Introduction to Refrigeration

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Chapter 1: Introduction to Refrigeration
• Refrigeration may be defined as the process of removing heat
or heat energy from a body, that is below the temperature of
its surroundings.
Refrigeration
Natural
Mechanical
Compression
Absorption: Ammonia and
water vapor
Steam jetMd Rahmatuzzaman Rana
AssistantProfessor, Dept of FET, SUST
Objectives
• To understand why the storage life of foods and agricultural
products can be extended by refrigeration
• To learn the adverse effects of low temperature for certain
commodities
• To refresh our knowledge about thermodynamics, in particular the
first law of thermodynamics, the phase diagram and the Mollier
diagram
Md Rahmatuzzaman Rana
AssistantProfessor, Dept of FET, SUST
Md Rahmatuzzaman Rana
AssistantProfessor, Dept of FET, SUST
Purpose of refrigeration of foods
- reduces postharvest losses
- preserves foods
- makes it possible to provide safe, wholesome food all consumers
have the right to expect
- by refrigerated transport, allows countries to export perishable
produce
Md Rahmatuzzaman Rana
AssistantProfessor, Dept of FET, SUST

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How does it work?
Fig. Gas production rate as a function of developmental stage
Md Rahmatuzzaman Rana
AssistantProfessor, Dept of FET, SUST
Q10 (reduction in reaction rate if temperature decreases by 10°C) typically
2.5 for biochemical reactions
• Respiration
• Texture change
• Microbial growth
Example
- Predictive microbiology: prediction of microbial growth/inactivation in
various conditions of temperature, gas concentration, salt etc.
- E.g. Pathogen Modeling Program
Md Rahmatuzzaman Rana
AssistantProfessor, Dept of FET, SUST
Negative effects of refrigeration: Chilling injury
- Variety of symptoms: lesions, brown heart in pineaple, mealiness in
tomato, gery discoloration in banana
- Most tropical fruits are affected
- Typically below 12°C, depending on cultivar
- Believed to be caused by imbalance between saturated and unsaturated
lipids
Md Rahmatuzzaman Rana
AssistantProfessor, Dept of FET, SUST
Md Rahmatuzzaman Rana
AssistantProfessor, Dept of FET, SUST

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Md Rahmatuzzaman Rana
AssistantProfessor, Dept of FET, SUST
Importance of refrigeration
- Number of farmers has gradually increased: <5% of world population
- Cold chains are vital to ensure safety, quality and commercial value of perishable
foodstuffs
- 75% of foods are processed, the majority involves one or more refrigeration steps
- Foods or ingredients are transported 2.5 times
- Approximately 70% of foods in developed countries are chilled or frozen when
produced, and 50% of all foods sold require refrigerated display at retailer level
Md Rahmatuzzaman Rana
AssistantProfessor, Dept of FET, SUST
A few numbers
- Annual purchase of chilled and quick-frozen foods is around 1200 billion
USD
- Refrigeration generates jobs: 1 out of 1000 jobs in industrialized
countries are in the equipment manufacturing field
- 300000 jobs in the USA and 70000 in Europe in the installation and
sectors
- Annual investment in refrigerating equipment totals 170 billion USD
- More than 1 billion domestic refrigerators/freezers in the world
- 300 m3 of cold storage worldwide
Md Rahmatuzzaman Rana
AssistantProfessor, Dept of FET, SUST
Simple cooling systems
• Pot – in – Pot (M. Bah Abba)
- Two ceramic pots
- Smallest pot inside largest pot
- Wet soil in between
- Cooling via evaporation: latent heat (enthalpy removed from pot
- Rolex award
- Simple design, local manufacturing possible
Md Rahmatuzzaman Rana
AssistantProfessor, Dept of FET, SUST

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Thermodynamics of phase change
Md Rahmatuzzaman Rana
AssistantProfessor, Dept of FET, SUST
Temperature Vs Total Heat
Md Rahmatuzzaman Rana
AssistantProfessor, Dept of FET, SUST
State diagram
Md Rahmatuzzaman Rana
AssistantProfessor, Dept of FET, SUST
Units of Refrigeration
• Unit of refrigeration: Tonne of refrigeration (TR)
• A tonne of refrigeration is defined as the amount of refrigeration effect
produced by the uniform melting of one tonne (1000kg) of ice from and at
0 oC in 24 hrs.
• Latent heat of ice is 335 KJ/Kg, hence,
• 1TR= 1000 Kg * 335 KJ/Kg in 24 hrs
= (1000*335)/ (24*60)
= 232.6 KJ/min (in actual 210 KJ/min or 3.5 KW)
Md Rahmatuzzaman Rana
AssistantProfessor, Dept of FET, SUST

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Coefficient of performance (COP)
• COP = Amount of heat extracted in the refrigerator (Q)/ amount of
work done (W)
= Q/W
• It is reciprocal of the efficiency of a heat engine
Md Rahmatuzzaman Rana
AssistantProfessor, Dept of FET, SUST
Difference between Heat engine, Refrigerator and heat pump
COP (E) = Work done
(W)/ Heat supplied (Q2)
=(Q2 – Q1)/ Q2
COP (R) = Heat taken
(Q2)/ Work done (W)
=Q1/ (Q2- Q1)
COP (P) = Heat delivered
(Q1)/ Work done (W)
= Q1/ (Q2- Q1)
= COP (R) + 1
Md Rahmatuzzaman Rana
AssistantProfessor, Dept of FET, SUST
Simple vapor compression refrigeration system
• It is named as vapor compression, due to low pressure vapor
refrigerant from the evaporator is changed to high pressure
vapor in the compressor.
• Refrigerant: NH3, CO2, SO2
• Advantage: small, less running cost, can be run over large
range of temperatures, COP is high.
• Disadvantage: Initial cost high, leakage problem
Md Rahmatuzzaman Rana
AssistantProfessor, Dept of FET, SUST
Md Rahmatuzzaman Rana
AssistantProfessor, Dept of FET, SUST

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Md Rahmatuzzaman Rana
AssistantProfessor, Dept of FET, SUST
Md Rahmatuzzaman Rana
AssistantProfessor, Dept of FET, SUST
Mechanism of a simple vapor compression refrigeration
system
1. Compressor: Low pressure, temp vapor refrigerant from evaporator drawn into
compressor at high temp through suction valve. The highly pressed and temp
vapor refrigerant is discharged into the condenser through discharge valve.
2. Condenser (cooler): Refrigerant cooled and condensed here. At this stage,
refrigerant gives up its latent heat to the surroundings condensing medium
(air/water).
3. Receiver: the condensed refrigerant stored here in a vessel.
4. Expansion valve (refrigerant control valve): allow the liquid high pressure
refrigerant in a controlled reducing rate. Some portion of refrigerant evaporates
here but greater are evaporate in evaporator.
Md Rahmatuzzaman Rana
AssistantProfessor, Dept of FET, SUST
5. Evaporator: The liquid-vapor refrigerant are passed through evaporator
(consists of coils of pipe) at low pressure and temperature. Here, the
liquid refrigerant absorbs its latent heatof vaporisation from the medium
(air, water or brine) which is to be cooled.
High pressure side: discharge line, condenser, receiver and expansion valve.
Low pressure side: evaporator, piping from expansion valve to evaporator and
the suction line.
Check the link for more: https://www.mobileair.com/refrigeration-cycle
Md Rahmatuzzaman Rana
AssistantProfessor, Dept of FET, SUST

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Pressure- enthalpy diagram
Md Rahmatuzzaman Rana
AssistantProfessor, Dept of FET, SUST
• Red line: temperature
• Green line: constant volume curve
• Black line: quality curves or mass fraction of vapor
Md Rahmatuzzaman Rana
AssistantProfessor, Dept of FET, SUST
Factors influencing COP
1. There is no pressure loss in condenser, evaporator, connecting pipes and
compressor valves.
2. There is no heat transfer form or to the system at any point except the
condenser and evaporator
3. Compression is isentropic.
4. Transmission efficiency is from motor to compressor is 100%
5. Condenser temperature increase then COP is decrease due to
decreasing refrigeration effect.
6. Subcooling
7. Superheating
Md Rahmatuzzaman Rana
AssistantProfessor, Dept of FET, SUST