16-02-2010, 10:15 PM
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FREE FROST HEAT EXCAHANGER
ABSTRACT
A new concept has emerged in the field of zero - emission vehicles : the cryogenic automobile . The cryogenic automobile uses nitrogen , stored in liquid state , as its working fluid . The liquid nitrogen is pressurized , then vaporized in an economizer through which the exhaust from the expander runs in counter flow . The resulting gaseous nitrogen is superheated in an ambient temperature gas which is injected into a quasi “ isothermal expander to produce the system™s motive work. The spent , low pressure nitrogen is exhausted back to the atmosphere.
One technical challenge which must be overcome is a heat exchange system that is structurally robust , works well under a variety of operating conditions and is not hampered by the buildup of frost. The formation of frost on sub “ambient heat exchangers increases conductive resistance to heat transfer , occludes air “side flow passages, and contributes to structural loading . A concept is described by which vaporization and superheating of the nitrogen is achieved in a frost “free manner .
1. INTRODUCTION
In September of 1990 , in an effort to improve local air quality , the California Air Resources Board enacted the Low Emission Vehicle (LEV) program. The LEV program established several categories of emission standards for cars and light trucks. The most stringent of these categories was for the zero-emission vehicle (ZEV). The LEV program requires that , by 2003 , each of the seven largest automobile manufacturers (Chrysler,Ford , General Motors , Honda, Mazda , Nissan and Toyota) produce and offer for sale ZEVs at a rate equal to 10 % of the automobile sales each company has in the state , or about 110,000 cars per year. Similar mandates have also been adopted by New York and Massachusetts.
The impetus for the LEV legislation is the desire to reduce air pollution. In urban areas of Southern California, vehicles account for over 50% of the air pollution emitted. In 1995, the South Coast Basin (which includes Los Angeles, Orange, and parts of San Bernardino and Riverside counties) experienced 98 days in which the EPA health standard for ground - level ozone was exceeded . Ground - level ozone can cause aching lungs , wheezing , coughing and headaches . Serious health problems can also arise for those people with asthma , emphysema and chronic bronchitis . Children appear to be at particular risk. A 1984 study conducted at USC showed that children raised in the South Coast Basin suffered a 10% to 15% decrease in lung function. The deleterious effects of gasoline and diesel powered vehicles are not limited to air quality in southern California . In half of the world's cities , tailpipe emissions are the single largest source of air pollution. Worldwide, automobiles account for half of the oil consumed and a fifth of the greenhouse gases emitted. This situation is not expected to improve in the near future, as the number of cars and light trucks in the world “ over 500 million “ is expected to double in the next thirty years. Most of this growth will occur in developing countries which have little or no emission controls.
Currently, the battery powered electric vehicle is the only commercially available technology that can meet ZEV standards. However, electric vehicles have not sold well. This is primarily due to their limited range, although anemic performance, slow recharge and high initial costs are also contributing factors. All of these issues can be traced directly to the limitations of electrochemical energy storage, particularly lead “ acid batteries. Lead - acid remains the dominant technology in the electric vehicle market, but only exhibit energy densities in the range of 30-40 W-hr/kg. This compares with about 3,000 W-hr/kg for gasoline combusted in an engine running at 28% thermal efficiency. Lead acid batteries can take hours to recharge and must be replaced every 2“3 years. This raises the specter of increased heavy metal pollution, were a lead-acid powered electric fleet ever to come to pass.
Even advanced battery systems, such as nickel- metal hydride, zinc-air, and lithium-ion suffer from slow recharge and high initial cost. Nickel- metal hydride batteries, often touted as the heir- apparent of lead- acid, still contain a heavy metal and must realize dramatic reductions in cost in order to be truly competitive. Lithium - ion batteries, considered by many to be the third - generation solution, must also contend with cost and demonstrate their safety to a wary public.
Another energy storage medium will be required to make ZEVs the non-mandated automobile choice of the car-buying public. If certain technical challenges can be over come, that energy storage medium may well be liquid nitrogen. Since 1993, the University of Washington has been researching the technical challenges involved in building and operating a vehicle powered by liquid nitrogen. Issues pertaining to frost-free heat exchanger performance, cryogenic equipment, cycle analysis, drive train selection and vehicle configuration are being investigated.
This paper describes the fundamental concepts of frost free heat exchanger for cryogenic automotive propulsion.
2. CRYOGENIC AUTOMOTIVE PROPULSION
The cryogenic automobile is a zero-emission vehicle. It operates on the thermodynamic potential between the ambient atmosphere and a reservoir of liquid nitrogen. One way to utilize that potential is through an open Rankine cycle. The liquid nitrogen is drawn from a tank, pumped up to the system pressure, then vaporized and superheated in a two-stage heat exchange system. The resulting high pressure, near- ambient temperature gas is injected into a quasi-isothermal expander which produces the system's motive work. The spent, low pressure gas is exhausted back to the atmosphere. Because a zero emission vehicle is required to produce no smog-forming tailpipe or evaporative pollutants, and because nitrogen gas is its only emission , the cryogenic automobile meets California's ZEV guidelines. Although the concept of a nitrogen powered automobile has been studied in the past,, there are two key technologies that have yet to be demonstrated: the quasi-isothermal expander and a frost-free liquid nitrogen heat exchange system. It is this heat exchange system that is the primary subject of this paper.
2.1. Overview
There are many thermodynamic cycles available for utilizing the thermal potential of liquid nitrogen. These range from the Brayton cycle, to using two- and even three-fluid topping cycles , to employing a hydrocarbon - fueled boiler for superheating beyond atmospheric temperatures. The easiest to implement, however, and the one chosen for this study, is shown in Fig. 1. This system uses an open Rankine cycle. It begins with a tank of liquid nitrogen stored at 77 K and 1 bar. The nitrogen is pumped, as a liquid, to the system's working pressure. This high pressure liquid flows into the economizer. The economizer is a shell-and-tube heat exchanger where the shell- side fluid is the exhaust from the expander. While this step is not necessary from an energy point of view, it does have the advantage of providing a frost-free pre-heat to the incoming liquid.
Fig. 1: Liquid nitrogen propulsion system.
Once through the economizer, the vaporized nitrogen enters the heat exchanger, which has a multi-element, tube-in- crossflow configuration. The exterior fluid is the ambient atmosphere, which is drawn through the core of the heat exchanger either by the motion of the vehicle or by a fan, depending on the operating regime. This heat exchanger must be able to operate across most of the spectrum of environmental and operating conditions without suffering the adverse effects of frost buildup.Upon leaving the heat exchanger, the working fluid is a high pressure, near-ambient temperature gas. It is injected into the expander which provides all of the motive work for the system. This can be either a positive displacement or turbine engine. Following expansion, the low-pressure exhaust is warm enough to be used in an economizer, where it preheats the incoming liquid, before finally being vented to the atmosphere.
3. EXPERIMENTAL FACILITIES
3.1 Test Vehicle
A test vehicle has been purchased to serve both as a proof-of-concept and as a rolling test-bed for further system refinements. The vehicle itself is a 1984 rumman-Olson Kubvan, pictured in Fig 2. To emphasize the near-term potential of this project, it has been christened the LN2000 .
Fig.2 : 1984 Grumman-Olson Kubvan.
This particular vehicle was originally electric , operating on a pack of 14 lead acid batteries which weighed over 450 kg. The running gear for this vehicle is from a right-hand drive 1984 Volkswagen Rabbit. The construction is welded frame with riveted body panels and is made entirely of aluminum .
Table 1 : Performance of the open Rankine cycle.
City Cycle Highway Cycle Straight and Level*
Avg. Mass Flow (g/s) 81 204 256
Max. Mass Flow (g/s) 297 298 256
Consumption (kg/km) 9.5 10.0 10.8
Avg. Power (kW) 4.0 10.7 13.0
Max. Power (kW) 13.0 13.0 13.0
* At a top speed of 85 km/h (53 mph).
The Kubvan was selected as a test-bed for several reasons. The volume available is well suited to the placement of the necessary equipment. The simplicity of its construction “flat sheet-metal body panels, aluminum frame, open interior “ is conducive to making modifications. Also, because it was originally designed to be electric, it operates with very few "hotel" functions such as air conditioning and rear window defrosting.
The Kubvan performance using the prototype LN2 propulsion system has been calculated and is given in Table 1. This employs an EPA approved Federal Urban Driving Schedule. Velocity dependent rolling resistance and aerodynamic drag were calculated , and correlated reasonably well with coast-down tests made with the vehicle.The equipment layout is shown in
Fig.3 . Most of the cryogenic plumbing is stainless steel, but the components attached to the economizer are aluminum . Low pressure plumbing utilizes large diameter rubber hose where possible. Each of the items pictured is described below .
Fig. 3 : Schematic of vehicle equipment layout.
3.2 Liquid Nitrogen Storage Tank
The dewar chosen for this application can hold 80 liters of liquid nitrogen at 24 bar with a daily boil-off rate of ~3%. The primary protection against over-pressure is a relief valve connected to the internal vessel. This valve also serves as the bleed for the boiloff gases, which are vented to the outside by a rubber hose. There are several other safety devices, providing multi-tiered protection against catastrophic rupture. The dewar is held in place at five attachment points: one on the roof, the other four on the floor.
3.3 Pressurization System
The pressurization system consists of two high-pressure nitrogen bottles stored under the rear deck of the Kubvan. The blow down system has the advantage of mechanical simplicity at the cost of increased weight and volume. Each of the nitrogen bottles has a mass of 40 kg. The volume of gas
required was calculated such that the pressurant tanks and the dewar get to within 3 bar of equilibrium just as the last of the liquid nitrogen is drained out. The pressurant tanks are initially filled to a starting pressure of about 133 bar. This is regulated down to the system pressure of 24 bar before being injected into the dewar. The hardware required for filling both the pressurant bottles and the dewar is attached to the vehicle.
3.3 Economizer
The economizer, is actually a pair of shell-and-tube heat exchangers, as shown in Fig.4. These heat exchangers operate in parallel, with the shell-side fluid being the exhaust from the expander. When operating at maximum mass flow, ~300 g/s, the economizer is designed to bring the liquid nitrogen to a quality of about 75%. This represents approximately one quarter of the total enthalpy change the nitrogen will experience before being injected into the expander. At lower mass flows, the vaporization will be complete.
Fig.4 : Economizer units with and without shell.
3.4 Ambient “Air Heat Exchanger
The ambient “air heat exchanger, is made up of 45 finned-tube elements. These elements are manifolded together , as shown in Fig.5 to make a staggered array of tubes in cross flow with the incoming air. Either the motion of the vehicle, or the two ducted fans located at the back of the van, draw the air through the heat exchanger. The air inlet consists of a sheet-metal scoop slung underneath the vehicle.
Fig. 5 Ambient-air heat exchanger assembly.
3.5 Expander
The expander chosen for the prototype vehicle is a 11.1 kW, radial piston air motor made by Cooper Power Tools. This motor, pictured in Fig 6. has a cast-iron block with five 7.5 cm cylinders. Each cylinder holds a steel piston attached to the single-throw crank shaft by a connecting rod. Lubrication is maintained by splash and by an oiler located near the gas inlet. The motor is attached to the front- wheel drive, 5-speed manual transmission by a custom-made aluminum gear-box. The output shaft of the motor drives a 15.24 cm diametral pitch (DP) spur gear. The input shaft to the clutch assembly has a 7.62 cm. DP spur gear, giving a 1:2 speed ratio through the gear box. The running gear is from a 1984 Volkswagen Rabbit and is right - hand drive.
Fig. 6: Motor and transmission assembly.
4 . AMBIENT-AIR HEAT EXCHANGER DEVELOPMENT
The goal of this research is to design a heat exchanger that can operate in a variety of environmental conditions and is structurally robust, while not being hampered by the build “ up of frost. Many approaches have been examined, but in general, strategies for dealing with frost formation fall into two categories: passive and active control. Passive control involves either preventing frost formation , or oversizing the heat exchanger such that frost build - up is unimportant on time - scales characteristic of automotive travel. The advantages of passive control are mechanical simplicity and reliability . One disadvantage is that passive systems are , in general , less flexible in dealing with off “ design operation .
Active control of frost formation entails allowing frost to accrete, and then removing it either thermally or mechanically . The advantage of active systems is that they are more operationally robust . Performance of an actively controlled heat exchanger is much less dependent on the ambient or operational conditions since these systems can be responsive to different frost loading conditions. Disadvantages include the power consumption and poor long “ term reliability .
Both options have been examined closely. Of the two, the passive control heat exchanger was chosen for the LN2000 as the best possible solution to the issue of frost formation. As of this writing , the ambient air heat exchanger has been built and pressure tested. The results presented in this chapter are calculated and have not yet been experimentally verified . Described here in is the theoretical modeling , design , and fabrication of the ambient “ air heat exchanger . Future plans include rigorous testing to verify the calculations and identify possible modifications for a second-generation heat exchanger.
4.1 Design
The operating environment of a typical automobile can be a demanding one. There are weather conditions such as rain, snow, or extreme heat to contend with. There are driving conditions such as rough pavement, gravel roads, potholes and speed-bumps, and there are situations such as stop-and-go traffic, long-distance highway travel, and cold-starts. Finally , automobiles have to meet rigorous safety, comfort, and reliability standards if they are to compete in today's marketplace. A heat exchanger for automotive application has to successfully meet all of these requirements .
4.2 Constraints
Along with meeting the necessary design requirements, the automotive heat exchanger has to also fit within constrained volume and mass envelopes. The LN2000 has a space in back , under the cargo deck , which provides a large volume and allows easy ducting of ambient air from beneath the vehicle. Furthermore, because the heat exchanger can be hung directly from the deck , structural modifications are minimal. The available space constrains the heat exchanger to dimensions of no more than 78 cm x 53 cm x 38 cm. The mass constraint is less well defined , but it does stand to reason that in an 1100 kg automobile with an 11 kW motor , less mass is better .
4.3 Configuration
To prevent frost buildup on sub ambient heat exchangers, the exterior surfaces must be kept above the freezing point of water. A method for achieving this is shown in Fig.7 The principle is the same as for multi “ fluid heat exchange systems , where heat is transferred from source to sink via a number of media operating in series. In this concept , that series of media is simply the nitrogen gas at different stages of its thermal history.
Fig.7: Frost free heat exchanger element.
Figure shows only three passes , but it is possible to use as few as two, or as many as can be fit within the outermost tube. Two concentric tubes, operating within a bundle in a shell-and-tube heat exchanger, are known as bayonets. These have been used for years to eliminated stresses due to differential thermal expansion between the shell and its tube bundle . More recently, a 1995 patent for a frost-tolerant heat exchanger described much the same idea as that presented here, but using only two concentric tubes in a variety of configurations .
The choice of three passes was based on two practical considerations. The first is that using an odd number of internal passages allows the inlet and outlet to be at opposite ends of the element , greatly simplifying the manifolding. The second is that going tomore internal passages such as five or seven can present problems with fabrication, cost and mass . This has to be weighed against the fact that more passes increase the robustness of the system . Robustness insures that frost “ free performance can be maintained in lower “ temperature environments .
Fig.8: Temperature profile of frost-free heat exchanger element.
An example of the behavior of the gas as it passes through the heat exchanger element is given in Fig.8. The black arrows represent heat transfer: from the ambient air to the wall , from the wall to the nitrogen and from each pass to the previous one. The exterior wall temperature is dependent on the heat transfer coefficient and fluid temperature in the outermost passage only .
Passive heat exchangers are more tightly coupled to environmental and operating conditions . In the case of this particular concept, when the outside temperature is lower than the freezing point of water, frost will form. In fact, frost will form even when the ambient temperature is above the freezing point of water, if the temperature difference from the air to the wall is too large .
5. ALTERNATIVE HEAT EXCHANGER DESIGN
Till now described a frost-free heat exchange system that relied on keeping the exterior wall temperature above the freezing point of water. The penalty for this type of passive heat exchanger is that the T which drives the heat transfer is necessarily small. To account for this , very large surface areas are required to keep the size of the heat exchanger reasonable ; i.e., 45 finned tubes . An active frost-control heat exchanger can exploit the large DT that occurs between the ambient atmosphere and a cryogenic fluid to achieve a very compact unit .
A heat exchanger has been developed which uses rotating brushes for active frost-control. As shown in Figure 9 , this heat exchanger consists of several multi “ pass tubes sandwiched between two sets of concentric rings . The cryogenic fluid (either liquid or gas ) flows through the tubing , running in cross flow with the ambient air. The rings provide structural support for the tubing , as well as added heat transfer area .
Threaded through the center of these rings is a shaft, to which are attached two brushes. The brushes engage the tube/ring structure from either side and rotate , driven by a motor (not pictured ) . As they rotate , accumulated frost is scraped free of the heat transfer surfaces and eventually falls out the bottom of the unit. Each of these tube/ring structures can serve as a stand-alone unit or they can be stacked together to form a multi-element heat exchanger . The entire assembly is enclosed in a cylindrical duct through which the air flows. For multi element designs, only one shaft is needed to drive the brush assembly for the entire heat exchanger .
Fig.9: Heat exchanger employing active frost removal.
Joining of the tube/ring structure can be accomplished by spot welding or, for larger assemblies , by dip brazing in molten salts . The result is a very rigid, yet lightweight structure which easy to manifold . A prototype active-control heat exchanger has been successfully fabricated and tested .
6. CONCLUSIONS
The cryogenic automobile is a potential contender in the ZEV market, provided certain key technologies are demonstrated . One of these technologies is the development of an all - weather heat exchange system . A heat exchanger that works well in a variety of operating conditions, is structurally robust, and is not hampered by the buildup of frost is a necessary technology for the cryogenic automobile. This paper describes the design and fabrication of such a system . Although experiments have yet to be conducted to determine the actual performance, numerical predictions have been given. until road-test experiments are conducted , the actual thermal performance will not be known with enough accuracy to support an overall recommendation for the final heat exchange system design .
7. REFERENCES
1. Dooley,J.L., and Hammond,R.P., The Cryogenic Nitrogen Automotive Engine
2. Websites of the University of Washington and University of North Texas
3. Saunders, E.A.D., Heat Exchangers :Selection Design and Construction
