7/23/2019 A REVIEW OF GAS-GAS HEAT RECOVERY SYSTEMS http://slidepdf.com/reader/full/a-review-of-gas-gas-heat-recovery-systems 1/39 eat Recorery Systems Vol 1 pp 3 to 41 Pergamon Press Ltd 1980 Printed in Great Britain A REVIEW OF GAS-GAS HEAT RECOVERY SYSTEMS D.A. REAY International Research & Development Co Ltd (UK), Fossway, Newcastle-upon-Tyne, NE6 2YD, U.K. Abstract--Gas-gas heat recovery systems are probably the most prolific in all application areas. and are certainly of considerable interest for rapid 'retro-fitting on industrial plant as an energy saving measure. They are of growing importance in the home, and their use in large HVAC systems is now mandatory in Sweden. A major development effort in equipment design to meet the requirements of more arduous environments and economic eonstraints is evident in many countrieS. This paper reviews this field, giving descriptions of equipment available and, for the more important systems, factors to be taken into account in their selection. i INTRODUCTION IT IS in the field of heat recovery from exhaust gas and air streams that in most cases the largest benefits from investment in energy conservation equipment can be realized. There are a considerable number of uses to which this waste heat can be put, and these depend to a large extent on the temperature and condition of the exhaust gases or air, the heat recovery equipment used, and the economic assessment of the overall system perform- ance. With regard to the uses which determine the type of heat exchanger to be used. we may identify three main areas, these being the heating of liquid, steam raising, and air heating. This paper is concerned with the last of these categories, the recovery of heat from exhaust gas or air streams for preheating of other air or gas streams. Within this category it is possible to identify three main application areas for the waste heat recovery equipment: (i) Use of process waste heat for preheating process supply air. (ii) Use of process waste heat for space heating and air conditioning. (iii) Recovery of exhaust heat from an air conditioning system in a commercial or domestic building for preheating supply air. (Note that in summer such a heat exchanger may also be used for precooling incoming air, effecting savings on the refrigeration load.) One can also identify a few more specialised areas of application, involving for example pollution control, recovery of heat from prime movers, and the use of incinera- tor waste heat. Most items of equipment described here can be used when the waste heat is at a sufficiently high temperature not to require 'upgrading' for reuse. However, heat pumps, which can use outside air or process air as a heat source, are also available. The heat pump, which is able to upgrade waste heat, may also be effectively used in some drying processes, and is dealt with here in this context. A point often overlooked when selecting heat recovery equipment, whatever the type, is that when installed in a new building or process, it can generally result in a reduction in size of, say, boiler or refrigeration plant. However, when retrofitted, the equipment, while relieving the load on steam raising plant, for example, may as a result cause it to operate at a lower efficiency. Such changes should be taken into account when carrying out an economic assessment, but, of course, such a loss in efficiency is unlikely to result in any dramatic reduction in the effectiveness-bf heat recovery equipment as an energy- saving technique. (In the same way, of course, extra fan power needed to overcome the pressure drop through a waste heat recovery unit should also be included in an operating cost balance sheet.)
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7/23/2019 A REVIEW OF GAS-GAS HEAT RECOVERY SYSTEMS
P e r g a m o n P r e s s L t d 1 9 80 P r i n t e d i n G r e a t B r i t a i n
A R E V I E W O F G A S - G A S H E A T R E C O V E R Y S Y S T E M S
D.A. REAY
International Research & Developmen t Co Ltd (UK ), Fossway,
Newcastle-upon-Tyne, NE6 2YD , U.K.
Abstract--Gas-gas hea t recovery systems are probably the mo st prolific in all application areas.
and are certainly of considerable interest for rap id 'retro-fitting on industrial plant as an energy
saving measure. Th ey are of growing importance in the ho me , and their use in larg e HVA C
systems is now mand atory in Sweden. A m ajor development effort in equipment design to meet
the requirements of mo re arduous env ironments and economic eonstraints is evident in many
countrieS.
This paper reviews this field, giving descriptions of equipm ent available an d, for the more
impo rtant system s, factors to b e taken into accou nt in their selection.
i I N T R O D U C T I O N
I T IS i n t h e f i el d o f h e a t r e c o v e r y f r o m e x h a u s t g a s a n d a i r s t r e a m s t h a t i n m o s t c a s e s t h e
l a r g e st b e n e fi t s f r o m i n v e s t m e n t in e n e r g y c o n s e r v a t i o n e q u i p m e n t c a n b e r e a li z ed . T h e r e
a r e a c o n s i d e r a b l e n u m b e r o f u s e s t o w h i c h t h i s w a s t e h e a t c a n b e p u t , a n d t h e se d e p e n d
t o a l a r g e e x t e n t o n t h e te m p e r a t u r e a n d c o n d i t i o n o f th e e x h a u s t g a s e s o r a i r, t h e h e a t
r e c o v e r y e q u i p m e n t u s ed , a n d t h e e c o n o m i c a s s e s s m e n t o f th e o v e r a l l s y s t e m p e r f o r m -
a n c e. W i t h r e g a r d t o t h e u s es w h i c h d e t e r m i n e t h e t y p e o f h e a t e x c h a n g e r t o b e u s e d . w e
m a y i d e n t i f y t h r e e m a i n a r e a s , t h e s e b e i n g t h e h e a t i n g o f l i q u id , s t e a m r a i s in g , a n d a i r
h e a t i n g . T h i s p a p e r i s c o n c e r n e d w i t h t h e l a s t o f t h e s e c a t e g o r i e s , t h e r e c o v e r y o f h e a t
f r o m e x h a u s t g a s o r a i r s t r e a m s f o r p r e h e a t i n g o f o t h e r a i r o r g a s s tr e a m s .
W i t h i n t h i s c a t e g o r y i t i s p o s s i b l e t o i d e n t i f y t h r e e m a i n a p p l i c a t i o n a r e a s f o r t h e w a s t e
h e a t r e c o v e r y e q u i p m e n t :
( i) U s e o f p r o c e s s w a s t e h e a t f o r p r e h e a t i n g p r o c e s s s u p p l y a i r .
(ii) U s e o f p r o c e s s w a s t e h e a t f o r s p a c e h e a t i n g a n d a i r c o n d i t i o n i n g .
(iii) R e c o v e r y o f e x h a u s t h e a t f r o m a n a i r c o n d i t i o n i n g s y s t e m i n a c o m m e r c i a l o r
d o m e s t i c b u i ld i n g f o r p r e h e a t i n g s u p p l y a i r. ( N o t e t h a t i n s u m m e r s u c h a h e a t
e x c h a n g e r m a y a l s o b e u s e d f o r p r e c o o l i n g i n c o m i n g a i r, e ff e c ti n g s a v i n g s o n t h e
r e f r i g e r a t i o n l o a d . )
O n e c a n a l s o id e n t i f y a f e w m o r e s p e c i a l i s e d a r e a s o f a p p l i c a t i o n , i n v o l v i n g f o r
e x a m p l e p o l l u t i o n c o n t r o l , r e c o v e r y o f h e a t f r o m p r i m e m o v e r s , a n d t h e u s e o f i n c in e r a -
t o r w a s t e h e a t .
M o s t i t e m s o f e q u i p m e n t d e s c r i b e d h e r e c a n b e u s e d w h e n t h e w a s t e h e a t i s a t a
s u f fi c ie n t ly h ig h t e m p e r a t u r e n o t t o r e q u i r e ' u p g r a d i n g ' f o r r eu s e . H o w e v e r , h e a t p u m p s ,
w h i c h c a n u s e o u t s i d e a i r o r p r o c e s s a ir a s a h e a t s o u r c e , a r e a l s o a v a i l a b l e . T h e h e a t
p u m p , w h i c h is a b l e to u p g r a d e w a s t e h e a t , m a y a l s o b e e f f e c ti v e ly u s e d i n s o m e d r y i n g
p r o c e s s e s , a n d i s d e a l t w i t h h e r e i n t h i s c o n t e x t .
A p o i n t o f te n o v e r l o o k e d w h e n s e l e c ti n g h e a t r e c o v e r y e q u i p m e n t , w h a t e v e r t h e t y p e,
i s t h a t w h e n i n s t a l le d i n a n e w b u i l d i n g o r p r o c e s s , it c a n g e n e r a l l y r e s u l t i n a r e d u c t i o n
i n s iz e o f , s a y , b o i l e r o r r e f r i g e r a t i o n p l a n t . H o w e v e r , w h e n r e t r o f i t te d , t h e e q u i p m e n t ,
w h i l e r e l i e v i n g t h e l o a d o n s t e a m r a i s i n g p l a n t , f o r e x a m p l e , m a y a s a r e s u l t c a u s e i t t o
o p e r a t e a t a l o w e r ef fi ci en c y. S u c h c h a n g e s s h o u l d b e t a k e n i n t o a c c o u n t w h e n c a r r y i n g
o u t a n e c o n o m i c a s s e s s m e n t , b u t , o f c o u r s e , s u c h a l o ss in e f f ic i e n c y i s u n l i k e l y t o r e s u l t
i n a n y d r a m a t i c r e d u c t i o n i n t h e e f f e c ti v e n e s s- b f h e a t r e c o v e r y e q u i p m e n t a s a n e n e r g y -
s a v i n g t e c h n iq u e . ( I n t h e s a m e w a y , o f c o u r se , e x t r a f a n p o w e r n e e d e d t o o v e r c o m e t h e
p r e s s u r e d r o p t h r o u g h a w a s t e h e a t r e c o v e r y u n it s h o u l d a l s o b e in c l u d e d i n a n o p e r a t i n g
c o s t b a l a n c e s h e e t . )
7/23/2019 A REVIEW OF GAS-GAS HEAT RECOVERY SYSTEMS
A n o t h e r a s p e c t o f p a r t i c u l a r i m p o r t a n c e i n p r o c e ss a p p l i c a t i o n s o f g a s - g a s h e a t r e c o v -
e r y e q u i p m e n t is fo u l in g . T h e a c c u m u l a t io n o f m a t t e r o n t h e h e a t e x c h a n g e s u r f a ce
a f f ec t s b o t h p r e s s u r e d r o p a n d h e a t t r a n sf e r , a n d d i s p o s a b l e h e a t e x c h a n g e r e l e m e n t s
( d e s c r ib e d e l se w h e r e in t h is J o u r n a l ) a r e o f f e r ed b y s o m e m a n u f a c t u r e r s a s a n a l t e r n a t i v e
t o r e g u l a r c l ea n i n g , w h i c h m a y b e d i f f ic u l t o r n e c e s s i ta t e p r o c e s s s h u t - d o w n f o r e x t e n d e d
p e r i o d s . T h e u s e o f d i s p o s a b l e u n i t s c a n g e n e r a l l y o n l y b e e n t e r t a i n e d i f t h e c o s t is l o w ,
h o w e v e r .
T h e d e s i g n e r o f g a s - g a s h e a t r e c o v e r y s y s t e m s m u s t , b e c a u s e, a t l e as t i n t h e c a s e o f
i n d u s t r i a l i n s t a l la t i o n s , th e m a j o r i t y o f u n i t s a re s t il l r e t r o f i t te d t o p l a n t , m a k e s u r e t h a t
t h e i n s t a l la t i o n is e as y t o i m p l e m e n t , a n d t h e u n i t h a s a m i n i m u m e f fe c t o n p l a n t
o p e r a t i o n . I t m a y b e s a i d t h a t p r a c t i c a li t ie s a s s o c i a t e d w i t h i n s t a l la t i o n , o p e r a t i o n , m a i n -
t e n a n c e a n d , o f c o u r s e , e c o n o m i c s , t a k e p r e c e d e n c e o v e r t h e r m a l d e s i g n . A fe w p o i n t s
i m p r o v e m e n t i n t h e r m a l e f f ic i en c y is m e a n i n g l e s s to t h e p r o c e s s p l a n t m a n a g e r i f t h e
d e s i g n d o e s n o t m e e t h i s m a n y o t h e r r e q u i r e m e n t s .
T h i s p a p e r r e v ie w s t h e f o l lo w i n g t y p e s o f ~ q u i p m e n t :
R o t a t i n g r e g e n e r a to r s
S t a t i c r e g e n e r a t o r s
P l a t e h e a t e x c h a n g e r s
R u n - a r o u n d c o i l s
C o n v e c t i o n r e c u p e r a t o rs
R a d i a t i o n r e c u p e r a t o r s
R e c u p e r a t i v e b u r n e r s
T h e r m o s y p h o n a n d h e a t p i p e h e a t e x c h a n g e r s
M u l t ip l e t o w e r e n t h a l p y e x c h a n g e r s
G a s - g a s h e a t p u m p s
T h e a b o v e s y s t e m s re p r e s e n t t h e v a s t m a j o r i t y o f c o m m e r c i a l l y a v a i l a b l e g a s - g a s u n i t s ,
a n d m o r e u n u s u a l o r s p e c u l a t i v e d e s i g n s w i l l b e d i s c u s s e d i n s e p a r a t e p a p e r s i n f u t u r e
i s su e s ( as w i ll d e v e l o p m e n t s a n d a p p l i c a t i o n s o f th e m o r e c o m m o n t y pe s ).
3. ROT ATING REGENERATORS
T h e r o t a t i n g r e g e n e r a t o r v a r i o u s l y k n o w n a s t h e h e a t w h e e l , r o t a r y a i r p r e - h e a t e r ,
M u n t e r w h e e l, o r L j u n g s t r o m w h e e l a ft e r i ts D a n i s h i n v e n t o r ,* h a s b e e n u s e d o v e r a
p e r i o d o f a b o u t 5 0 y e a r s fo r h e a t r e c o v e r y in l ar g e p o w e r p l a n t c o m b u s t i o n p r o c e s se s [ 2 ] .
I t h a s a ls o b e e n w i d e l y u s e d in a i r c o n d i t i o n i n g a n d a v a r i e t y o f i n d u s t r i a l p r o c e s s h e a t
r e c o v e ry a p p l i c a t i o n s - - i n 1 9 7 5 i t w a s e s ti m a t e d t h a t in E u r o p e a l o n e u p w a r d s o f 1 5 0 0 0
r o t a t i n g r e g e n e r a t o r s w e r e i n u s e [ 3 ] .
T h e o p e r a t i o n o f t h e r o t a t i n g r e g e n e r a t o r i s s h o w n i n F i g . 1. I n c o m m o n w i t h p l a t e
a n d h e a t e x c h a n g e r s , t h e r e g e n e r a t o r w h e e l s p a n s t w o a d j a c e n t d u c t s , o n e c a r r y i n g
e x h a u s t g a s a n d t h e o t h e r c o n t a i n i n g t h e g a s f l o w w h i c h i t i s r e q u i r e d t o h e a t . T h e g a s
f l o w s a r e c o u n t e r - c u r r e n t . A s t h e w h e e l r o ta t e s , it a b s o r b s h e a t f r o m t h e h o t g a s p a s s i n g
t h r o u g h i t a n d t r a n s f e r s t h e h e a t t o t h e c o o l e r g a s f l o w . A l a t e r d e v e l o p m e n t , t h e h y g r o -
s c o p i c w h e e l , i s a b le t o t r a n s f e r m o i s t u r e a s w e l l a s s e n s ib l e h e a t b e t w e e n t h e t w o d u c t s .
R o t a t i n g r e g e n e r at o r s, i n c o m m o n w i t h m a n y o t h e r h e a t r e c o v e r y s ys te m s , c a n b e u s e d
i n h o t c l i m a t e s f o r p r e - c o o l i n g a i r u s e d f o r c o n d i t i o n i n g l a r g e b u i l d i n g s , a n d t h e w h e e l
w o r k s e f fe c ti v e ly i n a p p l i c a t i o n s w h e r e t h e t e m p e r a t u r e d i f fe r e n c es b e t w e e n h o t a n d c o l d
a i r s t r e a m s a r e t o o l o w f o r e f f ec t iv e u s e o f r e c u p e r a t iv e h e a t e x c h a n g e r s .
M a n y m a n u f a c t u r e r s p r o d u c e t h r e e d if f er e n t t y p e s o f r o t a t i n g r e g e n e r a to r . T h e m o s t
c o m m o n f o r m u t i l i z e s a w h e e l m a d e u p f r o m a k n i t t e d a l u m i n i u m o r s t a i n l e s s s t e e l w i r e
ma t r i x . Th i s ma t r i x i s c he a p , a nd t he he a t t r a ns fe r e f f i c ie nc y i s h i gh a s t he a i r f l ow i s
e x p o s e d t o a l a r g e a m o u n t o f s u r f a c e a s it ~ p a ss e s t h r o u g h t h e w h e e l . H o w e v e r , t h e
* Some power generating plants use a second typ e of rotating regenerator, the Rothemuhle design, which
incorporates a station ary matrix with rotating hoo ds to d istribute the gas and air flows, as sho wn in Fig. 2.
However, few m anufacturersoffer hese commercially o r process plant app lications[1].
7/23/2019 A REVIEW OF GAS-GAS HEAT RECOVERY SYSTEMS
Fig. 6. Rotating regenerator used to recover heat from a coating and gravure plant.
n e c e s s a ry t o r e m o v e e x h a u s t e v e n i f h e a t r e c o v e r y w a s n o t u s ed . I n t h i s c a s e t h e p a y b a c k
p e r i o d o n t h e i n v e s t m e n t i s s u b s t a n t i a l l y l e s s t h a n t w o y e a r s .
Static regenerators
T h e s t a t ic r e g e n e r a t o r t h e o p e r a t i n g p r i n c i p l e o f w h i c h i s i ll u s-
t r a t e d i n F i g . 7 t a k e s s e v e r a l f o r m s a n d i s c o m m o n l y a s s o c i a t e d w i t h h i g h t e m p e r a t u r e
p r o c e s s e s s u c h a s g l a s s m e l t i n g f u r n a c e s . S t a t i c r e g e n e r a t i v e h e a t e x c h a n g e r s a r e i n e f f e c t
h e a t s t o r a g e d ev i ce s t h e h e a t s t o re o f te n b e in g r e q u i r e d t o f u n c t i o n o v e r a c o m p a r a t i v e l y
s h o r t c y c l e ti m e .
B y s w i t c h i n g t h e g a s f l o w s t h r o u g h t h e r e g e n e r a t o r c o r e t h e c o r e i s a l t e r n a t e l y e x p o s e d
t o t h e h o t e x h a u s t a n d c o l d s u p p l y a i r s t r e a m s . T h e e x h a u s t g a s h e a t s u p t h e c o r e
m a t e r i a l w h i c h m a y b e a s t a c k o f b r i c k s a s i n g l a ss m e l t i n g f u r n a c e r e g e n e r a t o r s a n d
t h e n t h e s u p p l y a ir i s d i r e c t e d t h r o u g h t h e c o r e p i c k i n g u p h e a t r e t a i n e d b y t h e c o r e
b e f o r e p a s s i n g t o t h e p r o c e s s w h e r e i t i s t o b e u s e d .
A c o m p a r a t i v e l y r e c e n t d e v e l o p m e n t h a s b e e n t h e d e s i g n a n d a p p l i c a t i o n o f s t a ti c
r e g e n e r a t i v e s y s t e m s i n d o m e s t i c h e a t r e c o v e r y u n i t s. W i t h t h i s e x c e p t i o n p r o g r e s s i n t h e
f ie l d o f r e g e n e r a t i v e h e a t e x c h a n g e r s o f t h i s t y p e a w a i t s f u r t h e r a p p l i c a t i o n o f h e a t
s t o r a g e s y s t e m s u n d e r d e v e l o p m e n t f o r l o n g e r t e rm s t o r a g e a n d m o r e s o p h i s t ic a t e d
c o n t r o l te c h n i q u e s p o s s i b ly m i c r o p r o c e s s o r - b a s e d .
4 P L A T E H E A T E X C H A N G E R S
T h e g a s - to - g a s p l a t e h e a t e x c h a n g e r ta k e s s e v e r a l f o r m s t w o o f w h i c h a r e i l l u s t r a t e d i n
F i g s 8 a n d 9 . T h e y n o r m a l l y in v o l v e a c r o ss - fl o w c o n f ig u r a t i o n a l t h o u g h s o m e a p p r o a c h
F l o w ~ - -
control
v o l v e s . ~ _
H e a t e d
supply
ai r A
1
Regenerator
core
[
t
Cooled
exhaust
e
i
G a s
f low
/
I
H o t SupPly
exhaust a i r
Fig. 7. Op eration of a static regenerator.
- -~ F low
c ont ro l
/ v a l v e s
7/23/2019 A REVIEW OF GAS-GAS HEAT RECOVERY SYSTEMS
A r e v i e w o f g a s g a s h e a t r e c o v e r y s y s t e m s 13
. . . . . . . .
Fig. 9. A p late heat exchanger emp loyingglass p lates.
s i o n p r o t e c t i o n ) , t h e p l a t e s m a y b e c o r r u g a t e d t o i n c r e a s e t h e h e a t t r a n s f e r s u r f a c e p e r
u n i t v o l u m e o f h e a t e x c h a n g e r , o r t o p r o m o t e t u r b u l e n c e . T h e s u r f a c e o f t h e M u n t e r s
E c o n o v e n t E X se r ie s o r E c o n o p l a t e i n N o r t h A m e r i c a ), a m o d u l e o f w h i c h i s i l l u s tr a t e d
i n F i g . 11 , i s t yp i c a l o f t he fo rm use d , t he p l a t e s i n t h i s c a se be i ng a l umi n i um. The
c o r r u g a t i o n s a l s o s e r v e a s p a t h w a y s f o r c o n d e n s a t e t r a n s p o r t a n d r e m o v a l t o t h e d r a i n
p a n a t t h e l o w e r a p e x o f t h e h e a t e x c h a n g e r , a n d p r e v e n t e n t r a i n m e n t o f t h e c o n d e n s a t e .
I t i s i n t e r e s t i n g t o n o t e t h a t t h e m o d u l e s m a y b e m o u n t e d i n p a r a l l e l t o c a t e r f o r h i g h
f l o w ra t es , o n e m o d u l e h a n d l i n g u p t o 1 .5 m 3 / s. M o d u l e s a r e a l s o r e m o v a b l e f o r c l e a n i n g .
A n o v e l f e a t u r e o f t h e M u n t e r s s y s t e m , w h i c h m a y a l s o b e a p p l i e d i n H V A C s y s t e m s , is a
f re e z e p ro t e c t i on de v i c e . I l l us t ra t e d i n F i g . 12 , t h i s sma l l l i d i s t r a ve rse d by a mot or
a c r o s s t h e o u t d o o r a i r s id e o f t h e h e a t e x c h a n g e r c o r e , t e m p o r a r i l y i n h i b i t i n g t h e c o o l i n g
e f fec t o f t he f re sh a ir , t hus pe rm i t t i ng t he c o re o f t he e xc h a ng e r t o i nc re a se i n t e m pe ra -
ture .
T h e s e l ec t io n o f p l at e h e a t e x c h a n g er s in c o m m o n w i t h m a n y o t h e r t y p e s o f h e a t
r e c o v e r y u n i t ) i s b e c o m i n g r e l a t i v e l y e a s y i n s o f a r t h a t m a n y m a n u f a c t u r e r s n o w p r o d u c e
s e l ec t io n c h a r ts , p a r t i c u l a r ly f o r u se a t t h e H V A C e n d o f t h e t e m p e r a t u r e s c a le . T h e
m o d u l a r c o n s t r u c t i o n o f m o s t u n i t s s im p l i fi e s t h i s p r o c e d u r e s ti ll f u r t h e r . F i g u r e 1 3
s h o w s a c a p a c i t y c h a r t f o r t h e M u n t e r s E c o n o v e n t E X 1 0 /1 1 p l a t e h e a t e x c h a n g e r . W i t h
t h e a s s is t a n c e o f a p s y c h o m e t r i c c h a r t , a n d k n o w l e d g e o f f lo w s , t e m p e r a t u r e s a n d h u m i -
d i t ie s o f t h e a m b i e n t a n d e x h a u s t a i r, it is p o s s i b l e t o s e l e ct t h e n u m b e r o f m o d u l e s
r e q u i r e d , p r e d i c t s u p p l y a n d e x h a u s t s i d e p r e s s u r e d r o p s , a n d t h e s u p p l y a i r c o n d i t i o n s
H.R.S. J I B
7/23/2019 A REVIEW OF GAS-GAS HEAT RECOVERY SYSTEMS
m a y b e u s e d o n l a r g e st e a m - ra i s in g p l a n t m u l t i -M W s iz e) t o r e c o v e r h e a t f r o m t h e
e x h a u s t g a s e s f o r b o i l e r a i r p r e h e a t i n g . T r a d i t i o n a l l y t h e l a r g e b o i l e r a i r p r e h e a t e r m a r k e t
h a s b e e n d o m i n a t e d b y r e g e n e r a ti v e h ea t e x c h a n g e rs , p ri n c i p a ll y t h e r o t a t i n g r e g e n e r a t o r
b u t . t o a l e s s e r e x t e n t , t h e R o t h e m u l e r e g e n e r a t o r . R e g e n e r a t i v e u n i t s a r e n o w r e c e i v i n g
c o m p e t i t i o n f r o m t h e r u n - a r o u n d c o i l, w h i c h, u s in g e c o n o m i z e r t e c h n o l o g y , c a n b e s u c -
c e s s fu l l y a p p l i e d i n t h is a p p l i c a t i o n . T h e l a r g e s t i n s t a l l a t i o n t o d a t e i n t h e U K is a t
S t a n l o w r e f i n e r y .
6. CONVECTION TUBULAR) RECUPERATORS
T h e t u b u l a r c o n v e c t i o n r ec u p e r a t o r c om e s i n m a n y f o rm s , d e p e n d i n g u p o n t h e o p e r a t -
i n g t e m p e r a t u r e r a n g e , p l a n t s i ze . a n d t h e t y p e o f f o u l i n g i n th e e x h a u s t g a s s t r e a m . I n
t h i s s e c t i o n a n u m b e r o f d i f f er e n t t y p e s a r e d e s c r i b e d a n d a p p l i c a t i o n s l is t ed . I t i s d i ff i c u lt
t o g e n e r a l i s e o n t h e r a t e o f r e t u r n f o l l o w i n g i n v e s t m e n t i n t h e s e u n i t s b e c a u s e o f t h e w i d e
v a r i e t y o f a p p l i c a t i o n s , b u t d a t a f r o m A i r F ro t ,~ q i ch , t a b u l a t e d l a t e r i n t h e s e c t i o n , g i v es
a n i d e a o f t h e p o t e n t i a l f o r th e i r p a r t i c u l a r u n i t i n s e v e r a l p r o c e s s i n d u s t r i es .
6 1 L o w t e m p e r a t u r e u n i ts
A s w i t h a n y t y p e o f h e at r e c o v e r y u n it . th e t e c h n o l o g i c a l p r o b l e m s t o b e o v e r c o m e i n
t h e d e s ig n a n d o p e r a t i o n o f t u b u l a r r e c u p e r a t o r s i n c re a s e w i th i n c r e a s i n g p r o c e s s e x h a u s t
g a s t e m p e r a t u r e s , a n d a t t e m p e r a t u r e s b e l o w a b o u t 2 5 0 ° C , s e v e r a l i n t e r e s t i n g t y p e s o f
t u b u l a r r e c u p e r a t o r e x i st w h ic h b e a r l i tt le r es e m b l a n c e t o t h e i r h i g h e r t e m p e r a t u r e
c o u n t e r p a r t s .
T h e m o s t i n t e r e s t i n g o f t h e s e is t h e g la s s t u b e r e c u p e r a t o r , a n e x a m p l e o f w h i c h ,
m a n u f a c t u r e d b y A i r F r o e h l i c h , is s h o w n i n F i g . 1 9. A g la s s t u b u l a r h e a t e x c h a n g e r l ik e
t h e g la s s p l a t e h e a t e x c h a n g e r d e s c r i b e d i n a n o t h e r s e c t i o n ) is v e r y e a s y t o c l e a n a n d h a s
e x c e l le n t r e s is t a n c e to c o r r o s i o n . I n c o m m o n w i t h al l o t h e r t u b u l a r r e c u p e r a t o r s , o n e g a s
s t r e a m f l o w s a c ro s s t h e tu b e s , w h i c h in t h is c a s e a r e n o t f i n n e d , a n d t h e o t h e r s t r e a m
p a s s e s t h r o u g h t h e t u b e s , e ff e c ti n g a c r o s s - fl o w a r r a n g e m e n t . O f c o u r s e , t h e t w o g a s
s t r e am s a r e c o m p l e t e l y s e a le d f ro m o n e a n o t h e r , t h u s n o c r o s s - c o n t a m i n a t i o n c a n o c c u r .
I n s o m e t u b u l a r r e c u p e r a t o r s , a n u m b e r o f p a s s e s o n t h e t u b e s i d e a r e u s e d , a n d t h i s is
i l l u s t r a te d l a t e r in t h e c a s e o f a h i g h t e m p e r a t u r e u n i t . I t i s a l s o p o s s i b l e , a l t h o u g h n o t
c o m m o n , t o p u t t h e d ir t y g a s s t re a m t h r o u g h t h e i ns i de s o f t h e t u b e s , w h i c h m a y , w i t h
s o m e t y p e s o f d e p o s i t s , b e e a s i e r t o c l e a n t h a n t h e o u t s i d e o f t h e t u b e s .
Fig. 19. A glass tube recuperator.
7/23/2019 A REVIEW OF GAS-GAS HEAT RECOVERY SYSTEMS
A r e v ie w o f g a s g a s h e a t r e c o v e r y s y s t e m s
o u o i
5 H e a t o u t
F i g . 2 7 . T h e h e a t p i p e .
29
c onde nsa t e t o t he he a t i npu t s e c t i on , t he he a t p i pe i s pa r t i c u l a r l y se ns i t i ve t o t he e f fe c t s
o f g r a v i t y , a n d h e n c e i ts i n c l i n a t i o n t o t h e h o r i z o n t a l . D e p e n d i n g o f c o u r s e o n t h e t y p e o f
w i c k u s e d a n d i t s p o r e s i z e , a h e a t p i p e o p e r a t i n g w i t h t h e e v a p o r a t o r b e l o w t h e c o n -
d e n s e r m a y b e c a p a b l e o f t r a n s p o r t i n g s e v e r a l t i m e s a s m u c h a s o n e h a v i n g t h e e v a p o r -
a t o r a b o v e t h e c o n d e n s e r . T h i s i s p a r t i c u l a r l y i m p o r t a n t i n l o n g h e a t p i p e s ( m o r e t h a n
500 mm i n l e ng t h) , a nd i t s i mpl i c a t i ons wi t h re spe c t t o he a t p i pe e xc ha nge rs a re d i sc usse d
be l ow.
I n c a s es w h e r e g r a v i t y a i d s r e t u r n o f t h e c o n d e n s a t e t o t h e e v a p o r a t o r s e c ti o n , i t is
p o s s i b l e t o o m i t t h e w i ck , e i t h e r i n w h o l e o r i n p a r t . T h e d e v i c e t h e n b e c o m e s a s i m p l e
t h e r m o s y p h o n . I t s h o u l d b e n o t e d h e r e t h a n s o m e m a n u f a c t u r e r s u se t h e t e r m ' t h e r m o s y -
p h o n ' to d e s c r i b e t h e b a s i s o f th e i r h e a t r e c o v e r y e q u i p m e n t , w h i le o t h e r s u s e ' h e a t p i pe
o r ' g r a v i t y - a s s is t e d h e a t p ip e '. ( I t i s b e c o m i n g i n c r e a s in g l y c o m m o n t o u s e s o m e f o r m o f
i n t e r n a l h e a t f l u x - e n h a n c i n g t o i m p r o v e i n t e r n a l h e a t tr a n s f e r c o e ff ic i en t s . L o n g i t u d i n a l
a n d c i r c u m f e r e n t i a l g r o o v e s , m e s h e s , a n d r o u g h e n e d s u r f a c es a r e d i sc u s s e d i n m o r e d e t a il
i n Dr . Gro l l ' s pa pe r i n t h i s i s sue . )
9.2. eneral description of the hea t exchanger
I n a g a s - g a s h e a t p i p e h e a t e x c h a n g e r t h e e v a p o r a t o r s e c t io n s o f t h e h e a t p i p e s s p a n
t h e d u c t c a r r y i n g t h e h o t e x h a u s t g a s , a n d t h e c o n d e n s e r s a r e l o c a t e d i n t h e d u c t t h r o u g h
whi c h t he a i r r e qu i r i ng p re -he a t i ng i s pa s s i ng , a s shown i n F i g . 28 .
S p l i t t e r p l a t e
Recoven
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F i g . 2 8 . L a y o u t o f h e a t p i p e h e a t e x c h a n g e r .
7/23/2019 A REVIEW OF GAS-GAS HEAT RECOVERY SYSTEMS
A s m e n t i o n e d a b o v e , h e a t p i p e s a r e i n f l u e n c e d i n t h e i r p e r f o r m a n c e b ~ t h e a n g l e o f
o p e r a t i o n . I n t h e h e a t p i p e h e a t e x c h a n g e r , t h e t u b e b u n d l e m a y b e h o r i z o n t a l , o r t i l t e d
w i t h t h e e v a p o r a t o r s e c t i o n s b e l o w t h e c o n d e n s e r s . B e c a u s e o f t h i s s e n s i t i v i t y , t h e a n g l e
o f t h e h e a t p i p e s m a y b e a d j u s t e d
n s tu
a s a m e a n s o f c o n t r o l l i n g t h e h e a t t r a n s p o r t .
T h i s i s a u s e f u l f e a t u r e i n a i r c o n d i t i o n i n g a p p l i c a t i o n s , a n d a n u m b e r o f p r o p r i e t a r y
u n i t s i n c o r p o r a t e t i l t c o n t r o l m e c h a n i s m s w h i c h , e i t h e r m a n u a l l y o r a u t o m a t i c a l l y , c a n
b e a d j u s t e d t o c a t e r f o r c h a n g e s i n h e a t t r a n s p o r t r e q u i r e m e n t s [ 1 6 ] .
T h e b a s i c f i n n e d h e a t p i p e h e a t e x c h a n g e r e x t e r n a l l y r e s e m b l e s a n a i r - c o o l e d c o n -
d e n s e r c o i l, c o m p l e t e w i t h f l a n g e d c a s in g a n d c o v e r s p r o t e c t i n g t h e e n d s o f th e t u b e s . T h e
m a i n e x t e rn a l d i f f e re nc e i s t he i nc orp or a t i o n o f a sp l i t te r p l a t e , v i si b le i n F i g . 28 , whi c h i s
use d pa r t l y t o supp or t t he h e a t p i pe s, whi c h c a n be se ve ra l me t re s i n le ng t h , bu t p r i ma r -
i ly t o p r e v e n t c r o s s - f l o w b e t w e e n t h e t w o a i r - s tr e a m s , e f fe c ti v e ly s e a l in g t h e m f r o m o n e
a n o t h e r . I n c o m m o n w i t h o t h e r a i r - c o o l e d h e a t e x c h a n g e r s , t h e f i n n i n g m a ~ b e a p p l i e d
i n d i v i d u a l l y to e a c h t u b e , u s i n g i n t eg r a l o r h e l i c a ll y w o u n d f i n s, o r m a y b e i n t h e f o r m o f
p l a t e s i n t o w h i c h t h e t u b e s a r e e x p a n d e d . I n t h e l a t t e r c a s e t h e c o n t a c t b e t w e e n t h e t u b e
a n d f i n r e s u l t s i n a h i g h e r t h e r m a l r e s i s t a n c e b e t w e e n t h e m , b u t t h e c o s t o f s u c h t u b e
b u n d l e s is g e n e r a l l y l o w e r . T u b e s a r e n o r m a l l y s t ag g e r e d , a n d t h e n u m b e r o f t u b e r o w s i n
t h e d i r e c t io n o f fl o w is t y p i c a l l y b e tw e e n f o u r a n d t en . { Tw o o t h e r c o n f i g u r a t i o n s w i ll b e
de sc r i be d l a t e r . )
M a t e r i a l s e l e ct i o n t b r th e h e a t p i p e s d e p e n d s u p o n t h e w o r k i n g f l u id c o n t a i n e d w i t h i n
t h e m , a s w e l l a s t h e e x t e r n a l e n v i r o n m e n t . W o r k i n g f l u i d s u s e d i n h e a t p i p e h e a t
e x c h a n g e rs r a n g e f r o m f l u o r o c a r b o n s a n d w a t e r t o h i g h t e m p e r a t u r e o r g a n i c f l u id s a n d ,
f o r s p e c ia l a p p l i c a t i o n s , li q u i d m e t a l s s u c h a s m e r c u r y a n d s o d i u m . D e p e n d i n g u p o n t h e
f l u id u s ed , a l u m i n i u m , c o p p e r a n d s t a in l e s s st ee l a r e s u i ta b l e c o n t a i n e r m a t e r i a ls , a n d
i d e n t i c a l m a t e r i a l s ar e u s e d f o r t h e f in s a l t h o u g h i n s o m e u n i t s a l u m i n i u m f in s m a y b e
a p p l i e d o n c o p p e r h e a t p ip es ). E n v i r o n m e n t a l f a c t o r s a ff e c ti n g t h e s e l ec t io n o f t h e t u b e
a n d f in m a t e r ia l a r e c o m m o n t o a ll o th e r t y p e s o f h ea t e x c h a n g e r - - t e m p e r a t u r e , c o r-
ros i on , e ros i on , e tc . S i mi l a r l y , fi n p i t c h i ng a nd t he sha pe o f t he f i n m a y be d i c t a t e d by
p r e s s u r e d r o p o r f o u l i n g c o n s i d e r a t i o n s .
W o r k r e p o r t e d i n J a p a n [- 17 ] d i r e c t e d a t a p p l y i n g h e a t p i p e s i n a g g r e s si v e e n v i r o n -
m e n t s s u c h a s a r e f o u n d i n s t e e l - w o r k s , h a s b r o u g h t t o g e t h e r t h e l o n g i t u d i n a l g r o o v e s
m e n t i o n e d e ar l ie r , a n d p r o t e c t i o n i n c o r r o s i v e e n v i r o n m e n t s . T h e m a i n f e a t u r e s o f t h is
d e v e l o p m e n t w o r k a r e a s f o l l o w s :
i) P r o d u c t i o n o f l o n g i t u d i n a l l y - f i n n e d h e a t p i p es i n t e rn a l s t r u c t u r e t u s i n g a h y d r o s t a t i c
e x t r u s io n p r o c es s w h i c h p r o m i s e s e c o n o m i c m a s s p r o d u c t i o n o f lo n g h e a t p i p e s h a v -
i n g s u p e r i o r h e a t t r a n s f e r c a p a b i li t ie s .
i i )
D e v e l o p m e n t o f e n v e l o p e m a t e r i a ls w h i c h c a n b e u s e d a s e x t e r n a l c l a d d i n g o n w a t e r -
f i ll e d c op pe r he a t p i pe s .
A s w i ll b e e m p h a s i s e d e l se w h e r e , w a t e r i s a g o o d w o r k i n g f l ui d , h a v i n g a ll t h e d e s i r a b l e
p r o p e r t i e s , i n c l u d i n g a h i g h l a t e n t h e a t . H o w e v e r . i t i s n o t c o m p a t i b l e w i t h m i l d o r
s t a i n l e s s s t e e l , a s n o n - c o n d e n s i b l e g a s e s c a n b e g e n e r a t e d w i t h i n t h e h e a t p i p e , a n d
t h e r e f o r e m u s t b e u s e d i n c o n j u n c t i o n w i t h a c o p p e r c o n t a i n m e n t v e s s e l . C o p p e r h o w e v e r
b e c o m e s r a p i d l y a n n e a l e d a b o v e 2 0 0 C , a n d t h e r e f o r e c a n n o t w i t h s t a n d h i g h i n t e r n a l
pre s sure s . I t a l so ha s a l ow re s i s t a nc e t o a t t a c k i n a c or ros i ve a t mosphe re .
B y c la d d i n g a c o p p e r t u b e w i t h a s t a i n l e ss s t ee l t u b e , f o r e x a m p l e , w i t h f i n s a p p l i e d t o
t h e s t e e l o u t e r s u r f a c e , a h e a t p i p e o r t h e r m o s y p h o n m a y b e c o n s t r u c t e d w h i c h r e t a i n s
t h e b e n e f it s o f w a t e r a n d c o p p e r / w a t e r c o m p a t i b i l i t y , w h i l e o f f e r in g t h e h i g h s t r e n g t h a n d
c o r r o s i o n - r e s i s t a n c e o f s t a in l e s s s te e l. A g a i n t h e p r o g r a m m e i n J a p a n i s d i r e c t e d a t u s i n g
a h y d r o s t a t i c e x t r u s i o n p r o c e s s f o r m a n u f a c t u r i n g t h e c l a d t u b e .
U n i t s iz e v ar ie s w i t h t h e a i r f lo w , a v e l o c i t y o f a b o u t 2 - 4 m / s b e i n g g e n e r a l l y a c c e p t e d
t o k e e p t h e p r e s s u r e d r o p t h r o u g h t h e t u b e b u n d l e t o a r e a s o n a b l e l e v e l . S m a l l u n i t s
ha v i ng a f a c e si z e o f 0 .6 m { l e ng th) x 0 .3 m {he i gh t) a re a v a i l a b l e , a n d t he l a rge s t s i ng l e
u n i t s e m p l o y h e a t p i p e s h a v i n g l e n g t h s i n e x c es s o f 5 m . a s u s e d i n l a r g e b o i l e r a i r
p r e h e a t e r s . A w i d e r f in p i t c h m a y b e u s e d o n t h e e v a p o r a t o r s e c ti o n s , w h e r e c o n t a m i -
7/23/2019 A REVIEW OF GAS-GAS HEAT RECOVERY SYSTEMS
Fig. 30. Fors ter Wheeler heat pipe a i r preheater .
B a r r a tt a n d H e n d e r s o n [2 0- 1 s t a te t h a t t h e r e q u i r e d p e r f o r m a n c e c o u l d b e o b t a i n e d
w h e n t h e h e a t p i p e s w e r e t il te d a t a n a n g l e o f 1 0 : t o t h e h o r i z o n t a l { e v a p o r a t o r b e l o w
co nden s er ) . W el d ed f in s t ee l tub i ng w a s u s ed fo r the hea t p i pes , a nd i t i s no t i cea b l e f ro m
F i g . 3 0 th a t t h e c o n d e n s e r s e c t i o n s a r e c o n s i d e r a b l y s h o r t e r t h a n t h e e v a p o r a t o r s e c t io n s .
T h i s i s d o n e t o m i n i m i s e t h e p r e s su r e d r o p o n t h e f l u e g a s s id e , a n d a l s o r e s u l ts f r o m t h e
fa c t tha t the a ir - s ide co nde ns er ) s ec t i o ns a re m o re e f f ec t i ve per un i t v o l u m e bec a us e they
a re a b l e to a cco m m o da te a h i g her f i n dens i ty , a s fo u l i ng i s un l i ke l y to o ccur .
The fue l f o r the f i red hea ter i s na tura l g a s , but pro v i s i o n i s m a de i n the i ns ta l l a t i o n fo r
s o o t b l o w e r s s h o u l d t h e f u e l b e c h a n g e d t o o i l a t s o m e f u t u r e t i m e .
R e f e r en c e h a s a l r e ad y b e e n m a d e t o w o r k i n J a p a n a t th e N a t i o n a l C h e m i c a l L a b o r a -
t o r y f o r I n d u s t r y [ 1 7 ] o n h e a t p i p e m a n u f a c t u r i n g t e c h n i q u e s . T w o n e w h e a t p i p e h e a t
e x c h a n g e r c o n c e p t s a r e b e i n g st u d i e d a t th i s l a b o r a t o r y f o r a p p l i c a t i o n s w h e r e a ) o p e r -
a t i o n i n h ig h l y f o u l e d e n v i r o n m e n t s a n d b ) h i g h e r p e r f o r m a n c e i s r eq u i re d .
W i t h r e g a r d t o o p e r a t i o n i n f o u l e d e n v i r o n m e n t s , a s w o u l d p r e d o m i n a t e i n a l m o s t a l l
e q u i p m e n t e m p l o y e d i n s te e l - m a k i n g p l a n t , a u n i t h a s b e e n d e s i g n e d w h i c h , i t i s c l a im e d .
c o u l d b e o p e r a t e d f re e fr o m p r o b l e m s o f d u s t a d h e s i o n . I l lu s t r a te d in F i g . 3 1 . t h e e v a p o r -
Table 3 . Boi l er a i r preheater data
Heat t ransported
F l u e g a s m a s s f l o w
Inlet a i r mass f low
F l u e g a s i n l e t t e m p .
F l u e g a s o u t l e t t e m p .
Air in l et t emp.
Air out l et t emp.
W o r k i n g f l u i d t em p .
M a x i m u m h e a t t r an s fe r p e r h e a t p i p e
N u m b e r o f h e a t p i p e s
H e a t p i p e l e n g t h
H e a t p i p e d i a m e t e r
F i n p i t c h
Pres sure drop on f lue gas s ide
Pres sure drop on air s ide
938 kW
36 700 kg/h
34500 kg/h
260°C
177:C
27°C
121°C
108 198°C
6.74 kW
144
4.57 m
5.1 c m
2.4/cm on air s ide
1.2/ cm on f lue gas s ide
57 Pa
249 Pa
7/23/2019 A REVIEW OF GAS-GAS HEAT RECOVERY SYSTEMS
5 0 . - \ \ \ . Ca rb on s te e l ( in du s t r ia l )
I % \ . . / . . . . . . .
30 J- - ~ x - /~ . , -- -Coppe r ( in du s t r ial )
J ~ ~ _ . ~ A l u m i n i u r n ( H V A C)
0 2 4 6 8 I0 1 2
3
H eat transfer surface area, m 2 x l 0 2
F i g . 3 4. C o s t s o f h e a t p i p e h e a t e x c h a n g e r s a s a f u n c t i o n o f s i z e a n d c o n s t r u c t i o n m a t e r i a l s .
R eco v e r i n g ap p r o x i m a t e l y 1 .5 M W , t h e u n i t w o u l d s av e f u el v a l u e s a t S 4 5 0 0 0 p e r
annu m, a t a cos t per ann um of $8000 (amo r t i zed over 10 year s a t 10% in teres t ). Inc lud ed
i n t h e co s t s w e r e a l l o w an ces f or fan p o w er an d m a i n t en an ce , t o t a ll i n g $ I 7 0 0 p e r an n u m .
T h e lif e o f t h e u n i t w as e s t im a t ed t o b e 1 5 y ea r s. C o s t i n g w as b a s ed o n o p e r a t i o n f o r
24 hou rs per day , 355 day s per year , and the e f f ec t iveness of the he a t exch anger was 60%.
10. G A S G A S H E A T P U M P S
T h e h ea t p u m p can i n m a n y i n s t an ces b e r eg a r d ed a s a h ea t r e co v e r y d ev i ce , an d t h is
i s par t i cu lar ly t rue when i t i s used as an a id to increased ef f i c i ency in dryer s . Whi le
i n d u s tr i a l ap p l i c a t i o n s o f h ea t p u m p s h av e b een i n ex i s ten ce si n ce th e I 9 4 0 's , t h e t o t a l
n u m b er o f i n s ta l l a ti o n s r em a i n s co m p ar a t i v e l y f ew . H o w ev e r , t h ei r ap p l i c a t i o n i n d r y e r
technolog y , which has be en dem ons t ra ted , a lbe i t on a smal l s ca le , to of fe r subs tan t i a l
p e r cen t ag e r ed u c t i o n s in d r y e r en e r g y co n s u m p t i o n , i s li k e ly t o b e o n e o f th e m a j o r
g r o w t h a r ea s o v e r t h e n ex t d ecad e .
H e a t p u m p s a r e m o r e w i d el y k n o w n t h r o u g h t he ir d o m e s t i c a n d c o m m e r c ia l a p p l i ,
c a t io n s , f o r h ea t i n g b u i l d i n g s u s i n g f r ee h ea t i n t h e a t m o s p h e r e o r t h e g r o u n d , f o r
red i s t r ibu t ing w as te hea t ava i l ab le w i th in bu i ld ings , and for us ing was te hea t f rom ref ri ,
g e r a t i o n s y s tem s in l a rg e o l d s t o r e s o r i c e r in k s . T h e h ea t p u m p w o u l d n o t b e i m m e d i -
a t e l y i n c l u d ed b y ev e r y o n e in a r ev i ew o f g a s - g as h ea t r e co v e r y d ev i ce s, b u t i ts i m p o r t -
ance jus t i f ies a s ec t ion here .
10.1.
peratin cycle
T h e r e a r e s ev e r al h e a t p u m p o p e r a t i n g c y cl es , t h e t w o m o s t c o m m o n b e i n g th e v a p o u r
co m p r es s i o n an d ab s o r p t i o n cy c le s . I n d r y i n g p r o ces s e s , i n v o l v in g g a s - g as h ea t t ran s fe r ,
t h e v ap o u r co m p r es s i o n cy c l e h a s p r ed o m i n a t ed t o d a t e , an d i t i s t h is o n w h i ch I w il l
co n cen t r a t e h e r e .
Th e hea t pum p cy c le i s i l lus tr a ted on a p res sure- -en tha lpy d iagram in F ig . 35 , and the
p r i n c i p a l co m p o n en t s a r e s h o w n i n t h e c i r cu i t d i ag r am i n F i g . 3 6 . T h e l o w g r ad e h ea t
i n p u t o ccu r s a t t h e ev ap o r a t o r , t h i s c au s i n g t h e h ea t p u m p w o r k i n g f l u i d ( t y p i ca l l y a
7/23/2019 A REVIEW OF GAS-GAS HEAT RECOVERY SYSTEMS