Thermodynamic heat pump and refrigeration cycles are the models A mathematical model uses mathematical language to describe a system. The process of developing a mathematical model is termed mathematical modelling . Mathematical models are used not only in the natural sciences (such as physics, biology, earth science, meteorology) and engineering disciplines, but also in the social sciences (such as economics, for heat pumps A heat pump is a machine or device that moves heat from one location to another location (the 'sink' or 'heat sink') using mechanical work. Most heat pump technology moves heat from a low temperature heat source to a higher temperature heat sink. Common examples are food refrigerators and freezers, air conditioners, and reversible-cycle heat pumps and refrigerators A refrigerator is a cooling appliance comprising a thermally insulated compartment and a heat pump—chemical or mechanical means—to transfer heat from it to the external environment, cooling the contents to a temperature below ambient. Cooling is a popular food storage technique in developed countries and works by decreasing the reproduction. The difference between a heat pump and a normal air conditioner is that a heat pump can be used to heat a home as well as cool it. Even though the heat pump can heat, it still uses the same basic refrigeration cycle to do this. In other words a heat pump can change which coil is the condenser and which the evaporator. In cooler climates it is common to have heat pumps that are able only to heat the house, making the pumps simpler and cheaper, since cooling is rarely necessary.
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Heat pumps
A heat pump has the ability to pump heat two ways. This allows the heat pump to both bring heat into a house and take it out. In the cooling mode a heat pump works the same as your typical A/C. It uses an evaporator to absorb heat from inside the house and rejects this heat outside through the condenser. The refrigerant is flowing in the normal direction to where the condenser and compressor are outside of the space to be conditioned, while the evaporator inside. The one key component that makes a heat pump different from an A/C is the reversing valve. The reversing valve A reversing valve is a component in a heat pump, that changes the direction of refrigerant flow. By reversing the flow of refrigerant, the heat pump refrigeration cycle is changed from cooling to heating or vice versa. This allows a residence or facility to be heated and cooled by a single piece of equipment, by the same means, and with the same allows for the flow direction of the refrigerant to be changed. This allows the heat to be pumped in either direction.
In the heating mode the outdoor coil becomes the evaporator, while the indoor becomes the condenser which absorbs the heat from the refrigerant and dissipates to the air flowing through it. The air outside even at 0 degrees Celsius has heat in it. With the refrigerant flowing in the opposite direction the evaporator(outdoor coil) is absorbing the heat from the air and moving it inside. Once it picks up heat it is compressed and then sent to the condenser(indoor coil). The indoor coil then rejects the heat into the air handler, which moves the heated air through out the house.
In the cooling mode the outdoor coil is now the condenser. This makes the indoor coil now the evaporator. The indoor coil is now the evaporator in the sense that it is going to be used to absorb the heat from inside the enclosed space. The evaporator absorbs the heat from the inside, and takes it to the condenser where it is rejected into the outside air.
Many heat pumps also use an auxiliary heat source. This means that, even though the heat pump is the primary source of heat, another form is put in place as a back-up. Electric, oil, or gas are the most commons sources. This is put in place so that if the heat pump fails or can't provide enough heat, the auxiliary heat will kick on to make up the difference.
Geothermal heat pumps A geothermal heat pump or ground source heat pump is a central heating and/or cooling system that pumps heat to or from the ground. It uses the earth as a heat source (in the winter) or a heat sink (in the summer). This design takes advantage of the moderate temperatures in the ground to boost efficiency and reduce the operational costs of heating are another form. They use the ground and water to work as the condensers and evaporators. They work in the same manner as an air to air heat pump, but instead of indoor and outdoor coils they use the earth as natural evaporators and condensers. These are very eco-friendly and are a cheaper alternative in the long run due to lower operating cost.
Heat naturally flows from hot to cold. Work In physics, mechanical work is the amount of energy transferred by a force acting through a distance. Like energy, it is a scalar quantity, with SI units of joules. The term work was first coined in 1826 by the French mathematician Gaspard-Gustave Coriolis is applied to cool a living space or storage volume by pumping heat from a lower temperature heat source into a higher temperature heat sink. Insulation The term thermal insulation can refer to materials used to reduce the rate of heat transfer, or the methods and processes used to reduce heat transfer. Heat energy can be transferred by conduction, convection, radiation or by actual movement of material from one location to another. For the purposes of this discussion only the first three is used to reduce the work and energy required to achieve and maintain a lower temperature in the cooled space. The operating principle of the refrigeration Refrigeration is the process of removing heat from an enclosed space, or from a substance, and moving it to a place where it is unobjectionable. The primary purpose of refrigeration is lowering the temperature of the enclosed space or substance and then maintaining that lower temperature. The term cooling refers generally to any natural or cycle was described mathematically by Sadi Carnot Nicolas Léonard Sadi Carnot was a French physicist and military engineer who, in his 1824 Reflections on the Motive Power of Fire, gave the first successful theoretical account of heat engines, now known as the Carnot cycle, thereby laying the foundations of the second law of thermodynamics. He is often described as the "Father of in 1824 as a heat engine A Carnot heat engine is a hypothetical engine that operates on the reversible Carnot cycle. The basic model for this engine was developed by Nicolas Léonard Sadi Carnot in 1824. The Carnot engine model was graphically expanded upon by Benoît Paul Émile Clapeyron in 1834 and mathematically elaborated upon by Rudolf Clausius in the 1850s and 60s.
Heat pump can be classified as:
- Vapor cycle,
- Gas cycle, and
- Stirling cycle A Stirling engine is a heat engine that operates by cyclic compression and expansion of air or other gas, the working fluid, at different temperature levels such that there is a net conversion of heat energy to mechanical work
Vapor cycle refrigeration can further be classified as:
- Vapor compression refrigeration Vapor-compression refrigeration is one of the many refrigeration cycles available for use. It has been and is the most widely used method for air-conditioning of large public buildings, private residences, hotels, hospitals, theaters, restaurants and automobiles. It is also used in domestic and commercial refrigerators, large-scale warehouses for
- Gas absorption refrigeration The absorption refrigerator is a refrigerator that uses a heat source to provide the energy needed to drive the cooling system. Absorption refrigerators are a popular alternative to regular compressor refrigerators where electricity is unreliable, costly, or unavailable, where noise from the compressor is problematic, or where surplus heat is
Vapor-compression cycle
The vapor-compression cycle is used in most household refrigerators as well as in many large commercial and industrial refrigeration systems. Figure 1 provides a schematic diagram of the components of a typical vapor-compression refrigeration system.
Figure 1: Vapor compression refrigerationThe thermodynamics In science, thermodynamics is the study of energy conversion between heat and mechanical work, and subsequently the macroscopic variables such as temperature, volume and pressure. The first to give a concise definition of the subject was Scottish physicist William Thomson who in 1854 stated that: of the cycle can be analyzed on a diagram[1][2] as shown in Figure 2. In this cycle, a circulating refrigerant A refrigerant is a substance used in a heat cycle usually including, for enhanced efficiency, a reversible phase change from a gas to a liquid. Traditionally, fluorocarbons, especially chlorofluorocarbons were used as refrigerants, but they are being phased out because of their ozone depletion effects. Other common refrigerants used in various such as Freon A chlorofluorocarbon is an organic compound that contains carbon, chlorine, and fluorine, produced as a volatile derivative of methane and ethane. A common subclass is the hydrochlorofluorocarbons (HCFCs), which contain hydrogen, as well. They are also commonly known by the DuPont trade name Freon. The most common representative is enters the compressor A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume as a vapor. The vapor is compressed at constant entropy Entropy is a measure of how disorganized a system is. It is an important part of the second law of thermodynamics. Thermodynamic systems consist of objects, e.g. atoms or molecules, which "carry" energy. In applied thermodynamics, as a matter of convention, entropy is measured in units of energy per temperature . If thermodynamic systems and exits the compressor superheated In physics, superheating is the phenomenon in which a liquid is heated to a temperature higher than its boiling point, without boiling. Superheating is achieved by heating a homogeneous substance in a clean container, free of nucleation sites, while taking care not to disturb the liquid. The superheated vapor travels through the condenser In systems involving heat transfer, a condenser is a device or unit used to condense a substance from its gaseous to its liquid state, typically by cooling it. In so doing, the latent heat is given up by the substance, and will transfer to the condenser coolant. Condensers are typically heat exchangers which have various designs and come in many which first cools and removes the superheat and then condenses the vapor into a liquid by removing additional heat at constant pressure and temperature. The liquid refrigerant goes through the expansion valve A thermostatic expansion valve is a component in refrigeration and air conditioning systems that controls the amount of refrigerant flow into the evaporator thereby controling the superheat at the outlet of the evaporator. This is accomplished by use of a temperature sensing bulb filled with a similar gas as in the system that causes the valve to (also called a throttle valve) where its pressure abruptly decreases, causing flash evaporation Flash evaporation is the partial vapor that occurs when a saturated liquid stream undergoes a reduction in pressure by passing through a throttling valve or other throttling device. This process is one of the simplest unit operations. If the throttling valve or device is located at the entry into a pressure vessel so that the flash evaporation and auto-refrigeration of, typically, less than half of the liquid.
Figure 2:Temperature–Entropy diagram The Carnot cycle is a particular thermodynamic cycle proposed by Nicolas Léonard Sadi Carnot in 1824 and expanded by Benoit Paul Émile Clapeyron in the 1830s and 40s. It is the most efficient existing cycle capable of converting a given amount of thermal energy into work or, conversely, creating a temperature difference by doing a given amountThat results in a mixture of liquid and vapor at a lower temperature and pressure. The cold liquid-vapor mixture then travels through the evaporator coil or tubes and is completely vaporized by cooling the warm air (from the space being refrigerated) being blown by a fan across the evaporator coil or tubes. The resulting refrigerant vapor returns to the compressor inlet to complete the thermodynamic cycle.
The above discussion is based on the ideal vapor-compression refrigeration cycle, and does not take into account real-world effects like frictional pressure drop in the system, slight thermodynamic irreversibility In thermodynamics, a reversible process, or reversible cycle if the process is cyclic, is a process that can be "reversed" by means of infinitesimal changes in some property of the system without loss or dissipation of energy. Due to these infinitesimal changes, the system is in thermodynamic equilibrium throughout the entire process. So during the compression of the refrigerant vapor, or non-ideal gas An ideal gas is a theoretical gas composed of a set of randomly-moving, non-interacting point particles. The ideal gas concept is useful because it obeys the ideal gas law, a simplified equation of state, and is amenable to analysis under statistical mechanics behavior (if any).
More information about the design and performance of vapor-compression refrigeration systems is available in the classic "Perry's Chemical Engineers' Handbook Perry's Chemical Engineers' Handbook was first published in 1934 and the most current eighth edition was published in October 2007. It has been a source of chemical engineering knowledge for chemical engineers, and a wide variety of other engineers and scientists, through seven previous editions spanning more than seventy years".[3]
Vapor absorption cycle
In the early years of the twentieth century, the vapor absorption cycle using water-ammonia systems was popular and widely used but, after the development of the vapor compression cycle, it lost much of its importance because of its low coefficient of performance The coefficient of performance or COP , of a heat pump is the ratio of the change in heat at the "output" (the heat reservoir of interest) to the supplied work (about one fifth of that of the vapor compression cycle). Nowadays, the vapor absorption cycle is used only where waste heat is available or where heat is derived from solar collectors A solar collector is a device for extracting the energy of the sun directly into a more usable or storable form. The energy in sunlight is in the form of electromagnetic radiation from the infrared to the ultraviolet (short) wavelengths. The solar energy striking the earth's surface at any one time depends on weather conditions, as well as.
The absorption cycle is similar to the compression cycle, except for the method of raising the pressure of the refrigerant vapor. In the absorption system, the compressor is replaced by an absorber which dissolves the refrigerant in a suitable liquid, a liquid pump which raises the pressure and a generator which, on heat addition, drives off the refrigerant vapor from the high-pressure liquid. Some work is required by the liquid pump but, for a given quantity of refrigerant, it is much smaller than needed by the compressor in the vapor compression cycle. In an absorption refrigerator, a suitable combination of refrigerant and absorbent is used. The most common combinations are ammonia (refrigerant) and water (absorbent), and water (refrigerant) and lithium bromide Lithium bromide, or LiBr, is a chemical compound of lithium and bromine. Its extreme hygroscopic character makes LiBr useful as a desiccant in certain air conditioning systems (absorbent).
Gas cycle
When the working fluid is a gas that is compressed and expanded but does not change phase, the refrigeration cycle is called a gas cycle. Air The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night. Dry air contains roughly (by volume) 78.09% nitrogen, 20.9 is most often this working fluid. As there is no condensation and evaporation intended in a gas cycle, components corresponding to the condenser and evaporator in a vapor compression cycle are the hot and cold gas-to-gas heat exchangers A heat exchanger is a device built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact. They are widely used in space heating, refrigeration, air conditioning, power plants, chemical plants, petrochemical plants, petroleum refineries, natural in gas cycles.
The gas cycle is less efficient than the vapor compression cycle because the gas cycle works on the reverse Brayton cycle The Brayton cycle is a thermodynamic cycle that describes the workings of the gas turbine engine, basis of the jet engine and others. It is named after George Brayton , the American engineer who developed it, although it was originally proposed and patented by Englishman John Barber in 1791. It is also sometimes known as the Joule cycle. The instead of the reverse Rankine cycle The Rankine cycle is a thermodynamic cycle which converts heat into work. The heat is supplied externally to a closed loop, which usually uses water as the working fluid. This cycle generates about 80% of all electric power used throughout the world, including virtually all solar thermal, biomass, coal and nuclear power plants. It is named after. As such the working fluid does not receive and reject heat at constant temperature. In the gas cycle, the refrigeration effect is equal to the product of the specific heat of the gas and the rise in temperature of the gas in the low temperature side. Therefore, for the same cooling load, a gas refrigeration cycle will require a large mass flow rate and would be bulky.
Because of their lower efficiency and larger bulk, air cycle coolers are not often applied in terrestrial refrigeration. The air cycle machine An air cycle machine is the refrigeration unit of the environmental control system used in pressurized turbine-powered aircraft. Normally an aircraft has two or three of these machines arranged in a system with each ACM called a "pack". The cooling process uses air instead of Freon in a gas cycle. No condensation or evaporation of a is very common, however, on gas turbine A gas turbine, also called a combustion turbine, is a rotary engine that extracts energy from a flow of combustion gas. It has an upstream compressor coupled to a downstream turbine, and a combustion chamber in-between. Gas turbine may also refer to just the turbine component-powered jet airliners In contrast to today's relatively fuel-efficient, turbofan-powered air travel, first generation jet airliner travel was noisy and fuel inefficient. These inefficiencies were addressed by the invention of turboprop and turbofan engines since compressed air is readily available from the engines' compressor sections. These jet aircraft's cooling and ventilation units also serve the purpose of pressurizing the aircraft cabin An aircraft cabin is the section of an aircraft in which passengers travel, often just called the cabin. At cruising altitudes, the surrounding atmosphere is too thin to breath without an oxygen mask, so cabin pressurization adapts the cabin to atmospheric pressures.
References
- ^ The Ideal Vapor-Compression Cycle
- ^ Scroll down to "The Basic Vapor Compression Cycle and Components"
- ^ Perry, R.H. and Green, D.W. (1984). Perry's Chemical Engineers' Handbook Perry's Chemical Engineers' Handbook was first published in 1934 and the most current eighth edition was published in October 2007. It has been a source of chemical engineering knowledge for chemical engineers, and a wide variety of other engineers and scientists, through seven previous editions spanning more than seventy years (6th Edition ed.). McGraw Hill, Inc.. ISBN ISBN 0-07-049479-7. (see pages 12-27 through 12-38)
- Notes
- Turns, Stephen (2006). Thermodynamics: Concepts and Applications. Cambridge University Press. pp. 756. ISBN The International Standard Book Number is a unique numeric commercial book identifier based upon the 9-digit Standard Book Numbering (SBN) code created by Gordon Foster, now Emeritus Professor of Statistics at Trinity College, Dublin, for the booksellers and stationers W.H. Smith and others in 1966 0521850428. http://books.google.com/books?id=fy5hs04OeMQC&dq=Heat+pump+and+refrigeration+cycle.
- Dincer, Ibrahim (2003). Refrigeration Systems and Applications. John Wiley and Sons. pp. 598. ISBN The International Standard Book Number is a unique numeric commercial book identifier based upon the 9-digit Standard Book Numbering (SBN) code created by Gordon Foster, now Emeritus Professor of Statistics at Trinity College, Dublin, for the booksellers and stationers W.H. Smith and others in 1966 0471623512.
- Whitman, Bill (2008). Refrigeration and Air conditioning Technology. Delmar.
External links
Categories: Thermodynamic cycles Categories: Thermodynamic processes | Engine technology | Heat pumps Categories: Pumps | Cooling technology | Heating
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