When operating in a refrigeration cycle, an air cycle unit can also produce heat at a useful ..... It has a double shaft where the indoor blower and outdoor.
DEPARTMENT OF MECHANICAL ENGINEERING
LAB MANUAL SUBJECT: REFRIGERATION & AIR CONDITIONING (2161908) th
B.E - 6 Semester (Mechanical Engineering) Compiled by Dr. D.B. Jani (PhD IIT-Roorkee)
Government Engineering College – Dahod Affiliated to GUJARAT TECHNOLOGICAL UNIVERSITY (GTU) 1
SR. NO.
NAME OF EXPERIMENTS
1
Study of air-refrigeration cycle.
2
Study vapor compression cycle.
3 4 5 6 7
Study of different vapor absorption refrigeration systems and to calculate the COP of Electrolux vapor absorption system. Study of various components of VCR system. To understand various tools used for refrigeration tubing and various operations like flaring, bending, brazing. To understand the different psychrometric processes and analyze the same using psychrometric chart. To calculate the cooling load of the confined space and compare the same with load estimation sheet.
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To study domestic refrigerator.
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To study air-washer.
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To study about window and split type air-conditioner.
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To study about different types of refrigerants.
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1. Aim:- Study of air-refrigeration cycle.
Why use air cycle? Air cycle systems have specific advantages that apply to all potential applications: • The working fluid (air) is free, environmentally benign, safe and non-toxic • Air cycle equipment is extremely reliable, reducing maintenance costs and system down-time • The performance of an air cycle unit does not deteriorate as much as that of a vapour-compression unit when operating away from its design point • When operating in a refrigeration cycle, an air cycle unit can also produce heat at a useful temperature. If this is used together with the cooling, highly efficient, low energy processes are possible • Air cycle units can produce a much higher temperature difference between the hot and cold sides compared to vapour-compression units. This means that: o Very cold air can be produced for nearcryogenic processes o Heat can be produced at a useful temperature, which, if used together with the cooling, can result in highly efficient, low energy processes.
How does air cycle work? Air cycle refrigeration works on the reverse Brayton or Joule cycle. Air is compressed and then heat removed, this air is then expanded to a lower temperature than before it was compressed. Work must be taken out of the air during the expansion, otherwise the entropy would increase. Work is taken out of the air by an expansion turbine, which removes energy as the blades are driven round by the expanding air. This work can be usefully employed to run other devices, such as generators or fans. Often, though, it is used to power a directly connected (bootstrap) compressor, which elevates the compressed (hot) side pressure further without added external energy input, essentially recycling the energy removed from the expanding air to compress the high pressure air further. The increase in pressure on the hot side further elevates the temperature and makes the air cycle system produce more useable heat (at a higher temperature). The cold air after the turbine can be used as a refrigerant either directly in an open system, or indirectly by means of a heat exchanger in a closed system. The efficiency of such systems is limited to a great extent by the efficiencies of compression and expansion, as well as those of the heat exchangers employed. Originally, slow speed reciprocating compressors and expanders were used. The poor efficiency and reliability of such machinery were major factors in the replacement of such systems with vapour compression equipment. However, the development of rotary compressors and expanders (such as in car turbochargers) greatly improved the isentropic efficiency and reliability of the air cycle. Advances in turbine technology, together with the development of air bearings and ceramic components offer further efficiency improvements. Combining these advances with newly available, compact heat exchangers, which have greatly improved heat transfer characteristics, makes competition with many existing vapour compression quite feasible. 3
The components of the air refrigeration system are shown in Fig.6.3 (a). In this system, air is taken into the compressor from atmosphere and compressed. The hot compressed air is cooled in heat exchanger up to the atmospheric temperature (in ideal conditions). 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 and absorbs the heat. The cycle is repeated again. The working of air-refrigeration cycle is represented on p-v and T-s diagrams in Fig.6.3 (b) and (c).
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Process 1-2 represents the suction of air into the compressor. Process 2-3 represents the isentropic compression of air by the compressor. Process 3-5 represents the discharge of high pressure air from the compressor into the heat exchanger. The reduction in volume of air from v3 to v5 is due to the cooling of air in the heat exchanger. Process 5-6 represents the isentropic expansion of air in the expander. Process 6-2 represents the absorption of heat from the evaporator at constant pressure.
Analysis of Bell-Coleman Cycle: The air refrigeration system works on Bell-Coleman cycle.
Assumptions: 1) The compression and expansion processes are reversible adiabatic processes. 2) There is a perfect inter-cooling in the heat exchanger. 3) There are no pressure losses in the system. COP = Net refrigeration effect/Net work sup plied
Work done per kg of air for the isentropic compression process 2-3 is given by, WC = Cp (T3 - T2 ) Work developed per kg of air for the isentropic expansion process 5-6 is given by, WE = Cp (T5 - T6 ) Net work required = Wnet = (WC - WE ) = Cp (T3 - T2 ) - Cp (T5 - T6 )
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2. Aim:- Study of vapor-compression refrigeration cycle.
Vapour compression refrigeration cycle A vapour compression refrigeration system is an improved type of air refrigeration system in which a suitable working substance, termed as refrigerant is used. It condenses and evaporates at temperatures and pressures close to the atmospheric conditions. The refrigerant used does not leave the system but is circulated throughout the system alternately condensing and evaporating. The vapour compression refrigeration system is now days used for all-purpose refrigeration. It is used for all industrial purpose from a small domestic refrigerator to a big air conditioning plant. The vapour compression refrigeration cycle is based on the following factor:
Refrigerant flow rate. Type of refrigerant used. Kind of application viz air-conditioning, refrigeration, dehumidification etc. The operation design parameters. The system equipments/ components proposed to be used in the system.
The vapour compression refrigeration cycle is based on a circulating fluid media, viz, a refrigerant having special properties of vaporizing at temperatures lower than the ambient and condensing back to the liquid form, at slightly higher than ambient conditions by controlling the saturation temperature and pressure. Thus, when the refrigerant evaporates or boils at temperatures lower than ambient, it extracts or removes heat from the load and lower the temperature consequently providing cooling. The super-heated vapour pressure is increased to a level by the compressor to reach a saturation pressure so that heat added to vapour is dissipated/ rejected into the atmosphere, using operational ambient conditions, with cooling medias the liquid from and recycled again to form the refrigeration cycle. The components used are:
1. Evaporator 2. Compressor 3. Condenser and receiver 4. Throttling device The working of vapour compression refrigeration cycle and function of each above component is given below. 7
Fig. 2.1 Components of vapour refrigeration system
(a)
Evaporator:
The liquid refrigerant from the condenser at high pressure is fed through a throttling device to an evaporator at a low pressure. On absorbing the heat to be extracted from Media to be cooled, the liquid refrigerant boils actively in the evaporator and changes state.
The
refrigerant gains latent heat to vaporizes at saturation temperature/ pressure and further absorbs sensible heat from media to be cooled and gets fully vaporized and super heated. The “temperature-pressure relation chart” table can determine the pressure and temperature in the evaporator. (b)
Compressor:
The low temperature, pressure, superheated vapour from the evaporator is conveyed through suction line and compressed by the compressor to a high pressure, without any change of gaseous state and the same is discharge into condenser. During this process heat is added to the refrigerant and known as heat of compression ratio to raise the pressure of refrigerant to such a level that the saturation temperature of the discharge refrigerant is higher than the
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temperature of the available cooling medium, to enable the super heated refrigerant to condense at normal ambient condition. (c) Condenser: The heat added in the evaporator and compressor to the refrigerant is rejected in condenser at high temperature/ high pressure. This super heated refrigerant vapour enters the condenser to dissipate its heat in three stages. First on entry the refrigerant loses its super heat, it then loses its latent heat at which the refrigerant is liquefied at saturation temperature pressure. This liquid loses its sensible heat, further and the refrigerant leaves the condenser as a sub cooled liquid. The heat transfer from refrigerant to cooling medium (air or water) takes place in the condenser. The sub-cooled liquid from condenser is collected in a receiver (wherever provided) and is then fed through the throttling device by liquid line to the evaporator. There are several methods of dissipating the rejected heat into the atmosphere by condenser. These are water-cooled, air cooled or evaporative cooled condensers. In the water-cooled condenser there are several types viz. Shell and tube, shell and coil, tube in tube etc. In Evaporative cooled condenser, both air and water are used. Air-cooled condensers are prime surface type, finned type or plate type. The selecting of the type depends upon the application and availability of soft water. (d) Throttling device: The high-pressure liquid from the condenser is fed to evaporator through device, which should be designed to pass maximum possible liquid refrigerant to obtain a good refrigeration effect. The liquid line should be properly sized to have minimum pressure drop. The throttling device is a pressure-reducing device and a regulator for controlling the refrigerant flow. It also reduces the pressure from the discharge pressure to the evaporator pressure without any change of state of the pressure refrigerant. The types of throttling devices are: Capillary tubes Thermostatic expansion valves Hand expansion valves Hand valves.
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The most commonly used throttling device is the capillary tube for application upto approx. 10 refrigeration tons. The capillary is a copper tube having a small dia-orifice and is selected, based on the system design, the refrigerant flow rate, the operating parameters (such as suction and discharge pressures), type of refrigerant, capable of compensating any variations/ fluctuations in load by allowing only liquid refrigerant to flow to the evaporator.
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3. Aim: Study of different vapor absorption refrigeration systems and to calculate the COP of Electrolux vapor absorption system. The vapour absorption refrigeration system is one of the oldest method of producing refrigerating effect. The principle of vapour absorption was first discovered by Michael Faraday in 1824 while performing a set of experiments to liquefy certain gases. A french scientist Ferdinand carre developed the first vapour absorption refrigeration machine in 1860. This system may be used in both the domestic and large industrial refrigerating plants. The refrigerant, commonly used in a vapour absorption system, is ammonia. The vapour absorption system uses heat energy, instead of mechanical energy as in vapour compression systems, in order to change the conditions of the refrigerant required for the operation of the refrigeration cycle. In the vapour absorption system (Fig.3.1), an absorber, a pump, a generator and a pressurereducing valve replace the compressor. These components in vapour absorption system perform the same function as that of a compressor in vapour compression system. In this system, the vapour refrigerant from the evaporator is drawn into an absorber where it is absorbed by the week solution of the refrigerant forming a strong solution. This strong solution is pumped to the generator where it is heated by some external source. During the heating process, the vapour refrigerant is driven off by the solution and enters into the condenser where it is liquefied. The liquid refrigerant then flows into the evaporator and thus the cycle is completed.
Fig. 3.1 Electrolux vapour absorption system 11
Working: The domestic absorption type refrigerator was invented by two Swedish engineers Carl Munters and Baltzer Van Platan in 1925 while they were studying for their under-graduate course of royal institute of technology in Stockholm. The idea was first developed by the ‘Electrolux Company’ of Luton, England. This type of refrigerator is also called three- fluids absorption system. The main purpose of this system is to eliminate the pump so that in the absence of moving parts, the machine becomes noise-less. The three fluids used in this system are ammonia, hydrogen and water. The ammonia is used as a refrigerant because it possesses most of the desirable properties. It is toxic, but due to absence of moving parts, there are very little changes for the leakage and the total amount of refrigeration used is small. The hydrogen being the lightest gas is used to increase the rate of evaporation of the liquid ammonia passing through the evaporator. The hydrogen is also non-corrosive and insoluble in water. This is used in the lowpressure side of the system. The water is used as a solvent because it has the ability to absorb ammonia readily. The strong ammonia solution from the absorber through heat exchanger is heated in the generator by applying heat from an external source usually a gas burner. During this heating process, ammonia vapour are removed from the solution and passed to the condenser. A rectifier or a water separator fitted before the condenser removes water vapour carried with the ammonia vapour, so that dry ammonia vapour are supplied to The condenser. These water vapour, if not removed, they will enter into the evaporator causing freezing and choking of the machine. The hot weak solution while passing through the exchanger is cooled. The heat removed by the weak solution is utilized in raising the temperature of strong solution passing through the heat exchanger. In this way, the absorption is accelerated and the improvement in the performance of a plant is achieved. The ammonia vapour in the condenser is condensed by using external cooling source. The liquid refrigerant leaving the condenser flows under gravity to the evaporator where it meets the hydrogen gas. The hydrogen gas which is being fed to the evaporator permits the liquid ammonia to evaporate at a low pressure and temperature according to Dalton’s principal. During the process of evaporation, the ammonia absorbs latent heat from the refrigerated space and thus produces cooling effect. The mixture of ammonia vapour and hydrogen is passed to the absorber where ammonia is absorbed in water while the hydrogen rises to the top and flows back to the evaporator.
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The main disadvantage of electrolux refrigerator is: It can not be used for industrial purpose as the COP of the system is very low.
Result: Vapour refrigeration refrigeration system. Viva voce Questions Q1 What is the function of water in this system? Q2 What is the function of the hydrogen in this system? Q3 What is the function of the generator in this system? Q4 What is the function of the rectifier/dehydrator in the system? Q5 Is there is compressor is present in this type of system?
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4. Aim: To study various components of VCR system. Indoor Side Components: The indoor parts of a window air conditioner include:
Cooling Coil with a air filter mounted on it. The cooling coil is where the heat exchange happen 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.
Outdoor Side Components: The outdoor side parts include: ·
Compressor is used to compress the refrigerant.
·
Condenser Coil is used to reject heat from the refrigeratn 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.
·
Fan Motor is located here. It has a double shaft where the indoor blower and outdoor propeller fan are connected together.
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.
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Usually, it has to be off for at least 3 minutes before turning on again to prevent it from being damaged. For mechanical control type, there is usually a caution to turn on the unit after the unit has turned off for at least 3 minutes. For electronic control, there is usually a timer to automatically control the cut-in and cut-out of compressor. 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 compressors are one of the most important parts of the refrigeration cycle. The compressor compresses the refrigerant, which flows to the condenser, where it gets cooled. It then moves to the expansion valve, and the evaporator and it is finally sucked by the compressor again. Types of compressors: Mainly following type of compressors are used. 1 Reciprocating, 2 Rotary, 3 Screw, 4 Centrifugal and 5 Scroll. All these have been described below briefly: 1) Reciprocating Compressors:
The reciprocating and screw compressors are best suited for use with refrigerants which require a relatively small displacement and condense at relatively high pressure, such as R-12, R-22, Ammonia, etc. The centrifugal compressors are suitable for handling refrigerants that require large displacement and operate at low condensing pressure, such as R-11, R-113, etc. The rotary compressor is most suited for pumping refrigerants having moderate or low condensing pressures, such as R-21 and R-114; this is mainly used in domestic. The reciprocating compressors are one of the most widely used types of the refrigerating compressors. They have piston and cylinder arrangement like the automotive engine. The reciprocating motion of the piston due to external power compresses the refrigerant inside the cylinder. There are three types of reciprocating compressors: hermetically sealed, semi-hermetically sealed and open type. The open of reciprocating compressors can be of single cylinder type or multi-cylinder type. 15
Fig. 4.1 Reciprocating air compressor. 2) Screw Compressors:
The screw compressors comprise of the pair of meshing screws between which the refrigerant gets compressed. They can produce high pressure for small quantity of gas. They consume less power than the reciprocating compressors and are being used widely. It can be used with refrigerants like R12, R22, and others.
Fig. 4.2 Screw, rotary and centrifugal compressors. 16
3) Rotary Compressors:
The rotary compressors have two rotating elements, like gears, between which the refrigerant is compressed. These compressors can pump the refrigerant to lower or moderate condensing pressures. Since they can handle small volume of the gas and produce lesser pressure, they are used in fewer applications. 4) Centrifugal Compressor:
The centrifugal compressors comprise of the impeller or the blower that can handle large quantities of gas but at relatively lower condensing pressure.
Result: Various components of room air conditioner have been studied. Viva Questions: 1. 2. 3. 4. 5.
What do you mean conditioning of air? Explain the working principle of air conditioning system? What are different types of air conditioning systems? What is the function of the blower? What is the function of the filter in front of the evaporator coil?
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5. Aim: To understand various tools used for refrigeration tubing and various operations like flaring, bending, brazing. PERFORMANCE CRITERIA ELEMENT 1.1 OHS procedures for a given work area are 1 Prepare to fabricate identified, obtained and understood through tubing and established routines and procedures attach 1.2 Established OHS risk control measures and fittings for procedures are followed in preparation for the refrigeration work. and/or air conditioning systems 1.3 Safety hazards which have not previously been identified are reported and advice on risk control measures is sought from the work supervisor. 1.4 The nature of work is obtained from documentation or from work supervisor to establish the scope of work to be undertaken. 1.5 Advice is sought from the work supervisor to ensure the work is coordinated effectively with others. 1.6 Sources of materials that may be required for the work are accessed in accordance with established routines and procedures. 1.7 Tools, equipment and testing devices needed to carry out the work are obtained and checked for correct operation and safety 2.1 Established OHS risk control measures and 2 Fabricate tubing and procedures for carrying out the work are attach followed. fittings for 2.2 Work in strict accordance with OHS refrigeration requirements and when necessary conducted and/or air within established safety procedures conditioning systems 2.3 Established methods used to cut, flare, swage, bend, silver braze tubing and fittings as they apply to the refrigeration/air conditioning equipment arrangements. 2.4 Refrigerant tubing and fittings are silver brazed with the use of dry nitrogen to prevent 18
ELEMENT
PERFORMANCE CRITERIA contamination. 2.5 Fabricate tubing and attach fittings are prepared efficiently without waste of materials or damage/contamination to apparatus and the surrounding environment or services and using sustainable energy practices. 2.6 Routine quality checks are carried out in accordance with work instructions/or specifications including dimensions and pressure testing.
3 Complete work and report
3.1 OHS work completion risk control measures and procedures are followed. 3.2 Work site is cleaned and made safe in accordance with established procedures. 3.3 Work supervisor is notified of the completion of the work in accordance with established procedures.
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REQUIRED SKILLS AND KNOWLEDGE: 7) This describes the essential skills and knowledge and their level, required for this unit. Evidence shall show that knowledge has been acquired of safe working practices and preparing refrigerant tubing and fittings. All knowledge and skills detailed in this unit should be contextualised to current industry practices and technologies. KS01-EJ102A techniques
Refrigerant tubing and fittings
Evidence shall show an understanding of cutting, bending and joining refrigeration piping and tubing tools, equipment and techniques, applying safe working practices and relevant Standards, Codes and Regulations to an extent indicated by the following aspects: T1 Piping
T2
T3
T4
Refrigeration & water grade copper tube Maintaining cleanliness (always capped, do not blow out with mouth etc) Soft and hard drawn tube Tubing applications (soft, hard, pair coil, water grade etc) Tube qualities - diameter, wall thickness (gauge) and pressure ratings (R410A etc) Pipe insulation (types - tube, slit tube, sheet etc and joining methods - glue, tape etc) Other tube materials (Bundy, steel, aluminum, brass) Cutting Cutting tools (Imps, normal & large pipe cutters, tube cutting rings etc) Precautions while cutting (sharp burrs, sharp blades etc) Deburring tools (reamers, deburrers etc) Bending Bending tools (springs, levers, presses etc) Precautions while bending (work hardening, collapsing etc) Bending hard drawn tube - the process of annealing Joining Flare nuts (plain, short barrel, frost proof, reducing) Flaring tools (flare block, eccentric with clutch for high pressure tube) 20
PERFORMANCE CRITERIA ELEMENT Precautions while flaring (deburred, length past block face, cleanliness) Swaging tools (punch, flare block, expander etc) Precautions while swaging (length past block face, tube shortening effect, cleanliness etc) Other tube fittings (BSP to flare elbows, tees, unions, plugs, flare washers, okrings etc) Thread sealants (tapes, pastes etc) Access valves (Schrader, piercing, cut-away of service valve/s) Precautions using access valves (refrigerant leakage, core removal, limitations on piercing valves etc) T5
T6
T7
Soldering and brazing equipment Gas types (oxy acetylene, air acetylene, propane, Mapp gas) Hazards associated with their use (cylinder transport, remove regulator, oil & oxy = bang) Personal safety (MSDS - oxy, acetylene, propane, MAPP gas) Flash back arrestors Setting up equipment (fitting regulator, adjusting pressures, tip selection) Igniting and flame types (flint guns, oxidising, neutral, carburising) Care and maintenance of equipment (hoses, regulator, tips, cylinders, flash back arrestors) Silver solder Types (yellow, brown, blue and their metal components) Personal safety (MSDS - silver brazing alloys) Flux and its use (dissimilar metals) Personal safety (MSDS - flux) Preparing surfaces (removing oxides, oils, applying flux) Soldering techniques Dry nitrogen Personal safety (MSDS - nitrogen) Applying dry nitrogen to a piping circuit Silver soldering copper to copper Silver soldering copper to dissimilar metals Annealing copper tube
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6.
Aim: To understand the different psychrometric processes and analyze the same using psychrometric chart.
In the design and analysis of air conditioning plants, the fundamental requirement is to identify the various processes being performed on air. Once identified, the processes can be analyzed by applying the laws of conservation of mass and energy. All these processes can be plotted easily on a psychrometric chart. This is very useful for quick visualization and also for identifying the changes taking place in important properties such as temperature, humidity ratio, enthalpy etc. The important processes that air undergoes in a typical air conditioning plant are discussed below. a) Sensible cooling: During this process, the moisture content of air remains constant but its temperature decreases as it flows over a cooling coil. For moisture content to remain constant, the surface of the cooling coil should be dry and its surface temperature should be greater than the dew point temperature of air. If the cooling coil is 100% effective, then the exit temperature of air will be equal to the coil temperature. However, in practice, the exit air temperature will be higher than the cooling coil temperature. Figure 28.1 shows the sensible cooling process O-A on a psychrometric chart. The heat transfer rate during this process is given by:
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b) Sensible heating (Process O-B): During this process, the moisture content of air remains constant and its temperature increases as it flows over a heating coil. The heat transfer rate during this process is given by:
c) Cooling and dehumidification (Process O-C): When moist air is cooled below its dew-point by bringing it in contact with a cold surface as shown in Fig.28.3, some of the water vapor in the air condenses and leaves the air stream as liquid, as a result both the temperature and humidity ratio of air decreases as shown. This is the process air undergoes in a typical air conditioning system. Although the actual process path will vary depending upon the type of cold surface, the surface temperature, and flow conditions, for simplicity the process line is assumed to be a straight line. The heat and mass transfer rates can be expressed in terms of the initial and final conditions by applying the conservation of mass and conservation of energy equations as given below:
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d) Heating and Humidification (Process O-D): During winter it is essential to heat and humidify the room air for comfort. As shown in Fig.28.5., this is normally done by first sensibly heating the air and then adding water vapour to the air stream through steam nozzles as shown in the figure.
e) Cooling & humidification (Process O-E): As the name implies, during this process, the air temperature drops and its humidity increases. This process is shown in Fig.28.6. As shown in the figure, this can be achieved by spraying cool water in the air stream. The temperature of water should be lower than the dry-bulb temperature of air but higher than its dew-point temperature to avoid condensation (T< T< TDPT w O).
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Questions and answers: 1. Which of the following statements are TRUE? a) During sensible cooling of air, both dry bulb and wet bulb temperatures decrease b) During sensible cooling of air, dry bulb temperature decreases but wet bulb temperature remains constant c) During sensible cooling of air, dry and wet bulb temperatures decrease but dew point temperature remains constant d) During sensible cooling of air, dry bulb, wet bulb and dew point temperatures decrease Ans.: a) and c) 2. Which of the following statements are TRUE? a) The sensible heat factor for a sensible heating process is 1.0 b) The sensible heat factor for a sensible cooling process is 0.0 c) Sensible heat factor always lies between 0.0 and 1.0 d) Sensible heat factor is low for air conditioning plants operating in humid climates Ans.: a) and d) 3. Which of the following statements are TRUE? a) As the by-pass factor (BPF) of the cooling coil increases, temperature difference between air at the outlet of the coil and coil ADP decreases b) The BPF of the coil increases as the velocity of air through the coil increases c) The BPF of the coil increases as the fin pitch increases d) The BPF of the coil decreases as the number of rows in the flow direction increase Ans.: b), c) and d) 4. Which of the following statements are TRUE? a) During cooling and humidification process, the enthalpy of air decreases b) During cooling and humidification process, the enthalpy of air increases c) During cooling and humidification process, the enthalpy of air remains constant d) During cooling and humidification process, the enthalpy of air may increase, decrease or remain constant depending upon the temperature of the wet surface Ans.: d) 5. An air stream at a flow rate of 1 kg/s and a DBT of 30C mixes adiabatically with another air stream flowing with a mass flow rate of 2 kg/s and at a DBT of 15oC. Assuming no condensation to take place, the temperature of the mixture is approximately equal to: a) 20oC b) 22.5oC c) 25oC d) Cannot be found Ans.: a) 6. Which of the following statements are TRUE? a) In an air washer, water has to be externally cooled if the temperature at which it is sprayed is equal to the dry bulb temperature of air b) In an air washer, water has to be externally heated if the temperature at which it is sprayed is equal to the dry bulb temperature of air c) In an air washer, if water is simply recirculated, then the enthalpy of air remains nearly constant at steady state 25
d) In an air washer, if water is simply recirculated, then the moisture content of air remains nearly constant at steady state Ans.: b) and c) 7. Which of the following statements are TRUE? a) When the enthalpy of air is equal to the enthalpy of saturated air at the wetted surface temperature, then there is no sensible heat transfer between air and the wetted surface b) When the enthalpy of air is equal to the enthalpy of saturated air at the wetted surface temperature, then there is no latent heat transfer between air and the wetted surface c) When the enthalpy of air is equal to the enthalpy of saturated air at the wetted surface temperature, then there is no net heat transfer between air and the wetted surface d) When the enthalpy of air is equal to the enthalpy of saturated air at the wetted surface temperature, then the wet bulb temperature of air remains constant Ans.: c) and d)
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7. Aim: To calculate the cooling load of the confined space and compare the same with load estimation sheet.
The heating and cooling load calculation is the first step of the iterative HVAC design procedure; a full HVAC design involves more than the just the load estimate calculation. Right-sizing the HVAC system, selecting HVAC equipment and designing the air distribution system to meet the accurate predicted heating and cooling loads, begins with an accurate understanding of the heating and cooling loads on a space. The Air Conditioning Contractors of America (ACCA) Manual J Version 8 provides the detailed steps required to calculate the heating and cooling loads. The accurate heating and cooling loads are used to right-size the equipment with ACCA Manual S Residential Equipment Selection, then to design the air distribution system and ductwork with ACCA Manual T Air Distribution Basics for Residential and Small Commercial Buildings and ACCA Manual D Residential Duct System Procedure. The Strategy Guideline: Accurate Heating and Cooling Load Calculations report provides information for the following groups: • Heating Ventilation and Air Conditioning (HVAC) Mechanical Contractors • HVAC System Designers • Builders • House Remodelers. calculations were manipulated for: • Outdoor/Indoor Design Conditions • Building Components • Ductwork Conditions • Ventilation/Infiltration Conditions • Worst Case Scenario (combining all the safety factors) Building Components Building construction, proper details, and materials are critical components of the heating and cooling load calculations. The R-value of the building wall, roof, and foundation construction components can be accurately calculated using the insulation levels specified combined with the remainder of the components that make up the construction assembly (i.e. drywall, sheathing, exterior siding materials, structural framing system, roofing materials, etc.). The window performance, described by the U-value and SHGC, must be known and accurately represented by the data input. Shading provided by the overhang of eaves, insect screens, and internal blinds or shades will reduce the sensible heat gain. If shading is ignored in the load calculation the cooling load will be inflated. 27
Heating and Cooling System Location and Duct Leakage: Best practice for HVAC design is to keep all ductwork within the conditioned space in order to eliminate the duct losses/gains to and from the outside conditions. Scenarios, such as the onestory slab-on-grade Orlando House, present challenges in keeping all ductwork inside conditioned spaces. In a slab-on-grade house, it is typical for an installer to put the HVAC system completely in the attic. Because it has a basement, the Chicago House does not present the same challenges to keeping the ductwork inside conditioned space. In a single-story house with a basement, the duct system is typically run in the basement, which is considered conditioned space provided the basement walls are insulated or there are supply registers in the basement. For ducts outside conditioned space, the heating and cooling loads are more sensitive to duct leakage and R-values of the duct insulation. Comfort - Space Temperatures Short cycling limits the total amount of air circulating through each room, and can lead to rooms that do not receive adequate duration of airflow. Short cycling of an oversized system can lead to comfort complaints when the spaces located further from the thermostat do not change temperature as quickly as spaces near the thermostat. Even in an energy-efficient house with an enhanced thermal enclosure, this can lead some rooms being colder during the heating season and warmer in the cooling season. In attempt to make the spaces further from the thermostat more comfortable, the occupant may set the thermostat set point higher, requiring additional energy. Comfort Humidity Control The risks associated with oversizing the cooling system, particularly in more humid climates, are also a concern. In the cooling season in humid climates, cold clammy conditions can occur due to reduced dehumidification caused by the short cycling of the equipment. The cooling system removes moisture from the air by passing the air across a condensing coil. The system must run long enough for the coil to reach a temperature where condensation will occur and an oversized system that short cycles may not run long enough to sufficiently condense moisture from the air. Excess humidity in the conditioned air delivered to a space may lead to mold growth within the house. Space Heat Gain The manner in which it enters the space – a. Solar radiation through transparent surfaces such as windows b. Heat conduction through exterior walls and roofs c. Heat conduction through interior partitions, ceilings and floors d. Heat generated within the space by occupants, lights, appliances, equipment and processes e. Loads as a result of ventilation and infiltration of outdoor air f. Other miscellaneous heat gains 28
Sensible heat - Heat which a substance absorbs, and while its temperature goes up, the substance does not change state. Sensible heat gain is directly added to the conditioned space by conduction, convection, and/or radiation. Note that the sensible heat gain entering the conditioned space does not equal the sensible cooling load during the same time interval because of the stored heat in the building envelope. Only the convective heat becomes cooling load instantaneously. Sensible heat load is total of a. Heat transmitted thru floors, ceilings, walls b. Occupant’s body heat c. Appliance & Light heat d. Solar Heat gain thru glass e. Infiltration of outside air f. Air introduced by Ventilation. Latent Heat Loads - Latent heat gain occurs when moisture is added to the space either from internal sources (e.g. vapor emitted by occupants and equipment) or from outdoor air as a result of infiltration or ventilation to maintain proper indoor air quality. Latent heat load is total of a. Moisture-laden outside air form Infiltration & Ventilation b. Occupant Respiration & Activities c. Moisture from Equipment & Appliances
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COOLING LOAD CALCULATION METHOD a. Transfer Function Method (TFM): This is the most complex of the methods proposed by ASHRAE and requires the use of a computer program or advanced spreadsheet. b. Cooling Load Temperature Differential/Cooling Load Factors (CLTD/CLF): This method is derived from the TFM method and uses tabulated data to simplify the calculation process. The method can be fairly easily transferred into simple spreadsheet programs but has some limitations due to the use of tabulated data. c. Total Equivalent Temperature Differential/Time-Averaging (TETD/TA): This was the preferred method for hand or simple spreadsheet calculation before the introduction of the CLTD/CLF method. Outdoor Design Weather Conditions ASHRAE Handbook 1993 Fundamentals (Chapter 26) list tables of climate conditions for the US, Canada and other International locations: In these tables: The information provided in table 1a, 2a and 3a are for heating design conditions that include: a. Dry bulb temperatures corresponding to 99.6% and 99% annual cumulative frequency of occurrence. b. Wind speeds corresponding to 1%, 2.5% and 5% annual cumulative frequency of occurrence, c. Wind direction most frequently occurring with 99.6% and 0.4% dry-bulb temperatures and d. Average of annual extreme maximum and minimum dry-bulb temperatures and standard deviations. Indoor Design Conditions and Thermal Comfort The indoor design conditions are directly related to human comfort. Current comfort standards, ASHRAE Standard 55-1992 [4] and ISO Standard 7730 [5], specify a “comfort zone,” representing the optimal range and combinations of thermal factors (air temperature, radiant temperature, air velocity, humidity) and personal factors (clothing and activity level) with which at least 80% of the building occupants are expected to express satisfaction. The environmental factors that affect the thermal comfort of the occupants in an air-conditioned space are mainly: a. Metabolic rate, expressed in met (1 met = 18.46 Btu/hr.ft2) determines the amount of heat that must be released from the human body and it depends mainly on the intensity of the physical activity. b. Indoor air temperature (Tr) and mean radiant temperature (Trad), both in °F. Tr affects both the sensible heat exchange and evaporative losses, and Trad affects only sensible heat exchange. c. Relative humidity of the indoor air in %, which is the primary factor that influences evaporative heat loss. d. Air velocity of the indoor air in fpm, which affects the heat transfer coefficients and therefore the sensible heat exchange and evaporative loss. e. Clothing insulation in clo (1 clo = 0.88 h.ft2.°F/Btu), affects the sensible heat loss. Clothing insulation for occupants is typically 0.6 clo in summer and 0.8 to 1.2 clo in winter.
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8. Aim: To study about domestic refrigerator. The household refrigerator works on vapour compression refrigeration cycle. The refrigerant vapour is compressed by means of compressor to a pressure at which temperature obtained at the end of compression will be more then atmosphere so that at this high temperature it will reject heat to atmosphere and will get condensed. The condensate is then allowed to pass through a capillary so that the pressure and temperatures and lowered. Capillary device acts as a throttling unit. At low pressure and temperature refrigerant is supplied to the evaporator where load is kept, it absorbs the heat and refrigerant get converted into gaseous phase and it is again supplied to compressor and cycle is repeated. The evaporator in the household refrigerant is always fitted in the cabinet of the refrigerator at the top potion and the concealed type of evaporator used. The condenser is mounted at the back of the cabinet. The expansion device used in household refrigerator is capillary tube. Capacity of household refrigerator is expressed in terms of litre. The refrigerators manufactured by various manufactures are available in capacities ranging from 90 litres to 380 litres. (The capacity of household refrigerator is expressed in terms of litre, it is defined as the amount of water occupied in the cabinet. It specifies the space available for keeping various commodities in refrigerator.)
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In the household refrigerator the air circulation inside the cabinet is maintained by natural convection. The temperature in freezer is around - 5 to -10 c, the temperature is increased at the bottom most portion where vegetable crisper is kept. Also there is provision for keeping stuff like eggs, water, etc. fitted in the door of refrigerator. The refrigerator body is insulated with insulating materials like PUF (Polyainthene foam). Magnetic strips are provided to avoid thermal leakage through doors. ATTAINMENTS OF FREEZING AND DEFROSTING IN REFRIGERATOR: Freezing and Defrosting done by two ways: 1. Thermostat 2. Defrosting Unit 1. Thermostat: Thermostat is used to control the temperature in the refrigerator by varying time to idle time ratio. The bulb of the thermostat is clamped to the evaporator or freezer. The thermostat bulb is charged with few drops of refrigerant. The temperature at which compressor motor starts, by closing the thermostat contacts is called cut-in temperature. Cut-out temperature is higher then cut-in temperature and the difference between the two is called differential. Higher is the differential, longer is the running time and less is the idle time of refrigerator. By changing range adjustment and differential, any cut-in and cut-out temperature can be adjusted for maintaining desired temperature in the refrigerator. As the temperature of the bulb increases, gas pressure in the bellow assembly increases, and this closes the compressor motor circuit and refrigerator starts. As the compressor runs, the thermostat bulb is cooled; gradually reducing the pressure in the bulb and this opens the circuit when desired temperature is attained. The refrigerator is provided with a control knob. By operating knob desired temperature can be maintained. 2. Defrosting: The freezing of moisture on evaporator coil is called as frosting. The frost thickness increases due to frequent door openings, as the frost thickness increases the heat transfer through the coil decreases. This increases the running time of refrigerator and hence the power consumption. Therefore regular defrosting must be done when frost thickness increases above certain limit. Generally following methods are used for defrosting. i) Defrosting by stopping unit: Stop the unit, keep door open and chill tray must be kept in defrost position till defrosting takes place. ii) Timer Defrosting: The most popular defrost system used in household refrigerator is clock timer defrost cycle. The number of defrost periods varies from one to four in 24 hours depending upon timer used. The timer contacts initiate either the defrost cycle or cooling cycle. When the timer is in the cooling cycle, the thermostat control the on-off periods of the compressor. When the timer is in the defrost cycle. The thermostat cannot turn the compressor ON. In other words, thermostat has no control on the compressor when the defrost timer is in the defrost position. The defrost cycle terminates approximately 20 minutes after being turned on. The defrost heater is wired in series with a bimetal thermostat whose contacts will open at some predetermined temperature, there by disconnecting the heater. The length and time it takes for the contacts of the bimetal thermostat to open is determined by the amount of frost on the evaporator. 32
DO AND DON’T – WHILE USING REFRIGERATOR: 1. The refrigerator should be placed away from the heat source such as sunrays, heating appliance, cooking gas, etc. 2. Install the refrigerator away from wall at least by one foot which provides good air circulation over condenser. 3. Hot fluids should not be kept in refrigerator. 4. Keep door openings at minimum. 5. Strongly flavored food must be kept wrapped. 6. Vegetables, fruits should be kept in polythene bags before placing into the refrigerator. 7. Clean with soft cloth. No soap, detergent should be used. IN HOLIDAYS: 1. Remove every stored item including ice trays. 2. Defrost refrigerator. 3. Make refrigerator dry. 4. Disconnect three-pin plug. 5. Leave the door slightly open for movement of fresh air. RESTARTING: 1. Clean the Refrigerator. 2. Connect 3-pin plug. 3. Load the refrigerator after temperature has stabilized.
Specifications of refrigerator:
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CONCLUSION: The domestic refrigerators now a day are becoming essential part of life. These refrigerators are available in different capacities as well as different working models. These are having single door double door options, frost free refrigerators; quick chill refrigerators are also available. To make the refrigerators smart now a day the condensers are sealed and refrigerators are mode flat back. The compressors used in household refrigerator are hermetically sealed reciprocating units. Now a days noise free rotary hermetically seals compressors are also used. The refrigerant R-12 which was popularly used in household refrigerators is discarded due to its ODP (ozone depletion potential). It is replaced by R-134(a).
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9. Aim: To study about air-washer.
An air washer is a device for conditioning air. As shown in Fig.28.10, in an air washer air comes in direct contact with a spray of water and there will be an exchange of heat and mass (water vapour) between air and water. The outlet condition of air depends upon the temperature of water sprayed in the air washer. Hence, by controlling the water temperature externally, it is possible to control the outlet conditions of air, which then can be used for air conditioning purposes.
In the air washer, the mean temperature of water droplets in contact with air decides the direction nd
of heat and mass transfer. As a consequence of the 2 law, the heat transfer between air and water droplets will be in the direction of decreasing temperature gradient. Similarly, the mass transfer will be in the direction of decreasing vapor pressure gradient. For example,
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a) Cooling and dehumidification: tw < tDPT: Since the exit enthalpy of air is less than its inlet value, from energy balance it can be shown that there is a transfer of total energy from air to water. Hence to continue the process, water has to be externally cooled. Here both latent and sensible heat transfers are from air to water. This is shown by Process O-A in Fig.28.11. b) Adiabatic saturation: tw = tWBT: Here the sensible heat transfer from air to water is exactly equal to latent heat transfer from water to air. Hence, no external cooling or heating of water is required. That is this is a case of pure water recirculation. This is shown by Process O-B in Fig.28.11. This the process that takes place in a perfectly insulated evaporative cooler. c) Cooling and humidification: tDPT < tw < tWBT: Here the sensible heat transfer is from air to water and latent heat transfer is from water to air, but the total heat transfer is from air to water, hence, water has to be cooled externally. This is shown by Process O-C in Fig.28.11. d) Cooling and humidification: tWBT < tw < tDBT: Here the sensible heat transfer is from air to water and latent heat transfer is from water to air, but the total heat transfer is from water to air, hence, water has to be heated externally. This is shown by Process O-D in Fig.28.11. This is the process that takes place in a cooling tower. The air stream extracts heat from the hot water coming from the condenser, and the cooled water is sent back to the condenser. e) Heating and humidification: tw > tDBT: Here both sensible and latent heat transfers are from water to air, hence, water has to be heated externally. This is shown by Process O-E in Fig.28.11. Thus, it can be seen that an air washer works as a year-round air conditioning system. Though air washer is a and extremely useful simple device, it is not commonly used for comfort air conditioning applications due to concerns about health resulting from bacterial or fungal growth on the wetted surfaces. However, it can be used in industrial applications.
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10.
Aim: To study about window and split type air-conditioner.
Window air conditioner is sometimes referred to as room air conditioner as well. 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 are 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. Figure: Components of air conditioning system Indoor Side Components. The indoor parts of a window air conditioner include: ·Cooling Coil with a air filter mounted on it. The cooling coil is where the heat exchange happen 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.
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Outdoor Side Components The outdoor side parts include: · Compressor is used to compress the refrigerant. · Condenser Coil is used to reject heat from the refrigeratn 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. · Fan Motor is located here. It has a double shaft where the indoor blower and outdoor propeller fan are connected together.
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. Usually, it has to be off for at least 3 minutes before turning on again to prevent it from being damaged. For mechanical control type, there is usually a caution to turn on the unit after the unit has turned off for at least 3 minutes. For electronic control, there is usually a timer to automatically control the cut-in and cutout of compressor. 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. Specifications of window type room air-conditioner: Compressor: - Hermetically sealed compressor having cooling capacity of (as per water cooler) Condenser: Air cooled condenser made up of copper pipe & Aluminum fins Evaporator: - cooling coil. Capillary Tube: Diameter : suitable Material : copper Heater : 300 watts Thermometer : Wet / Dry Type Thermostat : 5 degree C to 15 degree C Pressure &Vacuum Compound gauge : any available brand Temperature Measurement : Digital Temperature indicator is provided to measure temperature of refrigerant and water Temperature sensor: RTD PT-100 Type Voltmeter : 0-300 V Ammeter : 0-5 A Energy Meter : Single Phase Standard make On/Off switch, Mains Indicator etc
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Result: Various components of room air conditioner have been studied. Viva Questions: 1. What do you mean conditioning of air? 2. Explain the working principle of air conditioning system? 3. What are different types of air conditioning systems? 4. What is the function of the blower? 5. What is the function of the filter in front of the evaporator coil?
Split air-conditioner: A split air conditioner is a suitable alternative to wall, window, or centralized air conditioner systems. Often called mini-split, ductless split, or duct-free air conditioning, this system can adequately cool a standard-sized house without requiring extensive installation costs and efforts.
A split air conditioner is made up of two primary parts that a very familiar: the evaporator and the compressor. Both of these elements exist is more common central air units and wall air conditioners. The difference with a mini-split system is that they are separated into two different, distant components, one being outdoors and one being indoors. The outdoor section is a 40
compressor that initiates the cooling process, while the indoor component consists of an evaporator and fan. The two sections are connected with a set of electrical wires and tubing, also called lines, used to transport air between the two sections. It's these lines that allow the split AC to be considered ductless, and the fact that the wires and tubing are so small and discreet compared to large ducts is where the "mini" split name comes from. Function The compressor is controlled by an internal thermostat. As the thermostat detects warm air, it activates the outdoor compressor. The compressor circulates a refrigerant gas, increasing the pressure and temperature of the refrigerant as it compresses it through a series of pipes. The refrigerant then moves to the condenser for further processing. In the condenser, a cooling system removes heat from the high-pressure gas and the gas changes phase and becomes a liquid. This chilled liquid is pushed through tubing indoors until it reaches the evaporator system. Inside the home, the evaporator fan collects warm air and passes it through a chamber containing the chilled liquid refrigerant. The fan system blows this air, which has now been cooled, back into the room, lowering the overall temperature of the space. If the thermostat still detects air that is warmer than desirable, the process continues, and the refrigerant and any excess heat that remains in the system are passed back outdoors to the compressor in order to begin the cycle again. Benefits of Split Air Systems Less Energy Loss A Split air conditioner is compact and isolated between two localized component sections, so there is very little opportunity for heat and other energy to escape the system. Centralized air conditioning systems waste enormous amounts of energy due to heat exchange in the air conditioner duct system. However, this problem is virtually eliminated in a split air conditioner system. Less Heat Loss Split air conditioner systems are preferable to window and wall air conditioning units as well. Although the latter are small and easy to install, they do not provide reliable cooling to a large space or to multiple rooms. Even with thoroughly sealed windows and walls, these air conditioner units allow for heat to enter the space, partially negating the effects of the system. Targeted Heating and Cooling Additionally, it's possible to have more than one indoor evaporator and fan. You could have one in each room or area of your home and run them each independently with only one outdoor compressor. This combines the efficiency and customization of a space heater or fan with the convenience of central air. 41
Specifications of split type air-conditioner: General ● Brand
Carrier
● Type of Air Conditioner
Split
Category Type ● Model Number
Superia
Dimension and Weigth ● Weight (Indoor unit)
9 Kg
● Weight (Outdoor unit)
35 Kg
Capacity ● Nominal Capacity
1 Ton
Power ● Power input (watts)
950
Automatic Features ● Auto restart
Yes
Display Features ● Panel display type
LED
Accessories Included ● Remote Control
LCD
AC Accessories Dimensions ● Dimensions (Outdoor unit)
250 x 780 x 540 mm
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● Dimensions (Indoor unit)
198 x 905 x 275 mm
Cooling Feature ● Cooling capacity (w)
3575
Certifications and Standards ● Star Rating
5
Frequency Range ● Voltage / Frequency / Phase
230/50/1
Available Modes ● Sleep Mode
Yes
Functional Features ● Dehumidification
Yes
Convenience Features ● Night glow buttons on remote
Yes
Speed ● Speed Settings
Fan Motor (Indoor)- 3 / Fan Motor (Outdoor)- 1
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11.
Aim: To study about different types of refrigerants. Refrigerants are the life blood of the vapor compression system. The refrigerant flows continuously through the vapor compression cycle, absorbing the heat in the evaporator and releasing it in the condenser. It undergoes various phase changes while flowing though the cycle. Refrigerant is the life blood of the vapor compression cycle. It is the fluid that flows continuously through the refrigeration cycle or vapor compression cycle absorbing heat from the low temperature reservoir and throwing it to the atmosphere or any other high temperature reservoir. For different temperature conditions and applications different refrigerants are found to be suitable. There is no ideal refrigerant that can be used in all the conditions. Refrigerants are the working fluids used in the counter clockwise thermodynamic working cycles. Depending on temperature levels of the heat source and the heat sink, the application area of the working cycle can be refrigeration, air- conditioning, or heat-pumping. Refrigerant circulates within the refrigeration machine, absorbs the heat from the heat source at lower temperature level and rejects it into the heat sink at higher temperature level. Refrigerant selection involves compromises between conflicting desirable properties. The working fluid desirable properties are related to thermodynamic and physical properties which lead to efficient cooling or heating factor and effective design of equipment, such as high evaporation heat, high volumetric refrigeration capacity, low temperature at the end of the compression. Other physical properties comprise the favorable position of critical and the freezing point, low specific heat capacity, low specific volume, low viscosity and high thermal conductivity. Desirable chemical and safety properties comprise chemical stability within the working conditions in the refrigeration unit in the presence of used materials and lubricating oil, non-flammability, non-toxicity, good miscibility with oil. If possible, the refrigerant must be odorless, but easy detection in the air is desirable. Safety properties of refrigerants considering flammability and toxicity are defined by ASHRAE standard 34 A variety of refrigerants are used in vapor compression systems. The choice of fluid is determined largely by the cooling temperature required. Commonly used refrigerants are in the family of chlorinated fluorocarbons (CFCs, also called Freons): R-11, R-12, R-21, R-22 and R-502. The properties of these refrigerants are summarized in Table 4.1 and the performance of these refrigerants is given in Table 4.2. The choice of refrigerant and the required cooling temperature and load determine the choice of compressor, as well as the design of the condenser, evaporator, and other auxiliaries. Additional factors such as ease of maintenance, physical space requirements and availability of utilities for auxiliaries (water, power, etc.) also influence component selection.
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Conclusion Influence of refrigerant properties and refrigeration system design is significant and those properties influence the design of HVAC systems which contain refrigeration subsystems as well. In the future we may expect changes in regulation concerning refrigerants, the construction of systems that are suitable for the use of newly developed and natural refrigerants, the optimization of the system in the sense of compensating the lower efficiency of some refrigerants, but with keeping cost within acceptable limits.
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REFERENCES • 2001 ASHRAE Handbook of Fundamentals • 1997 ASHRAE Handbook of Fundamentals • ASHRAE Cooling and Heating Load Calculation Manual • ASHRAE Standard 62, Indoor Air Quality • Trane C.D.S. Program Description and Capabilities • Elite Software Program Description and Capabilities
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