Vapor_compression_refrigeration_system.pdf

VAPOUR COMPRESSION REFRIGERATION SYSTEM ARNAB GANGULY

INTRODUCTION • Refrigeration cycle can be classified as Gas cycle or Vapor cycle depending on the working fluid in the system • In Vapor cycle the refrigeration effect is produced by vaporization of the liquid refrigerant into vapor(in the evaporator). • During vaporization from liquid to vapor the latent heat of vaporization is absorbed from the surrounding. • Vapour cycles can be subdivided into vapour compression systems, vapour

absorption systems, vapour jet systems. Among these the vapour compression refrigeration systems are predominant.

Vapour Compression Refrigeration system (VCRS) •

Vapour compression refrigeration system is the most common cycle used for refrigeration and air conditioning devices eg. Domestic refrigerator , air conditioner

•

VCRS is based on vapor cycle refrigeration in which the refrigeration effect R.E (QL) is obtained by vaporization of liquid to vapor.

•

The refrigeration effect is given as

QL(KW/TR)= mR(Kg/s) × hfg(KJ/kg) Where mR is the mass flow-rate of refrigerant and hfg is the latent heat of vaporization of the refrigerant. •

The actual vapour compression cycle is based on Evans-Perkins cycle, which is also called as reverse Rankine cycle

Carnot cycle • Carnot refrigeration cycle is a completely reversible cycle and therefore it will have the maximum COP. • The Carnot cycle can be used as a thermodynamic cycle for VCRS for maximum COP • However as we will see later the actual VCRS is not based on Carnot VCRS.

Carnot cycle based VCRS Supercritical Point Liquid vapour line

Fig. 3.1 Schematic of Carnot based VCRS

Fig. 3.2 T-S diagram of Carnot cycle

Carnot cycle based VCRS The components of the VCRS are

• Evaporator • Compressor

• Condensor • Expander/turbine

Practical problems with Carnot cycle VCRS Wet compression •

During process 1-2, a mixture consisting of liquid and vapour have to be compressed isoentropically in the compressor. Such a compression is known as wet compression due to the presence of liquid.

•

In practice, wet compression is very difficult especially with reciprocating compressors. This

problem is particularly severe in case of high speed reciprocating compressors, which get damaged due to the presence of liquid droplets in the vapour. Turbine work •

The second practical difficulty with Carnot cycle is that using a turbine and extracting work from the system during the isentropic expansion of liquid refrigerant is not economically feasible, particularly in case of small capacity systems.

TYPES VAPOUR COMPRESSION REFRIGERATION SYSTEM • Cycle

with

superheated

vapor

after

compression • Cycle with subcoooling of liquid refrigerant after condenser and superheating of vapour refrigerant after evaporator

Vapour compression refrigeration system

Schematic of VCRS

T-S diagram of VCRS

P-H chart for VCRS

R.E.

Refrigeration effect and COP for VCRS • If mr is the mass flow-rate of the refrigerant in the system then • 𝑅. 𝐸

𝑄𝐿 = 𝑚𝑟 (ℎ1 − ℎ4)

• Compressor work 𝑊𝑖𝑛 = 𝑚𝑟(ℎ2 − ℎ1) • COP for the system =

(ℎ1−ℎ4) (ℎ2−ℎ1)

VCRS problem solving Problems on VCRS can be solved by using 2 methods Using tabulated values of Refrigerant properties Using P-H chart or graphical method

Simple VCRS system with saturated vapor at compressor inlet and saturated liquid at condenser outlet

R.E.

Problem • Problem 1. A Vapour compression refrigeration system uses methyl chloride and operates between temperatures limits of -10 C and 45 C . At entry to the compressor the refrigerant is dry saturated and after compression it acquires a temperature of 60 C. Find (i)the C.O.P of the refrigerator (ii) Ideal COP of the refrigerator Saturation Temp in C

Enthalpy in KJ/kg

Entropy in KJ/kgK

Liquid

Vapour

Liquid

Vapour

-10

45.4

460.7

0.183

1.637

45

133.0

483.6

0.485

1.587

Problem • Problem 1. A simple refrigerant R 134 a heat pump for space heating operates between temperature limits of 15 C and 50 C. The heat required to be pumped is 100 MJ/h. Determine 1.Dryness fraction of refrigerant entering the evaporator. 2. The piston displacement assuming a volumetric efficiency of 87 % 3. The theoretical power 4. The theoretical COP The specific heat of vapour is 0.996 KJ/kg K and the specific volume of R 134a is 0.04185 m3/kg. Pressure Saturati Enthalpy in KJ/kg (bar) on Temp Liquid Vapour in C

Entropy in KJ/kgK Liquid

Vapour

4.887

15

220.26

413.6

1.0729

1.7439

13.18

50

271.97

430.4

1.2410

1.7312

Problem • Problem 3 A refrigeration machine using R 12 works between the pressures of 2.5 bar and 9 bar. The vapour entering the compressor is dry saturated and there is no subcoooling in the condenser. If the capacity of the plant is 20 TR and the relative COP is 65 % determine (a) Actual COP (b)Actual Compressor work Pressure bar

Saturatio Enthalpy in KJ/kg n Temp in C

Entropy in KJ/kgK

Liquid

Vapour

Vapour

9

36

456.4

585.3

4.74

2.5

-7

412.4

570.3

4.76

P-H chart

SUBCOOLED REGION

SUPERHEATED REGION

TWO PHASE REGION

Solving VCRS problem using P-H chart A vapour compression system using R 12 operates between -15 C and 35 C as evaporator and Condenser temperature resp. Using P-H chart determine • COP • Mass flow rate of refrigerant per TR • Piston displacement per TR using volumetric efficiency of 80% • Heat rejected in condenser per TR • Ideal COP

Point

state

T

h

Specefic volume v

1

Saturated vapour

-15

344.8

0.09146 m3/kg

2

Superheat ed vapour

40

372

3

Saturated liquid

35

233.5

4

Liquid vapour mixture

-15

233.5

COP = 4.09 mR = 0.0316

V = 3.612 x 10-3 m3/s QH= 4.37 KW COP carnot = 5.16

MODIFICATION OF SIMPLE VCRS TO INCREASE THE EFFICIENCY OF THE CYCLE

 SUBCOOLING OF THE LIQUID REFRIGERANT IN THE CONDENSOR  SUPERHEATING OF THE VAPOUR REFRIGERANT IN THE EVAPORATOR BEFORE ENTERING THE CONDENSOR

SUBCOOLING OF THE LIQUID REFRIGERANT IN THE CONDENSOR

 WHAT IS SUBCOOLING ?

 It is possible to cool the refrigerant liquid in the condenser to a few degrees lower than the condensing temperature by adding extra area for heat transfer.  In such a case, the exit condition of the condenser will be in the subcooled liquid region. Hence this process is known as subcoooling of the refrigerant.

Advantages of Liquid Subcoooling • Subcoooling increases the refrigeration effect by reducing the throttling loss at no additional specific work input. • COP of the cycle increases • Subcoooling ensures that only liquid enters into the throttling device leading to its efficient operation

Advantages of Liquid Subcoooling There is an increase in R.E. due to subcoooling of the refrigerant as shown in the Fig. Without Subcooling R.E. = 4-1 With Subcooling R.E. = 4’-1

SUPERHEATING OF THE VAPOUR REFRIGERANT IN THE EVAPORATOR BEFORE ENTERING THE CONDENSOR

• It is possible to increase the temperature of the vapour refrigerant leaving the evaporator beyond the saturated vapour temperature

• This condition in which the vapour leaving the evaporator is in superheated state is known as Superheating.

Advantages of vapour • Superheating increases the R.E. in the evaporator but there is an increase in compressor work • The COP of the system may or may not increase with superheating. • A minimum amount of superheat is desirable as it prevents the entry of liquid droplets into the compressor.

Problem with subcoooling and superheating A vapour compression system using R 717 (NH3) operates between -15 C and 40 C as evaporator and Condenser temperature resp. The vapour is superheated by 5 C before entering the compressor and the liquid is subcooled by 5 C Using P-H chart determine • COP • Mass flow rate of refrigerant per TR • Piston displacement per TR using volumetric efficiency of 80% • Heat rejected in condenser per TR • Ideal COP

Point

state

T

h

Specefic volume v

1

Superheat ed vapour

-10

1457

0.513 m3/kg

2

Superheat ed vapour

130

1743.5

3

Subcooled liquid

35

364

4

Liquid vapour mixture

-15

364

COP = 3.81 mR = 0.00322

V = 2.065 x 10-3 m3/s QH= 4.44 KW COP carnot = 4.7

Methods of sub-cooling in a VCRS cycle • Sub cooling of liquid refrigerant by vapour refrigerant (Liquid to vapour heat exchanger) • Subcooling of liquid refrigerant by Liquid refrigerant (Liquid to Liquid heat exchanger )

Liquid – vapour heat exchanger

Liquid – vapour heat exchanger • A Liquid vapour heat exchanger is used for subcoooling of liquid in a VCRS cycle. • It is a counterblow heat exchanger in which the warm refrigerant liquid from the condenser exchanges heat with the cool refrigerant vapour from the evaporator. • Temperature of the refrigerant liquid at the exit of condenser is considerably higher than the temperature of refrigerant vapour at the exit of the evaporator and therefore it is possible to sub-cool the refrigerant liquid and superheat the refrigerant vapour by exchanging heat between them. • The main advantages of the Liquid vapour heat exchanger is it increases the RE for the system.

Liquid to Liquid heat exchanger Condenser T

HEAT EXCHANGER

EX

Evaporator

Liquid – Liquid heat exchanger • A Liquid Liquid heat exchanger is used for subcoooling of liquid in a VCRS cycle. • The hot condenser fluid exchanges heat with the cold refrigerant from the expansion valve which is bypassed from the evaporator • The cold refrigerant is superheated before it is mixed with the refrigerant coming out of the evaporator before it enters the compressor • The main advantage of the liquid liquid heat exchanger is higher subcooling can be obtained in the heat exchanger without any change in COP. • The Refrigerant entering the evaporator is always saturated vapour

Flash chamber • When the liquid refrigerant is passed through the expansion device some of the refrigerant evaporates and forms vapour. • This phenomenon is known as flashing • The vapour of the refrigerant doesn’t produce any refrigeration effect in the evaporator • This refrigerant vapour can be removed with the help of a flash chamber.

Flash chamber Condenser Saturated Vapour refrigerant

FLASH CHAMBER

m2 EX

m2 -m1 m1

Saturated Liquid refrigerant

Evaporator

T

Advantages of Flash Chamber • The COP and the RE of a simple VCRS with a flash chamber is same as that of the simple cycle • The mass flow rate of the refrigerant in the evaporator is less which results in reduction in the evaporator size • There refrigerant entering the compressor doesn’t contain any liquid which results in the durability of the compressor.

Effect of evaporator pressure on the performance of VCRS

Effect of Decrease in evaporator pressure with constant condenser pressure

Effect of evaporator pressure on the performance of VCRS • Decrease in refrigeration effect • Increase in volume of suction vapour and therefore higher capacity compressor is required • Decrease in volumetric efficiency • Increase in compressor work due to increase in pressure ratio and change from steeper isoentropic curve to flatter iso-entropic curves. • Decrease in COP

Effect of condenser pressure on the performance of VCRS

Effect of increase in condenser pressure with constant evaporator pressure

Effect of condenser pressure on the performance of VCRS • Decrease in refrigeration effect • Increase in mass flow rate for the same refrigeration capacity (TR) • Increase in compressor work • The increase in compressor work is due to higher pressure ratios and higher mass flow rate • Decrease in COP

Two stage VCR cycle The simple vapour compression system is a twopressure system. Systems with more than two pressures may arise either due to • Multistage or compound compression • Feeding of the refrigerant to a multievaporator system.

Multistage or Compound compression VCR cycle • When the pressure ratio between the condenser and evaporator is greater than 4 or 5 the work due to a single compression is very high. • This will happen either as a result of a very high condensing temperature, and/or a very low evaporator temperature. • Multistage or compound compression with interstage cooling is one effective method of reducing work of compression.

Two stage VCR cycle • Two stage VCR cycle with water intercooler • Two stage VCR with Flash intercooler and liquid sub cooler

Two stage VCR cycle with water intercooler PC Condenser

HP

5

4 Saturated Vapour refrigerant

EX

6

3 Water intercooler

Water in

PE 1 Evaporator

2 LP

Wate out

Two stage VCR cycle with water intercooler PC

PE

Two stage VCR cycle with Flash Intercooler

Two stage VCR cycle with Flash Intercooler •

For flash intercooling. the compressed vapors from the lower stage are led and bubbled through the liquid in the flash chamber.

•

The vapors are thus cooled to the saturation temperature at the pressure of the flash chamber and a part of the liquid evaporates which goes to the higher stage along with the vapors from the lower stage.

•

Flash intercooling thus enables the higher stage compression to take place along the steeper isentropic, nearer the saturated vapour line.

•

Thus, although the specific work is reduced in the high stage because of working along the steeper isentropic 3-4 instead of the isentropic 2-2'. the increase of the actual mass flow through the higher stage may increase the work of the higher stage.

•

It is found that in the case of refrigerants such as ammonia. the mass of the liquid evaporated for flash intercooling is extremely small because of its high latent heat of vaporization and the isentropic become very flat at higher temperatures. Hence flash intercooling will decrease the power requirement.

•

Flash intercoolers are. therefore. commonly used in multistage ammonia plants.

Two stage VCR cycle with Flash chamber 4

5

Condenser

m2

3 C 8 FLASH CHAMBER

EX1 6

7

EX2

Saturated Vapour refrigerant Saturated 2 Liquid refrigerant

Evaporator 9

C 1

m1

Multi-evaporators at different temperatures • If varied types of cooling loads are connected to the same refrigeration system. Each load may require an evaporator working at a different refrigeration temperature. The whole system may, therefore, be operated at a pressure equal to the lowest evaporator pressure leading to a single compressor system, or at various suction pressures with individual compressors for each evaporator, leading to a multi compressor system