Basic Mechanical Engineering - Air conditioning

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Information about Basic Mechanical Engineering - Air conditioning

Published on March 1, 2014

Author: msstevesimon


Air conditioning Prepared by M.S.Steve Assistant Professor Dept. of Mechanical Engineering Amal Jyothi College of Engineering Kanjirapally.

Definition  Process of treating the air so as to control simultaneously its temperature, humidity, purity and distribution to meet the requirements of controlled space.  Requirements of a conditioned space 1. Comfort and health of occupants 2. Needs of certain industrial processes 3. Efficient working of commercial premises/offices Page 2

Air conditioning  For human comfort ,air should have the following properties : 1. Temperature -22 °C to 27 °C 2. Humidity - 55% to 65% Relative Humidity 3. Velocity - 0.3 -0.5 m/s.  Air conditioning systems cool/heat the air, humidify/dehumidify the air to the above human comfort conditions depending on the temperature and humidity of the outside atmospheric air. Page 3

Applications  Industrial applications- To control chemical & bio chemical reactions .  Commercial applications- Theatres, hotels, banks etc.  Transport applications – automobiles, aircrafts,ships etc  Special applications – T .V centres , hospitals, computer centres etc. Page 4

Classifications  Depending up on the processes required for conditioning of outside air ,air conditioning system are classified in to : 1. Summer air conditioning- it involves Cooling & Dehumidification process. 2. Winter air conditioning – it involves Heating & Humidification process Page 5

Winter air conditioning  In winter outside temperature is low hence heating of air is needed for comfort.  Cycle of operations consists of 1. Air cleaning 2. Air heating 3. Humidification 4. Air distribution/circulation Page 6 Page 7

Winter air conditioning system  The outside air is sucked in through the damper and is filtered using an air filter.  The air is then passed through a preheat coil to heat the air to control the evaporation of water in the humidifier.  Steam is mixed with the air in required quantity to increase the humidity to 55% RH.  The air is then passed through a reheat coil to bring it to the comfort temperature of 22 °C.  The conditioned air is then supplied to the conditioned space by a blower.  Cold polluted air is discharged out and outside air is again sucked to repeat the cycle. Page 8

Winter air conditioning system These systems are used to heat the air and increase the humidity to acceptable levels. Page 9

Summer air conditioning  In summer outside temp is more and hence cooling of air is required Consists of 1. Air cleaning 2. Air cooling 3. Dehumidification 4. Air distribution/circulation Page 10 Page 11

Summer Air Conditioning  In this system air is cooled & dehumidified.  The outside air flows through the damper and is filtered using the filter.  The air is then passed through a cooling coil and the air gets cooled below the required temperature and is also dehumidified.  The cooled air is then passed through the water eliminator and water is collected in the sump.  After that air is made to pass through a heating coil to reach the desired comfort condition.  The conditioned air is now supplied to the space using the blower. Page 12

Summer Air Conditioning Page 13

Essentials of air conditioning systems Temperature control  Effective temp = 200C to 230C in summer  Effective Temp = 180C to 220C in winter  For Indian conditions 250C with 60% relative humidity to 300C with 45 % relative humidity, with air velocity not exceeding 10 m/min Page 14

 Humidity control During summer dehumidification is done so that relative humidity is 40 to 50% During winter humidification is done so that relative humidity is 50 to 60 % Page 15

Air velocity control High velocity of conditioned air may cause greater temperature difference between outside and inside. Page 16

Air quality control Air should be free from odour, toxic gases, bacteria and other micro-organisms Page 17

Essentials of air conditioning systems 1. Filtration 2. Heating (in winter) 3. Cooling (in summer) 4. Humidification 5. Dehumidification 6. Air circulation or distribution Page 18

1. Filtration  Should be capable of removing dust, ash, smoke, bacteria etc  Should be capable of holding moderate amount of dust without affecting the efficiency  Should offer low resistance to flow of air  Workable under sufficient range of velocities.  Should afford easy cleaning manually or mechanically Page 19

2. Heating (in winter)  Heating is necessary in winter so as to compensate heat loss from room.  Pre heating of incoming air may be done by passing over air furnaces or coils through which hot water is circulated Page 20

3. Cooling (in summer) Cooling of incoming air is necessary in summer. Principle of mechanical refrigeration is used. Page 21

4. Humidification Humidification or addition of water is necessary in winter because of low temp has less humidity. Incoming air is passed through pans of water or wet cloth strips Page 22

5. Dehumidification In this process certain amount of water is extracted from air. This is done in summer where incoming air is cooled. Adsorbents can be used to adsorb excess moisture from air . Page 23

Psychrometry  The properties of moist air are called as pshychrometric properties .  Although the principles of pshychrometry can be applied to any physical system consisting of gas vapour mixtures,the most common system of interest is the mixture of water vapour & air. Page 24

Terms used in psychrometry  Dry Air : Dry air is a mixture of oxygen, nitrogen, carbon dioxide, hydrogen, argon, neon ,helium etc.  Moist Air : It is an ordinary air, which is a mixture of dry air and water vapour.  Saturated Air : It is the air which contains maximum amount of water vapour which the air can hold at a given temperature and pressure.  Relative Humidity (RH) : It is the ratio of mass of water vapour in a given volume of moist air at a given temperature to the mass of water vapour contained in the same volume of moist air at the same temperature when the air is saturated.  Dry Bulb Temperature (DBT) : It is the temperature of air measured by an ordinary thermometer. Page 25

Terms used in psychrometry  Wet Bulb Temperature (WBT) : It is the temperature recorded by a thermometer, when its bulb is covered by a wet cloth and is exposed to a current of moving air.  Dew Point Temperature (DPT) : It is the temperature at which the condensation of moisture begins when the air is cooled at constant pressure.  Sensible Heat of Air : It is the enthalpy of dry air, which can be calculated by measuring its dry bulb temperature.  Total Heat : It is the sum of sensible heat of dry air and sensible plus latent heat of water vapour present in it. Page 26

Psychrometric chart Page 27

Thank You M S Steve @msstevesimon 2m.s.steve

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