Flash temperature

As stated earlier, the calculation of the bulk or average surface temperature between two mating surfaces is performed by classic heat transfer methods. In fact the only unique aspect to this problem is the heat generation term Q comes from frictional heat and is assumed to occur at the interface. In order to begin a thermal analysis, we calculate the total heat  

Occasionally, there will be chemical changes to the mating materials that are not explained by the bulk temperature estimates or other environmental effects. These changes can result in discoloration or reaction by-products that would not be predicted for the mating conditions. One possible reason can be found in Hash temperature rise. In general, the mating condition is not perfect but the load is carried by an area less than the calculated real area of contact. This area can approach the dimension scale of the asperities. When this happens, aU of the frictional energy is dissipated in a very small volume of material. The instantaneous temperatures can flash up to 50-100°C or more above the bulk interfacial temperature. 

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The equilibrium ratios are not fixed in a separation calculation and, even for an isothermal system, they are functions of the phase compositions. Further, the enthalpy balance. Equation (7-3), must be simultaneously satisfied and, unless specified, the flash temperature simultaneously determined. 

The temperature and composition of each feed stream and the stream ratios are specified along with a common feed pressure (significant only for the vapor stream) and the flash pressure. For an isothermal flash the flash temperature is also specified. Resulting vapor and liquid compositions, phase ratios, vaporization equilibrium ratios, and, for an adiabatic flash, flash temperature are returned. 

T temperature (K) of isothermal flash for adiabatic flash, estimate of flash temperature if known, otherwise set to 0 to activate default initial estimate. 

ERF error flag, integer variable normally zero ERF= 1 indicates parameters are not available for one or more binary pairs in the mixture ERF = 2 indicates no solution was obtained ERF = 3 or 4 indicates the specified flash temperature is less than the bubble-point temperature or greater than the dew-point temperature respectively ERF = 5 indicates bad input arguments. 

Figure 14-8 shows the effect of speed and load intensity on the flash temperature index. These curves are general in nature, since scoring is a function of pressure angle, lubrication, and tooth size. 

Figure 14-8. Scoring based on flash temperature index related to speed and torque. (Courtesy of Lufkin Industries, Inc.)...

P = vapor pressure of each component at the flash temperature... 

Zhu, D. and Hu, Y. Z., A Computer Program Package for the Prediction of EHL and Mixed Lubrication Characteristics, Friction, Subsurface Stresses and Flash Temperatures Based on Measured 3-D Surface Roughness," Tribol. Trans., Vol. 44, No. 3,2001, pp. 383-390. 

For two phases to exist the flash temperature must lie between the bubble point and dew point of the mixture. 

In many flash processes the feed stream is at a higher pressure than the flash pressure and the heat for vaporisation is provided by the enthalpy of the feed. In this situation the flash temperature will not be known and must be found by trial and error. A temperature must be found at which both the material and energy balances are satisfied. 

Flash point. The temperature at which enough vapor forms above a liquid (or solid) to form an ignitable mixture with the air near the surface. The lower the flash temperature, the more hazardous the substance. 

Physico-chemical property parameters, such as boiling points, lower explosion limits, and flash temperatures, are well-defined parameters. Some of them have been measured accurately and can be found in databases that have been refereed and vetted. Sometimes, they have not yet been measured accurately, and there exist only tentative values in a database. But once they have been measured and authenticated, these parameter values should be archived and should not change with time. 

Figure 10.3 Lower explosion limits and flash temperatures of paraffins and alcohols...

I was once working in a refinery that could not meet the flash-point specification for its diesel product. Flash point is the temperature at which a hydrocarbon will ignite, when exposed to an open flame. To raise the flash point of diesel oil, it is steam-stripped, to remove the lighter, more combustible components. I noticed that I could drain water from the bottom of the steam supply line to the diesel-oil stripper. I then screwed a steam trap, on to the i/4-in drain valve, on the steam supply line. The stripper bottoms temperature increased by 35°F, and the flash temperature of the diesel product increased from 120 to 175°F,... 

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 ... 

Since we already have a total molar flow of the debutanizer feed, 46.2 mol/h, divide 46.2 by 2 to get 23.1 mol/h. Here we must find a temperature at the 85-psig pressure to derive this 23.1 vapor mol/h feed rate. We also know that the bubble point of the feed is 203°F, so here we know to start increasing the flash temperature until this vapor rate value is found. 

When chemical reactions are involved in a process, it is important to know the reaction temperature. In the model described here, reactions at a point on the wafer surface are assumed to be driven by temperature excursions due to contact by passing pad asperities. This is known as flash heating. In systems in which there is dry sliding contact between two rough surfaces, it is known that flash temperatures at asperity contacts can be much higher than the average temperatures of the workpieces involved. In CMP, however, the contact is lubricated and cooled by the slurry, and this needs to be taken into account. In the case of polishing on a rotary tool, it is possible to derive a simple estimate of the mean reaction temperature, and it is this that we use in the chemical part of the two-step model. 

When an asperity is in contact with the wafer, the surface of the wafer and the contacting surface of the asperity will have the same temperature if temperature continuity applies, a common assumption in this kind of analysis [14]. Even when an asperity is not touching the wafer, the temperature of the asperity tip and that of the nearest point on the wafer should still be within a few degrees because of cooling provided by the thin ( 20 /an) slurry layer [21]. Thus, the mean temperature of points on the wafer that are immediately over pad summits should be nearly the same as the mean asperity tip temperature. This allows a shift in focus from the wafer to the asperities. It also suggests that the flash temperature experienced by a given point on the wafer depends on the thermal history of the asperity that it has encountered. 

The solution of the equations listed in Table 3.3.1 requires an iterative procedure. Thus, it is good strategy to examine the variables to determine if there are limits on their values. For example, the mole fractions of the components will vary from zero to one. This fact greatly simplifies the solution procedure. Also, the final flash temperature will lie somewhere between the bubble and dew-point temperatures. The bubble-point temperature is that temperature at which the first... 

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