topic 2 refrigeration notes

7/25/2019 Topic 2 Refrigeration Notes

1/56

Refrigeration

Topic 2


2/56

1. Introduction

! Refrigeration - method of reducing the temperature of a system below that of

the surroundings and maintaining it at the lower temperature by continuously

extracting the heat from it.

!The principle of refrigeration is based onsecond law of thermodynamics. Itstates that heat does not flow from a low temperature body to a high

temperature body without the help of an external work.


3/56

1. Introduction

! In refrigeration process, the heat is

continuously removed from a system

at lower temperature and transfer it

to the surroundings at a higher

temperature. This operation

according to second law ofthermodynamics can only be

performed by the aid of the external

work.

! Therefore in a refrigeration system,power is to be supplied to remove

heat continuously from the

refrigerator to keep it cool at a

temperature less than the

surroundings.! Note: For history of refrigeration, refer to Dossats

book Chapter 1.


4/56

1. Introduction

! Direct and reverse heat engines


5/56

1. Introduction

! Direct and reverse heat engines


6/56

2. Fundamental terms

! Refrigerant diagram

! The characteristics of a refrigerant can be illustrated in a diagram using the

primary properties as x- and y-axis. The primary properties are normally

chosen as energy content and pressure (P-h diagram).

! Energy content is represented by the thermodynamic property of specific

enthalpy - quantifying the change in energy content per mass unit of the

refrigerant as it undergoes processes in a refrigeration system.

! typically applicable interval for pressure is large. Therefore, diagrams use a

logarithmic scale for pressure.


7/56


! The diagram is arranged

so that it displays the

liquid, vapour and

mixture regions for the

refrigerant. Liquid is

found to the left (with a

low energy content) -

vapour to the right (with ahigh energy content).

! In between its the

mixture region which are

bounded by a curve -

called the saturationcurve. The fundamental

processes of evaporation

and condensation are

illustrated.


8/56



9/56

3. Basic refrigeration cycle


10/56

! Evaporator

! The pressure-enthalpy chart plots the

properties of a refrigerantrefrigerant

pressure (vertical axis) versus enthalpy

(horizontal axis).

! The cycle starts with a cool, low-pressure

mixture of liquid and vapor refrigerant

entering the evaporator where it absorbs

heat from the relatively warm air/water or

other fluid that is being cooled.

! This transfer of heat boils the liquid

refrigerant in the evaporator, and this

superheated refrigerant vapor is drawn to

the compressor.



11/56

! Compressor

! The compressor draws in the

superheated refrigerant vapor and

compresses it to a pressure and

temperature high enough that it can

reject heat to another fluid.

! This hot, high-pressure refrigerant

vapor then travels to the condenser.



12/56

! Condenser

! Within the condenser, heat is

transferred from the hot refrigerant

vapor to relatively cool ambient air or

cooling water.

! This reduction in the heat content of

the refrigerant vapor causes it to

desuperheat, condense into liquid,

and further subcool before leaving

the condenser for the expansion

device.



13/56

! Expansion valve

! The high-pressure liquid refrigerant flows

through the expansion device, causing a

large pressure drop that reduces the

pressure of the refrigerant to that of theevaporator.

! This pressure reduction causes a small

portion of the liquid to boil o", or flash,

cooling the remaining refrigerant to the

desired evaporator temperature.

! The cooled mixture of liquid and vapor

refrigerant then enters the evaporator to

repeat the cycle.



14/56



15/56

4. Refrigeration effect

! The change in enthalpy that occurs in

the evaporator is called the

refrigeration e"ect.

! This is the amount of heat that each

kg of liquid refrigerant will absorb

when it evaporates.

! In comparison, the same system

without subcooling produces less

refrigeration e"ect.


16/56

4. Refrigeration effect

! Superheating occurs inside the final length

of tubes at which temperature di"erence

between refrigerant and air is highest

! Such large temperature di"erence

increases the rate of heat transfer and therefrigerant vapor absorbs much heat.

! Liquid refrigerant completely evaporated

! Superheating shifts from the liquid/vapor

region to vapor

! It ensures the refrigerant vapor is

completely free liquid before entering the

compressor.


17/56

5. Heat pumps

! A heat pump extracts heat from a heatsource and rejects heat to air or water at a

higher temperature.

! Usually it is a packaged air conditioner or a

packaged unit with a reversing valve or

other changeover setup, in which the

refrigerant flow to the condenser is changed

to the evaporator.

! Alternatively, air passage through the

evaporator may be changed over to

passage through the condenser.

! A heat pump has all the main components

of an air conditioner or package unit: fan,

filters, compressor, evaporator, condenser,

and a throttling device.


18/56

6. Cycle analysis of ideal cycles


19/56



20/56



21/56

7. Cycle analysis of practical cycles


22/56



23/56



24/56

8. Components of vapour compression system


25/56

Example 1

Consider a vapour compression refrigeration cycle using R-134a based on

the following conditions:

Condenser temperature = 45C

Evaporator temperature = 10C

Sub-cooling at condenser = 3CSuperheating at evaporator = 3C

Compressor e#ciency = 90%

Plot the refrigeration cycle for the single stage compression.

Find the refrigeration e"ect and the COP.


26/56

Example 1 (solution)


27/56

9. Two-stage vapour compression system

! Consists of two stage of compressionsystem

! Comprise with a high stage compressor

and a low stage compressor or several

single stage compressors in series.

! The pressure between the dischargepressure of the high stage and suction

pressure of the low stage is called inter-

stage pressure.

! To achieve a larger temperature range

(cooler temperature) without requiring alarge pressure range in the compressor


28/56



29/56


! Volumetric e#ciency and compression ratio


30/56


! Vapor refrigerant at state point (1) enters the

first stage of the compressor in dry saturated

state.

! This vapor is compressed to the inter-stage

pressure Piat stage point (2)

! The mixture enters the second stage of

compressor at state point (3)

! Hot gas compressed to condensing pressure

Pcleaves compressor at state point (4)

! The hot gas is then discharged to the

condenser and condenses into liquid state at

state point (5).


31/56

Two-stage vapour compression system

! Upon passing the condenser, the sub-cooled

liquid refrigerant at state point (5') flows through

the high pressure side flow control device.

! A portion of liquid refrigerant evaporates into

the vapour form in the flash cooler at state point

(7) The flashed refrigerant cools the remainingportion of liquid refrigerant to the saturated

temperature at state point (8) at the inter-stage

pressure.

! The mixture of liquid and vapor refrigerant in

flash cooler is at state point (6).

! Liquid refrigerant flows through low pressure

expansion valve, a small amount of which is

pre-flashed and the liquid vapor mixture enters

the evaporator at state point (9). In the

evaporator, all liquid refrigerant is evaporated

into vapour form and flows to the first stage

inlet.


32/56


! The inter-stage pressure of 2-stage compound system

! The inter-stage pressure is selected such that the compression ratios at

various stages are approximately equal.

! The inter-stage pressure could be approximated by the following

equation:


33/56


! Defining the enthalpy of various state points in the P-h diagram:


34/56


! The portion of flashed vapour refrigerant in the flash cooler

! In the flash cooler, there is x kg of vaporized refrigerant cools down the

remaining liquid refrigerant (1-x) kg to the saturation temperature at the inter-

stage pressure.

From the heat balance point of view, we have

! Note that x is also the dryness fraction of the liquid-vapor mixture in the flashcooler at the inter-stage pressure. This equation could be expressed as


35/56


! Enthalpy of vapor mixture entering the second-stage compressor

! Heat balance at the mixing point before entering the second stage

compressor could be show as follows:


36/56


! Coe#cient of Performance of the 2-stage compound system with a flash

cooler

The refrigeration e"ect in evaporator Qre (per kg of refrigerant through

condenser) could be expressed as:

! Work input Winto the compressor (first and second stages) could be

expressed as:-


37/56


! Advantages of using two-stage vapour compression system compared with a single

stage system:

! The compression ratio of each stage in a two-stage system can be reduced so that

the volumetric e#ciency can be increased.

! In two-stage system, the liquid refrigerant can be sub-cooled to the saturation

temperature at the inter-stage pressure which in turn increases the refrigeration

e"ect.

! In two-stage system, the discharge gas from the low stage compressor can be de-

superheated that results in reduction of discharge temperature after the high stagecompression process.

! Disadvantage of multi-stages systems: High equipment cost due to complicated

equipment.


38/56

Example 2

Consider a vapour compression refrigeration cycle using R-134a based on

the following conditions:

Condenser temperature = 45C

Evaporator temperature = 10C

Sub-cooling at condenser = 3CSuperheating at evaporator = 3C

Compressor e#ciency = 90%

Plot the refrigeration cycle for the two-stage compression.

Find the refrigeration e"ect and the COP.


39/56



40/56

10. Vapour absorption system

! The vapour absorption refrigeration

system is a heat operated system. It

di"ers from the vapour compression

refrigeration system only in the manner

in which the circulation of the refrigerant

is achieved.

! In this system, the compressor is

replaced by an absorber, a generator

and a pump. The refrigerant used in this

system must be highly soluble in the

solution known as absorbent. Thesystem uses ammonia as refrigerant and

water as absorbent.

! Others: Water-LiBr, water-LiCl where

water is refrigerant


41/56


! The liquid refrigerant in the evaporator

absorbs the heat from the medium to be

cooled and it undergoes a change of

phase from liquid to vapour. The low

pressure vapour is then passed to the

absorber. In the absorber, the low

pressure ammonia vapour is dissolved inthe weak ammonia solution producing

strong ammonia solution at low

pressure.

!The strong ammonia solution is thenpumped to a generator through the heat

exchanger at high pressure. While

passing through the heat exchanger, the

strong ammonia solution is warmed up

by the hot weak ammonia solution

flowing from the generator to absorber.


42/56


! The warm strong ammonia solution is

heated by an external source in the

generator. Due to the heating, the

vapour gets separated from the solution.

The vapour which is at high pressure

and high temperature is condensed to

low temperature in the condenser bycold water circulation.

! The high pressure liquid ammonia then

passes through the expansion valve

where it is expanded to low pressureand temperature. The low pressure and

low temperature ammonia liquid again

enters the evaporator where it absorbs

the heat from the medium to be cooled

and the cycle continues.


43/56


! The P-h cycle is similar to that of a vapour compression cycle. Process 3 to 4

is due to the mixing in the evaporator. Process 1 to 2 is due to the absorption.


44/56


! The heat required to operate the

system may be obtained from

anywhere and is commonly a gas

flame (Electrolux refrigeration

system). This system is popular in

caravan refrigerators.


45/56

11. Comparison between ...

Vapour Compression Refrigeration System Vapour Absorption Refrigeration System

Works using mechanical energy Works using heat energy

Refrigerating capacity < 1000 tons Capacity is > 100 tons

COP is much higher ( 4 to 10) COP is less than 2

Noisy due to compressor Pump noise is less

Chances of leakage of refrigerant are more No leakage

Maintenance and operating cost is more Less maintenance cost

Smaller in size Larger in size

Wear and tear are more Less wear and tear

Uses Freon-12 or any other refrigerant Only Ammonia is used as refrigerant

Refrigerant vapour is compressed Refrigerant vapour is absorbed and heated


46/56

Example 3

A heat pump uses a vapour compression cycle with refrigerant 12. The

compressor is driven by a heat engine with a thermal e#ciency of 40%. Heat

removed from the engine in the cooling system is recovered. This amounts to

40% of the energy supplied in the fuel.

The heat pump cycle uses an ideal cycle with an evaporator at 5oC and a

condenser at 12.19 bar. The vapour is dry saturated at inlet to the

compressor. The condenser produces liquid at 45oC.

Calculate the thermal advantage (Coe#cient of Performance) for the heat

pump.

The plant is to deliver 40 kW of heat with fuel power input of 14.14 kW.

Determine the mass flow rate of refrigerant.


47/56



48/56

Example 4

The power input to the compressor of an ammonia vapour compression plant

is 8.2 kW. The mechanical e#ciency is 85%. The ammonia is dry saturated at

-6oC at inlet to the compressor. After compression the vapour is at 11.67 bar.

The compression has an isentropic e#ciency of 90%. The condenser

produces saturated liquid.

Calculate the following.

i. The flow rate.

ii. The coe#cient of performance for the refrigerator.

iii. The coe#cient of performance for the heat pump.


49/56



50/56

Example 5

A practical refrigerator using ammonia as the refrigerant operates with an evaporator

pressure 1.902 bar and condenser pressure of 15.54 bar. The refrigerator uses 2-

stage compression and expansion to improve e#ciency. The refrigerant enters the

low pressure (LP) compressor with a dryness fraction of 0.8 at 1.902 bar and enters

the high pressure (HP) compressor at 6.149 bar. The refrigerant at entry to the HP

expansion valve is sub-cooled to 38oC and entry to the LP expansion valve is wet

saturated at 10oC. The refrigerant expands in the HP expansion valve to a pressure

6.149 bar into a separator where the vapour is returned to the entry of the HP

compressor. The liquid refrigerant is expanded again in the LP expansion valve

before entry to the evaporator.

Evaluate the Coe#cient of Performance (COP) of the refrigerator.


51/56



52/56

Example 6

20 lbm/min of liquid water-ammonia solution at 150 psia, 220 F and

concentration of 0.25 lbm ammonia per lbm of solution is mixed in a steady-

flow adiabatic process with 10 lbm/min of saturated water-ammonia solution

at 150 psia and 100F. Find the enthalpy, concentration and temperature of the

mixture.


53/56



54/56

Example 7

Consider an aqua-ammonia absorption system and the following given data:

condenser pressure 200 psia

evaporating pressure 30 psia

generator temperature 240 F

temperature of vapour leaving dephlegmator 130 F

temperature of strong solution entering rectifying column 200 F

The heat exchanger lowers the temperature of the liquid, leaving the condenser at

10 F. States 1,3,4,7,8 and 12 are saturated. Pressure drop in components and

connecting lines is negligible. The system produces 100 tons of refrigeration.

Determine

(a) properties P, t, x andifor all state points of the system

(b) mass flow rate for all parts of the system

(c) power required for the pump, assuming 75% mechanical e#ciency

(d) system coe#cient of performance (COP)

(e) system refrigerating e#ciency.


55/56

Example 7

12

6

5

4

3

Weak liq. solution

0

07

8

10

9 1112

Strong liq.

solution


56/56


0

0

topic 2 refrigeration notes

Documents