Note: Descriptions are shown in the official language in which they were submitted.
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METHOD OF AND APPARATUS FOR HEATING
A ROAD SURFACE FOR REPAYING
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
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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. Patent No. 5,653,552 issued to Wiley and U.S.
Patent 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. Patents No. 5,653,522 and 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
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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. Patent No. 4,561,800 issued to Hatakenaka et al. and
U.S. Patent 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 periph-
eral 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
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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.
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Accordingly, it is preferable to heat the asphalt surface sufficiently
prior to rupturing.
Summar~i 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
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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
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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
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surface can be uniformly and consistently heated to optimal tempera-
tures 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 fox spent gases to be recollected
and directed by fan 26 back to heater 18 without undue restriction.
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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
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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 sof
tened. 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 l0a and lOb (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 10a, reforming drum 32,
heating apparatus lOb, and rupturing drum 34 are collectively moved
in the direction of the arrow in FIG. 2 over road surface 16, heating
apparatus l0a causes that portion of road surface 16 affected by
heating apparatus l0a 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.
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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 lOb access to a deeper level of
rocks; and (3) increase the surface area available to absorb heat from
the hot gases from heating apparatus lOb. Therefore, grooves 36
allow heat from heating apparatus lOb 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 .
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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.
