Continuous furnace heating capacity

Because of its small size and portabiHty, the hot-wire anemometer is ideally suited to measure gas velocities either continuously or on a troubleshooting basis in systems where excess pressure drop cannot be tolerated. Furnaces, smokestacks, electrostatic precipitators, and air ducts are typical areas of appHcation. Its fast response to velocity or temperature fluctuations in the surrounding gas makes it particularly useful in studying the turbulence characteristics and rapidity of mixing in gas streams. The constant current mode of operation has a wide frequency response and relatively lower noise level, provided a sufficiently small wire can be used. Where a more mgged wire is required, the constant temperature mode is employed because of its insensitivity to sensor heat capacity. In Hquids, hot-film sensors are employed instead of wires. The sensor consists of a thin metallic film mounted on the surface of a thermally and electrically insulated probe. 

The other common category of calorimetry is differential methods, in which the thermal behavior of the substance being measured is compared to that of a reference sample whose behavior is known. In differential scanning calorimetry (DSC), the instrument measures the difference in power needed to maintain the samples at the same temperature. In differential thermal analysis (DTA), the samples are heated in a furnace whose temperature is continuously changed (usually linearly), and the temperature difference between the sample and the reference sample as a function of time can yield thermodynamic information. DSC and DTA are most commonly used for determining the temperature of a phase transition, particularly for transitions involving solids. In addition, DSC experiments can yield values for the enthalpy of a phase transition or the heat capacity. Commercial DSC and DTA instruments are available. 

A continuous furnace may be heated so that the temperature of its zones is practically the same across the furnace. This temperature uniformity can be obtained by lengthwise firing in several zones (as illustrated by fig. 4.2), or by roof firing or side firing in several zones (as shown in fig. 4.3). In such furnaces, the heating capacity of a continuous furnace will equal or exceed the capacity of a same-size batch furnace. 

The slalemenls relating lo balch type and continuous furnaces are for lop-fired furnaces al a lemperafure corresponding lo fliat of fhe balch type. The heating capacity of such furnaces is determined by hearlh area, ceiling lemperafure, load absorptivity, time, and exposure of the load as well as composition and thickness of the load and of the poc. 

The heating capacity of continuous furnaces usually exceeds that of batch type furnaces of the same hearth areas because ... 

Thickness of heating stock does not limit heating capacity as much in continuous furnaces as it does in top-fired batch furnaces because heat can be imparted to the load from below. The limiting thickness depends on the thermal conductivity of the load and required temperature uniformity. 

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