Chapter 5

Greenhouse Ventilation and Cooling

(book excerpts)

Greenhouses structures by design gather light and to trap the considerable heat contained in sunshine. Greenhouses are so efficient at retaining relatively low levels of solar energy, that without specialized ventilation and cooling equipment, high greenhouse temperatures could severely impact plant growth. Greenhouse ventilation involves removing air from inside the greenhouse and replacing it with outside air. The purpose of ventilation is to control high temperatures during the summer 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. Three types of ventilation systems can be distinguished: 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. As water is evaporated, heat energy is lost from the air which reduces its temperature. The fan and pad system has been the standard system for evaporative cooling. Fog systems operate in much the same way as a fan and pad system except that fog systems add moisture directly to the greenhouse environment where it then evaporates. In both cases, ventilation is required to exhaust the humidified air and exchange it with drier air so that evaporative cooling can continue. Shade is also used to reduce the temperature and light inside a greenhouse. There are two primary options for providing shade: shading compound or whitewash; or shade curtains or shade cloth. Continuous positive air movement within greenhouses is highly desirable since it equalizes temperature, carbon dioxide, and humidity levels within the greenhouse. Through improved environmental conditions, healthier plants can be grown and problems with disease associated with high humidity or “stale air” lessened.

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

Topics Within This Chapter:

  • Natural Ventilation in Greenhouses
  • Wind Driven Ventilation
  • Buoyancy Driven Ventilation
  • Greenhouse Height
  • Types of Greenhouse Venting Systems
  • Sidewall Vents
  • Ridge Vents
  • Vent Operation
  • Greenhouse Sun Screens
  • Forced-Air Ventilation in Greenhouses
  • How to Size Exhaust Fans
  • Air Exchange Rates
  • Selecting Exhaust Fans for Forced-air Ventilation
  • Exhast Fans Static Pressure Rating
  • Determing the Number and Size of Exhaust Fans
  • Exhaust Fan Optimization
  • Exhaust Fan Staging
  • Air Intake Vents
  • Ventilation Placement
  • Determing Vent Inlet 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
  • Greenhouse Shading Compounds (White Wash)
  • Greenhouse Shade Curtains
  • Materials for Shade and Heat Retention
  • Operation of Greenhouse Shade Curtains
  • 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
  • Vapor Pressure Deficit
  • Reducing Greenhouse Humidity
  • Cultural Practices
  • Ventilation and Heating
  • Air Circulation
  • Bottom Heat
  • Greenhouse Design
  • Anti-Drip Plastic