Expander-compressor

In pre-boost applications, the turboexpander discharge pressure is considerably lower than the compressor inlet pressure. This situation requires special consideration when designing the turboexpander and auxiliary systems. In pre-boost designs, compressing the gas to a higher pressure produces more refrigeration in the turboexpander. Figure 3-9 shows a cross-section of an expander-compressor unit. 

Figure 3-9. Expander-compressor cross-section. (Source Atlas Copco.)...

A typical expander-compressor consists of three basic sections ... 

Off-design performance of an expander-compressor set can be derived from the curves typically provided by tlie equipment manufacturer. Actual... 

Expander-compressor shafts are preferably designed to operate below the first lateral critical speed and torsional resonance. A flame-plated band of aluminum alloy or similarly suitable material is generally applied to the shaft in the area sensed by the vibration probes to preclude erroneous electrical runout readings. This technique has been used on hundreds of expanders, steam turbines, and other turbomachines with complete success. Unless integral with the shaft, expander wheels (disks) are often attached to the shaft on a special tapered profile, with dowel-type keys and keyways. The latter design attempts to avoid the stress concentrations occasionally associated with splines and conventional keyways. It also reduces the cost of manufacture. When used, wheels are sometimes secured to the tapered ends of the shaft by a common center stretch rod which is pre-stressed during assembly. This results in a constant preload on each wheel to ensure proper contact between wheels and shaft at the anticipated extremes of temperature and speed. 

USING EXPANDER-COMPRESSOR CURVES TO PREDICT OFF-DESIGN PEFORMANCE... 

The expander-compressor will attain a speed of 5,000-15,000 rpm, depending on the design horsepower of the machine. If the design pressure of the compressor suction is higher than the design pressure of the expander discharge (pre-boost type), increase the speed to approximately 80% of the design speed as rapidly as possible. This... 

Agahi, R. R. and Ershaghi, B., 1993. Engineering of Turbo-expander-Compressor Units for Off-Shore Installation, Eifth International Congress on Eluid Maehinery, The Hague, Netherlands. 

Figure 7-24. Cryogenic expander-compressor. Courtesy of Mafi-Trench Corp.)...

API Standard 617, Axial and Centrijugal Compressors and Expanders— Compressors for Petroleum, Chemical, and Gas Industry Services,... 

Related Calculations. When specific thermodynamic charts, namely, enthalpy-temperature, entropy-temperature, and enthalpy-entropy, are not available for a particular system, use the generalized enthalpy and entropy charts to perform expander-compressor calculations, as shown in this example. 

Assume ideality for expander, compressor, and heat exchangers. 

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