Condenser. Evaporator Boiler

When the temperature of either fluid remains constant during the heat exchange resulting from a change of phase (condensation or evaporation), the LMTD method continues to apply, but it may be given a more convenient form. This is done in the next section. 

In a condenser, vapor condenses at a constant temperature (and pressure) while heating a cold fluid [Fig. 7.15(a)] and exits as a liquid. In an evaporator (or boiler), liquid evaporates at a constant temperature while cooling a hot fluid [Fig. 7.15(b)] and exits as a vapor. Two important features of condensers and evaporators are related to the fact that the temperature remains constant in the two-phase (condensing or evaporating) side  

The change of enthalpy flow in the two-phase side can no longer be expressed in terms of CAT, because AT = 0. In this case the change of enthalpy flow is mAh, m being the mass flow rate and Ah the difference between inlet and outlet 

The change of enthalpy is much larger in the two-phase side. Consequently, the mass flow rate is much smaller in the two-phase side. Assuming-the two-phase side to remain approximately stagnant, we may eliminate the effect of the type of heat exchanger and set F = 1. 

Introducing Eq. (7.35) into the second equality of Eq. (7.34), we obtain for the heat transfer area of a condenser 

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This problem was solved by the introduction of the Linde double-column system. Two rectification columns are placed one on top of the other (hence the name double-column system). In this system, liquid air is introduced at an intermediate point in the lower column. A condenser-evaporator at the top of the lower column provides the reflux needed for the rectification process to obtain essentially pure nitrogen at this point. In order for the column to also deliver pure oxygen, the oxygen-rich liquid (—45% oxygen), from the boiler in the lower column is introduced at an intermediate level in the upper column. The reflux and the rectification process in the upper column produce pure oxygen at the bottom and... 

Figure 7.15 Cold and hot fluid temperatures for condenser and evaporator (boiler), (a) Condenser, (b) evaporator (boiler).

A heat pump that uses the vapor-comprcssion refrigeration cycle and two connected (so that they operate together) three-way valves is schematically shown in Fig. 5.2-10. In this way the indoor coil is the condenser during the winter months and the evaporator or boiler during the summer. Similarly, the outdoor coil is the evaporator (boiler) during... 

D. Butterworth, Steam Power Plant and Process Condensers, in Boilers, Evaporators Condensers, S. Kakag (ed.), Ch. 11, pp. 571-633, Wiley, New York, 1991. 

Heat transfer systems are one of the basic requirements of thermal process technology. Depending on the purpose of the heat exchange, the terms evaporator, Uquefier or condenser, superheater, boiler, and many others are also used. 

The steam flash drum is a device for steam recovery. Flash steam occurs at the drum where steam condensate or boiler blowdown experiences a drop in pressure causing some of the condensate or boiler blowdown to evaporate forming steam and thus produces steam at the lower pressure (Figure 15.11). For low-pressure condensate, flash steam is negligible and thus it is not worth to recover. However, for medium-and high-pressure condensate, it is important to recover flash steam. 

Generally these are installed in exit gas streams of equipment like absoibers, condensers, evaporators, etc., for arresting the liquid droplets entrained in the gases. If the demisters are not installed, the hquid droplets can corrode or choke downstream equipment like heat exchanger tubes, damage tube sheets of heat exchangers, contaminate products (acid mist carryover from oleum boilers), cause atmospheric pollution (unabsorbed acid mist going out from chimneys). 

Based on service, heat exchangers can be classified as cooler, heater, condenser, and vaporizer such as evaporator, boiler, or reboiler. Cooler is used to cool a stream and heater is used to heat a stream. The stream can be vapor or liquid. Condenser is ased to condense a vapor stream partially or totally. Vaporizer is used to vaporize part of the liquid to vapor. There (s no phase change for cooler and heater, but there is phase change for condenser and vaporizer. 

Acid cleaners based on sulfamic acid are used in a large variety of appHcations, eg, air-conditioning systems marine equipment, including salt water stills wells (water, oil, and gas) household equipment, eg, copper-ware, steam irons, humidifiers, dishwashers, toilet bowls, and brick and other masonry tartar removal of false teeth (50) dairy equipment, eg, pasteurizers, evaporators, preheaters, and storage tanks industrial boilers, condensers, heat exchangers, and preheaters food-processing equipment brewery equipment (see Beer) sugar evaporators and paper-mill equipment (see also Evaporation Metal surface treati nts Pulp). 

Boiler feed-water systems that use dernineralized or evaporated makeup or pure condensate may be protected from caustic attack through coordinated phosphate and pH control. Phosphate buffers the boiler water, reducing the chance of large pH changes due to the development of high caustic or acid concentrations. Excess caustic combines with disodium phosphate and forms trisodium phosphate. Sufficient disodium phosphate must be available to combine with all of the free caustic in order to form trisodium phosphate. 

A recent development in heat recovery has been the heat tube. This is a sealed metal tube which has been evacuated of air and contains a small quantity of liquid which, for boiler applications, could be water. When heat from the flue gases is applied to one end of the heat pipes the water in the tube boils, turning to steam and absorbing the latent heat of evaporation. The steam travels to the opposite end of the tube which is surrounded by water, where it gives up its latent heat, condenses and returns to the heated end of the tube. Batteries of these tubes can be arranged to form units, usually as a water jacket around a section of a flue. 

NOTE When a boiler first starts up from cold, there is always some initial condensation arising from combustion (which generally falls to the rear of the boiler and may form a pool of water). This quickly evaporates and should not be confused with tube leaks that leave evaporated solids behind. 

Convective heat exchange, natural or forced Radiant heat transfer, e.g. furnaces Evaporation, e.g. in evaporators Condensation, e.g. in shell and tube heat exchanges Heat transfer to boiling liquids, e.g. in vaporizers, boilers, re-boilers ... 

Usually in evaporation the thick liquor is the valuable product and the vapour is condensed and discarded. There are, however, specific situations where the opposite is true. In this context, mention may be made of the fact that mineral-containing water is often evaporated to yield a solid-free product for boiler feed, for special process requirements, or for human consumption. This technique is often called water distillation, but technically it is evaporation. 

Example 23.1 A small package fire-tube boiler has makeup water that contains 500 ppm dissolved solids. The steam system operates with 50% condensate return. Estimate the blowdown rate. Assume that the maximum limit for the TDS is 4500 ppm. Assume that there are no solids in the evaporation or the condensate return. 

Superheaters and Reheaters A superheater raises the temperature of the steam generated above the saturation level. An important function is to minimize moisture in the last stages of a turbine to avoid blade erosion. With continued increase of evaporation temperatures and pressures, however, a point is reached at which the available superheat temperature is insufficient to prevent excessive moisture from forming in the low-pressure turbine stages. This condition is resolved by removing the vapor for reheat at constant pressure in the boiler and returning it to the turbine for continued ejq)ansion to condenser pressure. The thermodynamic cycle using this modification of the RanTine cycle is called the reheat cycle. 

Figure 17. Weak acid recovery plant used by Sachtleben Chemie (based on know-how of Bayer AG) a) Heat exchanger b) Evaporator c) Injection condenser d) Stirred salt maturing vessels e) Filter press f) Bunker for pyrites g) Coal silo h) Bunker i) Mixing screw unit j) Covered store for mixed filter cake k) Calcination furnace 1) Waste-heat boiler m) Cyclone n) Electrostatic precipitator o) Stirred tank p) Storage tank q) Pump r) Cooler...

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