Controlling Greenhouse Ventilation Inlets by Pressure Difference

Table 1. Standard air density as a function of elevation above sea level (at 70F and 50% relative humidity). M echanical ventilation can be divided into three types: negative-, positive-, and neutral-pressure ventilation. Negativepressure systems use fans to exhaust air, thereby creating a slight partial vacuum that draws fresh air in from 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. Positivepressure ventilation accomplishes the reverse. Fans force air into the air space, creating a slight over-pressure and expelling air from the air space through outlets located in the greenhouse walls or roof. Positive-pressure ventilation systems also are suited to simple ventilation and evaporative cooling. Neutral-pressure systems are less