Mechanical Engineering Module 4 (Thermal Systems and Management) PDF

Module IV-Thermal Systems and Management Heat in Electronic Devices: Modes of Heat Transfer, heat generation in electronics, temperature measurement, heat sink, Cooling of electronic devises: Active, Passive, and Hybrid Cooling. Refrigeration: Principle of refrigeration, Re...

Module IV-Thermal Systems and Management Heat in Electronic Devices: Modes of Heat Transfer, heat generation in electronics, temperature measurement, heat sink, Cooling of electronic devises: Active, Passive, and Hybrid Cooling. Refrigeration: Principle of refrigeration, Refrigeration effect, Ton of Refrigeration, COP, Refrigerants and their desirable properties. Principles and Operation of Vapor Compression and Vapor absorption refrigeration. Applications of Refrigerator. Air-Conditioning: Classification and Applications of Air Conditioners. Concept and operation of Centralized air conditioning system.  Modes of Heat Transfer Conduction: An energy transfer across a system boundary due to a temperature difference by the mechanism of inter­molecular interactions. Conduction needs matter and does not require any bulk motion of matter.  Convection: An energy transfer across a system boundary due to a temperature difference by the combined mechanisms of intermolecular interactions and bulk transport. Convection needs fluid matter.  Radiation: Radiation heat transfer involves the transfer of heat by electromagnetic radiation that arises due to the temperature of the body. Radiation does not need matter.  Heat in Electronic Devices All electronic devices and circuitry generate excess heat and thus require thermal management to improve reliability and prevent premature failure. The amount of heat output is equal to the power input, if there are no other energy interactions. There are several techniques for cooling including various styles of heat sinks, thermoelectric coolers, forced air systems and fans, heat pipes, and others. A common problem in product design— particularly in electronics cooling—is managing thermal conditions for optimal efficiency. The core of the challenge is designing energy-efficient microprocessors and printed circuit boards (PCBs) that will not overheat.  Thermal Management of Electronics: Active vs Passive Cooling Fundamentally, we can divide electronics cooling techniques into two categories: passive cooling and active cooling. Passive cooling utilizes natural conduction, convection, and radiation to cool a component. Active cooling requires the use of energy specifically dedicated to cooling the component. One example of these two cooling categories is in heat sink and heat spreader design, both of which utilize fundamental heat transfer principles.  Passive cooling Passive cooling is a form of cooling without any fans etc. The idea is to have the heat produced from the CPU transferred to another surface, and therefore cool the CPU down. The pros of passive cooling comes down to it being silent, which is something that a lot of people prefer. Another benefit comes down to it looking good, and it fits very well with the rest of the room it’s in. Passive cooling achieves high levels of natural convection and heat dissipation by utilizing a heat spreader or a heat sink to maximize the radiation and convection heat transfer modes. In architectural design, natural resources like wind or soil are used as heat sinks to absorb or dissipate heat. This leads to proper cooling of electronic products.  Active Cooling Active cooling is somewhat similar to passive. The idea is to have the heat generated by the CPU transferred onto another material (metal etc.), and then cooling it down with fans. Another thing it has over liquid-cooling, is the fact that if anything goes wrong, it’s only the fan on the cooler itself that can break, which limits the amount of damage that can be done. (except for CPU overheating damage). Active cooling, on the other hand, refers to cooling technologies that rely on an external device to enhance heat transfer. Through active cooling technologies, the rate of fluid flow increases during convection, which dramatically increases the rate of heat removal. Active cooling solutions include forced air through a fan or blower, forced liquid, and thermoelectric coolers (TECs), which can be used to optimize thermal management on all levels. Fans are used when natural convection is insufficient to remove heat.  Working Principle of Refrigerator Refrigerators work on the second law of thermodynamics. In the process of refrigeration, unwanted heat is taken from one place and discharged into another. The common refrigerator which we have in our homes, works on the principle of evaporation. A refrigerant is a substance used in a heat cycle to transfer heat from one area, and remove it to another. A refrigerant when passed through the food kept in the refrigerator, it absorbs heat from these items and transfers the absorbed heat to the surrounding with less temperature. The Components of Refrigerator There are four components in the refrigerator system. They are: Evaporator Compressor Condenser Expansion valve Components Evaporator: It is the main part of the refrigerator that helps to keep the device and things cool always. It features the tubes with high thermal conductivity that helps in absorbing the heat rejected by the fan or coil in the system. Compressor: It compresses the low-pressure, low-temperature vapor into a high-temperature, high-pressure vapor. The refrigerant is passed from the evaporator and is compressed in a cylinder to generate a high-temperature, high-pressure gas. Expansion valve helps to control the flow of refrigerant into the evaporator, or cooling coil. Expansion valve is also known as flow control valves. It is a sensitive small device that aids to sense the temperature change of the refrigerant. Condenser: It comprises a set of tubes with external fins placed back of the refrigerator. This component helps to convert the gaseous refrigerant into liquid form. Working Of The Refrigerator Vapour compression refrigeration cycle is followed for the refrigeration process. In this process, the Evaporator, Compressor, Condenser and expansion valves are connected to tubes made of copper or steel. Evaporator tube is placed throughout the refrigerator, when heat is absorbed the liquid refrigerant absorbs the heat and then converts into vapour. The heat absorbed is passed to the external environment through the compressor from vapour state to liquid state. This process repeats when the heat is absorbed and passed through the expansion valve to the evaporator. The above process helps to keep the refrigerator cool always. Refrigeration & air- conditioning  Refrigeration: It is defined as the process of removing heat from a substance under controlled conditions and reducing and maintaining the temperature of a body below the temperature of its surroundings by the aid of external work.  Refrigeration Effect: In a refrigeration system, the rate at which the heat is absorbed in a cycle from the interior space to be cooled is called refrigerating effect.  Unit of Refrigeration: The capacity of a refrigeration system is expressed in tons of refrigeration, which is the unit of refrigeration. A ton of refrigeration is defined as the quantity of heat absorbed in order to form one ton of ice in 24 hours when the initial temperature of the water is 0°C. In S.I. System, 1 Ton of Refrigeration = 210 kJ/min = 3.5 kW  Coefficient of Performance (COP): The performance of a refrigeration system is expressed by a factor known as the coefficient of performance. The COP of a refrigeration system is defined as the ratio of heat absorbed in a system to the work supplied. Mathematically: COP=Q/W Where Q = Heat Absorbed or Removed  REFRIGERANT: A Refrigerant is medium it continuously extracts the heat from the space within the refrigerator which is to be kept cool at temperatures less than the atmosphere and finally rejects to it to the surroundings. The most commonly used refrigerants are given below 1) Ammonia 2) Carbon dioxide 3) Sulphur dioxide 4) Methyl Chloride 5) Freon 1) Ammonia  Ammonia as a refrigerant is employed in refrigerators operating on the absorption principles. Because of its high latent heat (1300 kJ/kg at -15°C) and low specific volume (0.509mVkg at -15°C) it produces high refrigeration effects even in small refrigerators.  Since ammonia will not harm the ozone, it is environmental friendly. It is widely used in cold storage, ice making plants, etc.  It is toxic, flammable, irritating and food destroying properties makes it unsuitable for domestic refrigerators. 2) Carbon dioxide  The efficiency of the refrigerators using carbon dioxide refrigerant is low. Therefore it is seldom used in domestic refrigerators, but is used in dry ice making plants.  It is colourless, odourless, non-toxic, non-flammable and non-corrosive. 3) Sulphur dioxide  Earlier Sulphur dioxide was one of the most commonly used refrigerants in domestic refrigerators. Although it has better thermodynamic properties, it has low refrigerating effect and high specific volume, therefore large capacity high speed compressors are required.  Since it combines with water and forms sulfurous and sulfuric acids which are corrosive to metals, the refrigerators using Sulphur dioxide as refrigerant are seldom used. 4) Methyl Chloride  Methyl chloride was used earlier in domestic and small scale industrial refrigerators. Since it will burn under some conditions and slightly toxic, is not generally used. 5) Freon  Freon group of refrigerants is used almost universally in domestic refrigerators.  These refrigerants are colourless, almost odourless, non-toxic, non-flammable, non-explosive and non-corrosive, Freon-12 and Freon-22 are the two Freon refrigerants commonly used in domestic refrigerators and air conditioners.  Although these refrigerants are being now used extensively in the refrigerators and the air conditioners, it has been found that these refrigerants posing a major threat to the global environment through their role in the destruction of the ozone layer. PROPERTIES OF A GOOD REFRIGERANT  Thermodynamic properties 1) Boiling Point : An ideal refrigerant must have low boiling temperature at atmospheric pressure. 2) Freezing Point: An ideal refrigerant must have a very low freezing point because the refrigerant should not freeze at low evaporator temperatures. 3) Evaporator and Condenser Pressure: In order to avoid the leakage of the atmospheric air and also to enable the detection of the leakage of the refrigerant, both the evaporator and condenser pressures should be slightly above the atmospheric pressure. 4) Latent Heat of Evaporation : The latent heat of evaporation must be very high so that a minimum amount of refrigerant will accomplish the desired result, in other words, it increases the  Physical properties 1) Specific Volume: The specific volume of the refrigerant must be very low. The lower specific volume of the refrigerant at the suction of the compressor reduces the size of the compressor. 2) Specific heat of liquid and vapour: A good refrigerant must have low specific heat when it is in liquid state and high specific heat when it is vapourised. The low specific heat of the refrigerant helps in sub-cooling of the liquid a high specific heat of the vapour helps in decreasing the superheating of the vapour. Both these desirable properties increase the refrigerating effect. 3) Viscosity: The viscosity of a refrigerant at both the liquid and vapour states must be very low as improves the heat transfer and reduces the pumping pressure.  Safe working properties 1) Non-toxicity refrigerant: A good refrigerant should be non-toxic, because any leakage of the toxic refrigerant increase suffocation and poisons the atmosphere. 2) Corrosiveness : A good refrigerant should be non-corrosive to prevent the corrosion of the metallic parts of the refrigerators. 3) Chemical Stability: An ideal refrigerant must not decompose under operating conditions.  Other properties 1) Coefficient of Performance : The coefficient of performance of a refrigerant must be high so that the energy spent in refrigeration will be less. 2) Odour: A good refrigerant must be odourless, otherwise some foodstuff such as meat, butter, etc. loses their taste. 3) Leakage Tests: The refrigerant must be such that any leakage can be detected by simple tests. 4) Action with Lubricating Oil: A good refrigerant must not react with the lubricating oil used in lubricating the parts of the compressor. PARTS OF A REFRIGERATOR  TYPES OF REFRIGERATION SYSTEMS The refrigeration systems are mainly divided into two types, they are 1)Vapour Compression Refrigerator (VCR) 2)Vapour Absorption Refrigerator (VAR) )Vapour Compression Refrigerator (VCR) : 2) Vapour Absorption Refrigerator (VAR) AIR - CONDITIONING  ROOM AIR-CONDITIONER  HUMIDITY Humidity is defined as the moisture content present in the atmosphere. The atmosphere always contains some moisture in the form of water vapour. The maximum amount depends on the atmospheric conditions. The amount of vapour that will saturate the air increases with a rise in temperature. Humidity can be specified in three different ways. 1) Absolute humidity: The absolute humidity is defined as the, weight of water vapour contained in a given volume of air. It is expressed in grams of water vapour per cubic metre of air. 2) Specific humidity: The specific humidity is defined as the ratio of weight of water vapour to the total weight of air. It is expressed in grams of water vapour per kilogram of air. 3) Relative humidity: The relative humidity is defined as the ratio of the actual vapour content of the air  What Is Central Air Conditioning Really? The “central” in central air conditioning comes from the fact that the system conditions the entire building from one large central location in your home. This system cools and dehumidifies the air, then blows the cool air into your duct system and out through the supply air registers located in each room. A central AC’s main components are: Fans that pull air through the system Evaporator coils that change refrigerant from liquid to gas A compressor that pressurizes refrigerant gas Condenser coils that turn refrigerant gas into a liquid Refrigerant lines that carry refrigerant between the coils An expansion device that regulates refrigerant flowing to the evaporator Central air conditioning systems come in two basic designs: split and packaged. Split systems are the most commonly used type, and are split between an outdoor and an indoor unit, as the name suggests. In a packaged/ductless system, all the components are housed in one unit that is installed outside your home. Split and packaged systems both contain the same components and can provide the same amount of cooling Benefits of Central Air Conditioning System: Indoor comfort during warm weather – Central air conditioning helps keep your home cool and reduces humidity levels. Cleaner air – As your central air conditioning system draws air out of various rooms in the house through return air ducts, the air is pulled through an air filter, which removes airborne particles such as dust and lint. Sophisticated filters may remove microscopic pollutants, as well. The filtered air is then routed to air supply duct-work that carries it back to rooms. Quieter operation – Because the compressor-bearing unit is located outside the home, the indoor noise level from its operation is much lower than that of a free-standing air conditioning