413 Topic v-3 (Run-Around Coil Systems, Regenerative Heat Ex Changers and Pinch Technology)

8/8/2019 413 Topic v-3 (Run-Around Coil Systems, Regenerative Heat Ex Changers and Pinch Technology)

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1

Run-around Coil Systems:

Definition of Run-Around Coil Systems

Design Factors

Examples

ISAT 413 - Module V: Industrial Systems

Topic 3: Run-around Coil Systems,Regenerative Heat Exchangers,Pinch Technology



2

Run-around coil system of heat recovery

A run-around coil heat recovery system is the name

given to a linking of two recuperative heat exchangersby a third fluid which exchanges heat with each fluid

in turn as shown diagrammatically below.



3

Run-around coil system A run-around coil would be used in cases where the two

fluids which are required to exchange heat are too farapart to use a conventional direct recuperative heatexchanger. It is also desirable to use such an indirectsystem if there is a risk of cross-contamination betweenthe two primary fluids (e.g. when a particularly corrosive

fluid is involved, or when there is a risk of bacterialcontamination as in a hospital).

Advantage would be free choosing of working fluid.

Disadvantage would be low effectiveness of the HX.

Typical applications are recovery of energy from the airleaving a room or building to pre-heat the air entering;and recovery of energy from a corrosive gas for waterheating.



4

Run-around coil heat recovery between fluidswith the same thermal capacity

; and

i.e.

22112211

212121

C S C S S H S H

H C S

H H C C S S

H C S

t t t t t t t t

cmcmcm

t t t t t t cm

Q

cm

Q

cm

Q

!!

!!

!!!!!

H C

cmcm !



5

2

asexpressed becanrecoveryheatthehence

sinceand

2 thus,

2 and

2

:identicalareexchangersheattwotheAssume

21

21

111111

222

2222

111

1111

C

H

C H H

C

C C

C H H

C S C S H H

C H

S

C S S H

C H

C H

S

C S S H

C H

cm

UA

t t UA

cmt t

t t UAt t UAt t UA

t t

t t t t t UAUA

t t t

t t t t UAUA

!

!

!!!

!!!!

!

!

!!

H C cmcm !

Run-around coil heat recovery between fluidswith the same thermal capacity



6

ExampleA run-around coil heat recovery system similar to that on

slide 4 is used for a room in which the presence of bacteriarules out any possibility of air re-circulation or a directrecuperative heat exchanger. Air enters the room at 24oCand leaves at 20oC; the average outside air temperatureduring the annual period of use is 5oC. Assuming that the

mass flow rate of air is 2 kg/s, mean specific heat 1.005kJ/kg-K, that (UA)H = (UA)C = 4 kW/K, and that thespecific heat of the secondary fluid is 2.5 kJ/kg-K,calculate:

(i) the required mass flow rate of secondary fluid;(ii) the temperature of the air leaving the run-around coil;

(iii) the percentage energy saving by using the run-aroundcoil.



7

Example

39.4%100%

524

5512saving%

: bygivenis

savingenergy percentagethehence24to5from

heat bemustair herecovery theatheWithout t(iii)

512012

04155and

0415

012

42

5204

2

(ii)

804052012

01200512 (i)

21

21

!v

!

!!!

!

!

!

!!!

!v!!!

C

C

oo

o

C

C C

C

H

C H H

S

S s

C H S

cm

.cm

C C

C . .

.

cm

Qt t

kW .

.cm

t t Q

s / kg . . .

ccmm

K / kW . .cmcmcm





9

C S H cmcmcm {{

Round-around coil heat recovery betweenfluids of diff erent thermal capacity



10

C S H cmcmcm {{

Round-around coil heat recovery betweenfluids of diff erent thermal capacity



11

Idealized run-around coil system

_ a _ a H C C H S

C H C H S

cmcmc

cmcmm

!fluidsecondaryRequired



12

Example 5.7

(use I- NTU method to analyze run-around coil

heat recovery system)



13

Example 5.7A corrosive gas at a flow rate of 30 kg/s from a process at

300oC is to be used to heat 20 kg/s of water entering 10oCusing a run-around oil as shown on slides 8 & 12. Calculateusing the data given:

(i) the mass flow rate of secondary fluid required;

(ii) the effectiveness of the overall heat transfer;(iii) the exit temperature of the water;

(iv) the temperatures of the secondary fluid.

Data: Mean specific heat of gases, 1.2 kJ/kg-K; meanspecific heat of water, 4.2 kJ/kg-K; mean specific heat of secondary fluid, 3.8 kJ/kg-K; (UA) for the gas to secondaryfluid heat exchanger, 40 kW/K; (UA) for the secondaryfluid to water heat exchanger, 200 kW/K.



14

Example 5.7

_ a _ a

_ a s

kg .

. . .

. .

UAcmUAcmcUAUAcmcmm

H C C H S

C H C H S

0918200213040242083

2004024202130

bygivenisluidsecondaryoratelowmassrequiredThe(i)

!vvvv

vvv!

!

55042901

1

1

1 ess,effectivenHXHence

42908436 and

;926036

33333 since

1003

2001

401111 Since(ii)

57109260

57109260

1

1

.e .

e

Re

e

.cmcm R

. .

cm NT

. .

. .

R NT

R NT

max

min

min

overall

C _ S S H overall

!

!

!I

!!|

!!|

!!!

v

v

v

v



15

Example 5.7

C .t .

t . .

C .t .

t . .

t t cmt t cm

t t cmt t cm

t t cmQQ

oC

H

C C

o H

H

H H

C H H

C C C

C H H

H H H

minmax min

C H

478103002130102420550

5140103002130

3002130550

or

bygivenisesseffectivenHXof definitiontheSince(iii)

11

22

21

21

21

21

!vvvv!!I

!vv

vv!!I

!

!

!I

C .t t t .

t t t t

C .t .t t

t t t t

o S S C S H

C S C S H H

o S S C S H

C S C S H H

83110200514040

:2station@

311547820030040

:1station@

bygivenisfer heat transtheSince(iv)

222

2222

111

1111

!!

!

!!

!



16

Temperature changes for Example 5.7



17

Regenerative Heat Exchangers:

Definition of Regenerative HX Design Factors

Examples

In a regenerative heat exchanger (sometimes

called a capacitance heat exchanger) the hot

and cold fluids pass alternately across a

matrix of material; the matrix is heated up by

the hot fluid then cooled down by the cold

fluid so that the process is cyclic.



18

Stationary Regenerative Heat Exchanger

In (a) matrix B is hot and heats up the cold fluidwhile matrix A is heated by the hot fluid; in (b) thecold fluid is now heated by matrix A while the hotfluid re-heats matrix B; the valves are then switchedover and the cycle commences again as in (a) .



19

Rotary Regenerator or Thermal Wheel

A matrix of material is mounted on a wheel which isrotated slowly through the hot and cold fluidstreams as shown above. It is known as the thermalwheel, and Ljungstrom rotary regenerator after itsDanish inventor.



20

Influence of Matrix RotationalSpeed on Effectiveness

M

M

M

max

mi

U

U

c

.

mi

M

c

M c

M c

cm

cm

cm

e

e E

cm

cm

E E

!X!

|

!

±

±

À

±±

¿

¾

±

±

°

±±

¯

®

¼½

»

¬-

«!

and

;1

1 where,

9

11

1

1

931



21

Example 5.8 (A rotary regenerator)

A rotary regenerator is used to recover energy from

a gas stream leaving a furnace at 300o

C at a massflow rate of 10 kg/s. Heat is transferred to a massflow rate of air of 10 kg/s entering at 10oC. Thewheel has a diameter 1.5 m, giving an approximateface area of 1.6 m2, and a width of 0.22 m; the

matrix has a surface area to volume ratio of 3000m2/m3 and a mass of 150 kg; the rotational speed of the wheel is 10 rev/min. The heat transfer coefficientfor both fluid streams is 30 W/m2-K and the mean

specific heats at constant pressure for the gas and airare 1.15 kJ/kg-K and 1.005 kJ/kg-K; the specificheat of matrix material is 0.8 kJ/kg-K.

Calculate:



22


(i) the effectiveness of the heat exchanger;

K

k .cm .

. .

cm

cm

E E

K

k .cm .

e .

e

e

e E

. .

.

cm

cm .

.cm

UAU

m . .V

AV A

K .mhh

hhU

Ah AhUA

mi . .

mi

M

c

M . .

. .

U

U

c

max

mi

mi

C H

H C

C H

0510Where61709919

116360

9

11

208015060

10 ;6360

87401

1

1

1

874015110

005110 ;5761

1000005110

105615

1056300022061area,er eat trans

153030

3030111 bygivenisctcoe iciener heat transoverallThe

931931

12605761

12605761

1

1

2

2

!!À¿¾

°¯®!

±±

À

±±

¿

¾

±±

°

±±

¯

®

¼½

»¬-

«!

!vv!!

!

!

!vv!!!

vvv!!

!vv!¹ º

¸©ª

¨v!

!

v!

!!

v

v



23


(ii) the rate of heat recovery and the temperature of

the air at exit;

.C

.

t

k t .t t cm

k . .

.t t cm

E

oC

C C C mi

C H mi

189

0510

179910air,oratureexit tempe

179910005110Also,

1799103000051106170

6170

esse ectivenexchanger heatode initiontheFrom

1

121

21

!!

!vv!!

!vvv!

!

!



24


(iii) the air temperature at exit if the rotational speed

of the wheel is increased to 20 rev/min;

C .t

.t t cm

t t cm

t t cm

Q

. .

.

cm

cm

. .cm

cm

K

kW . NMccm

sec

min

min

.rev

min

.rev N

oC

C H min

C C min

C H min

. .

min

M

c

min

M M M

19310300631010

6310Therefore

63109839

116360

9

11

983005110

404080150

60

20

60

12020Given

1

21

21

21

931931

!v!

!!

!

!À¿¾

¯ !

±±

À

±±

¿

¾

±±

°

±±

¯

¼½

»¬-

«!

!v

!!vv!!

v!!



25


C . .t

.t t cm

t t cm

t t cm

Q

. .

.

cm

cm

. .cm

cm

K

kW . NMccm

sec

min

min

.rev

min

.rev N

oC

C H min

C C min

C H min

. .

min

M

c

min

M M M

817310300565010

5650Therefore

565099509

116360

9

11

9950005110

101080150

60

5

60

155Given

1

21

21

21

931931

!v!

!!

!

!À¿¾

°¯ !

±±

À

±±

¿

¾

±±

°

±±

¯

¼½

»¬-

«!

!v

!!vv!!

v!!

(iv) the air temperature at exit if the rotational speed

of the wheel is reduced to 5 rev/min;



26

Gas-Fired Regenerative Burners

In (a) the hot gases are fed back through the burner

and through a matrix to exhaust; while in (b) air isdrawn through the matrix and supplied with gas tothe burner where combustion takes place; twoburners are used in tandem so that continuouscombustion can take place.



27

Magnetic Heat Pump researchat Oak Ridge National Laboratory



28

Problem 5.9

Doubl e Acc u mul at or Regenerat iv e Heat Exchanger

A double accumulator as shown above is to beinstalled to recover energy from the air leaving a

building. The air leaves the building at 20oC at a rateof 2 kg/s and the mean outside air temperature forthe heating season is 5oC. Calculate the rate of therecovery, etc...



29

Pinch Technology Concepts:

Basic Concepts

Design Factors

Examples

For many years the approach to a large network of heatexchangers was either by µrule of thumb¶ or a

systematic mathematical examination of all possible

configurations to try to achieve the best layout.

Another approach to network design is given the nameProcess Integration, or Pinch Technology.



30

Heat Exchanger Temperature Profiles

The design of theheat exchanger isbased on theminimum allowabletemperature

difference betweenthe two streamsbeing 20K .Additional heatingand cooling are

required to achievethe desiretemperatures.



31

Simple Heat Recovery Scheme

kW

C

Q

C

eating ,external

40202

110-130

:heating/Externaldditional

!v!

v!

kW

C

Q

H

cooling ,external

30301

30-60

:cooling/Externaldditional

!v!

v!



32

Temperature-Heat Load representationof Heat Recovery scheme

The two linesrepresenting thestreams are positionedso as to show a regionof overlap whichrepresents the action of the HX in transferring140 KW. The minimumtemperature difference

occurs where the twolines are nearesttogether - this point iscalled the Pinch point .



33

Effect of µMoving¶ the Cold Stream

The effect of increasing the Pinchtemperaturedifference is twofold;the amount of heat

exchange between thetwo fluids is reducedand the externalduties are increased.Note that the slope of

the cold stream line isdetermined by thevalue of CC, which is1/CC = 0.5 K/kW.



34

Effect of µMoving¶ the Hot Stream The same argument could be used for positioning the hot

stream line. Thus we can say that the lines can be movedhorizontally within the limits of temperature and gradientuntil the nearest points are separated by the minimumallowable temperature difference, that is the Pinchtemperature d iff erence.

Also

To achieve the target for the external cooling duty of thehot stream, there must be no exter nal cool in g o f the hot

stream above the pinch.A similar argument applies to the external heating of thecold stream below the pinch: there must be no exter nal heat in g o f the cold stream below the Pinch.



35

Stream Netwroks:

Stream Network Concepts

Design Factors

Examples

Considering the design of a system of heat recovery

between two (or more) hot streams and two (or

more) cold streams to illustrate some fine points of

Pinch Technology.



36

Example 6.1 (Pinch Technology)

Streamnumber

Type Thermalcapacityrate, C(kW/K)

Initialtemp.(oC)

Finaltemp.(oC)

Rate of enthalpyincrease

(Cv(T)(kW)

1 Hot 2 200 60 -280

2 Hot 4 170 70 -400

3 Cold 3 40 175 +405

4 Cold 4.5 100 150 +225

- 50

The heat flow capacities and temperatures of four streams are

shown in the table below. For the purpose of definition, a hotstream is defined as one which requires cooling to reach its finaltemperature and a cold stream is one which requires heating toreach its final temperature. The minimum allowabletemperature difference between the streams is 20 K.



37

Composite Stream Heat Flow Capacities

(Hot Stream Composite)



38

Composite Stream Heat Flow Capacities

(Cold Stream Composite)



39

Hot and Cold Composite Curves



40

Combined Hot and Cold Composite Curves

From the graph the

following information canbe derived:

hot stream temperatureat the Pinch: 120oC

cold stream temperatureat the Pinch = 100oC

target external heatingload = ? kW

target external coolingload = ? kW

Cooling load will exceed the heating load by ? kW.



41

4

11

1

1

14

14

1

1

1

4

Heat ad

e m p e r a t u r e

alculati ns f Exter nal ling and Heating ads

target external

heating load = 90 kW

target external coolingload = 140 kW

Cooling load will exceed the heating load by 50 kW.

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413 Topic v-3 (Run-Around Coil Systems, Regenerative Heat Ex Changers and Pinch Technology)

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