Refrigeration and Air-Conditioning

Part #1

Important topics:-

❖ Refrigeration Cycles

❖ Refrigerants

❖ Basics of Psychometry

❖ Psychometric Processes

Refrigeration Cycle

Reversed Carnot Cycle

➢ Thermodynamic cycle and construction is as follows:

➢ Coefficient of performance (COP) :

Some Important Terms

➢ Refrigerants : A refrigerant is a substance or mixture, usually a fluid, used in a heat pump and refrigeration cycle. It is the carrier of the heat.

➢ Unit of refrigeration (A ton of refrigeration (TR or TOR)) : It is defined as the rate of heat transfer that results in the freezing of 1 ton of pure water at 0 °C to ice at 0 °C in 24 hours.

Vapour Compression Refrigeration System (VCRS Cycle)

➢ Construction of the system:

➢ COMPRESSION

The refrigerant enters the compressor at low temperature and low pressure. It is in a gaseous state. Here, compression takes place to raise the temperature and refrigerant pressure. The refrigerant leaves the compressor and enters to the condenser.

➢ CONDENSATION

The condenser is essentially a heat exchanger. Heat is transferred from the refrigerant to the environment. As the refrigerant flows through the condenser, it is in a constant pressure

➢ THROTTLING AND EXPANSION

When the refrigerant enters the throttling valve, it expands and releases pressure. Consequently, the temperature drops at this stage. Because of these changes, the refrigerant leaves the throttle valve as a liquid vapor mixture.

➢ EVAPORATION

At this stage of the Vapor Compression Refrigeration Cycle, the refrigerant is at a lower temperature than its surroundings. Therefore, it evaporates and absorbs latent heat of vaporization.

Thermodynamic Cycles is as follows :

➢ Process 1-2: Isentropic compression of saturated vapour in compressor

➢ Process 2-3: Isobaric heat rejection in condenser

➢ Process 3-4: Isenthalpic expansion of saturated liquid in expansion device

➢ Process 4-1: Isobaric heat extraction in the evaporator

➢ Refrigeration Effect :

➢ Work input:  

➢ COP:

Vapour Absorption Refrigeration Systems 

➢ Pumping this solution to a high pressure cycle by a simple pump

➢ An absorption refrigerator is a refrigerator that uses a heat source (e.g., solar energy, a fossil-fuel flame, waste heat from factories) to provide the energy needed for the cooling process.

➢ In this system mechanical compression process of vapor compression cycle is replaced by a thermal compression process. The thermal compression is achieved by the following process:

➢ Producing vapors from the solution by heating.

➢ Absorbing a fluid vapor into another carrier liquid.

➢ Construction is as follows :



COP of Ideal VARS

Important Points

➢ Most widely used system is NH3 – H2O system, where NH3 serves as refrigerant and H2O as transport medium.

➢ Other systems include water–lithium bromide(LiBr) and water–lithium chloride(LiCl) systems, where water serves as the refrigerant. These systems are limited to applications such as A-C where the minimum temperature is above the freezing point of water.

Reverse Brayton/ Joule Cycle

➢ It is a air or gas refrigeration cycle.

➢ Construction is as shown in figure.

➢ Here instead of throttling valve a expander as turbine is used.

➢ It is also known as Bell Coleman Cycle

➢ Thermodynamic cycle :

 ➢ Process 1-2: Reversible, adiabatic compression in a compressor

➢ Process 2-3: Reversible, isobaric heat rejection in a heat exchanger

➢ Process 3-4: Reversible, adiabatic expansion in a turbine

➢ Process 4-1: Reversible, isobaric heat absorption in a heat exchanger

➢ By thermodynamic relations we get

➢ COP of the cycle :

REFRIGERANTS

➢ The working substance that flows through a refrigerator and is capable of absorbing heat from the source ( which is at a lower temperature) and dissipate the same to the sink (which is a higher temperature than the source) either in the form of sensible heat (as in case of air refrigeration) or in the form of latent heat (as in the case of vapor Refrigeration) is called a refrigerant.

Primary Refrigerants

➢ Refrigerants that directly take part in the refrigeration process.

Secondary Refrigerants

➢ Refrigerants that are first cooled by the primary refrigerants and then further used for cooling purpose.

Halocarbon (CFC / Freon) Compounds 

➢ Chlorofluorocarbons (CFCs) are nontoxic, non-flammable chemicals containing atoms of Chlorine, Fluorine, and Carbon. They are considered as pollutants because they cause damage to the atmosphere

Nomenclature of CFC

R – (C-1)(H+1)F

➢ The number of chlorine atoms is found by

Cl = 2(C +1) − H − F

➢ where C, H, & F represent the number of Carbon, Hydrogen, and Fluorine atoms.

Examples of Halocarbon (CFC / Freon) Compounds 

Unsaturated Organic Compounds 

➢ Comprising of mainly hydrocarbon group with ethylene and propylene bases.

Nomenclature of UOC

R 1 – (C-1)(H+1)F

➢ The number of chlorine atoms is found by

Cl = 2C − H − F

Examples of Unsaturated Organic Compounds

Inorganic compounds

Nomenclature of Inorganic compounds

R 700 + Atomic Weight

Examples of Inorganic compounds 

Azeotrope compounds 

➢ Consist of mixtures of different refrigerants which don’t separate into their compounds with the change in pressure or temperature or both. They have fixed thermodynamic properties and act as pure substance.

Nomenclature of Azeotrope

Starts from R 500

Examples of Azeotrope compounds


SOME QESTIONS FORM THESE TOPICS

1.A Carnot cycle refrigerator operates between 250K and 300 K. Its coefficient of performance is:

(a) 6.0

(b) 5.0

(c) 1.2

(d) 0.8

2.Round the clock cooling of an apartment having a load of 300 MJ/day requires an air conditioning plant of capacity about

(a)1 ton

(b) 5 tons

(c) 10 tons

(d)100 tons

3.A refrigerator based on reversed Carnot cycle works between two such temperatures that the ratio between the low and high temperature is 0.8. If a heat pump is operated between same temperature range, then what would be its COP?

(a) 2

(b) 3

(c) 4

(d) 5

4.A heat pump for domestic heating operates between a cold system at 0°C and the hot  system at 60°C. What is the minimum electric power consumption if the heat rejected is 80000 kJ/hr?

(a) 2 kW

(b) 3 kW

(c) 4 kW

(d) 5 kW

5.A refrigerator working on a reversed Carnot cycle has a C.O.P. of 4. If it works as a heat pump and consumes 1 kW, the heating effect will be:

(a) 1 KW

(b) 4 KW

(c) 5 KW

(d) 6 KW

6.A refrigeration cycle operates between condenser temperature of +27°C and evaporator temperature of –23°C. The Carnot coefficient of performance of cycle will be _______

(A) 0.2

(B) 1.2

(C) 5

(D) 6

7.The door of a running refrigerator inside a room was left open. Which of the following statement is correct?

(A) The room will be cooled to the temperature inside the refrigerator.

(B) The room will be cooled slightly.

(C) The roam will be gradually warmed up.

(D) The temperature of the air in the room will remain unaffected.

8.The vapour compression refrigeration cycle is represented as shown in the figure below,with state 1 being the exit of the evaporator. The coordinate system used in this figure is

(A) p - h

(B) T - s

(C) p - s

(D) T - h

9.The values of enthalpy at the beginning of compression, at the end of compression and at the end of condensation are 185 kJ/kg, 210 kJ/kg and 85 kJ/kg respectively. What is the value of the COP of the vapour compression refrigeration system?

(a) 0·25

(b) 5·4

(c) 4

(d) 1·35

10.In a vapour compression refrigeration plant, the refrigerant leaves the evaporator at 195 kJ/kg and the condenser at 65 kJ/kg. For 1 kg/s of refrigerant, what is the refrigeration effect?

(a) 70 KW

(b) 100 KW

(c) 130 KW

(d) 160 KW

11.For a heat pump working on vapour compression cycle, enthalpy values of the working fluid at the end of heat addition process, at the end of compression process, at the end of heat rejection process, and at the end of isenthalpic expansion process are 195 kJ/kg, 210 kJ/kg, and 90 kJ/kg respectively. The mass flow rate is 0.5 kg/s. Then the heating capacity of heat pump is, nearly

(a) 7.5 kW

(b) 45 kW

(c) 52.2 kW

(d) 60 kW

12.The correct sequence of the given components of a vapour compression refrigerator is:

(a)Evaporator, compressor, condenser and throttle valve

(b)Condenser, throttle valve, evaporator and compressor

(c)Compressor, condenser, throttle valve and evaporator

(d)Throttle valve, evaporator, compressor and condenser

13.The compressor in the vapour compression system is replaced by which component in vapour absorption system :

a. an absorber

b. a generator

c. an absorber-generator

d. none of the above

14.For the same capacity of plant, the COP of the vapour absorption refrigeration system is

(a). lower than the COP of the vapour compression refrigeration system

(b). higher than the COP of the vapour compression refrigeration system

(c) same as the COP of the vapour compression refrigeration system

(d)cannot say

15.Lithium bromide in vapour absorption refrigeration system is used as

(A) Refrigerant

(B) Cooling substance

(C) Auxiliary refrigerant

(D) Absorbent

16.In vapour absorption refrigeration systems, which of the following fluids are commonly used?

(A) Air and water

(B) Sulphur dioxide and water

(C) Ammonia and water

(D) Freon and water

Read more:-

CLASSES NOTES FOR MECHANICAL ENGINEERING | I C ENGINE | WITH IMPORTANT QUESTION.

LESSON - 1

LESSON - 2

LESSON - 3

 

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