Surface temperature wiring

Martensite is a hard, nonductile microconstituent formed when steel is heated above its critical temperature and cooled rapidly. In the case of steel of the composition conventionally used for rope wire, martensite can be formed if the wire surface is heated to a temperature near or somewhat in excess of 1400°F (760°C), and then cooled at a comparatively rapid rate. The presence of a martensite film at the surface of the outer wires of a rope that has been in service is evidence that sufficient frictional heat has been generated on the crown of the rope wires to momentarily raise the wire surface temperature to a point above the critical temperature range of the steel. The heated surface is then rapidly cooled by the adjacent cold metal within the wire and the rope structure, and an effective quenching results. 

Hot plates. The electrically heated hot plate, preferably provided with three controls — Low , Medium and High — is of great value in the analytical laboratory. The heating elements and the internal wiring should be totally enclosed this protects them from fumes or spilled liquids. Electric hot plates with stepless controls are also marketed these permit a much greater selection of surface temperatures to be made. A combined electric hot plate and magnetic stirrer is also available. For some purposes a steam bath may be used. 

The other limit is the problem of temperature measurements. Classical temperature sensors could be avoided in relation to power level. Hence, temperature measurements will be distorted by strong electric currents induced inside the metallic wires insuring connection of temperature sensor. The technological solution is the optical fiber thermometers [35-39]. However, measurements are limited below 250 °C. For higher values, surface temperature can be estimated by infrared camera or pyrometer [38, 40], However, due to volumic character of microwave heating, surface temperatures are often inferior to core temperatures. 

Saito with a fine wire thermocouple embedded at the surface [3]. The scatter in the results are most likely due to the decomposition variables and the accuracy of this difficult measurement. (Note that the surface temperature here is being measured with a thermocouple bead of finite size and having properties dissimilar to wood.) Likewise the properties k. p and c cannot be expected to be equal to values found in the literature for generic common materials since temperature variations in the least will make them change. We expect k and c to increase with temperature, and c to effectively increase due to decomposition, phase change and the evaporation of absorbed water. While we are not modeling all of these effects, we can still use the effective properties of Tig, k, p and c to explain the ignition behavior. For example,... 

Additional Requirements. In addition to electrical and physical isolation requirements, the surface temperature of all equipment and wiring located in the hazardous environment must not exceed the values indicated in the standard. 

If the thermocouple wires are located in a hole or groove in a metal tube or plate, the fin effect will be remedied, but the heat flow pattern through the solid will be altered. The correct surface temperature can be computed by the relaxation method. This corrected method has been used for boiling studies (S2), but many workers have made no correction for embedded wires. 

Some other pathological phenomena connected with the existence of hysteresis loop have been reported in the literature. Frank-Kamenetskii (32) described the Buben experimental results for difference of surface temperatures for both steady states for the reaction between hydrogen and air on a Pt wire. These observations indicate a difference up to 1000°C if hydrogen with an excess of air was used, while the maximum temperature difference amounts to 250°C for air in excess of hydrogen. Frank-Kamenetskii explained this phenomenon by the thermal diffusion effects. 

No heating unit, with open flames or surface temperatures at or above the ignition point of natural gas ( 1000°F), should be installed in any area where the electrical recommendations require Division 2 wiring. 

The bare wire is unwound, sometimes by a controlled tension device, and is preheated to a temperature above the Tg or Tm of the polymer to be extruded this is done so that the layer next to the bare wire adheres to it, and to drive moisture or oils off the conductor surface. The wire is fed in the back of the cross-heat die and into a guider tube. Upon exiting the guider, it meets the molten plastic, which covers it circumferentially. Since the wire speed, which is controlled by a capstan at the end of the line, is usually higher than the average melt velocity, a certain amount of drawdown is imposed on the melt anywhere from a value slightly greater than unity to 4. 

In an electrical heater, energy is generated in a 1.5-mm diameter wire at a rate of 1200 W. Air at a temperature of 30°C is blown over the wire at a velocity that gives a mean heat transfer coefficient of SO W/m2°C. If the surface temperature of the wire is not to exceed 200°C, find the length of the wire. 

A 3.2-mm-diameter stainless-steel wire 30 cm long has a voltage of 10 V impressed on it. The outer surface temperature of the wire is maintained at 93°C. Calculate the center temperature of the wire. Take the resistivity of the wire as 70 pSl cm and the thermal conductivity as 22.5 W/m °C. 

A fine wire having a diameter of 0.001 in (2.54 x 10 5 m) is placed in a 1-atm airstream at 25°C having a flow velocity of 50 m/s perpendicular to the wire. An electric current is passed through the wire, raising its surface temperature to 50°C. Calculate the heat loss per unit length. 

A 0.13-mm-diameter wire is exposed to an airstream at -30°C and 54 kPa. The flow velocity is 230 m/s. The wire is electrically heated and is 12.5 mm long. Calculate the electric power necessary to maintain the wire surface temperature at I75°C. 

A 10-cm length of platinum wire 0.4 mm in diameter is placed horizontally in a container of water at 38°C and is electrically heated so that the surface temperature is maintained at 93°C. Calculate the heat lost by the wire. 

How much heat would be lost from a horizontal platinum wire. 1.0 mm in diameter and 12 cm long, submerged in water at atmospheric pressure if the surface temperature of the wire is 232°F ... 

A fine iron wire 0.025 mm in diameter is exposed to a high-velocity airstream at 10"6 atm, - 50°C, and M = 5. Estimate the wire surface temperature, assuming e = 0.4 and a = 0.9. Assume the radiation-surroundings temperature is the free-stream temperature. 

In addition, surface temperatures shall be limited to prevent any ignition of the potentially explosive atmosphere. All components (including the wiring)... 

As a second main feature of intrinsic safety, current densities in thin internal wires and printed board wiring may increase up to some 102 A/mm2, and the surface temperature of small components may exceed the limits of T classification by far. 

A 2-m-long, 0.3-cm-diameter electrical wire extends across a room at 15°.C, as shown in F(g. 1-34. Heat is generated in the wire as a result of resistance heating, and the surface temperature of the v/ire is measured to be 152°C in... 

Reconsider Prob. 1-95. Using EES (or other) soft-ware, plot the convection heat transfer coefftcteru as a ftinciion of the wire surface temperature in the range of 100°C to SOO C, Discuss the results. 

A 30-cm-long, 0.5-cm-diamcter electric resistance wire is used to detenniiie the convection heat transfer coefficient in air at 25°C experimentally. The surface temperature of the wire is measured to be 230°C when the electric power consumption is 180 W. If the radiation heat loss from the wire is calculated to be 60 W, Ihe convection heat transfer coefficient is (a) 186 W/m -X (b) 158 W/m -X... 

A room is heated by a 1.2 kW electric re.sistance heater whose wires have a diameter of 4 mm and a total length of 3.4 m. The air in the room is at 23°C and the interior surfaces of tlie room are at 17°C. The convection heal liansfer coefficient on the surface of the wires is 8 W/m °C. If the rates of heal transfer from the wires to the room by convection and by radiation are equal, the surface temperature of the wire is (a) 3534X (b) 1778X (c) 1772X... 

SOLUTION The base plate of an iron is considered. The variation of temperature in the plate and the surface temperatures are to be determined. Assumptions 1 Heat transfer is steady since there is no change with time. 2 Heat transfer is one-dimensional since the surface area of the base plate is large relative to its thickness, and the thermal conditions on both sides are uniform. 3 Thermal conductivity is constant. 4 There is no heat generation in the medium. 5 Heat transfer by radiation is negligible. 6 The upper part of the iron is well insulated so that the entire heat generated in the resistance wires is transferred to the base plate through its inner surface. 

Consider a long resistance wire of radius ri = 0.2 cm and thermal conductivity k i,e = 15 W/m °C in which heat is generated uniformly as a result of resistance heating at a constant rate of = 50 W/cm (Fig. 2- 61). The v/ire is embedded in a 0.5 cm-thick layer of ceramic whoso thermal conductivity is =1.2 W. m °C. If the outer surface temperature of the ceramic layer is measured to be T, = 45 C, determine the temperatures at the center of the resistance wire and the interface of the vrire and the ceramic layer under steady conditions. 

SS A 6-m-long 2-kW electrical resistance wire is made of 0.2-cm-diameter stainless steel k = 15.1 W/m °C). The resistance wire operates in an environment at 20 C with a heat transfer coefficient of 175 W/m C at the outer surface. Determine the surface temperature of Ihe wire (a) by using the applicable relation and (b) by. setting up the proper differeniial equation and solving it. Answers (a) 323 C, (fr) 323"C... 

A 6-mra-diameler electrical transmission line carries an electric current of 50 A and has a resistance of 0.002 ohm per meter lenglh. Determine the surface temperature of the wire during a windy day when the air temperature is 10°C and the wind is blowing across the transmission line at 40 km/l>. 

I Reconsider Prob. 7-49. Using EES (or Other) iBSa software, investigate the effect of the wind velocity on the Surface temperature of the wire. Let the wind velocity vary from 10 km/h to 80 km/h. Plot the surface temperature as a function of wind velocity, and discuss the results. 

A 300-W cylindrical resistance heater is 0.V5 m long and 0,5 cm in diameter. The resistance wire is placed horizontally in a fluid at 20"C. Determine the outer surface temperature of the resistance wire in steady operation if the fluid is (rt) air and (fc) water. Ignore any heat transfer by radiation. Use properties at 500°C for air and dO C for water. 

Water in a tank is to be boiled at sea level by a 1-cm-diameter nickel plated steel heating element equipped with electrical resistance wires inside, as shown in Fig. 10-16. Determine the maximum heat flux that can be attained in the nucleate boiling regime and the surface temperature of the heater in that case. 

P Water is boiled at atmospheric pressure by a horizontal polished copper heat-° ing element of diameter D = 5 mm and emissivity e = 0.05 immersed in v/a-ter, as shown in Fig. 10-17. If the surface temperature of the heating wire is 350°C, determine the rate of heat Iransfer from the wire to the water per unit length of the wire. 

After the pumps were started, and the liquid flow rate set to the desired value, the cartridge heaters were energized. Onee all the parameters reached steady-state, the values were recorded for 100 s at a sampling rate of 1 Hz. Thereafter, the power to the copper block was increased and a new set of data reeorded. The experiment was stopped when either the surface temperature was above the boiling point or when the temperature at the base of the eopper bloek rose above 350 °C, which could damage the Teflon jacket and the eleetrieal wires. In these experiments, the test fluids used were deionized water and FC-84. 

Wire of this variety is, in actuality, an electrical resistance. When a current of electricity is passed through a length of the wire, heat is produced. Most nichrome wire, like the more modern calrod typc units, will produce surface temperatures in excess of 1,000 degrees F. Nichrome wire may be purchased in almost any length desired or if secrecy is important, it may be removed from small heaters and toasters. It is also often found in the cheaper hot-plates, being embedded in a moulded refractory plate. 

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