HeatTransferEngineering, 30(9):720–735, 2009
`Copyright C(cid:1)(cid:1) Taylor and Francis Group, LLC
`ISSN: 0145-7632 print / 1521-0537 online
`DOI: 10.1080/01457630802678193
`
`Automotive Air-Conditioning
`Systems—Historical Developments,
`the State of Technology, and Future
`Trends
`
`RAMESH K. SHAH
`Energy Science and Engineering Department, Indian Institute of Technology Bombay, India
`
`Automotive air-conditioning (A/C) or mobile air-conditioning (MAC) systems have played an important role in human
`comfort and to some extent in human safety during vehicle driving in varied atmospheric conditions. It has become an
`essential part of the vehicles of all categories worldwide. After discussing the basic operation of the A/C system, a brief
`summary is provided on historical development of the vehicular A/C system, with refrigerant history from the inception of
`the A/C system to future systems: R12, R134a, and enhanced R134a A/C system, and next-generation refrigerants having no
`ozone depletion potential in the stratosphere and global warming potential less than 150. The discussion also includes an
`enhanced MAC system with R134a, and the direct and indirect emissions from vehicles impacting global warming due to
`the use of the A/C system. This would explain why we continue to change the refrigerants in the automotive A/C system in
`spite of billions of dollars of cost for the previous refrigerant change (from R12 to R134a). The system design considerations
`are then outlined for minimizing the impact of A/C operation on the vehicle fuel consumption. Finally, new concepts of
`design of A/C system and vehicle heat load reduction ideas are discussed to further minimize the impact of A/C system
`operation on the environment without impacting human comfort. It is anticipated that this article will provide the overall and
`detailed prospective of the A/C system developments and provide an opportunity to the researchers to accelerate research
`and development for the refrigerant changeover and A/C system and component optimization and cost reduction.
`
`INTRODUCTION
`
`According to the American Society of Heating, Refriger-
`ating and Air-Conditioning Engineers (ASHRAE), air condi-
`tioning (A/C) is the science of controlling the temperature, hu-
`midity, motion, and cleanliness of air within an enclosure. In
`a passenger/driver cabin of a vehicle, air conditioning means a
`controlled and comfortable environment in the passenger cabin
`
`This article is an updated and revised version of the keynote paper presented
`by R. K. Shah entitled “Automotive Air-Conditioning Systems—Historical De-
`velopments, the State of Technology and Future Trends” at the Third BSME-
`ASME Thermal Engineering Conference, Dhaka, Bangladesh, 2006.
`The author is thankful to G. D. Mathur of Calsonic Kansei, Robert Cum-
`mings, consultant, Dallas, TX, Dr. Tim Cowell of VTS Consult Ltd., and W.R.
`Hill of General Motors for critically reviewing the article and making specific
`suggestions.
`Address correspondence to Dr. Ramesh Shah, Energy Science and Engi-
`neering Department, Indian Institute of Technology Bombay, Powai, Mumbai
`400076, India. E-mail: rkshah@gmail.com
`
`during summer, winter, and rainy seasons, i.e., control of tem-
`perature (for cooling or heating), control of humidity (decrease
`in passenger cabin humidity), control of air circulation and ven-
`tilation (amount of air flow and fresh air intake vs. partial or
`full recirculation), defrost or defogging of the windshield, and
`cleaning of air from odor, pollutants, dust, pollen, etc. before
`entering the passenger cabin.
`While the A/C system provides comfort to the passengers in
`a vehicle, its operation in a vehicle has twofold impact on fuel
`consumption and subsequently on indirect tailpipe CO2 (indirect
`greenhouse gas [GHG] emissions): (1) burning extra fuel to
`power the compressor for the A/C operation, and (2) carrying
`extra A/C component weight in the vehicle all the time, whether
`the A/C is on or off. In addition, direct refrigerant emissions
`(system leakage, vehicle accidents, and losses at service and
`scrap) impact the GHG emissions. Also, the total operation time
`of the A/C system (to impact item 1) depends on the climatic
`condition of the concerned geographical region and the time of
`the year. The most important impact on the fuel consumption is
`
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`
`R. K. SHAH
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`721
`
`when the A/C is running. Clodic [1] reported the additional fuel
`consumption due to mobile air-conditioning (MAC) operation as
`2.5–7.5% (in the United States/Europe), considering the climatic
`conditions, engine type (diesel or gasoline), and user profile.
`Corresponding CO2 emission due to MAC operation is between
`150 and about 500 kg annual CO2 equivalent per year per vehicle
`in developed countries. The impact on the fuel consumption is
`somewhat more significant (maybe 10% or more) when the A/C
`is installed in compact and subcompact vehicles, as is the case
`in many Asian and other developing countries where mainly
`compact and subcompact cars are sold.
`In this article, the components and operation of the current
`A/C systems with some details on the components as back-
`ground information are first described. A brief history of the
`refrigerants and A/C system is then presented, followed by the
`developments of the major components of the A/C system. The
`SAE-initiated study is then summarized for an enhanced A/C
`system with R134a to reduce R134a emissions by 30% in the
`near future. Finally, the potential alternative refrigerant(s) se-
`lection and some historical attempts for alternate refrigerants
`(CO2, R152a, and HC blends) are briefly summarized for re-
`duction in global warming. While the auto A/C has become
`very sophisticated, the newer A/C systems are becoming more
`energy-efficient for desired high performance; the cost is contin-
`ually reducing due to global competition with the same or better
`durability and reliability. Finally, some discussion is provided
`on ongoing efforts on A/C system heat load reduction and new
`developments in auto A/C systems.
`
`BASIC OPERATION OF CURRENT AUTOMOTIVE
`A/C SYSTEMS
`
`Two major types of A/C systems are used in the vehicles:
`TXV-RD and OT-AD. The components of a typical modern
`TXV-RD and OT-AD systems are shown in Figure 1a and b,
`respectively. In the TXV-RD system, the refrigerant flow rate is
`controlled by the thermostatic expansion valve (TXV or TEV)
`by monitoring refrigerant superheat at the evaporator outlet. The
`receiver-dryer (RD) is placed ahead of the TXV for separation
`of liquid and vapor refrigerant and storing access refrigerant
`required during cool-down and sudden increase in heat load. In
`the OT-AD system, similar functions are achieved by a fixed-
`diameter orifice tube (OT) placed ahead of the evaporator and
`an accumulator-dryer (AD) placed after the evaporator. Further
`details of components are provided later in this section. Moisture
`ingression in the refrigerant loop in the A/C system occurs
`through the porosity of hoses, less than perfect refrigerant line
`joints, etc. If this moisture is not removed, it can internally
`corrode the evaporator, TXV, and OT, and clog the “orifice” of
`the TXV. Hence, a desiccant bag is placed in the RD and AD
`bottles, whichever is used in the system selected.
`The basic operation of this system is now described start-
`ing with the compressor. The primary function of the com-
`pressor is twofold: (1) Compress and pressurize relatively cool
`gaseous refrigerant from the evaporator outlet (suction line)
`heat transfer engineering
`
`Figure 1 Major types of automotive air-conditioning A/C systems used in
`vehicles: (a) TXV-RD, (b) OT-AD.
`
`with minimum compressor power, and (2) deliver maximum
`amount of high-pressure high-temperature gaseous refrigerant
`to the condenser. These two objectives are measured/quantified
`by isentropic and volumetric efficiencies of the compressor, re-
`spectively. The compressor is powered by a drive belt from the
`engine, and its rotational speed (revolutions per minute, rpm)
`is generally higher than the engine rpm, and is decided by the
`chosen pulley ratio. The compressor has an electrically oper-
`ated engagement clutch to turn the A/C system off or on. Next
`is the condenser in the refrigerant flow path; see Figure 1. The
`condenser is (and should be) located in front of the radiator. In
`automotive A/C systems, the condenser is typically a cross-flow
`heat exchanger in which air flows through the corrugated or flat
`louvered fins and the refrigerant flows through the flat multiport
`or round tubes (with or without microfins) in multiple passes in
`the direction perpendicular to the airflow. The condenser cools
`the high-pressure hot refrigerant gas coming from the compres-
`sor and converts it to liquid with generally a small pressure
`drop through the use of ambient air (relatively cool compared
`to the hot refrigerant) blown by the condenser/radiator fan. The
`exiting liquid refrigerant (subcooled in many cases) from the
`condenser is sent via a small tube (liquid line) to the RD (ap-
`plies only to an expansion valve system). The RD is a metal can
`with a desiccant bag inside and allows only liquid refrigerant
`to go out. It is usually located near the condenser outlet pipe.
`Nowadays the RD bottle is an integral part of the condenser in
`modern high performance A/C systems, and such a condenser
`is referred to as an integral receiver-dryer condenser (IRDC).
`In this case, refrigerant passes through the RD before leaving
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`722
`
`R. K. SHAH
`
`the condenser through the condenser last pass as subcooled
`liquid. The objective is to improve the degree of subcooling of
`the refrigerant at the condenser outlet by allowing only already
`condensed liquid refrigerant. There is a negligible pressure and
`temperature change in the refrigerant through the RD, except
`that the moisture is removed by the desiccant.
`Continuing the A/C system operation, as the high-pressure
`warm liquid exits the condenser and RD (Figure 1a) or condenser
`(Figure 1b), it passes through an expansion device (TXV or OT)
`which modulates the proper amount of the refrigerant flow rate
`going through the complete A/C system. Effectively, the TXV
`has a variable diameter orifice tube, and the OT has a fixed diam-
`eter orifice tube. Thus TXV allows more refrigerant flow rate at
`idle compared to that for the OT thus providing higher cooling
`at idle vehicle operating condition; ideally, both systems pro-
`vide the same maximum design cooling performance at the city
`traffic conditions (40–50 km/h vehicle speed). For a given vehi-
`cle, the TXV-RD system has slightly better cooling performance
`than the OT-AD system. This is because the OT-AD system has
`higher pressure drop in the refrigerant line (the suction line)
`between the evaporator and compressor, since the AD bottle
`introduces additional pressure drop impacting the A/C system
`performance. At higher vehicle speeds, the TXV maintains de-
`sired refrigerant superheat at the exit of the evaporator by allow-
`ing more refrigerant flow rate through the system to meet higher
`cooling requirement. The OT cannot control the refrigerant exit
`condition at the evaporator outlet, but the bubble point (where
`the refrigerant starts vaporizing) moves within the OT from the
`entrance (high evaporator cooling condition) to the exit end of
`the tube within the OT (low evaporator cooling condition) to
`provide the required refrigerant flow rate to the evaporator and
`the A/C system. The pressurized liquid passes through the ex-
`pansion device, with considerable reduction in the pressure and
`corresponding temperature.
`The cold liquid/vapor refrigerant mixture from the expansion
`device is fed to the evaporator in an HVAC module located under
`the dashboard in the passenger compartment. It cools fresh or
`recirculated warm air, which flows into the car interior with the
`help of a blower to cool the passenger cabin. As the air is cooled
`flowing through the evaporator on one fluid side, the liquid/vapor
`mixture of the refrigerant is heated on the other fluid side and
`evaporates. The evaporated refrigerant gas then returns via the
`“large” suction line (tube and hose) to the compressor “suction”
`port to begin this whole process again in the TXV-RD system. In
`the OT-AD system, an accumulator-dryer is placed between the
`evaporator and compressor. It separates and stores any liquid
`refrigerant coming out of the evaporator before going to the
`compressor since there is no superheat control at the evaporator
`exit in the OT-AD system.
`
`BRIEF HISTORY OF THE REFRIGERANT AND A/C
`SYSTEM
`
`With the invention of R12 in 1928 by GM researchers came
`the dawn of automotive air-conditioning. The first prototype
`heat transfer engineering
`
`self-contained system was installed in a 1939 Cadillac. Packard
`Motor Company (later merged with Chrysler) in 1939 was the
`first company to offer a complete auto air-conditioning system
`for cooling in summer and heating in winter using R12 re-
`frigerant. The first bus A/C prototype was developed in 1934
`by a joint venture between Houde Engineering Corporation of
`Buffalo, NY, and Carrier Engineering Corporation of Newark,
`NJ, and others followed. Initial air conditioners had a number
`of problems, and the Second World War hampered the produc-
`tion/progress. By 1953, many of the problems had been resolved
`and General Motors and Chrysler came back with improved air-
`conditioning and that luxury became a necessity for a common
`car owner! In 1953, the Harrison Radiator Division of General
`Motors came up with a revolutionary air conditioner that was
`totally spaced in the underhood (compressor and condenser) and
`dashboard (HVAC module and expansion device), eliminating
`it from being in the trunk, which was the common practice
`until then for all car manufacturers. The use of desiccant ma-
`terial to absorb moisture in the refrigerant line started in 1953.
`Detailed early history of the refrigerant, components, and devel-
`opment/penetration of A/C system in vehicles is given by Bhatti
`[2–4]. The following are the milestones of the development of
`the A/C system after 1953 (Bhatti [3]):
`• In 1955, GM developed the first A/C and heating unit that was
`front mounted, totally pre-assembled and pretested. By 1957,
`all car makers followed this design approach.
`• To provide the evaporator freeze protection, a hot gas bypass
`valve was introduced in the A/C system in 1956.
`• In 1957, air conditioning became a standard item in Cadillac
`Eldorado Broughams. The average price of all air conditioners
`sold in 1957 was $435.
`• The popularity of auto A/C soared and the number of installed
`A/C systems on the vehicle tripled from 1961 to 1964. During
`1963, Ford set the A/C unit price at $232.
`• In August 1965, GM crossed the 5 million A/C unit production
`mark. GM also introduced first the Climate Control System
`on Cadillac. Industry-wide penetration of A/C reached 70%
`by 1980.
`• Due to an oil embargo in 1973, an emphasis was placed on fuel
`economy. The Harrison Radiator Division of General Motors
`developed a cycling clutch orifice tube (CCOT) system re-
`placing the Frigidaire valve-in-receiver (VIR) system, which
`resulted in the compressor off for one-third of the time rather
`than continuously running, thus improving fuel economy. By
`1979, all GM vehicles used this CCOT system.
`• In 1974, the world came to know about ozone depletion in
`the stratosphere due to R12 use. Note that the ozone layer
`blocks ultraviolet rays that would otherwise cause skin can-
`cer for humans exposed to sun rays in sunbathing, common
`in Western countries. The Harrison Radiator Divison of GM
`analyzed nine refrigerants and by 1976 arrived at R134a as
`the replacement of R12, eliminating chlorine from the re-
`frigerant. However, there was no commercial availability of
`R134a then; Allied Chemicals, the major company conducting
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`R. K. SHAH
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`723
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`research on R134a then and located only 20 miles from Har-
`rison, would supply about 1 lb of refrigerant per week and
`the need was about 1000 lb per week for A/C system devel-
`opment work at Harrison in those days. So the development
`work continued slowly. Although the viability of R134a was
`proven by Harrison through wind tunnel tests on the 1978
`Chevrolet, and further development of A/C system was com-
`pleted with R134a, no interest was shown by the worldwide
`auto industry till the Montreal Protocol was adopted by the
`United Nations in September 1987. The first major revolution
`in the A/C system thus came starting in the 1990s with the
`replacement of R12 by R134a to eliminate the ozone deple-
`tion in the stratosphere by introducing a refrigerant having
`chlorine replaced by fluorine in its composition. The com-
`mercial production of R134a started with DuPont and ICI
`in 1990.
`• The changeover of R12 to R134a necessitated the following
`major changes in the A/C system: (1) a new condenser with
`about 20% higher condensing capacity to maintain the same
`operating system pressures so that a new compressor was
`not needed, (2) a change of the lubricant from mineral oil to
`compatible synthetic polyalkylene glycol (PAG) oil, (3) the
`expansion valve setting required a change for keeping cool-
`ing equivalent to R12 system, and (4) all rubber components
`required change from RBR to HNBR.
`• Conversion from R12 to R134a in the United States, Europe,
`and Japan took place during 1991–1994. The rest of the world
`has changed to R134a as the refrigerant for the A/C system in
`new vehicles during late 1990s and early 2000s.
`
`Global warming potential (GWP) was not an issue when
`changeover from R12 to R134a took place, although the global
`warming potential of R134a was significantly lower than R12,
`1430 versus 7800; nature’s own greenhouse gas, carbon diox-
`ide, is the basis for the GWP yardstick, having a GWP of 1.
`According to the European Commission F-gas regulation, the
`refrigerant in the A/C systems for all new vehicle models from
`2011 and all new vehicles manufactured from 2017 introduced
`in European Union (EU) must have a GWP of 150 or less. Sev-
`eral potential replacement refrigerants are considered and an
`account is provided in a later part of this article.
`
`COMPONENTS OF THE A/C SYSTEM
`
`Major components of an automotive A/C system are a com-
`pressor, a condenser, an expansion device, an evaporator, and a
`receiver-dryer or an accumulator-dryer, as described next with
`some historical development and/or current various designs in
`use. In addition, tubes and hoses are required to connect these
`components, and sensors for proper operation of the A/C sys-
`tem. With sensors becoming a very important part of the human
`comfort, they are also briefly described.
`
`Compressors
`
`The function of a compressor is twofold: to compress the
`refrigerant to the desired high pressure with minimum power
`requirement, and to deliver (circulate) the largest amount of re-
`frigerant volume to the A/C system. These two functions are
`measured in terms of isentropic efficiency and volumetric ef-
`ficiency, respectively. Major types of compressors used in the
`automotive A/C system are reciprocating compressors [fixed-
`displacement compressors (FDC), variable-displacement com-
`pressor (VDC)], and scroll and rotary compressors. These are
`shown in Figure 2. Fixed-displacement compressors were in-
`troduced with the beginning of the A/C development. In 1950s,
`the compressors weighed over 27 kg (60 lb). Today they weigh
`about 4–7 kg (9–15 lb). Along with the reduction in weight, the
`volumetric and isentropic efficiencies and durability/reliability
`have increased considerably and noise has reduced significantly.
`Reciprocating compressors have about 80% worldwide market
`share, and scroll and rotary compressors have about 20%. Gen-
`eral characteristics of compressors are summarized in Table 1.
`In a fixed-displacement compressor (FDC), rotary motion of
`the shaft-swash plate assembly is converted into the recipro-
`cating motion of the piston (fixed stroke length). Historically,
`first a single-acting piston compressor was introduced for the
`A/C application, and was further improved by modifying to a
`double-acting piston compressor for more power and better ef-
`ficiency. The refrigerant flow rate is maintained by the pressure
`differential between the suction plenum and discharge plenum.
`In this compressor, the displacement (swept volume by the pis-
`ton) does not vary with rpm. The refrigerant flow rate varies
`with the change in rpm only.
`In a variable-displacement compressor (VDC), rotational
`motion of the swash plate is converted into reciprocating motion
`of a variable stroke length depending on the A/C cooling load.
`There is only one single-acting piston. This is because there is
`a mechanism on the other side (left-hand side in Figure 2b) for
`changing the inclination of the swash plate, which results in the
`variable stroke length of the piston. The mechanism consists of
`the lug plate (and springs) (shown in Figure 2b) plus the shaft-
`swash plate subassembly. A control valve is provided to sense
`the variation in heat load and change the displacement of the
`compressor accordingly. The VDC is more efficient at all oper-
`ating conditions (from 5 to 95% compressor displacement) than
`the FDC in mild weather in spring/fall and summer, particularly
`in North America and Europe. With the change of swash plate
`angle, the compressor is operated with precise displacement at
`full stroke (≈ 60–74% volumetric efficiency). Where the am-
`bient conditions are very hot for most of the time in the year
`(e.g., in India, Mexico, etc.) and full compressor displacement
`is needed most of the time when the A/C is on, the FDC is
`primarily used due to its lower cost. The size of the VDC is
`approximately about the same as for the FDC for equivalent full
`stroke capacity, and hence the weight. VDC gives overall better
`performance in low/mild ambient conditions. Its performance
`
`heat transfer engineering
`
`vol. 30 no. 9 2009
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`

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`
`R. K. SHAH
`
`Figure 2 Compressor types: (a) fixed-displacement compressor, (b) variable-displacement compressor, (c) scroll compressor, and (d) rotary compressor. Courtesy
`of Subros Ltd., New Delhi, India.
`
`in hot ambient conditions is the same as that of the FDC of the
`same 100% full displacement. Its cost is about 20–40% higher
`than the equivalent full-performance FDC.
`A scroll compressor is a rotary-type compressor, as shown in
`Figure 2c, where a moving scroll has an orbital motion around
`a fixed scroll. Motion of the scroll pulls in gas between the
`fixed and orbiting walls and continuously compresses it toward
`the center. This means there is no re-expansion of the com-
`pressed gas as in the FDC/VDC. Hence, the volumetric effi-
`ciency is very high, ∼85–95%. Two types of scroll compressors
`are available, having fixed capacity and variable capacity. The
`advantages of the fixed scroll compressor over FDC are better
`performance at higher compressor speeds, higher volumetric
`efficiency, compact and low weight, and continuous compres-
`
`sion process ensuring smoother gas flow. Disadvantages are
`that very high machining accuracy is essential, manufacturing
`of parts with complex geometry is difficult, an almost equivalent
`cost for the same cooling capacity and somewhat less durabil-
`ity, and lower performance at low rotational speeds, particularly
`at idle. A variable-capacity scroll compressor is more complex
`in design. Packaging is slightly larger, as would be expected,
`mainly for a control valve in the rear head ∼15 mm longer. This
`compressor was used by GM in the past for minivans and some
`other luxury cars.
`A rotary compressor, shown in Figure 2d, consists of a rotary
`piston with vanes inside a cylinder, which rotates eccentrically.
`This eccentric rotational motion is used to compress the re-
`frigerant gas. It gives higher performance at lower compressor
`
`heat transfer engineering
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`vol. 30 no. 9 2009
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`Table 1 Important information for various compressors used in automotive air-conditioners
`
`R. K. SHAH
`
`725
`
`Compressor
`types
`
`Fixed-displacement
`compressors
`
`VDC, internally
`†
`controlled
`VDC, externally
`†
`controlled
`Scroll compressors
`
`120–170
`
`∼ 2.8
`
`120–170
`
`2.2–2.8
`
`60–115
`
`1.71–
`
`∼ 6
`
`4.9–6
`
`2.33–
`
`45–70
`
`60–74
`
`∼ 6.5
`
`45–75
`60∼80
`
`50∼70
`
`60–74
`
`5.3–5.4
`
`85–95
`
`4
`
`75–85
`
`2.9–2.6
`
`Rotary compressors
`
`70–142
`
`1.6–2.85
`
`3–6.4
`
`Compressor
`displacement,
`cc
`
`Compressor
`power at
`1800 rpm, kW
`
`Cooling
`capacity at
`1800 rpm, kW
`
`ηi
`Range, %
`
`ηv
`Range, %
`
`Compressor
`weight,
`kg
`
`Major
`advantages
`
`Major
`disadvantages
`
`80–200
`
`1.48–3.6
`
`2.94–7.2
`
`45–70
`
`50–69
`
`4.3–7.2
`
`Approximate
`worldwide
`business, %
`
`Approx. 66%
`decreasing
`
`Approx. 14%
`increasing
`
`Approx. 12%
`increasing
`
`Approx. 8%
`increasing
`
`Simple mechanism
`and reliable
`
`Better COP and
`human comfort
`Better COP and
`human comfort
`Better ηv and
`compact in size
`
`Low cost, compact
`in size and
`weight
`
`Lower ηv and high
`noise due to frequent
`on–off
`High cost and complex
`mechanism
`High cost and complex
`mechanism
`High cost,
`serviceability
`problem
`Performance
`deteriorates at higher
`speeds and
`unsuitable for larger
`loads.
`
`†
`
`Variable-displacement compressors (VDC): continuous type internally variable, i.e., mechanical control, continuous type externally variable, i.e., electro-
`mechanical control.
`
`speeds. It has a smaller number of parts. Hence, its cost is low.
`Major advantages are the simple mechanism, lower number of
`parts, cheapest, easy to manufacture, and better performance at
`vehicle idle conditions. The major disadvantages are that it is
`only applicable for lower cooling requirement (small cars), and
`compressor performance deteriorates at higher speeds. Because
`of the construction features, rotary and scroll compressors can
`tolerate some liquid better because the liquid refrigerant can
`escape from the compressor inlet to outlet without damaging
`internal parts.
`The compressor requires lubrication. Because of the con-
`struction features, it is not easy to separate and return the lu-
`bricant from the refrigerant at compressor outlet to compressor
`inlet (this development though is continuing). As a result, the
`lubricant circulates through the complete refrigerant circuit. In a
`properly designed system, refrigerant brings the right amount of
`lubricant to the compressor for lubrication. Mineral oil has been
`used with R12 refrigerant. Since it does not mix with R134a,
`a special synthetic lubricant such as PAG (polyalkylene glycol)
`or POE (polyol ester) is used with R134a. Note that POE oil
`is compatible with both R134a and R12 refrigerants, as well as
`with the residual mineral oil that may still be in the A/C system.
`It is often used when retrofitting an older R12 A/C system to
`R134a.
`
`Condensers
`
`In a condenser, refrigerant flows on the (flat or round) tube
`side, and the ambient air from the vehicle front grill flows on
`the fin side. The refrigerant is de-superheated, condensed, and
`subcooled in the condenser with cooler ambient air flowing on
`the fin side. The refrigerant rejects heat in the condenser, which
`is gained in the A/C system as follows: heat transfer in the
`evaporator from air to refrigerant (60–65%), compressor power
`heat transfer engineering
`
`input (30–35%), and heat gain (3–5%) in the refrigerant lines in
`the engine compartment.
`The following historical developments of condensers are
`summarized from Ravikumar et al. [5], and typical condenser
`development is shown in Figure 3. The mass production of the
`A/C system started with the 1954 Pontiac at the Harrison Radi-
`ator Division of General Motors Corporation, now Delphi Ther-
`mal and Interior System of Delphi Corporation. After trying out
`different designs during 1954–1956, production of condensers
`with round tube and flat fin design started with wavy fins in
`1956; in early 1980s, the louver fins replaced the wavy fins. In
`tube-and-fin condensers, tubes are mechanically expanded onto
`the fins, thus not requiring brazing and associated cost. However,
`the performance is also lower compared to serpentine (thin flat
`multiport tubes) and parallel flow (also referred to as multiflow
`or headered tube-and-center) condensers. The first-generation
`serpentine condensers were sold by Modine in 1957. Serpen-
`tine tube and corrugated louver fin condensers were introduced
`during the late 1970s through 1980s. Their heat transfer perfor-
`mance is higher than that for mechanically expanded round/oval
`tube-and-fin condensers for equivalent air-side pressure drop.
`
`Figure 3 Historical condenser developments, from Ravikumar et al. [5].
`vol. 30 no. 9 2009
`
`Page 6 of 17
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`

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`726
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`R. K. SHAH
`
`The parallel flow condenser was introduced in the late 1980s
`due to its higher performance (not achievable in a serpentine
`condenser for the same packaging envelope), required with the
`change of refrigerant from R12 to R134a. This condenser has
`extruded microchannel (flat) tubes and corrugated multilouver
`fins.
`Depending on the amount of the heat rejection required from
`the condenser, a number of different condenser designs are used
`worldwide today:
`
`1. For lower cooling requirements, mechanically expanded
`round/oval tube and louvered/wavy flat fins are most eco-
`nomical since this design does not require brazing. Since the
`1990s, microfins are being used in round tubes of round tube
`and flat louver fin condenser design; such designs have two
`tube rows; microfins increase heat transfer surface area and
`hence the performance. On the air side, improvements have
`been made in the design of multilouvers, and this fin design
`is still mostly used. These condenser designs have two tube
`rows, fin density 400–800 fins/m, fin thickness 0.075–0.125
`mm, core depth 25–44 mm, and tube diameter 6–7 mm. Both
`multilouver and wavy fin designs are being used.
`2. For medium cooling requirements, condensers with brazed
`serpentine multiport tubes and corrugated multilouver fins
`are also used; such a condenser has only one serpentine
`circuit for refrigerant flow and those with higher cooling
`requirement have two parallel serpentine circuits. Current
`designs have fin density 400–800 fins/m, fin thickness 0.075–
`0.125 mm, fin height 10–15 mm, core depth 25–38 mm, and
`tube height 3–5 mm.
`3. For even higher cooling requirements, brazed parallel flow
`condensers are used. This condenser has radiator-type con-
`struction with tanks on both sides of the condenser and re-
`frigerant having multipass flow, with the number of tubes
`reducing in succeeding passes due to reduction in refrigerant
`volume as it is condensing. Most such condensers have ex-
`truded multiport tubes on the refrigerant side, but alternative
`designs with folded tubes with internal fins have also been
`used. Current designs have narrow height tubes (0.9 to 1.2
`mm), thinner fins (0.075–0.1 mm), fin heights 5–9 mm, and
`core depth 16–25 mm.
`
`Usually the cooling performance of the mechanically ex-
`panded (e.g., tube and fin) condenser is low (about 2–4 kW)
`versus the brazed condenser’s (serpentine and parallel flow)
`performance of 3–10 kW or so. Thus there is some good over-
`lap of the performance, and the eventual decision in selection is
`primarily based on the cost.
`In the conventional automotive TXV-RD air-conditioning
`system, an RD and a condenser are separately mounted in the en-
`gine compartment. This design requires more packaging space
`in the engine compartment and adds cost to the air-conditioning
`system. In one of the latest developments in design of the con-
`denser, the RD bottle is integrated with one of the condenser
`heat transfer engineering
`
`tanks, thus reducing the space requirement and cost. This de-
`sign eliminates separate mounting space for the RD bottle, one or
`two brackets for receiver mounting, pipe connectors to connect
`the condenser and receiver, the larger quantity of the refriger-
`ant contained in the liquid line, and additional manufacturing
`operations at the car assembly line. Integrating the RD in the
`condenser by connecting it before the last pass of the refrigerant
`circuit allows the liquid coming out of the RD to be further sub-
`cooled in the last pass of the condenser, thus producing more
`◦
`C) in the condenser and resulting in higher A/C
`subcooling (5–7
`system performance with about 5–10% higher operating pres-
`sures on the refrigerant side. Also, now only one part (IRDC)
`is manufactured, thus reducing the manufacturing cost and the
`space requirement associated with the RD and piping) in the
`engine compartment, as mentioned earlier.
`
`Evaporators
`
`The function of the evaporator is to dehumidify and cool the
`ambient air going to the passenger compartment, thus reduc-
`ing the sensible and latent heat from the incoming air to the
`evaporator. The evaporator is located in the HVAC module be-
`fore the heater core, and the heater core is positioned at some
`angle with respect to the evaporator in most designs. Equal or
`lower amounts of airflow (than that