OPERATION OPTIMIZATION OF HOLDING FURNACES IN SPECIAL CASTING SHOPS

The object of research is the melting and casting system of special casting shops. The process is considered on the basis of simulation modeling of the requirements of chill or melts pressure casting machines and the capabilities of the melting and holding furnaces to meet this need. The modeling is carried out on the basis of typical solutions for the used brands of furnaces for the manufacture of aluminum alloys in non-ferrous casting shops of a machine-building enterprise specializing in the manufacture of casting in metal molds for engineering products. As a result of simulation modeling, a description is obtained of the influence of the intensity of applications on the melt on the part of chill machines or injection molding machines, and the time taken to complete these applications for the total costs of the implementation of the technological process. It is proposed to determine the total costs as the sum of the costs associated with the consumption of technological electricity, and the costs associated with the likely downtime of machines due to lack of metal. Moreover, the second component reflects the requirement for the performance of machines in terms of their actual operation. Therefore, the total cost of the process of chill casting or casting under pressure in terms of the consistency of the elements of the melting and casting system is chosen as the criterion for optimizing its operation. It is proposed to consider the melting and casting system as a queuing system (QS) with failures. Such a presentation is the most stringent in terms of requirements for ensuring a given performance. Using the study of the response surface, the optimization problem is solved according to the consistency of the intensity of requests for the melt and the time of their execution, which minimizes the total costs of the implementation of the technological process. Local optimal technological solutions are obtained that enable technologists to choose the most rational decisions for conducting a melting campaign using transfer furnaces with a capacity of 0.16–0.25 tons. Such solutions will provide the possibility of reducing the cost of manufacturing aluminum casting.


Introduction
In the conditions of mass production of special casting types in shops with chill machines or injection casting machines, it is necessary to ensure the specified perfor mance. However, it is difficult to organize the process in such a way as to harmonize the quality requirements of the resulting castings. If the decisions on the organiza tion of the process are wrong, there may be additional costs for the process. For example, the overrun of energy carriers or material resources is possible. Therefore, it is important to develop solutions in the field of control of TECHNOLOGY AUDIT AND PRODUCTION RESERVES -№ 6/1(44), 2018 ISSN 2226-3780 melting and casting systems in such a way as to ensure consistency between different management quality crite ria -performance and energy costs.

The object of research and its technological audit
The object of research is the melting and casting sys tem of special casting shops. This system includes mel ting furnaces, holding furnaces, diecasting machines or die casting machines. Technological audit may consist in the timing of the technological process, which is needed to determine two main parameters -the intensity of applica tions for the melt (λ, t/h) and the average service time of the application (μ = Τ -1 , h -1 ). In the absence of real data, simulation modeling is possible, allowing for typical performance indicators of special casting shops to obtain estimated performance characteristics of the melting and casting systems. This approach is justified, since the technical characteristics of the furnaces used are as well known as the technological capabilities of the used typical equipment.

The aim and objectives of research
The aim of research is determination of the optimal functioning parameters of the melting and casting system for special casting shops equipped with chill machines or injection casting machines.
To achieve this aim it is necessary to solve the fol lowing objectives: 1. To determine the ranges of typical technical charac teristics of the equipment for melting and casting systems.
2. To select optimization criteria. 3. To conduct simulation modeling of the functioning of melting and holding furnaces.

Research of existing solutions of the problem
Since the melting and casting systems must provide the machine with highquality alloy, special attention is paid to the technological issues of alloy production. They, in turn, relate either to a targeted impact on the processes of structure formation [1, 2], or to a complex effect on the melt in the process of its solidification [3,4]. However, issues related to the effect of the time of the proposed technological operations on the probability of equipment downtime are not considered. Obviously, this question is connected with the possibility of choosing a rational working space and capacity of the furnaces. In this part, it is necessary to talk about the possibility of creating new design solutions for furnaces, which are of fered by the world's leading manufacturers. In particular, BottaEngineeringSrl (Italy) electric furnaces are proposed in [5], in which the melting chamber is a monolith of special refractory concrete resistant to aluminum. The issue of energy saving is solved by highquality thermal insulation of the working space of the furnace. The furnace temperature is controlled automatically by two thermo couples, one of which is located in the pocket for metal sampling, and the other, which performs a safety function, in the furnace vault. In order to optimize the operating temperature, combustion control can be carried out using an onoff, minimummaximum method or with modulation. FometSrl (Italy) offers CR induction crucible furnaces operating at an industrial frequency for melting, holding and processing any nonferrous metals [6]. Highperfor mance melting furnaces for melting and storing molten aluminum, furnaces for injection molding machines, as well as large bath furnaces with electromagnetic or mechani cal molten aluminum circulation pumps, are offered by NovacEngineeringSrl (Italy) [7]. Analysis of these pro posed melting equipment solutions allows to see main trend: an attempt to automate and universal design and technological solutions. Such solutions are considered as an alternative to traditional approaches, which are cha racterized by a lack of a weak degree of automation and integration with other technological systems of the shop. The direction of development is seen in the combination of technological operations [8,9]. For example, it should be noted the revolutionary model of the company CIME CrescenziInductionMeltingSrl (Italy). CAP (CorelessAuto Pour) is a crucible furnace that can perform automatic casting under pressure in combination with any automatic molding line. In this furnace, an elliptical coil is used to improve the heating of the channels and increase the overall energy efficiency. As a result, in the SAR furnace channels are in the magnetic field of the induction coil and are heated constantly. Constant and uniform heating makes it possible to begin casting at any time, since metal hardening or slag sticking are excluded. Maintaining a con stant temperature reduces overheating to a minimum and facilitates selfcleaning of the furnace. This helps to in crease the time between the replacement of the lining. One of the features of the furnace is that in the absence of a metal ballast bath, the automatic melting furnace can be completely deenergized on weekends, after which work can be resumed with a semiliquid or solid charge [8].
Optimal control of the metallurgical process is a fea ture of the solutions proposed in [9]. In particular, the DuoMelt system allows for a smooth distribution of the power of the frequency converter between two furnaces operating in series. This leads to the possibility of full utilization of 100 % of the rated power constantly, to shorter downtimes, and therefore to an increase in melting per formance. Among the advantages of such solutions should be noted the possibility of simultaneous melting, holding and casting, as well as maximum flexibility and elimina tion of pauses when switching modes. Control using the DuoControl system implies computer integration. This ensures shorter downtimes, which results in higher melting rates. This is made possible by simultaneous melting, holding and casting.
The continuous monitoring and automatic control for all required functions and technological operations of the furnace during the melting cycle is provided by the JOKS melting processor. This processor controls the exchange of data and information with higherlevel control systems and provides logging and evaluation of operational data. Such solutions can be recognized as successful, however, the models and the IT solutions implemented on them in terms of automation and computer integration are not disclosed by manufacturers. Obviously, simulating the opera tion of the melting and casting system, it is necessary to take into account that the furnaces are energytechnology and heat engineering complexes, regardless of the method of energy supply [10,11]. Their thermal performance and energy capabilities can have a decisive influence on the ISSN 2226-3780 melting quality and the ability to provide consumers with a given amount of melt with the required properties.

Methods of research
As a research method, a mathematical apparatus is chosen that describes the operation of queuing systems, adapted to simulate the operation of melting systems in casting shops [12]. In particular, the most unfavorable from the point of view of evaluating the capabilities of the description system is considered -its representation by a queuing system with failures (QS). In accordance with this, analytical and economic performance criteria are calculated [13].
To evaluate the analytical criteria, the following pa rameters are calculated: -the initial probability of the state of the system, Р 0 ; -probability of failure in the service application, Р f ; -intensity of the flow of lost applications, Q l.a ; -probability that the application will be served, q; -intensity of the flow of served applications, Q s.a ; -average number of busy channels, m k ; -system load factor, Ψ.
where n -the number of channels in the QS service node.
where k -the number of the channel that serves the request (k = 1, 2, 3, …, n).
where Ψ -the system load factor (analogue efficiency).
To assess the economic criterion, the total cost of operating the system is calculated: where С 1 -the value of costs associated with system downtime; С 2 -the amount of costs as sociated with operating the system. Criterion (8) is minimized with respect to n, i. e., a value of n* must be found which turns criterion (8)

Research results
The results of the ridge analysis, which are final in solving the problem, are shown in Fig. 1, 2.
From Fig. 1, 2 it can be seen that in the area of restrictions imposed by the plan of the experiment, the optimal solutions are on the ridge lines I and IV. In this case, the first parametric equation of system (9) can be used to select pairs (λ-μ) that satisfy the optimal solu tions y* = n*.

SWOT analysis of research results
Strengths. The strength of this research is the ability to determine the optimal load of furnaces by a compro mise criterion of minimizing total costs for downtime and energy consumption. The resulting solutions are analytical and allow to perform calculations with the actual perfor mance of the melting and casting systems in the casting shop. This opens up prospects for reducing the cost of production.
Weaknesses. Weaknesses of this research are related to the fact that the obtained solutions are acceptable only within the considered range of values of the input vari ables. If the intensity of applications for the melt and the average time of their service are outside the limits of this area, the results will differ from received. Using the obtained optimal solutions without taking this circumstance into account may lead to incorrect conclusions regarding the expedient loading of the furnaces.
Opportunities. Additional opportunities when using the above results in an industrial environment are associated with the rationalization of the organization of the melting campaign. The organizational and technical solutions adop ted at the same time can contribute to the improvement of the performance of the melting and casting systems of the casting shop.
Threats. Obvious risks when using the obtained results are associated with the need to make changes to the mana gement system of the casting shop. And it is mandatory to adapt theoretical solutions to the real performance of the melting and casting equipment and chill machines (injection molding machines). Any wrong decision in this case can lead to unnecessary costs.

Conclusions
1. It is established that typical solutions for melting equipment -melting and holding furnaces -in aluminum casting shops involve the use of furnaces for which the intensity of applications for the melt is (0.1-0.5) t/h, and the reciprocal of the average time service requests is in the range (0.03-0.07) min -1 .

2.
As an optimization criterion, it is necessary to choose a compromise criterion that is formed from the costs of process electricity and costs due to equipment downtime due to the absence of a melt in a given amount. These two components of the criterion are competitive in rela tion to each other.
3. Imitating modeling establishes optimal solutions for loading furnaces. They are determined depending on the intensity of applications for the melt from the side of the chill machines or injection molding machines, and the average execution time of these applications. It is shown that such optimal solutions can be written in the parametric form n* = j(r(λ)). This representation allows to calculate the optimal furnace load depending on the restrictions imposed by the ranges of input variables. Such variables are the operating parameters of the melting and casting systems: the intensity of applications for the melt and the average time of their service, depending on the technical characteristics of the used furnaces.