Thermal efficiency of process heating equipment, such as furnaces, ovens, heaters, and kilns is the ratio of heat delivered to a material and heat supplied to the heating equipment.
The purpose of a heating process is to introduce a certain amount of thermal energy into a product, raising it to a certain temperature to prepare it for additional processing or change its properties. To carry this out, the product is heated in a furnace. This results in energy losses in different areas and forms as shown in sankey diagram figure 4.10. For most heating equipment, a large amount of the heat supplied is wasted in the form of exhaust gases.
These furnace losses include:
Stored Heat Loss: First, the metal structure and insulation of the furnace must be heated so their interior surfaces are about the same temperature as the product they contain. This stored heat is held in the structure until the furnace shuts down, then it leaks out into the surrounding area. The more frequently the furnace is cycled from cold to hot and back to cold again, the more frequently this stored heat must be replaced. Fuel is consumed with no useful output.
Wall losses: Additional heat losses take place while the furnace is in production. Wall or transmission losses are caused by the conduction of heat through the walls, roof, and floor of the heating device, as shown in Figure 4.11. Once that heat reaches the outer skin of the furnace and radiates to the surrounding area or is carried away by air currents, it must be replaced by an equal amount taken from the combustion gases. This process continues as long as the furnace is at an elevated temperature.
Material handling losses: Many furnaces use equipment to convey the work into and out of the heating chamber, and this can also lead to heat losses. Conveyor belts or product hangers that enter the heating chamber cold and leave it at higher temperatures drain energy from the combustion gases. In car bottom furnaces, the hot car structure gives off heat to the room each time it rolls out of the furnace to load or remove work. This lost energy must be replaced when the car is returned to the furnace.
Cooling media losses: Water or air cooling protects rolls, bearings, and doors in hot furnace environments, but at the cost of lost energy. These components and their cooling media (water, air, etc.) become the conduit for additional heat losses from the furnace. Maintaining an adequate flow of cooling media is essential, but it might be possible to insulate the furnace and load from some of these losses.
Radiation (opening) losses:Furnaces and ovens operating at temperatures above 540°C might have significant radiation losses, as shown in Figure 4.12 Hot surfaces radiate energy to nearby colder surfaces, and the rate of heat transfer increases with the fourth power of the surface's absolute temperature. Anywhere or anytime there is an opening in the furnace enclosure, heat is lost by radiation, often at a rapid rate.
Waste-gas losses: Waste-gas loss, also known as flue gas or stack loss, is made up of the heat that cannot be removed from the combustion gases inside the furnace. The reason is heat flows from the higher temperature source to the lower temperature heat receiver.
Air infiltration: Excess air does not necessarily enter the furnace as part of the combustion air supply. It can also infiltrate from the surrounding room if there is a negative pressure in the furnace. Because of the draft effect of hot furnace stacks, negative pressures are fairly common, and cold air slips past leaky door seals, cracks and other openings in the furnace. Figure 4.13 illustrates air infiltration from outside the furnace. Every time the door is opened, considerable amount of heat is lost.
Economy in fuel can be achieved if the total heat that can be passed on to the stock is as large as possible.
The efficiency of furnace can be judged by measuring the amount of fuel needed per unit weight of material.
The quantity of heat to be imparted (Q) to the stock can be found from
Q = m x Cp (t1 – t2)
Where
Q = Quantity of heat of stock in kCal
m = Weight of the stock in kg
Cp = Mean specific heat of stock in kCal/kgoC
t1 = Final temperature of stock desired, oC
t2 = Initial temperature of the stock before it enters the furnace, oC
Similar to the method of evaluating boiler efficiency by indirect method, furnace efficiency can also be calculated by indirect methods. Furnace efficiency is calculated after subtracting sensible heat loss in flue gas, loss due to moisture in flue gas, heat loss due to openings in furnace, heat loss through furnace skin and other unaccounted losses
In order to find out furnace efficiency using indirect method, various parameters that are required are hourly furnace oil consumption, material output, excess air quantity, temperature of flue gas, temperature of furnace at various zones, skin temperature and hot combustion air temperature. Instruments like infrared thermometer, fuel efficiency monitor, surface thermocouple and other measuring devices are required to measure the above parameters.
Typical thermal efficiencies for common industrial furnaces are given in Table: 4.1
An oil-fired reheating furnace has an operating temperature of around 1340oC. Average fuel consumption is 400 litres/hour. The flue gas exit temperature is 750 oC after air preheater. Air is preheated from ambient temperature of 40 oC to 190 oC through an air pre-heater. The furnace has 460 mm thick wall (x) on the billet extraction outlet side, which is 1 m high (D) and 1 m wide. The other data are as given below. Find out the efficiency of the furnace by both indirect and direct method.
1. Sensible Heat Loss in Flue Gas:
Theoretical air required to burn 1 kg of oil = 14 kg
Total air supplied = 14 x 2.33 kg / kg of oil
= 32.62 kg / kg of oil
Sensible heat loss = m x Cp x ΔT
Where ,
m = Weight of flue gas (Air +fuel)
= 32.62 + 1.0 = 33.62 kg / kg of oil.
Cp = Specific heat
ΔT = Temperature difference
Sensible Heat loss = 33.62 x 0.24 x (750– 40)
= 5729 kCal / kg of oil
2. Loss Due to Evaporation of Moisture Present in Fuel
Where,
M - kg of Moisture in 1 kg of fuel oil (0.15 kg/kg of fuel oil)
Tfg - Flue Gas Temperature, 0C
Tamb - Ambient temperature,0C
GCV - Gross Calorific Value of Fuel, kCal/kg
3. Loss Due to Evaporation of Water Formed due to Hydrogen in Fuel
Where, H2 – kg of H2 in 1 kg of fuel oil (0.1123 kg/kg of fuel oil)
4. Heat Loss due to Openings:
If a furnace body has an opening on it, the heat in the furnace escapes to the outside as radiant heat. Heat loss due to openings can be calculated by computing black body radiation at furnace temperature, and multiplying these values with emissivity (usually 0.8 for furnace brick work), and the factor of radiation through openings. Factor for radiation through openings can be determined with the help of graph as shown in figure 4.14. The black body radiation losses can be directly computed from the curves as given in the figure 4.15 below.
The reheating furnace in example has 460mm thick wall (X) on the billet extraction outlet side, which is 1m high (D) and 1m wide. With furnace temperature of 1340 0C, the quantity (Q) of radiation heat loss from the opening is calculated as follows:
| The shape of the opening is square and D/X | 1/0.46 = 2.17 |
| The factor of radiation (Refer Figure 4.14) | 0.71 |
| Black body radiation corresponding to 1340oC (Refer Figure 4.15 on Black body radiation) | 36.00 kCal/cm2/hr |
| Area of opening | 100 cm x 100 cm = 10000 cm2 |
| Emissivity | 0.8 |
| Total heat loss | 36 x 10000 x 0.71 x 0.8
204480 kCal/hr |
| Equivalent fuel oil loss | 20.45 kg/hr |
| % of heat loss through openings | 20.45 /368 x 100 = 5.56 % |
5. Heat Loss through Furnace Skin:
a. Heat loss through roof and sidewalls:
Total average surface temperature = 122oC
Heat loss at 122 oC (Refer Fig 4.12)= 1252 kCal / m2 / hr
Total area of heating + soaking zone = 70.18 m2
Total heat loss = 1252 kCal / m2 / hr x 70.18 m2
= 87865 kCal/hr
Equivalent oil loss (a) = 8.78 kg / hr
b. Total average surface temperature of
area other than heating and soaking zone = 80oC
Heat loss at 80oC = 740 kCal / m2 / hr
Total area = 12.6 m2
Total heat loss = 740 kCal / m2 / hr x 12.6 m2
= 9324 kCal/hr
Equivalent oil loss (b) = 0.93 kg / hr
Total loss of fuel oil = a + b = 9.71 kg/hr
Total percentage loss = 9.71 x 100 / 368
= 2.64%
6. Unaccounted Loss
These losses comprises of heat storage loss, loss of furnace gases around charging door and opening, heat loss by incomplete combustion, loss of heat by conduction through hearth, loss due to formation of scales.
Furnace Efficiency (Direct Method)
Heat input = 400 litres / hr
= 368 kg/hr
Heat output = m x Cp x ΔT
= 6000 kg x 0.12 x (1340 – 40)
Efficiency = 936000 x 100 / (368 x 10000)
= 25.43 %
Losses = 75% (app)
Furnace Efficiency (Indirect Method)
1. Sensible Heat Loss in flue gas = 57.29%
2. Loss due to evaporation of moisture in fuel = 1.36 %
3. Loss due to evaporation of water formed from H2 in fuel = 9.13 %
4. Heat loss due to openings = 5.56 %
5. Heat loss through skin = 2.64%
Total losses = 75.98%
Furnace Efficiency = 100 – 75.98
= 24.02 %
