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Greenhouse Heating. Principles and Hardware. Important Heat Units. Btu – British thermal unit – amount of heat required to raise 1 lb of water 1 ºF Boiler horsepower – 33,475 Btu Calorie – is the amount of heat required to raise 1 g of water 1 ºC 1 Kcal = 1000 cal = 3.968 Btu
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Greenhouse Heating Principles and Hardware
Important Heat Units • Btu – British thermal unit – amount of heatrequired to raise 1 lb of water 1 ºF • Boiler horsepower – 33,475 Btu • Calorie – is the amount of heat required to raise1 g of water 1 ºC • 1 Kcal = 1000 cal = 3.968 Btu • 1 Joule = .239 cal = .00095 Btu • 1 Watt = 1 J per second
Heating Basics • You must add heat at the rate it is lost • Heat is lost by: CONDUCTION, Infiltration, and Radiation • Many different factors play a role in the amountof heat that is lost (a few are listed here): • Type of glazing • Frame material choices • Local climate
Glazing Heat Loss • Worst – single layer of polyethylene film or single layer of glass • Best – 3 layer polycarbonate panels • Double Layer of Poly with dead air space brings the heat loss down to .70 Units Heat Loss Units
Greenhouse Air Infiltration • Varies with glazing material choice • You must also consider the quality and age ofthe construction • Variability among greenhouses is high EXAMPLE: An older glass greenhouse mayhave air infiltration rates 8 times that of a newhigh quality double-layer plastic film coveredhouse.
Types of Heating Systems • Unit Heater • Central Heat • Radiant Heat • Solar Heat
Important Heating Notes CH2O(s) + O2(g) -----> CO2(g) + H2O(g) Incomplete Combustion C=O Sulfur Dioxide
Unit Heaters • Cost including installation labor is $0.50 - $1.00/sq. ft. • More ideally suited for smaller greenhouses. • As greenhouse operations get larger centralheating systems become more economicallyfeasible. • Components • Fuel burner • Heat exchanger • Distribution • Controls
Unit Heaters • Heat supplied as needed • Available in horizontal and vertical designs • Must have a constant supply of oxygen • Exhaust stack must be tall enough above thegreenhouse roof to permit dissipation of thesmoke without re-entry into the greenhouse. • Openings for fresh air must be provided for unit heaters---or else a heater can use up all the oxygen in a house quickly.
Common Fuelsfor Unit Heaters • No. 2 oil • Kerosene • LP gas • Natural gas Many types of unit heaters can be refitted to burn a different type of fuel.
Unit Heaters You go outside to your doubly poly greenhouse early one winter morning and realize that the heater is off. You are naturally concerned about your Poinsettias as the Holidays are approaching. What should you do first? and Why? ALLOW FRESH AIR IN THE GREENHOUSE
HAF (Horizontal Air Flow) • Horizontal Airflow • Utilized to establish uniform temperatures in greenhouses • For individual greenhouses – two rows of fans installed along the length of the greenhouse • Fans should be 2-3 feet above the plants • For ridge and furrow fans should be installed along the center of the greenhouse
Central Heat • Consists of one or more boilers in a central location (steam or water) • Cost $2.50-$3.25 / sq. ft. of floor space • Boilers can burn cheaper fuels than unit orradiant heaters • Engineer Redundancy into the system whenever possible.
Suitable Boiler Fuels • Wood Chips • Logs • Coal • No. 4,5, and 6 fuel oil Many boilers can be retrofit to burn other fuels at a nominal cost.
Other Boiler Notes • Hot water boilers most popular worldwide • Temperature uniformity is greater than in steam systems • Larger reserve of heat • Due to lower operating temperatures you can place the pipes closer to the plants • For some crops Boilers can also be a good source of CO2 for greenhouse atmosphere enrichment.
Boiler Components • FIREBOX – where fuel is burned • Flue – provides a way for smoke, from thefirebox, to vent to the outside air • Heat exchanger – network of tubes either filled with or surrounded by water.
Heat Distribution • Pipe size and quantity • American greenhouse: 2” Fe pipe 180 F • Dutch greenhouse: 2” Fe pipe 203 F • Steam delivery: smaller pipe 215 F • Amount of pipe necessary depends on greenhouse, crop, local climate, and type of pipe to be utilized • Tables 3-3 and 3-4 give you the info necessary to estimate pipe length once you have calculated the necessary total heat requirement for the greenhouse.
Radiant Heater System • May be cheaper to operate than unit heaters•Emit infrared radiation • Plants and other objects are heated first beforethe air, so air temperatures can be 5 to 10 ºFcooler than a conventional house • Available in sizes from 20,000 to 120,000 Btu/hr
Radiant Heat Systems • Place above plants in lines running along the length of the greenhouse • Can be spaced as far as 40 ft apart Composition of reflector is important
Radiant Heat Advantages • Newer systems do not emit visible red light • More efficient fuel combustion • Cooler air temps reduce DF; why does this saveyou money? • Only 25% electrical consumption of othersystems.
Solar Heat • Tax incentives for solar heating are gone, until this winter petroleum fuel product costs were low. • There are commercial greenhouses with solar power – however some real costs may not have been included • Since fewer structures like this are built construction costs are generally higher.
