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Features of the Heat Engineering Calculation of the Enclosing Structures Industrial Buildings Operated in a Variable Thermal Conditions

In recent years’ special attention has been paid to energy conservation. The need to reduce the consumption of mineral raw materials and fossil organic fuel is primarily associated with a reduction their reserves and, consequently, a constant increase in the cost. Therefore, a very actual task is to reduce the energy costs for heating and ventilation of industrial enterprises. The use of on duty heating during the non-working period can significantly increase the energy efficiency of an industrial building. On the basis the thermal regime of an industrial building, considered in the article, is shown the need to develop an engineering method for calculating the process of non-stationary heat transfer in buildings operated with a stand-by heating mode. This article presents the method of heat engineering calculation of building enclosing structures used in buildings with variable power of the heating system, which allows to determine the minimum allowable thickness of insulation from the condition of limiting the heating time of the outer wall. An example of the design calculation of the wall sandwich panel an industrial building is considered. An assessment of the specific energy consumption for heating external walls was made. A study conducted by the authors, allows to determine the range of thickness the enclosing structures, operated in conditions of a variable thermal regime.


Introduction
Most production workshops do not work around the clock. Therefore, in single-shift or double-shift operation, there is a need to use the district heating during the non-working period. A typical graph of changes the internal air temperature in the production hall on weekdays is presented in figure 1.
The thermal regime of an industrial building is characterized by four stages. At the first stage, the building is operated under conditions of on duty heating. The estimated temperature of the internal air during off-hours is determined from the condition that there is no condensate on the internal surfaces of the building envelope, depending on the severity of physical labor according to the normative sources for heating buildings.
At the second stage, the heating of the room is carried out, during which the temperature of the inside air rises from value to. Operation of the building during working hours is characterized by the third stage. At the fourth stage, the workshop room is cooled after completion of the work. The process of heating the enclosing structures is non-stationary, which greatly complicates the method of heat engineering calculation. To assess the energy consumption for heating the building and the warm-up time of the enclosing structures, it is necessary to consider the process of non-stationary heat transfer. In work [1], using the operational method, the exact solution of the problem of nonstationary thermal conductivity of a single-layer and two-layer walls was obtained in the absence of heating on duty. When a solution was found, the initial wall temperature was taken to be equal to the outside air temperature.
An approximate analytical solution to the problem of unsteady heat transfer in a multilayer wall in the absence of heating on duty was obtained by B. A. Semenov in work [2]. In the works [3,4,5,6,7,9,10,11,12] the results of the study of the thermal conditions of rooms operated in intermittent heating conditions are presented by numerical methods.
In this connection, it became necessary to develop an engineering method for calculating the process of non-stationary heat transfer in buildings in the presence of heating on duty.
In work [8], the author's methodology based on the use of an approximate analytical method is described in detail.

The method of heat engineering calculation of the exterior walls industrial buildings' operated in the conditions of intermittent heating
According to standard sources on thermal protection of buildings the required thickness of the insulating material must be determined on the basis of the energy saving condition: where пр R 0 is the reduced resistance to the heat transfer of the outer wall, (m 2 •ºС)/W; тр R 0the required resistance to heat transfer of the outer wall, determined according to Order Book 50.1333.2012 by the magnitude of the degree-day of the heating period, (m 2 •ºС)/W.
The reduced resistance to the heat of the enclosing structure is determined by the formula where r -is the coefficient of heat engineering uniformity of the wall sandwich panel, determined by the results of certification tests.
The minimum allowable thickness of the insulation is determined from the condition of limiting the heating time the outer wall according to the formula given in [8].
According to the temperature chart the time of the room's heating н τ heating is 2 hours. The coefficient A depends on the thickness of the insulation layer. In order to simplify the solution of the problem, its value is preset.  The energy consumption for heating the outer wall is determined according to [6] by the formula: We neglect the accumulation of heat from metal layers due to the high thermal conductivity of steel. With this assumption, we consider the sandwich panel as a single-layer wall. In this case, the value according 2 τ ∆ to [6] is determined by the formula:  We specify the heating time of the outer wall according to [6] according to the formula

Discussions
The calculation results show that the outer wall of the production building with a standard insulation thickness of 80 mm heats up in 1.26 hours. This heating time does not exceed the permissible value, which is actually for enclosing structures operated in variable thermal conditions. According to the above method, it is also possible to determine the energy consumption for heating the production building, given the geometrical dimensions of the enclosing structures.
The choice of the thickness of the insulation the enclosing structure in this case requires a reasonable approach. The graph shown in figure 3, obtained for the considered wall by calculation, allows you to select other values of the standard thickness of the basalt mineral wool layer. This is possible due to the fact that the remaining layers in the design are low inertia and practically do not accumulate heat. In addition, with increasing thickness of insulation increases the energy consumption and warm-up time of the enclosing structure.