Patent Application
20160279826
The present invention relates to systems and methods for concreting, and, more specifically, to systems and methods for cooling aggregate or other materials.
Ready-mixed concrete manufacturers, in hot weather regions are faced with drops in compressive strengths of concrete produced in summer, as shown in
The weather conditions of many regions of the world are associated with hot weather for six to eight months per year. Northern and southern Africa, Arab peninsula, Southern Asia, Southern part of North America, Middle regions of Latin America, and northern Australia are the hottest regions in the world. The weather fluctuates between summer temperatures that approach 50° C. and winter temperatures that sink to 18° C. Relative humidity follows a similar pattern ranging between 5% and 90% from inland to coastal regions, respectively. Ready-mixed concrete manufacturers have to accommodate these extreme highs and lows in climate fluctuation.
Aggregate temperature plays a very important role in defining the concrete mix temperature. It has been shown that keeping the aggregate temperature about 10° C.-15° C. is adequate in achieving proper results. Cooling the concrete aggregate is one of the most effective methods to reduce the concrete mix temperature. Different methods have been developed for that purpose. In these methods cooling is obtained using; chilled-water, ice, chilled-air, or liquid Nitrogen. The key factor in choosing the proper method is the most economic technique without degrading the cement properties. The optimization analysis results will depend significantly on the amount of aggregates to be cooled down and the required temperature drop.
Cooling using liquid Nitrogen has many virtues, however, there has not been a great deal of testing. There is a lot more industry needs to know about how liquid nitrogen affects cement hydration products, concrete set, and concrete production equipment as well. Furthermore, there are safety issues that need to be addressed more fully. Chilled water is commonly used in reducing the aggregates temperature in hot weather. Cooling 600 m3 of aggregates at 7° C. requires about 300 m3 of chilled water storage which costs about $20,000. Ready mix concrete industries in hot regions, however, may require a temperature drop of about 35° C. for much bigger amount of aggregates. In such case although cooling by chilled water is an effective method it will be highly costly and not feasible to achieve such target. Furthermore, the concrete aggregates are required to be mostly dry before mixing to achieve lipophilic (oil-loving) surface for good bonding between the cement and the aggregate during mixing. Flake ice could be added to the mixing drum as a direct substitute for batched water on a pound-for-pound basis. It is reported that an ice making plant that delivers 10 tons of ice per day would cost about $300,000. Thus, medium production rate plants that require around 100 tons/hr of concrete will require a huge investment to accommodate for a proper ice maker plant. Chilled air is a preferred candidate, although this requires huge flow rates and extensive cooling systems. It is explicitly reported that chilled-air cooling is an economical option when large volumes of aggregates must be cooled with significant large temperature difference.
The above mentioned cooling methods are utilized through different cooling equipment. Among those are belt conveyors, cooling drums, chilled storage rooms, and a mix of those methods. Most of this equipment is meant for cooling and drying the foundry sand. It could be adopted, however, for sand and coarse aggregates cooling. The cooling drums are designed on the basis of well mixing the aggregates using radial webs. The cooling process in the existing drums in the market is based on mixing the aggregates to enlarge the contact area between the aggregates and the cooling air. Research shows that drying drums are very compact, efficient, and provide a high production rate. Cooling drums entail, the disadvantages of being quite expensive, consume high mechanical power, and are difficult to maintain. On the other hand, cooling using air jets over belt conveyors may be the cheapest and simplest method. However, the concrete industry reports long cooling time, low cooling efficiency, large occupied space and a low production rate. A combination of both methods is generally recommended. Three-stage cooling of return sand is effective and efficient when flash cooling and premixing are accomplished on a belt conveyor and final cooling is performed in a rotary sand blending, cooling, screening drum.
Convection to the cooling air is the main heat transfer theme in these previous designs. Cooling through the thermal contact between the aggregates and the cold belt conveyor or drum body as well as the mixing process was neither analyzed nor optimized. The heat flow during the cooling process, either by belt conveyors or drums, needs to be analyzed and optimized to achieve short and optimum cooling time with low cooling power. The main objectives of the current work are to propose optimized designs for belt conveyors system and drum cooling system to be used in cooling the concrete coarse and fine aggregates and to present a numerical simulation for the cooling process using the finite element method with the objective of optimizing the overall system performance.
Needs exist for improved systems and methods for cooling aggregates.
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:
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.
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.
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.
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.
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 m3/hr and approximately 20,000 m3/hr.
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.
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.
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 buckets-are next to one another in the radial and axial direction. Cooled aggregate may be removed from the drum via the aggregate outlet.
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.
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
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:
The above equation may be written in the following expanded form for 2-D approximation:
where kx, ky are the thermal conductivities in the x- and y-directions, respectively (W/m° C.), T is the temperature (° C.), qB is the internal rate of heat generation per unit volume (W/m3), p is the material density (kg/m3), c is the specific heat of the material (J/kg. ° C.), and t is time (sec).
The general boundary conditions that may be applied to Equation (2) may take one of the following forms:
This boundary condition is called the adiabatic or natural boundary condition, and may be expressed as:
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:
where h is the convection heat transfer coefficient, which may be temperature dependent (nonlinear), Ts is the surface temperature on Si and Tf is the fluid temperature, which may be constant or a function of boundary coordinate and/or time and kn is the thermal conductivity in direction n.
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
S(x,y,z,t),on surface Si (6)
In belt conveyor all surfaces, S1, S4 and S5 are in good cooling conditions, so all heat transfer coefficients at these surfaces are assumed 40 W/m2 ° C. In a cooling drum, internal surfaces of the drum are in worse cooling condition than external surfaces. So, hS1 is assumed 20 W/m2 ° C. and hS4 is assumed 40 W/m2 ° 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 S5 is loaded with convection and hS5 is assumed 40 W/m2 ° C. In closed external body, only internal surfaces of the drum body may be cooled with air jets, so surface S5 is assumed adiabatic.
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]·{
where [C] is the thermal capacity matrix given by:
[C]=∫Vρc[N]T[N]dV; (11a)
[Kh] is the thermal conductivity matrix given by:
[Kc]=∫V[B]T[K][B]dV; (11b)
[Kh] is the additional thermal conductivity matrix due to convection B.C. given by.
[Kh]=∫Sh[Ns]T[Ns]dS; (11c)
[Kh] is the additional thermal conductivity matrix due to convection B.C. given by:
[Kh]=∫Sh[Ns]T[Ns]dS; (11d)
{Q}S is the heat flux vector due to input surface flux B.C. given by:
{Q}s=∫Sqs[Ns]TdS; (11e)
and finally, {Q}h is the heat flux vector due to convection B.C. given by:
{Q}h=∫ShT2
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:
where a is a constant which is between 0 and 1 depending on the solution method used. The following methods are used frequently:
α=0 explicit Euler forward method.
α=1/2 implicit trapezoidal rule.
α=1 implicit Euler backward method.
The current analysis is solved using the implicit Euler backward method.
By referring to
Along the length of the drum, a total of ten compartments are modeled similar to the above model,
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.
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.
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.
This application is related to and claims the benefit and priority of U.S. Provisional Application No. 62/137,722 filed Mar. 24, 2015, the entirety of which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 62137722 | Mar 2015 | US |
