"Refrigeration and Air-Conditioning : Vapour Compression Refrigeration System & Refrigerants"

By Apoorbo Roy|Updated : August 26th, 2021

Complete coverage of syllabus is a very important aspect for any competitive examination but before that important subject and their concept must be covered thoroughly. In this article, we are going to discuss "Refrigeration and Air-Conditioning : vapour Compression Refrigeration System & Refrigerant" which is very useful for SSC JE Exams

INTRODUCTION

Refrigeration is the process of maintaining a space at a lower temperature than that of surrounding which can be done by various methods.

In the earlier time, it is done by keeping the ice at a top compartment of the space to be refrigerated and space below that will be maintained at lower temperature. Air will absorb heat from the surrounding and become warm & air will become lighter whereas the air at the top of the compartment remains cool because of the presence of ice & hence remains heavier. Thus, due to this density difference a circulation of air takes place which maintain the lower space of the compartment will be at a lower temperature than the surroundings.

The standard unit of refrigeration in vogue is ton refrigeration or simply ton denoted by the symbol TR.

1 ton of refrigeration – It is the amount that is to be removed to convert 1 short ton of water at 0°C to 0°C ice in 24 hours.

1 short ton = 907.185kg

Latent heat of fusion = 334.4kJ/kg

VAPOUR COMPRESSION SYSTEM

As reversed carnot is an ideal cycle which is not practically possible thus reversed Carnot cycle with minor modifications vapour as a refrigerant. The isothermal processes of heat rejection and heat absorption, replaced by condensation and evaporation respectively.

   Modifications in Reversed Carnot Cycle with Vapour As A Refrigerant

(i). Dry Versus Wet Compression

  • The isentropic compression process as shown in Figure involves the compression of wet-refrigerant vapour at 1’ to dry-saturated vapour at 2’. It is called wet compression.
  • With a reciprocating compressor, wet compression is not found suitable. It is, therefore, desirable to have compression with vapour initially dry saturated at 1 as shown in Figure or even slightly superheated if a reciprocating compressor is used. Such compression is known as dry compression.
  • The vapour is compressed till point ‘2’ at pressure PH which is the saturation pressure of the refrigerant corresponding to the condensing temperature tH, instead of being at 2”, which would be the state point if the Carnot cycle were to be executed.

  • It results in the discharge temperature t2 being higher than the condensing temperature tH & the refrigerant leaves the compressor superheated due to which compressor work will increase.
  • This increased amount of work of the cycle due to the substitution of wet compression by dry compression appears as the area 2-2’ – 2”, generally known as superheat horn.

Advantage of Dry compression over wet compression

(i) In case of wet compression, liquid refrigerant may be trapped in the head of the cylinder and may damage the compressor valves and the cylinder itself.

(ii) In case of wet compression, liquid-refrigerant droplets may wash away the lubricating oil from the walls of the compressor cylinder, thus increasing wear.

(iii) In case of dry compression, refrigeration effect is more as compare to wet compression.

(iv) Work input in case of wet compression is less as compare to dry compression.

(ii). Throttling Versus Isentropic Expansion

In refrigerating machines work input is given by compressor whereas in reversed carnot cycle, some of the work get extracted in isentropic expansion process in expander. 

REFRIGERANTS

Refrigerants are those working substance in a refrigerant cycle which undergoing a cyclic process & produces lower temperature by absorbing heat in evaporator side and rejects heat to surrounding to condenser side.

 Types of Refrigerants

 Primary Refrigerants

Primary refrigerants are those working substance in a refrigerant cycle which directly flows into refrigerating machine and produce cooling effect.

Ex: R – 11, R – 12, R – 22, R – 134

 Secondary Refrigerants

Secondary Refrigerants are those working fluids which are first cooled by primary refrigerants & then used for cooling at the desired place.

Ex: H2O, Brine solution, sometimes ammonia.

   DESIGNATION OF REFRIGERANTS:

Case-1 When the refrigerant is saturated Hydro carbon then:

Suppose, a refrigerant is designated by Cm Hn Fp Clg

Then, refrigerant will be saturated, if it follows criteria given below

n + p + q = 2m + 2

thus, refrigerant will be designated as

Example - R – 11 ⇒ R – 011

p = 1

n + p + q = 2m + 2

0 + 1 + q = 2 × 1 + 2

q = 3

Chemical formula CmHnFpClq

 

Example - R – 134

p = 4 

n + p + q = 2m + 2

2+ 4+ q = 2 2+2

q = 0

Chemical formula CmHnFpClq

Similarly, R – 11⇒ CF cl3

R – 12 ⇒ CFcl2

R – 22 ⇒ CHF2cl2

R – 134 ⇒ C2H2F4cl0

Note:

R – 134 is known as ecofriendly refrigerants, because of the absence of chlorine elements.

The chlorine element which is present in the commonly used refrigerator attacks the ozone layer which is situated in stratosphere which filters out the harmful ultraviolet radiations which are emitted by the sun. This chlorine element depletes the thickness of ozone layer. Therefore, we have to prefer those refrigerants which are having lower of ozone Depletion potential (ODP). The other alternatives are Hydrocarbon, Flouro carbon etc.

Case-2 When the refrigerants is unsaturated hydrocarbon:

Suppose, a refrigerant is designated by Cm Hn Fp Clg

Then, refrigerant will be unsaturated, if it follows criteria given below

n + p + q = 2m

thus, refrigerant will be designated as

You can avail of Online Classroom Program for all AE & JE Exams:

Online Classroom Program for AE & JE Exams (12+ Structured LIVE Courses and 160+ Mock Tests)

You can avail of BYJU'S Exam Prep Test Series specially designed for all AE & JE Exams:

BYJU'S Exam Prep Test Series AE & JE (160+ Mock Tests) 

Thanks, 

Team BYJU'S Exam Prep

Sahi Prep Hai to Life Set Hai !!!

Download BYJU'S Exam Prep APP , for best Exam Preparation , Free Mock Test, Live Classes

Posted by:

Apoorbo RoyApoorbo RoyMember since Sep 2020
Share this article   |

Comments

write a comment

AE & JE Exams

AE & JERVUNLUPSSSCSDEPSPCLPPSCSSC JEGPSCTNPSCAAIDFCCILUPRVUNLPSPCLOthersPracticeMock TestCourse
tags :AE & JE ExamsMechanical EngineeringNHPC JE ExamRSMSSB JE ExamGSECL JE ExamRRB JE ExamNWDA JE Exam

AE & JE Exams

AE & JERVUNLUPSSSCSDEPSPCLPPSCSSC JEGPSCTNPSCAAIDFCCILUPRVUNLPSPCLOthersPracticeMock TestCourse
tags :AE & JE ExamsMechanical EngineeringNHPC JE ExamRSMSSB JE ExamGSECL JE ExamRRB JE ExamNWDA JE Exam

Follow us for latest updates