heating system to provide dual temperature water:180°F in the finned tube and 110°F in the floor loops. A system providing 180°F water with a mixing valve to get the 110°F water could also be used. Natural convection air movement will develop in a closed greenhouse with warm air rising to cool against roof surfaces, then falling to the floor to be
warmed. The result is a warm air mass in the ridge volume with large temperature variations possible in both vertical and horizontal directions. Uniform temperatures in the crop zone can be obtained by using small fans to create slow horizontal movement of the air mass (Figure 5–8 on next page). Total fan capacity should be equivalent to moving one-quarter of the greenhouse volume per minute to assure relatively uniform distribution. Fans should be mounted high enough to be clear of personnel. Wire enclosures (guards) are necessary unless the fans are 7 ft. or higher above the floor. If
greenhouse plants have tall, dense foliage such as roses, tomatoes, or cucumbers, air movement can be provided by using perforated polyethylene film ducts that discharge within the crop. Overhead perforated PE ducts used to distribute warm air can also help maintain uniform temperatures. However, if they are too high, warm air may not reach the plants. Air leaving the perforations has to overcome buoyancy forces and does not have high enough velocity to reach the
floor if the tube centerline is more than eight feet above the floor. Cooling Equipment
Cooling equipment
includes such items as vents, fans, shading, evaporative pad systems, fogging systems, and control elements. Propeller blade fans used for greenhouse cooling are low pressure, medium volume fans, either direct-connected or belt-driven. Two-speed units are available to help match supply with demand. There are some fan and motor units available for which output varies from maximum to about 10% of maximum. Fans are ratedaccording to output in cubic feet per minute (cfm)
against pressure in inches of water. Most greenhouse fan assemblies, including housing, louvers, etc., are