A lockable drain cock is installed at the lowest end of the crankcase to permit removal of any water accumulation that has resulted from sweating of the crankcase walls. The crankcase of the compressor acts as the oil sump. The main components of a positive-displacement compressor's lubrication system consist of an oil pump, filter, and heat exchanger. The lubrication system must inject sufficient quantities of clean fluid to provide lubrication for the compressor's internal parts, such as pistons and lobes, and to provide a positive seal between moving and stationary parts. Positive-displacement compressors use their lubrication system to provide additional functions. Keith Mobley, in Fluid Power Dynamics, 2000 Positive-Displacement Compressors The following equation may be used for large, slow-speed compressors with compression ratios >2.5 and for gas specific gravity of 0.65. The HP required in a positive displacement compressor is usually estimated from charts provided by the compressor manufacturer.
Heat exchangers are used between stages of compression so that the compressed heated gas is cooled to the original suction temperature before being compressed in the next stage. The compression ratio per stage for positive displacement compressors is limited to 4.0 because a higher ratio causes a higher discharge pressure, which affects the valve life of positive displacement compressors. Positive displacement compressors may be single stage or multistage depending on the compression ratio required. Positive displacement compressors have pressure ranges up to 30,000 psi and range from very low HP to more than 20,000 HP per unit. They are also not very sensitive to the composition of the gas.
Positive displacement compressors have flexibility in pressure range, have higher efficiency, and can deliver compressed gas at wide range of pressures. However, centrifugal compressors have less efficiency than positive displacement compressors. They also operate at high speeds and are of balanced construction. They can handle larger volumes within a small area compared with positive displacement compressors. Centrifugal compressors have lower capital cost and lower maintenance expenses. Centrifugal compressors are more commonly used in gas transmission systems because of their flexibility. Centrifugal compressors, on the other hand, develop the pressure required by the centrifugal force resulting from rotation of the compressor wheel that translates the kinetic energy into pressure energy of the gas. These compressors have a fixed volume and are able to produce high compression ratios. Piston-operated reciprocating compressors fall within the category of positive displacement compressors.
The high-pressure gas is then released through the discharge valve into the pipeline. Positive displacement compressors generate the pressure required by trapping a certain volume of gas within the compressor and increasing the pressure by reduction of volume. Shashi Menon, in Transmission Pipeline Calculations and Simulations Manual, 2015 11 Types of Compressors: Centrifugal and Positive DisplacementĬompressors used in natural gas transportation system are either positive displacement type or centrifugal type.
0 kommentar(er)
