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Air-conditioning
MODULE 3.6
Department of Mechanical Engineering 1 Applied Thermodynamics
VJEC, Chemperi
Relationship of the refrigeration & air-conditioning fields
Cooling &
dehumidifying
operations in
air conditioning
Heating, humidifying,
and control of air
quality
Industrial refrigeration,
including
food preservation,
chemical & process
industries
Air conditioning Refrigeration
Department of Mechanical Engineering, VJEC 2 Applied Thermodynamics
Air-conditioning
In-order to achieve air-conditioning, following physical processes have to be carried out
1.Air Purification2.Temperature control3.Humidity control4.Air distribution
Department of Mechanical Engineering, VJEC 3 Applied Thermodynamics
Physical processes involved in air-conditioning
Air-conditioning
Air taken from atmosphere carries dust, bacteria and odourswhich are harmful for human health, in order to safeguard human health they should be removed
Air purification is carried out by one or more of the following methods
Air filtration: removing dust particles
Air sterilization: killing ingredients like bacteria
Odour suppression: removes foul smell causing agents from the air
Air ionization: is a device that uses high voltage to ionize air molecules, to remove impurities
Department of Mechanical Engineering, VJEC 4 Applied Thermodynamics
1. Air purification
the negative ions flood the room and seek out positively charged particles, such as dust, bacteria, pollen, smoke and many other allergens. The negative ions
and positively charged particles bond together. This creates dirt particles that are too heavy to keep floating around in the air, so they fall to the ground and can
be cleaned by normal means.
Air-conditioning
It is intended to regulate dry bulb temperature by simple heating or cooling, which may be associated with humidification process.
Cooling is achieved by passing air over evaporator coils of a refrigerating system, in that case relative humidity aspect is neglected.
In most of the cases, an indirect evaporating system is used for cooling the air, like chilled water
Heating of air is achieved by passing air over heated surfaces, that will be usually located outside the room. Heated air flows into the room by fan action.
Warm air heating system, hot water heating system, steam heating system, electrical heating are some of the commonly used heating systems.
Department of Mechanical Engineering, VJEC 5 Applied Thermodynamics
2. Temperature control
Air-conditioning
It is achieved either by humidification (increasing humidity) or de-humidification (decreasing humidity)
a) Humidification
It is accomplished by addition of steam or hot water to the air
Steam type humidifiers: steam is directly injected to the air, seldom used since it increases the temperature and may carry odours
Atomization type humidifiers: compressed air is used to make fine droplets of water, which is the passed to air-conditioned space
Impact type humidifiers: fine jet of water is directed against had targets, which forms fine spary
Forced evaporation type humidifier: here air is allowed to mix with vapourformed by evaporation
Department of Mechanical Engineering, VJEC 6 Applied Thermodynamics
3. Humidity control
Air-conditioning
b) De-humidification
it is the process of reducing water vapour content in air
It can be accomplished by using air washer or absorbents
In absorption method air is passed through a chemical (drying agent), moisture in the air enters into chemical combination with drying agent,
usually used drying agents are H2SO4, NH3, silica gel etc
Department of Mechanical Engineering, VJEC 7 Applied Thermodynamics
3. Humidity control
Air-conditioning
the object of air distribution is to create proper combination of temperature, humidity, and air motion in the room.
The variation of temperature within the room should not exceed 2 ْ C
Desirable air movement is 7.5-15 m/min
Flow direction towards the occupants is preferred, also downward flow is preferred over upward flow
Department of Mechanical Engineering, VJEC 8 Applied Thermodynamics
4. Air Distribution
Air-conditioning
Department of Mechanical Engineering, VJEC 9 Applied Thermodynamics
Comfort air-conditioning
Air-conditioning
While considering the air-conditioning for human comfort, following are required
Air purity
Air movement
Supply of oxygen and removal of carbon dioxide
Removal of heat dissipated by the occupants
Removal of moisture dissipated by the occupants
The comfort can be maintained only if the heat is removed at the rate that human body produces it
Heat transfer from the human body depend on Dry bulb temperature
Relative humidity
Air velocity
Department of Mechanical Engineering, VJEC 10 Applied Thermodynamics
Comfort air-conditioning
• There are two kinds of air conditioners available for regular domestic use.
1. Window AC
2. Split AC
Window Air-conditioning
Department of Mechanical Engineering, VJEC 13 Applied Thermodynamics
Window air-conditioning
Typical Window AC, available in market
Window Air-conditioning
Department of Mechanical Engineering, VJEC 14 Applied Thermodynamics
1. Window ACs
• As the name suggests these ACs are fitted in the window of the room.
• It consists of a case divided into two parts, namely indoor and outdoor parts
• Outdoor part contains Compressor and condenser, condenser fan
• Indoor part contains Expansion valve, Evaporator and evaporator fan
• Dampers are present in-front of indoor portion to control air flow
• They are cheaper and easy to install.
• There is no flexibility available in fixing of these ACs.
Window Air-conditioning
Department of Mechanical Engineering, VJEC 17 Applied Thermodynamics
Indoor unitOutdoor unit
Department of Mechanical Engineering, VJEC 21 Applied Thermodynamics
Split Air-conditioning
Split Air-conditioning
Department of Mechanical Engineering, VJEC 23 Applied Thermodynamics
• These ACs comprise of two parts: in-house unit and outside unit.
• These units are connected with conduit
• Outdoor unit contains Compressor and condenser, condenser fan
• Indoor unit contains Expansion valve, Evaporator and evaporator fan
• These are more expensive than the regular window ones but they are less noisy and give better distribution of air.
• Dampers are present in-front of indoor unit to control air flow
• They are energy efficient.
• Even rooms with no outdoor access, split ac can be installed
• Installing these units require renovation work, which can be expensive.
• The outdoor unit should be placed in such a way that air flow is not blocked.
• Moisture from the room, which gets collected as water also, needs to be drained out properly.
Department of Mechanical Engineering, VJEC 24 Applied Thermodynamics
Split Air-conditioning
1. Summer Air-conditioning
2. Winter Air-conditioning
Classification based on season/Temperature
• In the summer time you remove heat from the air, in the winter you add
heat to the air.
• In summer outside temp is more and hence cooling of air is required
• Consists of i) Air cleaning
ii) Air cooling
iii) Dehumidification
iv) Air distribution/circulation
• During summer dehumidification is done so that relative humidity is 40 to 50%
• Effective temp = 200C to 230C in summer
Department of Mechanical Engineering, VJEC 26 Applied Thermodynamics
1) Summer air conditioning
• In the summer an air conditioning unit removes moisture from the air allowing moisture to evaporate quickly from skin and gives crisp cold feeling. A dry space feels a lot cooler than a humid space at the same temperature.
• In winter outside temperature is low hence heating of air is needed for comfort.
• Cycle of operations consists of i) Air cleaning
ii) Air heating
iii) Humidification
iv) Air distribution/circulation
• During winter humidification is done so that relative humidity is 50 to 60%
• Effective Temp = 180C to 220C in winter
Department of Mechanical Engineering, VJEC 28 Applied Thermodynamics
2) Winter air conditioning
• Heating dries the air. So humidifier added to the heating system to increase humidity in winter. Humid air holds heat longer than dry air and will not evaporate moisture on skin and feels cold.