{"id":18065,"date":"2018-06-13T19:00:12","date_gmt":"2018-06-13T19:00:12","guid":{"rendered":"http:\/\/sitepourvtc.com\/?page_id=18065"},"modified":"2022-11-21T14:51:59","modified_gmt":"2022-11-21T14:51:59","slug":"heating-and-air-conditioning","status":"publish","type":"page","link":"https:\/\/sitepourvtc.com\/nuclear-engineering\/thermodynamics\/thermodynamic-cycles\/heating-and-air-conditioning\/","title":{"rendered":"Heating and Air Conditioning"},"content":{"rendered":"

According to the second law of thermodynamics<\/strong><\/a>, many thermodynamic processes proceed naturally in one direction but not the opposite. For example, when a temperature difference<\/strong> does exist, heat flows spontaneously from the warmer system to the colder system<\/strong>, never the reverse. In fact, such heat flow (from a colder body to a warmer system) would not violate the first law of thermodynamics<\/strong><\/a>, i.e., energy would be conserved. But it doesn’t happen in nature.<\/p>\n

Directions of thermodynamic processes<\/strong> are subject of the second law of thermodynamics, especially of the Clausius Statement of the Second Law<\/strong><\/a>, which states:<\/p>\n

“It is impossible to construct a device which operates on a cycle and whose sole effect is the transfer of heat from a cooler body to a hotter body.”<\/em><\/p>\n

\"Clausius<\/a>Heat cannot spontaneously flow<\/strong> from cold system to hot system without external work being performed on the system. This is exactly what refrigerators and heat pumps accomplish. In a refrigerator, heat pump, or air conditioner, heat flows from cold to hot, but only when forced by external work. These<\/strong> devices are driven by electric motors, requiring work from their surroundings to operate. There is no difference between the thermodynamics of refrigerators and heat pumps. Both work by moving heat from a cold space to a warm space.<\/p>\n

See also: What is Temperature<\/a><\/p>\n

See also: Heat in Thermodynamics<\/a><\/p>\n

While internal energy<\/strong><\/a> refers to the total energy of all the molecules within the object, heat<\/strong> is the amount of energy flowing<\/strong> spontaneously from one body to another due to their temperature difference. Heat<\/strong> is a form of energy, but it is energy in transit<\/strong>. Heat is not a property of a system. However, the transfer of energy as heat occurs at the molecular level due to a temperature difference<\/strong>.<\/p>\n

\"zeroth-law-of-thermodynamics-heat\"<\/a>Consider a metal block<\/b>\u00a0at high temperature that consists of atoms oscillating intensely around their average positions. At low temperatures<\/strong>, the atoms continue to oscillate but with less intensity<\/strong>. If a hotter block of metal is put in contact with a cooler block, the intensely oscillating atoms at the edge of the hotter block give off their kinetic energy to the less oscillating atoms at the edge of the cool block. In this case, there is energy transfer<\/strong> between these two blocks, and heat flows<\/strong> from the hotter to the cooler block by these random vibrations.<\/p>\n

In general, when two objects are brought into thermal contact<\/strong>, heat will flow<\/strong> between them until<\/strong> they come into equilibrium<\/strong> with each other. \u00a0When a temperature difference<\/strong> does exist, heat flows spontaneously from the warmer system to the colder system<\/strong>. Heat transfer occurs by conduction<\/strong> or by thermal radiation<\/strong>. When the flow of heat stops<\/strong>, they are said to be at the same temperature<\/strong>. They are then said to be in thermal equilibrium<\/strong><\/a>.<\/p>\n

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Heat Pump – Operating Principle<\/h2>\n
\"Heat<\/a>
Heat Pump, Refrigerator, Air Conditioner – basic principle of operation<\/figcaption><\/figure>\n

The term heat pump<\/strong> is usually reserved for a device that can heat a house in winter by using an electric motor that does work W to take heat Qcold<\/sub><\/strong> from the outside at low temperature and delivers heat Qhot<\/sub><\/strong> to the warmer inside of the house.<\/p>\n

The operating principle of refrigerators<\/strong>, air conditioners<\/strong>, and heat pumps<\/strong> is the same,<\/strong> and it is just the reverse <\/strong>of a heat engine<\/strong><\/a>. In general, a heat pump<\/strong> is a device that transfers heat energy from a heat source<\/strong> to a “heat sink<\/strong>,” but in this case, the transfer occurs in the opposite direction of spontaneous heat transfer by absorbing heat<\/strong> from a cold space<\/strong> and releasing it<\/strong> to a warmer one<\/strong>. As diagrammed in the figure, by doing external work W, heat is taken from a low-temperature region (heat source), and a greater amount of heat is exhausted at a higher temperature (heat sink).<\/p>\n

The most widely used thermodynamic cycle or method for heating, air-conditioning, refrigerators, and heat pumps is the vapor compression cycle<\/strong>.<\/p>\n

Vapor-compression Cycle – Vapor-compression Refrigeration<\/h2>\n
\"Vapor-compression<\/a>
Vapor-compression cycle – Thermodynamic cycle of heat pumps.<\/figcaption><\/figure>\n

The vapor-compression<\/strong> uses a circulating liquid refrigerant as the medium (usually R134a<\/strong>), which absorbs and removes heat from the space to be cooled and subsequently rejects that heat elsewhere. The figure depicts a typical, single-stage vapor-compression system<\/strong>. The typical vapor-compression system consists of four components:<\/p>\n