A Model of an Air-Conditioning Condenser and Evaporator With Emph

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)

=

m(

;

)

J tTLi

Lmi: ll

j J j

ll1

a

nd

f

.

+[

u

'

1

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1

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I )

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u

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,

. ('

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I

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JL l

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.l

l

whe

re

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ubs

cript

s

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and

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or

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nt, "a

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r

epre

sent

s

t

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th

inc

rem

ent alo

ng t

he tu

be .

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e p

aram

ete

rs

l i

s ted

1n

t

he ab

ove

equa

tion

s

follo

w

con

vent

ions

use

d in m

ost

o

f the

liter

atur

e,

nam

ely,

m

is

the

ma

ssflow,

i is

en

thal

py, lJ

is

over

all

heat

t

rans

fer co

eff i

cient

, A

is ar

ea,

and

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is

tem

per

atur

e.

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he

"per

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en t

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subs

crip

t re

fers

t

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e fac

t t

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h t

ube

is div

ided

int

o

n c r e

m ~ n l

s that

are

ref

erre

dl o

u ~

ele

men

ts. Si

nce

th

eheat

exc

hang

er is

d ivi

ded

into

inc

rem

ents

,the

area

, A,

c o r r

e s p o

n d ~ t

u an

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crem

ent

al are

a. A s

d isc

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d la

ter,

th

e

a

bove

eq

uatio

n alo

ng

wit

hthe

ref

riger

ant-

side

mo

men

tum

equ

atio

n

and

the

equ

ation

of

stat

e m

ust b e s

olve

d si

mul

tane

ousl

yov

er eac

h t

ube

incre

men

t.

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e d

iffer

ent i

al eq

uatio

n fo r

mo

men

tum

c

an

be

app

roxim

ate

dby a

f ini

te-d i

f fere

n ce

n

um

erica

l

f

o rm

as fo

llow

s:

[

( 1

I

)

(

z J -z

l_ , l -1

_1

)I

- P

G

-

-

~ +

J

- l

J

P

,

PJ-

l

4D

PJ

P

,_J

wh

ere

P

is

pre

ssur

e, G

i

s ma

ss fl

ux,

p

is

d

en si

ty , f

is fri

ction

fac

tor,

z is

posi

tin

an

d Dis

i

nsid

e

tub

e

d

iame

ter,

2

.3.

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erall

heat

tr

ans f

er coe

fficie

nt

'r

he e

qua

tion

fo r

the

over

all

he

attran

sfer

coef

ficien

L, 0

0

,

ca

n be

d er

ived

by

usin

g

Lhe

rm al

re

sist

ance

s. The

th

erm

al

r

es is

tanc

esa

re d

ef in

ed

as foll

ow s:

R

1

=

1

/h;

Ap;

R2 = l lh

d i

Ap

;

R3

=

tp/A

pm k

p

R4 =

1 h

eAp

o

R

5

= 1/h

duA

o

R

6 =

see d

iscu

ssio

n)

R

7

=

l lh o

A p o

c

onve

ctio

n in s

id e

the

tub

e

i

nsid

ed ep

osi t

tu

be w a

ll

con

tact

be

Lw ee

n fi

n

and

tub

e w

all

ou Ls

ide d

epo

sit

in

con

vec

tion

ou ts

ide

the

tube

The

v

alue

s

of

hdi

a

ndhdo

for t

he in

s id e

a

nd ou t

s ide

depo

sits

resp

ect iv

e ly

ap

proa

ch

in

finit

yfor a

cle

an t

ube su

rfac

e . T

he

pa

ram

eter

he is

the

equ

ival

en tconv

ecti

on

coe f

flcie

nl

for

fin

-tub

econt

act

t

herm

al resi

stan

ce b as

ed o

n L

he out

s ide

tub

e ar

ea. A

rece

nt

s

Lu dy

of

31

co i

ls fro

m 6

m

anuf

actu

rers

s

how

ed

tha

t

h

0

va

ries

co ns

ider

ab ly

, ra

ngin

g

fro

ml.0

45

k

W /m

2K

to

in f

in ity

[

12].

An

ave

rage

valu

e of1

4 kW

/m

2K wa

s u

sed

in

thi

s stud

y.

t is co

nven

ien

t to

c

o ns i

der

the

e

ffect

ofR

6 by

com

bini

ng

it wi

th

R7

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cr ib

ed i

n de

tail

in ref

eren

ce 1

3, th

e i

na U

0

equa

tion

iso

btain

ed by

com

bin

ing

the

abo

ve

res

istan

ces .

(51

wh

ere

......

27

6 -

r

~ ~

~ ~

~ ~

~ =

= ~

~ ~ _

j

.-

.

f

__

27

2

r

0

F

ig .6

.

0

Fig

B

I

I

I

I

I

4

5

POS

ITION

m

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vapo

rato

r.re

frig

eran

t

te

mpe

ra tu

res f

orsm

oot

h a

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en

han

ced tub

es

3

P

OSIT

ION

m

E

vap

ora t

ora

ir t

emp

erat

ures

fo r

smo

oth a

nd

enha

nce

d tub

es

Documents

A Model of an Air-Conditioning Condenser and Evaporator With Emph