Structural Heat Engineering

the scientific discipline that investigates the processes of heat and moisture transfer and air infiltration in buildings and structural components and develops design methods for calculating these processes; it is a branch of structural physics. Structural heat engineering draws upon related scientific fields, such as the theories of heat and mass transfer, physical chemistry, and the thermodynamics of irreversible processes, together with modeling methods and similarity theory, particularly for engineering calculations of heat and mass transfer. This ensures that useful results are achieved for a variety of external conditions and different combinations of areas and volumes inside buildings. Of great importance in structural heat engineering are field and laboratory studies of temperature and moisture fields in the enclosing structures of buildings, as well as the determination of the thermal characteristics of structural materials and components.

The methods used and information gained in structural heat engineering are applied in designing enclosing structures intended to create required temperature, humidity, and sanitary conditions in residential, public, and industrial buildings. In the designing of such structures, the effects of heating, ventilating, and air-conditioning systems are taken into account. The discipline has become more important as a result of the industrialization of construction and the substantial increase in the use of new building materials and lightweight structural components under various climatic conditions.

The problem of providing the desired heat-engineering properties for exterior enclosing structures is dealt with by imposing specifications for thermal stability and resistance to heat transfer. The permissible permeability of structures is limited by the given resistance to air infiltration. Normal moisture conditions are achieved in structures by reducing the initial moisture content of the material, by moistureproofing, and, in laminated structures, by the best recommended arrangement of layers made of materials with different properties.

The heat-transfer resistance must be high enough so that temperature conditions at interior surfaces are acceptable to occupants during the coldest period of the year. The thermal stability of structures is judged by the structures” ability to maintain relatively constant temperatures in rooms during periodic temperature variations in the atmosphere surrounding the structures and to maintain a relatively steady heat flow through the structures themselves. The overall thermal stability of a structure depends to a great extent on the physical properties of the materials used for the outermost layer, which are subject to sharp temperature variations. The methods used to calculate thermal stability are based on the solution of differential equations for periodically varying heat-transfer conditions. When the one-dimensional heat transfer inside enclosing structures is disturbed at local heat-conducting enclosures, panel joints, and corners, there arises an undesirable temperature drop at the structural surfaces facing a room, requiring a corresponding increase in the thermal insulating properties of the surfaces. Design methods for these cases involve a numerical solution for the differential equation of a two-dimensional temperature field (Laplace’s differential equation).

The temperature distribution in the enclosing structures of buildings changes when cold air infiltrates the structures. Air infiltrates chiefly through windows, joints between elements, and other areas where leaks occur; it also penetrates the enclosures themselves to a certain degree. Suitable methods have been developed for calculating changes in the temperature field during the steady infiltration of air. The resistance to air infiltration for all the elements of an enclosure must be greater than the standard values established by the Construction Code.

When the moisture conditions of enclosing structures are analyzed, consideration is given to the moisture-transfer processes that occur in response to a difference in transfer potential. Moisture transfer within the hygroscopic-moisture range for materials occurs chiefly as a result of diffusion in the vapor phase and the adsorption state; the transfer potential used in this case is the partial pressure of water vapor in the air that fills the material’s pores. In the USSR graphical analysis is widely used to calculate the probability and amount of condensed moisture within a structure during the diffusion of water vapor under standard conditions. A more accurate calculation can be obtained for transient conditions by solving the differential equations for the transfer of moisture; of particular value are various devices in computer technology, including those based on physical analogues, such as some fluid-jet devices.