Figure 4.12(a) Two-stage direct reliquefaction cycle with inter-stage cooling



Figure 4.12(b) Mollier diagram — two-stage direct reliquefaction cycle

The vapour from the first stage discharge — (b) in the diagrams — is taken to an inter­stage cooler where its superheat is reduced — (c) in the diagrams. The cooling medium is cargo liquid flashed down to intercooler pressure from the sea water-cooled condenser. The remaining parts of the cycle are similar to the single-stage cycle.


Cascade direct cycle

The cascade cycle is used for fully refrigerated cargoes where a special refrigerant such as R22* (see below) is used to obtain the lower carriage temperatures. Furthermore in these systems, refrigeration plant capacities are not so affected by sea water temperature changes compared with other reliquefaction cycles. For the carriage of ethylene this type of equipment is essential.

Figure 4.13 Simplified cascade reliquefaction cycle

The cascade system uses a refrigerant such as R22 to condense cargo vapours; a simplified diagram for this system is shown in Figure 4.13. The single-stage com­pression of cargo vapour is identical to the single-stage direct cycle, but the cargo condenser is cooled using R22 instead of sea water. The cargo, in condensing, evaporates the liquid R22 and the R22 vapour is then taken through a conventional R22 closed refrigeration cycle, condensing against sea water — hence the term cascade.

*Refrigerant gas - R22

As appropriate the indirect and direct cascade reliquefaction systems discussed in this book are assumed to use the refrigerant monochlorodifluoromethane which is more normally referred to by its refrigerant number R22. This material is an halo-genated chloro fluoro carbon (HCFC).

It is well suited for use in reliquefaction plants, particularly in reciprocating type compressors. This refrigerant is not specifically listed in the Montreal protocol to be phased out but a separate agreement, indicating that its eventual phasing out by the year 2020 is desirable, has been reached by all signatories to the Montreal protocol. Research into suitable replacements is under way with major chemical companies involved.

R22 has a very low toxicity; however, in the presence of a naked flame it breaks down into a toxic gas which has a very strong smell.

As per the Montreal Protocol, R22 will eventually be phased out in the not too distant future.

4.6 CARGO COMPRESSORS AND ASSOCIATED EQUIPMENT

The compressor is the heart of the reliquefaction plant. As far as LPG ships are concerned there are two main types of compressor: these are the reciprocating type and the screw type.


4.6.1 Reciprocating compressors

Older compressors were sometimes not of the oil-free type. This attracted the problems discussed in 2.8 and 7.6.1 because many liquefied gases can adversely affect the quality of the lubricating oil used in the machines. In using these older compressors, very careful control is required. In particular, sump heating systems are often fitted in order to evaporate any dissolved gases. In addition, the changing of lubricating oil between cargoes is usually necessary. Full data on the operation of these compressors should be available from manufacturers' handbooks.

For these reasons, the vast majority of reciprocating cargo compressors now found on board gas carriers are of the so-called oil-free type.


 




gas being compressed

 

same gas, not flowing cooling water oil


6 The piston rod is guided very accurate­ly by a guide bearing and crosshead.

1 The piston's surface is machined with labyrinth grooves, forming a succession of throttling points for gas blow-by.

2 The cylinder is water-cooled or heated and is similarly provided with grooves in the bore.

3 The gland consists of a system of graphite rings forming a labyrinth seal. Gas leakage at this gland is usually returned to the intake side of the compressor.

4 The distance piece gives clear segre­gation between the compression space and crank gear and prevents the part of the piston rod (with a molecular oil film) from entering the gland.

5 The oil wiper prevents oil creeping up the piston rod into the neutral space and thence into the gland.

6 The piston rod is guided very accurate­ly by a guide bearing and crosshead.

7 The guide bearing is lubricated and water-cooled.

8 The crosshead is lubricated and water-cooled.

9 The crankshaft is lubricated.


 


Дата добавления: 2018-02-28; просмотров: 2330; Мы поможем в написании вашей работы!

Поделиться с друзьями:






Мы поможем в написании ваших работ!