Subcooled liquid, pumps

To solve Equation 9.50, start by assuming a feed condition such that q can be fixed. Saturated liquid feed (i.e. q = 1) is normally assumed in an initial design as it tends to decrease the minimum reflux ratio relative to a vaporized feed. Liquid feeds are also preferred because the pressure at which the column operates can easily be increased if required by pumping the liquid to a higher pressure. Increasing the pressure of a vapor feed is much more expensive as it requires a compressor rather than a pump. Feeding a subcooled liquid or a superheated vapor brings inefficiency to the separation as the feed material must first return to saturated conditions before it can participate in the distillation process. 

The 15 ft of head is the available NPSH to this pump. Does this mean that pumps may have a substantial amount of available NPSH, even when their suction pressure is under a partial vacuum Yes, if we are pumping a subcooled liquid. But this is quite common, because the liquid stored in an ordinary atmospheric-pressure storage tank is almost always well below its boiling point—that is, the liquid is subcooled. 

The bypassed vapor heats up the liquid there, thereby causing the pressure to rise. WTien the bypass is closed, the pressure falls. Sufficient heat transfer surface is provided to subcool the condensate, (f) Vapor bypass between the condenser and the accumulator, with the condenser near ground level for the ease of maintenance When the pressure in the tower falls, the bypass valve opens, and the subcooled liquid in the drum heats up and is forced by its vapor pressure back into the condenser. Because of the smaller surface now exposed to the vapor, the rate of condensation is decreased and consequently the tower pressure increases to the preset value. With normal subcooling, obtained with some excess surface, a difference of 10-15 ft in levels of drum and condenser is sufficient for good control, (g) Cascade control The same system as case (a), but with addition of a TC (or composition controller) that resets the reflux flow rate, (h) Reflux rate on a differential temperature controller. Ensures constant internal reflux rate even when the performance of the condenser fluctuates, (i) Reflux is provided by a separate partial condenser on TC. It may be mounted on top of the column as shown or inside the column or installed with its own accumulator and reflux pump in the usual way. The overhead product is handled by an alter condenser which can be operated with refrigerant if required to handle low boiling components. 

Observations of the flow patterns inside of fin passage making the set of rectangular channels were done at apparatuses shown at Fig. 9. Subcooled liquid was pumped through electro-heating coil to provide a certain vapor quality of the flow. Then the flow was passed through adiabatic test section and later through the evaporator, for exception pulsation of flow, into the condenser. The test section can operate both in up... 

Figure 15. Subcooled liquid nitrogen catheter system. One nitrogen dewar is pumped on with a vacuum pump to cool it below 77 K.

Liquids are usually moved by pumps, generally rotating equipment. The same equations apply to adiabatic pumps as to adiabatic compressors. Thus, Eqs. (7.13) through (7.15) and Eq. (7.17) are valid. However, application of Eq. (7.14) for the calculation of Wj = AH requires values of the enthalpy of compressed (subcooled) liquids, and these are seldom available. The fundamental property relation, Eq. (6.8), provides an alternative. For an isentropic process,... 

Reversible, adiabatic (isentropic) pumping of the saturated liquid to the pressure of the boiler, producing compressed (subcooled) liquid. The vertical line (whose length is exaggerated in Fig. 8.3) is very short, because die temperature rise associated with compression of a liquid is small. 

The need for maintaining subcooled liquid in the supply tank is based upon the need for net positive suction head (NPSH), which is the total head of fluid in excess of vapor pressure, at the pump inlet. Years of experience in designing and testing pumps for fluids other than hydrogen have demonstrated the need for NPSH at the pump inlet to ensure proper operation. However, test data for hydrogen suggest that unusual suction performance can be expected. This expectation is especially valid if the available thermodynamic head at the pump inlet is considered. In this case, thermodynamic head is defined as the reduction in enthalpy of the fluid when expanded isentropically. This process involves vaporization, i.e., cavitation of the fluid. 

A simple Rankine steam plant is shown in Figure 23.14. Steam is supplied to a turbine at 1000°F and 2000 psia, where it is expanded in a turbine with an isentropic efficiency of 85% to a pressure of 1 psia. The exhausfed sfeam is condensed and cooled to a subcooled liquid at 85°F. It is returned to the boiler with a pump that has an isentropic efficiency of 85%. The properties for each poinf in fhe cycle are calculated using computer generated steam tables and shown in Table 23.1. Points 2s and 4s represent conditions that would be achieved with an isentropic pump and turbine, respectively. In accordance with the state... 

If the pressure is lower, we will not have complete condensation if it is higher, there is no problem for we will end up with a subcooled liquid. The latter is actually desirable in practice, to avoid problems in pumping a saturated liquid back into the column as reflux, and the remaining to storage. 

Common design error. Please refer back to Fig. 13.2. How can the liquid from the condenser rise to the higher elevation in the reflux drum, without being pumped Simple The pressure head of the liquid leaving the condenser is converted to elevation as the liquid flows up into the reflux drum. This works fine, as long as the liquid leaving the condenser is sufficiently subcooled. By sufficiently subcooled, I mean that when the lower-pressure liquid flows into the reflux drum, it has to be cold enough that it does not flash. 

Answer—yes But why Well, the liquid is cooled by 5°F after it leaves the drum. The cooled liquid is not in equilibrium with the vapor in the drum. It has been subcooled by 5°F. This means that the bubble-point liquid has been cooled, without altering its composition. The vapor pressure of the liquid has been reduced. As can be seen in Fig. 25.3, subcooling this particular liquid by 5°F reduces its vapor pressure by about 2 psi. As the specific gravity of the liquid is 0.58, this is equivalent to an increase in the NPSH by 8 ft. Once again, our objective is to increase the flow from 250 to 300 GPM. Figure 25.2 tells us that the required NPSH increases from 20 to 26 ft. However, when we subcool the liquid by 5°F, the available NPSH increases from 20 to 28 ft. As the available NPSH now exceeds the required NPSH by 2 ft, the flow can be increased without risk of pump cavitation. 

Liquids in storage tanks are almost always subcooled. This is so because otherwise the ambient vapor losses from the tank s vent, would be excessive. This creates the potential for a negative pump suction pressure. 

Using a pump to obtain the debutanizer pressure, the cooler is used to obtain a bubble point duty of 1.0 mmBtu/h. Next add a small subcooling duty. Use Table 2.1 and again the heptane liquid line data. 

The conditions of steam generation in the boiler are the same as in Example 8.1 8,600 kPa and 500°C. The exhaust pressure of the turbine, lOkPa, is also the same. The saturation temperature of the exhaust steam is therefore 45.83°C. Allowing for slight subcooling of the condensate, we fix the temperature of the liquid water from the condenser at 45°C. The feedwater pump, which operates under exactly the conditions of the pump in Example 7.10, causes a temperature rise of about 1°C, making the temperature of the feedwater entering the series of heaters equal to 46°C. 

Some subcooling of the condensate will usually be required to control the net positive suction head at the condensate pump (see Chapter 5) or to cool a product for storage. Where the amount of subcooling is large, it is more efficient to subcool in a separate exchanger. A small amount of subcooling can be obtained in a condenser by controlling the liquid level so that some part of the tube bundle is immersed in the condensate. 

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