Mechanical ventilation can be divided into three types: negative-, positive-, and neutral-pressure ventilation. Negative-pressure systems use fans to exhaust air, thereby creating a slight partial vacuum that draws fresh air from the outdoors. Negative-pressure ventilation systems are used most commonly for greenhouses in the United States and are well-suited for simple ventilation and evaporative cooling. Positive-pressure ventilation accomplishes the reverse. Fans force air into the airspace, creating a slight over-pressure and expelling air from the air space through outlets located in the green-house walls or roof.
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