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/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 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
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 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 !!!!