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--- firewood_weight_volume_relationship [2012/04/05 19:27]
ddrummond created
+++ firewood_weight_volume_relationship [2013/03/28 17:12] (current)
rsheridan consider leaving , include heating value per dry pound or ton between species, consider keeping it, check for editing on this page!,
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 ====== Firewood Weight/Volume Relationship ======
+===Heat value of Dry Wood===
+The heat value of wood **per unit of weight** is about the same for all species: 8,600 Btu per pound,
+ovendry weight. Exceptions to this rule are very resious species, which have slightly higher values.
+The heat value of hardwood bark is slightly lower than that of wood, while the energy content of pine
+bark is slightly higher.
+The heat value of wood **per unit volume**, at any given moisture content (MC), depends on its specific
+gravity or relative density. The higher the specific gravity (SG) of a wood, the denser the wood per
+unit of volume, and the higher its heat value.
+The table below groups common tree species by relative density classes. Each class represents a different
+range of specific gravities of wood, and each class has been assigned an average specific gravity factor.
+This factor is used to calculate the **approximate** amount of heat energy available in a given volume
+of dry wood.
+===Calculation of Gross Heat Value (GHV)===
+To calculate the GHV of a cord of dry wood, two assumptions are first made:\\
+. A cord(CD) of dry wood contains 80ft<sup>3</sup> of solid wood.\\
+. Each pound of ovendry wood, regardless of species can produce 8,600 Btu of heat energy.
+**Relative density classes, based on specific gravity, for common tree species, with average specific
+gravity factors.**
+^SG Range              ^Low-Density Woods  ^Medium-Density Woods    ^High-Density woods     |
+^                      ^Less that .50      ^.05-.59                 ^Greater than .59       |
+^Avg. SG Factor        ^0.45               ^0.55                    ^0.65                   |
+|Species, common names |Aspen              |Ash, green             |Ash, white            |
+|                      |Baldcypress        |Birch, paper            |Beech                  |
+|                      |Basswood           |Cherry, black           |Birches, sweet, yellow |
+|                      |Butternut          |Elms: American, Slippery|Dogwood                |
+|                      |Cedar              |Hackberry               |Elm, rock              |
+|                      |Cottonwood         |Holly, American         |Hickory                |
+|                      |Fir                |Magnolia                |Honeylocust            |
+|                      |Hemlock            |Maple, red              |Hophornbeam            |
+|                      |Maple, silver      |Pines, Loblolly, longleaf, pitch, pond, shortleaf, slash|Locust, black|
+|                      |Pines, Eastern white, Jack, Red, Sand, Spruce, Virginia|Sweetgum|Magle, sugar|
+|                      |Sassafras          |Tamarack                |Oak                    |
+|                      |Sycamore           |Tupelo                  |Osage-orange           |
+|                      |Willow, black      |Walnut, black           |Pecan                  |
+|                      |Yellow-poplar      |                        |Persimmon              |
+Specific gravity is based on ovendry weight and volume at 12% moisture content. The average specific
+gravity factors shown here were arbitrarily chosen to represent all species within a certain density
+class.
+The formula for **gross heat volume** is:
+GHV = SG factor X [6.23 lb./ft<sup>3</sup>(wt. of water)X 80 ft<sup>3</sup>/CD. X 8600Btu/ovendry lb.]
+[6.23 lb./ft<sup>3</sup>(wt. of water)X 80 ft<sup>3</sup>/CD. X 8600Btu/ovendry lb.] = 42,862,400 Btu/cord.
+This formula is used to derive approximate GHV's‡ for each of the three density classes:
+     Low Density Woods:
+        GHV = 0.45 X 42,862,400 Btu/cord = 19.3 million Btu per cord. (MMBtu/CD.)
+     Medium Density Woods:
+        GHV = 0.55 X 42,862,400 Btu/cord = 23.6 MMBtu/CD.
+     High Density Woods:
+        GHV = 0.65 X 42,862,400 Btu/cord = 27.9 MMBtu/CD.
+Use the following table to provide a more precise GHV, if a species' average specific gravity
+is known.
+**Gross heat value of ovendry wood by specific gravity**
+^Relative density or specific gravity   ^Gross heat value per cord (MMBtu) |
+|.30                                    |12.8                              |
+|.35                                    |15.0                              |
+|.40                                    |17.1                              |
+|.45                                    |19.3                              |
+|.50                                    |21.4                              |
+|.55                                    |23.6                              |
+|.60                                    |25.7                              |
+|.65                                    |27.9                              |
+|.70                                    |30.0                              |
+|.75                                    |32.1                              |
+|.80                                    |34.3                              |
+|.85                                    |36.4                              |
+|.90                                    |38.6                              |
+‡These GHV's a re only approximations of Btu's available from a cord of dry wood of a species group.
+If a more precise heat value is needed for a particular species, the average SG of  that species
+must be used in place of the density class SG factor. SG values for most woods can be found in
+__The Wood Handbook__ (USDA FS, For. Prod. Lab. Ag Hdbk No. 72)
+For example, if we knew the average SG of balsam fir were 0.36, we could use the preceding table to
+determine a more exact GHV for a dry cord of balsam fir: 15.4 million Btu. This figure is certainly
+a better approximation than the 19.3 milion Btu derived through the use of the first and the
+generalized GHV formulas for broad density classes.
+Note that the GHV's calculated by the density class method are only approximations; they are meant
+to provide relative values for large groups of woods. More realistic GHV's can be derived, however,
+by using the same general formula and by substituting a more precise SG value for the standard SG
+value shown in the first table.
+Also not, at this point, that **gross heat values** represent the **maximum Btu values for ovendry
+wood.** To find the actual, or net, heat value of a cord of green or partly dried wood, other
+factors must be considered, such as the amount of water in the wood and the combustion efficiency
+of the device in which the wood is to be burned.
+**Heat Value of Green Wood**
+All wood except that artificially dried in the laboratory contains some amount of water, commonly
+expressed as moisture content (MC). Though there are two accepted methods of calculating MC, most
+wood technologists calculate MC ont he basis of ovendry weight (ODWT), as follows:
+MC% = (Green weight-ODWT)/(ODWT) X 100  [Equation 1]
+In the above equation, the green weight (GWT) and ODWT of a given volume of wood must be known. If
+the MC and ODWT of a cord of wood are known, GWT can be calculated as follows:
+GWT = ODWT(1+(MC%/100)) [Equation 2]
+Using the same relationships, and knowing both GWT and MC, ODWT can be found this way:
+ODWT = (GWT/(1+(MC%/100)) [Equation 3]
+if MC, ODWT, and GWT of a cord are not known, but the SG of that wood is, we can calculate ODWT
+another way:
+ODWT/CD = SG X 6.23lb/ft<sup>3</sup> X 80ft<sup>3</sup>/CD. [Equation 4]
+All of these equations are helpful in calculating the **low heat value** (LHV) of wood. This value
+is defined as the amount of net energy available in a given volume of wood after accounting for the
+energy needed to evaporate the water in the wood, but before accounting for the the combustion
+efficiency of the device in which the wood is burned. After determining LHV, combustion efficiency
+can be factored in to arrive at the bottom line, net heat value (NHV.)
+===Calculation of Low Heat Value===
+To estimate LHV it is necessary to know the weight of water contained in a given volume of wood. That
+water must be evaporated before wood can burn. This water evaporation requires about 1,210 Btu per
+pound of water, and the total energy needed to evaporate all the water in the wood must be deducted
+from GHV to arrive and LHV.
+Take as an example a cord of "green" red maple. Assume approximate heat values are all that is
+required. The table above suggest the GHV for red maple to be 23.6 million Btu per cord. To simplify
+the process, assume green woods have an average MC of 75% ODWT basis. [The reported average MC for red
+maple is 70%]. At 75% MC, how many pounds of water are in a cord of red maple? This value would be the
+difference in weight between a green cord and an ovendry cord.
+Recall that equation 4 can be used to calculate ODWT/CD, if we know the SG. Because an approximation
+of LHV is all that is required for this example, use .55 as average SG (see table above).
+ODWT/CD = 0.55 X 6.23 X 80 = 2,741 pounds.
+With values for MC and ODWT, equation 2 can be used to calculate GWT:
+GWT/CD = 2,741(1+ (75%/100)) = 2,741(1.75) = 4,797 pounds
+Weight of water in the cord would be:
+Wt. of water/CD = GWT/CD - ODWT/CD = 4,797lb - 2,741pounds = 2,056 pounds
+The amount of energy needed to evaporate this weight of water is 2.5 million Btu per cord, i.e.,
+,210 Btu/lb of water X 2,056lb of water/CD
+Now deduct this value from GHV to arrive at LHV:
+LHV = 23.6 MMBtu/CD -2.5 MMBtu/CD. = 21.1MMBtu/CD
+===Calculation of Net Heat Value (NHV)===
+Calculation of net heat value requires reducing LHV to account for energy losses which occur during the
+combustion process. These Btu losses may be due to excessive air entering the combustion chamber,
+excessively high temperatures of flue gases, ro simple radiation losses. Btu losses are generally
+related stove or furnace efficiency.
+Efficiency of stoves, fireplaces, and other heating devices are affected by the design, quality of
+installation, location, indoor and outdoor temperatures, and use patterns. Fireplaces range in
+efficiency  from -10 to 15%, box stoves 20-40%, and airtight stoves 25-70%.
+To calculate NHV it is necessary to know which type of heating device will be used to burn the wood,
+and then to apply a combustion efficiency factor to the LHV.
+Assume an airtight stove is to be used to burn a cord of red maple, and that it's a new, carefully
+installed and properly located stove. Its combustion efficiency factor will be at least 60%.
+Net Heat Volume (NHV) = 0.60 X 21.1 million Btu per cord. = 12.7 million Btu per cord.
+For this example, under assumptions made, a green cord of red maple burned in a 60% efficient airtight
+stove would produce approximately 12.7 million Btu's net heat energy.
+If the average reported SG for red maple (0.54) and that species' average reported green MC (70%) had
+been used in the calculation of NHV (in an effort to derive a more precise value), the NHV would have
+been approximately 12.5 million Btu per cord, only 0.2 millin Btu off the approximation [1.6% error].
+   The process of calculation NHV can be summarized this way:
+. GHV (MMBtu/CD) = SG X 42.862 MMBtu/CD.
+. LHV (MMBtu/CD) = GHV - [1,120 Btu/lb X lb. water/CD]
+. NHV (MMBtu/CD) = Combustion efficiency factor X LHV
+When asserting the energy content of a cord of firewood, the most important parameter is the ovendry
+density for that species because a pound of dry wood of any species has about the same energy content.
+If all species were sold at the same price, the best buy would, of course, be the denser wood, assuming
+equal moisture contents. When buying wood by the pound, the most important parameter is moisture
+content, and the driest wood would be the best buy.

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