Airconditioning

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

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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

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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

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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.

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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

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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

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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

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4. Air Distribution

Air-conditioning

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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

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Comfort air-conditioning

• There are two kinds of air conditioners available for regular domestic use.

1. Window AC

2. Split AC

Window Air-conditioning

Window Air-conditioning

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Window air-conditioning

Typical Window AC, available in market

Window Air-conditioning

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Window Air-conditioning

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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

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Split Air-conditioning

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Split Air-conditioning

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Split Air-conditioning

Indoor unitOutdoor unit

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Split Air-conditioning

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Split Air-conditioning

Split Air-conditioning

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• 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.

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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

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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.

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Summer air conditioning

• 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

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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.

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2) Winter air conditioning