Defining a procedure for predicting the duration of the approximately isothermal segments within the proposed drying regime as a function of the drying air parameters

One of the main disadvantages of the recently reported method, for setting up the drying regime based on the theory of moisture migration during drying, lies in a fact that it is based on a large number of isothermal experiments. In addition each isothermal experiment requires the use of different drying air parameters. The main goal of this paper was to find a way how to reduce the number of isothermal experiments without affecting the quality of the previously proposed calculation method. The first task was to define the lower and upper inputs as well as the output of the "black box" which will be used in the Box-Wilkinson's orthogonal multi-factorial experimental design. Three inputs (drying air temperature, humidity and velocity) were used within the experimental design. The output parameter of the model represents the time interval between any two chosen characteristic points presented on the Deff -t. The second task was to calculate the output parameter for each planed experiments. The final output of the model is the equation which can predict the time interval between any two chosen characteristic points as a function of the drying air parameters. This equation is valid for any value of the drying air parameters which are within the defined area designated with lower and upper limiting values

Comparison and evaluation of recently reported methods for optimization of industrial drying regimes

In our previous studies we have presented the calculation method along with the procedure for setting up the non isothermal drying regime. Even though this method is harmonized, with the theory of moisture migration during drying and can be used to predict the optimal industrial drying regime and proper drying air parameters, up till now it was not compared with other models. The main goal of this paper, was to compare and evaluate our model with the one reported by the German group of authors in 2008. The first task was to create criteria for model evaluation. Five parameters were chosen: non-existence of cracks, total drying time, twist coefficient, chamber coefficient and flexural strength. The second task was to create a software tool, for modeling the first and second drying section of green clay masonry element, using the instructions provided within the published articles. The third task was to apply German and our procedure on the same clay raw material. Results have shown the absence of cracks on dried and fired samples. In the case of German method total drying time, as well as twist and camber coefficients were higher while the physico - mechanical properties were lower. Presented results have additionally validated that our dying model can be used for the accurate prediction of industrial drying kinetics and a reliable estimation of moisture transport during drying

Non isothermal drying process optimisation - Drying of clay tiles

In our previous studies we have developed a model for determination of the variable effective diffusivity and identification of the exact transition points between possible drying mechanisms. The next goal was to develop a drying regime which could in advance characterize the real non isothermal process of drying clay tiles. In order to do this four isothermal experiments were recorded. Temperature and humidity were maintained at 350C / 75%; 450C / 70%; 450C / 60% and 500C / 60%; respectively in each experiment. All experimentally collected data were analyzed and the exact transition points between possible drying mechanisms were detected. Characteristic drying period (time) for each isothermal drying mechanism was also detected. The real, non-isothermal drying process was approximated by 5 segments. In each of these segments approximately isothermal drying condition were maintained. Temperature and humidity of the drying air, in the first four segments, was maintained on the same level as in recorded isothermal experiments while in the fifth segment, it were maintained at 700C / 40%. The duration of the first four segments were calculated from the diagrams Deff - t respectively for each experiment. The clay tile in experiment five was dried without cracking using the proposed non isothermal drying regime

Setting up the drying regimes based on the theory of moisture migration during drying

Drying is energy intensive process which has important effect on the quality of the clay tiles that are dried commercially. Chamber and tunnel dryers are constantly improving. Better technical equipment and operational strategies have lead to higher quality of the dried clay products. The moisture migration during isothermal drying process can be visually traced on the curve that represents the relationship between variable effective moisture diffusivity (MR) with time (t). Proposed non isothermal drying regimes were consisted from several isothermal segments. For the first time, the choice of isothermal segments specification and its duration was not specified by experience or by trial-and-error method. It was detected from the isothermal curves Deff - MR in accordance with the theory of moisture migration during drying. Proposed drying regimes were tested. Clay roofing tiles were dried without cracks. Dried clay roofing tiles has satisfied all requirements defined in EN 1304 norm related to the shape regularity and mechanical properties

Characterization of drying behavior and modeling of industrial drying process

The general method for industrial chamber dryer optimization was reported in this paper. The first step in finding the most suitable drying regime is to characterized the clay raw material, especially its water loss at 2000C and to determine the critical drying rate inside the specially constructed laboratory dryer. These data provides us information if the product or the dryer is the bottleneck for the optimisation. If the optimization is justified geometry of the dryer, air mass flows, temperature, and humidity profiles inside the dryer as well as initial water content in the drying material, initial temperature of the products and the load of the dryer are required. Some of the previously mentioned data are only used to check if the chamber dryer is working properly, while the others are used as the initial parameters necessary for software simulation. In this paper two models for calculating the optimal drying parameters were used. The first model was developed from the comprehensive theory of the moisture migration during isothermal drying. The calculation software for setting up the non - isothermal drying regimes was reported in our previous papers. It is important to say that this model was not able to adequately predict the temperature raise within the drying products. In order to simulate the raise of the temperature of the roofing tiles during drying the second model was used. This simple receding drying front model was firstly reported by Kitcher. If both models are used simultaneously it is possible to calculate air temperatures, product temperature, absolute and relative humidity of the drying air, moisture content of the product, drying rate etc ... It is important to mention that this approach can lead to the recommendations for changes inside the dryer before an optimized situation is achieved. One example of such situations is described in this paper in details

Method for avoiding cracks during drying of masonry units made of illite raw material

Drying is one of the most important steps in the production process of masonry units. In order to prevent the formation of cracks during drying information's about the moisture migration rate variability as well as the material strength variability through drying are necessary. The main goal of this paper was to find a solution how to prevent the crack formation at the beginning (during the first hour) of the drying for the drying sensitive illite raw material. The first step was to record a series of isothermal Deff - MR curves at different drying air temperatures and constant drying air velocity and humidity. As it was already reported all moisture transport mechanisms during isothermal drying are visible on those curves. Characteristic spots registered on these curves were then transposed on the experimentally registered figure material strength us moisture content. It was found that the material strength for the cracked masonry units at the beginning of drying was around 0.09 MPa and that the time of cracking was near the characteristic spot B. Registered material strength and the crack time position (spot B) has additionally confirmed that the drying sensitivity of the raw material are obviously related with the present clay mineral constituents structure and the initial moisture content of the green heavy clay units. Using the Deff values registered for each experiment in the spot B we were able to calculate the maximal moisture transport rate and consequently the proper drying air parameters which are safe and which will not initialize the formation of the cracks at the beginning of the drying

The lumped approach in drying modeling of roofing tiles – variable effective diffusivity determination

In this paper, the drying modeling of roofing tiles was done using a "lumped" approach. In other words, several forces that are influencing internal moisture transport are combined to create effective moisture diffusivity. For this reason, the effective diffusivity coefficient was added to Cranck's diffusion equation. In their earlier works, the authors published the solution to the diffusion equation, which assumes that effective diffusivity is constant. It was demonstrated that, particularly if shrinkage is not taken into account during the computation stage, the simulated drying curves differ from the experimental results. The next step was to ascertain the time-dependent effective diffusivity and to more precisely anticipate the drying kinetic. The general functional relationship between effective moisture diffusivity and Fourier number was first determined to fulfill this assignment. Experimental proof of the proposed model was provided. Less than 3% of the outcomes from the simulation and the experiment deviated from each other. This was a resounding affirmation that effective diffusivity is not constant during drying and that all internal transport mechanisms are observable in their timedependent relation

Uticaj fenomena vlažnog širenja na degradaciju i trajnost opeka u zidanim konstrukcijama

Fenomen vlažnog širenja je spontani i dugotrajni proces koji se odigrava odmah po izlasku opeka iz peći i koji traje godinama, odnosno sve vreme dok materijal fizički egzistira. U principu, proces povećanja zapremine pečenih proizvoda u početku se odigrava većom brzinom i sa tokom vremena sve više se usporava, da bi se po isteku više godina sveo na praktično zanemarljive vrednosti. Posledice odigravanja ovog štetnog fenomena su veoma velike i u slučaju sprečenog širenja manifestuju se kao pucanje i raspadanje opeka a u najtežim slučajevima dolazi i do oštećenja dela ili cele zidane konstrukcije

Procedure for assessment of the clays suitability towards fast drying

During the last 10 years it was shown that Deff – MR curve pattern is unique and very useful. This ability was firstly embedded in the procedure for setting up the fast drying regimes several years ago. This incredible pattern was used again in this invited paper. Actually, if the test samples formed from the same clay do not crack during drying in at least two isothermal experiments in which drying air temperature was different, while drying air humidity and velocity were constant, the unique line can be constructed. This line is linking corresponding Deff – MR curves through the same characteristic point. For simplicity reasons it was chosen to construct and use only the unique line pulled through characteristic E points. Its slope was proposed as the objective criteria for evaluation of the clays suitability for the rapid drying application. It was found that clays, for which the unique line slope is larger, had better dimensional and mechanical properties and were more suitable for the construction of the fast drying regimes

Comparison of gamma ray effects on EPROMs and E2PROMs

This paper compares the reliability of standard commercial Erasable Programmable Read Only Memory (EPROM) and Electrically Erasable Programmable Read Only Memory (E2PROM) components exposed to gamma rays. The results obtained for CMOS-based EPROM (NM27C010) and E2PROM (NM93CS46) components provide the evidence that EPROMs have greater radiation hardness than E2PROMs. Moreover, the changes in EPROMs are reversible, and after erasure and reprogramming all EPROM components restore their functionality. On the other hand, changes in E2PROMs are irreversible. The obtained results are analyzed and interpreted on the basis of gamma ray interaction with the CMOS structure