Greenhouse ventilation has always been quite simple. Forty years ago, we opened more and more windows as the temperature rose inside the greenhouse. Today's staged systems are similar. Individual fans, louvers and possibly sidewalls are opened in sequence according to the greenhouse temperature.
Summer temperature conditions require the ventilation system to be designed to move one to one and one-half air exchanges per minute. This large volume of air is moved through the greenhouse because the air inside the greenhouse is heated so quickly by the solar input into the greenhouse. The greenhouse can be maintained at 8-10 degrees above the outdoor temperature (80 degrees outside, 90 degrees inside) if the fans move one air exchange per minute. More air movement yields less temperature rise in the greenhouse and less air movement results in more temperature rise.
For the last 20 years standard greenhouses have used a staged ventilation system that uses fans and louvers in increments to increase ventilation with increasing greenhouse temperatures. A standard 30' x 100' quonset greenhouse with the appropriate automatic ventilation system (air circulation system, 2 motorized louvers, 1 fan with high and low speed, 1 fan with high speed only) would be designed to move 27,000 cfm (cubic feet per minute) of air at maximum ventilation. An example of the staged ventilation to maintain greenhouse day temperatures at 70 degrees follows:
1. No ventilation - The greenhouse temperature is below the desired inside temperature of 70 degrees. The air circulation system (horizontal air flow, HAF, fan system or perforated poly tube, fan-jet, system) operates continuously. The heating system is set to maintain 60-62 degrees.
2. First stage - The greenhouse temperature rises to 70 degrees. Both motorized louvers open. Cool air is drawn into the greenhouse and mixed with the warm greenhouse air by the air circulation system and the greenhouse is cooled.
3. Second stage - The greenhouse temperature rises to 72 degrees. One ventilation fan is switched on at low speed and operates until the greenhouse is cooled to below 72 degrees. This fan capacity should be about one-third of the maximum fan capacity, or 9,500-10,000 cfm for the above greenhouse. The motorized louvers and circulation system continue to operate from stage one.
4. Third stage - The greenhouse temperature rises to 74 degrees. One fan switches from low speed to high speed and the second fan is switched on at high speed and operates until the greenhouse is cooled to below 74 degrees. The motorized louvers continue in the open position from stage 1, however, the air circulation system switches to off to allow air temperature stratification in the greenhouse and more efficient air flow at the plant canopy.
These stages are managed by thermostats placed near the plant canopy in the middle of the greenhouse, preferably in an aspirated box. These stages can also be managed by microprocessor controllers or greenhouse computers.
This standard ventilation system is quite effective at maintaining greenhouse temperatures, plus it is automatic. The greenhouse operates without regular attention from the greenhouse operator. The equipment and electrical details of this staged ventilation sequence are shown on Plan 6197 from the Biosystems and Agricultural Engineering Department.
This standard system is uncommon in tobacco greenhouses, yet it offers nearly optimal temperature control. The cost of this system is somewhat higher than a system that includes roll-up or roll-down side walls. However, this cost is easily offset by the increased quality of temperature control and the fact that the system is automatic and the greenhouse operator does not need to be present each day to open the walls for the final stage of ventilation. The added benefits of having good plants unaffected by adverse conditions could well be worth the added costs.
This standard staged ventilation system is typical in approximately 100 acres of commercial greenhouses in the state yet is rare in the 30-35 acres of tobacco greenhouses. Many of the commercial greenhouses have back-up generators to support the fans if power outages occur. Some commercial greenhouses have automated sidewall opening systems to meet the final stage of ventilation.