Download Refrigeration and Air Conditioning: Introduction, Principles, and Applications and more Assignments Electronics in PDF only on Docsity!
Refrigeration and Air Conditioning
Topics:
- Introduction
- Evaporation Process
- Refrigeration Effect
- Unit of Refrigeration & COP
- Important Refrigerants
- Refrigerating Systems: Air Refrigeration System, Ammonia Absorption Refrigeration System, Vapour Compression Refrigerating Systems
- Analysis of Vapour Compression Refrigerating Systems like COP, Mass Flow Rate, Heat rejected from Condenser, Power Consumption etc.
- Window and Split Air Conditioning System - Principles and Working There is no such thing as cold — only the absence of heat. When an air conditioner or refrigerator is cooling a space, do not think about it as adding cold air into the space. The purpose of the refrigeration cycle is to remove the heat in a given area and reject it outside. Less heat means a colder room!!!
• Introduction
For specific applications, efficiencies of both living and non-living beings depend to a great extent on the physical environment. The nature keeps conditions in the physical environment in the dynamic state ranging from one extreme to the other. Temperature, humidity, pressure and air motion are some of the important environment variables that at any location keep changing throughout the year. Adaptation to these many a times unpredictable variations is not possible and thus working efficiently is not feasible either for the living beings or the non- living ones. Thus, for any specific purpose, control of the environment is essential. Refrigeration and air-conditioning is the subject which deals with the techniques to control the environments of the living and non-living subjects and thus provide them comforts to enable them to perform better and have longer lives.
• Refrigeration
Literal meaning of refrigeration is the production of cold confinement relative to its surroundings. In this, temperature of the space under consideration is maintained at a temperature lower than the surrounding atmosphere. To achieve this, the mechanical device extracts heat from the space that has to be maintained at a lower temperature and rejects it to the surrounding atmosphere that is at a relatively higher temperature. Since the volume of the space which has to be maintained at a lower temperature is always much lower than the environment, the space under consideration experiences relatively higher change in temperature than the environment where it is rejected.
The precise meaning of the refrigeration is thus the following: Refrigeration is a process of removal of heat from a space where it is unwanted and transferring the same to the surrounding environment where it makes little or no difference. To understand the above definition, let us consider two examples from the daily life. It is a well-known fact that the spoilage of food and many other items reduces at a lower temperature. At a lower temperature, molecular motion slows down and the growth of bacteria that causes food spoilage also retards. Thus, to preserve many types of perishable food products for a longer duration , we use refrigerators in our homes, canteens, hotels , etc. The temperature of the food products has to maintained at a level below that of surroundings. For this we keep the food products in a refrigerator. The inside volume of the refrigerator where we store food products or any other items is much less than the volume of the room where the refrigerator is kept. The room in this case is the surrounding environment. Food products in the refrigerator initially were at a higher temperature than desired temperature, meaning that it had some unwanted heat. If its heat is removed, its temperature will decrease. The refrigerator removes unwanted heat from the food products and throws away that heat to the room – the surrounding environment of the refrigerator. The amount of heat makes a big difference in temperature inside the refrigerator and almost little or no difference in the temperature of the room. Figure: Spoilage of Food Products in a Refrigerator Slows Down. Temperature in the Refrigerated Space is Lower than the Room where Refrigerator is Kept. Refrigerator Throws Away Heat from the Food Products to the Room
flow of heat from a body at a lower temperature to a body at a higher temperature do not occur naturally. In practice these are achieved at the cost of external work (power) done on the water and the carrier of heat (here the refrigerant) with help of a mechanical device. Whether the space under consideration has to be maintained at a temperature lower or higher than the surrounding environment, to pump out or in the heat, external power is always required.
• Air Conditioning
Merely lowering or raising the temperature does not provide comfort in general to the machines or its components and living beings in particular. In case of the machine components, along with temperature, humidity (moisture content in the air) also has to be controlled and for the comfort of human beings along with these two important parameters, air motion and cleanliness also play a vital role. Air conditioning, therefore, is a broader aspect which looks into the simultaneous control all mechanical parameters which are essential for the comfort of human beings or animals or for the proper performance of some industrial or scientific process. The precise meaning of air conditioning can be given as the process of simultaneous control of temperature, humidity, cleanliness and air motion. In some applications, even the control of air pressure falls under the purview of air conditioning. It is to be noted that refrigeration that is control of temperature is the most important aspect of air conditioning. To understand the above definition in a better way, let us consider one example. In the summer, the temperature in Delhi is about 10 ºC higher than in Kolkata where temperature varies in the range of 32 ºC to 35 ºC. We feel uncomfortable in both places. Weather in Delhi is hot and dry (moisture content in the air is low) whereas in Kolkata it is (mild) hot but humid (moisture content in the air is very high). If we go to a hill-station, say Shillong in the summer, we feel comfortable there. Temperature there remains about 25 ºC and relative humidity of the air is also in the comfortable range, say about 65%. In Delhi, temperature is very high and humidity is low, whereas in Kolkota, temperature is low but humidity is high. In Delhi if there is a rain, we feel more comfortable whereas in Kolkata even with rain, the relative comfort is less. In Delhi temperature falls down and humidity also increases towards the comfortable value. In Koklata, temperature falls down but humidity still remains on the higher side. Thus, for comfort, both temperature and humidity have to be in the specified range. This is true for both human beings and scientific processes. Apart from the above two, from intuition one can also say that purity or cleanliness of the air is an essential item for the comfort and it has been established that the air motion is also required for the comfort condition. Depending upon the requirement, air conditioning is divided into the summer air conditioning and the winter air conditioning. In the summer air conditioning, apart from cooling the space, in most of the cases, extra moisture from the space is removed, whereas in the winter air conditioning, space is heated and since in the cold places, normally the humidity remains low, moisture is added to the space to be conditioned. The summer air conditioning thus uses a refrigeration system and a dehumidifier. The winter air conditioning uses a heat pump (refrigeration system operated in the reverse direction) and a humidifier. Depending upon the comfort of the human beings and the control of environment for the industrial products and processes, air conditioning can also be classified as comfort air conditioning and industrial air conditioning. Comfort air conditioning deals with the air conditioning of residential
buildings, offices spaces, cars, buses, trains, airplanes, etc. Industrial air conditioning includes air conditioning of the printing plants, textile plants, photographic products, computer rooms, etc. It has been mentioned above that the refrigeration and air conditioning are related. Even when a space has to be heated, it can be done so by changing the direction of flow of the refrigerant in the refrigeration system, i.e., the refrigeration system can be used as a heat pump. However, some section of the people, treat refrigeration exclusively the process that deals with the cooling of the space. They treat heating operation associated with the heat pump.
• Applications
➢ Comfort air conditioning of auditoriums, hospitals, residents, offices, hotels etc.
➢ Manufacturing and preservation of medicine.
➢ Storage and transportation of food stuffs such as dairy products, fruits, vegetables,
meat, fish etc.
➢ Processing of textiles, printing work and photographic materials.
➢ Manufacturing of ice.
➢ Spot heating
➢ Spot cooling
➢ Environmental laboratories
➢ Cooling of concrete for dam.
➢ Treatment of air for blast furnace.
➢ Processing of petroleum and other chemical products.
➢ Production of Rocket fuel.
➢ Computer functioning.
• Evaporation/ Evaporative Cooling
Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. Principle : Evaporation causes cooling because the process requires heat energy. The energy is taken away by the molecules when they convert from liquid into gas, and this causes cooling on the original surface. When a liquid evaporates, its molecules convert from the liquid phase to the vapor phase and escape from the surface. Heat drives this process. In order for the molecule to leave the liquid surface and escape as a vapor, it must take heat energy with it. The heat that it takes with it comes from the surface from which it evaporated. Since the molecule is taking heat with it as it’s leaving, this has a cooling effect on the surface left behind. Evaporation causes cooling naturally. Evaporation and Human Perspiration: An example of evaporated cooling is that of human perspiration. We have pores in our skin from which liquid water internal to our skin is escaping and converting to water vapor in the
In the heat pump, it is the condenser that performs the main function of heating the room, while the evaporator performs the function of absorbing the heat from the atmosphere.
Comparison of Heat Pump with Electric Room Heater
The main purpose of the heat pump is to heat the room when the atmospheric temperature is very low. The same purpose can also be served by the ordinary electric room heater. However, for heating the same size of the room and adding same quantity of heat to the room, the power consumed by the electric room heater is more than that consumed by the heat pump. On the other hand, the heat pump is much more expensive than the electric room heater. Thus, if you have to use the heater for longer hours of duration, the heat pump is more economical in the long-run, but if you have to use the heater less frequently, you can opt for the electric room heater since its initial cost is low.
- Refrigerating effect The rate of heat absorbed from a body to be cooled is called refrigerating effect. Or It is the amount of heat energy removed per unit time from the space to be cooled by the refrigeration process. It indicates refrigerator capacity. Unit: kJ/s or kW
- Refrigerant A refrigerant is a working substance used in refrigerating machines. It is a working medium through which heat is transferred from a cold body to a hot body. The refrigerants commonly used are: Air, Water, Ammonia (NH 3 ), Carbon dioxide (CO 2 ), Sulphur dioxide (SO 2 ) Freon group, Methyl chloride (CH 3 CI), Methylene chloride (CH 2 CI 2 ), Ethyl chloride (C 2 H 5 CI), R (CCl 2 F 2 ), R22 (CHClF 2 ), Chlorinated fluorocarbons (CFCs) etc.
Desirable Properties of Refrigerant
o Low Boiling Point o Low Freezing Point. o Low specific volume
o High Latent Heat of evaporation. o Chemically Inert & stable. o Low saturation pressure o Cheaper and easily available o High critical pressure o High COP o Non-Flammable o Non toxic o Should not react with lubrication oil of comp. o Should not be corrosive o Good thermal conductivity for rapid heat transfer
• Commonly used refrigerants
1. R11(Trichloro-monofluro methane) or Freon-11:
Properties: non-toxic, non-flammable, non-corrosive. Uses: It is used in small office building and factories for refrigeration.
2. R12 (Dichloro-difluro methane) or Freon-12:
Properties: non-toxic, non-flammable, non-explosive, High COP. Uses: It is used in domestic refrigerator.
3. R22 (Monochloro-difloro methane) or Freon-22:
Properties: non-toxic, non-flammable, non-explosive, required less compressor displacement. Uses: It is used in air conditioning.
4. NH 3 (Ammonia)
Properties: Highly-toxic, flammable, good thermal properties, highest refrigerating effect per kg of refrigerant. Uses: It is widely used in large industrial and commercial application. Mostly used with vapour absorption cycle like ice plants, cold storage etc.
5. CO 2 (Carbon dioxide)
Properties: Colourless, non-toxic, non-flammable, non-corrosive, low refrigerating effect per kg of refrigerant. Uses: It is used in marine refrigeration system.
6. Air
Properties: Colourless, non-toxic, easily available without cost, low COP. Uses: It is used in aircraft air-conditioning system.
• Vapour Compression Refrigeration (VCR) System
The Vapor Compression Refrigeration Cycle is nearly 200 years old and the cycle is still applicable in the industrial sphere. Basically, vapor-compression refrigeration employs a heat engine run backwards, so heat energy is taken from a cold reservoir and deposited into a hot reservoir. By the Second Law of Thermodynamics, heat energy does not spontaneously transfer from a cold to a hot reservoir. In order to have heat transfer in that direction (and not from hot to cold, as the system is naturally inclined to do), it is necessary to do work on the system. The vapor-compression uses a circulating liquid refrigerant as the medium (Such as R134a, ammonia) which absorbs and removes heat from the space to be cooled and subsequently rejects that heat elsewhere. The Vapor Compression Refrigeration Cycle involves four components: compressor, condenser, expansion valve/throttle valve and evaporator.
Step 1: Compression (Isentropic) The refrigerant enters the compressor at low temperature and low pressure. It is in a gaseous state. Here, compression takes place to raise the temperature and refrigerant pressure. The refrigerant leaves the compressor and enters to the condenser. Since this process requires work, an electric motor may be used. Compressors themselves can be scroll, screw, centrifugal or reciprocating types. Compressor is generally painted black and at the bottom side of domestic refrigerator. Compression work (Wc) = h 2 - h 1 (kJ/kg) Step 2: Condensation (Isobaric) The condenser is essentially a non-contact type heat exchanger. The superheated refrigeration vapour is converted in to saturated liquid refrigeration by rejecting heat in to atmosphere. It may be air-cooled or water cooled depending upon the capacity of refrigeration. In domestic refrigerator, condenser is on back side of refrigeration. It is having small diameter of tubes. Heat rejection in condenser (Qc) = h 2 - h 3 (kJ/kg) Step 3: Throttling and Expansion (Isenthalpic) When the refrigerant enters the throttling valve, it expands and releases pressure. Consequently, the temperature drops at this stage. Because of these changes, the refrigerant leaves the throttle valve as a liquid vapor mixture. Throttling valves play two crucial roles in the vapor compression cycle. First, they maintain a pressure differential between low- and high-pressure sides. Second, they control the amount of liquid refrigerant entering the evaporator. An expansion valve or capillary tube (a tube having very small diameter and long length) can be used as a throttling device. During this process, the enthalpy of refrigerant remains constant. h 3 = h 4 Step 4: Evaporation (Isobaric) At this stage of the Vapor Compression Refrigeration Cycle, the refrigerant is at a lower temperature than its surroundings. Therefore, it absorbs the heat from the space to be cooled and gets evaporated. Heat extraction from the refrigerant happens at low pressure and temperature. An evaporator is also known as cooling coil. Refrigerating (Cooling) Effect = h 1 – h 4 (kJ/kg) Suction line conveys the low pressure vapor from the evaporator to the suction inlet of the compressor. Discharge line delivers high pressure, high temperature vapor from the discharge valve of the compressor to the condenser. Receiver tank stores the liquid refrigerant coming from condenser and supplies it to the evaporator as per requirement. The liquid line carries the liquid refrigerant from receiver tank to the expansion valve.
and at the same time causes it to partly evaporate. Hence, fluid will come out from the expansion valve as a very wet vapour and at a very low temperature. At point 4, the very wet vapour refrigerant enters the evaporator pipes, and as its (wet vapour refrigerant) temperature is below that of the space to be cooled, it absorbs (takes) its latent heat of evaporation from the space to be cooled and further evaporates (the liquid part of the very wet vapor is changed into vapour), producing the refrigerating effect. The refrigerant will thus, leave the evaporator as a fairly dry vapour and will enter the compressor at point 1. This completes the cycle. Processes: 1 - 2 : Isentropic compression in compressor. 2 - 3 : Constant pressure heat rejection in the condenser. 3 - 4 : Throttling in an expansion valve. (Isenthalpic) 4 - 1 : Constant pressure heat addition in the evaporator. 𝐶𝑂𝑃 𝑜𝑓 𝑅𝑒𝑓𝑟𝑖𝑔𝑒𝑟𝑎𝑡𝑜𝑟 =
Factors affecting design of refrigeration system: ➢ Refrigerant ➢ Required cooling ➢ Load ➢ Ease of maintenance ➢ Physical space requirements ➢ Availability of utilities (water, power) Advantages of VCR System: ➢ It has smaller size for the given capacity of refrigeration. ➢ It has less running cost. ➢ It can be employed over a large range of temperatures. ➢ The coefficient of performance is high. Disadvantages of VCR System: ➢ The initial cost is high. ➢ The prevention of leakages of the refrigerant is the major problem in this system.
• Vapour Absorption Refrigeration (VAR) System
The major drawback of the vapour compression refrigeration system is that it requires a compressor to compress large volume of vapour which in turn requires large power for its operation. If some means are adopted to reduce this volume of vapour before it is compressed, there would be a reduction in both the bulk of the system and power required to operate it. Some liquids have the ability of absorbing large quantities of certain vapours, thus greatly reducing the total volume. A system that operates on this principle is called a vapour absorption system. Cold water in particular has the ability to absorb very large quantities of ammonia vapour. The solution formed in termed as aqua-ammonia. In this case a pump is used to increase the pressure of the solution to that desired in the condenser. In order to separate ammonia vapour from the solution, aqua-ammonia solution is heated in a heater or a generator. As the boiling temperature of ammonia is much lower than that of water, major part of the ammonia vapour is driven out of the solution by heating. The vapour then passes through the condenser, expansion (throttle) valve and evaporator as in case of vapour compression system. It will be seen from the diagram that an absorber, a pump and a generator (heater) replace the compressor of a vapour compression system described earlier. Otherwise the two systems are same, i.e., both systems are provided with a condenser, an expansion valve and an evaporator. At low pressure, fairly dry vapour leaving the evaporator passes to the absorber where it is dissolved in the weak ammonia solution contained in the absorber. The absorber is cooled by circulating cold water through water pipes. The strong warm ammonia solution thus formed in the absorber is then pumped to the generator and circulated through the system by the liquid
Mechanical energy is supplied through a compressor. Heat energy is utilized. Charging of the refrigerating to the system is easy. Charging of refrigerant is difficult. Energy supplied is ¼ to ½ of the refrigerating effect (less). Energy supplied is about one and half times the refrigerating effect (more). Preventive measure is needed, since liquid refrigerant accumulated in the cylinder may damage the cylinder. Liquid refrigerant has no bad effect on the system. Small systems are compact and large systems are bulky. Small systems are bulky and large systems are compact. Performance (COP and capacity) very sensitive to evaporator temperatures. Performance not very sensitive to evaporator temperatures. Regular maintenance is required. Very low maintenance is required.
• Air Refrigeration System
Refrigerators using air as the working medium (working substance) operate on either reversed Carnot cycle, or reversed Joule cycle, more commonly known as Bell-Coleman cycle. Air as a refrigerant has two outstanding advantages: it is available free of cost, and leakage will not cause any trouble. Thermodynamically, air is a poor refrigerant and was abandoned (given up)
with the development of vapour refrigerants with superior thermodynamic properties. However, air refrigeration is now increasingly used for cooling of aircrafts and cargo ships. In this system, air is taken into the compressor from atmosphere and compressed in an isentropic manner. Due to compression, pressure and temperature of air increases whereas volume decreases. The hot compressed air is cooled in a cooler (heat exchanger) up to the atmospheric temperature (in ideal conditions) at constant pressure. The cooled air is then expanded in an expander. The temperature of the air coming out from the expander is below the atmospheric temperature due to isentropic expansion. The low temperature air coming out from the expander enters into the evaporator (of refrigerated system) and absorbs the heat. This increases the temperature of air from T 4 to T 1. In order to reduce the work supplied & to increase the efficiency of the system work produced by compressor is used to drive the expander. The cycle is repeated again. Advantages: ➢ Air is a cheaper refrigerant and available easily compared to other refrigerants. ➢ There is no danger of fire or toxic effects due to leakage. ➢ The total weight of the system per ton of refrigerating capacity is less. Disadvantages: ➢ The quantity of air required per ton refrigerating capacity is far greater than other systems. ➢ The COP is low and hence maintenance cost is high. ➢ The danger of frosting at the expander valves is more as the air taken into the system always contains moisture.
• Air Conditioning System
Air conditioning may be defined as the simultaneous control of temperature, humidity (moisture content), motion of air, (movement and circulation), and purity (filtering and purification) of air within an enclosed space for the purpose of: ➢ Producing comfortable and healthy conditions for the occupants (persons) in residences, theatres, office buildings, hospitals, railway coaches, etc. ➢ Increasing efficiency of the employees working in factories, offices and stores. Conditioned air has a comfort effect, a health effect, and a psychological or emotional effect on human beings, and ➢ Making possible more complete environmental condition for manufacturing processes in industries - such industries include printing, textile manufacturing, chemical, pharmaceutical, etc.
• Window Air Conditioning System
These types of AC are designed to be fitted in window sills. A single unit of Window Air Conditioner houses all the necessary components, namely the compressor, condenser, expansion valve or coil, evaporator and cooling coil enclosed in a single box. Since a window AC is a single unit, it takes less effort to install as well as for maintenance. This is the most commonly used air conditioner for single rooms.
It is the simplest form of an air conditioning system and is mounted on windows or walls. It is a single unit that is assembled in a casing where all the components are located. This refrigeration unit has a double shaft fan motor with fans mounted on both sides of the motor. One at the evaporator side and the other at the condenser side. The evaporator side is located facing the room for cooling of the space and the condenser side outdoor for heat rejection. There is an insulated partition separating this two sides within the same casing. Front Panel The front panel is the one that is seen by the user from inside the room where it is installed and has a user interfaced control be it electronically or mechanically. Older unit usually are of mechanical control type with rotary knobs to control the temperature and fan speed of the air conditioner. The newer units come with electronic control system where the functions are controlled using remote control and touch panel with digital display. The front panel has adjustable horizontal and vertical (some models) louvers where the direction of air flow is adjustable to suit the comfort of the users. The fresh intake of air called VENT (ventilation) is provided at the panel in the event that user would like to have a certain amount of fresh air from the outside. Various Components and its functions Cooling Coil with an air filter mounted on it. The cooling coil is where the heat exchange happens between the refrigerant in the system and the air in the room. Fan Blower is a centrifugal evaporator blower to discharge the cool air to the room. Capillary Tube is used as an expansion device. It can be noisy during operation if installed too near the evaporator. Operation Panel is used to control the temperature and speed of the blower fan. A thermostat is used to sense the return air temperature and another one to monitor the temperature of the coil. Type of control can be mechanical or electronic type. Filter Drier is used to remove the moisture from the refrigerant. Drain Pan is used to contain the water that condensate from the cooling coil and is discharged out to the outdoor by gravity. Compressor is used to compress the refrigerant. Condenser Coil is used to reject heat from the refrigeration to the outside air. Propeller Fan is used in air-cooled condenser to help move the air molecules over the surface of the condensing coil.
Operations During operation, a thermostat is mounted on the return air of the unit. This temperature is used to control the on or off of the compressor. Once the room temperature has been achieved, the compressor cuts off. The evaporator blower fan will suck the air from the room to be conditioned through the air filter and the cooling coil. Air that has been conditioned is then discharge to deliver the cool and dehumidified air back to the room. This air mixes with the room air to bring down the temperature and humidity level of the room. The introduction of fresh air from outside the room is done through the damper which is then mixed with the return air from the room before passing it over the air filter and the cooling coil. The air filter which is mounted in front of the evaporator acts as a filter to keep the cooling coil clean to obtain good heat-transfer from the coil. Hence, regular washing and cleaning of the air filter is a good practice to ensure efficient operation of the air conditioner. The window air conditioner works on Vapour Compression Refrigeration cycle as discussed earlier. It is available in size up to 2 tons capacity. Disadvantages of Window Air Conditioner : ➢ It produces noise in the room because of compressor is very near to the room. ➢ The evaporator and condenser are enclosed in single unit. Therefore, evaporator cannot be used as an interior of room because condenser requires outside air for cooling. ➢ It requires appropriate size of window or hole in wall in order to fit conditioner.
• Split Air Conditioning System
The split air conditioner can be considered as bifurcation of window air conditioner into two units i.e., the indoor unit and the outdoor unit. The indoor unit consists of evaporator coil (cooling coil) with blower driven by an electric motor and capillary tube. The cooling coil is mounted in a cabinet and is fixed inside the room. Hence, it is named as indoor unit. The cooling coil may be wall mounted at certain convenient height or a floor mounted with suitable base frame or ceiling mounted. The outdoor unit consists of compressor and condenser with separate fan driven by an electric motor. They are kept inside the casing and mounted outside the room. The indoor and outdoor units are connected by small diameter tubes. Therefore, a small hole is required in wall for installation of split air conditioner. This modification in window air conditioner will eliminate all the disadvantages of window air conditioner. The split air conditioning units are available in capacity range 1 to 3 tons. The distance between indoor and outdoor unit should not be more than 10 to 12 m. Also, elevation difference between indoor and outdoor unit should not be more than 5 m. Working of split air conditioner is similar to that of window air conditioner as both work on vapour compression refrigeration (VCR) cycle.
