AIR CONDITIONERS AND REFRIGERATORS
A typical person in modern society may spend upto 90% of each day indoors. It is not surprising, therefore, that providing a healthy, comfortable indoor environment has become a major factor in our economy.
To an average person, air conditioning simply means “the cooling of air”. This definition is neither sufficiently useful nor accurate. To be more precise, air conditioning is the process of treating air in an internal environment to establish and maintain required standards of temperature, humidity, cleanliness, and motion. Most air conditioning systems are used for either human comfort or for process control.
Air conditioning is the process of treating air in an internal environment to establish and maintain required standards of temperature, humidity, cleanliness, and motion. This is how each of these conditions is controlled:
1. Temperature : Air temperature is controlled by heating or cooling the air. Cooling technically means the removal of heat, in contrast to heating, the addition of heat.
2. Humidity : Air humidity, the water vapour content of the air, is controlled by adding (humidification) or removing (dehumidification) water vapour from the air.
3. Cleanliness : Air cleanliness or air quality is controlled by either filtration, the removal of undesirable contaminants using filters or other devices or by ventilation, the introduction of outside air into the space which dilutes the concentration of contaminants. Often both filtration and ventilation are used in an installation.
4. Motion : Air motion refers to air velocity and to where the air is distributed. It is controlled by appropriate air distributing equipment.
Sound control can be considered an auxiliary function of an air conditioning system even though the system itself may be the cause of the problem. The air conditioning equipment may produce excessive noise requiring additional sound attenuating (reducing) devices as part of the equipment.
The above description does not imply that every HVAC (heating, ventilation and air conditioning) system regulates all of the conditions described. A hot water or steam heating system consisting of a boiler, piping, and radiation devices (and perhaps a pump) only controls air temperature and only during the heating season. These types of systems are common in many individual homes (residences), apartment houses, and industrial buildings.
A warm air system, consisting of a furnace, ducts, and air outlet registers, also controls air temperature in winter only. However, by the addition of a humidifier in the ducts, it may also control humidity in winter. Warm air systems are popular in residences.
Some residences have combination of air heating and air cooling equipment that provides control of temperature and humidity in both winter and summer. Some degree of control of air quality and motion is provided in air-type heating and cooling systems.
Air conditioning systems used for newer commercial and institutional buildings and luxury apartment houses usually provide year round control of most or all of the air conditions described. For this reason, it is becoming increasingly popular to call complete HVAC systems environmental control systems.
Most air conditioning systems are used for either human comfort or for process control. Air conditioning enhances our comfort. Certain ranges of air temperature, humidity, cleanliness, and motion are comfortable; others are not.
Air conditioning is also used to provide conditions that some processes require. For example, textile printing, and photographic processing facilities as well as computer rooms and medical facilities, require certain air temperature and humidity for successful operation.
COMPONENTS OF AIR CONDITIONING SYSTEMS
Heat always travels from a warmer to a cooler area. In winter, there is a continual heat loss from within a building to the outdoors. If the air in the building is to be maintained at a comfortable temperature, heat must be continually supplied to the air in the rooms. The equipment that furnishes the heat required is called a heating system.
In summer heat continually enters the building from the outside. In order to maintain the room air at a comfortable temperature, this excess heat must be continually removed from the room. The equipment that removes the excess heat is called a cooling system.
An air conditioning system may provide heating, cooling, or both. Its size and complexity may range from a single space heater or window unit for a small room to a huge system for a building complex. Most heating and cooling systems must have the following basic components:
1. A heating source that adds heat to a fluid (air, water, or steam).
2. A cooling source that removes heat from a fluid (air or water).
3. A distribution system (a network of ducts or piping) to carry the fluid to the rooms to be heated or cooled.
4. Equipment (fans or pumps) for moving the air or water.
5. Devices (e.g., radiation) for transferring heat between the fluid and the room.
ALL-WATER AIR CONDITIONING SYSTEMS
A typical hydronic (all water) heating system is shown in Fig. 52.1. Water is heated at the heat source (1) usually a hot water boiler. The heated water is circulated by a pump (2) and travels to each room through piping (3) and enters a terminal unit (4). The room air is heated by bringing it into contact with the terminal unit. Since the water loses some of its heat to the rooms, it must return to the heat source to be reheated.
If steam is used in a heating system, the components still work in the same manner, with the exception that a pump is not necessary to move the steam; the pressure of steam accomplishes this. However, when the steam cools at the terminal unit, it condenses into water and may require a condensate pump to return the water to the boiler.
A hydronic cooling system Fig. 52.2, functions in a similar manner to the hydronic heating system. Water is cooled in refrigeration equipment called a water chiller (1). The chilled water is circulated by a pump (2) and travels to each room through piping (3) and enters a terminal unit (4).
The warmer room air loses its heat to the cold water in the terminal unit. Since the water is now warmed, it must return to the water chiller to be recooled.
Hydronic systems are popular for HVAC systems that require both heating and cooling. This is because it is possible to use the same piping system for both by connecting a hot water boiler and water chiller in parallel, Fig. 52.3, using each when needed.
ALL-AIR AIR CONDITIONING SYSTEMS
All-air systems use air to heat or cool rooms. They may also have the added capability ofcontrolling humidity and furnishing outdoor ventilation, which hydronic systems cannot do.
A typical all-air heating and cooling system is shown in Fig. 52.4. Air is heated at the heat source (1), such as a furnace. It may also be a coil circulating hot water, or steam, heated by a boiler. The heated air is circulated by a fan (2) and travels to each room through supply air ducts (3). The supply air enters the room through outlets called air diffusers or registers (4) that are designed to provide proper air distribution in the room. When the warmed supply air enters the room, the room is heated. A humidifier (10) may also be included to maintain a comfortable room humidity in winter.
In summer, air is cooled when it flows over a cooling source (5), usually a coil of tubing containing a fluid cooled by refrigeration equipments. When the cooled supply air enters the room, the room is cooled.
Because a room’s size is fixed, the same volume of air that enters the room must also exit. This is usually accomplished by return air ducts (6). The air is then heated or cooled again and recirculated. An outdoor air intake duct (7) may be provided for introducing fresh outdoor air for increased air quality. Similarly, the same volume of air must be exhausted (8). Provision may be made for cleaning the air with air filters (9) and for humidifying the air (10).
REMOTE CONTROL BUTTONS
The outdoor unit has the other parts of the system like the compressor, air-cooled condenser, condenser fan and its own motor and is installed outside. The liquid and suction lines of the system have to be laid at site after the outdoor and indoor units are installed in position. The distance between the indoor and outdoor units has to be as small as possible. As this distance increases, the pressure drop in the suction and liquid lines also increases, resulting in reduction of the unit capacity.
Since the compressor (inside the outdoor unit) is remotely installed from the room to be air conditioned, the noise level will be appreciably lower than in the case of a room air conditioner. This is the advantage and the reason for opting for the split. Both room and split air conditioners and a typical remote control are shown in Fig. 52.5.
It is frequently desirable to combine water and air systems. For example, there may be instances when certain parts of a building may need cooling while others require heating simultaneously for providing comfort conditions in all areas. A typical application of this type is the air conditioning system for a big hotel. The heat removed from areas requiring cooling is transferred to the areas requiring heating. Such a system is called a heat recovery system, as distinguished from the heat pump.
UNITARY AND CENTRAL AIR CONDITIONING SYSTEMS
A unitary or package air conditioning system uses equipment where most or all of the basic components have been assembled in the factory e.g., room air conditioner (Fig. 52.5).
A central or built-up air conditioning system uses equipment centrally located in mechanical equipment rooms. Each piece of equipment is installed separately and connected on the job, rather than manufactured as a package.
SPLIT AIR CONDITIONERS
In split units the indoor and outdoor sections of the room air conditioner are separated out into two casings or units. The indoor unit, Fig. 52.5, consists of the evaporator coil, evaporator blower with its own separate motor, capillary tube, control panel, air filter, supply and return grills, etc. It is installed inside the room to be conditioned. It can be ceiling suspended, wall mounted, or kept on the floor as a console unit and is generally known as fan coil unit.
An environmental control system that includes cooling and dehumidification will require a means of removing heat from the conditioned spaces. Because heat flows from a higher to a lower temperature, a fluid with a temperature lower than the room design temperature must be made available, to which the excess room heat can be transferred.
A natural heat sink that is used occasionally for cooling water is atmospheric air. In climates where the humidity is extremely low, evaporative cooling of air may reduce both the air and water temperature low enough so that either can be used for cooling.
A refrigeration system extracts heat from a substance at a temperature lower than the ambient and transfers the extracted heat to the atmosphere at a temperature higher than the ambient. A refrigeration system is termed as :
1. A heat recovery system when its refrigeration effect is utilised for cooling an area or a fluid and the heat rejection is put to some beneficial use.
2. A heat pump when it is used for cooling during summer and heating in winter by incorporating suitable accessories for the change over from the cooling to heating modes and vice versa.
Food preservation, both for processing and storage, is one of the significant applications of refrigeration. Food processing calls for chilling, freezing, quick freezing, or freeze drying. Typical applications are the domestic refrigerators and home freezers, ice cream manufacturing and storage, drinking water coolers, beverage cooling, cold storages, process cooling of meat, fish, dairy products, fruits, vegetables, transport refrigeration etc. Refrigeration is very vital to the chilled and frozen-foods industry for maintaining the cold chain i.e. a supply of such foods from the farm to the consumer. Transport refrigeration is an important link in this chain. The conditions of temperature and relative humidity are dictated by the application for which the refrigeration system is intended.
Refrigerants are heat carrying mediums which during their cycle in the refrigeration system absorb heat at a low temperature level and discard the heat so absorbed at a higher level.
These refrigerants have been used since the 1930s because of their excellent characteristics. They have good physical properties for performance temperatures, pressure, oil mixing feature, heat transfer, specific, etc. They are non-toxic, stable, and inexpensive.
Chlorofluorocarbons (CFCs) are composed of chlorine, fluorine, and carbon atoms. Some in this group are R11, R12, and R114. Hydrochlorofluorocarbons (HCFCs) are composed of hydrogen, chlorine, fluorine, and carbon atoms. Some in this group are R122 and R123. Hydrofluorocarbons (HFCs) are composed of hydrogen, fluorine and carbon atoms. Some of these are R134a and R125.
A schematic flow diagram showing the basic components of the vapour compression refrigeration system is shown in Fig. 52.6. To aid in understanding, some typical temperatures for air conditioning applications are also indicated.
Refrigerant fluid circulates through the piping and equipment in the direction shown. There are four processes (changes in the condition of the fluid) that occur as it flows through the system.
Process 1–2 : At point (1), the refrigerant is in the liquid state at a relatively high pressure and high temperature. It flows to (2) through a restriction, called the flow control device or expansion device. The refrigerant loses pressure going through the restriction. The pressure at (2) is so low that a small portion of the refrigerant flashes (vapourises) into a gas. But in order to vapourise, it must gain heat (which it takes from the portion of the refrigerant that did not vapourise, thus cooling the mixture and resulting in low temperature at (2).
Process 2–3 : The refrigerant flows through a heat exchanger called the evaporator. This heat exchanger has two circuits. The refrigerant circulates in one, and in the other, the fluid to be cooled (usually air or water) flows. The fluid to be cooled is at a slightly higher temperature than the refrigerant, therefore heat is transferred from it to the refrigerant, producing the cooling effect desired. The refrigerant boils because of the heat it receives in the evaporator. By the time it leaves the evaporator (4), it is completely vapourised.
Process 3–4 : Leaving the evaporator, the refrigerant is a gas at a low temperature and low pressure. In order to be able to use it again to achieve the refrigerating effect continuously, it must be brought back to the conditions at (1)–a liquid at a high pressure. The first step in this process is to increase the pressure of the refrigerant gas by using a compressor. Compressing the gas also results in increasing its temperature.
Process 4–1 : The refrigerant leaves the compressor as a gas at high temperature and high pressure. In order to change it to a liquid, heat must be removed from it. This is accomplished in a heat exchanger called the condenser. The refrigerant flows through one circuit in the condenser. In the other circuit, a cooling fluid flows (air or water) at a temperature lower than that of the refrigerant. Heat therefore transfers from the refrigerant to the cooling fluid, and as a result, the refrigerant condenses to a liquid (1). This is shown in Fig. 52.7.
The refrigerant has returned to its initial state and is now ready to repeat the refrigeration cycle. Of course the processes are actually continuous as the refrigerant circulates through the system.
The absorption system uses the principle that some gases will be absorbed by certain other substances. There are many pairs of substances that have this affinity for one another. We are all aware of how table salt absorbs water vapour from the air, thus making it difficult to pour. Yet another combination is lithium bromide (LiBr) and water, lithium bromide will absorb large quantities of water vapour. This pair is used in many refrigeration systems.
The refrigerator is an essential part of almost every household for preserving food and thereby reducing wastage. The primary function of a refrigerator or freezer is to provide food storage space maintained at a low temperature for the preservation of food. Its essential secondary function is the formation of ice cubes for domestic consumption.
A storage temperature of 0° to 4°C (32° to 39°F) is satisfactory for the preservation of most of the fresh foods. For the short term storage of frozen foods, however, temperatures much below the freezing point are required. The evaporator in the domestic refrigerator, formed as a box, serves as a freezer for the storage of frozen food as well as for making ice cubes. It is mounted above the food storage space. The evaporator is
held at a temperature of about –18°C (0°F) and the general storage space is cooled by natural convection. Mechanical vapour-compression cycle as well as the absorption cycle are adopted for domestic refrigerators and freezers. The mechanical vapour-compression system has an edge over the absorption system because of its compactness and more efficient use of electrical energy. Hence the mechanical vapour-compression system is almost universally adopted. The rear view of a single door vapour-compression refrigerator is given in Fig. 52.9.
1. What are the components of an air conditioning system?
2. Differentiate between an all-air and an all water air conditioning system.
3. What are the different types of refrigeration systems? Explain briefly.
4. Explain in detail the refrigeration cycle.
5. Explain the working of a domestic refrigerator.
6. Write short notes on:
(a) Combination systems
(b) Split air conditioners
(d) Evaporative condenser
Fill in the Blanks
1. Cooling means ………………………… of heat.
2. Humidity is controlled by adding or removing ………………………. from air.
3. Air cleanliness is controlled by either …………………………… or ventilation.
4. An air conditioning system may provide heating or cooling or………………………. .
5. A natural………………………. is the atmospheric air.
6. Refrigerants are………………………. mediums.
7. The refrigerant loses………………………. going through the flow control device.
8. The heat………………………. has two circuits.
9. Compressing the refrigerant gas increases its temperature and………………………. .
10. The………………………. system works on the principle that some gases will be absorbed by certain other substances.
11. The vapour-compression system is………………………. .
12. The………………………. is the coldest part of a refrigerator.
Fill in the Blanks
2. water vapour
5. heat sink
6. heat carrying