Degree of superheat

G = Gas specific gravity = mol. wt./29 Pi = Valve inlet pressure, psia AP = Pressure drop across valve, psi Q = Gas flow rate, SCFH Qs = Steam or vapor flow rate, Ib/hr T = Absolute temperature of gas at inlet, °R T5I1 = Degrees of superheat, °F... 

The formation of droplets and their rapid, efficient vaporization is the reason that there is more vapor in the cloud than the amount which flashed off originally. Schmidli et al. (1990) determined that 5 to 50% of the mass of the original fuel can be found in droplets. This value depends upon initial mass and degree of superheat, that is, amount by which the fuel s temperature exceeds its boiling point. 

Figure 6.5. Fraction of pool and aerosol mass generated after loss of containment of small vessels containing CFG 114, depending on degree of superheat and degree of filling (Schmidli etal. 1990).

Small-scale experiments by Schmidli et al. (1990) showed that, as degree of superheat increases, the quantity of fuel forming a pool decreases and droplet formation increases. These results support the proposition that more fuel is involved in a BLEVE than calculated from flash evaporation. 

Such experiments should also determine the influences of fill ratio, pressure, substance, and degree of superheat on mass contributing to fireball generation. 

S° = Degrees of superheat in a steam condition, degrees I above saturated (not the actual temperature)... 

A few degrees of superheat are recommended (5-15°F), but if superheated steam is to be used, its effect must be considered in the ejector design. A high degree of superheat is of no advantage because the increase in available energ) is offset by the decrease in steam density [16]. 

The selection is dictated by economics governing the initial plant cost versus higher turbine output. Usually, the turbine exhaust steam is designed to be slightly superheated, which is desirable, as it allows for heat loss from the steam with minimum condensate losses. At low loads from the turbine, the degree of superheat can rise sharply, well in excess of the normal design conditions, and for this purpose, desuperheaters are often employed to trim the steam temperature at exhaust. 

Dryback boilers are still occasionally used when a high degree of superheat is required, necessitating a rear chamber to house the superheater too large for a semi-wet-back chamber. A water-cooled membrane wall chamber would be an alternative to this. 

For shell boilers, superheaters may be one of three types, depending upon the degree of superheat required. The first and simplest is the pendant superheater installed in the front smokebox (Figure 23.7). The maximum degree of superheat available from this would be around 45°C. The second pattern is again installed in the front smokebox but with this, the elements are horizontal U tubes which extend into the boiler smoketubes. The degree of superheat from this pattern is around 80°C. Third, a superheater may be installed in the reversal chamber of the boiler. A wetback chamber presents problems with lack of space, and therefore a semi-wetback, dryback or water-cooled wall chamber may be considered. Maximum degree of superheat would be around 100°C. 

SpGr = specific gravity of fluid, relative to water =1.0 St = dust hazard class St St = stainless steel SSU = viscosity Saybolt universal seconds °S = degrees of superheat, °F T = absolute inlet or gas temperature, degrees... 

The early vertical boilers of dry-top design (steam on one side and hot combustion gases on the other side) were subject to the risk of overheating in any fire tubes located above the waterline, but these boilers could provide relatively dry steam with some degree of superheat. 

The formation of superheater deposits in and around the superheater tube outlets and receiving header is a relatively common but serious superheater problem that may occur. These deposits are caused by contamination from BW carryover and also by gross contamination of attemperation water used to control the degree of superheat. 

In large boiler plants, carryover is measured by employing a singleport sampling nozzle connected to a steam supply line between the top drum and the superheater. Sampling from superheaters is difficult, however, because a pump is needed to inject cool condensate water into a double-walled sample probe (via an attemperating nozzle). This is to remove the degrees of superheat and thus reduce the tendency for any contaminants to deposit in or on the sample probe, rather than be collected with the steam. 

Hence the liquid must be superheated near the surface of the bubble, the extent of the superheat increasing with decrease in the radius of the bubble. On this basis it follows that very small bubbles are difficult to form without excessive superheat. The formation of bubbles is made much easier by the fact that they will form on curved surfaces or on irregularities on the heating surface, so that only a small degree of superheat is normally required. 

Thermodynamic and mechanical equilibrium on a curved vapor-liquid interface requires a certain degree of superheat in order to maintain a given curvature. Characteristics of homogeneous and heterogeneous nucleation can be estimated in the frame of classical theory of kinetics of nucleation (Volmer and Weber 1926 Earkas 1927 Becker and Doring 1935 Zel dovich 1943). The vapor temperature in the bubble Ts.b can be computed from equations (Bankoff and Flaute 1957 Cole 1974 Blander and Katz 1975 Li and Cheng 2004) for homogeneous nucleation in superheated liquids... 

If the degree of superheat is large, it will be necessary to divide the temperature profile into sections and determine the mean temperature difference and heat-transfer coefficient separately for each section. If the tube wall temperature is below the dew point of the vapour, liquid will condense directly from the vapour on to the tubes. In these circumstances it has been found that the heat-transfer coefficient in the superheating section is close to the value for condensation and can be taken as the same. So, where the amount of superheating is not too excessive, say less than 25 per cent of the latent heat load, and the outlet coolant temperature is well below the vapour dew point, the sensible heat load for desuperheating can be lumped with the latent heat load. The total heat-transfer area required can then be calculated using a mean temperature difference based on the saturation temperature (not the superheat temperature) and the estimated condensate film heat-transfer coefficient. 

Thus it can be seen that the degree of superheat is much greater in liquid metals than in water for the same pressure and cavity size, because of their much higher values of (7sat)2. Also, for the same cavity size, pressure, and heat flux, the time required to build the thermal layer as well as its thickness will be much greater in liquid metals than in other liquids (see Sec. 2.2.2). 

Bankoff calculated Tx by using Gunter s experimental data and obtained the interesting result that, in each series of runs, Tx rises steeply toward the saturation temperature as burnout is approached. This gives a fairly thick bubble layer, which increases the degree of superheat near the wall. Bankoff concluded that burnout occurs when the core is unable to remove the heat as fast as it can be transmitted by the wall layer. ... 

Both the bubble departure frequency / and the number of nucleation centers n are difficult to evaluate. These quantities are known to be dependent on the magnitude of the heat flux, material of construction of the tube, roughness of the inside wall, liquid velocity, and degree of superheat in the liquid elements closest to the tube wall. Koumoutsos et al. (K2) have studied bubble departure in forced-convection boiling, and have formulated an equation for calculating bubble departure size as a function of liquid velocity. 

It was found that the steam supply to the reactor was often superheated (just prior to shutdown to 330°C) [10], Although this degree of superheat would not grossly increase the temperature of the inner reactor wall in contact with the liquid (or the bulk liquid temperature) [11], it seems probable that any reaction material splashed onto and dried out at the top of the coil-heated wall would have become heated to a much higher temperature. Further detailed work on the thermal stability of the mixture showed that a previously unsuspected very slow exothermic decomposition existed, beginning at 180°C and proceeding at an appreciable rate only above 200°C, so that the exotherm was insufficient to heat the contents of the reactor from the last recorded temperature of 158°C to the decomposition temperature of 230°C in 7.5 h [12,13,14], It was concluded that an alternative (effectively an external) source of heat was necessary to account for the observed effect, and the residual superheat from the steam at 330°C seems to have been that source. 

This brief commentary on superheated liquids has indicated that they are readily formed if one prevents heterogeneous nucleation of vapor embryos. Also, there is a limit to the degree of superheat for any given liquid, pure or a mixture. This limit may be estimated either from thermodynamic stability theory or from an analysis of the dynamics of the formation of critical-sized vapor embryos. Both approaches yield very similar predictions although the physical interpretation of the results from both differ considerably. 

Ksh = Capacity correction factor, due to the degree of superheat in the steam. For saturated steam, use Ksh= 1. For other values, see Appendix E... 

Ksh = Capacity correction factor, due to the degree of superheat in the steam. 

If the pressure over a solution is reduced below the partial pressure of the solvent over the solution, then the solution is said to be superheated. The degree of superheat is represented by the difference between the equilibrium partial pressure of the solvent over the solution and the total pressure (vacuum level) over the solution Pi — Pq- The higher the vacuum level, the higher the superheat at a given concentration and temperature. Increasing the temperature at a fixed pressure level, of course, also increases the superheat. 

Example 8.1 The Degree of Superheat and Vapor Volume for a Desired Separation Level We consider a 10,000 ppm styrene-PS solution at 220°C, which has to he devolatilized to 1000 ppm. Disregarding the rate of devolatilization, we wish to determine the minimum superheat necessary in order to achieve the required separation. We assume that... 

When does a liquid boil Clearly, boiling at constant pressure—say, atmospheric pressure—begins when we increase the temperature of a liquid or solution and the vapor pressure reaches a pressure of one atmosphere. Alternatively, the pressure over a liquid or solution at constant temperature must be reduced until it reaches the vapor pressure at that temperature (e.g., vacuum distillation). Yet it is well known that liquids can be superheated (and vapors supersaturated) without the occurrence of phase transfer. In fact, liquids must always be superheated to some degree for nucleation to begin and for boiling to start. That is, the temperature must be raised above the value at which the equilibrium vapor pressure equals the surrounding pressure over the liquid, or the pressure must be reduced below the vapor pressure value. As defined earlier, these differences are called the degree of superheat. When the liquid is superheated, it is metastable and will reach equilibrium only when it breaks up into two phases. 

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