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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
SUMMER COOLING
• Active– Fan-and-pad
cooling– Fog
• Passive– ventilators
• HAF fans
• Convection tube cooling
WINTER COOLINGWINTER 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
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/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
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/ft2; a six inch 350 cfm/ft2
• By way of comparison excelsior pads can only support an airflow rate of 150 cfm/ft2
• 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 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 astandard
• In warmer climates 1 volume per minuterecommended roughly 11-17 cfm/ft2
• As elevation increases so must the rate of air removal. Why?
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/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 waterdroplet 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 inletventilators
• 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 mistsystem. 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 thefloor cooling the plant growing area
• Pressurizing fan must move as much air as the exhaustfan.
• 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 andCooling Systems
Remember: there are some spring andfall days when you may have to use,summer heating, winter cooling andsummer cooling systems all on thesame day
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