COURSE NO : ME 01

REFRIGERATION

Refrigeration cycles transfer heat from a lower temperature region to a higher temperature one, at the expense of work input to the system.

Practical refrigeration cycles include:

Vapour-Compression Cycle: Refrigerant is vaporised and condensed alternatively at different pressure levels.

Vapour-Absorption Cycle: The refrigerant is dissolved in a liquid before it is compressed.

Gas (air) refrigeration cycle: The refrigerant is in gaseous form throughout the cycle.

Other systems: Thermo-electric Refrigeration, Vortex-Tube Refrigeration etc.

Both may operate on the same thermodynamic cycle, but at different reference w.r.t environmental temperature.

The objective of a refrigerator is to remove heat from a cold space/medium.

The objective of a heat pump is to supply heat to a warm space/medium.

Same machine can run as a refrigerator or a heat pump.

COP = Co-efficient of performance

COP refrigerator = Cooling effect/ Work input

COP heat pump = Heating effect/ Work input

COP heat pump = COP refrigerator +1

EER = Energy Efficiency Rating, It shows the BTU of heat removed per W-h electricity

EER = 3.412 COP

Typical COPref = 2.5-3.5, EER = 8-12

Refrigeration Capacity

Ton of Refrigeration: It is the cooling capacity that can freeze 1 ton (2000 Ib) of liquid water at 0(C into ice at 0(C in 24 hours.

1 ton Refrigeration = 200 BTU/min = 12000 BTU/h = 211kJ/min = 3024 Kcal/h = 3.516 kW

Refrigeration Performance: Co-efficient of Performance (COP)

COPref = Cooling effect / Work Input Ranging 3-6

EER = Energy Efficiency RatingEER = 3.412 COP Ranging 8-12

APPLICATIONS OF REFRIGERATION

Ice productionWater CoolingCold StorageFridges

Air ConditioningLiquefaction of Gases

PROPERTIES OF A REFRIGERANT

Latent heat of Vaporization should be high

Specific heat should be low

Critical Temperature and pressure should be above the operating range

COP should be high, ie. Less power requirement for cooling in desirable

Evaporator pressure must be above atmospheric (easier detection), Condenser pressure should not be too high (higher equipment cost)

Thermal conductivity should be high

Freezing point should be below the lowest operating temperature

Refrigerant must be chemically stable, non-corrosive

Refrigerant must be non-toxic, non-explosive

It should have appropriate dielectric properties

The cost of the refrigerant is a significant criteria, specially for large units

RIGERGERANT TYPES

CFC Refrigerants : R-11, R-12 (CF2CL2, Di-chloro Di-floro Methane), Chlorine of Chloro-Floro-Carbon (Freon) breaks up the atmospheric Ozone, causing depletion of Ozone layer.

HCFC Refrigerants : Hydrogenated CFCs, eg. R-22, Structure contains H atoms which react with Chlorine first as they breakup, saving the O3.

HFC Refrigerants : Hydrogenated Floro-Carbons, R-134a, does not contain any Chlorine atom, so much more environment friendly.

The problem was detected in early 70s ; Montreal Protocol was signed in 1992 ; Hopefully the depletion level will go back to pre-70 condition by 2025.

Other Refrigerants : NH3 (R-717), CO2 (R-744), Water (R-718), Air (R-729) etc.

AZEOTROPES : It is a mixture of two or more substances that behave like as if they are a compound. They can not be separated by distillation and evaporates and condenses like a single substance. However the properties of an Azeotrope are different from those of the constituents. Example R 502 (48.8% R22, 51.2% R115 by mass).

Practical Vapour Compression Cycle with super heating and sub cooling.

Both measures enhance the cooling effect in the evaporator.

Components of Typical Air Conditioning System (Vapour-Compression)

Schematic Diagram of a NH3-H20 Absorption System

It takes much less power to pressurize the gas once it is dissolved in a liquid. However additional heat is necessary to release the high pressure gas.

Refrigerant : NH3Absorbent : H2O

Some applications use the exhaust heat of a power plant to drive an absorption chillier. Such combined heat and power applications are examples of Cogeneration.

The absorption refrigerator is a refrigerator that uses a heat source (e.g. exhaust heat, solar, kerosene-fueled flame) to provide the energy needed to drive the cooling system. Absorption refrigerators are a popular alternative to regular compressor refrigerators where electricity is unreliable, costly, or unavailable, where noise from the compressor is problematic, or where surplus heat is available (e.g. from turbine exhausts or industrial processes). An absorption refrigerator changes the gas back into a liquid using a different method that needs only heat, and has no moving parts. An absorption refrigerator changes the gas back into a liquid using a different method that needs only heat, and has no moving parts. An absorption refrigerator dissolves the regrigerant vapour into a liquid solution allowing it to be pressurised using a pump and changes the gas back into a liquid using a different method that needs only heat, and has no moving parts.

CASCADE REFRIGERATION

In some industrial applications moderately low (eg. -75(C) temperatures are required, which involve a temperature range too large for a single vapour compression refrigeration cycle to be practical. A large temperature range also means a large pressure range in the cycle and poor performance of a reciprocating compressor. This is achieved by operating two or more suitable refrigeration cycles in series, where the condenser of one acts as the evaporator of the cycle above. Such systems are known as cascade refrigeration cycle. Same or different refrigerants may be used in cycles that work in series. Such refrigeration applications at very low temperature is termed as Cryogenics.

2-stage cascade refrigeration systems with R-22 and R-13

AIR CONDITIONING

Broadly Air conditioning is the process of a combination of control of

Temperature, Humidity,Cleanliness, Noise - aimed for human comfort.

Air conditioning is the automatic control of temperature and humidity to values which provide a sense of comfort for human beings.

This is achieved by

Regulation of Temperature and Humidity

Supply and control of Fresh Air

Filtration of removal of dust particles

Proper distribution of conditioned air

Noise control (indirect control)

CLASIFICATIONS

Based on type of Application :

Comfort Air Conditioning : Purpose is to create atmosphere conducive to human beings - eg. Residential/Commercial buildings, Auditorium, Community centres, Hotels.

Industrial air conditioning : Purpose is to create atmospheric conditions suitable for conducting certain manufacturing, production, physical and chemical processing eg. Textile, Pharmaceuticals and Electronic industry.

Special Air conditioning : Purpose is to create a special environment where the temperature, relative humidity, air purity etc needs to be modulated for a certain task eg. space training, military training, amusement/sports resorts etc.

Based on type of configuration :

Unitary Systems (Direct Expansion Systems)

These are air conditioning units which are mass produced, which are installed within or near the conditioned space. Here the air comes in to direct contact of the cooling coils in which the refrigerant evaporates extracting heat. They are manufactured to limited capacity (much less than 50 tons) and the system layout can not be changed very much.

This type of air conditioners are packaged as self-contained Window type and divided Split type packages.

Window Air conditioner (Room Air Conditioner) :

These are self contained units mainly used for residential or office air conditioning. Typical unit capacity varies 0.5 ton 3 tons. These may be window mounted or through the wall mounted. Components consist of hermetically sealed compressor, direct expansion coil, condenser, filters and fans. Manual and remote control of cooling and ventilation are generally provided.

Portable Air Conditioning Systems

These system can be used inside any room of a building. The trolley mounted unit is self containing, but fitted with a thermally insulated duct to remove the heat as a hot air blast. The construction is very similar to window type air conditioning.

Split Air Conditioning System

Here the evaporator coil with a fan is installed in the conditioned space, while the remote unit contains the compressor, condenser and the expansion device. This system allows keeping noise, vibration and maintenance away from the conditioned space. Typical units are made from 1.5 tons 25 tons capacity. However ventilation arrangement may be needed to circulate fresh air in the conditioned space.

The Domestic Fridge

Domestic refrigerator contains - a hermetically sealed compressor pressurising the refrigerant, a condensing unit located at the back an expansion device(capillary tube) and a evaporator. The evaporator is located at the ice chamber, which really cools the system. Depending on the load requirement the compressor may be kept on and off intermittently. The storage space with sub-zero temperature is called freezer and remaining cold space is termed as refrigerator. Often gross/net storage space is expresses in m3 or cft.

ALL AIR SYSTEM

In all air system meets all the cooling required by cold air supplied by a central air handling unit (AHU), which is distributed to various parts of a building as per requirement. The air from the room is returned back to the AHU, mixed with some fresh ventilation, cooled and sent to the rooms again. This system is more expensive but makes good humidity control.

ALL WATER SYSTEM

In all water system meets all the cooling required by chilled water supplied by a central refrigeration system, which is distributed to various parts of a building as per requirement. In each room Fan-Coil Units (FCU) are used to cool the room air. In the FCU a fan draws in a mixture of room air with some fresh air and blows it across the cooling colid of chilled water. This system is less costly but can not achieve good humidity control.

Typical Fan Coil Unit (FCU)

WATER-AIR SYSTEM

This system employs a combination of both air and water circulation to control the room air condition. This may require less air to be circulated through AHU as part of the heat is taken away by smaller FCU. Make the system more cost effective in large or high rise buildings.

COOLING LOAD

Cooling load in a room can be estimated by summing the possible heat gains through various ways. These may include -

Standard calculation procedures are used for the cooling load calculation, eg. using ASHRAE code.

ASHRAE : American Society of Heating, Refrigeration and Air-Conditioning Engineers.

Refrigeration and Air Conditioning