Greenhouses. Planning A Home Greenhouse

Heat is removed from the air to change water from

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Unformatted text preview: air to change water from liquid to a vapor. Moist, cooler air enters the greenhouse while heated air passes out through roof vents or exhaust louvers. The evaporative cooler works best when the humidity of the outside air is low. The system can be used without water evaporation to provide the ventilation of the greenhouse. Size the evaporative cooler capacity at 1.0 to 1.5 times the volume of the greenhouse. An alternative system, used in commercial greenhouses, places the pads on the air inlets at one end of the greenhouse and uses the exhaust fans at the other end of the greenhouse to pull the air through the house. Controllers/Automation Automatic control is essential to maintain a reasonable environment in the greenhouse. On a winter day with varying amounts of sunlight and clouds, the tem- 7 6 ' D This is an example of a curved-eave lean-to built on a two-story house. 8' 16' Figure 4. Use the greenhouse’s dimensions to determine the necessary heating system capacity. This discussion is a bit technical, but these factors must be considered when choosing a greenhouse. Note the effect of each value on the outcome. When different materials are used in the construction of the walls or roof, heat loss must be calculated for each. For electrical heating, convert Btu/h to kilowatts by dividing Btu/h by 3,413. If a wood, gas, or oil burner is located in the greenhouse, a fresh-air inlet is recommended to maintain an oxygen supply to the burner. Place a piece of plastic pipe through the outside cover to ensure that oxygen gets to the burner combustion air intake. The inlet pipe should be the diameter of the flue pipe. A piece of plastic pipe about the size of the flue pipe through the outside cover to a point near the burner combustion air intake would be adequate. This ensures adequate air for combustion in an airtight greenhouse. Unvented heaters (no chimney) using propane gas or kerosene are not recommended. Example. If a rigid-frame or post and rafter freestanding greenhouse 16 feet wide by 24 feet long, 12 feet high at the ridge, with 6 feet sidewalls, is covered with single-layer glass from the ground to the ridge, what size gas heater would be needed to maintain 60 °F on the coldest winter night (0 °F)? Calculate the total outside area (Figure 4): two long sides two ends roof gable ends A straight-eave lean-to greenhouse can fit under the roof of a single-story house. 6 ' 2' 4 1. A is the total exposed (outside) area of the greenhouse sides, ends, and roof in square feet (ft2). On a quonset, the sides and roof are one unit; measure the length of the curved rafter (ground to ground) and multiply by the length of the house. The curved end area is 2 (ends) × 2/3 × height × width. Add the sum of the first calculation with that of the second. 2. u is the heat loss factor that quantifies the rate at which heat energy flows out of the greenhouse. For example, a single cover of plastic or glass has a value of 1.2 Btu/h × ft2 × °F (heat loss in Btu’s per hour per each square foot of area per degree in Fahrenheit); a double-layer cover has a value of 0.8 Btu/h × ft2 × °F. A table of u values is provided in Extension Bulletin 351 Greenhouse Heating, Circulation, and Ventilation Systems. The values allow for some air infiltration but are based on the assumption that the greenhouse is fairly airtight. 3. (Ti – To) is the maximum temperature difference between the lowest outside temperature (To) in your region and the temperature to be maintained in the greenhouse (Ti). For example, the maximum difference will usually occur in the early morning with the occurrence of a 0 °F to –5 °F outside temperature while a 60 °F inside temperature is maintained. Plan for a temperature differential of 60 to 65 °F. The following equation summarizes this description: Q = A × u × (Ti – To). 2 × 6 ft × 24 ft = 288 ft2 2 × 6 ft × 16 ft = 192 ft2 2 × 10 ft × 24 ft = 480 ft2 2 × 6 ft × 8 ft = 96 ft2 A = 1,056 ft2 Select the proper heat loss factor, u = 1.2 Btu/h × ft2 × ° F. The temperature differential is 60 °F – 0 ºF = 60 °F. Q = 1,056 × 1.2 × 60 = 76,032 Btu/h (furnace output). An even-span attached to a garage allows a larger greenhouse in a limited space. Free-standing greenhouses allow more location choices and can be larger than attached greenhouses. Air Circulation Installing circulating fans in your greenhouse is a good investment. During the winter when the greenhouse is heated, you need to maintain air circulation so that temperatures remain uniform throughout the greenhouse. Without air-mixing fans, the warm air rises to the top and cool air settles around the plants on the floor. Although this is a relatively small greenhouse, the furnace output is equivalent to that in a small residence such as a townhouse. The actual furnace rated capacity takes into account the efficiency of the furnace and is called the furnace input fuel rating. 6 A window-mounted unit extends a house’s growing space. Figure 2. Different t...
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