Chapter 6

Greenhouse Ventilation and Cooling

(book excerpts)

Greenhouses are so efficient at retaining relatively low levels of solar energy, that without ventilation and cooling, high greenhouse temperatures could severely impact plant growth. The purpose of greenhouse ventilation is to control high temperatures caused by the influx of solar radiation, to maintain relative humidity at acceptable levels, to provide uniform air flow throughout the entire greenhouse, and to maintain acceptable levels of gas concentration in the greenhouse. There are three basic ventilation and cooling systems used in greenhouse production systems, natural, forced-air, and evaporative cooling. Natural ventilation is driven by two mechanisms, namely the pressure field induced by the wind around the greenhouse and the buoyancy force induced by the warmer and more humid air in the greenhouse. Forced-air ventilation is accomplished by fans that are capable to move large quantities of air at relatively low pressure drop. Evaporative cooling is a process that reduces the temperature of air by the evaporation of water into the airstream. The fan-and-pad system have been the standard system for evaporative cooling. An evaporative cooling pad is installed in the ventilation opening, cooling the incoming air. As the air moves through the greenhouse towards the exhaust fans, it picks up heat from the greenhouse environment. Therefore, pad-and-fan systems experience a temperature gradient between the inlet (pad) and the outlet (fan) side of the greenhouse. Fog systems can also be installed throughout the greenhouse, resulting in a more uniform cooling pattern compared to the pad-and-fan system. In both cases, ventilation is required to exhaust the humidified air and exchange it with drier air so that evaporative cooling can continue. Shade curtains help reduce the energy load on a greenhouse crop during warm and sunny conditions and they help reduce heat radiation losses at night. Movable curtains can be operated automatically with a motorized system that is controlled by a light sensor and/or timer. Even low-cost greenhouses can benefit from the installation of a shade system.

Click on the following topics for more information on greenhouse ventilation and cooling.

Topics Within This Chapter:

  • Seasonal Ventilation Requirements for Greenhouses
  • Winter Ventilation
  • Summer Ventilation
  • Spring–Fall Ventilation
  • Natural Ventilation in Greenhouses
  • Wind Driven Ventilation
  • Buoyancy Driven Ventilation
  • Greenhouse Height
  • Types of Greenhouse Venting Systems
  • Sidewall Vents
  • Ridge Vents
  • Vent Operation
  • Open-Panel Greenhouses
  • Greenhouse Sun Screens
  • Forced-Air Ventilation in Greenhouses
  • Ventilation System Performance
  • Static Pressure
  • Cooling Effectiveness of Forced-Air Ventilation
  • Greenhouse Ventilation System Capacity
  • Determining Ventilation Rate for a Greenhouse
  • Exhaust Fans
  • Exhaust Fans Air Exchange Rates
  • Selecting Exhaust Fans for Forced-Air Ventilation
  • Exhaust Fans Static Pressure Rating
  • Determining the Number and Size of Exhaust Fans
  • Exhaust Fan Optimization
  • Air Intake Shutters
  • Ventilation Placement
  • Determining Air Intake Vent Size
  • Thermostat Selection and Placement
  • Fan and Pad Evaporative Cooling Systems
  • Operating Fan and Pad Evaporative Cooling System
  • Greenhouse Temperature Gradient
  • Eveporative Cooling Pads
  • Determining Cooling Pad Size
  • Determining Pump Capacity for Pads
  • Determining Sump Tank Volume
  • Location of Exhaust Fans and Cooling Pads
  • Thermostats
  • Fan and Pad Evaporative Cooling System Maintenance
  • Pads
  • Recirculation Pumps
  • Distribution Headers
  • Greenhouse Fog Systems
  • Fog System Configurations
  • Pumps and Nozzles
  • Control Systems
  • Greenhouse Shading
  • Interior versus Exterior Curtains
  • Operation of Greenhouse Shade Curtains
  • Greenhouse Shading Compounds (White Wash)
  • Greenhouse Humidity Control
  • Relationship Between Temperature and Humidity
  • Dew Point Temperature
  • Humidity-Measuring Instruments
  • Sling Psychrometer
  • Hand Held Humidity Meters
  • Wireless Temperature and Humidity Sensors
  • Reducing Greenhouse Humidity
  • Cultural Practices
  • Ventilation and Heating
  • Air Circulation
  • Bottom Heat
  • Greenhouse Design
  • Anti-Drip Plastic
  • Vapor Pressure Deficit