Download - IX. REDUCING GREENHOUSE TEMPERATURE
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IX. REDUCING GREENHOUSE TEMPERATURE
A. temperature regimes– range 50-80 deg, stress above 90 deg
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Greenhouse effect
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B. Temperature in greenhouse determined by:
1. Solar energy– Greenhouse effect:
• Solar energy- short wave radiation passes through
glass
• Thermal radiation- longer waves trapped
– shading in greenhouse
– time of year
– time of day
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2. Exterior ambient temperature
3. Moisture transpired– Temperature in empty greenhouse greater than
in greenhouse with plants
4. Evaporation of water from soil, walls etc.
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C. Techniques to reduce temperature
1. Shading - reduce energy absorbeda. Shading compound
• Permanent shade after application• Removed Aug – Sept
b. Shade curtain• Remove during cloudy weather
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2. Greenhouse ventilationa. Definition• movement of outside air through
greenhouse
b. Characteristics of ventilation system1) capacity to reduce temperature2) uniform temperature3) automated4) minimum and maximum ventilation
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c. Time of year
1) Winter ventilation • Late fall, winter, early spring: • outside air cool • Minimum to maximum ventilation
usually adequate
2) Summer ventilation
1) Temperature control difficult• Shading and ventilation• Maximum ventilation
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d. Types of Systems
1) Natural ventilation
–Establish convection currents
• Ridge vent and side vents
• Open area
-40% of floor space
• Removable side wall
• Removable roof
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Natural ventilation
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Natural ventilation
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Natural ventilation
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Natural ventilation
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2) Motorized Systems - minimum • Mix cold with warm ghs. air• Poly tube system with vent• Low speed fan with vent• One fan or several turned
on, with vent
3) Motorized systems – maximum• Outside directly into
greenhouse • Temperature dependent• Air flow rate
-3/4 to 1 1/4 air ex. /min -7-8° rise
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Air exchange vs temperature
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e. Control of Systems1) Natural ventilation
Manual
Automatic
Thermostat
Computer
2) Motorized ventilation– thermostats– fan(s) 1 or 2 speed– Vents
• Fully open or closed
• modulating
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3) End wall ventilation1) Fans on one end wall2) Vents on other end wall3) Fan spacing 25’4) 1 air exchange/min
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4) Side wall ventilation
–problem mixing cold air
–Air exchange rate vs velocity
• 1 exchange 30’ x 100’
-end wall - 100 ft/min
-side wall - 30 ft/min
–Fan spacing- max 25 ft
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Side wall ventilation
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Natural ventilation