Covering - HRT 221 Greenhouse Structures and Management...

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Unformatted text preview: HRT 221 Greenhouse Structures and Management John Biernbaum Department of Horticulture Topics for Week 4 Review types of Structures Alternative or less common types Functional Features & Design Criteria Structural Materials Greenhouse Coverings Selection criteria Materials Alternative Greenhouses Sawtooth Only found in warm climates without snow like southern Florida. Designed to capture wind for cooling, to reduce light and limit rain/water on foliage plants for indoor use. Old greenhouse for botanical collection. Near the San Francisco Bay Bridge. Geodesic dome for botanical collection. Other Types of Greenhouses Gothic Pit or in ground Air inflated Standardization National Greenhouse Manufactures Association http:/ Published Guidelines Not all Companies are members Function: Strength Dead Load Structure and long term weight Live Load Short term weight (15 lbs/sq ft floor) Snow load 15 lbs/sq ft of floor minimum Wind load Lateral and lifting forces to resist 80 mph wind Foundation Vertical, horizontal and lifting forces. Structure and Light Orientation covered previously Roof slope 35 degrees preferred 26 to 28 (6:12 slope) degrees common Minimize structural members to minimize shading Also minimize equipment Structure and Climate Condensation and snow removal Pad/vent to fan distance (100200') Passive ventilation Vents, end walls, side walls Heating efficiency Surface area to floor area Infiltration or air leakage rate Working Height Gutter height has increased from as low as 8' to 10 to 12' as common and 14' for some structures. Impact of higher gutter: Improved working environment Greater air mass reduces rapid temperature fluctuations Equipment movement in greenhouse Floors Humidity Management Dry floors reduce humidity Crop movement by carts, etc Prevention of runoff to groundwater Minimize application is best prevention Cost of Cement is an issue Structural Materials Strength Weight Durability Maintenance Cost Initial Long term Structural Materials Wood Steel Galvanized Availability, treatment, maintenance Most common material (Gatorshield) One steel company supplies most of the greenhouse manufacturing companies Avoid reactive chemicals fertilizer Light weight, strong, low maintenance High cost; used for glass. Aluminum Coverings: Selection Criteria Cost Initial and annual Glass - $1.00 to $2.00/sq ft (surface) lasts 20+ years Plastic at $0.10/sq ft x 2 layers x 5 replacements in 20 years = $1.00/sq ft +labor Structure, heating and light differences must also be considered. Coverings: Selection Criteria Life Expectancy Photodegradation is the primary breakdown of plastics Greenhouse plastics have UV or ultraviolet stabilizers needed for multiyear use Oxidation Surface erosion Coverings: Selection Criteria Strength and Weight Structural support weight varies Dead load Shattering resistance to breakage by rock, hail, etc Crazing star like cracks at point of impact reduces light diffusion Tensile strength resistance to tearing Light Transmission: Light Reflected Transmitted Absorbed heat energy Indirect or Diffused Direct Definitions Transparent Translucent Opaque Direct light Diffuse light Light Transmission Amount passing through (inside) X 100 = Percent Light Transmission Amount of light at Surface (outside) Thermal Qualities Expansion and contraction Plastic films from winter to summer Rigid plastic sheets Heat retention conductance Thermal radiation Infrared or long wave radiation Glass opaque, plastic file transparent Plastic films can have IR additives Other Factors Flammability Insurance Shading How is light reduced? Taxes Heating and Fuel Costs Labor to change plastic film Coverings Glass Rigid Structured Sheets Fiberglass Polycarbonate Acrylic Films polyethylene Types of Glass Float Glass ($0.60/sqft) (Nelson Prices) Single strength (homes) double for greenhouses Shatters into sharp shards Tempered Glass (($0.75/sqft) Stronger Shatters to small pieces Laminated tempered Glass ($4.00 sq ft) Low iron tempered Glass ($0.85/sqft) Hammered Glass diffuses light Older structures had many small panes or lites of glass That required more sashbars and sealing. New structures have larger pieces of glass. One piece of glass from gutter to ridge. Glass Characteristics Light Transmission One of the highest 88% to 92% Strength Cost Initial high, justified for high light crops Thermal Properties Single layer, high conductance Needs a thermal blanket Rigid Structured Sheets Polyvinylchloride (PVC) Fiberglass at Home Depot Not recommended short life Being replaced with polycarbonate or acrylic Polycarbonate Acrylic Less rigid, able to curve more Lower price, flame resistant, less hail damage Higher price, flammable, hail damage Rigid Structured Sheets Forms Single layer flat Single layer corrugated Double layer ribbed 8 mm - $1.90 16 mm - $2.60 Aluminum extrusion to hold costs an additional $1.50 per sq ft of panel Corrugated Fiberglass Panels Diffuses light Fiberglass Bloom: resin wears away exposing the fibers. Collects dirt and reduces light. Not available in double layers so low insulation and heat retention value. Double Layer Polycarbonate or Acrylic Attaching Panels Thermal expansion and contraction requires special mounting. Large Roof Panels Sheet Characteristics Light Transmission Depends on thickness and type high 88% to 92% Strength good except crazing Cost Thermal Properties Initial high, justified for high light crops Single layer, high conductance Double layer provides good insulation Polyethylene Films Many sizes (widths) available Length typically 100' or 150' 3 to 4 years longevity Longer with lower light 6 mil (0.006 inch) thick Typically used as an air inflated double layer for the insulation and structure Inflation Fans One Large Fan Distributed Moisture Between Layers Pressure Between Layers Manometer to measure pressure inch difference in loop Effects resistance to wind Too much pressure stresses film Polyfilm over glass was investigated at Penn State around 1980. Light reduction reduced yield of roses more than money saved. Triple layer in spring for condensation drip reduction. Not really used. Reduce energy loss of end walls. Film Options Polyvinyl chlorides, polyvinyl fluorides, polyesters Only narrow widths available Some longer life films being tested. Infrared additives Anticondensate additives Light diffusing additives Greenhouses do get damaged and destroyed Wind damage Snow Damage Next On Schedule Light and Lighting Systems Benches (Wednesday, 9/29) First Exam (Friday, 10/1) ...
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This note was uploaded on 07/25/2008 for the course HRT 221 taught by Professor Biernbaum during the Fall '04 term at Michigan State University.

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