Heat Requirement Calculations for Greenhouses

Transcription

1 Environment The successful operation of any greenhouse requires the maintenance of an inside temperature near the optimum level for plant growth. The exact inside temperature to be maintained will depend on the crop being grown. Generally, designs are for 65 o F inside capability with thermostatic adjustment for exact conditions per horticultural recommendations. Other climatic factors include relative humidity, air movement, and carbon dioxide. Temperature and relative humidity are normally controlled by the heating and ventilation equipment. Continuous air circulation, especially in the wintertime, is important for distribution of heat and uniformity of inside conditions by preventing air stagnation and stratification. Carbon dioxide enrichment of the air for greater plant growth and production is sometimes profitable. Information on heating, circulation, and ventilation is included in the following pages. More detailed information can be found in existing publications of Georgia Cooperative Extension Service. Heating Heat Requirements and Fuel Costs Factors primarily affecting the heat requirements of a greenhouse are: 1. The external environmental condition 2. The size of the greenhouse. 3. The structural nature of the greenhouse. 4. The number of layers of glazing material used to cover the house. The following pages permit determination of heating requirements and annual fuel costs for a particular greenhouse type, size, and single or double-layer covering. Heat Requirement Calculations for Greenhouses It is useful to know some heat loss calculation procedures to predict heating loads and identify areas of the greenhouse with the most heat loss. Heat loss by conduction may be calculated with the following equation. Q = U A (Ti - To) where: Q = heat transfer rate, BTU/hr. U = heat transfer coefficient, BTU/hr.-ft. 2 o F A = surface area, ft. 2 Ti-To = air temperature difference between inside and outside, o F. Sometimes "R" values (the resistance to heat flow) are listed instead of "U" values. The relation between "U" and "R" is: U = 1

2 R The conduction heat transfer equation using "R" can then be written as: Q = A(Ti-To) R Frequently it is more convenient to work with "R" values when dealing with insulation, as the added effect of insulation can be determined quickly by simply summing the "R" values of materials in the heat flow path. For example, from Table L4.7, the "R" value for a single layer of glass is equivalent to 0.88 and for 1-inch-thick styrofoam, Adding styrofoam to a single-layer glass surface will give the wall an insulation value of R = = 4.88 (or a "U" value of 1/4.88 = 0.204). Note that high "R" values and low "U" values indicate less heat flow.

3 Table L4.7: Heat Transfer Coefficients For Construction Materials sq.ft./btu) Materials R Value** (hr.of Glass, single layer 0.88* Glass, double layer, 1/4 in. space 1.54* Glass, triple layer, 1/4 in. space 2.13* Clear polyethylene film, single layer (2, 4, or 6 mil) 0.87* Clear polyethylene film, double layer, separated (2, 4, or 6 mil) 1.43* Polyethlene film, double layer, separated over glass 2.00* Fiberglass 1.00* Double acrylic (Acrylite SDP TM ) 1.78* Double polycarbonate (Tuffak-Twinwal TM ) 1.61* Face Brick, 4 in. thick 0.44 Concrete block, 8 inch 1.96 Concrete block, 8 inch plus l inch foamed urethane 7.69 Concrete block, 8 inch plus 1 inch foamed polystyrene 5.55 Concrete, poured, 6 inch 1.33 Cement asbestos board, 1/4 inch 0.91 Cement asbestos board, 1/4 inch plus l inch foamed urethane 7.14 Cement asbestos board, 1/4 inch plus l inch foamed polystyrene 4.76 Microfoam TM 1/4 in. thick 1.08 Polystyrene (beadboard or loose fill), 1/2 in. thick 2.10 Polystyrene (beadboard or loose fill), 3/4 in. thick 3.05 Polystyrene (beadboard or loose fill), l inch thick 4.00 Extruded polystyrene (Styrofoam) 1 inch thick 5.40 Polyurethane foam (applied at site), 1 inch thick 7.30 Plywood 1/2 inch 0.62 Plywood 1 inch inch nominal softwood 1.79 Expanded vermiculite (6-6 lb./cu.ft., 1 inch thick) 2.20 Curtain Materials Al/Temp TM, aluminum down 1.43 aluminum up 1.18 Al/Blac TM 1.37 Duracote #2425 (Foylon TM ) 2.63 Black Sateen 1.54 Black poly, 6 mil 1.05 Reemay TM, spunbound polyester, Vinyl (aluminized polyester laminated vinyl) 4.5 mil 2.15 ** The R value represents the resistance to heat flow at the thickness listed. The higher the R value the better the insulating property. * Includes effects of surface coefficients. Acrylite S.D.P. TM CY/RO Industries Microfoam TM DuPont Al/Blac TM Simtrac, Inc. Reemay TM DuPont Al/Temp TM Simtrac, Inc. Styrofoam TM Dow Chemical Foylon TM Duracote Corp Tuffak-Twinwall TM Rohm and Haas Co.

4 Infiltration heat loss can be significant and should be calculated and added to conduction heat losses. The equation for infiltration heat transfer is: Q where: Q = 0.02 x Vol x NC x (Ti-To) = heat transfer rate, Btu/hour Vol = greenhouse volume, ft. 3 NC = number of air exchanges per hour (Table L4.8). Ti-To = air temperature difference between inside and outside, o F. Table L4.8: Natural Air Exchanges For Greenhouses Construction System Air Exchanges per Hour* New Construction, glass or fiberglass 0.75 to 1.5 New Construction, double layer plastic film 0.5 to 1.0 Old Construction, glass, good maintenance 1 to 2 Old Construction, glass, poor condition 2 to 4 *Low wind or protection from wind reduces the air exchange rate. Section Break (Next Page)

5 Greenhouse Surface Areas and Volume Calculations Once dimensions are known and listed, the area and volumes can be calculated with the following equations for the appropriate greenhouse style: Single gable greenhouse (Figure L4.1). Wall area = 2 (F x C) End area = (2xFxB) + (GxB) Roof area = 2 (DxC) Foundation Area = 2 (ExC) + 2(ExB) Volume = A x B x C = 1/2 (BxG)xC

6 Gutter-connected gable greenhouses (N = number of greenhouses) (Figure L4.2) Wall area = 2 (FxC) End area = [2 (FxB) + 2(GxB)] x N 2 Roof area = [2 (DxC)] x N Foundation area = 2 (ExC) + [2 (ExB) x N] Volume = [A x B x C + 1/2 (BxG) x C] x N

7 Gutter-connected, curved-roof greenhouses (N = number of greenhouses) (Figure L4.3) Wall area = 2(AxC) End area = [8/3 (HxB) + AxB] x N Roof area = (DxC) x N Volume = [4/3 (HxBxC) + (AxBxC)] x N Quonset-style greenhouse (Figure L4.4) End area = 4/3 (HxB) Roof area = D x C Volume = 4/3 (HxBxC)

8 Figure L4.4: Quonset-style Greenhouse.