tailieunhanh - Marine Machinery 7 E Part 10

Tham khảo tài liệu 'marine machinery 7 e part 10', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 350 Refrigeration The use of fresh water from a central cooling system closed circuit avoids the problem of sea-wafer corrosion. Evaporators in the direct expansion system shown in Figure evaporation takes place in air coolers consisting of pipe grids plain or finned enclosed in a closely fitting casing through which air from the holds or chambers is circulated by forced or induced draught fans. This type of evaporator can be operated either partly flooded fully flooded with incorporated accumulators or dry. In the latter the refrigerant flow is controlled at the expansion valve in such a way that as it passes through the grids it is completely vaporized and slightly superheated. Where brine is used as the secondary refrigerant evaporators may be of the shell and tube type. In a shell and tube evaporator the area of tube surface in contact with the liquid refrigerant determines its performance. The brine to be cooled may be circulated through the tubes with the refrigerant being on the outside of them. This involves either a high liquid level in the shell or the placing of the tubes in the lower part of the shell only the upper part then forming a vapour chamber. Modern flooded evaporators incorporate finned tubes. Shell and tube evaporators Figure may also be of the dry expansion type in which the refrigerant passes through the tubes and the brine is GAS outlet liquid outlet REFRIGERANT FEED b Figure a Flooded-type evaporator b Dry expansion-type evaporator Refrigeration 351 circulated through the shell. The advantages of this type are a smaller refrigerant charge and a more positive return of lubricating oil to the Compressor- Expansion valves The expansion valve is the regulator through which the refrigerant passes from the high pressure side of the system to the low pressure side. The pressure drop causes the evaporating temperature of the refrigerant to fall below that of the evaporator. Thus for example the refrigerant can be boiled off by an