The figure shows a schematic diagram of a simple steam plant. High-pressure superheated steam leaves the boiler, which is an element of the steam generator, and enters the turbine. The steam expands in the turbine and thereby performs work, which causes the turbine to move an electric generator.
Low pressure steam leaves the turbine and enters the condenser, where heat is transmitted from the steam (causing it to condense) to the cooling water.
Because very large amounts of water are required, power plants are located near rivers or lakes. When available water is limited, a cooling tower may be used. In the cooling towers, part of the water evaporates, so that the temperature of the remaining water drops. The pressure of the condensate, when leaving the condenser, is increased by means of a pump that makes it flow inside the steam generator.
In many steam generators an economizer is used. The economizer is simply a heat exchanger in which the heat is transmitted from the products of combustion to the condensate, increasing its temperature, but without evaporation. In other sections of the steam generator, the heat from the combustion products is transmitted to the water, causing its evaporation. The temperature at which evaporation occurs is called the saturation temperature. The steam then flows through another heat exchanger called a superheater, where the steam temperature rises well above the saturation temperature.
- Plastics industry
o Plastic production
Control devices for the transport and distribution of material for fluid, valve actuation and silo closure.
o Manufacture of plastic parts
Adjusting the calender rollers, knife drive, deep-drawing closing devices, welding and pressing devices, tape advance control, forming devices, gluing, actuation of safety devices such as windows and doors on machines and facilities, molding machines, cutting devices to measure.
o Manufacture of rubber parts
Safety devices, command and work actuation for chained transport and production devices, closing devices in mixers and vulcanization facilities, control devices.
Conservation of mass and control volume
A control volume is a volume in space in which we are interested, for a particular study, or for analysis. It is called the control surface that surrounds the control volume and is always a closed surface.
The size and shape of the control volume are completely arbitrary and delimited as best suited for the analysis to be done.
The surface can be fixed or it can move or expand. However, the surface must still be defined in relation to the coordinate system. In some analyzes it may be convenient to consider the coordinate system rotating or moving and describe the control surface relative to the system.
The mass, as well as heat and work, can cross the control surface, and the mass in the control volume, as well as the properties, of this mass, can also change with respect to time.
The figure shows a schematic diagram of a control volume, with heat transmission, arrow work, mass accumulation within the control volume, and moving limit.
Let's first consider how the law of conservation of mass and control volume are related, then consider the mass that flows in and out of it. Control volume and the net increase in mass within that volume.
During a time interval δ t, let the mass δm i enter the control volume and the mass δ m e leave the control volume. Now, let us call the mass within the control volume m t at the beginning of the time interval, and m t + δ t the mass at the end of the interval. So, by the principle of conservation of mass, we can write:
A simple power plant is an example of a thermal machine in this restricted sense. Each component in this plant can be analyzed using steady state and steady flow processes, but considering them as they can be treated as thermal machines where water (steam) is the working substance. A quantity of QH heat that is transmitted from the high temperature body either from the products of combustion in the home or from a reactor or from a secondary fluid, which has been heated in the reactor. In the figure we see schematically that the turbine drives the pump, and tells us what the net work is during the cycle. The quantity of heat QL is transferred to a low temperature body, which is generally the cooling water of the condenser; in this way,The simple steam power plant is a thermal machine in the strict sense of the word, because it has a working substance to which and from which heat is transmitted and which performs a certain amount of work when subjected to a cycle.
Another example of a thermal machine is the thermoelectric power generation system, where heat is transmitted from a high temperature body to the hot junction (QH) and heat is transmitted from the cold junction to the surrounding medium (QL). The work is carried out in the form of electrical energy; Because there is no working substance, we do not generally think that this system operates under a cycle; however, if we adopt a microscopic point of view, we could think of the flow of electrons as such.
Furthermore, as in the case of the steam power plant, the states at each point in the thermoelectric power generator do not change over time under steady-state conditions.
Vapor Compression Refrigeration Cycles
The ideal vapor compression refrigeration cycle is seen in the figure below, in the 1-2-3-4-1 cycle. Low pressure saturated steam enters the compressor and undergoes an adiabatic, reversible compression, 1-2. The heat is transferred to constant pressure in process 2-3, and the working substance leaves the condenser as a saturated liquid. An adiabatic throttling process follows for 3-4, then the working substance evaporates at constant pressure for 4-1, completing the cycle.
The similarity between this cycle and the Rankine cycle is evident, since it is the same cycle, but reversed, except that an expansion valve replaces the pump. This throttling process is irreversible, while the Rankine cycle pumping process is reversible. The divergence of this ideal cycle, with the Carnot 1′-2′-3-4′-1 ′ cycle, is noticeable in the T - s diagram. The reason for the divergence is that it is much more convenient to have a compressor that operates only steam and not a mixture of liquid and steam, as would be necessary during the 1′-2 ′ process of the Carnot cycle.
It is virtually impossible to compress (in a reasonable relationship) a mixture such as that represented by the l 'state, and to maintain the balance between the liquid and the vapor, because there must be a heat and a mass transferred across the limits of phase. It is much easier for the expansion process to take place irreversibly in an expansion valve, than it does in an expansion device, which receives saturated liquid, and discharges a mixture of liquid and vapor, as would be required in process 3- 4'.
For these reasons, the ideal refrigeration cycle by vapor compression is indicated in the previous figure as the 1-2-3-4-1 cycle.
Divergence between actual vapor compression refrigeration cycle and ideal cycle
The actual refrigeration cycle diverges from the ideal cycle, primarily due to the pressure drop associated with fluid flow and heat transmission to, or from, the surrounding medium. The actual cycle can be represented approximately as indicated in the following figure.
The steam entering the compressor will probably be overheated. During the compression process there are irreversibilities and heat transmission to or from the surrounding medium, depending on the temperature of the refrigerant and the external medium. Therefore, entropy could increase or decrease during this process; irreversibility and heat transmission to the refrigerant cause an increase in entropy and the heat transmitted from the refrigerant causes a decrease in entropy. These two possibilities are represented by the two dotted lines 1-2 and 1-2 ′. The pressure of the liquid leaving the condenser will be less than the pressure of the steam entering and the temperature of the refrigerant in the condenser will be somewhat higher than that of the external medium, to which the heat is then transmitted.Generally, the temperature of the liquid leaving the condenser is lower than the saturation temperature and drops somewhat more in the pipe between the condenser and the expansion valve; This represents, however, a benefit, since as a result of this heat transmission, the refrigerant enters the evaporator with a low enthalpy and this allows for greater heat transmission to the refrigerant in the evaporator.
Frequently it is necessary to have a source of dry air, to keep under pressure the telephone cables or other similar installations. The figure shows in diagram a device to provide dry air. The air is compressed to 11.6 kg f / cm2, cooled to 21.l ° C in a cooler and a counter-flow heat exchanger. Finally it is cooled to 1.67 ° C by heat transmission to the refrigerant in the evaporator of the refrigeration cycle. The condensed water in these processes is separated from the air and leaves through an automatic ejector. The remaining air-water vapor mixture is used as a cooling medium in the heat exchanger and its pressure reduced to 1.76 kgf / cm2, to be used in the programmed application.
Industrial applications of the enthalpy of combustion
The air separation plant
A process of great industrial significance is the air separation plant in which it is separated into its components. Oxygen, nitrogen, argon and rare gases are widely used in various industrial, research, special testing and consumer goods applications. The air separation plant can be considered as an example of two large fields: the chemical process industry and the cryogenic field.
Basic cooling in the liquefaction process is provided by different procedures. One is the expansion of air in a machine. During this process, the air produces work and, as a result, the temperature drops. The other procedure is to pass the air through a throttle valve, designed and located so that there is a substantial decrease in its temperature.
High pressure dry air enters a heat exchanger. The temperature drops its way through the changer. At an intermediate point in the changer, some of the air is removed to flow through the expansion machine. The remaining air passes through the rest of the heat exchanger through the throttle valve. The two streams meet again at a pressure of 5 to 10 atmospheres and enter a distillation column called the high pressure column. The function of the distillation column is to separate the air into its various components, mainly oxygen and nitrogen. Two streams of different compositions flow from the high pressure column to the upper column, past the throttle valves. One of them is abundant liquid of oxygen that comes out of the bottom of the lower column, and the other current,abundant in nitrogen, flows through the subcooler. The separation is completed in the upper column.
The Rocket Machine
The advent of shells and satellites has brought the rocket machine to prominence as a power plant. Rocket machines can be classified as either liquid propellant or solid propellant, depending on the fuel used. This has been used with great success in initial boosting of jet-assisted aircraft, military projectiles, and space vehicles. These rockets are simpler, both in the basic equipment required for their operation, and in the problems logically involved in their use for military service.
Some power plants such as the simple steam plant that we have considered many times, operate in a cycle; that is, the working substance undergoes a series of processes and finally returns to its initial state. In other power plants such as internal combustion machines and gas turbines, the working substances are not cycled.
Simple Vapor Compression Refrigeration Cycle
The refrigerant enters the compressor as a slightly superheated low pressure vapor. It exits the compressor and enters the condenser as steam at slightly elevated pressure; There it condenses as a result of heat transmission to the cooling water or to the outside environment. The refrigerant then leaves the condenser as a high pressure liquid. The pressure of the liquid decreases as it flows through the expansion valve, and as a result, some of the liquid immediately turns into steam. The remaining liquid, now at low pressure, evaporates in the evaporator as a result of heat transmission from the refrigerated space. This steam then enters the compressor.
The air separation plant
A process of great industrial significance is the air separation plant in which it is separated into its components. Oxygen, nitrogen, argon and rare gases are widely used in various industrial, research, special testing and consumer goods applications. The air separation plant can be considered as an example of two large fields: the chemical process industry and the cryogenic field.
Basic cooling in the liquefaction process is provided by different procedures. One is the expansion of air in a machine. During this process, the air produces work and, as a result, the temperature drops. The other procedure is to pass the air through a throttle valve, designed and located so that there is a substantial decrease in its temperature.
High pressure dry air enters a heat exchanger. The temperature drops its way through the changer. At an intermediate point in the changer, some of the air is removed to flow through the expansion machine. The remaining air passes through the rest of the heat exchanger through the throttle valve. The two streams meet again at a pressure of 5 to 10 atmospheres and enter a distillation column called the high pressure column. The function of the distillation column is to separate the air into its various components, mainly oxygen and nitrogen. Two streams of different compositions flow from the high pressure column to the upper column, past the throttle valves. One of them is abundant liquid of oxygen that comes out of the bottom of the lower column, and the other current,abundant in nitrogen, flows through the subcooler. The separation is completed in the upper column.
Bibliography
- Garzón G. Guillermo, " Fundamentals of General Chemistry ", Second Edition, Editorial: Mc Graw Hill, Mexico City, 1986, Pag: 244 - 245 GORDON J. VAN WYLEN AND RICHARD E. SONNTAG " Fundamentals of THERMODYNAMICS ", First Edition, Editorial: Limusa, SA México, 1967. Pages: 39-41, 125-126, 200-201.MARON AND PRUTTON, “ Fundamentos de FISICOQUÍMICA ”, Editorial: Noriega - Limusa, México, DF, 1990 Pages: 237-238,239-243,245.252-253.Whittaker Roland M, “ General Chemistry ” Editorial: CECSA, México, DF, 1984, Page: 150 - 151
Author Ing. Iván Escalona
Logistics Consultant, Mobile Phone: 044 55 18 25 40 61 (Mexico)
Industrial Engineer
[email protected], [email protected]
Note: If you want to add a comment or if you have any doubts or complaints about any published work (s), you can write me to the emails indicated, indicating which work was the one you reviewed by writing the title of the work (s), also where you are from since you are dedicated (if you study or work) Being specific, also age, if you do not indicate them in the mail, I will delete the mail and I will not be able to help you, thank you.
- University Studies: Interdisciplinary Professional Unit of Engineering and Social and Administrative Sciences (UPIICSA) of the National Polytechnic Institute (IPN)
- Patoyac School Center, (Incorporated at UNAM)
Origin: Mexico
Download the original file
