valproducts.com/Air/EvapFog.html
  • Fog Cooling
    • Initial cost usually close to that of fan and pad cooling systems (water quality determining factor)
    • Operating cost less than fan-and-pad cooling
    • Dispersion of water particles in the greenhouse air where they extract heat from the air as they evaporate.
    • Rate of cooling increases proportionately as water droplet size decreases.
    • Systems allow near 100 percent cooling efficiency and wet bulb temperatures can essentially be obtained
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  • Fog Cooling II
    • Exhaust fans still used
    • Fog nozzles installed just inside the inlet ventilators
    • Roughly half the exhaust fan capacity of fan- and-pad cooling systems is necessary
    • High water quality is critical
    • Can also be used with plant propagation systems
    • – Disease occurrence much lest than with a mist system. Why?
  • Fog Cooling Advantages
    • There is less electrical consumption
    • Heat rise across the greenhouse is controlled
    • Cooler average temperatures can be achieved across the greenhouse
    • System is good substitute for mist systems on propagation benches.
  • Effectiveness of Winter Cooling
    • Ventilators ‘used’ to be the only way to winter cool – problems
    • Convection-tube and HAF eliminate horizontal temperature gradient problems
    • Both modern systems circulate air in the greenhouse
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  • Active Winter Cooling
    • Convection tube cooling
      • Exhaust fan turned on
      • A louver opens in the gable
      • A pressurizing fan in the end of the polyethylene tubes turns on
      • Cool air mixes with greenhouse warm air and galls to the floor cooling the plant growing area
    • Pressurizing fan must move as much air as the exhaust fan.
    • 2 cfm required
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  • HAF fans
    • Similar to convection-tube-system
    • Requires HAF fans in the place of convection tubes
    • HAF fans can be used for air circulation when neither heating nor cooling is in operation
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  • Integrating of Heating and Cooling Systems
    • 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
  • BRING CALCULATORS and TEXTBOOK on Thursday !!!!