Condensate, flashing flow

Pressure drop, chart, 102, 103, 111 Computer aided drafting, 17 Condensate, flashing flow, 135-142, 147 Charts, 142, 143 Control valve pressure drop, 90 Calculations, 90-96 Cost estimates, plant, 45-49 Accounting, 48... 

From assumed feed temperature (forward feed) or feed flow (backward feed) to the first effect and assumed steam flow, calculate evaporation in the first effect. Repeat for each succeeding effect, checking intermediate assumptions as the calculation proceeds. Heat input from condensate flash can be incorporated easily since the condensate flow from the preceding effects will have already been determined. 

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. 

Flashing flow and condensing flow are two examples of multiphase flow with phase change. Flashing flow occurs when pressure drops below the bubble point pressure of a flowing liquid. A frequently... 

VELOCITY OF FLASHED CONDENSATE MIXTURE, ft/min. TOTAL FLOW OF MIXTURE IN CONDENSATE HEADER, Ib/h. WEIGHT FRACTION OF CONDENSATE FLASHED TO VAPOR. FLASHED STEAM FLOWRATE, Ib/h. 

When a liquid at its boiling point flows in a pipe, the pressure decrease due to friction will cause the pressure of the liquid to fall below its saturation pressure, and the liquid will boil. This type of flo i, called flashing flow, is important in the design of boilers, steam condensate lines, ete. 

When reboiler steam flow is unmetered, it can often be measured by running a hose to a 45-gallon drum and timing it Check whether this technique can be used. Remember that steam condensate flashes unless it is cooled, giving rise to incorrect readings. It may be necessary to cool the condensate or run it into a drum partially... 

The discharge capacity of the trap depends on the flow area of the valve orifice, the pressure drop across it, and the inlet temperature of the condensate. There is a considerable problem in measuring the pressure drop because hot condensate flashes as it passes through the valve orifice. Trap capacity is not truly defined by orifice size and pressure differential. Pressures upstream and downstream of the trap are also subject to variation, depending on calandria performance, flowrates, temperatures, and system back pressure. The orifice may never be fully open to flow because of the valve design. Nor can flow coefficients be measured with the same precision as for control valves, since the valve stem in the trap is often not definitely located with reference to the orifice. 

On the downstream side of the steam condensate control valve or steam trap, divert the condensate into an empty 55-gal. drum. Note the time it takes for the drum to fill. Add 5% (for low-pressure steam) to 15% (for high-pressure steam) to the observed pounds per hour of flow. This accounts for condensate flashing as it drops to ambient pressure. For instance, a reboiler having a duty of 25 MM BTU/hr will fill a 55-gal. drum in about one minute. 

As the feed-to-steam ratio is increased in the flow sheet of Fig. 11-125 7, a point is reached where all the vapor is needed to preheat the feed and none is available for the evaporator tubes. This limiting case is the multistage flash evaporator, shown in its simplest form in Fig. 11-125 7. Seawater is treated as before and then pumped through a number of feed heaters in series. It is given a final boost in temperature with prime steam in a brine heater before it is flashed down in series to provide the vapor needed by the feed heaters. The amount of steam required depends on the approach-temperature difference in the feed heaters and the flash range per stage. Condensate from the feed heaters is flashed down in the same manner as the brine. 

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