Chapter: Mechanical and Electrical - Thermal Engineering - Refrigeration and Air Conditioning

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Refrigeration

Refrigeration is a pro cess in which work is done to move heat from one location to another. The work of heat trannsport is traditionally driven by mechanical work, but can also be driven by heat, magnetism , electricity, laser, or other means.


REFRIGERATION


CONCEPT OF REFRIGERATION

 

Refrigeration is a pro cess in which work is done to move heat from one location to another. The work of heat trannsport is traditionally driven by mechanical work, but can also be driven by heat, magnetism , electricity, laser, or other means.

 

How does it work?


Thermal energy moves from left to right through five loops of heat trans fer:

 

1)    Indoor air loop

2)    Chilled water loop

3)    Refrigerant loop

4)    Condenser water loop

 

5)    Cooling water loop


 

SIGNIFICANCE

 

Refrigeration has had a large importance on industry, lifestyle, a griculture and settlement patterns. The idea of preserving food dates back to the ancien t Roman and Chinese empires. However, refrigeration technology has rapidly evolved in the last century, from ice harvesting to temperature-controlled rail cars. In order to avoid food spoilage, refrigeration plays an important role in day to day life, similarly, Air conditioning is also an important technological system to prevent the human from the hot atmosphere during summer seasons.

 

CLASSIFICATION OF REFRIGERATION SYSTEM

Types of Refrigeratio n

 

         Vapour Compression Refrigeration (VCR): uses mechanical energy

 

         Vapour Absorption Re frigeration (VAR): uses thermal energy

 

Vapour Compression Refrigeration

 

         Highly compressed flu ids tend to get colder when allowed to expand

 

         If pressure high enough

 

         Compressed air hotter than source of cooling

 

         Expanded gas cooler than desired cold temperature

 

•   Lot of heat can be rem oved (lot of thermal energy to change liquid to vapour)

 

•   Heat transfer rate re mains high (temperature of working fluid mu ch lower than

 

what is being cooled)

Vapour Compression Refrig eration Cycle


Evaporator

 

Low pressure liquid re frigerant in evaporator absorbs heat and changes to a gas

 

Compressor

 

The superheated vapo ur enters the compressor where its pressure is raised

 

Condenser

 

The high pressure sup erheated gas is cooled in several stages in the co ndenser

 

Expansion

 

Liquid passes through expansion device, which reduces its pressure an d controls the flow into the evaporator

 

Type of refrigerant

 

         Refrigerant determine d by the required cooling temperature

 

         Chlorinated fluorocarb ons (CFCs) or freons: R-11, R-12, R-21, R-22 and R-502

 

Choice of compressor, design of condenser, evaporator determined by

 

         Refrigerant

 

         Required cooling

 

         Load

 

         Ease of maintenance

 

         Physical space requirements

 

         Availability of utilities (water, power)

 

 

Vapour Absorption  Refrigeration

 


Evaporator



Absorber

 


High pressure generator


Condenser


Evaporative Cooling

 

         Air in contact with water to cool it close to ‘wet bulb temperature’

 

         Advantage: efficient cooling at low cost

 

         Disadvantage: air is ri ch in moisture

 

COMPARISON BETWEEN VAPOR COMPRESSION AND A BSORPTION SYSTEM


 

PERFORMANCE

 

Assessment of Refrigeration

 

         Cooling effect: Tons of Refrigeration

 

1 TR = 3024 kCal /hr heat rejected

 

         TR is assessed as:

 

TR = Q x×Cp x× ( Ti  To) / 3024

 

Q =       mass flow rate of coolant in kg/hr

 

Cp =     is coolant specific heat in kCal /kg °C

 

Ti = inlet, temperature of coolant to evaporator (chiller) in 0°C To = outlet temperature of coolant from evaporator (chiller) in 0°C

 

Specific Power Consumption (kW/TR)

 

         Indicator of refrigeration system’s performance

 

         kW/TR of centralized chilled water system is sum of

 

         Compressor kW/TR

 

         Chilled water pump kW/TR

 

         Condenser water pump kW/TR

 

         Cooling tower fan kW/TR

 

Coefficient of Performance (COP)

 

         The  performance  of  refrigerators  and  heat  pumps  is  expressed  in  terms  of

coefficient  of performance (COP), defined as


Measure

 

         Airflow Q (m3/s) at Fan Coil Units (FCU) or Air Handling Units (AHU): anemometer

 

         Air density r(kg/m3)

 

         Dry bulb and wet bulb temperature: psychrometer

 

         Enthalpy (kCal/kg) of inlet air (hin) and outlet air (Hout): psychrometric charts

 

APPLICATIONS OF REFRIGERATRION

 

Ø Metal workers

Ø Oil refineries

Ø Chemical plants

 

Ø  Petrochemical plants

Ø  Transporting temperature-sensitive foodstuffs

Ø Dairy products

 

 


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