Part2Lecture8-9 - Air Circulation(Lecture 8 In a closed...

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Fall 2002 Biernbaum, MSU HRT 221, pg 46 Air Circulation (Lecture 8) In a closed greenhouse there is little natural air movement except temperature currents from heat stratification. The air in the greenhouse should be kept circulated to: 1. Prevent temperature stratification, from roof to floor 2. Provide uniform temperatures in the greenhouse 3. Prevent high humidity pockets and moisture condensation on leaf surfaces 4. Maintain the carbon dioxide (CO 2 ) level around the plants. Forced air circulation should be provided. The air circulation system may be present as part of the ventilation or heating system. There are three basic methods of circulating greenhouse air: 1. Overhead unit heaters with horizontal fans or fan jets Fan capacity should be about one forth of the house volume. If the house width is less than 30 feet can use one, if greater than 30 feet two fans should be used. The fans can be located in the center of the house or the ends.
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Fall 2002 Biernbaum, MSU HRT 221, pg 47 2. Overhead unit heaters or fan jets with perforated convection tubes Tube should have one square foot cross sectional area per 1000 cfm air flow. The tube is inflated and air comes out the holes. 3. Horizontal air flow (HAF) (most common today) Air flow can be provided by locating several small fans around the greenhouse. The goal is to get 40 feet per minute air flow for good mixing. There usually is a fan every 50 feet with the first fan 15 to 20 feet from the end of the house and the last one 40 to 50 feet from the opposite end of the house. Air flow can be tested by using a smoke bomb or generator in the greenhouse to check air patterns or by checking temperature uniformity around the greenhouse.
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Fall 2002 Biernbaum, MSU HRT 221, pg 48 Temperature Control (Lecture 8 and 9) Nelson, Chapter 5, pgs 173-185 Temperatures can be monitored and controlled by simple bimetallic thermostats or by a temperature sensor and a step controller or an environmental control computer. There is a wide range in cost, from perhaps $50 for a thermostat, to $1000 to $3000 for a step controller, and from $5000 to $20,000 for an environmental control computer. The level of sophistication or options available is also very different. The development of temperature control strategies is very important but can get more
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Part2Lecture8-9 - Air Circulation(Lecture 8 In a closed...

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