Texts for Additional Reading
Transformation of Heat into Work
The heat engines that play such a large part in modern life depend on the transformation of heat into work. The healed steam in the cylinder of a steam engine does work in pushing the piston hack. This work is available for driving the machinery connected to the engine. A gasoline engine can drive an automobile or a tractor only when it is supplied constantly with heat from the exploding gasoline in the cylinders. In these cases, heat is transformed into work.
First Law of Thermodynamics. The first law of thermodynamics is a special case of the law of conservation of energy. It is implied in (lie definition of the mechanical equivalent of heat and may be expressed by the equation
W—JH
where W = the work measured in work units
H = heat measured in heat units
J = the mechanical equivalent of heat
More specifically, the law states that when any mechanical change occurs in an isolated system, the energy of the system remains constant. Ideal may be transformed into work or work into heat, but the total energy of the system remains unchanged. In other words, the first law of thermodynamics states that in the transformation of work into other forms of energy or in the transformation of one form of energy into other forms of energy, no energy is ever created or destroyed.
The energy before and after the transformation is always the same. This law in its general form can not be proved by experiment hut conclusions based on It have always been confirmed by experiment.
Second Law of Thermodynamics
The second law of thermodynamics states the conditions under which heat may be transferred from one body to another. It is in effect a statement of the fact that heat naturally flows from a place of higher to one of lower temperature but never in the reverse direction. An analogue may make the meaning clearer. Water may flow from a higher to a lower level with the performance of work. Meat may flow from a higher to a lower temperature with the performance of work. To cause water to flow from a lower to a higher level requires that external work be done on It. To cause heal to How from a lower to a higher temperature also requires the performance of external work. The natural tendency of heat to flow from a higher to a lower temperature makes it possible for a heal engine to transform heat into work. On the contrary, a mechanical refrigerating machine must transfer heal from a colder to a hotter body. Work must be done on such a machine to make this transfer. The following is one form of statement of the law.
It is impossible for any kind of a machine working in a cycle to transfer heat from a lower to a higher temperature unless external work, is done on itA similar statement of the water analogy would be: It is impossible for a pump working in a cycle to transfer water from a lower to a higher level unless external work is done on it. The law cannot be proved by direct experiment. It is a generalization based on the fact that in all human experience no contradictions of the law have been found. It merely states that heat of itself can flow only from higher to lower temperatures and no exceptions to this rule are known.
Steam Engine.
In a steam engine a closely fitting piston moves in a cylinder that is connected to the steam chest by means of two pipes, which are provided with valves, serve alternately as inlet and exhaust for the steam. As the piston moves forward, steam enters through A and the used steam is forced out through B. When the piston moves in the opposite direction, steam enters the cylinder at B and used steam is forced out at A. With this simple arrangement, the steam would leave the cylinder on exhaust at a temperature nearly as high as that at which it entered it. A considerable quantity of heat would thus be earned to the condenser or the outside air and lost so far as useful work is concerned. In order to prevent this waste as far as possible, an automatic cutoff is provided. When the piston has moved through about one-fourth of its stroke, this slide valve automatically cuts off the supply of steam.
Alter this cutting off of the steam from the steam chest the steam that has already entered the cylinder expands and pushes the piston forward, through the remainder of the stroke. During this expansion, piston is doing work, the pressure of the steam is being reduced, the temperature of the steam Is lowered. The heat contained in steam is thus converted into useful work.
When the piston has reached the end of this stroke, the slide valve opens A and connects B to the steam chest. Live steam is now again admitted to the cylinder behind the piston and 11 pushes the piston toward the left. The dead steam in front of the piston is forced through A. When, as before, the piston has made about one-fourth of its stroke, the slide valve closes B, and the steam behind the piston expands until the piston has reached the cud of its stroke. The cycle is then repeated.
The pressure of the steam in the boiler is regulated by means of a pop valve, which allows the steam to escape when the pressure exceeds a certain value.
CIVIL ENGINEERING
When n structure is planned, it is desirable to know the type of lighting system to be used, the general layout of the fixtures, the location and size of the electrical control room or switchboards, and the approximate sizes and locutions of conduits. The pattern made by the system should have •tome symmetry and uniformity, should be an obvious and harmonious part of the structure, and should fit in with the framing.
When these plans are in the making, it is important to consider maintenance problems the cleaning of luminaries and the replacing of bulbs. The fixtures in offices and multistory factories should be located so that they may be reached by means of stepladders, specially designed movable platforms, or ladders mounted on wheels.
I his is not so easy to accomplish in the case of high, single-story mill buildings in which the lights are attached to or near the roof trusses. Sometimes a workman may reach them by riding the crane bridges, by going along catwalks, or by riding on wheeled towers pushed along the aisles between machines, provided the lights have been located with this in mind. It is foolish to waste power burning dirty lights in dusty fixtures, the combination perhaps being only 60 to 75 per cent efficient. Yet, if cleaning them is dangerous and difficult, they will be poorly maintained. Although overhead lights might be suspended on pulley systems so that they could be lowered within reach of the floor, this is not usually practicable, and the multitudinous ropes and loops of wire arc unattractive. Lights should be held firmly in a stationary position.
INDIRECT SYSTEMS
Indirect systems are chiefly used for the continuous heating of a number of rooms or large buildings from one, Centre source, b the name Central Heating, This does not necessarily imply that the heating source is strictly central, indeed it may be at a distance from the building.
This class of system finds its greatest application in large buildings of all types.
The advantages of the indirect systems are:
( a )Fuel and ashes are kept outside the occupied spaces.
( b )Individual tines arc not requiring.
( c )Cleanliness.
( d )Equable temperature maintained in all parts.
The disadvantages are:
( a ) Heat is lost from piping where this is outside the occupied rooms. This loss can be minimized by proper insulation of the pipes.
( b ) Labour is required for stoking and removal of ashes. This applies only with solid fuel and can be greatly reduced with automatic firing.
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