Furnaces losses from

Qloss stack loss from furnace, boiler, or gas turbine (kJ s )... 

The efficiency of an induction furnace installation is determined by the ratio of the load usehil power, P, to the input power P, drawn from the utihty. Losses that must be considered include those in the power converter (transformer, capacitors, frequency converter, etc), transmission lines, cod electrical losses, and thermal loss from the furnace. Figure 1 illustrates the relationships for an induction furnace operating at a constant load temperature with variable input power. Thermal losses are constant, cod losses are a constant percentage of the cod input power, and the usehd out power varies linearly once the fixed losses are satisfied. 

The heat balance shows that the heat loss from the furnace walls is only ca 11% of the energy suppHed by the fuel and just slightly more than the sensible heat loss with the slag. The principal heat loss is in the stack gases and is equivalent to ca 30% of the energy suppHed by the fuel. 

Radiation differs from conduction and convection not only in mathematical structure but in its much higher sensitivity to temperature. It is of dominating importance in furnaces because of their temperature, and in ciyogenic insulation because of the vacuum existing between particles. The temperature at which it accounts for roughly half of the total heat loss from a surface in air depends on such factors as surface emissivity and the convection coefficient. For pipes in free convection, this is room temperature for fine wires of low emissivity it is above red heat. Gases at combustion-chamber temperatures lose more than 90 percent of their energy by radiation from the carbon dioxide, water vapor, and particulate matter. 

Rhodium-platinum alloys containing up to 40% Rh are used in the form of wire or ribbon in electrical resistance windings for furnaces to operate continuously at temperatures up to 1 750°C. Such windings are usually completely embedded in a layer of high-grade alumina cement or flame-sprayed alumina to prevent volatilisation losses from the metal due to the free circulation of air over its surface. Furnaces of this type are widely employed for steel analysis, ash fusions and other high-temperature analytical procedures. 

In moM furnaces operating under suction, there is also some leakage of air into the setting and, consequently, the excess air leaving the furnace and the unit is greater than that at the fuel-burning equipment. Another loss that must be considered is the radiation loss from the unit setting. 

FIG. 5-4 Thermal circuit for Example 1. Steady-state conduction in a furnace wall with heat losses from the outside surface by convection (hc) and radiation (hR) to the surroundings at temperature Tsur. The thermal conductivities kD, kg, and ks are constant, and there are no sources in the wall. The heat flux q has units of W/m2. 

A 5-m-long section of an air beating system of a house passes through an unheated space in the basement Fig. ] 20). The cross section of the rectangular duct of the heating system is 20 cm X 25 cm. Hot air enters the duct at 100 kPa and 60 C at an average velocity of 5 m/s. The temperature of the air in the duct drops to 54°C as a result of heat loss to the cool space in the basement. Determine the rate of heat loss from the air in the duct to the basement under steady conditions Also, determine the cost of this heat loss per hour if the house is heated by a natural gas furnace that has an efficiency of 80 percent. and the cost of the natural gas in that area is 1.60Aherm (1 therm = 105,500 kJ). 

A plane furnace surface at 150°C covered with 1-cm-(hick insulation is exposed to air at 30°C, and the combined heat transfer coefficient is 25 VV/m "C. The thermal conductivity of insulation is 0.04 W/m °C. The rale of heat loss from the surface per unit surface area is (a) 35W (h)414W (c) 300W... 

Consider a funiace wall made of sheet metal at an average temperature of 800°C exposed to air at 40°C. The combined heat transfer coefficient is 200 W/m C inside the furnace, and 80 W/ra °C outside. If the thermal resistance of the furnace wall is negligible, the rale of heat loss from the furnace per uiiit surface area is... 

Hot au at 60 C leaving the furnace of a house enters a 12-m-long section of a sheet metal duct of rectangular cross section 20 cm X 20 cm at an average velocity of 4 m/s. The thermal resistance of the duct is negligible, and the outer surface of the duct, whose einissivity is 0.3, is exposed to the cold air at 10°C in the basement, with a convection heal transfer coefficient of 10 W/m °C. Taking the walls of the basement to be at 10°C also, determine (a) the temperature at which the hot air will leave the basement and (ft) the rale of heat loss from llie hot air in the duct to ihe basement. 

Steam is generated in a gas furnace that has an efficiency of 78 percent, and the plant pays SI. 10 per therm (I therm 105,500 kJ) of natural gas. The plant operates 24 h a day 365 days a year, and thus 8760 h a year. Deteimine the annual cost of the heat losses from the steam pipe for this facility. 

During a plant visit, it was observed that a 1.5-ni-high and I-m-wide section of the vertical front section of a natural gas furnace wall was too hot to touch. The temperature measurements on the surface revealed that the average temperature of the exposed hot surface was 110°C, while the temperature of the surrounding air was 25°C. The surface appeared to be o.xidized, and its emissivity can be taken to be 0.7. Taking the temperature of the surrounding surfaces to be 25 C also, determine the rale of heal loss from this furnace. 

Tlie furnace has an efficiency of 79 percent, and the plant pays 1.20 per therm of natural gas. If the plant operates 10 ha day, 3 to days a year, and thus 3100 h a year, determine the annual cost of the heat loss from this vertical hot surface on the front section of the furnace wall. 

Radiation heat loss from the fryer, furnace, and other ancillaries. 

In order to investigate convective heat and mass transfer, a laboratory reactor has been set up at the Research Centre Karlsruhe [16], see Fig.5. A hot gas flow is used to heat up the packed bed, which is located in a furnace pot suspended from a weighting cell. To minimise heat loss from the packed bed, the reactor wall is heated. Therefore radial temperature gradients are assumed to be negligible. Thermocouples arc used to determine the temperature distribution over the height. The maximum height of the packed bed in the reactor is 210 mm. 

Substrate heating is required in CVD reactors. Since the films are grown under isothermal conditions, the substrate must be held at a constant growth temperature for an extended period of time. The achievement of this requirement is facilitated in two ways, and reactors are classified into two groups depending on how the substrate is heated. In a hot-wall design, the entire reactor is placed in a tube furnace and the substrate, the region of forced gas convention, and the walls of the reactor are maintained at the same temperature. Of course the ends of the reactor are cooler than the middle because of heat loss from the ends and the introduction of cold gas at the entrance of the reactor. To remedy this imbalance, a three-zone furnace, with independent feedback control for each zone, is usually employed. Substrates are loaded only in the portion of the reactor where the temperature can be accurately maintained. 

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