Abstract:
A method and apparatus for heating an asphalt-paved road surface by forcing gases heated by a heater against that road surface and then returning those gases to the heater for reheating and recirculation, wherein the temperature of the returning gases is measured by a temperature sensor, and the heater is automatically adjusted so that the temperature of the gases being directed against the road surface is automatically decreased as the temperature of the returning gases increases. This prevents damage to the asphalt and premature rupturing of the road surface.

Description:
TECHNICAL FIELD 
     The present invention relates to a method of, and apparatus for, heating an asphalt-paved road surface. 
     BACKGROUND 
     As used herein, the term “asphalt” is meant to include macadam and tarmac. As is known in the art, asphalt-paved road surfaces typically comprise a concrete mixture of asphalt cement (typically a black, sticky, petrochemical binder) and an aggregate comprising appropriately sized stones, gravel, and/or sand. The asphalt concrete mixture is usually laid, compressed, and smoothed to provide an asphalt-paved road surface. 
     Conventionally, in repair work of roads paved with asphalt concrete, the road surfaces are, prior to the repair work, softened through heating thereof by road surface heating apparatuses, each typically mounted on a vehicle. It is necessary to soften old asphalt so that a satisfactory bond is formed between the old asphalt and subsequently applied new asphalt. Softening is also required where portions of the old asphalt are to be recycled into the new pavement. Such recycling generally operates on the premise of (1) heating the paved surface to soften an exposed layer of asphalt using direct flames, infrared burners, radiant heaters, hot air blowers, or other heating means; (2) mechanically breaking up the heated surface, typically using devices such as rotating, toothed grinders; screw auger/mills; and rake-like scarifiers; (3) applying fresh asphalt and/or asphalt rejuvenant to the heated, broken asphalt; (4) distributing the mixture over the road surface; and (5) compacting or pressing the distributed mixture to provide a recycled asphalt-paved surface. 
     U.S. Pat. No. 5,653,552 issued to Wiley and U.S. Pat. No. 5,791,814 issued to Wiley et al. discuss in great detail a number of problems in respect of heating, softening, and rupturing old asphalt. For example, asphalt concrete (especially the asphalt cement within it) is susceptible to damage from heat, usually when the asphalt mixture is heated to a temperature above approximately 320° F. (160° C.), although old asphalt pavement can usually be heated to a temperature approaching the flash point of asphalt (about 210° C. or 410° F.) since the surface oil on the pavement has typically been worn off, washed away, or severely oxidized. Another problem in this regard is the increasing difficulty of heating asphalt concrete as the depth of the layer being heated increases. A further problem results from excess heating and/or smoking of the asphalt surface which can lead to a negative impact on the environment. 
     To this end, U.S. Pat. No. 5,653,522 and U.S. Pat. No. 5,791,814 discuss in detail numerous attempts in the prior art to deal with the inherent difficulty of adequately and uniformly heating an asphalt surface in an efficient manner while minimizing or eliminating burning and smoking of the asphalt surface. Much of this effort involved utilizing relatively complicated means to distribute heat through the asphalt surface after rupturing thereof, often requiring further heating of the ruptured asphalt surface to facilitate heat distribution therethrough. These complicated processing means were typically cumbersome and large, yet were necessary due to the inability to preheat the unruptured asphalt surface adequately without overheating thereof. 
     U.S. Pat. No. 4,561,800 issued to Hatakenaka et al. and U.S. Pat. No. 4,559,922 issued to Crupi et al. teach the use of blowing hot air (or other gases) to heat and soften a road surface, and then recirculating and reheating those hot gases. In particular, each patent teaches an apparatus having an enclosure with a bottom peripheral wall positionable in engagement with a road surface, a heater, a duct for conveying hot gases from the heater to the enclosure and to the road surface under the enclosure, a duct for recycling gases after their contact with the road surface back to the heater for reheating, and a fan for circulating the hot gases through the ducts. According to these patents, using blowing hot gases resolves problems with direct flame and radiant heating means for heating a road surface, namely that the combination of the heat applied in accordance with those means and the oxygen in the atmosphere tended to oxidize the asphalt and drive off relative volatile components, thereby causing deterioration of the quality of the asphalt as well as releasing objectionable pollutants into the air. In contrast, by recirculating hot gases, minimal pollutants are released into the surrounding atmosphere and the hot gases can be controlled so as to have very little oxygen, thereby minimizing oxidation of the asphalt. Another problem with direct flame and infrared burners is that they result in non-uniform heating of the road surface, with the portions closely confronting the burners being overheated and burned, while other portions are underheated. By instead blowing hot gases uniformly across the road surface, Hatakenaka et al. and Crupi et al. provide a means of more uniformly heating a road surface. 
     Hatakenaka et al. goes one step further than Crupi et al., in that Hatakenaka et al. also teaches the use of a thermal control to maintain the hot gases in the duct leaving the heater at a constant, pre-determined temperature. However, this still would not prevent the possibility of overheating the road surface since Hatakenaka et al. does not disclose means by which the temperature of the road surface is taken into account and by which the constant, pre-determined temperature of the hot gases can be automatically reduced as the road surface approaches the flash point of the asphalt. 
     All of the prior art continues to exhibit a persistent problem in controlling the heat source in such a manner that the heat being produced is relative to only what is required to efficiently heat the asphalt material without causing damage. In addition, the prior art processes and apparatuses generally require that the material be heated only from the top of the road surface or in combination with a secondary heating step that applies heat to ruptured material. While rupturing the material and exposing more free oil creates a black surface that more readily absorbs infrared waves, it has been discovered that heating this exposed oil in loosened material greatly increases the amount of deterioration in the asphalt due to heat. Accordingly, it is preferable to heat the asphalt surface sufficiently prior to rupturing. 
     SUMMARY OF INVENTION 
     The present invention provides an improved method and apparatus for heating an asphalt-paved road surface to soften it prior to initiating repair work. This method and apparatus involves forcing gases heated by a heater against that road surface and then returning those gases to the heater for reheating and recirculation, wherein the temperature of the returning gases is measured by a temperature sensor, and the heater is automatically adjusted so that the temperature of the gases being directed against the road surface is automatically decreased as the temperature of the returning gases increases. This prevents damage to the asphalt and premature rupturing of the road surface. 
     In particular, the method according to the present invention comprises: (1) heating gases adjacent the heater; (2) forcing gases heated by the heater into contact with the road surface; (3) collecting the gases after they have been forced against the road surface and returning them to the heater; (4) measuring the temperature of the gases as they are returned from the road surface to the heater; and (5) adjusting the heater so that the temperature to which gases are heated by the heater decreases as the temperature of the returned gases increases. To this end, a road surface heating apparatus according to the present invention comprises: (1) a heater for heating gases; (2) at least one manifold for directing heated gases from the heater against a road surface; (3) an enclosure for trapping gases exiting that manifold against the road surface; (4) a fan for returning gases which have been directed against the road surface back to the heater for reheating and recirculating; and (5) a temperature sensor for measuring the temperature of gases being returned to the heater, wherein the heater of this heating apparatus provides heated gases at a temperature that decreases as the temperature measured by the temperature sensor increases. This apparatus could further comprise a venturi valve through which the fan directs returning gases at high velocity to the heater, creating a low pressure area sufficient to allow the heater to be no more than a natural aspirated burner introduced into the air stream to reheat the gases and combust any fumes collected. The temperature sensor can be a simple thermocouple. 
     The efficiency of this method and apparatus can be improved even further by doing the following: once the road surface has been heated for a period of time according to the method and apparatus described above, grooves can be pressed into the heat-softened road surface (for example, by a flanged reforming drum) without rupturing it. This prepares the road surface for further and deeper and more effective heating in accordance with the method and apparatus described above. 
     After completion of heating and softening of a road surface in accordance with the method and apparatus described above, the road surface can then be ruptured (for example, by a rupturing drum) and the ruptured material can then be reused in new pavement for the road surface. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a schematic view of a road surface heating apparatus according to an embodiment of the present invention. 
     FIG. 2 is a schematic view of road surface heating apparatuses according to the present invention used in combination with a reforming drum and a rupturing drum. 
     FIG. 3 is a horizontal cross-sectional view of a road surface after the reforming drum of FIG. 2 has been applied thereto. 
    
    
     DESCRIPTION 
     The present invention provides an improved method and apparatus for heating an asphalt road surface without overheating the asphalt material. The method and apparatus according to the invention directs impinging jets of hot gases at the road surface in such a way and with sufficient velocity so as to prevent the buildup of a cooler boundary layer near the top of the road surface. Further, rather than simply directing hot air (or other gases) of a constant, pre-determined temperature at the road surface, the present invention takes into account the temperature of the road surface and automatically adjusts the temperature of the impinging hot gases to only what is required to heat the road surface to the desired temperature. In this way, the road surface can be uniformly and consistently heated to optimal temperatures for repairing or recycling the asphalt pavement without damaging the road surface. 
     Referring to FIG. 1, a heating apparatus  10  that uses circulating hot gases to heat a road surface, according to one embodiment of the present invention, comprises an enclosure  12  having a flexible skirt or other bottom peripheral wall  14  positionable in engagement with a road surface  16  for containing hot gases therein. Heating apparatus  10  further comprises a heater  18  for heating the hot gases in a combustion chamber  20 , and manifolds  22  which direct impinging jets of hot gases from combustion chamber  20  through apertures  24  to the road surface  16  where heat from those hot gases is absorbed. Enclosure  12  largely prevents the release of gases to the surrounding atmosphere. A fan  26 , which can be a simple mild steel fan, collects the spent gases from the enclosure  12  after the gases have contacted the road surface  16  and then fan  26  accelerates and drives the returning gas at high velocity through a venturi valve  28  back to heater  18 . The venturi valve  28  creates a low pressure area in combustion chamber  20  sufficient to allow heater  18  to be no more than a natural aspirated burner introduced into the air stream to reheat the gases and combust any fumes collected, thereby eliminating the need for an expensive pressure burner. The manifolds  22  are spaced to provide ample room therebetween for spent gases to be recollected and directed by fan  26  back to heater  18  without undue restriction. 
     It has been discovered that the temperature of the road surface  16  is substantially directly proportional to the temperature of the gases leaving the road surface and being recirculated back to heater  18 . Accordingly, heating apparatus  10  further comprises a temperature sensor  30  which measures the temperature of the gases returning to the heater  18 , and adjusts heater  18  accordingly so as to provide impinging hot gases of a temperature suitable to heat road surface  16  to an optimal temperature, Temperature sensor  30  can be a simple thermocouple or any other temperature measuring device. 
     In practice, when first exposed to the heating process, road surface  16  absorbs a great deal of energy and so the temperature of the returning gases is low. As the temperature of the road surface  16  rises, less energy is transferred and the temperature of the returning gases also rises. At a preset point, the operating level of heater  18  is reduced to reduce the temperature of the gases being directed at the road surface  16 , thereby preventing damage to the road surface  16 . 
     For example, heater  18  initially provides hot gases at a temperature not higher than 1000° F. (approximately 538° C.) to start heating road surface  16 . After a period of time, the temperature of road surface  16  will typically rise as high as 320° F. (160° C.) at which point heater  18  will provide gases at a temperature of only approximately 700° F. (371° C.). Theoretically, although uncommon, the temperature of road surface  16  may increase to as high as 350° F. (approximately 177° C.), at which point heater  18  will have been adjusted to provide hot gases at a temperature of only approximately 650° F. (343° C.). 
     Except for a minor amount of leakage of gases to the atmosphere from the enclosure  12  between the bottom peripheral wall  14  and road surface  16 , the hot gases are continually recirculated and reheated. The heater  18  can be readily operated with a fresh air intake just sufficient to effect combustion of fuel, so that the hot gases directed at the road surface  16  may be substantially free of oxygen, with there subsequently being no oxidation of the asphalt being softened. Further, any fumes produced by heating the asphalt in the road surface  16  will be directed to and incinerated by heater  18 . 
     FIG.  2  and FIG. 3 illustrate a manner in which the efficiency of the method and apparatus according to the invention can be improved even further. FIG. 2 illustrates how two heating apparatuses  10   a  and  10   b  (each identical to heating apparatus  10  described above) can be combined with a flanged reforming drum  32  and a rupturing drum  34  to efficiently heat and remove material from road surface  16 . As heating apparatus  10   a , reforming drum  32 , heating apparatus  10   b , and rupturing drum  34  are collectively moved in the direction of the arrow in FIG. 2 over road surface  16 , heating apparatus  10   a  causes that portion of road surface  16  affected by heating apparatus  10   a  to soften, after which the flanges of reforming drum  32  press grooves  36  into softened road surface  16  by compacting the softened material without rupturing it, resulting in road surface  16  having a dense and grooved, but unruptured, surface as shown in FIG.  3 . This serves to (1) increase the density of road surface  16 , causing rocks embedded in road surface  16  to move into contact with one another and thereby enhancing heat transfer from rock to rock; (2) press a depressed groove into road surface  16 , thereby allowing the jet of hot gases from heating apparatus  10   b  access to a deeper level of rocks; and (3) increase the surface area available to absorb heat from the hot gases from heating apparatus  10   b . Therefore, grooves  36  allow heat from heating apparatus  10   b  to penetrate deeper and faster over a larger surface area without exposing the oil in the material to more damage. As rocks and densely packed material transfer heat more efficiently than oil or loose materials, the use of reforming drum  32  improves the effective heating of road surface  16 . In other words, the rock component of road surface  16  has a much higher rate of thermal conductivity than the asphalt cement component, making it advantageous to apply heat to the existing polished road surface  16  rather than a ruptured, loose, oil covered surface as taught by the prior art. Only after road surface  16  is thoroughly and deeply heated and softened does rupturing drum  34  rupture the heated material so that it can be improved and perhaps pressed into a recycled asphaltic surface. 
     The method and apparatus according to the present invention can be used to advantage with any of the asphalt recycling processes described in the prior art, and are suitable for use both as a static process or a moving process utilizing one or more heating apparatus  10  in a modular fashion to improve performance and efficiency. 
     As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.