Hot mix temperature

Energy Saving. Reduced fuel requirements for heating the aggregate can be realized by decreasing the hot-mix temperature (estimated... 

Fig. 3.18. More excess air and more recirculated gases reduce the temperature rise of the oven gases, lowering the hot-mix temperature. Courtesy of Dick Bennett s Energy Notes in the Sept. 1999 issue of Process Heating.

Added benefits from preheating combustion air are faster burning, resulting in a hotter burner wall, and lower flue gas exit temperature. The desired prompt heat release is difficult to evaluate. An interesting facet of the available heat charts (figs. 5.1 and 5.2) is that the curves jc-intercepts (where available heat is zero) are theoretical adiabatic flame temperatures, or hot-mix temperatures mentioned earlier. For the... 

Fig. 7.6. Percent excess air necessary to maintain a required hot mix temperature when burning natural gas or distillate oil with cold air. (See also figure 3.18.) Example To find the amount of excess air necessary to keep the hot mix below 2400 F, enter the vertical scale at 2400 F. Then move right to the curve, then down. Read 75% excess air.

Rg. 7.7. Percent excess oxygen needed to maintain a required hot mix temperature when burning natural gas or distillate fuel oil using nonpreheated air. 

Asphalt concrete is primarily used as a structural pavement surface constructed over a subgrade and a subbase. It is designed to support the traffic load and distribute the load over the roadbed. Asphalt concrete pavements can be constructed using hot mix or cold mix asphalt. Hot mix asphalt is a mixture of tine and coarse aggregate with asphalt cement binder that is mixed, placed, and compacted in a heated condition. Cold mix asphalt is a mixture of emulsified asphalt and aggregate, produced, placed, and compacted at ambient air temperature. Cold mix asphalt pavement usually requires an overlay of hot mix asphalt or surface treatment to resist traffic action. 

The results tend to verify the conclusions established earlier by Shell, that as long as mix temperatures are maintained below 300°F, concentrations of H S and SO are well within the safety limits suggested by the American Conference of Governmental Industrial Hygienists [ 29]. All of the field trials conducted both in the United States and Canada utilized conventional batch type hot mix plants and paving equipment. These field sections have been under post-construct ion evaluation for more than four years and are still performing satisfactorily. 

Hot mixing may be conducted either in kneaders of the Werner-Pfleiderer type with a heating jacket or in disk mixers (Fig. 167). These mixers consist of a round plate of copper, brass or bronze, approximately 2 mm dia., with beaded edges, which is heated with hot water to a temperature of 80-90°C. The material is mixed with bronze blades and kneaded with a bronze roller. 

Apart from oxidation of the lubricant and the metal surfaces, there can be complex tribo-chemical reactions. Chemical reactions at the surfaces can be stimulated by different factors. One factor is heating due to friction. This can either be a global effect (elevated mean temperature of surfaces and lubricant) or a localized phenomenon. Especially in situations where mixed or boundary lubrication exists, the direct contact of surface asperities can lead to high flash temperatures. At these hot spots temperatures in excess of 1000°C promote chemical reactions and surface melting. Other factors promoting chemical reactions are ... 

In hot mixing or elevated-temperature curing of an epoxy system, vapor pressure could also be of concern relative to the quality of the adhesive bond. If the components in an epoxy system become too hot, boiling can occur, resulting in gas bubbles. If gas bubbles become trapped in the cured adhesive film, they can lead to reduction of cohesive strength and stress risers. For many adhesive applications, particularly those in the electrical and electronic industries (due to possible ionization of air voids), complete removal of any gas bubbles from the epoxy is essential. 

In addition to the problems associated with excessive mixing and compaction, a series of tests was designed to illustrate the extent to which the structural integrity of the S-A-S material would be affected by a loss of process control. The two operational problems simulated were material handling below the solidification temperature of the sulfur (240° F) and a loss of temperature control at the hot-mix plant, causing a temperature rise above 320°F. 

Hot-mix paving mixtures are prepared at a lower temperature (30°C below the usual temperature) because of the lower viscosity of S/A binder. This results in lower energy consumption, saving 30% fuel and using less electric energy. 

Application of hot-mix paving mixtures is performed more easily with conventional equipment. Moreover, the distance of truck transportation is improved, because of the lower compacting temperature, which can be below the melting point of sulfur. In addition, with some formulations, compaction is not necessary. 

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