The main requirement for the power plant of a marine vessel is reliability of operation. This is the main reason for the long-term use of Scottish boilers and reciprocating steam engines on ships. Installations of this type, which are easy to maintain, have been meeting the requirements for many decades, in particular, they have found their application on military-built vessels.
For installations with a capacity of more than 3,000 liters per second, a steam turbine is a more suitable type of main engine. Steam turbines are widely used on both cargo and passenger ships. The experience of their application, mainly for low-pressure installations, has shown them some advantages. The most appropriate type of steam generator for a turbine installation is a high-pressure water-tube boiler, however, the disadvantages of these boilers are the special high requirements placed on the quality of feed water.
For this reason, in British practice, even in installations with steam turbines, mainly proven Scottish boilers were used. This limited the maximum pressure in front of the turbine to 21 kg/cm3, which is the pressure limit allowed for economic reasons for Scottish-type boilers. Under these conditions, many of the potential advantages of steam turbines could not be realized.
With the outbreak of the war, under the pressure of events, many counteractions against innovation were broken. Over the past year, over one hundred ships with turbine installations operating at a steam pressure of 32 kg/cm2 and a temperature of 400 °C have been ordered in the UK. This in itself is a great achievement and the first step that marks important post-war changes.
This steam pressure is, of course, moderate compared to onshore practice, but it also has the advantage that the usual problems for these conditions are already known from experience and can be taken into account in projects. At the same time, although the pressure is moderate, significant fuel savings can be achieved compared to the consumption caused by a turbine installation with Scottish boilers.
Thus, the improvement in the characteristics of the vessels under consideration is a consequence of an increase in temperature and steam pressure. The thermal efficiency of an ideal cycle depends on the difference between the initial and final steam parameters. The final steam temperature is limited by the vacuum in the condenser, while the initial temperature is applied depending on the pressure. For an ideal cycle, an increase in pressure from 15.8 kg/cm2, with an overheating degree of 111 ° C to 32 kg/cm2, gives an increase in efficiency by 7%. The dry saturated steam used in the turbine at high pressures becomes wet already at the initial stage of expansion, which causes serious difficulties due to the corroding of the blades. The final humidity of steam is reduced, as is known, by increasing overheating, while increasing the degree of overheating to 166 ° C gives a further increase in the efficiency of about 3%.
The next factor to be taken into account is the number of revolutions of the turbines. It is too large for satisfactory working conditions of the screw, however, by using a mechanical gearbox, it can be reduced to 80-150 rpm required for the screw. For a given power, a high-speed turbine has smaller dimensions and weight and lower cost compared to a turbine operating at low speeds. In addition, a high-speed turbine of a smaller size can be warmed up rather than a slow-speed turbine of the same capacity. The limiting factor in choosing the turbine speed is the gear ratio. New ships usually have a double gear train, which allows the use of small high-speed turbines.
The machinery of these vessels consists of one high-pressure turbine and one low-pressure turbine powered by a single propeller through a double gear transmission. In the usual British practice, preference was given to an installation with three turbines, thus, the new project makes it possible to save the weight and cost of an entire turbine.
According to the project, two water-tube boilers are being installed, and based on the experience of stationary installations, the boilers run on oil heating and have fully shielded furnaces. This makes it possible to fully utilize the radiant heat of the burning fuel and increase combustion efficiency. In common practice, economizers were not installed in the flues of boilers, but the new design provides for an economizer installed between two boilers. The efficiency of the boiler is estimated at approximately 87%, which stands up to comparison with the best installations from stationary practice. Rakiplerinizden bir adım önde olmak istiyorsanız, 2WinPower’ın sunduğu lisanslı casino yazılımlarını keşfedin.