Mechinopedia For Mechanical Engineers

  Advantages Of  Multistage Air Compressor : 1. Work Done per kg of air is reduced in Multistage compression with intercooler as compared to single compression for the same delivery pressure. 2. Better mechanical balance can be achieved with multistage compressors. 3. It reduces the leakage loss considerably. 4. Volumetric Efficiency is Improved by Increasing Number of Stages. 5. It gives more uniform torque, and hence smaller size of flywheel is required. 6. Lower operating Temperature permits the use of  cheaper materials for construction. 7. Better Lubrication due to lesser working Temperature. 

 1. The Quantity of refrigerant used per ton of refrigeration is high as compared to other systems.   2. The COP of the system is very low. Therefore running cost is high.  3. The danger of frosting at the expander valves is more as the air contains moisture content. 

 Advantages Of  Air Refrigeration : 1. The refrigerant used namely air is cheap and easily available. 2. There is no danger of fire or toxic effects due to leakages. 3. The weight to ton of refrigeration ratio is less as compared to other systems. Disadvantages Of Air Refrigeration : 1. The quantity of  Refrigerant used per ton of refrigeration is high as compared to other systems. 2. The COP of the system is very low. Therefore running cost is high. 3. The danger of frosting at the expander valves is more as the air contains moisture content. 

An ideal Refrigerant should possess the following desirable properties :  1. The Refrigerant should have low freezing point.  2. It must have high critical pressure and temperature to avoid large power requirements.  3. It should have low specific volume to reduce the size of the  compressor.   4. It should be non - flammable , non - explosive , non - toxic and non - corrosive.  5. It should give high C.O.P in the working temperature range. This is necessary to reduce         the running cost of the system.  6. It must have low specific heat and high latent heat.  7. It should be odorful for leak detection.  8. It should be of low cost.    

 Summer :    Inside Temperature - 24* + 1* C  RH - 50 - 60%  RH  Air Movement 4.5 -7.5 m/min. Winter : Inside Temperature - 20* + 1* C RH - 35 - 40 %  RH

EFFECTIVE TEMPERATURE : Effective temperature is defined as that temperature of saturated air at which the subject would experience the same feeling of comfort as experienced in the actual unsaturated environment. FACTORS AFFECT EFFECTIVE TEMPERATURE :  1. Climatic and Seasonal Differences. 2. Clothing. 3. Age and Sex. 4. Activity. 5. Stay duration. 6. Air Velocity.

1. Supply of  O2 and Removal Of  CO2. 2. Removal Of  Heat Occupants. 3. Removal Of  Moisture Of  Occupants. 4. Good Air Distribution. 5. Maintaining Air Purity. 

 i ) Solar gain through glass planes. ii ) Solar gain through roof and walls. iii ) Heat gain from occupants. iv ) Heat gain from appliances and lights. v ) Duct leakage.  vi ) Infiltration. vii ) Vapor Transmission.

 I.) Based On Construction Of Components :  a ) Unitary Systems  b)  Central Systems   c) Package Systems  d) Split Units. II.) Based On Fluid Flow Methods :  a ) Direct expansion (DX) System,  b ) Chilled water (DX) System,   c ) Chilled water air washer System.

ROOM AC SYSTEM OR UNITARY AC SYSTEM : This is the simplest type of air conditioning system, assembled inside a casing.  The unit consists of :  i) Refrigeration system (involving the refrigerant), ii) Control System ( Thermostat and selector switch ). iii) Electrical protection system (motor overload switches and winding protection thermostat on the compressor motor), iv) Air circulation System ( Fan motor, Centrifugal blower ), v) Exhaust System .  The working of this system can be divided into various circuits. They are, I) Room air circuit :   * Air from the room to be conditioned is drawn by a fan through a filter.   * The hot air is passed through evaporator and gets cooled.   * This cool air is passed to the room thereby makes it conditioned. II) Refrigerant circuit :   * The refrigerant (which was liquid) becomes evaporated by the heat given by the uncooled room air.   * This vapor is compressed in the compressor and goes to the condenser, ...

The conditioning of air during winter season is totally different from summer air conditioning. It is necessary to heat air for comfort condition. But the R.H of the air decreases as it is heated, therefore , it becomes necessary to increase the R.H by humidifying the air. The air conditioning for winters heating and humidifying. The heating of air is generally done in two stages to simplify the controls. The winter conditions at Aurangabad are 15*C and 70% relative humidity. The required comfort conditions are 24*C and 60% relative humidity. The air is first passed through air filter to remove dust and then it is passed over heating coil. The temperature of air coming out of heating coil is controlled by the energy input to the coil. Then the air is passed through humidifying chamber where it is humidified with help of water sprays. The air coming of humidifier passed through eliminator to remove the water sprays. The air coming out of  the humidifier passed through eliminator to ...

 a) Summer Air - Conditioning For Hot and Wet Weather :      When the air is hot and wet, the treating of the air in air - conditioning system differs from other  methods. Because air contains more quantity of water vapour than the required quantity. So, the air conditioning system has to remove the water vapour by dehumidifying.      During hot weather, air is first passed through air filter to remove the dust and then it is passed through the cooling coil to remove water vapor. At that time, the temperature of air becomes below its dew -  temperature due to reduction in surface temperature of cooling coil. This surface  temperature of coil is   controlled by varying the quantity of  coolant.      During wet weather, the air coming out from the cooling coil is passed over the heating coil to heat   up  to the required temperature. Then the heated air is directly supplied to air conditioned sp...

Air conditioning Systems become popular necessary for industrial and public buildings such as offices, halls, cinemas, etc. to supply a controlled atmosphere. The comfort conditioned required in an air - conditioned space is 24*C dry bulb temperature and 60% relative humidity. Many states in India like Delhi, Rajasthan, West Bengal , Chhattisgarh having very high temperature (above 40*C) during summer and very low temperature during which created discomfort to the men. Therefore, It is necessary to have the summer air-conditioning as well as winter air-conditioning. In further topics both the types are explained in Detail.  So , The two types of Air conditioning are , * Summer Air - Conditioning System. * Winter Air - Conditioning System. 

The air quantity selected should offset the room sensible and latent heat load. It should also handle the total sensible and latent heat loads i.e., including the outdoor (fresh) air loads, etc. The air quantity can be determined from the formula, Q da = ERSH / ( ( t - tadp ) (1 - BF) 60 * p * C ) where, Q da    - Dehumidified air quantity (m3 / min),             ERSH - Effective Room Sensible Heat (kJ / hr),             t          - Room Design Temperature,             t adp   - Apparatus Dew point ,            C         - Specific Heat of Air,            BF       - Bypass factor of  the coil,            P          - Density Of Air. 

  1. Sensible Heat Factor ( SHF ) :   This is defined as the ratio of sensible heat (SH) to the total heat (TH) Of the given system.   i. e , SHF = SH / TH = SH / (SH + LH)    TH = SH + LH   2. Room Sensible Heat Factor ( RSHF ) :    RSHF is the ratio of room sensible heat (RSH) to room total heat (RTH). i.e,    RSHF = RSH / RTH = RSH / ( RSH + RLH )    RTH = RSH + RLH  3. Effective Room Sensible Heat Factor ( ERSHF ) :   Ratio of effective Room Sensible Heat to the Effective Room Total Heat is called ERSHF.  i .e , ERSHF = ERSH / ERTH = ERSH / ( ERSH + ERLH )  

The total heat load of the room is contributed by various sources. The total heat load can generally be divided into two broad headings.  1. Sensible heat load,  2. Latent heat load.  1. Room Sensible heat Load ( RSH ) : a) Solar gain - glass ( Q1 ) :  Radiation from sun can enter the conditioned space through glass and heat is absorbed by materials and air in the conditioned space.  Heat gain = A* R * MF  A    - Area of  Glass,  MF - Multiplying factor for type of glass/shading, R    - Solar gain. b) Solar transmission through walls and roof (Q2) :  As a result of  direct absorption of  the radiation of the sun , temperature of  walls and roof rises. This causes larger difference in temperature with respect to the conditioned space. c) Transmission through partitions and glass (Q3) :  In addition to the above , heat gain is possible through partitions and glass due to temperature difference.  d) H...

 When air is passed through cooling coil, it may get humidified or not. For Dehumidification, the coil is to   be kept at a mean temperature (t s ) which is below the dew point temperature (DPT) of the entering air (t d ). This temperature t s   is called 'Apparatus dew point' of the coil or simply ADP. As the temperature determines the condition of the air supplied. It is to be controlled to obtain the desired conditions. To find out the air quantity required and to select the equipment ADP is an important parameter. 

 As a result of  BF, the room sensible heat and latent heat (room total heat) go up due to the addition of sensible and heat of  the bypassed fresh air. This means that, the system has to remove a larger quantity of  heat due to B.F.  a) Effect Of Smaller BF : 1. Requires higher ADP ( Apparatus Dew Point ) 2. Requires less air, this reduces the size of  the fan and motor.  3. Provides more heat transfer area. 4. Smaller piping as less chilled water is sufficient.  b) Effect Of  Larger BF : 1. Requires lower ADP. This means lower refrigerant temperature at evaporator. Size and Capacity requirement of refrigeration plant is increased.  2. Requires more air - larger fan and motor required.  3. Less heat Transfer area. 4. Larger Piping is required because of more chilling water requirement.   

 By Pass factor Depends on the Following Parameters : 1. Pinch of fins. (As pitch decreases, BF also increases). 2. Number of coil tubes (rows). ( More number of coils less BF). 3. Velocity of air over the coil. (More the velocity of air, more the B.F).  4. Direction of airflow.  

For ventilation purposes, fresh air is drawn from outside and mixed with room air (return air) in the usual A/c system. This mixture ( Room air + Fresh air ) is then passed through cooling coil or chilled water washer. Thereby, the mixture is treated and this is sent to the room again. All the air passing through the cooling coil cannot come into contact with it. This is because of the gaps between fins and tubes. Thus, this portion of air which has no contact with cooling coil actually 'by passes' it. It comes out of the cooling coil at the same condition at which it entered and it is called 'By pass air'.  The state of  the contacted air is that of  the saturated air at the temperature of  the surface. The uncontacted (bypassed) air remains at the entering state.  The By pass Factor (BF) is defined as the 'ratio of  the  amount of  uncontacted (bypassed) air to the total amount of air passing through the surface'. i . e, By Pass factor,   ...