Basic Turbine Arrangements

INTRODUCTION
Steam turbines are classed as external combustion engines since their primary source of heat energy, which may be coal, crude oil or distilate fuel, is burned not within the turbine casing proper, but within the furnace of a boiler. Through absorption of this heat energy by water within the boiler, steam is generated under pressure and this becomes the secondary source of heat energy. By means of approriate piping and control valves the steam is conducted to the turbine wherein it impacts upon the rotor blades to produce rotation, thereby expending its heat energy in the process.

(Whereas Diesel engines are classed as internal combustion engines since their primary source of heat energy, namely, various grades of distilate fuel, is burned within the combustion chamber of each cylinder, in the pressence of highly compressed air. This produces a high pressure gas which expands against the pistons, pushing them downward within the cylinders. Each piston is connected to a common crankshaft by means of an articulated linkage known as a connecting rod. Vertical movement of the piston within the cylinder is transmitted through the connecting rod to the crankshaft, thereby causing the latter to rotate.)

SINGLE ROTOR TURBINE
Or full expansion turbine, in which the steam expansion is completed through the several stages of the single rotor. This type of prime mover is nornally used for turbo-electric propulsion and turbo-generator (auxiliary) applications. In the case of single rotor main "propulsion turbines" (as opposed to "turbo-generator turbines"), this unit usually contains both the ahead and astern elements.

CROSS COMPOUND TURBINE
A cross compound turbine is normally a two-element plant consisting of a high pressure rotor and a low pressure rotor, each directly coupled to its respective drive pinion engaged with the main reduction gear, either directly, as in the case of single reduction, or through an intermediate gear, as in the case of double reduction gearing. In a cross compound turbine, heat energy of the steam is only partially expended within the stages of the high pressure rotor from where it exhausts into the low pressure rotor, to complete its final expansion.

HIGH PRESSURE TURBINE
This is the high pressure element of a cross compound turbine.

LOW PRESSURE TURBINE
This is the low pressure element of a cross compound turbine. In most cross compound plants the low pressure casing also incorporates the astern turbine element.

ASTERN TURBINE
This is the element used for driving the ship in the astern direction. In the case of single rotor turbines it is incorporated within the same rotor as the ahead turbine. In cross compound units it is usually housed within the low pressure turbine casing and in a few plants it may be enclosed within a separate housing.

AUXILIARY MACHINERY AND SYSTEMS
Regardless of the type of propulsion plant installed, every ship requires a number of auxiliaries for purposes of vessel operation, life support and safety. The function of Ship Service generators, for instance, is applicable in varying degrees, to all three purposes since so many of their respective sub-systems are either electrically powered or controlled. Operational support equipment includes steering gear, coolant circulating pumps, fluid transfer pumps, fuel pumps, heat exchangers and deck machinery. Potable water systems, sanitary systems, space illumination, food refrigeration and preparation, space heating and ventilation) are all essential life support systems. Typical safety systems include fire fighting equipment, navigation lights and detection gear, anchoring and mooring capability, de-watering machinery such as ballast and bilge pumps, whistle, bells, alarms, communications and life saving equipment.

Many of these auxiliary equipment systems are either electric, electronic, pneumatic, hydraulic, mechanical or a combination thereof, while some may be steam powered and others powered by internal combustion engines. The majority of them are spread throughout the ship, being connected by steam pipes, oil pipes, air ducts) electric cables, rods, levers, valves and switches. Operation and maintenance of most machinery below decks falls under the jurisdiction of the Chief Engineer who is also responsible for maintenance of many life support and safety systems. The end users however, may include deck personnel, cooks, medical department, engineers, radio operators, navigation officers and all crew members, plus passengers.

HORSEPOWER EQUIVALENCE, APPLICATIONS AND CALCULATIONS
One horsepower is equal to the effort required to raise a mass of 33,000 pounds through a height of one foot within the time of one minute. Therefore, the work equivalence of one horsepower may be expressed as follows:

a. 33,000 foot pounds per minute.

b. 550 foot pounds per second.

c. 1,980,000 foot pounds per hour.

There are several other terms to express the energy equivalence of ONE HORSEPOWER HOUR (1.0 H.P. Hour) of which the following are but a few examples:

a. 42.418 British thermal units per minute.

b. 0.7457 Kilowatts.

c. 4,562.42 Kilogram/meters per minute.

d. 10.688 Kilogram/calories per minute.

POWER TO DRIVE PUMPS: H.P. = Gallons per minute x total head (including friction) divided by 3.96 x efficiency of pump

POWER TO DRIVE FANS: H.P. = Cubic feet of air per minute x air pressure, in H20" divided by 6.350 x efficiency of fan

POWER TO DRIVE WINCHES: H.P. = Line pull (in pounds) x Line speed (in feet/min.) divide by 33,000 x efficiency of winch 33,000 x efficiency of winch

ENGINE HORSEPOWER CALCULATION
There are several means of determining the horsepower of a Diesel engine, these include:

a. Power test run coupled to a dynamometer, to obtain a torque reading in lbs.-ft., which is then multiplied by r.p.m. and divided by 5250.

b. Power test run coupled to a generator, to obtain a reading in kilovolt amperes, which is then multiplied by the power factor and divided by 1.34.

c. Power test run using a pressure volume indicator (under normal load) to obtain a reading of indicated mean effective pressure. This reading is calculated from an indicator card pressure-volume diagram, produced by the pressure volume indicator instrument, as it is attached to the test cock of each cylinder in turn. This form of measurement can be taken on a weekly basis while the ship is underway. The results serve as an indication of how well the individual cylinders of an engine are balanced.

The indicated mean effective pressure reading thus obtained may then be factored into a formula for determining the indicated horsepower of the total engine. Indicated horsepower is the total power actually developed by the engine. Brake horsepower is the balance of power available at the flywheel after subtracting friction horsepower, that is, the power consumed by the engine in driving itself. The formula is as follows:

I.H.P. = P x L x A x N x nc divided by 33,000

where: P = Indicated mean effective pressure, pounds per square inch.

L = Length of piston stroke, in feet.

A = Area of piston head in square inches.

N = Number of power strokes per minute.

nc + Number of cylinders.