GREENHOUSE MGMT I
GREENHOUSE MGMT I HORT 4050
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This 35 page Class Notes was uploaded by Mack Koelpin on Saturday September 12, 2015. The Class Notes belongs to HORT 4050 at University of Georgia taught by Thomas in Fall. Since its upload, it has received 90 views. For similar materials see /class/202008/hort-4050-university-of-georgia in Agricultural & Resource Econ at University of Georgia.
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Date Created: 09/12/15
Greenhouse Cooling Why is Cooling Needed 0 Solar radiation is the heat input for the earth can radiate as much as 277 Btuftzlhr onto the surface of the earth on summer s day in coastal and industrial areas may only be 200 Btulftzlhr Up 85 of this radiation may enter the greenhouse most of the IR heat becomes trapped inside greatly increases the greenhouse temperature 0 Mechanism is needed to remove this trapped heat 1 Greenhouse Cooling Types of Cooling Systems 0 Summer cooling oWinter cooling Summer Cooling Passive Cooling Systems 0 Open ridge and side wall vents temperatures can still rise 20 F above outdoor in oldstyle greenhouses ridge vent area is only about 10 of the total roof area newer passive cooling greenhouses have ridge vent areas equal to 20 to 40 of the total roof area 0 can maintain temperatures closer to outdoor temperatures 1 Summer Cooling Passive Cooling Systems 0 Other modifications include rollup side walls for ventilation roof ventilators on plastic filmcovered greenhouses retractable roof greenhouses for almost 100 ventilation area Summer Cooling Passive Cooling Systems 0 Passive cooling is cheaper to operate than active cooHng 0 Initial construction cost 125Ift2 of greenhouse floor area to add ventilators to a polyethylene film covered greenhouse Summer Cooling Active Cooling Systems Takes advantage of heat absorption from air during the evaporation of water 0 Two major types fan and pad fog cooling oAccurately designed fan and pad system should be able to lower the temperature of incoming air by 80 of difference between the dry and wet bulb temperatures 1 Summer Cooling Active Cooling Systems 0 Dry bulb temperatureactua air temperature measured with an ordinary thermometer Wet bulb temperaturethe air temperature if enough water were to be evaporated into it to saturate the air measured with a psychrometer wet sock is placed over the bulb of a thermometer air is drawn across the moistened sock to allow for evaporative cooling 1 Summer Cooling Active Cooling Systems Wet bulb temperature is what the air can be cooled to if 100 efficient evaporative cooling system 0 Fan and pad systems only reach about 80 efficiency fog systems about 95 0 Fan and pad cooling systems 075 to 125Ift2 of greenhouse floor area PSYCHROMETRIC CHART HlV A60 d0 ON Od 83d EMULSION d0 SNIV dS o O 0 0 O o O 8 N 9 9 8 a r 8 n q n 8 9 O O N I DRY BULB TEMPERATURE 39F Summer Cooling Physics of Evaporative Cooling 0 Use evaporation of water to convert sensible heat into latent heat thus reducing the temperature of the air oAbout 1060 Btu s of heat are absorbed out of the air for every pound of water evaporated 0 Conversion of energy from sensible heat into latent heat does not change total energy content of the air lowers the temperature of the air and increases the water vapor content tranfers energy to water vapor increases the relative humidity Summer Cooling Physics of Evaporative Cooling 0 Process of cooling air through the evaporation of water is called adiabatic cooling adiabaticpertaining to or designating a reversible thermodynamic process executed at constant entropy loosely occurring without gain or loss of heat oAbility to reduce sensible heat of air depends on how much water we can evaporate into the air Summer Cooling Physics of Evaporative Cooling 0 Amount of water that can be evaporated into air depends on the original temperature of the air sensible heat measured with a dry bulb thermometer o the warmer the air the more water it is capable of holding note the grains of moisture per pound of dry air along the right axis of the psychrometric chart 7000 grains of water 1 lb of water i 20 PSYCHROMETRIC CHART 65 7O DRY BULB TEMPERATURE 39F I GRAINS OF MOISTURE PER POUND OF DRY AIRI Summer Cooling Physics of Evaporative Cooling 0 Amount of water that can be evaporated into air depends on how much water is already in the air the moisture content 0 the grains of water difference between the original RH and 100 RH is the evaporative cooling capacity if we could achieve 100 efficiency Summer Cooling Physics of Evaporative Cooling 0 Amount of water that can be evaporated into air depends on the density of the air owaterholding capacity of air is based on lbs of water per lb of air not per ft3 of air 0 related to the barometric pressure and thus the elevation under normal greenhouse temperatures and given a BP of 2992 1 lb of air occupies about 135 ft3 1 Summer Cooling Physics of Evaporative Cooling 0 Amount of water that can be evaporated into air depends on the density of the air 0 systems designed based on this density to pull enough air through the greenhouse less dense air air at higher altitudes greater volume per pound of air greater volumes of air must be pulled through to achieve design levels of cooling 1 Summer Cooling Physics of Evaporative Cooling 0 Amount of water that can be evaporated into air depends on the efficiency of the evaporative system ofan and pad systems 80 fog systems perhaps closer to 95 Summer Cooling Evaporative Cooling Examples Tucson Arizona dry bulb temperature 100 F wet bulb temperature 70 F elevation 2400 feet above sea level 0 Raleigh NC dry bulb temperature 94 F wet bulb temperature 77 F elevation 360 feet 1 Need 28 grainslb PSYCHROMETRIC CHART Need 48 grainslb 50 40 30 20 65 7O 80 DRY BULB TEMPERATURE 39F GRAINS OF MOISTURE PER POUND OF DRY AIR Summer Cooling Evaporative Cooling Examples Tucson 80 efficiency 80 of the wet bulb 100 F 08 x 100 F 70 F 76 F since elevation is 2400 feet we need to increase the flow of air through system to account for the lesser density of the air 148 ft3llb rather than 135 ft3llb need a flow rate about 1096x greater than if our elevation were under 1000 feet 1 Greenhouse Heating Central Heating Systems 0 System one or more boilers in a central location distribution pipes piping steam or hot water into the greenhousealmost all are hot water today boiler components heat distribution system 250 to 325Ift2 of greenhouse floor area more expensive to install than unit heaters but can burn cheaper fuels and require less maintenance Greenhouse Heating Central Heating Systems 0 Boilers firebox flue and heat exchanger will only cover hot water boilers heat exchangertubes filled with water surrounded by flue gases g tubes filled with flue gasses surrounded by water locate boilers in service building not humid greenhouse locate stack so no shadows and no smoke towards greenhouses Greenhouse Heating Central Heating Systems 0 Boilers fire tube boilers gases runs through tubes flue ltubes surrounded by water 0 requires a large volume of water to fill boiler high mass boiler slow to heat up yet slow to cool down 0 can burn wood coal oil or natural gas Greenhouse Heating Central Heating Systems 0 Boilers water tube boilers owater runs through tubes or thin plates fins l gasses surround the tubes chamber is the flue needs only a small volume of water to fill boiler low mass boiler quick to heat up yet quick to cool down burns natural gas or propane no soot 0 less expensive and smaller than fire tube type i Greenhouse Heating Central Heating Systems 0 Heat distribution hot water or steam leaves the boiler heat exchanged in greenhouse via pipe coils unit heaters or both 2inch diameter pipe is used for hot water 180 F water is pumped through pipes Greenhouse Heating Central Heating Systems 0 Pipe placement wall pipe coils should supply enough heat to replace loss through walls 0 low and against curtain wall 0 leave about 2 clearance around pipes stacking reduces Btu output Table 34 0 finning increases Btu output up to 4X 1 Greenhouse Heating Central Heating Systems 0 Pipe placement overhead pipe coils located above plants must force heated air down pipes out of the way but shadows 0 may be needed for snow melt placed under gutters Greenhouse Heating Central Heating Systems 0 Pipe placement inbed pipe coils along edges of ground beds beneath benches owithin ground beds roses 0 within the concrete floor more later puts the heat near plants Greenhouse Heating Central Heating Systems 0 Additional heat distribution convection tubes also for winter cooling HAF Unit heaters no firebox just heat exchange coils and a fan can be used with convection tubes or HAF can replace overhead gutter heat needs Greenhouse Heating Central Heating Systems 0 lnfloor piping pipes placed in concrete floorbidirectional flow uses cooler 90 to 120 F water can be as high as 140 F systems output between 20 and 30 Btuft2 of floor polyvinyl chloride pvc polybutylene crosslinked polyethylene pex and synthetic rubber epdm piping much less than metal I easier to fit but takes more piping than inbed distribution why i Greenhouse Heating Central Heating Systems 0 lnfloor piping very efficient especially for crops grown on the floor will dry floor rapidly if flood floor irrigation is used still need some sidewall and overhead heating total system including concrete floor floor pipes wall pipes overhead pipes boiler installation and controllers costs 425 to 500Ift2 Environmental Control Systems Types of Controls 0 Manual controls night watch personnel monitoring open vents each morning large temperature variability Environmental Control Systems Types of Controls 0 Thermostat controls bimetallic and coil thermostats are not very accurate cost from 50 to 600 each each device requires separate thermostat no coordination among equipment possible avoid overlap of set points among devices 1 T t d39 I Blmetalllc Thermostat PM m M z 2 Tempurature pointer I 4 Cam 5 Calibration screw 3 Dial lack screw 6 Blue Contact screw 7 Differential dial an white 6 Matt screw 9 Contact arm 0 P lt 9 ll Bimemnic strip
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