Patent Application
20070084225
None.
1. Field of the Invention
The present invention relates to an open air cycle cooling system and more particularly, to an air cycle machine with a pair of heat exchangers and a bypass system which recirculates the expanded airflow through the second heat exchanger.
2. Related Art
It is generally known that an air cycle machine can be used in cooling and refrigeration systems, such as taught by U.S. Pat. No. 6,622,499. It is also known that, for air cycle machines that recirculate the expanded, cooled airflow, such air cycle machines may also include a recuperative heat exchanger in addition to the primary heat exchanger, such as taught by U.S. Pat. Nos. 4,295,518, 6,381,973 and 6,672,081. Typically, a regenerative heat exchanger, or recuperator, is positioned upstream of the expander to use the expanded airflow further reduce the temperature of the compressed airflow before it enters the expander. In prior art systems, the expanded airflow that flows through the recuperator to cool the compressed airflow is drawn from the compartment being cooled. There is no bypass between the expander and the compartment which would route the expanded airflow, the entire flow or a portion thereof, away from the compartment and to the recuperator. The lack of the bypass may not be a significant issue for a cooling system aboard an aircraft because low humidity compressed air is readily available from the ambient air at the majority of flight conditions. However, in most ground-based cooling systems, humidity must be removed to avoid its negative effects on the cooling ability of the air cycle machine.
One way to control humidity is to use a closed air cycle system, in which dry air recirculates between the compressor and expander without ever being discharged into the compartment being cooled. Since the working fluid is sealed within the system and not blown into the compartment, the fluid is not limited to dry air and could be any gaseous fluid or even a liquid, such as taught by U.S. Pat. No. 4,984,432. However, such closed configurations require yet another heat exchanger through which the expanded, cooled air is passed and a fan or blower which draws the air from the compartment being cooled through this additional heat exchanger. For open air cycle systems, where the same air that is passed through the compressor and expander is used to cool the compartment, humidity is generally controlled through the use of a variety of water separators. For many land-based open air cycle systems, the air entering the compressor is usually drawn from the compartment, and some systems also permit the intake of ambient air rather than air from the compartment. However, such land-based open air cycle systems are more inefficient during their initial starting phase than when operating at steady state conditions, resulting in higher power requirements of the system during the startup. Therefore, for most of these systems, the power units are sized for the work required in the startup phase and are not optimized for steady state operations, even though the systems are run in steady state conditions most of the time. Accordingly, there remains a need for an air cycle system that does not require an additional heat exchanger and that can be optimized for steady state operations.
The present invention is directed to an open air cycle system which has a variable bypass between the expander and the regenerative heat exchanger.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.
The accompanying drawings, which are incorporated in and form a part of the specification, illustrate the embodiments of the present invention and together with the description, serve to explain the principles of the invention. In the drawings:
Referring to the accompanying drawings in which like reference numbers indicate like elements,
The bypass system 26 of the present invention generally includes a bypass duct 44 and a mixer 46 situated between the bypass duct 46 and the supply duct 36. The mixer 46 is in fluid communication with the compressor 18 via a counter-path circuit 48 that directs the expanded airflow 12″ through the regenerative heat exchanger 22. As the expanded airflow 12″ passes through the regenerative heat exchanger 22, it cools the compressed airflow 12′ that is about to enter the expander 24. As the compressed airflow 12′ is cooled, and as the airflow's temperature falls below the dew point, the moisture in vapor state is condensed into liquid state and is extracted from the airflow 12 by the water separator 34.
To direct the airflow 12 through the counter-path circuit 48, the mixer 46 preferably includes a valve 50 and a controller 52. One route 54 through the valve 50 directs the airflow 12 to the bypass duct 44 and another route 56 directs the airflow 12 to the supply duct 36 into the compartment 14. The controller 52 operates the valve 50 to select between these alternate routes 54, 56.
As with other known air cycle systems, the compressor 18 can be powered by a combination of an electric motor 58 and work extracted from the expander 24. As illustrated in
It will be appreciated that the compartment 14 can have multiple inlets 38 and outlets 42, and at a minimum, has one of each. As illustrated in
As with other air cycle systems, the air cycle system 10 of the present invention operates according to the second law of thermodynamics by using work to move heat from a cooled space and reject it to a hot space. Performance of the air cycle machine 16 can be evaluated using the coefficient of performance (COP) as defined in Equation 1 below.
COP=(Heat RemovedCompartment)/(Work InputMachine) [Eq. 1]
The COP is the ratio of the heat transferred out of the cooled compartment 14 to the amount of work required for the air cycle machine 16 to perform the heat transfer. It is generally appreciated that the COP ratio can be larger than unity, with larger values generally indicating a better air cycle system because more heat is removed for a given amount of work. The COP ratio is usually dependent on operating conditions, such as the temperatures of the cooled space and the hot space to which heat is to be rejected.
The theoretical maximum performance of a refrigerator under specific hot space and cooled space temperature conditions is given by the reversed Carnot cycle. This cycle consists of a gas that undergoes four ideal processes: a reversible adiabatic (no heat transfer) compression, a reversible isothermal (constant temperature) compression, a reversible adiabatic expansion and then a reversible isothermal expansion. The theoretical COP for a Carnot refrigeration machine is defined by Equation 2 below.
In Equation 2, Thot and Tcooled are the temperatures of the hot space and the cooled space, respectively. This perfect cycle cannot be achieved because ideal reversible processes are impossible in a real machine, but the perfect cycle is helpful to compare real cooling machines and processes. Accordingly, all real air cycle machines have a COP less than COPCarnot.
In operating the air cycle machine 16 according to the present invention, less work is required than typical open air cycle systems because the working fluid can be initially sealed within the machine during the startup operating condition, such as in a closed air cycle system. In this startup phase, all of the airflow is diverted by the bypass system 26. Accordingly, only a fraction of the work is necessary to cool and dry the air within the closed system. As the sealed airflow is cooled and dried during the startup phase, the bypass system 26 can divert some of the cooled air into the compartment 14 and draw the remaining air from the compartment 14 or from the ambient environment 32. Accordingly, a port 62 with a flapper valve 64, or other type of flow control device, may be included in one of the ducts 28 returning from the expander 24, through the bypass system 26 and/or the compartment 14, to allow the compressor 18 to pull air into the system 10 directly from the ambient environment 32. It will be appreciated that, for geothermal air cycle machines, the ambient heat exchanger 20 could use a liquid, such as water or other coolant, as the ambient cooling environment 32. As the system 10 reaches steady state performance, where the compartment has reached a desired operating condition, all of the air returning through the bypass system 26 can be drawn from the compartment 14.
In view of the foregoing, it will be seen that the several advantages of the invention are achieved and attained. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. As various modifications could be made in the constructions and methods herein described and illustrated without departing from the scope of the invention, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative rather than limiting. Therefore, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments but should be defined only in accordance with the following claims appended hereto and their equivalents.
