Air Conditioning Compressors - Delphi

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e q u i r e d c o o l i n g i s t h u s a ch i eve d b ut at the ex p e n s e o f t h e wo r k n e cessar y t o drive the c o m p r e s s o r. heat rejected to ambient expansion device T h e A u t o m o t i v e 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 Th e a u t o m o t i ve a p p l i c ation places ver y s p e c i a l d e m a n d s o n t h e air c o n ditioning sys- t e m . A t y p i c a l ve h i c l e s ys t em has a similar c o o l i n g c a p a c i t y t o t h a t r e q uired for the air c o n d i t i o n i n g o f a s m a l l h o use d e spite the va s t d i ffe r e n c e i n vo l u m e s t o b e c ooled. The r e a s o n s fo r t h i s a r e t wo fold. Firstly cooling d u t y p e r u n i t vo l u m e i s m u ch higher for the ve h i c l e b e c a u s e h e a t t ransfe r c o efficients b e t we e n h o t a m b i e n t a ir a n d t h e outside s u r fa c e s a r e m u ch h i g h e r d u e t o movement o f t h e ve h i c l e t h r o u g h t h e a i r. Secondly t h e p r o p ortion o f t h e e nclosure consisting o f g l a s s i s ve r y h i g h fo r t h e vehicle – a fa c t o r t h a t m a ke s t h e e ffe c t o f direct solar r a d i a t i o n h e a t i n g ve r y h i g h. O n t op of this a p a r t i c u l a r l y d e m a n d i n g r e q u i r ement is to c o o l t h e cabin ve r y r a p i d l y a ft e r t he vehicle h a s b e e n s o a ke d i n a n a m bient t e mperature o f 4 0 ° C or h i g h e r. A t t h e s t a r t o f the cool d ow n t e mperatu r e s i n t he c a bin c an be as h i g h a s 6 0 o r 7 0 ° C . condenser evaporator S ch e m a t i c o f a i r c o n d i t i o n i n g c yc l e compressor heat absorbed from cabin A n o t h e r s i g n i f i c a n t way i n w h ich a utomotive a i r c o n d i t i o n i n g d i ffe r s from t h e domestic o r c o m m e r c i a l ve r s i o n is t h e q u estion of c o m p r e s s o r d r i ve . I n t he ve h icle the com- p r e s s o r i s b e l t d r i ve n by t h e e n g i ne so that i n d e p e n d e n t c o n t r o l over t h e c ompressor s p e e d i s n o t p o s s i b l e . This o bv i ously has s i g n i f i c a n t i m p l i c a t i o n s fo r sys t e m control a n d m e a n s t h a t t h e r e c an b e calls for high s ys t e m perfo r m a n c e a t t imes when the c o m p r e s s o r s p e e d i s ve r y l ow. A n import ant implication of this is that the compressor drive shaft must pass out of the compressor casing with the result ant potential for refrigerant leakage. Hermetic compressors are not possible and ver y effective shaft seals must be used. A second implication of the external drive is that the compressor must be engine mounted so that lengths of flexible hose must be introduced to accom- modate relative movement bet ween engine and chassis mounted components. As with other air conditioning applications system efficiency is ver y import ant and the automotive application is no exception. While the characteristics of all the major components in the system contribute to overall efficiency any compressor inefficiencies must be kept to a minimum. A further feature of the compressor that requires serious con- sideration is its intrinsic noise generation - modern vehicle noise requirements mean that the compressor must be ver y quiet and vibration free. And finally, three further requirements of the compressor, specific to the automotive application, but which apply equally to ever y vehicle component – are stringent constraints on size, weight and cost. Th e ch a l l e n g e o f c o m p r e s s o r p a ck a g i n g As a result of all these constraints deriving from the use of an engine driven compressor, it is often suggested that an electrically driven system would provide a much better solution. The reason that it is not feasible on current vehicles is insufficient electrical power is available and its application must await the widespread implement ation of high powered and efficient integrated st arter/ generators. The applications in which an electrical compressor would make real sense however are in hybrid and fuel cell vehicles where sufficient electrical power is readily available.
Th e h i g h e ff i c i e n c y o f m o d e r n e n g ines can l e a d t o t h e s i tu a t i o n i n which i n s u ff icient re- j e c t e d h e a t i s ava i l a b l e fo r comfo r t heating . Th i s fa c t h a s p r o m p t e d t h e s u g g estion that t h e a i r c onditioning s ys tem b e o p erated in r eve r s e a s a h e a t p u m p t o s u p p l e ment the e n g i n e h e a t a n d t h e v i a b i lit y o f the concept h a s b e e n d e m o n s t r a t e d o n a n u m b er of plat- fo r m s . Th e i m p l i c a t i o n s fo r t h e c o mpressor a r e t h a t t h e r e q u i r e d o perating life can be n e a r l y d o u b l e d a n d t h e c ompressor ambient s a fe o p e r a t i n g t e m p e r a tu r e r a n g e must be ex t e n d e d d ow nwa r d s t o minus 20°C. D e l p h i ve h i c l e w i t h H e a t Pu m p s u p p l e m e n t a r y h e a t i n g R e f r i g e r a n t I s s u e s Th e a u t o m o t i ve a i r c o n ditioning industr y r e s p o n d e d p r o - a c t i ve l y t o t h e d emands of t h e 19 8 7 M o n t r e a l Pr o tocol a n d by 1994, 2 ye a r s e a r l y, t h e ch a n g e ove r f r o m R12 to R 13 4 a wa s c o m p l e t e fo r a ll ve h icle produc- t i o n l i n e s i n t h e d eve l oped wo rld. It soon b e c a m e a p p a r e n t t h a t t h is m ay n ot be the e n d o f t h e s t o r y a n d w i th i ncreasing pene- t r a t i o n o f a i r c o n d i t i o n i n g i n t h e European m a r ke t t ogether w i t h t he d e mands of the Kyo t o Pr o t o c o l p r e s s u r e has b u ilt to make ch a n g e s t o s t i l l f u r t h e r r e d uce the potential c o n t r i b u t i o n o f a i r c o nditioning systems t o g l o b a l wa r m i n g . A n ew D i r ective has r e c e n t l y c o m p l e t e d i t s p r o g r ess t h r ough the E u r o p e a n l e g i s l a t i ve p r o cess. I t r e q uires the e l i m i n a t i o n o f R 13 4 a f r om a ll n ew models f r o m 2 011 a n d f r o m a l l n ew ve h icles by 2017. Th e c u rrently c o n s i d e r ed alternatives are R 15 2 a w i th a g l o b a l wa rming p o t e ntial less t h a n 10 % o f t h a t o f R 13 4 a, o r t h e so-called ‘ n a tu r a l ’ r e f r i g e r a n t R 744 ( carbon dioxide). R 15 2 a h a s t h e r m o d y n amic characteristics ve r y s i m i l a r t o t h o s e o f R 13 4a a nd will be d i r e c t l y u s a b l e w i t h current c ompressor t e ch n o l o g y. I t s p o t e n t i a l d r aw back is a d e g r e e o f f l a m m a b i l i t y that h as made the i n d u s t r y reluct a n t t o c onsider i t. More re- c e n t l y o t h e r l a r g e ch e m i c al c o m panies have a n n o u n c e d t h e p o t e n t i a l ava ilabilit y of new a l t e r n a t i ve s . L i tt l e i s k n ow n a b o u t them at t h i s s t a g e b u t i t i s a s s u m e d t hat they too wo u l d f u n c t i o n w i t h c u rrent c ompressor t e ch n o l o g y. Th e r e m u s t b e a q u estion as to w h e t h e r t h e n e c e s s a r y d eve l o pment and testing and then implement ation of the necessar y manufacturing capacit y can be completed in time to meet the requirements of the European legislation. If R744 becomes the chosen route the implications for compressor technology are ver y far-reaching . Air conditioning systems with R744 need compressors with displacement of only a fifth of that required for R134a but operating at much higher pressures. Discharge pressures of 120 bar are normal compared with a t ypi- cal 18 bar for an R134a system. As can be imagined, these factors have big implications for the compressor layout and structural strength requirements and also represent a particularly severe shaft seal challenge. Pressure - MPa 100 10 1 100 -40°C 0°C 200 40°C 80°C 120°C 160°C 200°C 0°C 40°C 300 400 Enthalpy - kJ/kg 500 I d e a l i ze d p r e s s u r e e n t h a l py d i a g r a m o f C O 2 c yc l e C o m p r e s s o r Te c h n o l o g i e s Air conditioning began to become available for vehicles in the early 1950s. From the beginning a number of different compressor technologies have been used. The earliest systems used piston compressors in various configurations. The Frigidaire F5, a five piston wobble plate configuration, used in the earliest GM vehicles with air conditioning was first produced in 1956 – it weighed in at 18kg! The R4 compressor with a scotch yoke mechanism was first produced by Harrison (now Delphi) in 1974 and remained in pro- duction until 1995. It was 190mm in overall diameter and weighed 9kg . A n e a r l y c o m p r e s s o r, t h e D e l p h i R 4 80°C 120°C 600 160°C 200°C 700
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Air Conditioning Compressors - Delphi

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