Electric heated block

Decomposition products of powder layers deposited on hot surface may flash, inducing a devastating dust explosion. The flash point of the decomposition products is determined in the apparatus shown in Figure 3.81. 2 g of powder sample is heated by an electric heating block at a rate of 4°C/min under an oxygen stream of 1.5 dm /h. At every 1 °C temperature rise, the electrodes arc. The flash point is the lowest temperature at which the decomposition products flare up. 

Q Fig, 5. Sublimation in a stream of gas (1) glass tube, (2) electric heating block, (3) platinum boat, (4) layer of cotton wool, (5) wet paper. 

Terminal block Vor single point power supply with electric heat) DCE units... 

Electrical heating is accomplished with resistance bauds or ribbons which must be electrically insulated from the machine body but in good thermal contact with it. The heaters must be carefully spaced to avoid a succession of hot and cold areas. Sometimes they are mounted in aluminum blocks shaped to conform to the container walls. Their effective temperature range is 150 to 500°C (about 300 to 930°F). Temperature control is precise, maintenance and supervision costs are low, and conversion of electrical energy to useful heat is almost 100 percent. The cost of electrical energy is usually large, however, and may be prohibitive. 

The common heater block shown in Fig. 19.23 can itself be subject to corrosion leading to different heat transfer conditions for different tubes, and in some later versions of the apparatus individual short heating jackets are used for each tube, which are heated with oil from either a steam-heated or electrically heated heat exchanger. This modification not only avoids corrosion problems but also obviates the necessity to machine a length of the outside of each tube to fit the semi-circular notches in the single heater block. The oil flow is adjusted to give an oil temperature of 95°C at each outlet. 

Measurements of the true specific heats at low temperatures have been carried out by Eucken, who worked in such a way that to a weighed quantity of the substance in the form of a block, or in a proper isolated vessel, a known quantity of heat was added by means of an electrically heated platinum spiral, the resistance of wThich at the same time served to measure the temperature. The quotient of the electrical energy spent by the rise of temperature gives the specific heat. The correction for cooling (in vacuum of 5 Jo mm.) amounted to 20 per cent., and for heat capacity of the apparatus 5 per cent., yet the results are stated to be accurate to 1 per cent. 

Several forms of apparatus employing electrical heating will be described. A simple form may be readily constructed from a domestic electric iron of 400-500 watts rating. The handle is removed, and two holes of 8 mm. diameter are drilled through the base (ca. 11 mm. thick) so that they meet in the centre of the block. One hole is for a 360 thermometer (small bulb) the other hole is spare and can be used for comparison with a standard thermometer. The heater is mounted on a sheet of thick asbestos board which is fixed to an appropriate wooden base. The wires from the heating unit are connected to two insulated terminals fitted on the board (Fig. 11, 11, 1). The rate of heating is controlled by either of the following methods ... 

Rotary vacuum evaporator, 40 °C bath temperature Dry-block bath, electrically heated, temperature 75 °C Mechanical shaker (universal shaker)... 

Adiabatic calorimeters have also been used for direct-reaction calorimetry. Kubaschewski and Walter (1939) designed a calorimeter to study intermetallic compoimds up to 700°C. The procedure involved dropping compressed powders of two metals into the calorimeter and maintaining an equal temperature between the main calorimetric block and a surrounding jacket of refractory alloy. Any rise in temperature due to the reaction of the metal powders in the calorimeter was compensated by electrically heating the surrounding jacket so that its temperature remained the same as the calorimeter. The heat of reaction was then directly a function of the electrical energy needed to maintain the jacket at the same temperature as the calorimeter. One of the main problems with this calorimeter was the low thermal conductivity of the refractory alloy which meant that it was very difficult to maintain true adiabatic conditions. 

Pa, catalog (1970), p 640] another is Ma-quenne Block (electrically heated metal block with. holes for capillary tubes thermometer)... 

The gel block (Figure 3) consists of an aluminum casting 48 inches long by 4 inches wide by 1 inch deep with electrical heating elements at one end and facilities for water cooling, if required, at the other end. The block is lagged and is fitted with... 

The two principal types of melting point apparatus in common use are those in which heating of the capillary tube is by means of a heated liquid bath and those in which heating is carried out in, or on, an electrically-heated metal block. 

For these investigations the metallic samples were fixed in cylindrical autoclaves and installed in an insulated block which was thermostated by electric heating. The autoclaves were kept in a continuous motion throughout the experiment. Thereby a steady contact of the material samples with the gas as well as the liquid phase of the reaction fluid could be maintained. After an operation time of two or three weeks at constant temperature the autoclaves were opened, the samples were weighed, visually examined and a part of them analyzed. The residual samples were put back in the autoclaves for further treatment. 

The feed tank is mounted on the ceiling using chains and a weighing cell. Continuous weighing of the feed vessel stands for an exact mass balance not influenced by pulsation of the feed pump, volumetric or viscosity effects. To ensure that the connected feed pipes do not block with solidifying liquid they are equipped with an explosionproof electric heating wire which holds an adjustable temperature. 

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