Abstract:
A system for automatically controlling the buoyancy of a lighter-than-air craft by condensing water ballast from engine exhaust gases.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to field of lighter-than-air craft and more specifically to a system for stabilizing the weight of such airships by making water ballast from the exhaust gases created by burning hydrocarbon fuel for propulsion or other purposes. 
     In such airships, the buoyancy is usually controlled by ballast which is added and/or dumped to compensate for lift variables such as helium volume (changes due to leakage and atmospheric heating) and, more importantly, the weight of fuel consumed. In the case of long range maritime airships, this ballast is customarily sea water which necessitates frequent descents to the surface to take on additional ballast. Not only is this operation hazardous in high wind, high seas conditions, but, especially in the case of maritime surveillance airships, this operation reduces the effectiveness of the surveillance during those periods when the airship is at the surface taking on ballast. Moreover, considerable time and skill is needed on the part of the crew to continuously calculate the state of buoyancy and manage the ballast accordingly. 
     It has already been suggested to simply condense water from engine exhaust gases and store this water as ballast. See, for example, U.S. Pat. Nos. 1,426,047; 1,576,859; 1,598,002; 1,645,065; 1,653,603; 2,310,767 and 2,479,766. However, the excess weight or bulk, parasitic power loss, and lack of dependability of previous designs has limited their value to various minor degrees of success for over fifty years. 
     Some of the more serious problems of these prior systems are discussed in the aforementioned U.S. Pat. Nos. 1,653,603 and 2,078,532. 
     Therefore, it is an object of the present invention to overcome the Problems and disadvantages of previous systems and provide an improved means for automatically controlling the buoyancy of a lighter-than-air craft. 
     SUMMARY OF THE INVENTION 
     In accordance with these objectives, the present invention, in its most general sense, comprises the combination of an internal combustion engine, an exhaust gas precooler, a water separator, and optionally a control computer. 
     The internal combustion engine is most preferably a recuperated gas turbine type of engine since that type produces a much cleaner and steady exhaust gas flow at a relatively low temperature (e.g. below about 800° F.) compared to other types of engines (e.g. above about 1000° F.). Such engines are also ideally suited for propulsion and as auxiliary power units. Modern aircraft often have several gas turbines in operation at any one time but it is not required that all of the exhaust gas from every engine be treated by the system of the present invention. Since the combustion of conventional hydrocarbon fuels in a gas turbine is highly efficient, each engine will produce about 1.3 times as much water vapor, by weight, than the amount of fuel consumed. 
     The exhaust gas precooler is a key element for reducing the parasitic losses of this system to acceptable levels. It is most preferably a rotatable, high efficiency heat exchange element made of a honeycomb or porous metal or ceramic and is designed to transfer heat between two gas streams of dissimilar temperature. It performs this function by continuously rotating a short cylinder or disc between the two streams so as to alternately heat that portion of the disc matrix exposed to the hot stream and then give up that heat as the disc rotates, moving the hot segment into the cooler gas stream. Thus, the hot exhaust gas enters the precooler and gives up thermal energy to the rotating matrix so that the gas leaving the precooler is at a significantly lower temperature. A temperature drop of over 600° F. has been found to be practical. It is preferred that the exhaust gas not be cooled below its dew point in the precooler since the condensation of water vapor is more efficiently carried out in the water separator section. 
     The water separator consists of a vapor cycle refrigeration loop (e.g. Freon compressor-condenser-evaporator) in which the cold evaporator heat exchange unit is exposed to the flow of precooled exhaust gas to further lower its temperature. When the exhaust gas is cooled to or below its dew point, water vapor is condensed. The water is extracted from the system and conveyed to the ballast tanks. The degree of cooling in the water separator, and hence the rate of water extraction, is governed by the loading on the refrigerant loop and more particularly by the speed of the compressor motor. The compressor is preferably electrically driven and controlled by a subcomponent of the airship&#39;s on-board flight data computer. 
     A ballast control means or computer automatically increases or decreases the refrigeration rate so as to maintain sufficient ballast to compensate for fuel consumed and other buoyancy factors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     While this specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention, it is believed that the invention, objects, features, and advantages thereof may be better understood from the following detailed description of a presently preferred embodiment when taken in connection with the accompanying drawing in which: 
     FIG. 1 is a diagrammatic illustration of the presently preferred embodiment of the instant invention showing the exhaust gas flow path through the overall system and the ballast control means which governs a refrigeration loop to make water ballast to compensate for fuel consumed by the engines; and 
     FIG. 2 is a more detailed illustration of the rotary precooler of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, the apparatus of the present invention includes an internal combustion engine (10) which burns conventional liquid hydrocarbon fuel supplied from fuel tanks (90) through fuel supply lines (95). Preferably, the amount of fuel consumed is monitored by any conventional means. For example, a fuel flow measuring device (97) may be placed in the supply line (95) or a fuel level detector (92) may be placed in the tanks (90). 
     The hot (typically above 700° F.) engine exhaust gases are collected, for example by ducting (11), and, in this preferred embodiment, split into two streams (12 and 16) for treatment. Each of the streams is passed through a first rotating precooler wheel (20,21) where each gives up a substantial portion of its heat to the precooler matrix. The now cool (typically below about 100° F.) gas streams (13,17) pass through the water separator unit (30) which contains cold refrigeration coils (31,32). The exhaust gases are chilled sufficiently below the dew point (e.g. about 40° F.) for water vapor to be condensed out as liquid water which flows through a drain (82) to a water storage tank (80) for use as ballast. The now cold (e.g. 40° F.) exhaust gas streams (14,18) are passed through a second rotating precooler wheel (20,21) where they absorb heat from the precooler matrix. The warm (e.g. 670° F.) gas streams (15,19) are then vented to the atmosphere and discarded. 
     Note that similar results could be obtained with only one precooler wheel if the exhaust gases are not split into two streams but rather are passed through the hot section of the wheel (shown as area 28 in FIG. 2) to give up most of its heat to the wheel matrix (26). The one gas stream is then passed through the water separator unit (30) as before to condense out water vapor. Finally, the one gas stream is passed through the cool section (27) of the one precooler wheel where it absorbs heat from the wheel matrix (26) before being discharged to the atmosphere. 
     An important aspect of these embodiments is the rotating precooler wheel, shown in more detail in FIG. 2, which is generally known in the heat-exchange art for other uses. It is designed to transfer heat from a hot gas stream into a cooler gas stream by alternately heating and cooling a thermal mass. It is constructed as a rotatable disk (20) containing a porous matrix or core (26) which could be metal but is preferably a ceramic such as magnesium aluminum silicate, or aluminum silicate, or other similar materials. These materials have demonstrated superb thermal stability and corrosion resistance in gas turbine exhaust applications. The manufacturing technology used to make these precooler cores is similar to that used in the production of automotive catalytic converter substrates. They may be formed by extruding sections of &#34;green&#34; ceramic material followed by assembling the sections and firing the ceramic to complete the process, or they may be laminated from a corrugated ceramic &#34;paper&#34; which is wrapped in a spiral fashion around a mandrel and then fired. The extrusion process has the advantage of matrix uniformity and virtually no length restrictions, whereas the wrapped configuration has minimal wall porosity. More details of the manufacturing process are disclosed in U.S. Pat. No. 4,256,172 and the references cited therein, all of which are incorporated herein by reference. 
     Since this precooler core physically removes heat from the exhaust stream at one point and redeposits it back into the stream at another point, it must rotate in order to function. One way to accomplish this is to provide a large external ring gear (not shown) resiliently mounted to the precooler core which can be driven by an electric motor (22,23). Preferably, the precooler (20) is also provided with sliding gas seals (25) to direct the gas flow through the proper section of the precooler without significant leakage or bypassing. 
     The preferred counterflow configuration, in which the hot and cold gas streams flow in opposite directions through the precooler, provides for very low fouling of the heat exchange matrix. This avoids one of the more serious problems of the prior art. 
     The water separator unit (30) is essential to the functioning of the rotary precooler since it chills the exhaust gas stream, in order to condense out the water vapor, and it is this chilled gas stream which provides the cold sink for the precooler. It is similar to any conventional refrigeration system in that a vapor (e.g. Freon) compressor (40) is driven by an electric motor (45) to pump Freon around a loop consisting of an air cooled condenser coil (50), an expansion valve (55), and evaporator coils (31,32) in the water separator unit (30). Preferably, the condenser (50) is cooled by an electric fan (52) but it could also be cooled by the normal air flow past the ship. 
     While the water separator could be set to run continuously to condense out water ballast, it is preferred that it be automatically controlled to produce only as much water as is necessary. In this preferred embodiment, a control means (60), such as a component of the on-board flight data computer, receives signals related to the factors which affect the buoyancy of the airship and then sends signals (61) to the compressor motor (45) and/or signals (63) to a water dump valve (85) to manage the ballast accordingly. 
     For example, one signal (62) reports on the current amount of water ballast in the tanks (80) while another signal (64) is related to the amount of fuel consumed by reporting either the current fuel level from a sensor (92) in the fuel tanks (90) or by integrating the total fuel flow through sensor (97). Other signals (66,68) may be related to the helium volume, atmospheric variables such as temperature and pressure, or the pilot&#39;s desire to ascend or descend. 
     While the invention has been described in terms of one preferred embodiment, it is expected that various alterations, modifications, or permutations thereof will be apparent to those skilled in the art. Therefore, it is intended that equivalents be embraced within the spirit and scope of the invention as it is defined by the appended claims.