Part2Lecture4-7 - Heating (Lecture 4 and 5) Nelson, Chapter...

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Fall 2002 Biernbaum, MSU HRT 221, pg 38 Heating (Lecture 4 and 5) Nelson, Chapter 3 When designing a heating system for a greenhouse, the first step is calculating how much heat energy must be supplied. The amount of heat necessary to keep the greenhouse warm can be determined by adding up the amount of heat that is lost and must be replaced. The calculation is made for heat loss under the average highest heat loss conditions. The average winter low temperature for the coldest month is used. The second step is selecting an economical and efficient heating system to supply the heat energy. Heat is a form of energy that can be transferred when a temperature difference exists. Heat can be transferred by one of three methods. conduction : transmission through a solid convection : air currents or movement, infiltration radiation : energy from a warm body or mass to a cool body or mass as electromagnetic waves Diagram of greenhouse energy balance.
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Biernbaum, MSU HRT 221, pg 39 Heat energy is measured in British Thermal Units or BTU's. A BTU is the amount of energy needed to raise one pound of water one degree fahrenheit. One pound of water is approximately equal to one pint or 16 fluid ounces. One gallon of water equals 8.33 pounds. One cubic foot of water holds 7.4 gallons, and would require 58 BTU to raise the temperature one degree Fahrenheit. In contrast, 1 cubic foot of air requires 0.02 BTU to raise the temperature one degree fahrenheit. One boiler horsepower is equal to 33,475 BTUs. Heat loss calculation (This is different then the method covered by Nelson, but similar to that covered in Greenhouse Engineering). Heat loss in BTU/hour = conductive heat loss of the greenhouse surface area Area (ft 2 ) x temperature difference ( o F) x u value (BTU/ft 2 /hour and o F) + the conductive heat loss of the perimeter perimeter length (ft) x temp dif ( o F) x u value (BTU/ft/hour and o F) + the convective or infiltration heat loss Volume (ft 3 ) x temp dif ( o F) x 0.02 BTU/ft 3 and o F x air exchanges/hour A u value is a heat transfer coefficient which is a constant for different types of greenhouse coverings. U values are listed in the texts. The u value is the inverse of an R value. The u value has dimensions of BTU/ft 2 /hour and o F. R value
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Part2Lecture4-7 - Heating (Lecture 4 and 5) Nelson, Chapter...

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