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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
SUMMER COOLING • Active – Fan-and-pad cooling – Fog • Passive – ventilators WINTER COOLING • HAF fans • Convection tube 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/OPENROOF 3. HTM
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
http: //www. munters. com/home. nsf/FS 1? Read. Form&content=/products. nsf/By. Key/OHAA-55 GSWH http: //okfirst. ocs. ou. edu/train/meteorology/Heat. Transfer. 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
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?
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
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 60 ft for the 4 inch system and 50 ft for the 6 inch system 1/8 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/ft 2 of floor space as a standard • In warmer climates 1 volume per minute recommended roughly 11 -17 cfm/ft 2 • 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 1. Calculate the standard cfm = Greenhouse area X 8 cfm/ft 2. Correct for the standard rate of air removal using the larger of Fhouse or (Fvel) 3. Fhouse = Felev X Flight X Ftemp 4. Total cfm = standard cfm X (Fhouse or Fvel) 5. Select the fans to install
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
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/Evap. Fog. 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
Fog Cooling II • Exhaust fans still used • Fog nozzles installed just inside the inlet ventilators • Roughly half the exhaust fan capacity of fanand-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
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
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
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 !!!!