Superheat effect, condensation

Since the degree of superheat effects also the average temperature at which heat is absorbed, in the results presented in Figure 3.8 we keep the boiler pressure constant at 35 bar, and vary the temperature of the superheated steam. The condenser operates as before at 1 bar. Again, as predicted by the Carnot cycle, higher operating temperatures lead to increased efficiencies. 

F Saturated Evaporating, 75°F Suction Superheat (Not Included in Refrigeration Effect), 100°F Saturated Condensing ... 

Where carryover occurs, much of the solids content is deposited in the first parts of the steam and condensate system, such as superheaters, but the balance can be transported all the way back to the pre-boiler system and from there to the boiler itself. Thus, a chain of cause and effect may once again develop in a manner similar to the progression of problems in other areas of the boiler system. 

Distilled water is produced from sea water by evaporation in a single-effect evaporator working on the vapour compression system. The vapour produced is compressed by a mechanical compressor of 50 per cent efficiency, and then returned to the calandria of the evaporator. Extra steam, dry and saturated at 650 kN/m2, is bled into the steam space through a throttling valve. The distilled water is withdrawn as condensate from the steam space. 50 per cent of the sea water is evaporated in the plant. The energy supplied in addition to that necessary to compress the vapour may be assumed to appear as superheat in the vapour. Calculate the quantity of extra steam required in kg/s. The production rate of distillate is 0.125 kg/s, the pressure in the vapour space is 101.3 kN/m2, the temperature difference from steam to liquor is 8 deg K, the boiling-point rise of sea water is 1.1 deg K and the specific heat capacity of sea water is 4.18 kJ/kgK. 

If the water were to be injected into a cold engine cylinder, the flash steam would immediately condense and there would be no pressure rise. To overcome this problem, the cylinder head and walls are heated and supply additional heat to the wet steam entering the cylinder. The atomised water droplets experience extremely high collision rates with the cylinder walls because of the explosive effect of the flash process. The tiny size of the droplets, coupled with high collision rates ensure rapid absorption of heat allowing them to be quickly converted to steam which is then heated further to superheat. 

The most commonly used steam is 100 psig with 10-15° superheat, the latter characteristic in order to avoid the erosive effect of liquids on the throats of the ejectors. In Figure 7.31 the steam consumptions are given as lb of motive steam per lb of equivalent air to the first stage. Corrections are shown for steam pressures other than 100 psig. When some portion of the initial suction gas is condensable, downward corrections to these rates are to be made for those ejector assemblies that have intercondensers. Such corrections and also the distribution of motive steam to the individual stages are problems best passed on to ejector manufacturers who have experience and a body of test data. 

The effect of superheat of the vapor will next be considered. In the previous analysis it was assumed that the vapor reservoir was at the saturation temperature. If the vapor reservoir is superheated to temperature, 7", the enthalpy of the vapor, hv, condensing onto the film will be ... 

A superheated vapor may be condensed directly from the vapor state by contact with a surface that is below the saturation temperature or the dew point of the vapor. The heat-transfer coefficient is predicted by the equations in this section if the saturation temperature (not the superheat temperature) is used as the effective temperature for heat transfer, i.e., a desuperheater/condenser may be... 

Equation (11.7) is based on the assumption that the vapor enters the condenser as saturated vapor (no superheat) and the condensate leaves at condensing temperature without being further cooled. If either of these sensible-heat effects is important, it must be accounted for by an added term in the left-hand side of Eq. (11.7). For example, if the condensate leaves at a temperature 7 that... 

The effect of superheat on the rate of heat transfer depends upon whether the temperature of the surface of the tube is higher or lower than the condensation... 

Effect of Superheat. When the vapor is superheated (i.e., Tg > Ts) and the cold wall temperature is less than the vapor temperature but greater than the saturation temperature, no condensation occurs. Instead, the vapor is cooled by single-phase free or forced convection... 

Although sensible heat transfer coeflicients are considerably lower than condensing coeflicients, heat transfer rates are quite high in the desuperheating zones of distillation condensers. The low heat transfer coefficient in the desuperheating zone is compensated for by the higher temperature difference between the superheated vapor and the coolants (compared to the temperature difference between the saturated vapor and the coolant). In most cases, moderate variation in superheat has little effect on condenser performance and is seldom troublesome in distillation operation. 

Dryer steam should be free of superheat. Although the condensate inside the dryer desuperheats the steam, a separate desuperheater is used in some cases. The effect of superheating above 100°C is controversial. 

B. Vapor-Recompression Evaporation. The existence of the BPR in a solution means that the condensing temperature of the vapor raised in an evsqiorator will be lower than the boiling point of the solution from which it came. In other words, the vapor as it forms is superheated. When the vapor is used in another effect, the superheat provides very little thermal energy, and the vapor temperature quickly drops to the saturation temperature of pure water at the operating pressure. 

Differences in performance between the three different screens are due to the effect of the screen thickness and porosity on the overall heat transfer across the LAD screen. Differences in performance between pressurants are due to modified heat and mass transport at the screen pore L/V interface through evaporation (GHe) and/or condensation (GH2 or GN2). Differences in performances between the two liquids are explained through the differences in superheats required to initiate boiling in the liquid. 

Superheater. The allocation of the superheater to the postboiler group rather than to the boiler itself is purely arbitrary. Problems in the superheater are somewhat similar to both those of the boiler and those of the return and condensate system. For that reason it serves as an effective transition problem between the two. The attack on snperheater tubes can be attributed to three corrosive factors ... 

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