Film coefficient internal

Double-Pipe Scrapea-Surface Crystallizer This type of equipment consists of a double-pipe heat exchanger with an internal agitator fitted with spring-loaded scrapers that wipe the wall of the inner pipe. The cooling hquid passes between the pipes, this annulus being dimensioned to permit reasonable shell-side velocities. The scrapers prevent the buildup of solids and maintain a good film coefficient of heat transfer. The equipment can be operated in a continuous or in a recirculating batch manner. 

In a countercurrent-flow heat exchanger, 1.25 kg/s of benzene (specific heat 1.9 kJ/kg K and specific gravity 0.88) is to be cooled front 350 to 300 K with water at 290 K. In the heat exchanger, tubes of 25 mm external and 22 mm internal diameter are employed and the water passes through the tubes. If the film coefficients for the water and benzene are 0.85 and 1.70 kW/nr K respectively and the scale resistance can be neglected, what total length of tube will be required if the minimum quantity of water is to be used and its temperature is not to be allowed to rise above 320 K ... 

Water at 293 K is heated by passing through a 6.1 m coil of 25 mm internal diameter pipe. The thermal conductivity of the pipe wall is 20 W/m K and the wall thickness is 3.2 mm. The coil is heated by condensing steam at 373 K for which the film coefficient is 8 kW/m2 K. When the water velocity in the pipe is I tn/s, ils outlet temperature is 309 K. What will the outlet temperature be if the velocity is increased to 1.3 m/s, if the coefficient of heat transfer to the water in the tube is proportional to the velocity raised to the 0.8 power ... 

If an internal cooling coil is used (as discussed in the next section) with a 0.0763 m diameter, the velocity in the tube is 3.7 m/s and the film coefficient is ha = 3200 J s-1 K-1 m-2, giving a TJ = 800 J s 1 K 1 m-2. ft is clear that the once-through cooling water system is by far the most inferior of the three systems. 

A smooth tube in a condenser which is 25 mm internal diameter and 10 m long is carrying cooling water and the pressure drop over the length of the tube is 2 x 104 N/m2. If vapour at a temperature of 353 K is condensing on the outside of the tube and the temperature of the cooling water rises from 293 K at inlet to 333 K at outlet, what is the value of the overall heat transfer coefficient based on the inside area of the tube If the coefficient for the condensing vapour is 15,000 W/m2 K, what is the film coefficient for the water If the latent heat of vaporisation is 800 kJ/kg, what is the rate of condensation of vapour ... 

Individual mass-transfer coefficient, m/s or ft/s for external film for internal diffusion... 

The overall coefficient Kc depends on the external film coeffient, kc exl, and on an effective internal coefficient, kc int. The external film coefficient can be estimated from the correlations for packed beds given in Section 2.6.5. The internal coefficient may be estimated by (Vermeulen et al., 1973)... 

If the velocity in the tubes was high enough to ensure turbulent flow, the difference in film coefficients would be reduced, but not by enough to favor internal flow. 

To adopt a common procedure here, the external film coefficient is expressed in terms of the Nusselt number. The internal coefficients, however, are given indirectly by the transfer efficiency, E , representing the fractional approach to the maximum possible heat transfer. Thus, by definition. 

Epq = modulus of elasticity of composite section, psi hi, ho = film coefficients, inside or outside, Btu/ft -hr/°F P = internal pressure, psig Q = heat loss through wall, Btu/ft -hr T, = temperature, outside ambient, °F T(, = temperature, outside ambient during construction, °F... 

In epitaxial films on the substrate, which is usually much thicker than the film, the internal mechanical strains originate from mismatch between film and substrate lattice constants and their thermal expansion coefficients. The technological defects and imperfections can also be the sources of internal strains. The mechanical strains U can be either compressive or tensile, their values are around 1 GPa [6] and their relaxation occurs via misfit dislocations creation. However, there exists certain critical thickness he, such that these dislocations appear Sith>hc only. The calculations had shown [7] that he 1/1/ and for PbTiOs films on MgO or SrTiOs substrates he — 0.5 nm or 8.3 nm respectively. 

One proposed simplified theory (4) provides reasonably accurate predictions of the internal flow characteristics. In this analysis, conservation of mass as well as angular and total momentum of the Hquid is assumed. To determine the exit film velocity, size of the air core, and discharge coefficient, it is also necessary to assume that a maximum flow through the orifice is attained. 

At X-ray fluorescence analysis (XRF) of samples of the limited weight is perspective to prepare for specimens as polymeric films on a basis of methylcellulose [1]. By the example of definition of heavy metals in film specimens have studied dependence of intensity of X-ray radiation from their chemical compound, surface density (P ) and the size (D) particles of the powder introduced to polymer. Have theoretically established, that the basic source of an error of results XRF is dependence of intensity (F) analytical lines of determined elements from a specimen. Thus the best account of variations P provides a method of the internal standard at change P from 2 up to 6 mg/sm the coefficient of variation describing an error of definition Mo, Zn, Cu, Co, Fe and Mn in a method of the direct external standard, reaches 40 %, and at use of a method of the internal standard (an element of comparison Ga) value does not exceed 2,2 %. Experiment within the limits of a casual error (V changes from 2,9 up to 7,4 %) has confirmed theoretical conclusions. 

Additive, slip An additive modifier that acts as an internal lubricant which exudes to the surface of the plastic during and immediately after processing providing the necessary lubricity to reduce or eliminate coefficient of friction in molded parts, film, etc. products. 

Checking the absence of internal mass transfer limitations is a more difficult task. A procedure that can be applied in the case of catalyst electrode films is the measurement of the open circuit potential of the catalyst relative to a reference electrode under fixed gas phase atmosphere (e.g. oxygen in helium) and for different thickness of the catalyst film. Changing of the catalyst potential above a certain thickness of the catalyst film implies the onset of the appearance of internal mass transfer limitations. Such checking procedures applied in previous electrochemical promotion studies allow one to safely assume that porous catalyst films (porosity above 20-30%) with thickness not exceeding 10pm are not expected to exhibit internal mass transfer limitations. The absence of internal mass transfer limitations can also be checked by application of the Weisz-Prater criterion (see, for example ref. 33), provided that one has reliable values for the diffusion coefficient within the catalyst film. 

Increasing the velocity of flow of the liquor through tubes results in a significant increase in the liquid-film transfer coefficient. This is achieved in the forced circulation units where a propeller or other impeller is mounted in the central downcomer, or a circulating pump is mounted outside the evaporator body. In the concentration of strong brines, for example, an internal impeller, often a turbine impeller, is fitted in the downtake, and this form... 

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