Efficiency of the Air Heater in a Heat Recovery System at Different Thermophysical Parameters and Operational Modes of the Boiler

We have examined, for the plate heater included in a heat recovery system of the boiler plant, the influence of its thermophysical parameters on the losses of exergetic power under different operational modes of the heating boiler. A procedure for the calculation of losses in a given heat recovery unit is based on an integrated approach that combines exergetic methods with the methods of thermodynamics of irreversible processes. A mathematical model includes a differential equation of the exergy balance and an equation of thermal conductivity for an air heater under boundary conditions of the third kind. The differential equation of exergy balance has been solved jointly with the equation of thermal conductivity. The result of solving them is the obtained estimation dependences for determining the losses of exergetic power associated with the processes of heat transfer.We have calculated losses of exergetic power in the examined heat recovery units at a change in the coefficient of thermal conductivity of the plate, in the heat transfer coefficient from flue gases and an operational mode of the boiler. The derived dependences on a thermal conductivity coefficient for the considered operating modes of the boiler have two distinct sections, along the first of which there is a relatively small increase in the losses of exergetic power while reducing the coefficient of thermal conductivity, along the second ‒ the loss of exergetic power in a heat recovery unit increase relatively sharply. For the considered sequence of regimes of the boiler a transition from its maximum heat output to the minimal one is accompanied by a decrease in the losses of exergetic power. A similar character is also demonstrated by the dependence on a thermal conductivity coefficient of the relative contribution of losses of exergetic power in the heat transfer processes to their totals in a heat recovery unit. In this case, there are minor differences in the relative contribution of these losses under different operational modes of the boiler. The heat transfer coefficient from the side of flue gases within a framework of a single operating mode of the boiler affects less significantly, compared to the thermal conductivity coefficient of the material of a heat exchange surface, the losses of exergetic power in the heat transfer processes. We have established regions of change in the thermal conductivity coefficient, as well as operational modes of the boiler, in the range of which the losses of exergetic power in a heat recovery unit are minima

Establishment of Regularities of Influence on the Specific Heat Capacity and Thermal Diffusivity of Polymer Nanocomposites of A Complex of Defining Parameters

This paper reports a series of experimental studies to establish regularities of the integrated effect exerted on the specific heat capacity of polymer nanocomposites by such factors as the temperature regime of their production, the value of the mass fraction of the filler, and the temperature of the composite material. The studies were conducted for nanocomposites based on polypropylene filled with carbon nanotubes. When obtaining composites, the method of mixing the components in the melt of the polymer was used. During the studies, the temperature of nanocomposites varied from 295 to 455 K, the mass fraction of the filler ‒ from 0.3 to 10 %. The basic parameter of the technological mode for obtaining composite materials, the value of overheating the polymer melt relative to its melting point, varied in the range of 10...75 K. It is shown that the temperature dependence of the specific heat capacity of the considered composites is sensitive to changes in the overheating of the polymer melt only in the region maximum values of the specific heat capacity. Concentration dependences of the specific heat capacity of the considered nanocomposites at different values of their temperature and the level of overheating of the polymer melt have been built. The studies have been carried out to identify the effects of the influence of the above parameters on the coefficient of thermal diffusivity of nanocomposites. It has been established, in particular, that an increase in the level of overheating the polymer could lead to a very significant increase in the coefficient of thermal diffusivity, which is all the more significant the higher the proportion of filler and the lower the temperature of the composite material. It is shown that the level of overheating the polymer melt relative to its melting point is a parameter that can be used as the basis for the creation of polymer composite materials with specified thermophysical properties

Comparison of the Features of the Formation of Joints of Aluminum Alloy 7075 (Al-Zn-Mg-Cu) by Laser, Microplasma, and Laser-microplasma Welding

This paper reports a study into features of the formation of structures of permanent butt joints of plates with a thickness of 1.5 mm made from the high-strength aluminum alloy 7075 of the Al-Zn-Mg-Cu system. Welding by melting these joints was performed using three techniques: laser, microplasma, and hybrid laser-microplasma. To implement the latter two, a compressed arc on a multipolar asymmetric current was used. The purpose of the research was to establish the tendency to the formation of characteristic defects and the possibility of their elimination. It has been determined that during laser welding a small (~5 %) volumetric fraction of defects in the form of pores is formed, residual welding deformations are minimized. There is a decrease in the hardness of the melted metal by 15 % with a simultaneous increase in the hardness of the heat-affected zone (HAZ) by 8...12 % relative to the base metal. In the melted metal, cavities up to 100 μm in size are formed, which are the center of the origin of hot cracks with a length of 25‒30 μm. There are oxide inclusions in the root part of the seam. With microplasma welding, the volume fraction of defects of the melted metal in the form of pores with a size of 10...105 μm increases (up to 25 %). The hardness of the melted metal is reduced by 30 % with the hardness of the HAZ metal close to the base metal. In laser-microplasma welding, the volumetric fraction of defects of the melted metal in the form of pores with a size of 15...25 μm is reduced to 5 %. The hardness of the melted metal is reduced by 15...20 % with the hardness of the HAZ metal close to the base metal. In the lower part of the melted metal, cavities of ~100 μm are formed. No microcracks were found in the seam metal. Analysis of the research results showed the advantage of the laser-microplasma technique. This method reduces the use of laser energy by 40...50 %, the lifetime of the welding pool (0.03...0.05 s) approaches laser welding, it eliminates the danger of burnout of alloying elements

Analyzing Metallurgical Interaction During Arc Surfacing of Barrier Layers on Titanium to Prevent the Formation of Intermetallics in Titanium-steel Compounds

This paper considers a possibility to obtain high-quality butt junctions of bimetallic sheets from steel clad with a layer of titanium, with the use of barrier layers. The task that was tackled related to preventing the formation of Ti-Fe intermetallic phases (IMPs) between the steel and titanium layer. The barrier layers (height ~0.5 mm) of vanadium and copper alloys were surfaced by arc techniques while minimizing the level of thermal influence on the base metal. To this end, plasma surfacing with a current-driving wire and pulsed MAG surfacing were used. The obtained samples were examined by methods of metallography, X-ray spectral microanalysis, durometric analysis. It has been established that when a layer of vanadium is plated on the surface of titanium, a defect-free structure of variable composition (53.87–65.67) wt % Ti with (33.93–45.54) wt % V is formed without IMPs. The subsequent surfacing of steel on a layer of vanadium leads to the formation of eutectics (hardness up to 5,523 MPa) in the fusion zone, as well as to the evolution of cracks. To prevent the formation of IMPs, a layer of bronze CuBe2 was deposited on the surface of vanadium. The formed layer contributed to the formation of a grid of hot cracks. In the titanium-vanadium-copper transition zones (0.1–0.2 mm wide), a fragile phase was observed. To eliminate this drawback, the bronze CuBe2 was replaced with bronze CuSi3Mn1; a defect-free junction was obtained. When using a barrier layer with CuSi3Mn1, a defect-free junction was obtained (10–30 % Ti; 18–50 % Fe; 5–25 % Cu). The study reported here makes it possible to recommend CuSi3Mn1 as a barrier layer for welding bimetallic sheets "steel-titanium". One of the applications of the research results could be welding of longitudinally welded pipes of main oil and gas pipelines formed from bimetallic sheets of steel clad with titanium