Critical surface temperature

Thomson, H.E., Drysdale, D.D., and Beyler, C.L., An experimental evaluation of critical surface temperature as a criterion for piloted ignition of solid fuels, Fire Safety Journal, 13, 185-196, 1988. 

Reliability A stable temperature control, combined with an excellent heat transfer and a uniform temperature profile (no hot spots) in the fluidized bed, easily achieves an onstream time >99% per year. A specially designed raw-material sparger system allows operation spans of two years without maintenance. Larger heat-transfer area allows a higher steam temperature and pressure in the cooling coils, which improves the safety margin to the critical surface temperature where hydrochloric acid dewpoint corrosion may occur. 

Electrode Walls. Development of durable electrode wads, one of the most critical issues for MHD generators, has proceeded in two basic directions ceramic electrodes operating at very high surface temperatures (>2000 K) for use in channels operating with clean fuels such as natural gas, and cooled metal electrodes with surface temperatures in the range 500—800 K for channels operating with slag or ash-laden flows. 

Properties. Hydroxypropylcellulose [9004-64-2] (HPC) is a thermoplastic, nonionic cellulose ether that is soluble in water and in many organic solvents. HPC combines organic solvent solubiUty, thermoplasticity, and surface activity with the aqueous thickening and stabilising properties characteristic of other water-soluble ceUulosic polymers described herein. Like the methylceUuloses, HPC exhibits a low critical solution temperature in water. 

The observation may be by a lamp illuminating the surface and a photocell to detect the scattered light due to the water droplets on the surface. The accurate measurement of the surface temperature, which is the dewpoint temperature, is critical. If a coolant is used, a close approximation for the surface temperature is the fluid temperature otherwise a small thermocouple or resistance sensor can be attached to or embedded into the surface. 

Nucleate boiling is boiling at the tube surfece at a temperature difference between outside tube surface temperature and the fluid body, less than the critical temperature difference. At and beyond the critical temperature difference, metastable and film boiling take place. These produce lower transfer coefficients as the temperature difference increases. 

Martensite is a hard, nonductile microconstituent formed when steel is heated above its critical temperature and cooled rapidly. In the case of steel of the composition conventionally used for rope wire, martensite can be formed if the wire surface is heated to a temperature near or somewhat in excess of 1400°F (760°C), and then cooled at a comparatively rapid rate. The presence of a martensite film at the surface of the outer wires of a rope that has been in service is evidence that sufficient frictional heat has been generated on the crown of the rope wires to momentarily raise the wire surface temperature to a point above the critical temperature range of the steel. The heated surface is then rapidly cooled by the adjacent cold metal within the wire and the rope structure, and an effective quenching results. 

Infrared thermometers or spot radiometers are designed to provide the actual surface temperature at a single, relatively small point on a machine or surface. Within a predictive maintenance program, the point-of-use infrared thermometer can be used in conjunction with many of the microprocessor-based vibration instruments to monitor the temperature at critical points on plant machinery or equipment. This technique is typically used to monitor bearing cap temperatures, motor winding temperatures, spot checks of process piping temperatures and similar applications. It is limited in that the temperature represents a single point on the machine or structure. However when used in conjunction with vibration data, point-of-use infrared data can be a valuable tool. 

If the temperature is changed the miscibility of the liquids alters, and at a particular temperature the miscibility may become total this is called the critical solution temperature. With rise of temperature the surface of separation between the liquid and vapour phases also vanishes at a definite temperature, and we have the phenomenon of a critical point in the ordinary sense. According to Pawlewski (1883) the critical temperature of the... 

During the first period of drying, the liquid that covers the particle external surface and is present in the macropores evaporates. The material structure does not affect the rate of evaporation. The liquid evaporates with the rate at which heat is supplied to the surface. The rate of drying is thus limited by heat transfer between the particles and their surroundings. The temperature at the particle surface remains constant. If heat is delivered by convection this temperature is the wet-bulb gas temperature. In case of radiation (e.g. microwave driers) or conduction (e.g. indirect contact driers) the surface temperature ranges between the wet-bulb gas temperature and the boiling point of the liquid. The moisture content at the end of the constant rate of drying period is called the critical moisture content. 

The maximum heat flux achievable with nucleate boiling is known as the critical heat flux. In a system where the surface temperature is not self-limiting, such as a nuclear reactor fuel element, operation above the critical flux will result in a rapid increase in the surface temperature, and in the extreme situation the surface will melt. This phenomenon is known as burn-out . The heating media used for process plant are normally self-limiting for example, with steam the surface temperature can never exceed the saturation temperature. Care must be taken in the design of electrically heated vaporisers to ensure that the critical flux can never be exceeded. 

In the meantime other experiments have also improved our range of observational results. For example, Watts et al. carried out experiments very similar to the NO/Ag(lll) experiments described above.32 A critical difference in this work was the substitution of Cu(110) in place of the Ag(lll). Despite the chemically distinct metal surface, nearly identical results were obtained as those in Refs. 24 and 25, including surface temperature and incidence energy dependence. While it is not unlikely that the bond softening of NO is similar on Ag(lll) and Cu(110), there is no a priori reason to believe that these two metals would exhibit the same incidence energy and surface temperature dependence in vibrational excitation experiments. More importantly, there has not been a theoretical attempt to explain why these two chemically distinct systems behave so similarly within the context of electronically adiabatic models. 

For concurrent spread, the growth rate can be much faster, and therefore the critical condition can be reached at lower compartment temperatures. The dependence of the concurrent flame spread area on both Q and the surface temperature of the material make this spread mode very feedback sensitive. 

Generally, the occurrence of a specific mode is determined by droplet impact properties (size, velocity, temperature), surface properties (temperature, roughness, wetting), and their thermophysical properties (thermal conductivity, thermal capacity, density, surface tension, droplet viscosity). It appeared that the surface temperature and the impact Weber number are the most critical factors governing both the droplet breakup behavior and ensuing heat transfer. I335 412 415]... 

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