Figure 4.10(a) Examples of indirect cooling cycles
Reference to Table 2.5 shows that with respect to propylene oxide it is unlikely, but dependent on ambient conditions, that refrigeration will be required on voyage.
Two indirect cycle systems are shown diagramatically in Figure 4.10(a).
4.5.2 Direct cycles
Direct cycle is descriptive of a system where the boil-off is compressed, condensed and returned to the tank. This is the most common system, but may not be employed for certain gases (see IGC Code, Chapter 17 and 4.5.1 above).
There are three main types of direct cycle reliquefaction plant and these are described in the following sections.
Single-stage direct cycle
The single-stage direct cycle system is particularly suited to the semi-pressurised carrier.
A simplified diagram of single-compression reliquefaction is shown in Figures 4.11 (a) and (b). This cycle is suitable where suction pressures are relatively high, as in the carriage of semi-pressurised products. Boil-off vapours from the cargo tank are drawn off by the compressor — (a) in the diagrams. Compression increases the pressure and temperature of the vapour — to (b) in the diagrams. The high temperature allows it to be condensed against sea water in the condenser — at (c) in the diagrams. The condensed liquid is then flashed back to the tank via a float controlled expansion valve — at (d) in the diagrams. The liquid/vapour mixture being returned the cargo tank may be either distributed by a spray rail at the top of the cargo tank or taken to the bottom of the tank to discourage re-vaporisation. The spray rail is normally used when the tank is empty and bottom discharge when the tank is full (see also 2.19 and Figure 2.16).
Figure 4.11 (a) Single-stage direct reliquefaction cycle
Figure 4.11 (b) Mollier diagram — single-stage direct reliquefaction cycle
Two-stage direct cycle
Although two-stage direct cycle equipment is relatively uncommon, it is used for those liquefied gas carriers handling a wide range of products. For grades such as butadiene and vinyl chloride its fitting is essential.
A simplified diagram showing two-stage reliquefaction is given in Figures 4.12(a) and (b). The two-stage cycle with inter-stage cooling is used where suction pressures are low and, as a result, compression ratios high (assuming sea water condensing) compared to the single-stage cycle. Two-stage compression (with inter-stage cooling) is necessary to limit the compressor discharge temperature which increases significantly with the higher compression ratio. This is particularly important for cargoes such as butadiene and vinyl chloride (see also 7.6).
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