Refrigeration – Air compressor – cooler and expander type – Motor-type expander
Patent
1993-11-17
1995-03-07
Capossela, Ronald C.
Refrigeration
Air compressor, cooler and expander type
Motor-type expander
62510, F25D 900
Patent
active
053947096
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
Conventional gas compressors and expanders are often classified in two groups corresponding to the principle of pressure change, i.e. static or dynamic action machines. They all have in common that the pressure change takes place more or less adiabatically, i.e. with relatively little exchange of heat with the surroundings, since the surface available for heat transfer during the process is much too small to allow any appreciable deviation from this regime. This causes a loss of power compared with a theoretical isothermal process.
Theoretical explanations how such power losses can be reduced by making the process of an essentially adiabatic compressor approach the isotherm by staging and intercooling will be found in almost any elementary text book on thermodynamics, e.g. in the book entitled "Technisches Handbuch Verdichter" third addition, p. 42-43. Usually, however, the problem is to find a practical and economical way of performing such processes.
One common design of a static or positive displacement machine is the reciprocating or rotating piston compressor. These types are normally used in a single stage up to a ratio of discharge to suction pressure of 6-8 and some times even higher, depending on the properties of the gas to be pumped and other working conditions. Consequently the adiabatic loss becomes quite important. Only at very high overall pressure ratios will a machine with two or more stages be used since this is an expensive solution. The power saving at moderate pressure ratio is not sufficient to pay for this more complicated design.
Another popular positive displacement type of compressor/expander is the screw machine. Its operational properties are similar to those of a piston machine, although there is a tendency to use it at even higher pressure ratios in a single stage.
Turbo machines operate on the dynamic principle, converting high flow velocities into pressure, and are used extensively for large flow volumes. Although the pressure ratio per stage is limited, in particular for compressors, intercooling, or heating between stages is rarely done. Due to the particular design conditions of such machines it would be too complicated and expensive to provide for bringing the gas out and back again for each stage. Only in the case of very nigh overall pressure ratios, when some intercooling or heating is unavoidable, is this done by using two or more machines in series, each containing a fair number of stages, and executing the heat exchange in transferring the gas from one unit to the next. The adiabatic power loss becomes at least as large as for the common positive displacement machines.
Gear type machines are extensively employed as pumps and motors in hydraulic power systems. With a nearly incompressible liquid working medium, normally oil, they can operate with very high efficiency at extreme pressure ratios. Sometimes similar machines are used as expanders in pneumatic systems for the operation of small power tools or starting of internal combustion engines. In such cases, with single stage operation and relatively large pressure ratio, the power efficiency becomes very poor.
A somewhat similar design, the "Roots blower", is sometimes used as a compressor for low pressure ratios. The common type uses two lobes, but three or up to four lobes are also found. Since the rotors are not fit to transmit power, they have to be synchronized by a separate set of gears. Two or three pairs of rotors are some times used in series in order to increase the pressure.
It has been proposed to use multistage gear or Roots machines for the expansion or compression in open systems, i.e. systems open to the atmosphere. Some patents pertaining to such applications can be referred to: be used as an internal combustion motor with expansion of the exhaust gas through one or several stages of a gear expander with increasing flow volume. type compressor, where all the stages are equal in design and volume capacity, sucking in air in parallel from the atmosphere at the same press
REFERENCES:
patent: 3848422 (1974-11-01), Schibbye
patent: 4149585 (1979-04-01), Sterlini
patent: 4311021 (1982-01-01), Leo
patent: 4967565 (1990-11-01), Thompson et al.
patent: 5014518 (1991-05-01), Thompson et al.
"Verdichter", Technisches Handbuch, 1966, Bohm et al.
Capossela Ronald C.
Sinvent A/S
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