Hvac

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Hvac

  1. 1. HVAC Purposes • Maintain ambient conditions within acceptable limits (temperature, humidity, contamination) • Protect the staff and equipment against specific risks from inside the buildings (anoxia, explosion, fire) and outside • Monitor the release of air from the controlled areas in normal operation • Contain the radioactivity released during a DBC or DEC event Maintaining a stable temperature and appropriate humidity levels, HVAC plays an important role in ensuring the safety and smooth operation of nuclear plants. HVAC stands for Heating, Ventilation and Air Conditioning. The objective of HVAC is to control the temperature of air along with control of moisture, filtration of air, etc. In this article we discuss HVAC for beginners and the essentials of comfort zone engineering along with psychometric chart. Basic Need for HVAC The objective of HVAC are to control the temperature of air inside the designated “Air Conditioned” space along with control of moisture, filtration of air and containment of air borne particles, supply of outside fresh air for control of oxygen and carbon dioxide levels in the air conditioned space, and finally control of the
  2. 2. movement of air or draught. All these factors comprise of a successful HVAC system. Air conditioning has changed over the years from just cooling of a space to the effective control of all the above parameters. HVAC has its history in 1851 when Dr. John Gorrie was granted a US patent for a refrigerating machine. Though refrigeration in that period was concerned with the preservation of frozen meat for transport and making of ice, gradually the concept of cooling for comfort arose. In 1902, the New York Stock Exchange was one of the first air conditioned buildings. Air conditioning ranges from simple air conditioning by use of window AC and split AC where cooling, and minor filtration and dehumidifying, is done to complicated air conditioning systems used in process plants, ships, museums, and centrally air conditioned buildings. Heating: To increase the temperature by adding thermal energy to a space. Cooling: To decrease the temperature by removing thermal energy from a space. Humidifying: The process of increasing the relative humidity of a space by addition of water vapor or steam. Dehumidifying: The process of removing the water vapor or humidity of a space. Cleaning: The process of removing dust, pollens, smoke and contaminants from air inside the space. Ventilating: The process of adding external air to freshen up the air and maintaining gas ratio. Air movement: To control the movement of the supplied air so that the inhabitants of the space do not feel discomfort. The level of comfort of a person is very subjective and depends on the following factors: age, physical activity, health, and clothing. It is further dependent on other factors like air temperature, air velocity, humidity, and radiant temperature. This means that even if the air temperature is 24 deg C (75 deg F), but the humidity is high at 90% a person will not feel comfortable or vice versa. Even if the humidity is at 90 % but the air temperature is 18 deg C (64 deg F), an individual may feel
  3. 3. comfortable. However, if both the temperature and humidity are comfortable but the speed of the air is high, a person may feel draught effects and wind chill factor comes in effect. Similarly all the other parameters may be normal, but the subject is sitting in front of an un-tinted window and may feel radiant heat that may make him uncomfortable. Thus all parameters have to be taken into consideration, and the comfort zone is dependent on the extremities of the points from which you feel comfortable to uncomfortable. Thus within a comfort zone most of the people will feel comfortable if all these points are addressed. Basic Principles of HVAC System Design, Operation and Determining Capacity Requirements Referenced here are informational resources regarding the basics of HVAC design and operation. When upgrading a current HVAC system or getting ready to build a new system in a residential or commercial building, one of the most important design considerations is correctly calculating the heating and cooling loads that the system will support. These calculations are described in complete detail in the articles below and sample calculations are given to help you achieve the fullest understanding of how to make the calculations properly possible. After reading through this section, you will have the knowledge and tools required to make good HAVC system design decisions. Soundproofing of Air Conditioning Ducts Soundproofing air conditioning ducts is to prevent the unwanted noise generated from the air conditioner. It is generated by the air handlers in the ducts which contains blowers that create vibrations transmitted as noise. It can be mitigated by absorbent materials and vibration isolators. a vibrator isolator is used. It’s a device that is fitted before and after the air handler structure. It is also installed in between the fan components and also other components which are bound to create vibration. It has a rubberized component that prevents the vibrations created by the air handler from passing on to the nearby ducts, thereby preventing noise. The fan is also an important source of
  4. 4. noise coming out from the air conditioning. This is prevented by placing the fan on a spring suspension and not allowing its vibration to pass through the floor. Another method of reducing noise from coming in through the ducts is to use sound absorbent materials. One can go for sound air conditioning absorbent material, which is in the form of a thick paper and can be placed along the ducts to prevent noise from coming through. Sound dampers can also be fit along the ducts to reduce sound. Seeking professional help to clean your ducts regularly is important. Splitting of air ducts resulting in air leaks can also result in noise. Right kind of insulation is needed to prevent this. Acoustic insulation is also available as a soundproofing material for air conditioning ducts. It’s a complex material made of porous mat and synthetic acoustic membrane. It’s easy to handle and is very effective for soundproofing air conditioning ducts When you are planning to install a new HVAC system in your home or office, it is important to select the air-conditioner of proper tonnage and specifications. They will measure various dimensions of your space which is to be cooled, including that of walls and roofs, and find out the heat gained by them. They will also consider the number of windows, type of windows, blinds and their exposure to sun and accordingly decide on the heat gained by them. The heat emitted by lights and other electrical appliances is also considered. One of the most important parameters to consider is the number of people that will occupy the room or the office. After measuring the total amount of heat generated in the home or office per hour, the HVAC designer will suggest you the HVAC system of proper tonnage so that you will feel comfortable in your room or office without excessive burden of electricity bills. Designing the HVAC system for your house or office is as crucial as designing the complete house or office. Don’t ignore this important factor. Comfort at reasonable cost is ensured.
  5. 5. Factors Affecting HVAC Designing and Heat Load Calculations The heat is generated in the air-conditioned space from various sources. To maintain the comfort conditions inside the room the total heat generated inside the room per hour should be removed completely. Here are various sources of heat that affect HVAC designing and heat load calculations, these are: 1) Heat gained by the walls: The walls of the room gain heat from the sun by way of conduction. The amount of heat depends on the wall material and its alignment with respect to sun. If the wall of the room is exposed to the west direction, it will gain maximum heat between 2 to 5 pm. The southern wall will gain maximum heat in the mid-day between 12 to 2 pm. The heat gained by the wall facing north direction is the least. The heat gained by the walls in day-time gets stored in them, and it is released into the rooms at the night time thus causing excessive heating of the room. If the walls of the room are insulated the amount of heat gained by them reduces drastically. 2) Heat gained by the roof and partitions: If the roof is exposed directly to the sun, it absorbs maximum heat. If there is other room above the air-conditioned room, then the amount of heat gained by the roof reduces. The heat gained by the partitions of the room depends upon the type of partition. 3) Heat gained by the windows: Windows of the room are exposed directly to the surrounding and the heat from the sun enters the room by radiation. As in the case of the walls, the heat gained by the rooms through windows depends on their alignment. If there are sufficient curtains on the windows and the external awning the amount of heat gained by radiation reduces. The type of glass doors on the windows also affects the amount of heat gained through the windows by radiation. 4) Heat generated by the people: The people inside the room generate lots of heat. The heat dissipated by working people is more than from sitting people. 5) Heat generated by the electrical appliances: Heat is generated by electrical appliances like lights, motors, coffeemakers, electronic equipments, etc. should also be considered for heat load calculations, which is also called cooling load calculations. 6) Heat gain from outside air: Outside air is normally at a greater temperature
  6. 6. than the room temperature. When this air comes inside the room, it brings certain amount of heat along with it. A good HVAC designer will thoroughly consider all the sources of heat inside the room and find out the total amount of heat generated inside the room per hour. These days it has become easier for engineers to design HVAC as various softwares are available which can also be integrated with AutoCAD. Based on these heat load calculations, they will suggest you the air-conditioning system of proper capacity for your room. How Heating and Ventilation Loads are Assessed Factors of Heating and Ventilating Loads The following factors should be taken into consideration while determining the heating and ventilating loads of a structure: Amount of heat distributed through the area exposed to locations which are not heated. Amount of heat necessary for the air to become hot, which penetrates from the cracks adjacent to the windows and doors, and also from the entrance, when persons come in or go out of the building. Heat which is necessary to make the air that has come mechanically into the building as ventilation air hot. A variety of other loads. Design Calculations The basis for determining the heating load are the constant average temperature for winter nights and anycontinuous supply of heat present at all times. The quantity of heat accumulated by the building must be taken into consideration as well as the energy of any cooling equipment. Method for Estimating Heating Load The normal process for estimation of heat load is as follows: Carry out an assessment of the weather conditions prevailing outside the building, including humidity, temperature, path of wind and speed. Determine the desirable inside air temperature to be maintained. Assess the temperature in adjoining locations which are not heated.
  7. 7. Choose the coefficient of heat transmission. Establish the outside areas by which heat is dissipated. Estimate the losses by heat transference from glass, bricks, and base in the building. Calculate the heat loss from the underground area. Structure Of Building Design calculations, and the selection of heating system, are largely based on the structure of the building and its use, including the exposure of building to wind, the direction of the sun, periods of occupancy and part of day when the building will be used, and the expected financial impact of the equipment an components of refrigeration system; A compressor is a mechanical device like a pump that is used in various different applications. Find out here what a compressor does and what its different types are. All refrigeration and air-conditioning systems have four basic parts: the compressor, condenser, throttling or expansion valve and the evaporator. In refrigeration and air-conditioning units the heat is taken from the low temperature reservoir and thrown to the high temperature reservoir; hence this process requires external power which is given to the compressor. The compressor sucks low pressure and low temperature refrigerant from the evaporator and compresses it to high pressure and high temperature gaseous state. The larger the size of the refrigeration plant, the larger the compressor will be and the more power will be required What Is a Compressor and How Does it Work? An HVACR compressor is a refrigerant gas pump in which the evaporator supplies gaseous refrigerant at a low pressure and increases it to a greater pressure. Upon being compressed, the temperature and pressure of the vapor are increased. The gaseous refrigerant is delivered to the condenser at a pressure at which condensation occurs at an appropriate temperature. A compressor consists of two components: the power source and the compressing mechanism (piston, vane, etc.). In the case of an air compressor, the compressing
  8. 8. mechanism is in fact compressing the atmospheric air. The aircompressor works in the following way: Air enters the piston or vane, and it is compressed by increasing its pressure and at the same time decreasing its volume. As soon as the pressure reaches a maximum set by the operator or the manufacturer, a switch mechanism prevents any further air intake in the compressor. The compressed air is used and pressure levels decrease. As soon as the pressure reaches a minimum, also set by the operator or the manufacturer, the switch allows air to enter the compressor. This procedure is repeated as long as the compressor is being used. Performance Factors for Compressors Factors which affect the performance of compressors are: speed of rotation pressure at suction pressure at discharge and type of refrigerant being used Similar compressors can operate at different capacities by varying their refrigerants and compressor horsepower input. When purchasing any type of compressor, the buyer should check certain characteristics that include the machine configuration, the operation type, the price, and the operating cost. In any case, he should check the performance of the compressor and consult with the manufacturer about the most suitable and safest compressor for his budget and requirements. Types of Compressors
  9. 9. The image above shows the available types of compressors. The most common ones used in refrigeration are described below: Rotary: Compressors of the rotary type are generally low capacity equipment, used normally in home refrigerators and freezers, and not used for air conditioning. These compressors can consist of one vane, which is placed in the body, and sealed against the rotor, or multivane rotary, with vanes located in the rotor. Centrifugal Compressors: These compressors revolve at high speed, and refrigerant is compressed by the application of centrifugal force. These compressors are normally used with refrigerants possessing higher specific volumes, which need lower compression ratios. Multi-stage units can be used to attain greater discharge pressures, and the number of stages is determined by the discharge temperature of the gas as it exits from the rotor. These compressors are utilized for water chilling in air conditioning and for low temperature freezing purposes. Reciprocating Compressor: These compressors have pistons, and move in cylinders. Types of reciprocating compressors are: Open Compressors: One extremity of the crankshaft is drawn out of the crankcase, due to which multiple drives can be used with the compressor. A mechanical seal is used to check external seepage of refrigerant and oil, and escape of air towards the inside. These compressors are driven by electric motors or internal combustion engines. With belt drive, changes in speed are achieved by altering the dimensions of the pulleys, while with direct drive units the compressor is planned to operate at the speed of motor. Hermetic Compressors: These compressors are serviceable hermetic, in which motor and compressor are enclosed in the same housing, while the welded
  10. 10. hermetic type has the compressor and motor sealed in a welded steel shell. Condensers are commonly used in both indoor and outdoor air-conditioning systems. A condenser is simply a heat exchanger, what is its purpose in the air- conditioning systems and what different types of condensers exists, find out here.. There are four main parts of refrigerating and air-conditioning systems, these are: compressor, condenser, throttling or expansion valve and the evaporator. The refrigerant leaving the compressor is in the gaseous state and at a high pressure and temperature. This refrigerant then enters the condenser where it loses the heat to the coolant, which can be air or water. After passing through the condenser the refrigerant gets condensed but still remains at high pressure. It comes out in a partially liquid and gaseous state and then enters the throttling or expansion valve. Purpose Of A Condensor The purpose of a condenser in the cycle of compression refrigeration is to change the hot gas being discharged from the compressor to a liquid prepared for use in the evaporator. The condenser accomplishes this action by the removal of sufficient heat from the hot gas, to ensure its condensation at the pressure available in the condenser. The heat is shifted to another medium, like water or air, to cool the condenser There are three types of condensers: air cooled, water cooled and evaporative. These have been described below.
  11. 11. 1) Air cooled condensers: Air cooled condensers are used in small units like household refrigerators, deep freezers, water coolers, window air-conditioners, split air-conditioners, small packaged air-conditioners etc. These are used in plants where the cooling load is small and the total quantity of the refrigerant in the refrigeration cycle is small. Air cooled condensers are also called coil condensers as they are usually made of copper or aluminum coil. Air cooled condensers occupy a comparatively larger space than water cooled condensers. Air cooled condensers are of two types: natural convection and forced convection. In the natural convection type, the air flows over it in natural a way depending upon the temperature of the condenser coil. In the forced air type, a fan operated by a motor blows air over the condenser coil. 2) Water cooled condensers: Water cooled condensers are used for large refrigerating plants, big packaged air-conditioners, central air-conditioning plants, etc. These are used in plants where cooling loads are excessively high and a large quantity of refrigerant flows through the condenser. There are three types of water cooled condensers: tube-in-tube or double pipe type, shell and coil type and shell and tube type. In all these condensers the refrigerant flows through one side of the piping while the water flows through the other piping, cooling the refrigerant and condensing it. 3) Evaporative condensers: Evaporative condensers are usually used in ice plants. They are a combination of water cooled and air cooled condensers. In these condensers the hot refrigerant flows through the coils. Water is sprayed over these coils. At the same time the fan draws air from the bottom side of the condenser and discharges it from the top side of the condenser. The spray water that comes in contact with the condenser coil gets evaporated in the air and it absorbs the heat from the condenser, cools the refrigerant and condenses it. Types Of Water Cooled Condensers Types of water cooled condensers generally used are as follows: Double Pipe Condenser: These condensers, normally used in ammonia systems, consist of a pipe inside another pipe, or a tube within a tube. The tubes are positioned horizontally to form a vertical bank. The whole unit is fixed on a wall, and the water-ammonia mixture enters the sections at the bottom, flowing up
  12. 12. through the inner pipe. The compressed refrigerant vapor goes through the top section flowing downward, due to which a counter flow is created. Double Tube Condenser: It is a version of the double type condenser in which water flows inside the tubes, and refrigerant flows through the annular space between the two tubes. A compact condensing unit is created by placing the compressor inside the condenser. Shell And Coil Condenser: It consists of a copper coil located in a steel shell. Water runs through the coil, and refrigerant vapor is discharged from the compressor, condensing on the outside of the cold tubes. This type of condenser is economical, but its maintenance is difficult. If a leakage occurs in the coil, it is essential to remove the head of the shell, and withdrawal of coil from the shell. Shell And Tube Condenser: It is comprised of a large number of tubes located in a steel shell, with water flowing inside, and vapor flowing around the tubes. The vapors condense on the exterior surface of the tubes, and drops to the bottom of the condenser. A considerable amount of condensing surface is available in a smaller place. Capacity Of Water Cooled Condensers The capacity of a water cooled condenser depends on the temperature of the water, the amount of water circulated, and the temperature of the refrigerant gas. The capacity will increase with the temperature difference between the refrigerant gas and water. Temperature difference can be enlarged by increasing the condensation pressure, or decreasing the temperature of the water entering, or by increasing the amount of water to maintain a low water temperature. Throttling or Expansion Devices for Refrigerator and Air-Conditioner Systems and air-conditioning systems are comprised of four important components: the compressor, thecondenser, the throttling or expansion device and the evaporator. The refrigerant keeps on flowing through these components in a continuous cycle. The high pressure and medium temperature condensed refrigerant leaves the condenser and enters the throttling or expansion valve. In the throttling valve the pressure of the refrigerant reduces suddenly and excessively. With this the temperature of the refrigerant also reduces drastically.
  13. 13. This low pressure and low temperature liquid refrigerant then enters the evaporator and absorbs heat from the substance or the space to be cooled. The throttling valve is fitted between the condenser and the evaporator. The throttling or expansion device is in the form of a small orifice. When refrigerant passes through this small orifice its pressure reduces suddenly due to the friction. The rate of the flow of refrigerant through the throttling device depends on the size and opening of the orifice. It also depends on the difference in pressure on the evaporator and the condenser sides. There are different types of throttling devices, but in refrigerating and air- conditioning systems, the two most commonly used types are: capillary tube and thermostatic expansion valve. These have been described below: 1) Capillary tube: Instead of the orifice, the capillary is small diameter tubing that offers the restricted flow of the refrigerant. Its internal diameter ranges from 0.020 to 0.090 inches depending upon the capacity of the refrigerating or air-conditioning system. The pressure drop attained through the capillary depends upon its diameter and length. Capillary tubing made of copper is most commonly used. Capillary tubing is used for small refrigerating and air-conditioning systems like household refrigerators, water coolers, deep freezers, window air-conditioners, split air-conditioners, small packaged air-conditioners etc. For systems in which capillary tubing is fitted, technicians have to be very careful of refrigerant charging as the overcharging can lead excessive high discharge pressures from the compressor, which leads to over loading of the compressor and the chances of refrigerant leakages from the system are also increased. 2) Thermostatic expansion valves: The thermostatic expansion valve is not controlled by the temperature. It works automatically maintaining proper flow of the refrigerant depending upon the heat load in the evaporator. Apart from reducing the pressure of the refrigerant, the thermostatic expansion valve also keeps the evaporator active. These days thermostatic expansion valves used with solenoid valves are more common.
  14. 14. Thermostatic expansion valves are used extensively in medium and large sized refrigerating and air-conditioning systems. They can be used for large water chilling plants, brine chilling plants, large packaged air-conditioners, central air- conditioning plants etc. Evaporators for Refrigerator and Air-conditioner systems The refrigerant undergoes various changes throughout the vapor compression cycle and it is in the evaporator where it actually produces the cooling effect. The evaporator is usually a closed insulated space where the refrigerant absorbs heat from the substance or food to be cooled. he refrigerant undergoes various changes throughout the vapor compression cycle and it is in the evaporator where it actually produces the cooling effect. The evaporator is usually a closed insulated space where the refrigerant absorbs heat from the substance or food to be cooled. The cooling effect is produced by the refrigerant rotating continuously in the refrigerating or vapor compression cycle. The refrigerant gets compressed and superheated in the compressor and then loses heat in the condenser. In thethrottling valve the pressure and temperature of the refrigerant is reduced suddenly and drastically. The low temperature liquid refrigerant enters the evaporator and produces the chilling effect for a refrigerator and cooling effect for an air- conditioner. The space comprising the evaporator is an enclosed space. For instance, in the case of a household refrigerator, the small enclosed freezer section has an evaporator embedded into it. In the case of the deep freezer the evaporator is enclosed in the space where ice or ice cream is to be made. The evaporator section of refrigerators is usually insulated by using insulating materials like polyurethane foam (PUF). The low temperature refrigerant flowing through the evaporator absorbs heat from the food, substance or any other enclosed space and gets converted into a gaseous state as its temperature rises. This is then sucked by thecompressor, which compresses it, keeping the cycle of refrigerant continuous. In the case of air-conditioners the evaporator is also called the cooling coil.
  15. 15. Usually the fan would pass the hot room air over the evaporator coil, which is chilled, hence the air gets cooled. This air is then supplied into the room, where it creates the cooling effect by absorbing heat from the room. Evaporators are of various types. Evaporators used for industrial refrigeration and air-conditioning purposes are very large and also called chillers. They are usually made in the form of shell and tube types with two possible arrangements: namely, dry expansion evaporators and flooded evaporators. In dry expansion evaporators the refrigerant usually flows through the tube side while the liquid to be chilled flows through the shell side. The flooded system is used where large quantities of liquids have to be cooled to extremely low temperatures. Since the load in such cases is very high, a large quantity of refrigerant flows through these evaporators. In flooded evaporators the refrigerant will usually pass through the shell side while the liquid to be chilled will pass through the tube side. For smaller and home purposes there are three types of evaporators: bare-tube type, plate-surface type and finned evaporators: In bare-tube evaporators the refrigerant flows through the bare-tube and the fluid to be chilled flows directly over it. Plate-surface evaporators are used in household refrigerators. These evaporators are formed by welding together two plates that have grooves on their surface. When they are welded, the closed grooves form a sort of the tubing through which the refrigerant flows. Finned evaporators are commonly used in window, split and packaged air- conditioners. They are in the form of a copper coil over which several fins are welded to increase the cooling area of the evaporator. Hot air passes over this evaporator and gets chilled as it enters the room 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.
  16. 16. Here are certain properties that all refrigerants should possess so that they can be considered for use in refrigerationand air-conditioning systems: 1) Safe properties: The refrigerants that are used in refrigerating and air- conditioning systems should be harmless to the environment and not contribute to the depletion of the ozone layer, or to the increase in earth’s warming potential also called the greenhouse effect. Since many years a number of chlorofluorocarbons (CFCs) have been used as the refrigerants that cause excessive damage to the ozone layer when they are leaked to the atmosphere. What makes CFCs even more damaging is that they have a very long atmospheric life, which in certain cases can be 100 years. This means that once this refrigerant is leaked in the atmosphere it will keep on damaging it for 100 years. 2) Toxicity: Refrigerants are used extensively in household and commercial refrigeration and air-conditioning systems. These units lie in close proximity to human beings, and technicians frequently come in contact with the refrigerants. Hence it is vital that the refrigerants used in refrigeration and air-conditioning systems should be non-toxic in nature, so that even if they are leaked in the atmosphere they won’t have any damaging effects to human life. Presently, ammonia is the only toxic refrigerant being used to a large extent though its applications are limited to packing plants, ice plants and large cold storage facilities. 3) Flammability and explosiveness: Most of the refrigerants being used today are non-explosive and nonflammable. This is again very important to ensure the safety of humans that are using the refrigerating and air-conditioning systems. Ammonia is slightly flammable and explosive, but its effects can be nullified by taking some precautionary measures. 4) Economic considerations: The refrigerant used in the vapor compression cycle of the refrigeration or air-conditioning system should produce maximum refrigerating effect. That’s means it should have high coefficient of performance and consume less power for producing certain refrigerating effect. The cost of the refrigerant itself should be low enough.
  17. 17. Earlier refrigerants had very damaging effects for the atmosphere, but now a number of new and safe refrigerants have been discovered and are fast replacing the older ones. In some developing countries the damaging refrigerants are still being used extensively though their use has been restricted. Selection of Refrigerants Keeping Things Cool Any substance, which absorbs heat through vaporization or expansion, is called a refrigerant. In broader sense, the term refrigerant is also applied to such cooling systems as cold water or brine solutions. As commonly interpreted, refrigerants include those working mediums that pass through the cycle of evaporation, recovery, compression and condensation. Thus, circulating cold mediums are not primary refrigerants, nor are cooling systems such as ice and solid carbon dioxide. Desirable Properties Of Refrigerants Desirable refrigerants are those which possess chemical, physical, and thermodynamic properties that permit their efficient application and service, in practical designs of refrigerating equipment. In addition, if the volume of charge is large, there should be no danger to health and property in case of its escape. A great variety of substances, such as butane, carbon tetrachloride, ethane, and hexane have been applied to refrigeration systems, but found to have little practical use. These materials are either highly explosive or flammable, or possess other combinations of undesirable properties. Different Refrigerants Air Air is one of the earliest refrigerants and was widely used in World War I whenever a completely nontoxic material was required. Although air is free of cost and completely safe, its low coefficient of performance makes it unable to compete with the modern nontoxic refrigerants. Ammonia Ammonia is one of the oldest and widely used of all refrigerants. It is flammable and highly toxic. It is widely used in commercial and large industrial reciprocating compression systems where high toxicity is secondary. Carbon Dioxide It is a colorless and odorless gas, which is heavier than air. It is nontoxic and nonflammable but has extremely high operating pressures. In former years it was used for marine refrigeration, theatre air conditioning systems, and for hotel refrigeration systems. Freon Refrigerants These refrigerants use ethane and methane as bases, and are the most important group of refrigerants being used in modern technology. These are used in a variety of applications, such as reciprocating compression refrigeration, and rotary compressors. Others Other refrigerants are methyl chloride, sulphur dioxide, hydrocarbon refrigerants, methyl chloride, and azeotropes.
  18. 18. Selection Of Refrigerants No substance has proved to be the ideal working medium, under all operating conditions. The characteristics of some refrigerants make them desirable for use with reciprocating compressors. In some applications toxicity is of negligible importance, whereas in others, such as comfort cooling, a nontoxic and nonflammable refrigerant is essential. Therefore, in selecting the correct refrigerant, it is necessary to determine those properties which are most suitable, and to choose the most closely approaching ideal for the particular application.

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