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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 Fall '04
 BIERNBAUM
 Energy, Heat, Heat Transfer, Biernbaum

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