AIR CYCLE • An integral part of air-conditioning system. Refrigeration

cycle is part of air cycle. • Both Air cycle and Refrigeration cycle is not independent

but inter dependent.• Indoor air can be too hot, too cold, too wet, too dry etc.

These can be changed by treating the air.• The treatment of air includes cooling of hot air, heating of

cool air, adding or removing moisture, creating air movement etc.

• The treatment provided in the air cycle is part of the air-conditioning.

• Major components of air cycle - Fan, Supply air Duct (SAD), Supply Air Grille (SAG), Space to be conditioned, Return Air Duct (RAD), Return Air Grille (RAG) with filter, heating or cooling coil.

COMPONENTSFAN• Circulates, creates air movement & Forces air from

supply duct to room• Can be regulated to prevent draft SAD – Supply Air Duct• For uniform air distribution, Minimum turns & lengthSAG – Supply Air Grille• Uniform distribution by jet or flow or combination of both• Number of outlets and their design is vital for comfort.ROOM SPACE• Most important component, & a must for working of an

air cycle• The material used for floor, wall, ceiling etc are very

important to prevent heat loss or gain.

RAG – Return Air Grille• Allow used air to escape through outlets & best location is

opposite SAGRAD – Return Air Duct• From RAD the air passes through this duct & filter to the

Cooling coil.• RAD can be eliminated if plenum is used.FILTERS• To remove dust, dirt, pollutants, contaminants etc, location

in RAD • Materials - spun glass, plastic etc, • Attract and capture dust particles with the help of electricityCOOLING COIL OR HEATING CHAMBER• Located only after the filter to prevent accumulation of dust

and dirt over coil and before SAD• If the air is too dry, moisture is added with shallow pan of

water, above heating chamber OR after the cooling coil in the SAD

• Cooling coil can be used for cooling air or to reduce the humidity

REFRIGERANT CYCLEREFRIGERANT• Any liquid can be used for absorbing heat by

evaporation. Water can also be used as a refrigerant but its boiling and freezing temperature is too high for ordinary cooling purposes.

• A change of state of refrigerant from liquid to vapour and back to liquid helps to absorb or discharge large quantity of heat efficiently. This has been efficiently used in refrigeration cycle and Air-conditioning.

• The selection of refrigerant depends on specific application and the context.

Any refrigerant must satisfy two main conditions.1. It must readily absorb heat at the temperature required

by the product load.2. For economy and continuous cooling, the system must

use the same refrigerant over & over again.

OTHER FEATURES OF REFRIGERANT• To absorb heat from a cooling coil by evaporating at low

temp & Pr. • It gives up heat by condensation at a higher temp and

pressure.• Ammonia (R-717) - earliest refrigerant. Boiling point –28 F

but it is very corrosive and toxic. • R12 is Trichloride di fluro methane & its boiling point is -

21.7 F. • Some refrigerants boils at very low temp, as low as -40

deg C• A boiling refrigerant absorbs huge quantity of heat from

surroundings hence are most suited for refrigeration purpose.

• Some refrigerants cause damage to the ozone layer• Ozone friendly refrigerants - gradually replacing old types

Secondary Refrigerant• Large a/c areas • Cost of main

refrigerant • pumping to AHU/FCU • Water or brine is

chilled in the evaporator (chiller)

• pumped to AHU or FCU at different places through insulated pipes

• Secondary refrigerent runs in evaporator coil in AHU or FCU

REFRIGERATION• It is the process of producing and maintaining

temperature below the surrounding atmosphere• it is nothing but the removal of heat from the

substance to be cooled to reduce its temperature below atmosphere

• Temperature and humidity is controlled by refrigeration process

• One ton of refrigeration is equivalent to the amount of heat required to be removed from 1 ton of water (2100 Lb) at 32 F in 24 hours

• One ton refrigeration (TR) = 1200 British thermal units (BTU)

REFRIGERATION CYCLE

• Common refrigeration can be accomplished by several different processes. In the great majority of applications vapor compression system is adopted. However vapor absorption system and steam jet vacuum system is also successfully used in many applications.

• Compressor, the main component in vapor compression system involves with two types of pressure. First, evaporating at low pressure and second, condensing at high pressure.

REFRIGERATION CYCLE

REFRIGERATION CYCLE

REFRIGERATION PROCESS• Liquid refrigerant evaporates in cooling coil

(evaporator). • Refrigerant boils at very low temp like, R-22 at-

40deg C• The cooling coil with refrigerant is in contact with

air surrounding it. Chilled water (Secondary refrigerant) is distributed to AHU or FCU in coils.

• Fins are used to get clear contact of all air that is passing through the cooling coil.

• This air is distributed to the conditioned spaces through ducts

• The hot refrigerant comes back to evaporator again in a cycle as cold refrigerant

BASIC LAWS OF REFRIGARATION • Fluids absorb heat while changing from liquid to

vapor.• Vapor releases heat while changing from vapor to

liquid • Heat cannot be destroyed but can be transformed

from one substance to another.• Heat flows from higher temperature to substance at

lower temperature • Heat and other forms of energy are mutually

convertible• Temp of a liquid can be raised by increasing

pressure• Temp of a liquid can be lowered by reducing

pressure• Boiling temperature of refrigerant is very low

A TYPICAL AC UNIT

COMPRESSORDESIRED TEMP IS ACHIEVED IN EVAPORATOR BY THISTYPES• It is similar to automotive engine making alternate suction and

compression strokes in a cylinder equiped with suction and discharge valve. It is suitable for small displacement volumes. It is efficient at high condensing pressures and high compression ratios. Its adaptability to different refrigerant is also an advantage. The other advantages are it is durable, simple in design and economical.

Open type compressor:• Earlier version of compressor in which only the piston and cylinder

alone were sealed. A crank shaft was extended around the body with seal around to prevent leakage of refrigerant as well as oil. It is very heavy, large in size, noisy and seal failure was common. Due to these reasons it is not used frequently.

Accessible type Hermetics (Semi Hermatics)• driven by electric motor mounted directly on the crank shaft -

compact, efficient, maintenance free and economical. - capacity of 17.5kw to 35kw

• Welded Hermetic motor compressor:• It is small in size and used for 17.5kw and below capacity. All the

parts including the electric motor is housed in a shell and sealed by welding. Hence field repair is not possible. It is very widely used for unitary air-conditioning system.

• In this there are three types are available as below.• Reciprocating compressor: Available from very small fractional

capacitiy and upto 35kw.• Scroll compressor: Very efficient and saves power. Available upto

50kw. The plus points are, high efficiency, less noise, high reliability and low maintenance. Tandem scroll type is an advanced type and is very energy efficient type.

• Rotary compressor: It is less noisy when compared with reciprocating type. Due to this they are widely used in Window AC units and to some extent in Split units.

CONDENSER

COMPRESSOR

• COMPRESSOR IN REFRIGERATION CYCLE• The hot refrigerant vapour from cooling coil

moves to compressor. • The compressor compresses the hot vapour in

to a high-pressure vapour. • This increases the temperature of vapour

further. • The compressor automatically maintains low-

pressure condition in cooling coil and creates high-pressure condition in condenser.

CONDENSER• Heat exchange takes place between refrigerant (hot high

pressure vapor) and air) or waterAIR COOLED CONDENSER• Very commonly adopted type, inexpensive, easy to install

and maintain• Fan forces large quantity of air through compact

condenser assembly.• Satisfactory in all climates.WATER COOLED CONDENSER• Economical - water available in abundance or as recycled• Water has excellent heat transfer characteristics• Types - Shell and coil, Shell and tube, Tube with in tube• Requires a cooling tower• spray ponds – Water aerated and cooled in the form of

fountain jets• Requires plumbing & pump due to cooling tower• Cold weather - freezing of water may create problem

CONDENSER IN REFRIGERATION CYCLE

• High-pressure hot vapor refrigerant passes to condenser• The condenser motor drives a condenser fan • Blown air receives heat - refrigerant vapor turns to

liquid• This liquid is collected in a receiver. • Remember this liquid is high-pressure and high temp• Refrigerant movement from condenser to evaporator is called as ‘liquid line’• To remove heat – air – Air cooled condenser - water - water cooled condenser

EXPANSION VALVE• Liquid refrigerant under high pressure flows through

this• Release of pressure reduces temp of liquid - cold

refrigerant• Capillary tube or Restrictor is an alternative to EV • Cold refrigerant pass through cooling coil & cycle

continues• Expansion valve also controls proper flow of

refrigerant• Other flow control devices equivalent to EV areAutomatic expansion valve, Hand expansion

valve,Thermal expansion valve, Low side floats, High side floats, Capillary tube

EVAPORATOREVAPORATOR OR COOLING COIL• Located at low pressure of the refrigeration cycle. Here

refrigerant boils or evaporates, absorbing heat & changes into vapor.

TYPES OF EVAPORATORS• BLOWER COIL OR FORCED CONVECTION TYPE –

Here the refrigerant evaporates in side the tube with fin or plate around, extracting heat from air blown through the coil / fin.

• DIRECT EXPANSION TYPE –Refrigerant is fed directly into the coil through a metering device like expansion valve, capillary tube etc. The heat is absorbed directly through the walls of the evaporator.

• CHILLER – In a cylinder the coils with refrigerant is placed and secondary refrigerant like water or brine is fed into it. This chilled water / brine and is pumped to AHU or FCU

FCU & CHILLER

EVAPORATOR OR COOLING COIL IN REFRIGERATION CYCLE

• The cold refrigerant liquid moves through cooling coil and picks heat from the surrounding air.

• Surrounding air looses heat & becomes cold. Refrigerant boils and turns to vapor.

• Refrigerant from evaporator to compressor as vapor is called as ‘suction line’