Ventilation and Cooling

How many fans are needed andwhat do I need to know beforehaving them installed?The size of fans selected determines the numberof fans required. Adequate fans should be usedto provide a spacing of not more than 25 feetalong the exhaust side of the greenhouse. Whenpossible, the fans should be located on the downwindside of the greenhouse. When three orfewer fans are used in a given installation, one ofthem should be a two-speed fan to provide formore flexibility of ventilation. Fans should beguarded properly to prevent workers or animalsfrom coming in contact with any moving parts.For most reliable fan performance, only use fansthat have been tested and rated by the AirMovement and Control Association (AMCA)standard test code and which bear the AMCACertified Rating Seal.Where should the cooling padbe placed?The cooling pad should be continuous along theentire side or end of the greenhouse. Pad height isdetermined by dividing the total pad area by thelength. Pads should be confined and secured in away that provides uniform airflow, prevents sagging,avoids puncturing holes or large openings inthem and promotes uniform water flow throughthe entire length of the pads. The pads should beinstalled for ease of removal and withstand normalhandling and usage. Whenever possible, the airinlet should be constructed in such a way that itcan be readily opened and closed without removingthe pads. It is preferable to have the padassembly located inside the air inlet opening; thiswill produce less turbulent airflow through thehouse. For such an arrangement the air inlet openingneed not be continuous but should be at leastwell distributed.What if the pad is located outsidethe air inlet opening?If the pad is located outside the air inlet opening, itshould be continuous, have no large obstructionsand should be centered in elevation on the centerof the pad so that the airflow is horizontal as itleaves the opening. When the height of the padexceeds that of the air inlet opening, the pad willextend above and/or below the opening. Whenthis occurs, the pad should be set back from thewall or sash at a distance of at least half theamount of the extension to provide ample room forair to pass uniformly through the entire pad. Ifpossible the pads should be located on the prevailingwind side of the greenhouse. When the greenhouseis sheltered from prevailing winds by anotherbuilding or greenhouse located within 25 feet,the location of the pads in relation to prevailingwinds is not significant.How and why should I use baffles?The cooling air passing through the house willtend to diverge at about a 7-degree-angle or onefoot in every eight. Vertical baffles usually areinstalled particularly with the house, to reducemixing and keep cool air down at plant level. Thebaffles should be transparent, spaced approximately30 feet and held in a fixed vertical plane, andtheir lower edges should be well above plant level.In greenhouses containing raised benches, a substantialvolume of air may travel under the benchesto the fan without removing much heat. A bafflecovering two-thirds of the distance from the benchto the ground will force most of the cooling air tocrop level for more effective plant cooling.How do I maintain my water system?All elements of the water system should be keptwell covered, to protect it from insects and windborndirt, which will ultimately cause clogging.The water returned to the sump should bescreened. The ends of the water distribution pipesshould be provided with removable caps or valvesto allow easy flushing out for cleaning. Becausewater is used up in the evaporative cooling process,a supply of makeup water is needed. An automaticfloat valve should be used for this purpose and alsoto maintain a proper water level in the sump.Are winter and summer ventilationsystems the same?Essentially winter and summer ventilation systemsare two separate systems having different characteristicsand requirements. However, they must tie inwith each other to switch from one system to theother during spring and fall. The transition is veryimportant as it determines the inside to outside temperaturedifference available on a mild, sunny day,and it establishes the airflow design capacity of thewinter ventilation system.Do I need to ventilate differentlyin the winter?Mechanical ventilation essentially provides thesame benefits regardless of the season, but wintercalls for different airflow principals. In the winter,outdoor air is too cold to introduce directly on theplants. The goal of winter ventilation is to introducethe cold air in a turbulent manner. This causesthe cold air to mix with the warmer air in the greenhouse,without producing cold drafts at plant level.This mixing is the result of using small high-velocityjets. In a greenhouse it is desirable to have manysmall, well-distributed openings rather than onelarge one for winter ventilation. It is important thatall areas of the greenhouse are at the same temperature.To achieve this, the ventilating system mustdistribute the air uniformly throughout the houseand maintain positive air movement and continuouscirculation.Can I use gravity to circulate theair in my greenhouse?A powered ventilating system has a real advantageover gravity systems that rely on thermal air currentsbecause it has the energy required to provideuniform air distribution and mixing. Fans thatmechanically ventilate greenhouses, combinedwith perforated transparent plastic tubes, make anideal system for introducing cold air into a greenhousein the winter without cold drafts.What do I use thermostats for inmy greenhouse?Thermostats or controllers are used to turn fanson and off as required to meet changes in outdoorclimate conditions and thereby maintain moreuniform greenhouse temperatures with loweroperation costs.What do I use a humidistat for?During warm weather a Humidistat can be used tocontrol the pump of the cooling pad system to helpprevent excessive greenhouse humidity. A humidistatthat is wired to operate exhaust fans also canhelp prevent excessive humidity.How can I find out more aboutventilation?You can find out more about ventilating andcooling greenhouses by contacting the NGMAfor a free copy of the ventilation guidelines ordownload it from the web: www.NGMA.comVISIT US ON THE WEB:WWW.NGMA.C OM20 W. Dry Creek Circle, Suite 110Littleton, CO 80120800-792-NGMA (6462)303-798-1338 • FAX 303-798-1315