Fuel heating values
To determine the cost of fuel per Btu you must know its heating value, which is how many Btu are produced by a given amount of the fuel. Most liquid fuels, including propane, are purchased by the gallon. Natural gas is usually purchased by the cubic foot (CF) or by 100 cubic feet (CCF), but is sometimes purchased by the therm (100,000 Btu). Thus, you must know how many Btu the fuel produces per gallon, per pound, per cubic foot, or per therm, as applicable. The heating values of various fuels are shown in Figure 18.
Figure 18. High and Low Heating Values
Type
of Energy |
Gross
or HHV |
Net
or LHV |
| No. 2 Fuel Oil |
140,000 Btu/gal |
132,000 Btu/gal |
| No. 5 Fuel Oil |
150,000 Btu/gal |
143,250 Btu/gal |
| Propane (LPG) |
90,500 Btu/gal |
84,345 Btu/gal |
| Natural |
1,040 Btu/Cu Ft |
905 Btu/Cu Ft |
| Gas |
104,000 Btu/CCF |
90,500 Btu/CCF |
| Electricity |
3,413 Btu/Kwh |
| CCF stands for 100 cu ft. The actual heating values of various fuels vary from one region to another. However, the values used here are for fuels commonly used in the US. Electrical energy is based on a resistive load. HHV and LHV do not apply to electricity. |
Two types of heating values are used to indicate the amount of heat a fuel can produce: LHV (low heating value) and HHV (high heating value). LHV is the net or useable amount of heat (Btu) produced by a fuel. HHV is the gross amount of heat (Btu) produced by a fuel. The difference between HHV and LHV is due to hydrogen in the fuel, which is normally lost as water vapor or moisture.
Unfortunately, the difference between the two values is not widely known or understood. Most suppliers of fuel oils and gas cite gross values for their fuels, but do state whether they are net or gross. Moreover, their representatives may not know that there is a difference. Thus, you should assume that Btu values shown on fuel bills are gross values unless specifically stated otherwise. And when calculating fuel costs you need to take the difference into account. Both values are sometimes used in calculating the amount of fuel required for heating asphalt and drying aggregate. However, it is usually more appropriate to use LHV.
To get an idea of the difference, compare the heating values of the most commonly used fuels. No. 2 fuel oil has a LHV of approximately 132,300 Btu per gallon. It has a HHV of approximately 140,000 Btu per gallon, a difference of 7700 Btu. One cubic foot of natural gas has a LHV of approximately 905 Btu. It has a HHV of approximately 1,040 Btu per cubic foot, a difference of 135 Btu.
A therm is 100,000 Btu (Figure 19). It takes approximately 0.758 gallons of No. 2 fuel oil at LHV to produce one therm. It takes approximately 100 cubic feet (1 CCF) of natural gas at LHV to produce one therm. Suppliers of natural gas may bill their customers either by the hundred cubic feet (CCF) or by the therm. It is important to note that natural gas suppliers normally calculate a therm based on the HHV of natural gas, which takes approximately 96.154 cubic feet (0.961 CCF) per therm.
Figure 20 compares the cost of fuel per Btu. This can serve as a guide to selecting a fuel from those that have acceptable levels of emissions.
Figure 19. Working With Therms
Type
of Energy
|
No. of Therms
(100,000 Btu) |
Amount Required For One Therm (100,000 Btu) |
| Gross or HHV |
Net or LHV |
Gross or HHV |
Net or LHV |
| No. 2 Fuel Oil |
1.400 per gal |
1.320 per gal |
0.714 gal |
0.758 gal |
| No. 5 Fuel Oil |
1.500 per gal |
1.433 per gal |
0.667 gal |
0.698 gal |
| Propane (LPG) |
0.905 per gal |
0.843 per gal |
1.105 gal |
1.186 gal |
Natural
Gas |
1.040 per CCF |
0.905 per CCF |
0.962 CCF |
1.105 CCF |
| 0.010 per cu ft |
0.009 per cu ft |
96.154 cu ft |
110.497 cu ft |
| Electricity |
0.034 per Kwh |
29.300 Kwh |
| CCF stands for 100 cu ft. The actual heating values of various fuels vary from one region to another. However, the values used here are for fuels commonly used in the US. Electrical energy is based on a resistive load. HHV and LHV do not apply to electricity. |
Figure 20. Equivalent Energy Costs. Click here to expand table in a new window.
Monitoring fuel usage
It is smart to separate costs for drying aggregate from the costs for heating your asphalt. Installing a fuel, gas or electric meter solely for the asphalt heating system separates those costs. Knowing the true costs for asphalt heating makes it possible to accurately determine the cost benefits of upgrading heating equipment. It is especially helpful when comparing the costs of heaters with different thermal efficiencies as discussed in subsequent portions of this paper.
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