Refrigeration Lecture 1

5/20/2018 Refrigeration Lecture 1

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Course 11127 - Sustainable Heating and Cooling

Refrigeration

Refrigeration Processes

Cold Vapour Compression

Absorption

AdsorptionSteam Ejector Refrigeration

Cold Gas (Air) Cycle

Peltier Cooling

Magneto-Electric Refrigeration


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Evaporation

The Coffee Cup Experiment (1)

thermo-

meter

hot coffee

very thin

plastic film

-32 % cooling

thin layer

of

olive oil

-40 % cooling100 % cooling

MILD AIRSTREAM


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Evaporation

The Coffee Cup Experiment (3)

Time Temperature

min. Oil Surf. Vaporiz.

0 55,0 C 55,0 C1 54,0 C 53,1 C

2 53,0 C 51,3 C

3 52,0 C 49,5 C

4 51,0 C 47,8 C

5 50,1 C 46,1 C

6 49,1 C 44,5 C

7 48,2 C 43,0 C

8 47,3 C 41,5 C

9 46,4 C 40,1 C

10 45,6 C 38,7 C

11 44,7 C 37,3 C

12 43,9 C 36,1 C

13 43,1 C 34,8 C

14 42,3 C 33,6 C

15 41,5 C 32,4 C

30

40

50

60

0 5 10 15

Time / minutes

Temperatu

re/C

WITH

Vaporization

WITH

Oil Surface

MILD AIRFLOW


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1,1,1,2-Tetrafluoroethane, R-134a, or

HFC-134a, is a haloalkane refrigerant withthermodynamic properties similar to R-12

(dichlorodifluoromethane), but with less

ozone depletion potential. It has the

formula CH2FCF3, and a boiling point of

26.3

C


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Evaporation

Introductory Remarks

Refrigeration with the commonly used "ColdVapour Process" needs basically nothing else

than an apparatus, called the "evaporator".

In this heat exchanger the necessary energy forevaporating a liquid is supplied by a warmerenvironment, which eventually "looses" heat andbecomes colder. That's all. The rest inrefrigeration technology is needed to makeevaporation (continuously) happen.

This means that all the complicated andexpensive gear around the evaporator is anecessity, but does not make "cooling" any more.The evaporator is the only and really importantpart, the rest is technical "neccessity".


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Group discussion

Topic:

How to make the evaporation continuouslyhappen?

Task:

Make a sketch to show the principle of the

cooling machine you designed.


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Vapour compression refrigeration cycle


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Condenser

Compressor

Evaporator

Expansion(Throttling)

Device

Low temperature

Low pressure gas

high temperature

high pressure gas

Low temperature

high pressure liquid

Low temperature

low pressure liquid

Qcool

Qrej.

Vapour compression refrigeration cycle


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Condenser

Compressor

Evaporator


Device

Low temperatureLow pressure gas

high temperature

high pressure gas

Low temperature


Low temperaturelow pressure liquid

Qcool

Qrej.

Replace the compressor


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Absorption refrigeration cycle


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Two ways of raising pressure

Absorber

Generator

Pump

Low trmp.

low pressurerich solution

Low temp.

high pressure

rich solution

high temp.

high pressure

poor solution

high temp.

low pressure

poor solution

Qg

Qa


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Refrigeration Processes

Compressor

Evaporator

Condenser

Exp.

Device

Compressor

Exp.

Device

Evaporator

Condenser Gascooler

Gasheater

Expansion

Machine

Desorber

"Generator"

Absorber

Solution

Heat Exch.

Solution

PumpQcool Qcool Qcool

Qrej. Qrej. Qrej.

Qabs

Qdes

Mechanical Vapour

Compression

(Thermal)

Absorption System

Mechanical Cold Gas

Compress./Expansion

el. el.heat

"rich" solution"poor" solution

R e f r i g e r a n t R e f r i g e r a n t


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Steam Ejector Refrigeration (2) ("Thermal Compressor")

Mixed Vapour Qcat Condensing

Pressure pc

Motive

Steam Qst

pst

Suction Vapour Qe

at EvaporatingPressure pe

Mixing PipeDiffusor

Motive Steam Nozzle

pst

pc

pe

pst

pc

pe

Pressure Diagram


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Ejector refrigeration cycle


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Steam ejector refrigeration cycle


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Peltier Cooling

Peltier (and Seebeck) Effect

Material A

Material B

Voltage

Material B

"cold" "warm"


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Magnetocaloric effect


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Analogy - Cold Gas and Magnetic Refrigeration

Heat Source

OUT

Heat SourceIN

Heat Sink

IN

Heat Sink

OUT

HEATING:

Compression of gas

(air) which heats up

HEAT REJECTION:

Cooling of gas (air) rsp.

gaschamber with a

cooling fluid (e.g.

cooling water)

REFRIGERATION

EFFECT:

Expansion of gas (air)

which is cooled below

ambient temperature

COOLING:

Heating of the - now

"cold" - gas chamber

by a warmer fluid which

is consequently cooled

Cold Gas Refrigeration (Analogy)

HEATING:

Magnetization of (ferro-

magnetic) heat exchan-

ger which consequently

heats up

HEAT REJECTION:

Cooling of heat

exchan-ger with a

cooling fluid (e.g.

cooling water)

REFRIGERATION

EFFECT:

De-Magnetization of

the heat exchanger

which is cooled belowambient temperature

COOLING:


"cold" - heat exchanger



S

N

S

N

S

N

S

N

Heat SinkIN

Heat Sink

OUT

Heat SourceIN

Heat Source

OUT

Magnetic Refrigeration

Heat Source

OUT

Heat SourceIN

Heat SinkIN

Heat Sink

OUT

HEATING:

Compression of gas

(air) which heats up

HEAT REJECTION:

Cooling of gas (air) rsp.

gaschamber with a

cooling fluid (e.g.

cooling water)

REFRIGERATION

EFFECT:

Expansion of gas (air)


ambient temperature

COOLING:


"cold" - gas chamber



Cold Gas Refrigeration (Analogy)

HEATING:

Magnetization of (ferro-

magnetic) heat exchan-

ger which consequently

heats up

HEAT REJECTION:

Cooling of heat

exchan-ger with a

cooling fluid (e.g.

cooling water)

REFRIGERATION

EFFECT:

De-Magnetization of

the heat exchanger


ambient temperature

COOLING:


"cold" - heat exchanger



S

N

S

N

S

N

S

N

Heat SinkIN

Heat Sink

OUT

Heat SourceIN

Heat Source

OUT

Magnetic Refrigeration


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The "classic" Cold VapourProcess

Definitions and Key Figures


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Energy performance of a refrigeration system

what you get cooling(refrigeration)COP

what you spend energy demand= =

Coefficient of Performance (COP)


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Coefficient of Performance

COP

The COP is NOT an "efficiency" BUT an energy

characteristic.

It describes:

"Efficiency" in refrigeration applies for:

- Carnot efficiency(system efficiency), and

- Isentropic efficiency(compressor efficiency)

what you get cooling(refrigeration)COPwhat you spend energy demand

= =


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Calculation of COPthe ideal Carnot(Refrigeration) Cycle

W=(Tc-Te)(S1,2-S3,4)

qe=Te(S1,2-S3,4)

COP= qe/W = Te(S1,2-S3,4)/(Tc-Te)(S1,2-S3,4)= Te/(Tc-Te)

W

qe


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System Efficiency: COP and "Carnot Efficiency"

Carnot COPC is the "benchmark" for cold vapourcompression cycles.

We learnt:

Carnot COPc is a function of Tc and Te only

e e eC

c e

q Q TCOP

w P T T= = =

&


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Example:

At 0 C evaporation and 35 C condensation, a real refrigerating

system has refrigerating capacity of 100 kW and power demand

of 21 kW. What is the COP and the Carnot efficiency of the

refrigerating system?

COPreal = 100/21 = 4,8

Carnot POCc= (0+273)/(35-0) =7.8

Carnot efficiencyreal

C

C

COP 4,8h 0, 62 62 %

COP 7,8

= = = =


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System Efficiency: COP and "Carnot Efficiency"COP as function of compressor size


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Cold Vapour Processtheideal Carnot processwith a "real" fluid

0,00

1,00

2,00

3,00

4,00

5,00

0,0 2,0 4,0 6,0 8,0 10,0Entropy s

Tempe

ratureT Problem !


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Standard vaour compression cycle

0,00

1,00

2,00

3,00

4,00

5,00

0,0 2,0 4,0 6,0 8,0 10,0Entropy s

Tempe

ratureT

liquid(b

ubbl

e)line

vapour(dew)line


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Pressure-Enthalpy diagram


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Mollier Diagram for Ammonia (R 717)

XXX

C ld V P


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Cold Vapour Processthe modified Carnot Cycle ("Plank process") with a "real" fluid

isentropic compression


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The standard vapour compression cycle expressed by

p-h and T-s diagram

Pre

ssurelogp

0,00

1,00

2,00

3,00

4,00

5,00

0,0 2,0 4,0 6,0 8,0 10,0Entropy s

Temp

eratureT


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Isentropic and polytropic compression (R 717)

0/35 C0/35 C

1

2is 2poly

4

3


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Isentropic Efficiency

Describes the compressor "quality"

Compressor power demand shown as enthalpy

difference between point 1 and 2 (see previous slide).

2 1

2 1

2 1

2 1

compressor

compressor compressor polytropic

compressor is

compressor

compressor polytropic

is is

real is

is

is

real

h h h

h h h h

h h h

h h h

=

>

= =


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Isentropic Efficiency

We can read from the h, log p diagram:

t p h v

C bar kJ/kgK m3/kg1 0 4,3 1.460 0,29 Qe kW

2is 80 13,5 1.620 P kW 15 21

2poly 110 13,5 1.690 Qc kW 115 121

3 35 13,5 360 COP - 6,7 4,8

4 0 4,3 360 is - 100% 70%

From the h, log p - diagram Calculated values

Ammonia R 717, evaporation 0 C, condensation 35 C Refrigerating Capacity: 100 kW

100

isen-

tropic

poly-

tropic

1 3,4

2 1.( )

eR

is R

ispoly

is

Qrefrigerant mass flow mh h

isentropic power demand P m h h

Ppolytropic power demand P

=

=

=

&&

&

A t l i l d ith t d d l


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Actual vapor-compression cycle compared with standard cycle


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Condenser

Evaporator


Device

Low temperature

Low pressure gas

high temperature

high pressure gas

Low temperature


Low temperature

low pressure liquid

Qe

Qc

Absorber

Generator

Pump

Low trmp.

low pressure

rich solution

Low temp.

high pressure

rich solution

high temp.

high pressure

poor solution

high temp.

low pressure

poor solution

Qg

Qa

Absorption refrigeration cycle


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Heat-operated refrigeration cycle as combination of a

power cycle and a refirgertion cycle


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COP of absorption refrigeration cycle


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Example:

What is the COP of an ideal heat-operated refrigeration

system that has a source temperature of heat of 100C, arefrigerating temperature of 5 C, and an ambient

temperature of 30 C?

Solution

As Ts increase, the COP increase

As Tr increase, the COP increase

As Ta increase, the COP increase

Refrigeration Lecture 1

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