Threshold temperature

FIGURE 11.56 Hour / room air temperatures in the occupied zone (T,) are plotted as a funaion of the daily I -h maximum air temperature, according to Swiss standard SIA 382/2. Limit threshold temperatures are adopted to the working conditions in the hall. Ventilation openings are open 0-24 h, but only if T, > I6°C and r (T., = actual outdoor temperature)... 

From the results obtained by thermal decomposition of both low-molecular weight vicinal dichlorides in the gas phase [74,75] and of the copolymers of vinyl chloride and /rthermal instability of PVC to the individual head-to-head structures. Crawley and McNeill [76] chlorinated m-1,4-polybutadiene in methylene chloride, leading to a head-to-head, and a tail-to-tail PVC. They found, for powder samples under programmed heating conditions, that head-to-head polymers had a lower threshold temperature of degradation than normal PVC, but reached its maximum rate of degradation at higher temperatures. 

In the early phases of this study, temperature surveys were run on various catalysts in order to determine the threshold temperature for CO methanation. The data in Table XVII, calculated for 0.25-in. C150-1-02 catalyst, are rather typical. 

A hydrogen bomb, which uses nuclear fusion for its destructive power, is three bombs in one. A conventional explosive charge triggers a fission bomb, which in turn triggers a fusion reaction. Such bombs can be considerably more powerful than fission bombs because they can incorporate larger masses of nuclear fuel. In a fission bomb, no component of fissionable material can exceed the critical mass. In fusion, there is no critical mass because fusion begins at a threshold temperature and is independent of the amount of nuclear fuel present. Thus, there is no theoretical limit on how much nuclear fiiel can be squeezed into a fusion bomb. 

Industrial preparation of 4-cyano-3-nitrotoluene by heating the reaction components at around 170°C for 6 h led to an explosion in 1976. Subsequent investigation by DSC showed that the cyano compound in presence of the starting materials exhibited an exotherm at 180°C. After 6 h reaction, this threshold temperature fell to 170°C. Isothermal use of a safety calorimeter showed that a large exotherm occurred dining the first hour of reaction and that, in absence of strong cooling,... 

Immediate protection with low conductivity materials, reactive materials respond when threshold temperature is reached. 

From everyday experience, we know that an egg will not denature at room temperature, however long it is left. We are not saying here that the egg denatures at an almost infinitesimal rate, so the lack of reaction at room temperature is not a kinetic phenomenon rather, we see that denaturation is energetically non-spontaneous at one temperature (25 °C), and only becomes spontaneous as the temperature is raised above a certain threshold temperature, which we will call T criticai) (about 70 °C for an egg). 

The second RPT criterion relates to the temperature of the hot liquid. That is, this temperature must exceed a threshold value before an RPT is possible. From one theory of RPTs, the superheated-liquid model (described later), this criterion arises naturally, and the threshold hot-liquid temperature is then equal to the homogeneous nucleation temperature of the colder liquid T. This temperature is a characteristic value for any pure liquid or liquid mixture and can be measured in independent experiments or estimated from theory. From alternate RPT theories, the threshold temperature may be equated, approximately, to the hot fluid temperature at the onset of stable film boiling. 

A corollary, often appended to the second RPT criterion, is that RPTs are more difficult to achieve as the temperature of the hot liquid increases beyond the threshold temperature. For very high hot-liquid temperatures, some external pressure pulse or shock becomes necessary to initiate an RPT. 

Low extract yields were obtained either by short reaction times at high temperatures or by more extended reaction at low temperatures. The designations of high and low temperature depend upon the individual coal and, more especially upon the coal rank. The evidence accrued in this research has shown that there exists a threshold temperature below which the potential for liquids formation is minimal and above which conversion can proceed at an appreciable rate. 

In the following, we summarize the main features of practical relevance. The position and amplitude of the activity hysteresis depends mainly on partial pressure of O2 and on the nature of the support material. The threshold temperatures of both decomposition and re-formation of PdO increase markedly with O2 partial pressure. Data on the effect of the support material are reported in Table 12.2. 

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