Grower 101: Using Evaporative Cooling, Part II

The heat of the summer is right around the corner, and thatmeans more energy is being used to keep your greenhouse cool. As you learned inPart I in the March issue of GPN, fan and pad systems are great for cooling,but there are more options: swamp coolers, mist and fog systems, and fan-generatedfog units.

Swamp coolers

Typically known as “swamp coolers,” self-containedevaporative cooling units are mounted on a concrete pad adjacent to thegreenhouse. A unit consists of a metal enclosure that contains a blower or fan andeither cellulose pads and a water pump or a polyester belt that rotates in apan of water. The dry outside air drawn in through the pads or belt picks upmoisture before it enters the greenhouse. After the cool air is heated insidethe greenhouse, it is exhausted through the louvers or vents, taking the heatwith it. These units work well in naturally vented greenhouses.

Portable swamp coolers are also available. They are usuallyplaced inside the door of a greenhouse on the end opposite where the fans arelocated. Air that enters through the door is conditioned and picks up heat asit travels toward the fans. A hose connection and electric supply are needed.One unit will cool up to 1,200 sq.ft. Measurements taken last summer in aConnecticut greenhouse showed that air temperature in a shaded greenhouse withswamp coolers was several degrees below outside ambient.

Mist and fog systems

Research has shown that fine mist or fog is more effectivethan cooling pads or swamp coolers. Most mist and fog systems utilizehigh-pressure nozzles to form fine water droplets. Mist contains droplets inthe 50- to 100-micron size (thickness of a human hair = 0.004 inches = 100microns). Fog contains droplets of 0.05-50 microns. The smaller the dropletsize, the quicker it absorbs heat and changes to the vapor state. Largerdroplets take longer to change and can end up wetting the surface they fall on.Therefore fog is more desirable for evaporative cooling whereas mist is usuallybetter for plant propagation.

A high-pressure pump is necessary to develop the forceneeded to form mist or fog. Depending on the nozzle style, it takes 60-500 psito form mist droplets and 500-1,200 psi to form fog. High-pressure piping isalso necessary. Particular care has to be given to filtration to prevent nozzleclogging. Because so little water is needed, (typical nozzle output is0.5?1.0 gal. per hour) some growers use demineralized, bottle water orrain water. Anti-drip nozzles are also installed to prevent draining the systemafter the water is shut off.

Most fan systems are set up with many of the nozzles overthe intake louvers and vents, with a small percentage distributed throughoutthe crop area. Building shelters over the intake end of the greenhouse andmounted nozzles within this will restrict any dripping to outside thegreenhouse. In natural ventilation systems, the nozzles are uniformlydistributed throughout the greenhouse.

Fan-generated fog units

An alternative system utilizes afan with a special hub and blade assembly. Water is fed into the hub andchanneled to the four blades. As the water exits the end of the blade, itencounters a shearing speed and atomizes into fine fog droplets. The fandistributes the fog in the greenhouse. Sizes are available that will cool up toseveral thousand square feet of growing area.

With all systems, addingexcessive moisture to the greenhouse can increase the incidence of disease.

Usually, evaporative cooling isthe last stage of cooling in the greenhouse. If the fan system or naturalventilation cannot satisfy the environmental needs, the evaporative coolingsystem activates. Control can be from a thermostat, controller or computer. Ifyou need to keep the temperature down for crop production this summer, tryevaporative cooling.

John Bartok is an agricultural engineer and extension professor-emeritus in the Natural Resource Management and Engineering department at the University of Connecticut, Storrs, Conn. He may be reached at jbartok@rcn.