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
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.
C. Techniques to reduce temperature 1.Shading - reduce energy absorbed a.Shading compound Permanent shade after application Removed Aug – Sept b.Shade curtain Remove during cloudy weather
2. Greenhouse ventilation a. Definition movement of outside air through greenhouse b. Characteristics of ventilation system 1) capacity to reduce temperature 2) uniform temperature 3) automated 4) minimum and maximum ventilation
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
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
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