Evaporators Fired heaters

Costs for forced circulation evaporators, long tube evaporators, and agitated falling-film evaporators are presented in Figs. 15-26 and 15-27. Information on purchased cost of specialized heaters is given in Figs. 15-28 and 15-29, while Figs. 15-30 and 15-31 present costs for process furnaces and direct-fired heaters. 

Absolute Humidity—the ratio of mass of vapor (moisture) to mass present in the carrier gas stream. Example 0.02 pounds of water per pound of air. This number can be used to find the relative humidity on the psychrometric charts. It is also useful for cumulative quantities in a stream due to such items as products of combustion (when a gas fired heater is used), and evaporation and ambient quantities. This is necessary for calculating condenser or venting amounts. 

Feedforward control was not widely used in the process industries until the 1960s (Shinskey, 1996). Since then, it has been applied to a wide variety of processes that include boilers, evaporators, solids dryers, direct-fired heaters, and waste neutralization plants (Shinskey et al., 1995). However, the basic concept is much older and was applied as early as 1925 in the three-element level control system for boiler drums. We will use this control application to illustrate the use of feedforward control. 

Infrared lamps and heaters. Infrared lamps with internal reflectors are available commercially and are valuable for evaporating solutions. The lamp may be mounted immediately above the liquid to be heated the evaporation takes place rapidly, without spattering and also without creeping. Units are obtainable which permit the application of heat to both the top and bottom of a number of crucibles, dishes, etc., at the same time this assembly can char filter papers in the crucibles quite rapidly, and the filter paper does not catch fire. 

Fig. 14. Production diagram of vinyltrichlorosilane 1,3- tanks 2 -evaporator 4, 5 - rotameters 6, 12 - towers with CaCl2 7 - heater 8 - reactor 9 - expander 10, 13 - coolers 11- fire-resistant apparatus 14 - buffer capacity 15 - absorber 16 -collector.

The drying air is cooled in a scrubber/condenser where the water, evaporated in the spray dryer, is removed. The air is subsequently recycled to the direct gas-fired air heater. A vent will exhaust a volume of drying air equal to the volume of combustion products from the gas burner. However, most of the air is recycled in the system. 

The critical regions in oil-fired furnaces are the evaporator tubes, steam superheater tubes, air heater, and channel. Evaporator tubes are affected by hot gas corrosion from hydrogen sulfide as the temperature exceeds 280°C. Superheater tubes suffer from sulfate/sulfite corrosion induced by molten sulfates above 620"C. Superheater tube holders undergo vanadic corrosion caused by molten vanadate species in the range of 550 C-600 C. Air heaters are subject to low-temperature corrosion from liquid sulfuric acid at 100°C-140°C. Flue gas channels suffer from acid deposits at the dew point (Balajka 1980). 

It is good practice to unplug all heaters when not in use as some older heaters have malfunctioned in the off position or when the thermostat safety cutoff failed. These have resulted in fires in unattended laboratories (See Incident 7.3.1.3 and Figure 7.3.1.4). It is prudent to only operate these devices when you are present. Do not leave heating devices on overnight. Do not use heaters with plastic components on top of hot plates and do not use plastic water baths with immersion heaters - use metal water baths. Covering baths may prevent evaporation. 

This heat transfer method is mainly used to enhance the drying capacity in coaters when the web is wet Infrared heaters are usually gas fired. The gas heats a mesh to a temperature of about 900 to 1100 °C. The low thermal inertia of the mesh allows fast control of the mesh temperature and the heating rate as well as preventing fires in the case of sheet breaks. In some cases electrical heaters are in use with temperatures up to about 700 °C, exhibiting a fast cool down of the emitter plates. Infrared drying units need sufficient air flow in order to carry off the evaporated water and to prevent coat quality problems. 

---