Flash steam

Vacuum Treatment. Milk can be exposed to a vacuum to remove low boiling substances, eg, onions, garlic, and some silage, which may impart off-flavors to the milk, particularly the fat portion. A three-stage vacuum unit, known as a vacreator, produces pressures of 17, 51—68, and 88—95 kPa (127, 381—508, and 660—711 mm Hg). A continuous vacuum unit in the HTST system may consist of one or two chambers and be heated by Hve steam, with an equivalent release of water by evaporation, or flash steam to carry off the volatiles. If Hve steam is used, it must be cuUnary steam which is produced by heating potable water with an indirect heat exchanger. Dry saturated steam is desired for food processing operations. 

Evaporation and Impurity Removal. Evaporation over and above that obtained in the cooling areas from flashed steam is usually required... 

The flashed steam method is less efficient and its requirements for steam properties—cleanliness, high temperature, and high pressure— are usually unavailable in most geothermal fields. The situation is different with the binary cycle system, which is quite efficient and widely used. This wet system involves the transfer of heat from the hot well stream into a more manageable boiling fluid to generate power through a turboexpander. 

For the frequent case of flashing steam-condensate lines, Ruskan supplies the handy graph shown above. 

Where Water Hammer Occurs. Water hammer can occur in any water supply line, hot or cold. Its effects can be even more pronounced in heterogeneous or biphase systems. Biphase systems carry water in two states, as a liquid and as a gas. Such a condition exists in a steam system where condensate coexists with live or flash steam in heat exchangers, tracer lines, steam mains, condensate return lines and, in some cases, pump discharge lines. 

Condensate normally flows at the bottom of a return line. It flows because of the pitch in the pipe and also because of the higher velocity flash steam above it, dragging it along. The flash steam moves at a higher velocity because it moves by differential pressure. 

In our glass pipe demonstrator, cold water is used to simulate condensate and air pressure is applied to simulate the flash steam in the top portion of the pipe (Fig. 2). 

C 0.35 Recovery of low level waste heat f space heating, district heating syste Absorption cooling. Recovery of steam condensate and flash steam. Heat pump for evaporation, drying, etc. 3r m. 

Often in plant operations condensate at high pressures are let down to lower pressures. In such situations some low-pressure flash steam is produced, and the low-pressure condensate is either sent to a power plant or is cascaded to a lower pressure level. The following analysis solves the mass and heat balances that describe such a system, and can be used as an approximate calculation procedure. Refer to Figure 2 for a simplified view of the system and the basis for developing the mass and energy balances. We consider the condensate to be at pressure Pj and temperature tj, from whence it is let down to pressure 2. The saturation temperature at pressure Pj is tj. The vapor flow is defined as V Ibs/hr, and the condensate quality is defined as L Ibs/hr. The mass balance derived from Figure 2 is ... 

Figure 2-54. Sarco flashing steam condensate line sizing flow chart. By permission, Spirax-Sarco, Inc., Allentown, Pa. [59].

Because flashing steam-condensate lines represent two-phase flow, with the quantity of liquid phase depending on die system conditions, these can be designed following the previously described two-phase flow methods. An alternate by Ruskin [28] uses the concept but assumes a single homogeneous phase of fine liquid droplets dispersed in the flashed vapor. Pressure drop was calculated by the Darcy equation ... 

Rgure 2-55. Flashing steam condensate line sizing chart. By permission, Ruskin, R. R, Calculating Line Sizes for Flashing Steam Condensate, Chem. Eng., Aug. 18,1985,... 

This relation is handled very similar to the flash steam separation. 

In an efficient and economical steam system, this so-called flash steam will be utilized on any load that can make use of low-pressure steam. Sometimes it can be simply piped into a low-pressure distribution main for general use. The ideal is to have a greater demand for low-pressure steam at all times than the available supply of flash steam. Only as a last resort should flash steam be vented to atmosphere and lost. 

Instead, the flash vessel is primarily a flash steam separator. Its shape and dimensions are chosen to encourage separation of the considerable volume of low-pressure steam from the small volume of liquid. 

Alternatively, tables such as Table 22.7 or charts such as that shown in Figure 22.12 will allow the proportion of flash steam available between two pressures to be read off directly, within the more usual pressure ranges. 

The steam/process air heater with multiple coils typifies the kind of application on which recovery of flash steam... 

Condensate from the low-pressure coil together with that from the flash vessel will then drain to a collecting tank, or direct to a condensate pump, for return to the boiler plant. If the pressure of the flash steam is left to find its own level it will often be sub-atmospheric. As the condensate must then drain by gravity through the steam traps these also must be sufficiently below the condensate drain points to provide an appropriate hydraulic head, and a vacuum breaker fitted above the coil. The alternatives are to allow the condensate to drain directly to a condensate pump, or to supply additional low-pressure steam through a pressure-reducing valve, to maintain a positive pressure in the coil and flash vessel. 

Somewhat similar arrangements can be used when radiant panels or unit heaters heat large areas. Some 10-15 per cent of the heaters are separated from the high-pressure steam supply and supplied instead with low-pressure steam flashed off the high-pressure condensate. The heating demands of the whole area remain in step, so supply and demand for the flash steam are balanced. 

In other cases, flash steam is utilized on equipment, which is completely separated from the high-pressure source. Often the low-pressure demand does not at all times match the availability of the flash steam. A pressure-reducing station is often needed to make up any deficit and a surplus valve is required to vent any flash steam in excess of the amount being condensed. 

The pressure drop needed to carry the condensate and any flash steam along the line. 

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