T-140: Heater thermal efficiency, Impact of efficiency

Technical Paper T-140
Heating and Storing Asphalt at HMA Plants

Publication No. T-140
download T-140 in PDF format

Pages: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12,
13, 14, 15, 16, 17, 18, 19, 20, 21

Heater thermal efficiency
Thermal efficiency affects the amount of fuel an asphalt heater uses. Fuel costs are a significant part of the overall operating costs of a HMA facility. Although the thermal efficiency of a direct-fired tank is very similar to that of a hot oil heater, this discussion applies specifically to hot oil heaters.

The thermal efficiency of a hot oil heater relates the amount of heat (Btu) that the burner produces to the amount of heat actually transferred to the thermal fluid flowing through its coil (Figure 21). Thus, a heater that is 85 percent efficient uses 85 percent of the heat produced to heat the fluid and wastes 15 percent.

Figure 21. Heater Thermal Efficiency.

All heat that does not go into the thermal fluid is wasted. Consequently, all heat that goes out the exhaust stack is wasted heat. And so is any heat that is lost by air leakage or as the result of poor insulation of the heater shell.

Temperature of the exhaust gas is a good indication of efficiency where most wasted heat goes out the stack. The lower the temperature of the exhaust gas, the higher the efficiency.

The formula for calculating the net (LHV) thermal efficiency of a heater is shown in Figure 21. However, there is an easier method of determining a reasonably accurate indication of efficiency for hot oil heaters used in the asphalt industry. This can be done by measuring the stack temperature and using a bar chart that shows efficiencies for various stack temperatures (Figure 28).

Heaters that are 15 or 20 years old are apt to have efficiencies much lower than those available now. A more efficient one could save a lot of money on fuel costs and pay for itself in a very short time. One with an optional combustion air pre-heater will save even more.

The thermal efficiency of a direct-fired tank is very similar to a hot oil heater. It relates the amount of heat the burner produces to the amount of heat actually transferred to the asphalt surrounding the fire tube. The efficiency of a direct-fired tank is about 2 percent higher than that of a hot oil heater because of operating temperatures.

Impact of efficiency
A small difference in efficiency makes a big difference in how much fuel a heater burns over its life. But it’s easy to overlook the amount of fuel a heater uses. That’s because many asphalt plants don’t meter fuel for the heater separately from the dryer. Both are usually lumped together. Consequently, most operators probably don’t know how much fuel the heater alone uses. And because the dryer burns fuel at a much higher rate than the heater, the heater is often ignored.

Its easy to calculate the operating cost of a heater when you know its thermal efficiency and input rating. Figure 22 shows how to calculate the operating cost per hour for a heater that operates on No. 2 fuel oil.

Figure 22. Calculating Heater Fuel Costs Per Hour.

HEATER
EFFICIENCY

COST PER HOUR

50 PERCENT

1,000,000 Btu per hour		1
	X		X $1.00 = $15.15
132,000 Btu per gallon		0.50

60 PERCENT

1,000,000 Btu per hour		1
	X		X $1.00 = $12.63
132,000 Btu per gallon		0.60

70 PERCENT

1,000,000 Btu per hour		1
	X		X $1.00 = $10.82
132,000 Btu per gallon		0.70

80 PERCENT

1,000,000 Btu per hour		1
	X		X $1.00 = $9.47
132,000 Btu per gallon		0.80

85 PERCENT

1,000,000 Btu per hour		1
	X		X $1.00 = $8.91
132,000 Btu per gallon		0.85

Heating load = 1,000,000 Btu per hour. No. 2 fuel oil LHV (low heating value) = 132,000 Btu per gallon. No. 2 fuel oil cost = $1.00 per gallon.

The heater at a typical asphalt plant runs 24 hours a day, 7 days a week, about 40 weeks a year. It may easily run a total of 134,400 hours during its life-span of about 20 years. That’s a lot of running hours, many times more than the dryer runs during those same years.

The amount of fuel a heater burns over its life and its cost depends on the heater’s efficiency. Figure 23 shows a comparison of fuel costs for heaters with efficiencies of 50 to 88 percent. Compare the differences in fuel costs over the life of the heaters. A heater with 85 percent efficiency saves a staggering $256,684 over one 70 percent efficient. Even more astounding, a heater that is 85 percent efficient saves $838,503 over one 50 percent efficient. And boosting efficiency from 85 percent to 88 percent with an air pre-heater saves an additional $40,836, far exceeding its extra cost.

Figure 23. Long Term Fuel Costs For Various Efficiencies.
Heater Efficiency (%)	Gallons per hour	Total Gallons	Total cost
50	15.15	2,036,364	$2,036,364
60	12.63	1,696,970	$1,696,970
70	10.82	1,454,546	$1,454,546
80	9.47	1,272,727	$1,272,727
85	8.91	1,197,861	$1,197,861
88	8.61	1,157,025	$1,157,025
Heat demand = 1 million Btu. Fuel = No. 2 diesel oil (132,000 Btu per gallon). Fuel cost = $1.00 per gallon. Operation = 24 hours per day, 7 days a week, 40 weeks a year, 20 years (heater life).