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

• 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?

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 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

BRINGCALCULATORS

andTEXTBOOK on Thursday !!!!