A Small Capacity Steam-ejector Refrigerator Experimental Investigation of a System Using Ejector With Movable Primary Nozzle

7/25/2019 A Small Capacity Steam-ejector Refrigerator Experimental Investigation of a System Using Ejector With Movable Primary Nozzle

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ELSEVIER

Int J. Refrig. Vo l. 20, No. 5, pp. 352 -358, 1997

1997 Elsevier Science Ltd and IIR

All rights reserved. Printed in G reat Britain

PII:SO 140-7 007(9 7)000 08-X 0140-7007/97/$17.00 + .00

A smal l capaci ty s team-ejector refr igerator: exper imenta l

invest igat ion o f a sys tem us ing e jector wi th movable

primary nozz le

S a t h a A p h o r n r a t a n a

D e p a r t m e n t o f M e c h a n i c a l E n g i n e e r i n g , S i r in t h o r n I n t e r n a t i o n a l

I n s t i t u t e o f T e c h n o l o g y , T h a m m a s a t U n i v e r s i t y , P . O . B o x 2 2

T h a m m a s a t R a n g s i t P o s t O f f ic e , P a t u m t h a n i 1 21 21 , T h a i l a n d

l a n W . E a m e s

I n s t i t u te o f B u il d in g T e c h n o l o g y , D e p a r t m e n t o f A r c h i t e c t u r e a n d

B u i ld i n g T e c h n o l o g y , T h e U n i v e r s i t y o f N o t t i n g h a m , U n i v e r s i t y P a r k ,

N o t t i n g h a m N G 7 2 R D , U K

R e c e i v e d 2 2 J u l y 1 9 9 6 ; r e v is e d 6 J a n u a r y 1 9 9 7 ; a c c e p t e d 6 F e b r u a r y 1 9 9 7

This paper desc r ibes an exper imenta l s tudy of a s team-e jec tor r e f r ige ra tor us ing an

e j e c to r w i th a p r im a r y noz z l e t ha t c ou ld be m ove d a x i a l ly w i th in t he m ix ing c ha m be r

sect ion . The e ffects on coeff ic ient o f pe r forman ce and cool ing capac i ty prod uced by

adjus t ing the pos i t ion of the nozz le were s tudied . Th e exper imen ta l r ig and me thod a re

desc r ibed and resul t s a re presented which c lea r ly show the benef i t of us ing such a pr im ary

nozzle. C opy r ight Elsevie r Sc ience Ltd an d I IR .

(Keywords: refrigerating machine;ejector; design; performance)

R 6fr ig6rateur de p et i te pu i ssan ce 5. e ject ion de vapeur:

E t ude exp6r i m ent a l e d ' un sys t 6m e u t i l i sant un 6 j ec t eur

b u s e p r i m a i r e m o b i l e

Cet article ddcrit l '~tude exp~rimentale d'un r~frig~rateur i t ~jection de vapeur uti l isant un

~jecteur i t buse prim aire qu i pe ut se dkplacer dans l 'ax e de la cham broe de m~lange . O n a

~tudi~ les e f fe ts de la posi t ion de la buse sur le C O P e t la puissa nce f r igori f ique . On d~crit le

banc d 'essai e t la m~th ode ; on pr~s ente les r~sul tats qui me t tent en ~vidence les avantages de

l 'u t il isat ion d 'un te l systbm e. Elsev ier Sc ienc e L td e t I IR .

(Mo ts cles: mach ine frigorifique; 6jecteur; concep tion; performan ce)

An e jec tor r e f r ige ra tor i s s imi la r to an absorpt ion

re f r ige ra tor , s ince both can b e powered by low grad e

hea t ene rgy, wi th the addi t ion of a smal l quant i ty of

work input r equi red to c i rcula te the working f lu ids .

The r e f o r e , bo th sy s t em s c a n c onve r t w a s t e he a t t o

use ful r e f r ige ra t ion and may be cheaper to opera te

tha n c onve n t iona l va po u r c om pr e s s ion cycl es w hose

energy input i s en t i re ly in the form of mechanica l

w or k . W he n c om pa r e d w i th a n a bso r p t ion sys t e m ,

an e jec tor sys tem is r e la t ive ly s imple to cons t ruc t ,

opera te and cont ro l ; i t a l so uses only s ingle com-

ponent working f lu id ( re f r ige rant only) . Even i t s

coef ficient of pe r fo rman ce (COP ) i s r e la t ive ly low bu t

i t s c a p i t a l c o s t a nd m a in t e na nc e shou ld m a ke i t

becom e a se r ious compe t i tor wi th a ny o the r cyc le 1

Figure 1

show s a s c he m a t i c d i a g r a m o f a n e j e c to r

re f r ige ra t ion cyc le . High pressure and h igh tempera -

ture re f r ige rant vapour i s evolved in a boi le r to

produce the pr imary (mot ive ) f lu id . This en te r s the

pr imary nozz le of the e jec tor , where i t expands to

produ ce a su personic f low tha t c rea tes a low pressure

region wi th in the mixing chamber . This region o f low

pr e s su r e d r a w s va p our f r om the e va po r a to r ( s e cond -

a ry f low) . The pr imary and secondary f lu ids a re

c om bine d in t he m ix ing c ha m be r o f t he e j e c to r . A t

the m ix ing c ha m b e r ' s t h r oa t , a t r a nsver se shoc k - w a ve

3 5 2

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A small capacity steam ejector refr igerator 3 5 3

Figure l

Figure 1

boiler

A steam ejector refrigeration cycle

Cv cle JHgor(fique g~ ~jection de vapeur

is induced to create a compression effect. The mixed

stream is then discharged, via a diffuser, to a

condenser, where the vapour is condensed. The

liquid refrigerant accumulated in the condenser is

returned to the boiler via a pum p whilst the remai nder

is expanded through the throttling valve to the

evaporator, thus completing the cycle. As the work

input required to circulate the fluid is typically < 1

of the heat supplied to the boiler, the COP may be

given as:

heat input at the evaporator

COP -

heat input at the boiler

Theoretical and experimental studies of a small

capacity steam-ejector refrigerator have been pre-

sented previously1. These studies showed that COP

and cooling capacity are both dependent on the

operating temperatures. The area ratio of the primary

nozzle throat to the diffuser throat was also impor-

tant. For fixed boiler and evaporator temperatures,

when the condenser pressure is allowed to be

decreased below a certain critical value the COP and

cooling capacity were found to be constant and the

ejector entrained the same amount of the secondary

fluid. This phenomenon may occur due to the flow

choking within the mixing chamber. Therefore, an

increase in boiler pressure or a reduction in condenser

pressure may not always increase the COP and

cooling capacity. Furthermore, if condenser pressure

is increased to a value greater than a certain critical

value, the ejector mar lose its function completely,

causing the CO P and cooling capacity to drop sharply

to zero. In a previous paper ~, it was concluded tha t,

for a given condenser pressure (limited by the cooling

water temperature) and evaporator temperature

(limited by the cooling application heat source

temperature ), a steam ejector refrigerator will provide

its maximum performance when the boiler pressure is

adjusted in order to allow the ejector to operate

precisely at its critical condenser pressure condition.

Also, when the geometry of the ejector is fixed,

cooling capacity can only be increased by reducing the

boiler temperature as the condenser pressure falls or

I JR 20 5

by allowing the evapora tor temperature to rise, which

may not be possible.

From the literature, the performance of an ejector

has been shown to be dependent on the position of the

primary nozzle2'3. The effect of the nozzle pos ition on

ejector performance has not been clearly explained. In

practice, ejectors are usually designed with a fixed

primary nozzle position. The opt imum position of the

nozzle within the mixing chamber being experimen-

tally determined. The ESDU design guide2 suggests

that the nozzle should be placed at a distance of 0.5

1.0 length of the mixing chamber's throat diameter

upstream of the mixing chamber inlet. However,

because of the complex nature of the flow structure,

it is difficult to give precise recommendations for

the optimum nozzle position. In this current paper,

the ejector used was designed so that the primary

nozzle exit position, relative to the mixing chamber,

could be adjusted in order to maximize the system

performance when the operating conditions were

difference from the design point. Tests were conduc-

ted with various boiler, evaporator and condenser

saturation temperatures.

An experimental refr igerator

An experimental refrigerator with a cooling capacity

of 2kW was constructed. Water was used as the

refrigerant. F i g u r e 2 shows a schematic diagram of the

system. The boiler design was based on the thermo-

syphon principle. Its maximum heating capacity was

s u p e r h e a t e r

evaporator

s t e a m

b o i l e r Y

c o n d e n s e r

8

t o d r a i n

c o o l i n g w a t e r

s o l e n o i d v a l v e

Figure

2 Schematic diagram of the experimental steam

ejector refrigerator

Figure 2 Sch em a du r{jHg~rateur e xper ime ntal h ~/ection de

vapeur


3/7

54

S . A p h o r n r a t a n a a n d I . W . E a m e s

primary nozzle

throat diameter (mm ) 2.0

exit diameter (mm ) 8.0

4 0 r a m I_ 1 0 0r am _ ,4 0 m m ,_ 2 1 0mm

............ Lm-; .............. ....L m - ; .................................................... - - 2 ] ..... .....

- ~ [ ~ _~ - 4 0 m m ~ ]

nozzle exit position (NXP)

Figure 3 F low bound aries of the experimental steam ejector

F ig u r e 3 Lirnites d'~coulement de l'gjecteur de vapeur experimental

20

15

10

I I I I t I

-30 -10 10 30

ev ap o r a to r co o l in g lo ad 6 8 5 W

"1 Tboiler = 120C , Peon = 30 m bar

T b o i l e r 120C, Peon = 35 mbar

~" ~ " ~ O Tboiler = 130C, Peon = 35 mbar

~ Tboiler = I30C , Pcon = 40 mbar

m i x i n g c h a m b e r i n l e t

NXP (mm)

50

Figure 4 Variations of the eva pora tor temperatures with the NX P

F ig u r e 4

Variations de temperatures h l'~vaporateur en fon ctio n de la position de la

b u s e ( N X P )

7 k W , p r o v i d e d b y t w o 3 .5 k W e l e c t r i c h e a t e r s . T h e

e v a p o r a t o r w a s b a s e d o n a f l a s h t y p e d e s i g n . A s i n g l e

3 . 2 5 k W e l e c tr i c h e a t e r w a s u s e d t o s i m u l a t e t h e

e v a p o r a t o r h e a t l o a d . A l l t h e e l e c t ri c h e a te r s w e r e

c o n t r o l l e d u s i n g v a r i a b l e t r a n s f o r m e r s . A s h e l l a n d

c o i l c o n d e n s e r w a s u s e d , c o o l e d b y w a t e r t a k e n f r o m

t h e l a b o r a t o r y ' s c o o l i n g to w e r . T h e b o i l e r w a s

c o v e r e d w i t h a 3 0 m m t h i c k n e s s o f g l a s s f ib r e w o o l

w i t h a l u m i n i u m f o i l b a c k i n g . T h e e v a p o r a t o r w a s

c o v e r e d w i th a 2 0 m m t h i c k ne s s o f n e o p r e n e f o a m

r u b b e r . T h e a v e r a g e b o i l e r h e a t l o s s w a s e s t i m a t e d t o

b e 2 5 % o f t h e e l e ct r ic a l p o w e r i n p u t a n d t h e a v e r a g e

e v a p o r a t o r u n w a n t e d h e a t g a i n w a s f o u n d t o b e

c a

1 2 % 1

T w o c i r c u l a t i o n p u m p s w e r e e m p l o y e d i n t h e

s y s t e m ; a p n e u m a t i c d i a p h r a g m p u m p w a s u s e d t o

r e t u r n t h e l i q u i d w a t e r c o l l e c t e d i n t h e c o n d e n s e r t o

t h e b o i l e r a n d e v a p o r a t o r , a n d a m a g n e t i c a l l y c o u p l e d

c e n t r i fu g a l p u m p w a s u s e d t o c i r c u l a t e w a t e r t h r o u g h

t h e e v a p o r a t o r .

F ig u r e 3 s h o w s a s e c t i o n a l d r a w i n g o f t h e t e s t

e j e c to r . I t w a s d e s ig n e d b a s e d o n m e t h o d s p r o v i d e d i n

t h e l i t e r a t u r e I - 3 . T h e n o z z l e w a s m o u n t e d o n a

t h r e a d e d s h a f t w h i c h a l l o w e d t h e d i s t a n c e b e t w e e n

t h e n o z z l e e x i t a n d t h e m i x i n g c h a m b e r i n l e t t o b e

a d j u s t e d i n o r d e r t o d e t e r m i n e t h e i n f l u e n c e o f t h e

n o z z le p o s i t i o n o n t h e p e r f o r m a n c e o f t h e e j e c t o r. T h e

n o z z l e e x i t p o s i t i o n ( N X P ) w a s d e f i n e d a s t h e d i s t a n c e

b e t w e e n t h e n o z z l e e x it a n d t h e m i x i n g c h a m b e r i n l e t

p l a n e s a s s h o w n i n

F ig u r e 3 .

T h e N X P h a s a p o s i t i v e

v a l u e w h e n t h e n o z z l e i s p l a c e d i n s i d e t h e m i x i n g

c h a m b e r , a n d i s n e g a t i v e w h e n o u t s i d e t h e m i x i n g

c h a m b e r .

T h e b o i l e r, c o n d e n s e r a n d e v a p o r a t o r w e r e c h a r g e d

w i t h d e i o n i z e d w a t e r . T h e p e r f o r m a n c e o f t h e

e x p e r i m e n t a l r e f r ig e r a t o r w a s o b t a i n e d b y m e a s u r i n g

t h e t i m e a v e r a g e d e l e c t r ic p o w e r i n p u t t o t h e

e v a p o r a t o r a n d g e n e r a t o r h e a t e r s o v e r a s te a d y s t a te

r u n n i n g t i m e o f 3 0 - 6 0 m i n .

Optimu m no zzle position

T h e s e t e s t s w e re c o n d u c t e d b y s e t t i n g t h e e v a p o r a t o r


4/7

A small capacity steam ejector refrigerator 55

20

.,:-d

i.

G

5

g

o

evaporator cooling load 685 W

boiler temperature 140C

condenser pressure

~7 35 mbar

+ 40 mbar

c,._.. A 45 mbar

0 50 mbar

a......~....~ [] 55 mbar

60 mbar

mixingchamber nlet NXP (ram)

f

-30 -10 10 30 50

Figure

5 Variations of the evaporator temperatures with the NXP

Figure 5 Variations de tempdratures h l'~vaporateur en fon ctio n de la position de la

b u s e ( N X P )

heat load cons tant at 685 W (110 4-0.5 V across the

heater). At each setting of the boiler and condenser

conditions, the primary nozzle position was varied

and the evaporator temperature was measured. If the

compression ratio (condenser to the evaporator

pressures) increased while the entrained fluid flow

was fixed (fixed cooling capacity), it was indicated

that the ejector performance was improved. The

optimum nozzle position was thought to be at the

point where the mini mum evaporator temperature

was achieved. The results of these experiments are

shown in F ig u r e 4 and 5. It was found that:

For fixed boiler and condenser pressures, the

evaporator temperature decreased as the primary

nozzle position was moved into the mixing

chamber. The temperature can be reduced to a

minimum level at some nozzle positions. Further

movement of the nozzle into the mixing chamber

caused the evaporator temperature to rise.

-- For a fixed nozzle position , the evaporator

temperature dropped when the condenser pres-

sure was reduced or when the boiler pressure was

increased. However, the effect of the condenser

and boiler pressures were reduced when the NXP

was large.

The optimum nozzle position was shown to

depend on the boiler and condenser pressures.

Increasing the condenser pressure or reducing the

boiler pressure moved the optimum nozzle posi-

tion into the mixing chamber, and vice versa.

According to the tests' results, a single optimum

nozzle position cannot be defined to meet all

operating conditions. Each operating condition

required a particular nozzle position. Similar experi-

ments were conducted by Hamner4 using RII as

the working fluid and with zero secondary flow

(the evaporator was isolated from the ejector). In

Hamner 's case, the optim um nozzle posit ion was

determined by measuring the minimum pressure in

the mixing chamber. However, the pressure varied

only very slightly with the nozzle position. According

to Zeren5, the pressure in the mixing chamb er was not

only a function of the boiler and condenser pressures

but also the mass flow rate of the secondary fluid.

From F ig u r e s 4 and 5, the optimum NXP was

found in the range of 0-15ram within the mixing

chamber inlet section. This is in contrast to the

recommendation from ESDU 2 which suggested pla-

cing the nozzle exit 0.5-1.0 length of the mixing

chamber's throat diameter upstream of the start of

the mixing chamber (equivalent to an NXP of - 9 to

-18mm). This may be due to the fact that the

primary steam pressure used is relatively low com-

pared with the pressure that is commonly used

in industrial applications (2-3.6 bar compared with

5-20 bar).

E f f e c t o f t h e n o z z l e p o s i t io n o n t h e s y s t e m p e r f o r m a n c e

These tests were conducted by setting the evapo-

rator's thermo stat at 5C and the boiler's thermostat

at 130C. For each NXP, the condenser pressure was

varied from below to above the critical value. Dur ing

the tests, the electric power inp ut to the boiler and the

evaporator were measured. Refrigerator COP based

on electric power input, to both the boiler and

evaporator, is a measure of overall performance and

includes all the unwanted heat losses and gains to the

system.

F ig u r e 6 (see also T a b le 1 ) shows the effect of the

nozzle position on COP. For fixed boiler and

evaporator temperatures, the COP and cooling

capacity can be varied as much as 100 by changing

on the nozzle position. Moving the nozzle into the


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3 5 6

S A p h o r n r a t a n a a n d I W E a m e s

0 .3 0

0 .2 4

0 .1 8

0. t2

0 .0 6

25

I

e~

28

I I I i

tt A . . zx

~ o u

0 .0 0

3 0

4O

T a b l e

1 P e r f o r m a n c e o f t h e e x p e r i m e n t a l r e f ri g e r a t o r a t

c r it i ca l c o n d e n s e r p r e s s u r e o p e r a t i o n w i t h v a r i o u s N X P s

T a b l e a u 1 Per form ance du r~ frig~ra teur expdr im en ta l fon c-

t ionnan t en press ion cr i t ique au condenseur , avec d i f f~ren tes

pos i t ions de la buse

C o n d e n s e r

P r e s s u r e

N X P ( m m ) T e m p . ( C ) ( m b a r ) Q ev ap ( W ) C O P

31

I _ _ ~ _ _ L ~ _ _ I

1 1

5 0

34 37

I [ I i

Tcon (C)

boiler temperature 130C

boiler pressure 2.7

bar

evaporator emperature 5C

A NXP = 4 mm

o NXP = 11 mm

V NXP = 26 mm

[] NXP = 41 mm

O NXP = 56 mm

Pco n (mb a r )

I 1

6 0

Figure

6 V a r i a t i o n s o f e x p e r i m e n t a l p e r f o r m a n c e ( C O P ) w i t h N X P a n d

c o n d e n s e r p r e s s u r e f o r t h e e v a p o r a t o r t e m p e r a t u r e o f 5 C a n d t h e b o i l e r

t e m p e r a t u r e o f 1 3 0 C

F i g u r e 6

V a r i a t io n s d e l a p e r f o r m a n c e e x p ~ r i m e n t a le ( C O P ) o n f o n c t i o n d e l a

pos i t ion d e la buso ( N X P ) e t de la press ion au condenseur , pou r une tempera ture h

lYvap ora teu r de 5 deg C e t une tempOra ture au bou i l leur de 130 deg C

m i x i n g c h a m b e r c a u s e d t h e C O P t o fa ll a n d c o o l in g

c a p a c i t y t o d e c r e a s e w h e n t h e b o i le r i n p u t w a s

m a i n t a i n e d c o n s t a n t . H o w e v e r , t h e c y cl e c o u l d b e

o p e r a t e d a t a h i g h e r c r i ti c a l c o n d e n s e r p r e s s u r e . B y

r e t r a c t i n g t h e n o z z l e f r o m t h e m i x i n g c h a m b e r , t h e

C O P a n d c o o l i n g c a p a c i t y c a n b e i n c re a s e d a t t h e

e x p e n s e o f t h e cr i ti c a l c o n d e n s e r p r e s s u r e .

-4 2 9 .5 4 1 7 5 4 0 .2 2 3 8

1 1 2 9 .8 4 2 6 5 0 0 .1 9 2 9

2 6 3 0 .8 4 4 5 2 8 0 .1 5 6 7

4 1 3 1 .2 4 5 4 5 8 0 .1 3 6 0

5 6 3 1 .9 4 7 3 7 2 0 .1 1 0 4

E v a p o r a t o r t e m p e r a t u r e , 5 C ; b o il e r t e m p e r a t u r e , 1 3 0 C ;

b o i l e r p r e s s u r e , 2 . 7 b a r ; b o i l e r h e a t i n p u t 3 3 6 9 W .

O p e r a t i o n a n d c o n t r o l o f a s t e a m e j e c t o r r e fr i g e r a t o r

T h i s s e c t i o n g iv e s m e t h o d s o f o p e r a t i n g a n d c o n t r o l-

l i n g a s m a l l s c a l e s t e a m e j e c t o r r e f r i g e r a t o r u s i n g a n

e j e c t o r w i t h a m o v e a b l e p r i m a r y n o z z l e p o s i t i o n .

P e r f o r m a n c e m a p s w e r e c o n s t r u c t e d s o t h a t t h e

e j e c t o r c o u l d b e t u n e d b y v a r y i n g t h e b o il e r t e m p e r a -

0 .6

0 .5

2 2 2 6 3 0 3 4 3 8

I I I I I I I I I I I I I I I I I

Tco n ( C)

e~

T boiler = 1 20 C - - N X P = 2 6 m m

. . . . . . . . N X P = I 1 m m

0 ,4 - - / 1 2 5 C

, 1 3 0 C

0 3 . / , , - 1 4 0 o c

0 .2 - ' - ~ ~ a - g ~ . . / ~ T e v a p = 1 0 .0 o c

" ' : - ~ - ~ _ 2 , ' ~ - - /

01 :/- 50oc

~ Peon (mb a r )

I

0 , 0 I i i i i I l l

2 5 3 5 4 5 5 5 6 5

F i g o r e 7 P e r f o r m a n c e c h a r a c te r i s t ic ( C O P ) o f t h e e x p e r i m e n t a l s t e a m e j e ct o r

r e f r i g e r a t o r

F i g u r e 7 Caract~;ristique de perf orm anc e (C O P ) du r~frig~rateur exp~;rimental ? t

~ jection de vapeur


6/7

A sm al l capa ci ty steam ejector refr igerator

57

1400

1000

600

200

22 26 30 34 38

i L i J i i i

Tcn (C)

- ,~- __/_ . __ , "

Tboiler = 120 C 125oc 130oC

25

. / f -

7 I I

- - N X P = 26 m m

135C

, f 140C

t - - ~ . . . . k , \

Z;--

~ k~ - . . . 5 . 0o C

Tevap = 10.0C

. . . . . . . . NXP = 1 l mm ~ Pcon ( mba r )

I

I I I I I I I I

35 45 55 65

F i gu r e 8 Pe r f o r m ance cha r ac t e r i s t ic (coo l i ng capac i t y ) o f t he expe r i men t a l s te am

e j ec t o r r e f r i ge r a t o r

F i gu r e 8 Carac tkr i s t ique de per forman ce (pu i ssance f r igor i f ique) du r~ fr ig~rateur

experimental h ~ject ion de vapeur

Tab l e 2 Pe r f o r m ance a t o f f de s i gn ope r a t i ons o f the s t e am e jec t o r r e f r i ge r a t o r a s shown i n

Figures 7

and 8*

T a b l e a u 2

Per form ance d u rOfrigOrateur ~ dject ion de vapeur en cas de fon ctio nne me nt hors des norm es de conception, selon les

f igures 7 e t 8

Hea t i npu t ( W ) Tem per a t u r e ( C)

O p e r a t i n g p o i n t C o n d e n s e r

on t he f i gu r e s NX P ( mm ) CO P Eva por a t o r Bo i l e r Evap or a t o r Bo i l e r p r e s su r e ( mba r )

a 26 0.210 710 3369 7.5 130.0 45.7

b 26 0.210 710 3369 7.5 1300 42.5

c 26 0.210 710 3369 7.5 130.0 25.0

d 26 0.225 710 3160 7.0 127.2 42.5

e 26 0.240 752 3138 7.5 126.9 42.5

f 26 0.278 936 3369 10.0 130.0 47.6

g 26 0.197 690 3510 7.5 131.7 47.6

h I1 0.210 710 3369 6.0 130.0 42.5

i 11 0.261 850 3257 7.5 128.5 42.5

* The da t a p r ov i ded i n t h i s t ab l e a r e ob t a i ne d g r aph i ca l l y f r om t he f igu re s

t u r e a n d p r e s s u r e o r N X P i n o r d e r t o o b t a i n t h e

m a x i m u m p e r f o r m a n c e w h e n t h e c o n d e n s e r p r e s su r e

i s c h a n g e d d u e t o t h e v a r i a t i o n o f t h e e n v i r o n m e n t

t e m p e r a t u r e

T h e e x p e r i m e n t s s h o w e d t h a t, f o r g i ve n e v a p o r a t o r

a n d c o n d e n s e r p re s su r e s, m a x i m u m C O P a n d c o o l in g

c a p a c i t y w e r e o b t a i n e d w h e n t h e c y c l e w a s o p e r a t e d

a t a b o i l e r t e m p e r a t u r e t h a t a l l o w e d t h e e j e c t o r

t o o p e r a t e a t i ts c r i t i c a l c o n d e n s e r p r e s s u r e T h e

p e r f o r m a n c e m a p s i n F i g u r e s 7 a n d 8 w e r e c o n s t r u c -

t e d f r o m d a t a t a k e n a t c r i t i c a l c o n d e n s e r p r e s s u r e

o p e r a t i o n f o r th e N X P s o f 1 1 a n d 2 6 m m . T h e b o i l e r

a n d e v a p o r a t o r i s o t h e r m l i n e s s h o w n i n th e f ig u r e s

c a n b e u s e d o n l y w h e n t h e e j e c t o r w a s o p e r a t e d a t i ts

c r it ic a l c o n d e n s e r p r e s s u re . A n e x a m p l e o f u s i n g t h es e

f i g u r es i s n o w g i v e n .

R e f e r r i n g t o F i g u r e s 7 a n d 8 ( d a t a f o r e a c h

o p e r a t i n g p o i n t o n t h e s e f i g u r e s a r e p r o v i d e d i n

T a b l e

2 ) ; I t is a s s u m e d t h a t , t h e c y c l e i s n o r m a l l y

d e s i g n e d t o o p e r a t e a t p o i n t a ( r e f e r e n c e d t o s o l i d

l in e c u r v e s ) w i t h a N X P o f 2 6 m m a n d c r i t ic a l

c o n d e n s e r p r e s s u re o f 4 5 . 7 m b a r . I f t h e c o n d e n s e r

p r e s s u r e f a ll s to 4 2 . 5 m b a r , d u e t o a r e d u c t i o n o f i t s

c o o l i n g w a t e r t e m p e r a t u r e , w h i l e t h e b o i l e r a n d

e v a p o r a t o r t e m p e r a t u r e s a r e h el d c o n s t a n t , t h e c y cl e

w i ll th e n o p e r a t e a t p o i n t b w i t h t h e C O P w i t h t h e

c o o l i n g c a p a c i t y e s s e n t ia l l y r e m a i n i n g t h e s a m e * . A s

t h e b o i l e r t e m p e r a t u r e i s f ix e d, f o r a n y c o n d e n s e r

p r e s s u r e < 4 5 .7 m b a r , t h e C O P a n d c o o l i n g c a p a c i t y

e s s e n t i a l l y r e m a i n c o n s t a n t a s s h o w n b y t h e h o r i z o n -

t a l l in e a - b - c , b u t t h e e j e c t o r is n o l o n g e r o p e r a t i n g a t

i ts c ri ti c a l c o n d e n s e r p r e s s u r e . I n o r d e r t o i m p r o v e

c y c l e p e r f o r m a n c e w h e n t h e c o n d e n s e r p r e s s u r e i s

* F o r t h i s c a se , t he evap or a t o r a nd bo i l e r i so t he r m l i ne s can

no t be u sed t o i nd i ca t e t he evapor a t o r and bo i l e r t empe r a -

t u r e s a s i t does no t op e r a t e w i t h c r it i c a l condense r p r e s su r e

(see

Table 2

f o r ope r a t i ng cond i t i ons ) . It mus t be no t ed t h a t

t he bo i l e r and evapor a t o r t empe r a t u r e s g i ven by t he

i o s t he r m l i ne s can be u sed on l y when t he e j ec t o r is ope r a t ed

wi th cr i t ica l condi t ion.


7/7

3 58

S A p h o r n r a t a n a a n d I W E a m e s

reduced from 45.7 to 42.5mbar, the ejector must

be allowed to operate at a new critical condenser

pressure (42.5mbar) by using one of the following

methods (see Figures 7 and 8):

Constant cooling capacity, lower evaporator

temperature

Point d with N XP of 26 mm referenced to solid line

curves). If the boiler temperature is reduced to

127.2C when the condenser pressure is decreased

to 42.5 mbar, the cycle operating point will move to

point d with the critical condenser pressure of

42.5 mbar. This produces a constant cooling capacity

at a lower evaporator temperature (7.0C). The COP

is also increased as the primary nozzle is always

choked, reducing the boiler temperature automati-

cally reduces heat input to the boiler.

Point h with NXP o f 11 mm referenced to dotted line

curves): If the boiler temperature is maintained at

130.0C and the nozzle is retracted to NXP of

l l m m when the condenser pressure falls to

42.5mbar, the cycle operating point will be moved

to point h with the critical condenser pressure of

42.5 mbar (from the figures, point h and b are shown

to be the same, however, point h is operated with

critical condition and referenced to the dotted line

curves while point b is not operated with critical con-

dition). This causes the COP and the cooling capa-

city to remain constant, but at a lower evaporator

temperature (6C).

Constant evaporator temperature, higher cooling

capacity

Point e with NX P o f 26m m referenced to solid line

curves): If the cycle is already operating at point d,

and the boiler temperature is slightly further reduced

from 127.2 to 126.9C, the evapora tor temperatu re

will return to 7.5C and the cycle operating point

will move to point e with the critical condenser pres-

sure of 42.5 mbar. The COP and the cooling capacity

will rise.

Point i with NX P o f l l mm referenced to dotted line

curves): If the cycle is already operating at point h

with an N XP of 11 mm and the boiler temperature

is reduced from 130.0 to 128.5C, the evaporator tem-

perature will increase back to 7,5C and the cycle can

be operated at point i with the critical condenser pres-

sure at 42.5 mbar. This causes the COP and cooling

capacity to be increase.

If the condenser pressure is increased higher than

the design point such as on a hot day points f and g

show the possible operating conditions when the

condenser pressure is increased to 47.6mbar. As

the condenser pressure is increased higher than the

critical value, in order to establish a new critical

condenser pressure, the cycle must operate with a

higher boiler pressure and fixed evaporator tempera-

ture (which reduces cooling capacity and COP) or a

higher evaporator temperature and fixed boiler

temperature (which increases cooling capacity and

c o P ) .

C o n c l u s i o n s

This paper describes the experimental studies of a

small scale steam ejector refrigerator using ejector

with adjustable primary nozzle position. The test

showed that a single optimum primary nozzle

position cannot be defined to meet all operating

conditions. Each operating condition requires a

particular optimum nozzle position. The COP and

cooling capacity can be varied as much as 100 by

changing the nozzle position. Moving the nozzle into

the mixing chamber caused the COP and cooling

capacity to decrease when the boiler input and

temperature was main tained constant. However, the

cycle could be operated at a higher critical condenser

pressure. When the nozzle was retracted from the

mixing chamber, the COP and cooling capacity was

found to increase, but the critical condenser pressure

was reduced.

The use of an ejector with movable primary nozzle

provides a more flexible opera tion t han a totally fixed

geometry unit. An increase in the cooling capacity can

be achieved by retracting the nozzle from the mixing

chamber as the condenser pressure falls without

changing either the evaporator or boiler tempera-

tures. In practice, the nozzle position may be

automatically controlled by monitoring the saturated

temperatures and pressures in the boiler, evaporator

and condenser.

R e f e r e n c e s

1 Eam e s , I . W . , Aphor nr atana, S . , Haid e r , H. A theore-

tical and experimental study of a small scale steam

jet refrigerator. Int. J. Refrig. (1995) 18 378-386

2 ESDU, Ejector and jet pump, data item 86030, ESDU

International Ltd, London, UK, (1985)

3 Ke e nan, J . H. , Ne um ann, E. P . , Lus twe rk , F . An inves-

tigation of ejector design by analysis and experiment.

ASME J. Appl. Mech., (1950), Sept. 299-309

4 Hamner, R. M. An investigation of an ejector-com-

pression refrigeration cycle and its applications to

heating, cooling, and energy conservation. Ph.D. the-

sis, The University of Alabama, Birmingham, USA

(1978)

5 Zeren, F. Freon-12 vapor compression jet pump solar

cooling system. Ph.D. thesis, Texas A&M University,

USA (1982)

Documents

A Small Capacity Steam-ejector Refrigerator Experimental Investigation of a System Using Ejector With Movable Primary Nozzle