Dry-heat temperatures

Heat Autoclaving or dry heat Temperature-sensitive coloured solution Sealed tubes partly filled with a solution which changes colour at elevated temperatures rate of colour change is proportional to temperature, e.g. Browne s tubes Temperature, time... 

For dry-heat temperatures other than 170°C, Fh values are used. The Fh concept is comparable to the Fq concept for moist heat sterilization and references lethality to equivalent times at 170°C. Fh values are shown in units of minutes or seconds, and the calculations of Fh use the same equations as the calculations of Fq. The only difference is that a z value of 20° C is substituted for 10°C.t ... 

There are three primary and three secondary variables involved with the mode of action of dry heat. Temperature, water content, and time are the primary variables, and the secondary variables are open and closed systems, physical and chemical properties of... 

There is basically no endotoxin destruction at temperatures below 80°C, and the D values can be as high as 20 min for dry-heat temperatures of around 170°C. The second-order models give a better estimate of endotoxin destruction kinetics at temperatures above 250°C than in the 170-250°C range. Generally, the following conditions are required for endotoxin destruction ... 

Each of these processes imposes different demands on the packaging materials. Dry heat (temperatures of 160-180°) can only be withstood by glass, metals and a few selected plastics. Glass often uses 320°C or slightly higher for 3 1 min. 

In the absence of precise knowledge of the dry heat inactivation characteristics of the actual microbiogical contaminants of the product, the reference microorganisms for overkill (twelve D-values> and SAL targets are spores of B, subtilis var niger. A D)7Q-value of at least 1.S min may be assumed for these spores. For dry heat temperatures other than 170 C, there Is a concept comparable lo the Fq concept for steam steriltzarion. It is termed the concept and references lethality to equivalent times at 170 C. The units of F -value are minutes or seconds. Calculations of F i use the same equations as calculations of but the z-value of 20 C is sub huted in the case of dry heat... 

While temperature transmitters are checked and calibrated, sensors may not be checked directly. The operation of some, notably those on the outlets of the electrolyzers, cannot be verified by liquor circulation before startup. A common assumption is that thermocouples (T/Cs) and resistance temperature devices (RTDs) work correctly. If desired, they can be checked by a hot box, if one is available, or by immersion in a flask of hot water or other appropriate fluid. A hot box is a dry-heated temperature-controlled block with drilled holes to accept RTD or T/C sensors. No further calibration of the instrument is necessary after installation, and the DCS indication should be checked against a thermometer of known accuracy. 

Dry heating—High dry-heat temperatures, as in toasting bread, can cause considerable loss. 

Heat-reactive resins are more compatible than oil-soluble resins with other polar-coating resins, such as amino, epoxy, and poly(vinyl butyral). They are used in interior-can and dmm linings, metal primers, and pipe coatings. The coatings have excellent resistance to solvents, acids, and salts. They can be used over a wide range of temperatures, up to 370°C for short periods of dry heat, and continuously at 150°C. Strong alkaUes should be avoided. 

Iron Browns. Iron browns are often prepared by blending red, yellow, and black synthetic iron oxides to the desired shade. The most effective mixing can be achieved by blending iron oxide pastes, rather than dry powders. After mixing, the paste has to be dried at temperatures around 100°C, as higher temperatures might result in the decomposition of the temperature-sensitive iron yellows and blacks. Iron browns can also be prepared directiy by heating hydrated ferric oxides in the presence of phosphoric acid, or alkaU phosphates, under atmospheric or increased pressure. The products of precipitation processes, ie, the yellows, blacks, and browns, can also be calcined to reds and browns. 

Dry-heat sterilization is generally conducted at 160—170°C for >2 h. Specific exposures are dictated by the bioburden concentration and the temperature tolerance of the products under sterilization. At considerably higher temperatures, the required exposure times are much shorter. The effectiveness of any cycle type must be tested. For dry-heat sterilization, forced-air-type ovens are usually specified for better temperature distribution. Temperature-recording devices are recommended. 

Superheated steam results when steam is heated to a temperature higher than that which would produce saturated steam. The equiUbrium between hquid and vapor is destroyed, and the steam behaves as a gas. It loses its abiUty to condense into moisture when in contact with the cooler surface of the article to be sterilized. This process resembles dry-heat sterilization more than steam sterilization and, under ordinary time—temperature conditions for steam sterilization, does not produce stetihty. 

Application Techniques, Structural Variations, and Fastness Properties. When appHed to polyester fiber, many of the disperse dyes originally developed for ceUulose acetate were found to be deficient in Hghtfastness, build-up properties, and especially fastness to the high temperatures employed in the newer dyeing and finishing, printing, and Thermosol (dry heat) processes. 

Figure 5 shows conduction heat transfer as a function of the projected radius of a 6-mm diameter sphere. Assuming an accommodation coefficient of 0.8, h 0) = 3370 W/(m -K) the average coefficient for the entire sphere is 72 W/(m -K). This variation in heat transfer over the spherical surface causes extreme non-uniformities in local vaporization rates and if contact time is too long, wet spherical surface near the contact point dries. The temperature profile penetrates the sphere and it becomes a continuum to which Fourier s law of nonsteady-state conduction appfies. 

Golorfastness to Heat Treatment. To test for fastness to dry heat, ISO 105-P01 the specimen is sandwiched between adjacent fabrics and placed under slight pressure between heated surfaces where the temperature of the surface is 150, 180, or 210°C for 30 s. The effect on the shade of the pattern and adjacents is then assessed. 

Trim Coolers Conventional air-cooled heat exchangers can cool the process fluid to within 8.3°C (15°F) of the design dry-biilb temperature. When a lower process outlet temperature is required, a trim cooler is installed in series with the air-cooled heat exchanger. The water-cooled trim cooler can be designed for a 5.6 to 11.1°C (10 to 20°F) approach to the wet-biilb temperature (which in the United States is about 8.3°C (15°F) less than the diy-bulb temperature). In arid areas the difference between diy- and wet-bulb temperatures is much greater. 

When the mean annual temperature is 16.7°C (30°F) lower than the design dry-bulb temperature and when both fans in a bay have automatically controllable pitch of fan blades, annual power required has been found to be 22, 36, and 54 percent respectively of that needed at the design condition for three process services [Frank L. Rubin, Tower Requirements Are Lower for Air-Cooled Heat Exchangers with AV Fans, Oil Gas J., 165-167 (Oct. 11, 1982)]. Alternatively, when fans have two-speed motors, these dehver one-half of the design flow of air at half speed and use only one-eighth of the power of the full-speed condition. 

Wet-bulb temperature is the dynamic equilibrium temperature attained by a water surface when the rate of heat transfer to the surface by convection equals the rate of mass transfer away from the surface. At equilibrium, if neghgible change in the dry-bulb temperature is assumed, a heat balance on the surface is... 

Example 3 Air Heating Air is heated by a steam coil from 30 F dry-bulb temperature and 80 percent relative humidity to 75 F dry-bulb temperature. Find the relative humidity, wet-bulb temperature, and dew point of the heated air. Determine the quantity of heat added per pound of dry air. 

Solution. Figure 12-8 shows the path on a psychrometric chart. The leaving dry-bulb temperature is obtained directly from Fig. 12-2 as 72.2 F. Since the spray water enters at the wet-bulb temperature of 70 F and there is no heat added to or removed from it, this is by definition an adiabatic process and there will be no change in wet-bulb temperature. The only change in enthalpy is that from the heat content of the makeup water. This can be demonstrated as follows ... 

The calculations are made as follows. The exchanger is divided into small increments to allow numerical integrations. A tube wall temperature is first calculated and then QAV. The gas temperature and composition from an increment can then be calculated. If the gas composition is above saturation for the temperature, any excess condensation can occur as a fog. This allows the degree of fogging tendency to be quantified. Whenever possible, experimental data should be used to determine the ratio of heat transfer to m.ass transfer coefficients. This can be done with a simple wet and dry bulb temperature measurement using the components involved. 

---