Heating equipment electric resistance heaters

Water is to be heated from 15 C to 65 C as it flows through a 3 cm-internal-diameter 5-m-long tube (Fig. 8-30). The tube is equipped with an electric resistance heater that provides uniform heating throughout the surface of the tube. The outer surface of the heater is well insulated, so that in steady operation all the heat generated in the heater is transferred to the water in the tube. If the system Is to provide hot water at a rate of 10 L/min, determine the power rating of the resistance heater. Also, estimate the inner surface temperature of the tube at the exit. 

SOLUTION Water is to he heated in a tube equipped with an electric resistance heater on its surface, The power rating of the heater and the inner surface temperature at the exit are to be determined. 

In industrial processes heat energy is transferred by a variety of methods, including conduction in electric-resistance heaters conduction-convection in exchangers, boilers, and condensers radiation in furnaces and radiant-heat dryers and by special methods such as dielectric heating. Often the equipment operates under steady-state conditions, but in many processes it operates cyclically, as in regenerative furnaces and agitated process vessels. 

The dryer provides heat to volatilize the solvent and a means to carry the solvent away from the coating. Efficient hardware is used to minimize energy costs. The dryers may be equipped with the appropriate pollution abatement devices to meet both OSHA and EPA standards. Dryers commonly use hot air both to provide heat and to carry away the solvent. The air may be heated by steam or by heat exchange with flue gases. Flue gases from the combustion of natural gas maybe used directly in place of hot air. Infrared radiant energy from gas combustion or electric resistance heaters is sometimes used. Conduction heat transfer from heated... 

Eor the experimental determination of the temperature dependence, the whole sample with the electrical leads has to be in temperature equilibrium at least for the time ofmeasure-ment. Due to the sniall size of the samples, the van der Pauw technique is particularly suited for such measurements and some corresponding measurement systenns are equipped with an integrated temperature chamber. A very flexible and convenient temperature control can be realized with a vacuum chamber, with the sample cooled by the Joule-Thompson effect (expansion cooling of N2 from a capillary nozzle), or heated by a resistive heater. In this way, temperature dependent measurements in the range between 80 and 580 K can be performed and the temperature can be set very accurately, which is also an important precondition for the Hall measurement (Section Hall effect measurement ). Simpler systems are using a cryostat with liquid nitrogen or helium, which allows measurements at temperatures down to 77 and 4 K, respectively. 

Consider a thin 16-cm-long and 20-cin-wide horizontal plate suspended in air at 20°C. The plate is equipped with electric resistance heating eletnents with a rating of 20 W. Now Ihe heater is turned on and (he plate temperature rises. Determine Ihe temperature of the plate when steady operating conditions are reached. The plate has an emissiviiy of 0.90 and the surrounding surfaces are at 17"C. 

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. 

In newly equipped wells the liners of the casing column near the casing shoes should be made of heat resistant materials. The regulation of bottomhole temperature is easier when electric heaters are used than it is with flame heaters. So, when shallow oil-bearing beds are to be ignited with the help of electric heaters, one can use ordinary materials for bottomhole construction (Fig. 65a). 

---