Question answers on air conditioning

make it easier to understand. Knowledge

increases to handle difficult situations.

Air conditioning is of two types. These are

comfort and industrial air conditioning.  

Comfort is for living beings. Industrial air

conditioning increases production with

improved quality.

Fans are used in refrigeration and air

conditioning. These are with evaporators

and condensers. Requires reliable &

efficient fans.

Q. Definition of Air conditioning

It is a process of controlling simultaneously temperature,
humidity, purity and velocity of air. i.e.
(i) Temperature between 22 to 270C
(ii) Relative humidity 50 to 60 %
(iii) Dust particles of size >   100  microns are not permitted
(iv) Velocity 8 to 10 m/s

Q.  Explain inside design conditions (IDC).

These refer to
(i) for human comfort
(ii) industrial processes.
Human Comfort
People sat in different halls under different inside conditions. They gave their response. Finally decided the combination of temperature, humidity, velocity and purity. These gave maximum comfort to larger population.
These conditions are
Temperatures                       22-270C
Relative Humidity              35-60%
Velocity of air                        8-12 m/min
Purity of air                            Cleans the air with particles of sizes up tp 100 um
Contaminants are present. Table gives their permissible sizes.
Table: Contaminants




< 100 um
From natural and mechanical processes
o.1 – 0.3 um
Incomplete combustion
< 1 um
Condensation of vapor
2-60 um
Condensation of vapor
60-200 um
Atomizing and spraying
0.003 – .06 um
0.4 – 5.0 um
Fungal spores
10-100  um
Industrial Processes need
Brewery                                             5-100C,                       30-50 % RH
Textile industry                             24-260C                      50-85 % RH
Drug industry                                  24-260C                     20-40 % RH
Printing industry                            20-300C                     45-80 % RH


These are average outside conditions for the major portion of summer at a certain city. These are ODC summer conditions. Cooling  gives comfort. Similarly there will be outside winter conditions based on major period of winter. Hence, it will require heating to provide comfort. Thus ODC differ from state to state and country to country.
Window air conditioner,
Air conditioning of cinema halls,
Libraries air conditioning,
Air conditioning of computer centers,
Hotel air conditioning
Air conditioning of cars, buses, trains and aircrafts
Hospitals  air conditioning
Air conditioning of an air-ports
Mall air conditioning
Industrial applications such as in textiles, printing, manufacturing, photographic


(i) Precision measurements for performance testing of materials, equipment and processes.
Research in electronics and biotechnology areas require very clean atmosphere.
(ii) Multi-color printing requires control over temperature & humidity. Dry the ink of one color before it goes to the second press. This way the colors do not smudge. There is no shrinkage of paper & no distortion of picture.
(iii) Make precision Parts under controlled conditions to maintain close tolerances.
(iv) Used in textile Industry for the manufacture of better quality thread
(v)  Air conditioning provides dry clean air to the pharmaceutical Industries.
(vi)  Maintain photographic material for filmmaking at low temperature and low humidity. High temperature and humidity deteriorates the photographic material.
(vii) The yield of farm animals like Jersey cows decreases drastically during summer months. Low temperature helps in more efficient digestion of food, increase in weight of cow and the milk yield.
(viii) Used in computer rooms which require control of temperature, humidity and perfect cleanliness. The temperature & relative humidity is around 25oC and 50%. Micro filters keep rooms clean.  Dust  spoils the compact discs and the printers.
(ix) Power plants use air conditioning for working persons and the microprocessors.

Q. Differentiate between split A.C system and window A.C system.

Window A.C.

 Fit the air conditioner in the window. Keep the cooling coil  inside of  room. Condensing unit is outside of the room. Hence there are changes in the window. Hot air from the condenser is available in the corridor which is not desirable. But the original cost, installation and operation charges are less as compared to the split A.C. system.

 Split A.C. System

 Place evaporator cabinet anywhere in the room. Place the condensing unit outside the room. Lengths of tubes, number of joints and  bends increase.  The chances of leakage are more because of more joints. The pressure drop increases and hence the running charges increase. But room is neat and clean.  No changes are in window as in a window air conditioner.

Q. What are the various losses in the ducts of air conditioning systems?

Losses in a duct

(i) Frictional loss:

There is a pressure losses due to friction. These are proportional to velocity and length and inversely proportional to diameter. Minimize these losses by taking diameter of the duct equal to diameter of the fan at the inlet.

(ii) Shock loss

 This type loss occurs wherever there is a sudden change in the direction of flow. Even gradual transition in area of flow causes this loss. For example a taper of 3.50 produces 22% loss of velocity head pressure. An angle of 300 or more causes a pressure drop equivalent of velocity head pressure.

(iii) System effect loss

These are due to flow pattern changing at entrance or at the exit of the duct.

(iv) Leakage loss of cold air

(v) Heat gains through ducts

It increases the cooling required. Hence, is a loss in operating cost.

 Q. What is comfort zone?

It is an area on the Psychrometric chart. Obtain it from comfortable conditions of temperature and relative humidity. The ranges are 22-270C with RH 35 % to 65 %.

Q.  Differentiate between Ventilation load and Infiltration load.

Ventilation Load

 It is a heat load on the air conditioning plant due to fresh outside air entering. In the absence of any specific information, assume it as 12 air changes per hour. One air change is volume of room.

Infiltration Load

 It is also a heat load due to outside air. Outside air enters due to door openings while entering or leaving the room. For each door opening, assume amount of outside air entering as 2 air changes per hour.

Q. Write the function and advantages of cooling towers.


Cooling towers cool the hot water from the condenser.  Use same water time and again. It reduces the cost of water  in the condenser.

Advantages of cooling tower in refrigeration industry are

(i) Space requirements are very less,

(ii) Cost of water significantly reduces.

Q. Which air conditioner do you recommend for small residences /offices?.

(a) Use Window type air conditioner.

(i)  Each separate room uses a separate air conditioner.

(ii)  This is convenient.

(iii) Its initial & running cost is less.

(b) Split air conditioner

It has more initial & running cost. This is because of more length of pipes and more pressure drops. Otherwise it looks good. Makes no changes to the windows.

Q. Discuss the various types of Fans/Blowers used in Refrigeration.

Fans and blowers provide air for ventilation and cooling purpose.  Evaporators and condensers use fans. These fans generate pressure to move air.
Difference between Fans, Blowers and Compressors
Method of moving air is different. The pressure created is different in fans, blowers and compressors. These are different in terms of specific ratio. It is the ratio of discharge pressure to the suction pressure.
Type of Equipment                                        Pressure ratio
Fan                                                                       1 to 1.11
Blower                                                                 1.11 to 1.20
Compressor                                                        Greater than 1.20

Refrigeration systems have to runs 24 hours a day. Reliable and efficient fans are required. There are two types of fans.

Axial Fans

In axial flow, air enters and leaves the fan axially. There is no change in direction.  Axial flow fans are  of three types

(i) Vane axial

(ii) Tube Axial

(iii)Propeller type

Centrifugal Fans—–Air enters axially then moves radially and but leaves tangentially. In centrifugal fans airflow changes direction twice, once when entering and second when leaving. Centrifugal fans are also of three types
(i) Forward Curved

(ii) Straight Radial

(iii)  Backward Curved


Centrifugal fans                                          Axial flow fans

(a)  Handle large volume of air                handle less volume of air

(b)  Air enters axially and leaves       air enters and leaves axially


(c)  These are forward curved & radial          There are Vane axial        straight & backward curved                                                                                                                                           Tube Axial and                                                                                     Propeller                                                                                               type.


Selection of Fan and blower depends on the volume flow rate, pressure difference , space limitations, and efficiency. Efficiencies of fans & blowers are given below.
Type of Equipment                                      Approximate % Efficiency
Axial Fan                                                        ——–
Vane axial                                                      80
Tube Axial                                                      70
Propeller                                                         50
Centrifugal Fans                                         ——
Backward curved                                           80
Straight Radial                                                70
Forward curved                                              60


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