Heat engineering - calculation of a pit-type chamber

Contents

Introduction

1. Characteristics of manufactured materials and products

2. Description of processes occurring at thermal

treatment of wall panels of heavy concrete

3. Selection of method and mode of heat treatment, heat carrier and thermal installation

4. Calculation of pit steaming chamber

5. Tasks of thermal plant automation

6. Decisions to ensure safety, safety and

environment during operation of the thermal plant

Conclusion

List of sources used

Introduction

Thermal methods for accelerating concrete hardening are currently the most efficient and versatile, and therefore widely used in the production of prefabricated reinforced concrete products and structures.

Curing of concrete and concrete goods at the plants at a usual temperature (15....20 wasps) is irrational as it is too long, reduces turnover of forms, delays release of finished goods .

The essence of thermal moisture treatment of concrete and reinforced concrete products lies in the fact that with an increased relative humidity of the environment (more than 95%) and a temperature of 60-100 0C and more, the rate of cement hydration reaction increases (10-20 times), the concrete hardening process accelerates, and the product acquires the strength necessary for their disassembly, transportation and installation in a shorter time.

Description of Thermal Processes

Treatment of wall panels of heavy concrete

The steaming process consists of four steps:

1) Exposure - consists in the fact that the freshly molded product is left at ambient temperature for some time, which can be from several minutes to several hours and even a day. Preliminary maintenance of molded articles prior to heat treatment is intended to provide the concrete with the minimum required initial (critical) strength of the concrete, at which it can perceive thermal impact under the accepted mode without violating its structure. The optimum holding time depends on the type and grade (activity) of the cement, the initial water content of the concrete mixture, the temperature of the medium and the use of chemical hardening accelerators. It should be an average of 2-4 hours, in some cases and more, but not less than 1-2 hours. With an increase in the critical strength of concrete before the start of heat treatment, it is possible to increase the intensity of its heating and thus, by spending time on preliminary holding of concrete, save time on the speed of its heating. Maintenance prior to the start of the heat treatment can be avoided if a pre-heated concrete mixture is used.

2) Temperature rise - during this period, the molded product after preliminary holding or immediately must reach the specified maximum steaming temperature. Raising the temperature of concrete (or medium in the chamber) is the most important stage of heat treatment, since possible violations in the structure of concrete occur precisely at this stage of heat treatment. The magnitude and nature of structural disturbances depend on the strength achieved by the concrete by the beginning of the heat treatment, on the rate of rise of the temperature of the medium in the chamber and a number of other factors that contribute to or impede the development of destructive processes. According to the existing "Instructions for heat treatment with steam of concrete and reinforced concrete products at factories and landfills," the permissible rate of medium temperature rise in the chamber with critical strength of concrete of 1-2 kgf/cm2 should not exceed 10 ° per 1h, and with critical strength of 5-6 kgf/cm2 - 40 ° per 1h. In cases where pre-aging of the molded articles is difficult to practice and the articles are subjected to heat treatment at a strength lower than critical, the temperature rise should be performed slowly, starting, for example, from 10 ° per hour, with an increase in its rate as the strength of the concrete increases (up to 30 ° per hour in the last stage of heating the concrete); this is especially necessary when the molded article, being in a mould, has large exposed surfaces or is processed on a pallet without on-board equipment. The rate of temperature rise can be increased without first holding in case of hard metal heat treatment

Cheches with a tight lid. If there is a load installed on the cover, the intensity of heating of the concrete can reach 100 ° per hour or more.

3) Isothermal heating (isothermal) - after the temperature rises to the specified maximum, there should be a period when the product, a certain time is held at a constant maximum temperature. The temperature in the steaming chamber at the isothermal heating stage (reference temperature) is mainly determined by the concrete grade and ranges from 65 to 100 ° C. The duration of the isothermal, in turn, is determined by the reference temperature. At low temperature, the duration of isothermal heating increases significantly, which is undesirable due to a decrease in the turnover of chambers and shapes of products. However, with an increase in the reference temperature, the probability of a lack of strength during subsequent hardening of concrete increases compared to concrete hardened under normal temperature-moisture conditions. Thus, the duration of holding the articles at the accepted highest temperature of isothermal heating in this type of cement depends on the strength of the concrete to be obtained at the end of the heat treatment. At the same time, it should be taken into account that the increase in strength with an increase in the duration of heating after reaching 65-75% of the design occurs with a progressive deceleration and that it is clearly impractical to continue processing until the concrete reaches full design strength.

4) Cooling - the period of concrete cooling (chamber cooling) is also an important step in which it is necessary to protect concrete from significant temperature differences in the product. During this period, a gradual, as uniform as possible throughout the section of the product, temperature decrease should be ensured. Temperature difference by the moment of article removal from the chamber between its surface and ambient air temperature shall not exceed 4045 ° С.

An introduction to the practice of the steaming shop showed that at present the control and regulation of the steaming cycle of precast reinforced concrete is carried out as follows:

At the reinforced concrete factory there is a list of manufactured products, each of which has a name, a unique cipher and has technical characteristics (length, width and height). For each product, there is information on the optimal steaming mode. Optimum steaming mode includes time of preliminary holding, time of temperature rise, duration of isothermal, temperature at which isothermal process takes place, duration of heat-cooling and duration of cooling with ventilation.

The steaming operation mode is set by the steaming process and may not match the optimal one. During the summer time period, the steps of pre-holding, heat-cooling or ventilation cooling may be omitted and the isothermal step may be shortened. In winter time, the duration of any TP stage can be increased.

The TVO process is carried out in steaming chambers. Each camera is of a certain type, has a unique number, contains a certain number

sensors and has technical characteristics (length, width and height).

In any chamber, only one batch of homogeneous products is steamed per day,

which is uniquely determined by the camera number in which it is placed and the start date of the PI.

After submerging the railway products into the chambers and closing them, a special worker is a steamer, every hour he measures the temperature using an ordinary technical thermometer with a division price of 1 ° С. Due to the inertia of the thermal processes and the fact that the visual reading does not achieve the required accuracy, errors in measuring temperature reach 6 ° 8 ° C.

The steamer records the measurement data in a special log (steamer log), which indicates the measurement time and temperature in the chamber .

Conclusion

The pit chamber in my project is used for thermal moisture treatment of floor panels measuring 6 × 3 × 0.14 m, made of heavy concrete of the 200 grade in an aggregate - in-line manner. The capacity of the line is 18000 m3/year. Steam is used as coolant. The heat utilization factor is [theta] = 93.7%. Duration of heat-moisture treatment - 11 hours. The section of the pit chamber, the hydraulic lock assembly, the vertical risers for steam distribution are shown in the drawing of the format A1.

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