Chapter 3

Greenhouse Ventilation and Cooling

(draft 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 severely impact plant growth. Ventilation is an air exchange process that replaces the warm moist air inside the greenhouse with cooler, often less moist, outside air. Ventilation can be provided by natural or forced air systems. 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 ventilation is accomplished by fans that are capable to move large quantities of air at relatively low pressure drop. Proper ventilation not only cools the greenhouse, but also reduces the humidity level inside the greenhouse and replenishes the carbon dioxide that plants consume during daylight hours in the process of photosynthesis. At the same time, ventilation can reduce the concentration of pollutant gases and during winter, in cases where the heating unit is installed in the greenhouse, keep the combustion of the fuel at high efficiency since the lack of adequate oxygen results in incomplete combustion and carbon monoxide buildup. Techniques for cooling greenhouses can be organized into several categories, including pad and fan evaporative cooling systems, mist and fog systems, and shade control. Pad and fan systems draw outside air through a wet porous pad causing water to evaporate. This results in lowering the vapor pressure deficit, raising the relative humidity, and decreasing the air temperature. Mist and fog systems operate in much the same way except that they 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. Shading does not actually cool but rather reduces the amount of solar radiation reaching the plant. It reflects or absorbs incoming solar radiation before it reaches the crop.

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

Within This Chapter: Greenhouse Ventilation and Cooling

  • Introduction to Greenhouse Ventilation and Cooling
  • Natural Ventilation Systems
  • Buoyancy Driven Ventilation
  • Wind Driven Ventilation
  • Greenhouse Vents
  • Roof (Ridge) Vents
  • Sidewall Vents
  • Size of Vents
  • Vent Location
  • Vent Operation
  • Insect-proof Screens for Vents
  • Greenhouse Height
  • Orientation of the Greenhouse
  • Greenhouse Sun Screens
  • Forced-air Ventilation
  • Forced-air Ventilation Rates
  • NGMA Air Exchange Rate Standards
  • Fan Staging
  • Selecting Fans for Forced-air Ventilation
  • Fan and Louver Location for Forced-air Ventilation
  • Thermostat Selection and Placement
  • Air Distribution with Forced-air Ventilation
  • Horizontal Air Flow Fan System
  • Polytube Systems
  • Maintenance of Forced-air Ventilation Fans
  • Fan and Pad Evaporative Cooling Systems
  • Temperature Gradient
  • Systems Efficiency
  • Determining Greenhouse Ventilation Requirements
  • Exchange Rate Adjustments
  • Total Air Flow Required
  • Eveporative Cooling Pads
  • Cooling Pad Types
  • Determining Cooling Pad Size
  • Pump and Sump Tank Operation
  • Determining Pump Capacity
  • Determining Sump Tank Volume
  • Thermostats
  • Fan and Pad Evaporative Cooling System Maintenance
  • Pads
  • Recirculation Pumps
  • Filters
  • Distribution Headers
  • Greenhouse Mist and Fog Systems
  • Fog Systems
  • Application
  • Water Quality
  • Control Systems
  • Greenhouse Shading
  • Shading Compounds (White Wash)
  • Shade Curtains
  • Screen Materials
  • Operation of Greenhouse Shade Screens
  • Curtain Configurations
  • Mechanical Drive Systems
  • Greenhouse Curtain Support Systems
  • Greenhouse Humidity Control
  • Relationship Between Temperature and Humidity
  • Measuring Humidity with a Sling Psychrometer
  • Measuring Dew Point with a Sling Psychrometer
  • Methods in Reducing Greenhouse Humidity
  • Cultural Practices
  • Bottom Heat
  • Anti-drip Plastic
  • Ventilation and Heating
  • Air Movement
  • References