ME 6404 – THERMAL ENGINEERING

UNIT – V – REFRIGERATION

Presented by

A. SANKARA NARAYANA MURTHY,Assistant Professor,

Dept. of Mechanical Engineering,Kamaraj College of Engineeriechnology,

Virudhunagar

REFRIGERATOR

• The transfer of heat from a low-temperature region to a high-temperature one requires special devices called refrigerators.

• The objective of a refrigerator is to remove heat (QL) from the cold medium; the objective of a heat pump is to supply heat (QH) to a warm medium.

HEAT PUMP• Heat pump supplies heat to the higher-temperature

region from lower temperature by giving work as input.

• Heat pumps and refrigerators are essentially the same devices; they differ in their objectives only.

for fixed values of QL and QH

TYPES OF REFRIGERATION SYSTEM

• Vapour Compression Refrigeration (VCR): uses mechanical energy

• Vapour Absorption Refrigeration (VAR): uses thermal energy

• Steam Jet Refrigeration system

• Liquid N2 bath

• Ice bunk cooling system

• Etc…

THE PRESSURE-ENTHALPY DIAGRAMThe process of the vapor compression refrigeration cycle may conveniently be displayed on a diagram having pressure and specific enthalpy as coordinates.

Below the critical point (CP) the saturated liquid line (SL) and saturated vapor line (SV) enclose a two-phase (wet) region between them. To the left of the saturated liquid line lie states which have lower temperature than the saturation temperature at a given pressure.

These are states of sub-cooled liquid.

THE PRESSURE-ENTHALPY DIAGRAM

To the right of the saturated vapor line lie states which have higher temperature than the saturation temperature at a given pressure.

These are states of superheated vapor. The area to the left of the liquid line is called the subcooled liquid region,

and the area to the right of the vapor line is called the superheated vapor region.

Within the two-phase region the horizontal lines of constant pressure are also lines of constant temperature

In the superheat region the lines of constant temperature leave the saturation line as indicated.

As the pressure diminishes in the superheat region, the lines of constant temperature tend to become lines of constant enthalpy, i.e. vertical on the diagram, indicating that the vapor is beginning to behave like an ideal gas with its enthalpy independent of pressure.

Lines of constant specific entropy and lines of constant specific volume , are shown in the superheat region.

REFRIGERANT IN VAPOR COMPRESSION REFRIGERATION

The working substance in a refrigeration system is called the refrigerant. There are lots of refrigerants, including gas, liquid and solid refrigerants. There are many natural and artificial substances have been used in mechanical

driven and thermal driven vapor compression refrigeration systems. In lithium bromide vapor absorption refrigeration system, H2O is used as a

refrigerant and LiBr is an absorbent ; in NH3 vapor absorption refrigeration system, NH3 is a refrigerant; is an absorbent.

Water H2O is also used as a refrigerant both in vapor adsorption and in vapor jet refrigeration cycles. In mechanical driven vapor compression refrigeration, NH3, CO2, chlorofluorocarbons (CFCs), hydro chloro fluoro carbons (HCFCs), hydro fluoro carbons (HFCs), azeotropic and zeotropic mixtures, inorganic compounds, hydrocarbons, and others are used as refrigerants.

REFRIGERATION CHARACTERISTICS OF REFRIGERANTS

The pressure- enthalpy diagram is the usual graphic means of presenting refrigerant properties and its cycles.

A typical vapor compression refrigeration cycle has been shown in figure

REFRIGERANT PROPERTIES1. Appropriate temperature and pressure characteristics

The saturated pressure with temperature is an important property of refrigerant.

1) It is desired for the pressure at evaporating temperature to be above atmospheric, to avoid inward leakage of air.

2)The pressure at the corresponding condensing temperature should not be excessive, so that extra strength high-side equipment is not required.

3) Low compression ratio is desirable, because the degree of complication and difficulty of a compressor increases directly with the compression ratio.

4) Discharge temperature of compressor should not be excessive, to avoid problems as breakdown or dilution of the lubricating oil, decomposition of the refrigerant, or formation of contaminants such as sludge or acids. All of these can lead to compressor damage.

REFRIGERANT PROPERTIES

2. High latent heat of vaporization and low specific volume of the refrigerant at the entry to compressor

A high latent heat of vaporization and a low specific volume of the refrigerant at entry to the compressor are desirable for smaller equipment and pipe size at given cooling capacity.

High latent heat means there is a high refrigeration effect. For example, R11 has a much larger specific volume of suction

vapor of compressor than those of refrigerants of R22, R502 and R717.

That means it requires a higher volumetric flow rate to produce the same amount of cooling capacity.

Therefore, R11 is usually used with centrifugal compressors because they are good at handing large volumetric flow rate.

REFRIGERANT PROPERTIES

3. Lower compression work

In order to get high COP, both high refrigeration effect and low compression work must be considered in combination.

For example, R717 (ammonia ) has a refrigerating effect q1 much larger than other refrigerants, but its compression work w is also high, as a result, COP of ammonia has the same order of magnitude as that of the other refrigerants.

REFRIGERANT PROPERTIES

4. Some Important Physical/Chemical Properties of Refrigerants

Any substance which has appropriate thermal properties can be used as a refrigerant, but in practice the choice is limited by many factors such as toxicity, flammability, chemical stability, and the behaviors of the refrigerant with lubricating oil, water and construction materials.

VAPOR-COMPRESSION REFRIGERATION (VCR) CYCLE

The vapor-compression refrigeration cycle has four components:

1. Evaporator,2. Compressor, 3. Condenser, and 4. Expansion (or throttle) valve

In a basic vapor-compression refrigeration cycle, the refrigerant enters the compressor as a saturated vapor and is cooled to the saturated liquid state in the condenser.

It is then throttled to the evaporator pressure and vaporizes as it absorbs heat from the refrigerated space

VCR - Cycle

VCR - Cycle The principal work and heat transfer that

occurs in the system are shown below, these quantities being taken as positive in the directions indicated by the arrows in the Fig. 6.4.

In the analyses, each component is first separately considered.

The evaporator, in which the desired refrigeration effect is achieved, will be considered first.

Considering a control volume enclosing the refrigerant side of the evaporator, conservation of mass and energy applied to this control volume together give the rate of heat transfer per unit mass of refrigerant flow in the evaporator as:

1 4e

eQq h hm

VCR - Cycle Next consider the compressor. It is usually adequate to assume that there is no heat transfer to or

from the compressor. Conservation of mass and energy rate applied to a control volume

enclosing the compressor then give:

For a control volume enclosing the refrigerant side of the condenser, the rate of heat transfer from the refrigerant per unit mass of refrigerant is:

2 1i

iWw h hm

2 3c

cQ

q h hm

VCR - Cycle Finally, the refrigerant at state 3 enters the expansion valve and

expands to the evaporator pressure.

This process is usually modeled as a throttling process in which there is no heat transfer, i.e., for which

In the vapor-compression system, the net power input is equal to the compressor power, the expansion valve involving no power input or output.

Using the quantities and expressions introduced above, the coefficient of performance, COP, of the vapor- compression refrigeration system is given by:

34 hh

1 4

2 1

//

e e

i i

q Q m h hCOPw W m h h

LIQUID SUBCOOING In practice some degree of subcooling

may be acquired, and the point 3 moves to the left of the saturated liquid on the pressure-enthalpy diagram, as shown in Figure.

Subcooling is the process of cooling condensed gas beyond what is required for the condensation process.

Subcooling is sensible heat and is measured in degrees.

If it was possible to further cool down the liquid to some lower value, say upto 3’, then the net refrigeration effect will be increased by

1 4' 1 4 4 4' 3 3'( ) ( )h h h h h h h h

Subcooling of the liquid and superheating of the vapor

LIQUID SUBCOOING

The volume refrigerating effect is of course increased by subcooling in the same way as the specific refrigerating effect.

Since the specific work of compression remains the same, the coefficient of performance is improved.

The subcooling may be achieved by any of the following methods:

(i) By passing the liquid refrigerant from condenser through a heat exchanger through which the cold vapor at suction from the evaporator is allowed to flow in the reversed direction.

(ii) By making use of enough quantity of cooling water so that the liquid refrigerant is further cooled below the temperature of saturation.

VAPOR SUPERHEATING If the vapor at the compressor entry is in the

superheated state 1’, which is produced due to higher heat absorption in the evaporator, then the refrigerating effect is increased as

The specific work of reversible adiabatic compression is increased by superheat. This is indicated in Figure by the decreased gradient of the line of constant entropy though point 1’ in the superheat region compared with the gradient of the line though point 1.

Although the specific work of reversible adiabatic compression is increased by superheat, so is the specific refrigerating effect, however, their ratio, COP, may increase, decrease or remain unchanged depending upon the range of pressure of the cycle.

Subcooling of the liquid and superheating of the vapor

1' 4 1 4 1' 1( ) ( )h h h h h h

VAPOR SUPERHEATING Here, it is necessary to consider the effect of heat transfer to the

refrigerant vapor in the suction pipe from the evaporator to the compressor.

The suction line usually passes though warm surroundings, then heat transfer to the vapor can take place, which will cause the temperature increase.

This will cause a part of refrigerating capacity loss to refrigerate engine room, so it will be called “useless refrigerant capacity”.

And the capacity used to refrigerate things which need refrigeration will be called “useful refrigerant capacity”.

In practice, useless refrigerant capacity loss should be decreased, and should increase useful capacity as possible.

CASCADE REFRIGERATION SYSTEMS

• Very low temperatures can be achieved by operating two or more vapor-compression systems in series, called cascading. The COP of a refrigeration system also increases as a result of cascading.

MULTISTAGE COMPRESSION REFRIGERATION SYSTEMS

•When the fluid used throughout the cascade refrigeration system is the same, the heat exchanger between the stages can be replaced by a mixing chamber (called a flash chamber) since it has better heat transfer characteristics.

VAPOUR ABSORPTION REFRIGERATION (VAR) SYSTEM

• When there is a source of inexpensive thermal energy at a temperature of 100 to 200°C is absorption refrigeration.

• Some examples include geothermal energy, solar energy, and waste heat from cogeneration or process steam plants, and even natural gas when it is at a relatively low price.

• Vapour Absorption refrigeration (VAR) systems involve the absorption of a refrigerant by a transport medium.

• The most widely used system is the ammonia–water system, where ammonia (NH3) serves as the refrigerant and water (H2O) as the transport medium.

VAR SYSTEM• Other systems include water–lithium bromide and water–lithium chloride

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.

• Compared with vapor-compression systems, ARS have one major advantage: A liquid is compressed instead of a vapor and as a result the work input is very small (on the order of one percent of the heat supplied to the generator) and often neglected in the cycle analysis.

• ARS are often classified as heat-driven systems.• ARS are much more expensive than the vapor-compression refrigeration

systems. They are more complex and occupy more space, they are much less efficient thus requiring much larger cooling towers to reject the waste heat, and they are more difficult to service since they are less common.

• Therefore, ARS should be considered only when the unit cost of thermal energy is low and is projected to remain low relative to electricity.

• ARS are primarily used in large commercial and industrial installations.

VAR SYSTEM

VAR SYSTEMThe COP of actual absorption refrigeration systems is usually less than 1.

Air-conditioning systems based on absorption refrigeration, called absorption chillers, perform best when the heat source can supply heat at a high temperature with little temperature drop.

VCR CYCLE COMPONENTS

• Refrigerant • Evaporator/Chiller • Compressor• Condenser• Receiver• Thermostatic expansion valve

(TXV)

REFRIGERANT

• Desirable properties:– High latent heat of vaporization - max cooling– Non-toxicity (no health hazard)– Desirable saturation temp (for operating pressure)– Chemical stability (non-flammable/non-explosive)– Ease of leak detection– Low cost– Readily available

29

LOW SIDE OPERATION

• Refrigerants have low boiling points• When liquid boils, it absorbs large amounts of heat• Amount of heat absorbed in evaporator is proportional to

amount of refrigerant boiled• High side Components

• Expansion device• Evaporator• Accumulator (if equipped)

30

EXPANSION DEVICES• The expansion device separates the high side from the low

side and provides a restriction for the compressor to pump against.

• There are two styles of expansion devices:• The TXV can open or close to change flow. It is controlled

by the superheat spring, thermal bulb that senses evaporator outlet temperature, and evaporator pressure

• Most OTs have a fixed diameter orifice

A TXV controls the refrigerant flow from the high pressure side to the evaporator. A receiver dryer is mounted in the liquid line of all TXV systems.

TXV SYSTEM

An OT controls the refrigerant flow from the high pressure side to the evaporator. An accumulator is mounted in the suction line of all OT systems.

OT SYSTEM

33

THERMAL EXPANSION VALVES, TXVs

•The three major types of expansion valves:

•Internally balanced TXVs are the most common.

•Externally balanced TXVs are used on some larger evaporators.

•Block valves route the refrigerant leaving the evaporator past the thermal sensing diaphragm so a thermal bulb is not needed.

Internally Balanced

Externally Balanced

Block Valve

34

THERMAL EXPANSION VALVES, TXVS

• Variable valve that can change size of opening in response to system load

• Opens or closes depending on evaporator pressure and temperature

35

EVAPORATOR OPERATION

Hot, liquid refrigerant flows through the expansion device in the low side to become a fine mist.

Refrigerant boils or evaporates to become a gas inside the evaporator.

The boiling refrigerant absorbs heat from the air during this change of state.

EVAPORATOR/CHILLER

• Located in space to be refrigerated• Cooling coil acts as an indirect heat exchanger• Absorbs heat from surroundings and vaporizes

– Latent Heat of Vaporization– Sensible Heat of surroundings

• Slightly superheated (10°F) ensures no liquid carryover into compressor

37

ACCUMULATORS

Accumulators are used in the suction line of all OT systems.

The accumulator:

•separates liquid refrigerant so only gas flows to the compressor.

•Allows oil in the bottom of the accumulator to return to the compressor.

•provides storage for a refrigerant reserve.

•contains the desiccant bag for water removal.

•provides a place to mount low pressure switches and sensors.

38

HIGH SIDE OPERATION• Takes low pressure vapor from evaporator and returns high

pressure liquid to expansion device• Must increase vapor temperature above ambient temperature for

heat transfer to occur resulting in change of state from vapor to liquid

• High side Components• High begins at compressor and ends at expansion device

• Compressor• Condenser• Receiver-drier (if equipped)

39

COMPRESSORSThere is a large variety of compressors. Some of variations are:

The compressor manufacturer

Piston, vane, or scroll type

The piston and cylinder arrangement

How the compressor is mounted

Style and position of ports

Type and number of drive belts

Compressor displacement

Fixed or variable displacement

40

SCOTCH YOKE COMPRESSORS

A Scotch yoke compressor has two pairs of pistons that are driven by a slider block on the crankshaft. The pistons are connected by a yoke.

Pistons Yoke

Discharge Reed

Suction Reed

41

CONDENSER OPERATION

Hot, high pressure gas is pumped from the compressor to enter the condenser. The gas gives up its

heat to the air passing through the condenser. Removing heat from the hot gas causes it to change state and become liquid.

42

CONDENSER TYPES

Condensers A and C are round tube, serpentine condensers.

Condenser B is an oval/flat tube, serpentine condenser.

Condenser D is an oval/flat tube, parallel flow condenser.

Flat tube condensers are more efficient.

43

SERPENTINE CONDENSER

Refrigerant flows from the upper inlet to the bottom outlet through two tubes. These tubes wind back and forth though the condenser.

44

PARALLEL FLOW CONDENSER

Refrigerant flows from the upper inlet to the bottom outlet through groups of parallel tubes. Some carry refrigerant from the right to the left, and others move it back to the right side.

45

HEAT EXCHANGERS Condensers have to move heat from the refrigerant to the air. Evaporators must move heat from air to the refrigerant. Both require a lot of contact area for both air and refrigerant. Both require free movement of air and refrigerant.

46

RECEIVER DRYERSA receiver dryer is mounted in the liquid line of a TXV system. It is used to:

•to store a reserve of refrigerant.

•hold the desiccant bag that removes water from the refrigerant.

•filter the refrigerant and remove debris particles.

•provide a sight glass so refrigerant flow can be observed.

•provide a location for switch mounting.

Barb Connections, Note Sight Glass

Male Flare Connections

Male O-ring Connections, Note Switch

PROBLEM-1

An ideal vapor-compression refrigeration cycle operates at steady state with Refrigerant 134a as the working fluid. Saturated vapor enters the compressor at 2 bar, and saturated liquid exits the condenser at 8 bar. The mass flow rate of refrigerant is 7 kg/min. Determine

a)the compressor power, in kW

b)the refrigerating capacity, in tons

c)the coefficient of performance

Analyzing vapor-Compression Refrigeration Systems

Ideal Refrigeration Cycle

An ideal cycle has no irreversibilities within the evaporator and condenser, and there are no frictional pressure drops. Compression is isentropic. The T-s diagram is shown on the next slide.

Process 1-2s: Isentropic compression of the refrigerant;

Process 2s-3: Heat transfer from refrigerant to outside air, at constant pressure;

Process 3-4: Throttling process to a two-phase mixture at lower pressure;

Process 4-1: Heat transfer to the refrigerant as it flows at constant pressure through the evaporator;

Analyzing vapor-Compression Refrigeration Systems

As the refrigerant passes through the evaporator, the heat transfer per unit mass of refrigerant flowing is:

A ton of refrigeration is equal to 200 Btu/min or 211 kJ/min.Work done by compressor per unit mass flow of refrigerant is

flowmasstrefrigeranismhhmQin

);( 41

);( 12 hhmWc

Analyzing vapor-Compression Refrigeration Systems

Heat rejected by the refrigerant:

Expansion valve:

Coefficient of performance:

);( 32 hhmQout

34 hh

;)()(

12

41

hhhh

mWmQ

c

in

SOLUTION

Let us first get the properties at each state in the cycle.

State 1: p1 = 2 bar, sat vapor. h1 = 241.30 kJ/kg, s1 = 0.9253 kJ/kg.K

State 2: p2 = 8 bar, s2 = s1, h2 = 269.92 kJ/kg

State 3: p3 = 8 bar, sat. liquid, h3 = 93.42 kJ/kg

State 4: Throttling process, h4 = h3 = 93.42 kJ/kg

a)The compressor power is:

CW m h h kg s kJ kg kW( ) ( / )( . . ) / . 2 17 269 92 241 30 3 3460

Solution

b) The refrigerating capacity is

c)The coefficient of performance is

inQ m h h kg kJ kg tons kJtons

( ) ( /min)( . . ) / / / /min.

1 4 7 241 30 93 42 2114 91

h hh h

( ) ( . . ) .( ) ( . . )

1 4

2 1

241 30 93 42 5 17269 92 241 30

• Problem-2• A refrigerator uses refrigerant-134a as the working fluid and

operates on an ideal vapor-compression refrigeration cycle between 0.12 and 0.7 MPa. The mass flow rate of the refrigerant is 0.05 kg/s. Show the cycle on a T-s diagram with respect to saturation lines. Determine:

• a) the rate of heat removal from the refrigerated space,• b) the power input to the compressor, • c) the rate of heat rejection to the environment, and • d) the coefficient of performance.

Solution

Answers: (a) 7.41 kW, 1.83 kW, (b) 9.23 kW, (c) 4.06

Problem-3Consider an ideal refrigeration cycle which uses R-12 as

the working fluid. The temperature of the refrigerant in

the evaporator is –20C and in the condenser it is 40C.

The refrigerant is circulated at the rate of 0.03kg/s.

Determine the coefficient of performance and the

capacity of the plant in rate of refrigeration.

For each control volume analyzed, the thermodynamic

model is the R-12 tables. Each process is SSSF with no

change in kinetic or potential energy.

Control volume: Compressor.Inlet state: T1 known, saturated vapor; state fixed.

Exit state: P2 known(saturation pressure at T3).

At T3=40C

2 1

2 1

cw h hs s

2

1

1 2

2

2

2 1

0.9607

178.61 /0.7082

50.8211.38 /

32.77 /

g

o

c

P P MPa

h kJ kgs s

T Ch kJ kgw h h kJ kg

Control volume: Expansion valve.Inlet state: T3 known, saturated liquid; state fixed.

Exit state: T4 known.

Control volume: Evaporator.Inlet state: State 4 known.Exit state: State 1 known.

3 4 74.53 /h h kJ kg

1 4 104.08 /

3.18

3.12

L

L

c

q h h kJ kgqw

Capacity kW

• Problem-3

Consider a 300 kJ/min refrigeration system that operates on an ideal

vapor-compression refrigeration cycle with refrigerant-134a as the

working fluid. The refrigerant enters the compressor as saturated

vapor at 140 kPa and is compressed to 800 kPa. Show the cycle on a T-

s diagram with respect to saturation lines, and determine the:

a) quality of the refrigerant at evaporator inlet,

b) coefficient of performance, and

c) power input to the compressor.