Chapter 1 - Introduction to Refrigration and Air Conditioning.pdf

Course: Refrigeration and Air-Conditioning(ME 331) Spring Semester- 2014 Instructor: Dr. Abdul Waheed Badar Course Designation

Core

Credit

3

No. of Sessions/Week

2

Total Session Duration

3 hrs

Mechanical Engineering Dept. CEME NUST

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Course: Refrigeration and Air Conditioning (ME 331) Recommended Books:  Refrigeration & Air-Conditioning by Wilbert F. Stoecker / Jerold W. Jones

 Air Conditioning Principles and Systems by Edward G. Pita  Refrigeration & Air-Conditioning by RC Arora  Modern Refrigeration & Air-Conditioning by Althouse, Turnquist, and Bracciano  Refrigeration and Air Conditioning Technology by William C. Engineering Dept. CEME NUST 2 Whitman, William M.Mechanical Johnson, John A. Tomczyk, Eugene Silberstein

Course Contents S.No.

Description

1.

Introduction to Refrigeration and Review of Fundamentals

2.

Vapor Compression Cycle

3.

Multi-pressure Systems

4.

Air Cycle Refrigeration System

5.

Absorption Refrigeration System

6.

Refrigerants

7.

Psychrometry

8.

Air Conditioning Systems

9.

Heating and Cooling Load Calculations

10.

Fluid Flow in Piping and Ducts

11.

Fans and Air Distribution Mechanical Engineering Dept. CEME NUST

3

Probable Grading Policy

1. Quizzes

10 %

2. Sessional Exam # 1

15 % Subject to Change

3. Sessional Exam # 2

15 %

4. Projects/Presentations

10 %

5. Final Exam

50 %

Total

100 %


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Introduction Refrigeration History



One of the greatest concerns of mankind has been Preservation of Foods like various meats and vegetables



Storage in Caves where the temperature remained lower than the outside air



Refrigeration was first employed by the Romans, Greeks, and Egyptians.

 

The First Methods of Cooling Foods consisted of either placing the foods in vessels immersed in Streams of Cool Water or storing them in holes in the ground Natural Ice: Melting of the ice absorbs some of the heat of the foods and reduces the temperature


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

Manufactured Ice: Beginning of the 19th century, ice boxes were used in England

o

At about the same time, some improvements in the 6 construction of household refrigerators are made.



 1910: Mechanical Domestic Refrigeration was first appeared  By 1918: Kelvinator produced the first Automatic Refrigerator

for the

American market

o o  

1918-1920: 67 machines were sold Now over 10 Million units are sold each year

1928: First of the Sealed Automatic Refrigeration units was introduced by General Electric 1927: Automatic Refrigeration Units, for the comfort cooling as a part of airconditioning were appeared

 1940: All domestic units were of the sealed type. .  From late 1930’s, Air-conditioning of Automobiles has also grown rapidly. Mechanical Engineering Dept. CEME NUST

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 

Starting in the 1960s, Home Air Conditioning Market experienced tremendous growth By 1990, the Automobile Air-conditioner became as standard as the automatic transmission


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Introduction How cold preserves food

  



Spoiling of food is actually the growth of bacteria in the food. Cold or low temperature slows up the growth of these bacteria and prevent foods from spoil. Slow freezing of the food results in the formation of large crystals which ruptures the food tissue. Fast freezing at very low temperatures (0 to -15 oF) forms small crystals and the food tissues are not injured. .


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Introduction How cold preserves food


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Introduction

Application of Refrigeration

Ice Making

Preservation and transportation of perishables

Special Industrial Processes

Air Conditioning

Chemicals manufacturing, Petroleum Refineries, paper and pulp industries etc. Cold Store, Refrigerated Transport, Domestic refrigerators and Freezers


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Introduction

  

Refrigeration Process of reducing and maintaining the temperature of space or substance below the temperature of the surroundings A continuous process of extraction of heat from a body whose temperature is already below its surrounding temperature. Process of removing heat from a place where it is not wanted and transferring that heat to a place where it makes little or no difference

Air-Conditioning

 

Treatment of air so as to simultaneously control its Temperature, Moisture Content, Cleanliness, Odor and Circulation, as required by occupants, a process, or products in the space . Process that heats, cools, cleans, and circulates air and control its moisture content on a continuous basis. Mechanical Engineering Dept. CEME NUST

12

Introduction Relation between Refrigeration and Air Conditioning


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Introduction Important Definitions/Concepts Heat



Heat is a form of energy

o o o

flows from a warmer to a cooler substance associated with the motion of atoms or molecules capable of being transmitted through solid and fluid media by Conduction, through fluid media by Convection, and through empty space by Radiation


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Introduction Important Definitions/Concepts Modes of Heat Transfer


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Introduction Important Definitions/Concepts Modes of Heat Transfer


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Introduction Important Definitions/Concepts Sensible Heat



Heat which causes a change in temperature of a substance, either

o o

By adding heat By removing heat

temp is raised OR temp is lowered

Latent (Hidden) Heat



Heat which brings about a change in state/phase with no change in temperature

o o

Solids become liquids, liquids become gas changes of state occur at the same temperature and Pressure combinations for any given substance Mechanical Engineering Dept. CEME NUST

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Introduction Important Definitions/Concepts Latent (Hidden) Heat—contd-DE → 2257 kJ/kg

Water → Steam

Latent Heat of Vaporization

ED → 2257 kJ/kg

Steam → Water

Latent Heat of Condensation

BC → 335 kJ/kg

Ice → Water

Latent Heat of Fusion

D

E

CB → 335 kJ/kg B

Water → Ice

C

A Latent Heat of Solidification

 Two latent heats for each substance, solid to liquid (Melting and Freezing) Mechanical Engineering Dept. CEME NUST and liquid to gaseous (Vaporizing and condensing)

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Introduction Important Definitions/Concepts Application / Significance of Latent Heat of Evaporation

 When a substance passes from a liquid to a vapor its ability to absorb heat is very high

 It takes five times as much energy (heat) to boil water than it takes to warm it up from 0 to 100 oC, (i.e. 418 kj/kg vs 2257 kj/kg)

 Having sweat evaporate off our skin is so helpful in keeping us cool

Absorption

of heat by changing a liquid to a vapor, and discharge of that heat by condensing the vapor is the keystone to the whole Mechanical Refrigeration Process


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Introduction Important Definitions/Concepts Saturation temperature

 Temperature

at which a fluid will change its state from liquid to vapor or conversely from vapor to liquid

 If

pressure is low, the saturation temperature is low. saturation temperature is high

If pressure is high,

Brine Water

 Salt, sodium chloride de NaCI), or calcium chloride (CaCl2), added to water, raises the temperature at which the water will boil

 It also lowers the temperature at which it will freeze  To get the lower temperature required in some

instances, ice and salt

mixtures are used → which may melt at -18 oC


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Introduction Important Definitions/Concepts Enthalpy

 Measure of the heat content of a substance  all the heat in one pound or one kg of a substance calculated form an accepted reference temperature of for example 32 oF or 0 oC

o o o

For Water, 0 enthalpy is at 0 oC and 100 kPa For Refrigerants, - 40 oC and 100 kPa For air, 25 oC and 100 kPa

H = M × sp. Heat × Δt

 Specific enthalpy is enthalpy per unit mass, h = H/M Cryogenics

 Creating and using temperatures in the range of 115 K down to 0 K (or -157 oC

down to -273 oC). Mechanical Engineering Dept. CEME NUST

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Introduction Important Definitions/Concepts

Cryogenic Range


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Introduction to Refrigeration Refrigeration Process

   

Heat that leaks into the refrigerator from any source must be removed by the refrigerator’s Heat-Pumping Mechanism Heat has to be pumped from the cool, 2°C interior of the refrigerator to the warmer, 22°C air in the room Power is required for pumping heat up the temperature scale from a 2°C box temperature to a 22°C room temperature Refrigeration concepts utilized in the Residential Air Conditioner are the same as those in the Household Refrigerator

o

Heat leaks into the House just as heat leaks into the Refrigerated Compartments in the refrigerator Mechanical Engineering Dept. CEME NUST

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Introduction to Refrigeration Temperature and Pressure Relationship



Temperature at which a liquid boils is not constant, but varies with the pressure


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 When

the pressure in the jar reaches the pressure that corresponds to the boiling point of water at 70°F (21 oC), the water will start to boil and vaporize.

⇒

Boiling Point of water can be changed and controlled by controlling the vapor pressure above the water Mechanical Engineering Dept. CEME NUST

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 By lowering the pressure in a jar to 0.122 psia will cause the water to boil at 5oC

 By circulating this water boiling at 5oC through a cooling coil and passing the room air over it, would absorb heat from room air As air is giving up heat to the coil, the air leaving the coil is cold 5oC Water used in this way is called a Refrigerant 24oC

13oC

Refrigerant is a substance that can be changed readily to a vapor by boiling it and then changed to a liquid by condensing it Mechanical Engineering Dept. CEME NUST

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Introduction Mechanical Refrigerator Evaporator

Condenser Cabnit

1.5oC, 2bar Referigerant Control Room Temp. (22 oC)

Insulation

Compressor

Refrigerator Temp (2 oC)

Liquid Refrigerant Receiver (E.g R-12) Mechanical Engineering Dept. CEME NUST

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Introduction Mechanical Refrigerator


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Introduction Window Air-Conditioner 13oC

38oC

66oC 10oC -7oC 30oC

24oC

38oC Mechanical Engineering Dept. CEME NUST

29

Introduction Window Air-Conditioner


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Introduction Refrigerant



Working substance used in a refrigeration system to produce cold

Refrigeration Effect



The quantity of heat which unit mass of refrigerant absorbs from the refrigeration space is known as the refrigeration effect

Units of refrigeration capacity



Rate at which heat is removed from the refrigeration space.

o o o

rated in kJ/s, or Btu/h or in terms of ice melting equivalent kilowatt (kW) tons of refrigeration (TR) Mechanical Engineering Dept. CEME NUST

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Introduction Units of refrigeration capacity Tons of Refrigeration

 

Refrigerating System having a capacity of one ton is the one that has a cooling capacity equivalent to the melting of one ton of ice in 24 hours. If 288,000 Btu are required to make one ton of ice

288000Btu / 24 hr = 12000 BTU/hr, to make 1 ton of ice in one day.

o

1 ton refrigeration = 200 Btu/min = 3.517 kJ/s = 3.517 kW = 4.713 HP


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Introduction Example A refrigeration system produces 40 kg/hr of ice at 0oC from water at 25oC. Find the refrigeration effect per hour and TR. Take latent heat of solidification of water at 0oC as 335 kJ/kg and specific heat of water 4.19 kJ/kg oC. Example 200 kg of ice at – 10 oC is placed in a bunker to cool some vegetables. 24 hours later the ice has melted into water at 5 oC. What is the average rate of cooling in kJ/hr and TR provided by the ice? Assume Specific heat of ice, Cp,i = 1.94 kJ/kg oC Specific heat of water, Cp,w = 4.1868 kJ/kg oC Latent heat of fusion of ice at 0oC, L = 335 kJ/kg. Mechanical Engineering Dept. CEME NUST

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Chapter 1 - Introduction to Refrigration and Air Conditioning.pdf

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