Q.1 What is the basic principal of refrigeration and what do you understand by refrigeration cycle? Ans.1 Refrigeration is a process in which work is done to move heat from one location to another. In other words it is a process in which heat is absorbed from lower temperature region and resent or transfer it to the high temperature region. Basic principle of refrigeration is to pass a colder liquid known as the refrigerant, continuously around the object which is to be cooled. This will take heat out from the object which is to be cooled. Due to the temperature difference object ejects heat to the refrigerant liquid. The refrigerant is in turn gets heated up due to heat absorption. A refrigerant is a substance or mixture, usually a fluid, used in a heat pump and refrigeration cycle. A refrigerant is a fluid used for heat transfer in a refrigeration system. Most refrigerants absorb heat during evaporation at low temperature and low pressure and reject heat during condensation at a higher temperature and higher pressure In most cycles it undergoes phase transitions from a liquid to gas and back again. There are two Types of refrigerants : primary and secondary. Primary refrigerants- it undergoes phase transition from a liquid to a vapor and back again. Secondary refrigerants- it doesn’t undergoes any phase transitions and always remains in liquid form. Application of refrigeration : Probably the most widely used current applications of refrigeration are for air conditioning of private homes and public buildings, and refrigerating foodstuffs in homes, restaurants and large storage warehouses. The use of refrigerators in kitchens for storing fruits and vegetables. It is also used in medicine and sterilization. Metal workers use refrigeration to temper steel and cutlery. In transporting temperature-sensitive foodstuffs and other materials by trucks, trains, airplanes and seagoing vessels, refrigeration is a necessity. Methods of refrigeration can be classified as non-cyclic, cyclic, thermoelectric and magnetic. 1) Cyclic method : This consists of a refrigeration cycle, where heat is removed from a low-temperature space or source and rejected to a high-temperature sink. Cyclic refrigeration can be classified as: • Vapor cycle (a) Vapour-compression refrigeration (b) Vapour-absorption refrigeration • Gas cycle : Gas is used instead of refrigerant. Gas has been moved from higher pressure to lower pressure which reduces the temperature. Its process is same as the other methods. When the working fluid is a gas that is compressed and expanded but doesn't change phase, the refrigeration cycle is called a gas cycle. Air 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 in gas cycles. The gas cycle is less efficient than the vapor compression cycle because the gas cycle works on the reverse Brayton cycle instead of the reverse Rankine cycle. 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 needs a large mass flow rate and is bulky. Vapour compression refrigeration method : it is the most common method of transferring heat from a low temperature to a high temperature. It is used in general refrigerators, compressors, etc. in this method both the temperature and pressure increases. firstly compression is done where vapor is compressed, than vapor are cooled and converted into liquid state and hence condensation occurs, than further expansion is carried out where vapor become partially liquid and partially gaseous. Lastly, evaporation is carried out. Here electricity is less consumed and external heat is not required. Vapour absorption refrigeration method : it is quite similar as that of the vapour compression method except vapour instead of compression undergoes absorption. High temperature and pressure is released. Electricity consumed is more and external heat is required. 2) Non-cyclic method : In non-cyclic refrigeration, cooling is accomplished by melting ice or by subliming dry ice (frozen carbon dioxide). These methods are used for small-scale refrigeration such as in laboratories and workshops, or in portable coolers. Q.2 What do you understand by air cooling and air conditioning system. Explain with sketches. Ans.2 Air cooling is a method of dissipating heat. It works by making the object to be cooled have a larger surface area or have an increased flow of air over its surface, The most important benefit of using air coolers is that they are relatively cost effective. They AIR COOLING AIR CONDITIONING VAPOUR CONDENSATIO CYCLE VAPOUR ABSORPTION CYCLE