Pumping incompressible liquids

New perspectives arising from isothermal oxidation. The next chapter of this book describes the greatly altered perspective of the fuel cell industry, when Grove s ideas are updated. The second chapter describes the detail of Regenesys, or ESS-RGN. This system has changed hands, as noted above, and information is available from http //www.vrbpower.com/. (The initials VRB stand for Vanadium Redox Battery, a low-power alternative to Regenesys.) The new 2005 VRB Power Systems shorthand is ESS-VRB for 2.5 to 10 MW and ESS-RGN for 10 to 100 MW. In Chapter 2 the reader will be acquainted with ESS-RGN, one of the two VRB fuel cell systems (incompressible liquid based) which can be termed complete . The redox battery uses small pumps as circulators. 

Because liquids are incompressible. Equation 5.2 may be used to calculate the work required to pump a liquid. The kinetic energy term is small compared to the other terms and is neglected. Therefore, Equation 5.2 reduces to... 

The energy required to pump a liquid food through a pipe line can be calculated from the mechanical eneigy balance (MEB) equation. The MEB equation can be used to analyze pipe flow systems. For the steady-state flow of an incompressible fluid, the MEB can be written as follows (Brodkey, 1%7) ... 

Also a pump is a chemodynamic machine. The experimental setup is shown in Fig. 9.9. We want to pump an incompressible liquid. We sketch here the steps only briefly ... 

As an example of energy transfer by work, consider Figure 9.6(aL where an incompressible liquid at 25 C having a specific volume, V, of 0.001 m /kg is pumped continuously at a rate wof... 

The pumps used in handling these high-pressure liquids can suffer considerable damage from cavitation. Incompressible liquids will not compress, nor will they withstand tension thus if the suction inlet to a pump is restricted the fluid will release any contained air to form cavities. This condition seriously affects the performance of the pump, can cause damage to its rotor and generates a great deal of noise. Gas or air entrained in a hydraulic fluid is detrimental to its effectiveness as a power transmission medium. 

Fans and compressors are gas movers, analogous to pumps as liquid movers. The term "gas mover" will be used when discussing the characteristics of faiK and compressors collectively. The geometry of gas movers is somewhat similar to that of pumps however, operating parameters and safety considerations are quite different. Fans and compressors must handle gases that are compressible and extremely sensitive to temperature and pressure changes, while pumps handle liquids that are relatively insensitive to pressure and temperature changes and can be considered incompressible. 

Pumps are a mechanical device that forces a fluid to move from one position to another. Usually a pump refers to the mechanical means to move incompressible (or nearly incompressible) fluid or liquid. Pumps are our earliest machine and are to this day one of our most numerous mechanical devices. 

Air or gas compressors are very similar in design and operation to liquid pumps discussed earlier. The air and gas compressor is a mover of compressed fluids the pumps are movers of basically incompressible fluids (i.e., liquids). 

As liquids are essentially incompressible, less energy is stored in a compressed liquid than a gas. However, it is worth considering power recovery from high-pressure liquid streams (> 15 bar) as the equipment required is relatively simple and inexpensive. Centrifugal pumps are used as expanders and are often coupled directly to pumps. The design, operation and cost of energy recovery from high-pressure liquid streams is discussed by Jenett (1968), Chada (1984) and Buse (1985). 

The scope of coverage includes internal flows of Newtonian and non-Newtonian incompressible fluids, adiabatic and isothermal compressible flows (up to sonic or choking conditions), two-phase (gas-liquid, solid-liquid, and gas-solid) flows, external flows (e.g., drag), and flow in porous media. Applications include dimensional analysis and scale-up, piping systems with fittings for Newtonian and non-Newtonian fluids (for unknown driving force, unknown flow rate, unknown diameter, or most economical diameter), compressible pipe flows up to choked flow, flow measurement and control, pumps, compressors, fluid-particle separation methods (e.g.,... 

As an example of a simple application of Bernoulli s equation, consider the case of steady, fully developed flow of a liquid (incompressible) through an inclined pipe of constant diameter with no pump in the section considered. Bernoulli s equation for the section between planes 1 and 2 shown in Figure 1.5 can be written as... 

Example 1.1. Figure 1.1 shows a tank into which an incompressible (constant-density) liquid is pumped at a variable rate (ftVs)- This inflow rate can vary with... 

Flat Spiral Pump A spiral flight of height H is welded to a stationary disk creating a spiral channel of constant width W, as shown in the accompanying figure. By placing a second disk over the channel, a flat spiral pump is created. Clockwise rotation of the upper disk pumps liquid from the outer inlet port to the inner exit port. Derive an expression for the flow rate of an incompressible Newtonian fluid in isothermal flow. 

As an example of a dynamic process, consider the process in Figure 1, which is a tank into which an incompressible (constant density) liquid is pumped at a variable feed rate FL (m3 s ). This inlet flow rate can vary with time because of changes in operations upstream of the tank. The height in the tank is h (m) and the outlet flow rate is F (m3 s ). Liquid leaves the tank at the base via a long horizontal pipe and discharges into another tank. Both tanks are open to the atmosphere. F h and Fean all vary with time and are therefore functions of time t. 

In the past, it was not uncommon for workers to bolt a heat exchanger onto an HPLC pump and call it an SFC pump. Unfortunately, many people still think that if the fluid is at a temperature and pressure defined as being a liquid, it becomes incompressible. This is emphatically not true. At 5°C, the volume of the fluid can change up to 20% over the normal pressure range of SFC. Decreasing the pressure much below 60 bar will still result in the fluid expanding into a low-density gas. 

In this case the heat input, , is zero and there is no mechanical power output, but a mechanical power supplied, Ps. Hence P = —Ps. There will be no significant difference in height over the pump, so Zi = i2. In addition, the almost incompressible nature of a liquid means that the specific volume will be essentially constant, allowing us to write ... 

A convenient small-scale continuous unit is shown in Fig. 10-5. The materiftl to be hydrogenated, if a liquid, is fed into the small high-pressure pump—OF, if a solid, it is dissolved in a suitable solvent. All that the pump needs to do is to put sufficient pressure on the liquid to raise it to the pressure on the reactor- Binoe liquids are relatively incompressible, the work expended in this operation is small- The material is heated near the inlet... 

Liquids can be expanded or compressed just as gases can. For technical reasons, the design of expansion and compression units for liquids is different from that of gases, but the thermodynamics analysis is the same. A turbine for liquids is usually called an expander, and a compressor for liquids, a pump. The analysis is based on the same equations as for gases, but we treat the subject separately because the relative incompressibility of liquids allows us to use short-cut approximations for the enthalpy and entropy changes of the fluid. The starting point is eqs. (5.20) and (5.30). 

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