Figure 4.14 Sulzer oil-free compressor
In the Sulzer oil-free compressor shown in Figure 4.14, sealing between the piston and cylinder wall, and between the piston rod and gland, is achieved by the use of machined labyrinths. Consequently, no lubrication is needed for those spaces in the compressor swept by cargo vapours. The absence of any contact at the seals limits wear and lubricating oil consumption is minimal. The oil-free side of the compressor and the lubricated crank are separated by oil scraper rings mounted on the piston rod. The rod also carries a ring which prevents any residual oil film from creeping up the rod. The distance between the crank and gland is such that the oily part of the piston rod cannot enter the oil-free gland. Should any gas leak through the gland, it is returned to the suction side. The crankcase and separation space are kept under suction pressure. Where the crankshaft leaves the case, it is fitted with a shaft seal operating in oil.
Although the Sulzer compressor is oil-free in the compression chamber, it is common practice to change the lubricating oil with each change of cargo. This is to cover the question of compatibility of the lubricating oil grade with the next cargo (see 7.6.1).
Capacity control of the compressor is achieved by lifting suction valves during the compression stroke. The plate lifters are normally hydraulically operated with the fluid pressure being provided by the lubricating oil pump. When the compressor is shut down, the cargo vapour in the crankcase can condense, giving rise to lubricating problems. To avoid this, provision must be made for crankcase heating when the compressor is idle. When the compressor is running, cooling must be provided for the crankcase, for the crossheads and for the guide bearings. Normally, a closed cycle glycol water system provides for the heating — when the compressor is shut down — and for cooling, when the compressor is running.
Figure 4.15 Linde oil-free compressor
Another common type of reciprocating oil-free compressor is shown in Figure 4.15. This machine is manufactured by Linde. Such a compressor has PTFE piston rings instead of the labyrinth piston in the Sulzer machines. Volumetric efficiencies tend to be higher with the PTFE ring design.
4.6.2 Screw compressors
Screw compressors for use with liquefied gas cargoes can be either dry oil-free or oil-flooded machines. In the dry machines, the screw rotors do not make physical contact but are held in-mesh and driven by external gearing. Due to leakage through the clearances between the rotors, high speeds are necessary to maintain good efficiency (typically 12,000 rpm). Figure 4.16 is a diagram of a typical rotor set with the common combination of four and six lobes. The lobes inter-mesh and gas is compressed in the chambers numbered 1,2,3, in the diagram which are reduced in size as the rotors turn. The compressor casing carries the suction and discharge ports.
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The oil-flooded machine relies on oil injection into the rotors and this eliminates the need for timing gears. Drive power is transmitted from one rotor to the other by the injected oil. This also acts as a lubricant and coolant. Because the rotors are sealed with oil, gas leakage is much less and, therefore, oil-flooded machines can run at lower speeds (3,000 rpm). An oil separator on the discharge side of the machine removes oil from the compressed gas.
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