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).

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.

⇐ Предыдущая28293031323334353637Следующая ⇒

Поделиться: