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VI. GREENHOUSE COVERINGS

VI. GREENHOUSE COVERINGS. A. Selection - factors to consider 1. Photosynthesis Transmission vs plant reception 2. light quality 400-800 nanometers. 3. durability Initial vs long term 4. Initial & maintenance cost 5. energy savings 1 layer vs 2 layers . B. Covering types. 1. Glass

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VI. GREENHOUSE COVERINGS

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  1. VI. GREENHOUSE COVERINGS A. Selection - factors to consider 1. Photosynthesis • Transmission vs plant reception 2. light quality • 400-800 nanometers

  2. 3. durability • Initial vs long term 4. Initial & maintenance cost 5. energy savings • 1 layer vs 2 layers

  3. B. Covering types 1. Glass • 40 years; high cost • transmission 90%-97% • Size 18" x 18", 24-39" wide x up to 65" long • Frame: usually aluminum; galvanized iron, wood • low maintenance • Energy air leaks • 2 layers ? • Reglazing every 15-20 yrs

  4. 2. Plastic film a. Polyethylene CH2 = CH2 • short life 2-4 years • deterioration- UV light,O2 and heat • Prevention: UV inhibitors • anti-oxidants • eliminate black surfaces • Transmission • 1 layer 90% • 2 layers 80-83%

  5. a. Polyethylene, contu. • structure • light weight • aluminum or steel • Loss of heat: • I.R. radiation loss high • Condensation • Tight house, little air exchange

  6. b. Vinyl 1) Polyvinyl chloride CH2 = CH – Cl 2) Polyvinyl acetate CH2 = CH - OCCH3 - 0 • 4-5 yrs • non UV resistant • attracts dirt

  7. Polyvinyl fluoride (tedlar) CH2 = CH - F • 10-15 yrs • stretched over frame

  8. 3. Rigid plastics • Polyvinyl chloride – CH2 = CH - Cl • corrugated • 4-5 yrs with UV inhibitors • more expensive than polyethylene

  9. b. Fiberglass reinforced plastic (FRP) • -C-O-C-O-CH2 CH2-O l l l l 0 0 • corrugated panel • transmission 90-92% • surface may degrade • treated with tedlar • 5-6 yrs; 15 with tedlar • light transmission scattered

  10. c. Acrylic profiled sheet • transmission 80% • Energy savings: 40% over 1 layer glass • Strong structure • Expensive

  11. d. Polycarbonate profiled sheet • transmission 80% • UV inhibitors increases life • e. Polycarbonate corrugated panel • transmission 90-92%

  12. 4. New developments • inert gas between layers of glass • Chemical solutions in rigid plastic channels

  13. C. Comparison - coverings

  14. 1. Light transmission • a. quality • All allow 400-800 nanometers Plant Growth

  15. b. Transmission • 1 layer 90%; • 2 layers 80% • direct vs diffused

  16. obstructions

  17. Heating • Tight vs loose • Polyethylene, fiberglass, acrylics and polycarbonates • .5-1 air exchange per hour • Glass • .5-2 air exchange per hour • 2 air exchanges/ hour 10-15% of energy • infiltration through cracks, vents, doors etc. • Greater heat loss

  18. Greenhouse Construction Factors, C, for the Common Types of Greenhouses in Use Today All metal (good tight glass house -20 or 24 in. glass spacing) 1.08 Wood & steel (good tight glass house -16 or 20 in. glass spacing) (Metal gutters, vents, headers. etc.) 1.05 Wood houses (glass houses with wood bars, gutters, vents, etc.- up to and including 20 in. glass spacing) Good tight 1.00 Fairly tight 1.13 Loose 1.25 FRP covered wood houses .95 FRP covered metal houses 1.00 Double glazing with 1. air space .70 Plastic covered metal houses (single thickness) 1.00 Plastic covered metal houses (double thickness) .70 -------------------------------------------------------------------------------------------------------- Standard heat loss values for transparent components of greenhouses such as gables and roofs transparent side walls and ends as well as covering are multiplied by a factor (C) to correct them for the type of construction.

  19. CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 CO2 Air depleted CO2 Air depleted CO2 Air with. > CO2 Air with. < CO2 Air with. > CO2 Air with. < CO2 CO2 CO2 CO2 CO2 CO2 CO2 Polyethylene double layer Glass

  20. b. Conduction & Radiation • Heat transfer coefficient • BTU / hr / ft2 / 10 F temp. differential • 1 layer same for all materials • 2 layers 40% energy savings • polyethylene • polycarbonate • Acrylite

  21. Heat transfer through Transparent coverings * BTU/ hr / ft2 / 10F

  22. c. Thermal radiation (radiant energy) loss • low • Glass, fiberglass, acrylic, polycarbonate • high • polyethylene • condensation reduces losses

  23. New film blocks thermal radiation loss New films also reduce dripping

  24. D. Air Inflated Double Layer Plastic 1. Attachment • 2 layers of polyethylene • Air inflated with small 1/10 hp fan • Air tight • Ideally like a balloon 2. Purpose • 40% less energy cost 3. Principles • Air tight • Ideally like a balloon • Create dead air space • Static air • Reduce heat transfer

  25. 4. Installation a. calm, cool day b. tightness • expansion and contraction • warm day - too loose • cold day - too tight c. Inflate with outside air • Principles • % Relative humidity • Dew point

  26. Attachment of polyethylene to frame Older method New systems Polylock

  27. d. Space between layers • .75 - 4" ideal; least convection • 4 - 18" greater convection e. Inflation pressure • .2 - .5 water column • greater in high wind • deflate in snow storm • Reduce pressure by closing vent

  28. D. Air inflation management • Replace leaked air • Source of air leak • Gaps in locking system • Puncture • Nails, Splinter, Metal frame

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