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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
VI. GREENHOUSE COVERINGS

A. Selection - factors to consider

1. Photosynthesis

  • Transmission vs plant reception

2. light quality

  • 400-800 nanometers
slide2
3. durability
  • Initial vs long term

4. Initial & maintenance cost

5. energy savings

  • 1 layer vs 2 layers
b covering types
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
2 plastic film
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%
slide5

a. Polyethylene, contu.

  • structure
    • light weight
    • aluminum or steel
  • Loss of heat:
    • I.R. radiation loss high
  • Condensation
    • Tight house, little air exchange
b vinyl
b. Vinyl

1) Polyvinyl chloride

CH2 = CH – Cl

2) Polyvinyl acetate

CH2 = CH - OCCH3 - 0

  • 4-5 yrs
  • non UV resistant
  • attracts dirt
slide7
Polyvinyl fluoride (tedlar)

CH2 = CH - F

    • 10-15 yrs
    • stretched over frame
3 rigid plastics
3. Rigid plastics
  • Polyvinyl chloride –

CH2 = CH - Cl

    • corrugated
    • 4-5 yrs with UV inhibitors
    • more expensive than polyethylene
slide9
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
slide10
c. Acrylic profiled sheet
  • transmission 80%
  • Energy savings: 40% over 1 layer glass
  • Strong structure
  • Expensive
slide11

d. Polycarbonate profiled sheet

    • transmission 80%
    • UV inhibitors increases life
  • e. Polycarbonate corrugated panel
    • transmission 90-92%
slide12
4. New developments
  • inert gas between layers of glass
  • Chemical solutions in rigid plastic channels
slide14

1. Light transmission

    • a. quality
  • All allow 400-800 nanometers

Plant Growth

slide15

b. Transmission

        • 1 layer 90%;
        • 2 layers 80%
        • direct vs diffused
slide17
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
slide18
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

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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.

slide19

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Polyethylene

double layer

Glass

slide20
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
slide22
c. Thermal radiation (radiant energy) loss
  • low
    • Glass, fiberglass, acrylic, polycarbonate
  • high
    • polyethylene
    • condensation reduces losses
slide23

New film blocks thermal radiation loss

New films also reduce dripping

slide24
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
slide25
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
attachment of polyethylene to frame
Attachment of polyethylene to frame

Older method

New systems

Polylock

slide27
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
slide28
D. Air inflation management
  • Replace leaked air
  • Source of air leak
    • Gaps in locking system
    • Puncture
        • Nails, Splinter, Metal frame