Системы теплоснабжения
Основное назначение любой систему теплоснабжения состоит в обеспечении потребителей необходимым количеством теплоты заданных параметров.
В зависимости от размещения источника теплоты по отношению к потребителям системы теплоснабжения разделяются на децентрализованные и централизованные.
В децентрализованных системах источник теплоты и теплоприемники потребителей совмещены в одном агрегате или размещены столь близко, что передача теплоты от источника до теплоприемников может производиться без промежуточного звена – тепловой сети.
В системах централизованного теплоснабжения источник теплоты и теплоприемники потребителей размещены раздельно, часто на значительном расстоянии, поэтому передача теплоты то источника до потребителей производится по сетям.
Системы децентрализованного теплоснабжения разделяются на индивидуальные и местные.
В индивидуальных системах теплоснабжение каждого помещения (участок цеха, комната, квартира) обеспечивается от отдельного источника. К таким системам, в частности, относится печное и поквартирное отопление. В местных системах теплоснабжение каждого здания обеспечивается от отдельного источника теплоты, обычно от местной котельной. К этой системе, в частности, относится так называемое центральное отопление зданий. В зависимости от степени централизации системы централизованного теплоснабжения можно разделить на следующие четыре группы:
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групповые – теплоснабжение группы зданий;
районное – теплоснабжение нескольких групп зданий;
городское – теплоснабжение нескольких районов;
межгородское – теплоснабжение нескольких городов.
Процесс централизованного теплоснабжения состоит из трех последовательных операций: а) подготовки теплоносителя; б) транспорта теплоносителя; в) использования теплоносителя.
Подготовка теплоносителя производится в специальных так называемых теплоподготовительных установках на ТЭЦ, а также в городских, районных, групповых (квартальных) или промышленных котельных.
Транспортируется теплоноситель по тепловым сетям. Используется теплоноситель в теплоприемниках потребителей. Комплекс установок, предназначенных для подготовки, транспорта и использования теплоносителя, составляет систему централизованного теплоснабжения.
Для транспорта теплоты на большие расстояния применяются два теплоносителя: вода и водяной пар. Как правило, для удовлетворения сезонной нагрузки и нагрузки горячего водоснабжения в качестве теплоносителя используется вода, для промышленной технологической нагрузки используется пар.
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Text 2
Putting heat to work
Changing heat into motion. Mechanical energy and heat energy are related. For example, mechanical energy is changed into heat by friction between the moving parts of a machine. Heat energy, in turn, can be changed into mechanical energy by heat engines.
Heat engines can be divided into two groups: (1) e xternal-combustion engines and (2) internal-combustion engines. External-combustion engines use heat produced outside the engine. Such engines include gas and steam turbines and reciprocating steam engines. Internal-combustion engines produce heat inside the engine from burning fuels. Such engines include diesel and gasoline engines, jet aircraft engines, and rocket engines. Heat is a form of energy. Heat and energy cannot be seen, but the work they do can. For example, the burning of fuel in the engines of a jet airplane creates hot gases. These gases expand and provide the power that moves the plane.
Temperature and heat. All things are made up of atoms or molecules, which are always moving. The motion gives every object internal energy. The level of an object's internal energy depends on how rapidly its atoms or molecules move. If they move slowly, the object has a low level of internal energy. If they move violently, it has a high level. Hot objects have higher internal energy levels than do cold objects. The words hot and cold refer to an object's temperature.
Temperature is an indication of an object's internal energy level. A thermometer is used to measure temperature. Thermometers have a numbered scale so that temperature can be expressed in degrees. The two most common scales are the Fahrenheit and the Celsius, or centigrade, scales.
The temperature of an object determines whether that object will take on more internal energy or lose some when it comes into contact with another object. If a hot rock and a cold rock touch each other, some of the internal energy in the hot rock will pass into the cold rock as heat If a thermometer were placed on the hot rock, it would show the rock's temperature falling steadily. A thermometer on the cold rock would show a steadily rising temperature. Eventually, the thermometers on the two rocks would show the same temperature. Then, no further flow of heat would occur.
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Just as water flows only downhill, so heat flows only down a "temperature hill," passing from an object at a higher temperature to an object at a lower one. The greater the difference in temperature between two objects, the faster the heat will flow between them.
It is important to recognize that temperature and heat are not the same thing. Temperature is simply an indication of the level of internal energy that an object has. Heat, on the other hand, is the passage of energy from one object to another.
The three units most commonly used to measure are British thermal units (Btu's), calories, and joules. One Btu is the quantity of heat needed to raise the temperature of 1 pound of water 1° F. One calorie is the quantity of heat needed to raise the temperature of 1 gram -water 1° C The calorie used to measure food energy 1,000 times as large as this calorie. The Btu is generally used in engineering, and the calorie in the sciences ~ joule can be used for measuring all forms of energy~ including heat. One joule is the amount of energy used or work done—when a force of 1 newton moves an object 1 meter in the direction of the force..
1. Study the following words:
external-combustion engines, reciprocating, expand, temperature hill, the burning of fuel, determine, eventually, steadily.
2. Complete the sentences with appropriate words from the text:
a) Heat energy, in turn, can be changed into mechanical energy by …...
b) …..is the quantity of heat needed to raise the temperature of 1 pound of water 1° F..
c) ….. is simply an indication of the level of internal energy that an object has.
d) One ….. is the quantity of heat needed to raise the temperature of 1 gram -water 1° C.
e) ……is the amount of energy used or work done—when a force of 1 Newton moves an object 1 meter in the direction of the force..
3. Define the part of the speech of the following words:
Violently, determine, related, downhill, touch
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Text 3
Disorder. Temperature and internal energy tell part of the story about heat. To tell the whole story need to see what happens to the atoms or molecules an object when heat flows into it.
As heat enters an object, that object's atoms or molecules move around more. The more heat that flows the more the object's atoms or molecules move and the more disorderly they become. For exam water molecules in a snowflake have an orderly pattern. But if a snowflake is taken into a warm room, it will melt and become a drop of water. Heat changes the orderly pattern of the snowflake into disorder. Scientists term entropy to describe the amount of disorder object.
Heat flowing into an object increases the internal energy and disorder in that object Usually, the added heat also raises the temperature of the object. On other hand, heat flowing out of an object decreases the internal energy and disorder in that object. Usually the heat loss also lowers the temperature of the object.Heat passes from one object or place to another by three methods: (1) conduction, (2) convection, and (3) radiation.
Conduction is the movement of heat through a material. When heat travels by conduction, it moves through a material without carrying any of the material with it for example, the end of a copper rod placed in a fire quickly becomes hot. The atoms in the hot end begin to vibrate faster and strike neighboring atoms. These atoms then vibrate faster and strike adjoining atoms. In this way, the heat travels from atom to atom until it reaches the other end of the rod. But during the process, the atoms themselves do not move from one end to the other.
Convection is the transfer of heat by the movement of a heated material. For example, a hot stove in a room heats the air around it by conduction. This heated air expands and so is lighter than the colder air surrounding it. The heated air rises, and cooler air replaces it. Then the cooler air near the stove becomes warm and rises. This movement of heated air away from a hot object and the flow of cooler air toward that object is called a convection current. The current of air carries heat to all parts of the room. Convection occurs in liquids as well as in gases. For example, convection currents will form in a pan of cold water on a hot stove. As the water near the bottom of pan warms up and expands, it becomes lighter than old water near the top of the pan. This cold water sinkand forces the heated water to the top. The convection current continues until all the water reaches the same temperature.
1. Study the following words:
Disorderly, snowflake,convection current, sink, pan, current, heated material,
Strike, copper rod, liquids, warm up.
2. Read and translate these sentences:
a) When heat travels by conduction, it moves through a material without carrying any of the material with it for example, the end of a copper rod placed in a fire quickly becomes hot.
b) Conduction is the movement of heat through a material.
c) This cold water sinkand forces the heated water to the top.
3.Choose the right words:
a) Usually, the added heat also raises/ reduces the temperature of the object.
b) Conduction / Convection is the movement of heat through a material.
c) As heat enters/ arrives an object, that object's atoms or molecules move around more.
d) For example, a hot stove in a room heats the air around it by conduction/disorder.
4. Say in what situations these words and word combinations are used in the text:
Entropy,convection current, internal energy, neighboring atoms, heated water
5. Put questions to the words given in bold type:
a) This movement of heated air away from a hot object and the flow of cooler air toward that object is called a convection current.(1)
b) Scientists term entropy to describe the amount of disorder object.(2)
c) The heated air rises, and cooler air replaces it
Text 4
Radiation. In conduction and convection, moving particles transmit heat. But in radiation, heat can travel througha vacuum, which has no particles. In any object, moving atoms or molecules create waves of radiant energy. These waves are also called infrared rays. Hot objects give off more infrared rays than do cold objects. Infrared rays travel through space in much the same way as water waves travel on the surface of a pond. When theradiant energy strikes an object, it speeds up atoms or molecules in that object. Energy from the sun travels through space to the earth. These rays warm earth`s surface. Insulation is a way to control the movement of heat by keeping it in or out of a place. For example, houses insulated to keep the heat inside in winter and out-in summer. People use three methods of insulation because heat can travel in any one of three ways. Certain materials, such as plastic and wood, make good insulators against the movement of heat by conduction. This is why many pots and pans have plastic or wood handles. The metal utensil itself heats rapidly by conduction, but the handle stays cool. The movement of heat through the air by convection can be controlledby blocking the space between a hot and cold with "dead air." For example, the layer of air between a storm window and the inner window acts as an insulator.
Surfaces that reflect infrared rays can insulate heat traveling by radiation. For example, shiny metal roofs reflect sun's rays.
Text 5
Heat pump is a device that takes heat from one area and delivers it to another area at a higher temperature. In heating a building, a heat pump absorbs heat from outside the building and delivers it inside. In the summer, the same heat pump can be reversed so that it cools the building and discharges heat outside. A household refrigerator is a type of heat pump. A refrigerator absorbs heat from the food inside it and discharges the heat to the surrounding room air.
The fluid that circulates through a heat pump is called a refrigerant. For heating, a cold refrigerant first flows through coils of pipe that are exposed to an outside heat source. The heat source may be outside air, well water, or even the ground. The refrigerant absorbs heat from these sources, then goes to a compressor which increases its temperature and pressure. The refrigerant then flows to a heat exchanger, which resembles an automobile radiator, and gives up its heat to room air which is circulating through the exchanger. The refrigerant then passes through a valve or through a restrictor called a capillary tube. The valve or tube lowers the pressure of the refrigerant, resulting in a drop in temperature. The cycle is repeated as the refrigerant circulates again through the coils of pipe and picks up heat from the heat source.
For cooling purposes, valves reverse the direction of the refrigerant flow. The refrigerant vapor flows from a compressor at high temperature and pressure through the outside coils. At this point, water, earth, or outside air absorbs heat from the hotter refrigerant This takes place even if the outside source is warm, because it is cooler than the refrigerant. The refrigerant then passes through a valve that lowers its pressure, thus decreasing the temperature. In the heat exchanger, the refrigerant absorbs heat from the room air. The refrigerant then returns to a compressor and the cycle is repeated. Heat pumps are controlled by thermostats that sense the temperature of the room and turn the compressor on and
off
1. Study the following words:
Device, heat pump, fluid,capillary tube, valve, take place, decreasing, heat pumps,
Tube, restrictor, reverse, coils, sense
2. Read and translate these sentences:
a) A refrigerator absorbs heat from the food inside it and discharges the heat to the surrounding room air.
b) The fluid that circulates through a heat pump is called a refrigerant.
c) Heat pumps are controlled by thermostats that sense the temperature of the room and turn the compressor on and off.
3. Put questions to the words given in bold type:
a) The refrigerant then passes through a valve or through a restrictor called a capillary tube.
b) The refrigerant then passes through a valve or through a restrictor called a capillary tube.
c) Heat pumpis a device that takes heat from one area and delivers it to another area at a higher temperature.
d) The valve or tube lowers the pressure of the refrigerant, resulting in a drop in temperature.
e) A refrigerator absorbs heat from the food inside it and discharges the heat to the surrounding room air.
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