Turboexpander efficiency

When discussing efficiencies, the two most important design parameters are the isentropic enthalpy drop across the expander and the volumetric flow rate at the expander outlet. Stress limits the rotor tip speed to a certain extent, but more often the speed is determined by the enthalpy drop. The outlet volumetric flow rate likewise controls the expander wheel flow area. These parameters within mechanical limits determine the basic configuration of the hydraulic channel, which in turn directly affects the turboexpander efficiency. 

The turboexpander in combination with a compressor and a heat exchanger functions as a heat pump and is analyzed as follows In Fig. 29-44 consider the compressor and aftercooler as an isothermal compressor operating at To with an efficiency and assume the working fluid to be a perfect gas. Further, consider the removal of a quantity of heat by the tumoexpander at an average low temperature Ti-This requires that it dehver shaft work equal to Q. Now, make the reasonable assumption that one-tenth of the temperature drop in the expander is used for the temperature difference in the heat exchanger. If the expander efficiency is and this efficiency is mul-... 

The radial reaction design has been selected for turboexpanders primarily because it attains the highest efficiency of all turbine designs. However, it has several addition features which favor this apphcation ... 

Efficiency for a turboexpander is calculated on the basis of isentropic rather than polytropic expansion even though its efficiency is not 100 percent. This is done because the losses are largely introduced at the discharge of the machine in the form of seal leakages and disk friction which heats the gas leaking past the seals and in exducer losses. (The exducer acts to convert the axial-velocity energy from the rotor to pressure energy.)... 

If hquid droplets form as ihe gas is expanded in the turboexpander, one s first thought may be that a radial inflow design is the last diing to use, but the following explanation will show that this is the only design that can accomphsh expansion efficiently. 

Size, rotating speed, and efficiency correlate well with the available isentropic head, the volumetric flow at discharge, and the expansion ratio across the turboexpander. The head and the volumetric flow and rotating speed are correlated by the specific speed. Figure 29-49 shows the efficiency at various specific speeds for various sizes of rotor. This figure presumes the expansion ratio to be less than 4 1. Above 4 1, certain supersonic losses come into the picture and there is an additional correction on efficiency, as shown in Fig. 29-50. 

For many years, turboexpanders have been used in cryogenic processing plants to provide low-temperature refrigeration. Power recovery has been of secondary importance. Expander efficiency determines the amount of refrigeration produced and, in gas process plants, the amount of product usually depends on the available refrigeration. Accordingly, there is a large premium on efficiency and, of course, on reliability. 

Dust-laden streams can also cause operational problems. A turboexpander that can efficiently process condensing streams (gas with fog droplets suspended) can usually handle a stream with suspended solid particles, as long as the particle size does not exceed 2-3 p. The newer designs reduce erosion of expander back rotor seals by disposing of... 

Numerous applications where the recovery of power is important are being explored and exploited to an increasing degree. These are classified as turboexpander applications because of the importance of reliability and high efficiency. Turboexpanders meet these requirements and are available in the needed capacity ranges. A 5,000 hp (3,727 kW) compressor-loaded turboexpander is shown in Figure 2-10. 

The efficiency of the compression effect is high and its quantity large, favored by high pressure. However, it would not have the opportunity to act to this degree in the absence of the turboexpander. 

In summary, starting with 105°F gas at atmospheric pressure, the theoretical work necessary to liquify one pound of methane is 510.8 Btu or 352 hp/MMcfd. The simplified liquefaction process, as illustrated, uses a turboexpander/compressor and a small propane refrigeration unit. The 41.25% efficiency breaks down as follows one-fourth contributed by the turboexpander/compressor at 35.8% efficiency one-sixteenth contributed by the mechanical propane refrigeration unit at 43% efficiency, at a moderate temperature where its efficiency is high and a large fraction—eleven-sixteenths—contributed at 58.2% efficiency by compression and Joule-Thomson condensation energy. 

Most ethylene plants operate continuously with the expanders operating at or near design conditions. If necessary, due to their unique design characteristics, radial inflow turboexpanders can accommodate a wide range of process conditions without significant losses in thermal or mechanical efficiency. Expanders may be loaded with booster compressors, gear-coupled generators, dynamometers, or other in-plant mechanical equipment such as pumps. In ethylene plants, turboexpanders are typically used in eitlier post-boost or pre-boost applications. 

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. 

The turboexpander dry gas seal consists of the conventional dry gas seal mating ring and primary ring, an outboard labyrinth, an inboard labyrinth, and tlie cavity to be vented, if desired. Tlie outboard labyrinth reduces warm seal gas leakage to the process side efficiency deterioration is thus minimized. The inboard labyrinth, on one hand, provides an additional seal between the process and lubricating fluids. On the other hand, it allows injection of an inert gas, if desired. In the latter case, inert gas leaks to the bearing side and to the cavity between the... 

Turboexpander sensitivity to process gas inlet pressure. Eigure 7-12a shows tliat TTE is more sensitive to a reduction than to an increase in the inlet gas pressure. Eor example, a 20% reduction of inlet gas pressure results in a 4% reduction of design TTE, while a 20% increase in inlet gas pressure produces only a 0.25% reduction in tliermal efficiency. 

To extract die EPBC liquids, die plant employs a cryogenic temperature process in combination with a refluxed demedianizer process in two trains. The two turboexpander trains are die heart of die process and bodi have operated at efficiencies higher dian originally expected at die design stage. 

The turboexpander lowers the temperature of the product to -100°F, causing it to liquify. Now at 350 psig pressure, the liquid from this process enters the demethanizer tower where it mingles with the previously introduced stream of liquid. The turboexpanders provide a 92% recovery rate while the former system, a backup Joule-Thomson valve, was able to provide only a 60% recovery rate. The volume of gas entering the turboexpanders can vary up to 10% yet, the different flowrates do not significantly affect the efficiency of these units, which are rated at 2,400 hp at 16,000 rpm. 

To meet sales specifications, gas produced at the wellheads must be free of water and hydrocarbon liquids. Twin turboexpanders are a key component in this process, providing dewpoint control with optimal efficiency. Initial processing takes place at the wellhead platforms, where methanol is injected to inhibit hydrate formation. A corrosion inhibitor is also added to prevent gas from damaging downstream equipment. 

Radial-inflow turboexpanders can be designed to handle relatively large amounts of condensation with vei7 little loss of efficiency. Axial turboexpanders can also tolerate some condensation, but usually at a loss of efficiency. 

If the energy of compression is not recovered in the heat pumps, the liquefaction efficiency will be low (35-60%). If the letdown valves are replaced by turboexpanders (ETG-2 and ETG-3), which will recover some of the compression energy during pressure letdown, and if helium or neon refrigerants are used, the liquefaction efficiency can theoretically reach 80% (see Table 1.46). 

The turboexpander is also a hydraulic turbine used for flashing liquids and liquids releasing dissolved gases as discussed by Swearingen [42]. Capacities range from 50 to 1,000 hp (39.3 to 746 kW), suction pressures from 1,000 to 1,500 psia (69 to 103 bar) and discharge pressures from 50 to 200 psia (3.45 to 13.79 bar). In an illustrative example, Swearingen cites an isentropic efficiency... 

Turboexpanders can be classified as either axial or radial. Axial flow expanders nave either impulse or reaction type blades and are suitable for multistage expanders because they permit a much easier flow path from one stage to the next. However, radial turboexpanders nave lower stresses at a given tip speed, which permits them to run at higher speeds. This results in higher efficiencies with correspondingly lower energy requirements. As a consequence, most turboexpanders built today are of the radial type. 

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