Thermally sensitive materials

An ideal clothing fabric, in terms of thermal comfort, should have the following attributes (1) high thermal resistance for protection from the cold (2) low water vapour resistance for efficient heat transfer under a mild thermal stress conditions and (3) rapid liquid transport characteristics for transferring heat efficiently and eliminating unpleasant tactile sensations due to perspiration under high thermal stress conditions. 

US Army Research Institute of Enviromnental Medicine (USARIEM) studies have examined the effects of temperature and wind chill on a human body and indicated that the sensations of warmth and cold at wind velocities between 2 to 32 m/s may be as below  

Wind chill depends on the temperature as well as velocity of the wind. In cold climates, consideration of the wind chill effect is very important, because 80% of heat losses are due to wind chill. 

Wind chill factor can be derived from the Siple Passel formula (source Slater K, J. Text. Inst. 1986 77(3) 157-171)  

The skin temperature under calm conditions is taken as 33 °C. The significance of the wind chill factor may be understood by reviewing its effect on the freezing time of exposed flesh. At a wind chill factor of 1000, an individual would feel very cold and at 1200, bitterly cold. When the wind ehill factor is 1400, the exposed flesh would freeze in 20 min, at a wind chill factor of 1800, the exposed flesh would freeze in 10 min, and when the wind ehill faetor is 2400, exposed flesh freezes in 1 min only. 

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In a plasma-activated reaction, the substrate temperature can be considerably lower than in thermal CVD (see Ch. 5, Sec. 9). This allows the coating of thermally sensitive materials. The... 

Evaporation, Heat exchange, condensing vapours U.sually not May become necessary when processing thermally sensitive materials or if fouling of heat-transfer surfaces is possible. 

Intentional heating of a vessel containing thermally sensitive material, due to lack of recognition of the runaway hazard or other reason... 

Avoid heating coils, space heaters, and all other heat sources for thermally sensitive materials. 

Several types of hazardous chemical reactivity, with 36 percent attributed to chemical incompatibility, 35 percent to runaway reactions, and 10 percent to impact-sensitive or thermally sensitive materials. 

Impact-sensitive or thermally sensitive materials (i.e., self-reactive chemicals)-When subjected to heat or impact, these chemicals may rapidly decompose, resulting in a potentially explosive release of energy. 

In this Chapter, fundamentals of design criteria in relation to processes and equipment are reviewed for dense-gas-extraction from solid matrices. Although, as mentioned in previous chapters, numerous dense gases can be used as solvents. In the following discussion we concentrate on the most extensively used gas-carbon dioxide. The reason for this is its nontoxic, non-flammable and inert nature, the possibility of gentle treatment of thermally sensitive materials, and the fact that it is inexpensive and an environmentally acceptable material. 

Figure 3.14. The lower ends of fractionators, (a) Kettle reboiler. The heat source may be on TC of either of the two locations shown or on flow control, or on difference of pressure between key locations in the tower. Because of the built-in weir, no LC is needed. Less head room is needed than with the thermosiphon reboiler, (b) Thermosiphon reboiler. Compared with the kettle, the heat transfer coefficient is greater, the shorter residence time may prevent overheating of thermally sensitive materials, surface fouling will be less, and the smaller holdup of hot liquid is a safety precaution, (c) Forced circulation reboiler. High rate of heat transfer and a short residence time which is desirable with thermally sensitive materials are achieved, (d) Rate of supply of heat transfer medium is controlled by the difference in pressure between two key locations in the tower, (e) With the control valve in the condensate line, the rate of heat transfer is controlled by the amount of unflooded heat transfer surface present at any time, (f) Withdrawal on TC ensures that the product has the correct boiling point and presumably the correct composition. The LC on the steam supply ensures that the specified heat input is being maintained, (g) Cascade control The set point of the FC on the steam supply is adjusted by the TC to ensure constant temperature in the column, (h) Steam flow rate is controlled to ensure specified composition of the PF effluent. The composition may be measured directly or indirectly by measurement of some physical property such as vapor pressure, (i) The three-way valve in the hot oil heating supply prevents buildup of excessive pressure in case the flow to the reboiier is throttled substantially, (j) The three-way valve of case (i) is replaced by a two-way valve and a differential pressure controller. This method is more expensive but avoids use of the possibly troublesome three-way valve.

Kettle reboilers consist of a bundle of tubes in an oversize shell. Submergence of the tubes is assured by an overflow weir, typically 5-15 cm higher than the topmost tubes. An open tube bundle is preferred, with pitch to diameter ratios in the range of 1.5-2. Temperature in the kettle is substantially uniform. Residence time is high so that kettles are not favored for thermally sensitive materials. The large shell diameters make kettles uneconomic for high pressure operation. Deentraining mesh pads often are incorporated. Tube bundles installed directly in the tower bottom are inexpensive but the amount of surface that can be installed is limited. 

Kuts et al. [88, 89] patented a spray dryer with rotating impinging streams for solutions or suspensions of thermal sensitive materials, as shown in Fig. 6.2. The drying chamber is a horizontal cylinder, 1.2 m in diameter and 4 m long. The main airflows at 150°C enter axially the drying chamber from the two sides with the sprayed... 

It is proposed to use a distillation system working at about 5mbar to separate thermally sensitive material from crude process material. The distillation system is to be evacuated with a Roots pump (-Sth = 500 m3 h"1) connected to a dry backing pump with a pumping speed of 200 m3h-1. The Roots pump is to be switched on at 50 mbar. 

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