Aggregate cooling for hot weather concreting

Abstract

Systems and methods for cooling aggregate are described. Systems and methods may include a drum. The drum may include an aggregate inlet; a plurality of buckets arranged in a ring around a circumference of the drum, and a plurality of rings arranged along the length of the drum, wherein each of the plurality of buckets has openings that open into the interior of the drum; one or more radial donuts or radial webs within the interior of the drum; an aggregate outlet; and a cooling air supply.

Claims

1. A system for cooling aggregate, the system comprising: a drum comprising: an aggregate inlet; a plurality of buckets, in which holes are designed, arranged in a ring around a circumference of the drum, and a plurality of rings arranged along a length of the drum, wherein each of the plurality of buckets has openings that open into an interior of the drum; one or more radial donuts or radial webs within the interior of the drum; an aggregate outlet; a cooling air supply; and an external body that entirely encapsulates the plurality of buckets, wherein one or more buckets define respective second openings that expose an aggregation disposed in the one or more buckets, wherein the external body comprises a plurality of openings that are in direct communication with external atmospheric conditions, and wherein the second openings are in direct communication with the plurality of openings of the external body to provide direct external cooling of the aggregation through external walls of the buckets.

2. The system of claim 1, wherein the plurality of buckets is twelve buckets.

3. The system of claim 1, wherein the buckets have a depth of approximately 1 m.

4. The system of claim 1, wherein the buckets have an opening angle of approximately 45 degrees, and start at an end of a radial donut.

5. The system of claim 1, wherein the predetermined width is approximately 0.6 m.

6. The system of claim 1, wherein the cooling air supply is counter current to the direction of travel of the aggregate through the drum.

7. The system of claim 1, wherein the drum is rotated to move the aggregate along a length of the bucket which contains holes, between the plurality of buckets and along the length of the drum.

8. A method for cooling aggregate, the method comprising: supplying hot aggregate to an end of a drum, wherein the drum comprises an aggregate inlet; a plurality of buckets in which holes are designed, arranged in rings around a circumference of the drum, and a plurality of rings arranged along a length of the drum, wherein the plurality of buckets has openings that open into an interior of the drum; one or more radial donuts or radial webs within the interior of the drum; and an aggregate outlet; rotating the drum to cause movement of the aggregate along a length of the bucket which contains holes, between the plurality of buckets and along the length of the drum; and supplying cooling air to the interior of the drum, wherein the drum further comprises an external body that entire encapsulates the plurality of buckets, wherein one or more buckets define respective second openings that expose an aggregation disposed in the one or more buckets, wherein the external body comprises a plurality of openings that are in direct communication with external atmospheric conditions, and wherein the second openings are in direct communication with the plurality of openings of the external body to provide direct external cooling of the aggregation through external walls of the buckets.

9. The method of claim 8, wherein the radial donuts move the aggregate along the length of the drum.

10. The method of claim 8, wherein the cooling air is supplied in a counter current to the movement of the aggregate along the length of the drum.

11. The method of claim 8, further comprising removing cooled aggregate from the aggregate outlet.

12. The method of claim 8, wherein the plurality of buckets is twelve buckets.

13. The method of claim 8, wherein the buckets have a depth of approximately 1 m.

14. The method of claim 8, wherein the buckets have an opening angle of approximately 45 degrees, and start at an end of a radial donut.

15. The method of claim 8, wherein the predetermined width is approximately 0.6 m.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate preferred embodiments of the invention and together with the detailed description serve to explain the principles of the invention. In the drawings:

(2) FIG. 1 shows an effect of aggregate temperature on 28-day concrete compressive strength.

(3) FIG. 2 shows a cross section view of a drum according to an exemplary embodiment.

(4) FIG. 3 shows a perspective, partial cross section view of the drum of FIG. 2 according to an exemplary embodiment.

(5) FIG. 4 shows an exemplary cross section of a drum showing a model and boundary conditions of the empty and filled buckets according to an exemplary embodiment.

(6) FIG. 5 shows a schematic of initial conditions calculation mechanisms for drum buckets after one third of the drum revolution according to an exemplary embodiment.

(7) FIG. 6 shows a schematic of initial conditions calculation mechanisms for drum compartments after one complete revolution of the drum according to an exemplary embodiment.

(8) FIG. 7 shows a graph of average steady state temperature of the aggregates at the drum exit according to an exemplary embodiment.

(9) FIG. 8 shows a graph of aggregate steady state temperatures through drum buckets according to an exemplary embodiment.

(10) FIG. 9 shows a graph of aggregate average temperature at the exit of the cooling drum at different cooling rates according to an exemplary embodiment.

(11) FIG. 10 shows a schematic of aggregate steady state temperature distributions in Celsius through drum buckets in the closed drum body design according to an exemplary embodiment.

(12) FIG. 11 shows a schematic of aggregate steady state temperature distributions in Celsius through drum buckets in the open drum body design according to an exemplary embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(13) Systems and methods are described for using various tools and procedures for aggregate cooling. In certain embodiments, the tools and procedures may be used in conjunction with regulating temperature of materials. The examples described herein relate to aggregate cooling for illustrative purposes only. The systems and methods described herein may be used for many different industries and purposes, including concrete, drilling, and/or other industries completely. In particular, the systems and methods may be used for any industry or purpose where cooling of solid or other materials is needed.

(14) Cooling System

(15) Systems may include a drum for cooling aggregate. The drum may be made of sheets that form various buckets. The sheets may be steel, alloys or other similar materials. In certain embodiments, there may be twelve buckets, but other numbers and configurations may be possible. Each bucket may have a predetermined depth. In certain embodiments, the buckets may have a depth of approximately 1 m. In certain embodiments, the depths of the buckets may be the same or may vary. Each of the buckets may have an opening angle of approximately 45 degrees that may start at an end of a radial donut that has a predetermined width. Other opening angles may be used. The predetermined width may be approximately 0.6 m. In certain embodiments, the empty drum may have a repeated pattern.

(16) FIGS. 2 and 3 show a cross section of an exemplary cooling drum. Radial donuts (rings) and axial webs may be welded inside the drum in such a way to facilitate slow axial movement of the aggregate as the drum rotates. Radial donuts (rings) are steel rings of various thickness, such as for example, steel rings with thickness of 2-5 mm. In some alternatives, the dimension of the Radial donuts (rings) may depend on the drum size. In some alternatives, the external diameter of the Radial donuts (rings) may be the same as that of the drum, which ranges from approximately 3 to approximately 4.5 m. In some alternatives, the internal diameter of these Radial donuts (rings) may range from approximately 1 to approximately 1.5 m. In some alternatives, these Radial donuts (rings) may be evenly distributed throughout the drum length, for example, with a distance of approximately 1 to approximately 1.5 m between each pair of consecutive Radial donuts (rings). Axial webs are steel strips that may be the same thickness as the Radial donuts (rings). For example, these Axial webs may be 12, 16, or 20 Axial webs evenly distributed circumferentially along the whole drum length. Openings in the external body of the drum may enlarge the cold surface areas in-contact with aggregates and also enhance the cooling of the internal radial donuts.

(17) In certain embodiments, a system for cooling aggregate may include a drum. The drum may include one or more aggregate inlets to allow access to the interior of the drum. The drum may include a plurality of buckets arranged around the circumference of the drum in a ring. There may be one or more rings of buckets along the length of the drum. In certain embodiments, each ring of buckets may include various numbers of buckets. In certain embodiments, each ring may include 12 buckets. Each of the plurality of buckets may have an opening that opens into the interior of the drum. Each bucket may have a depth of between approximately 75 cm and approximately 150 cm. In certain embodiments, the depth of each bucket may be approximately 1 m. Each of the buckets may have a width of between approximately 100 cm and approximately 200 cm. In certain embodiments, the width of the buckets may be approximately 1 m. Each of the buckets may have an opening angle of between approximately 18 degrees and approximately 30 degrees. In certain embodiments, the opening angle may be approximately 30 degrees. Dimensions may be the same or different for each of the plurality of buckets. The opening angle may start at the end of a radial donut. Radial donuts may spiral along the interior of the drum to provide for movement of the aggregate along the length of the drum.

(18) The drum may also include an external body. The external body may include a plurality of openings to provide for air flow. This air flow may provide for external cooling of the aggregate via the walls of the buckets.

(19) A cooling air supply may provide cooling air to the interior of the drum. The cooling air supply may provide air via jets or nozzles. Cooling air flow rates may be between approximately 750 m.sup.3/hr and approximately 20,000 m.sup.3/hr.

(20) Cooling Method

(21) Certain methods may provide methods for cooling aggregate. The methods may utilize cooling systems as described herein. In certain embodiments, hot aggregate may be supplied to an end of a drum. The hot aggregate may be supplied at a rate of between approximately 1 ton/hr. and approximately 200 ton/hr. Aggregate may be any suitable material. In certain embodiments, the aggregate particle size may be larger than 5 mm. In certain embodiments, the aggregate patch must be dry.

(22) The drum may be rotated at speeds from approximately 1 rpm to approximately 40 rpm. The drum may be rotated around the axis running the length of the approximate center of the drum. Rotation of the drum may cause movement of the aggregate between the plurality of buckets and along the length of the drum. Residence time in the drum may be approximately 15 minutes to approximately 1 hr.

(23) While the aggregate progresses through the drum, cooling air may be supplied to the interior of the drum. In certain embodiments, the cooling air may be supplied in a direction counter current to the movement of the aggregate through the drum. The radial donuts may move the aggregate along the length of the drum. The rotation of the drum may also move the aggregate from one or more buckets to one or more adjacent buckets, where adjacent bucketsare next to one another in the radial and axial direction. Cooled aggregate may be removed from the drum via the aggregate outlet.

(24) Heat transfer may include thermal contact resistance between cooling system components, random mixing between the hot and cold aggregates, cooling of the rubber belt or of the drum steel body before returning back to be filled with aggregates. The thermal properties of the aggregates bed, rubber belt, aluminum fins, and drum steel body are shown in Table 1. The thermal properties of the aggregates bed are assumed based on 20% porosity.

(25) TABLE-US-00001 TABLE 1 Thermal Specific Density Conductivity K heat Cp (kg/m.sup.3) (W/m .Math. C.) (J/kg .Math. C.) Air 1.2 0.025 1012 Aggregates Bed 1800 0.5 835 Rubber 1522 0.16 2010 Aluminum 2700 273 876 Steel 7800 75 460

(26) In certain embodiments, a model may be developed where the drum may be assumed to have four buckets filled with 0.6 m aggregate height all the time. The rest of buckets may be assumed empty. After one third of a drum revolution the four buckets are assumed to fully pour the aggregate into the neighboring four buckets. Three buckets are modeled as shown in FIG. 4. Bucket I is filled with aggregates representing sector I in the drum. The other two buckets II, and III, are empty representing sectors II and III, respectively. The donuts of the drum are assumed to have a pitch of 1 m. This means that for each drum revolution, the aggregates moves forward one meter.

(27) The first law of thermodynamics states that thermal energy is conserved. Specializing this to a differential control volume gives the three-dimensional conduction equation that may be written in the form:

(28) $\begin{array}{cc}c\frac{T}{t}=-.Math.q+q^B& \left(2\right)\end{array}$

(29) The above equation may be written in the following expanded form for 2-D approximation:

(30) $\begin{array}{cc}c\frac{T}{t}=\left[\frac{}{x}\left(k_x\frac{T}{x}\right)+\frac{}{y}\left(k_y\frac{T}{y}\right)\right]+q^B& \left(3\right)\end{array}$

(31) where kx, ky are the thermal conductivities in the x- and y-directions, respectively (W/m C.), T is the temperature ( C.), q.sup.B is the internal rate of heat generation per unit volume (W/m.sup.3), is the material density (kg/m.sup.3), c is the specific heat of the material (J/kg. C.), and t is time (sec).

(32) The general boundary conditions that may be applied to Equation (2) may take one of the following forms:

(33) No Heat Flow (Adiabatic or Natural Boundary):

(34) This boundary condition is called the adiabatic or natural boundary condition, and may be expressed as:

(35) $\begin{array}{cc}\frac{T}{n}=0,\mathrm{on}\mathrm{surface}{S}_i& \left(4\right)\end{array}$
Convection Heat Exchange:

(36) When there is a convective heat transfer on a part on the body surface, Si, due to contact with a fluid medium, it can be written as:

(37) $\begin{array}{cc}-k_n\frac{T}{n}=h\left(T_s-T_f\right),\mathrm{on}\mathrm{surface}{S}_i& \left(5\right)\end{array}$

(38) where h is the convection heat transfer coefficient, which may be temperature dependent (nonlinear), T.sub.s is the surface temperature on S.sub.i and T.sub.f is the fluid temperature, which may be constant or a function of boundary coordinate and/or time and k.sub.n is the thermal conductivity in direction n.

(39) Prescribed Temperature (Dirichlet BC):

(40) The temperature may be prescribed on a specific boundary of the body. The prescribed temperature may be constant or a function of boundary coordinate and/or time:
T=T.sub.S(x,y,z,t), on surface S.sub.i(6)

(41) FIG. 4 shows the repeated pattern of the cooling drum. Both models are bounded by symmetry planes S.sub.2 and S.sub.3. The boundary conditions applied to both simulations, belt conveyor and cooling drum, are defined for the five surfaces S.sub.1, S.sub.2, S.sub.3, S.sub.4, and S.sub.5 as follows:

(42) $\begin{array}{cc}{K\frac{T}{y}.Math.}_{S_1}=h_{S_1}\left(T_{S_1}-T_{}\right)& \left(7\right)\\ {K\frac{T}{n}.Math.}_{S_4}=h_{S_4}\left(T_{S_4}-T_{}\right)& \left(8\right)\\ {K\frac{T}{x}.Math.}_{S_{2,3}}=0\mathrm{Due}\mathrm{to}\mathrm{Symmetry}& \left(9\right)\\ {K\frac{T}{n}.Math.}_{S_5}=h_{S_5}\left(T_{S_5}-T_{}\right)& \left(10\right)\end{array}$

(43) In belt conveyor all surfaces, S.sub.1, S.sub.4 and S.sub.5 are in good cooling conditions, so all heat transfer coefficients at these surfaces are assumed 40 W/m.sup.2 C. In a cooling drum, internal surfaces of the drum are in worse cooling condition than external surfaces. So, h.sub.S1 is assumed 20 W/m.sup.2 C. and h.sub.S4 is assumed 40 W/m.sup.2 C. In a cooling drum, two designs are modeled and compared; opened and closed external bodies. In opened external body, the internal and external surfaces of the drum body will be cooled with air jets. Consequently, surface S.sub.5 is loaded with convection and h.sub.S5 is assumed 40 W/m.sup.2 C. In closed external body, only internal surfaces of the drum body may be cooled with air jets, so surface S.sub.5 is assumed adiabatic.

(44) Following the normal finite element discretization and assembly procedures for Equation (2), the model ends up with the global finite element equations in the following form:
[C].Math.{{dot over (T)}}+[[K.sub.c]+[K.sub.h]].Math.{T}={Q}.sup.s={Q}.sup.h(11)
where [C] is the thermal capacity matrix given by
[C]=.sub.Tc[N].sup.T[N]dV;(11a)
[K.sub.c] is the thermal conductivity matrix given by
[K.sub.c]=.sub.T[B].sup.T[K][B]dV;(11b)
[K.sub.c] is the additional thermal conductivity matrix due to convection B.C. given by:
[K.sub.h]=.sub.Sh[N.sup.s].sup.T[N.sup.s]dS;(11c)
[K.sub.h] is the additional thermal conductivity matrix due to convection B.C. given by:
[K.sub.h]=.sup.Sh[N.sup.s].sup.T[N.sup.s]dS;(11d)
{Q}.sup.s is the heat flux vector due to input surface flux B.C. given by
{Q.sup.s}=.sub.Sq.sup.s[N.sup.s].sup.TdS;(11e)
and, finally, {Q}.sup.h is the heat flux vector due to convection B.C. given by
{Q}.sup.h=.sub.ShT.sub.c.sub.c[N.sup.s].sup.TdS;(11f)

(45) It should be noted that Equation (11) does not involve radiation boundary conditions and internal heat generation which are not a factor in the current simulation. Time discretization of Equation (11) using the a-method results in the following final equation to be solved for the linear transient simulation:

(46) $\begin{array}{cc}\left[K^c+K^h+\frac{1}{t}C\right]T={}^{l+t}\left(Q^S+Q^h\right)-\left[K^c+K^h\right]{}^{t}T& \left(12\right)\end{array}$

(47) where a is a constant which is between 0 and 1 depending on the solution method used. The following methods are used frequently:

(48) =0 explicit Euler forward method.

(49) = implicit trapezoidal rule.

(50) =1 implicit Euler backward method.

(51) The current analysis is solved using the implicit Euler backward method.

(52) Modeling of the Cooling Drum

(53) By referring to FIG. 5, three repeated half buckets are modeled for approximating the cooling drum; buckets I, II, and III. The model assumes four buckets are always filled with aggregates and the rest eight buckets are empty. Bucket I approximates the filled buckets, while bucket II, and III approximate the rest empty buckets. Each one full rotation is divided to three load steps. For load step i, temperatures of buckets I, II, and III nodes, are obtained from the heat transfer analysis for one third of the time of one full revolution. Then, the initial condition of load step i+1 are obtained as follows; temperatures of the drum body of the upper bucket (I) are transferred to the middle bucket (II), those of the middle bucket (II) are transferred to the lower bucket (III) and those of the lower bucket (III) are transferred back to the upper bucket (I). The temperatures of the aggregate contents are reapplied and distributed to random locations within the aggregate to simulate the mixing process. For each drum revolution this mechanism is repeated three times namely; i, i+1, and i+2 load steps.

(54) Along the length of the drum, a total of ten compartments are modeled similar to the above model, FIG. 6. After one full revolution, the calculated temperatures of the aggregate of each compartment are moved forward to the next compartment with random allocations and the temperature of the aggregate in the first compartment is initialized again to 50 C. simulating a new aggregate fed to the drum. Finally the resulted temperatures of the aggregate of the last compartment are averaged to report the final output temperature T.sub.out, FIG. 6.

(55) Cooling Drum

(56) FIG. 7 shows the aggregate temperatures at the exit of the cooling drum for both configurations. In this exemplary embodiment, the first patch leaves the drum after 10 minutes, for 1 rpm drum rotational speed. There is no difference between the opened and closed drum designs at the early time of the simulation. This may be attributed to the fact that the drum body temperature is initially set at 15 C. and that temperature is increased and reach a steady state value after enough time of contact with the hot aggregate. It is noted that by the time the drum body gains heat reaching steady state, the difference in results is quite significant and reaches up to 5 C. This difference also increases along the length of the drum. The average temperature of the aggregates at each bucket within the drum length is shown in FIG. 8. If the aggregate stays more time in the drum, these differences will be more significant. Most manufacturers would place a maximum time stay limit of approximately 30 minutes in the drum. The gained temperature due to the drum body is also more significant as the aggregate moves towards the exit of the drum. Drum rotation of 1 rpm reduce the temperature of 90 tons per hour. For higher rate production, the final temperature will be bit higher. FIG. 9 shows the drum cooling performance at rotation of 3 rpm which serves production rate of 270 tons of aggregates per hour. The temperature distributions of the aggregate within the drum buckets for the opened and closed drum designs are compared in FIGS. 10 and 11.

(57) Cooling concrete aggregates is a crucial factor in hot weather regions to retain the concrete strength. Existing cooling methods in the literature are not optimized for power and cooling time minimization. Most of the existing designs perform some mixing of the aggregates in a cold environment. These designs do not utilize the full advantages of proper mixing, free falling of aggregates, extended belt surface area for heat convection with belt conveyors, and extended metal surface area for heat convection with cooling drums.

(58) Certain embodiments utilize cooling system configurations as described herein for hot weather concreting. In certain embodiments these configurations may include a belt conveyor for small production rates, such as, but not limited to, approximately 3-15 tons/hr, and rotating drums for high production rates, such as, but not limited to, approximately 90-270 tons/hr. Modeling has shown the significant impact of the mixing process on the cooling efficiency. The simulation results showed the importance of enhancing the cooling conditions of the drum body by cooling the drum body as well as cooling the hot aggregate to get lower aggregate output temperature with reduced power.

(59) Although the foregoing description is directed to the preferred embodiments of the invention, it is noted that other variations and modifications will be apparent to those skilled in the art, and may be made without departing from the spirit or scope of the invention. Moreover, features described in connection with one embodiment of the invention may be used in conjunction with other embodiments, even if not explicitly stated above.