Local boiling

Ca.ustic Corrosion. Concentration of caustic (NaOH) can occur either as a result of steam blanketing (which allows salts to concentrate on boiler metal surfaces) or by localized boiling beneath porous deposits on tube surfaces. 

If the responsible specific corrodent cannot be entirely removed from the environment, it may be beneficial to reduce its concentration, since susceptibility to SCC is frequently concentration dependent. However, if this method of elimination is chosen, care must be taken to avoid conditions that will increase concentration locally, such as evaporation, localized boiling, alternate wet and dry conditions, crevices, and deposits. 

Sub-cooled boiling in this region the liquid next to the wall has reached boiling point, but not the bulk of the liquid. Local boiling takes place at the wall, which increases the rate of heat transfer over that given by forced convection alone. 

Note that the local boiling void calculated this way is independent of channel length. The prediction of the point of bubble departure (void detachment) is, however, important in predicting the subcooled boiling void. Rouhani (1967) assumed... 

Subcooled nucleate boiling is frequently called local boiling or surface boiling. 

Liquid core temperature and velocity distribution analysis. BankofT (1961) analyzed the convective heat transfer capability of a subcooled liquid core in local boiling by using the turbulent liquid flow equations. He found that boiling crisis occurs when the core is unable to remove the heat as fast as it can be transmitted by the wall. The temperature and velocity distributions were analyzed in the singlephase turbulent core of a boiling annular flow in a circular pipe of radius r. For fully developed steady flow, the momentum equation is given as... 

The flow friction of a channel having subcooled local boiling behaves in a manner similar to the flow friction of a channel having a rough surface. From air-water data obtained at low void fractions (Malnes, 1966), the two-phase flow friction factor without bubble departure was found (Tong, 1968b Thorgerson, 1969)to be... 

For convenience of application, a simplification of subcooled boiling length dnb is suggested to be measured from the inlet rather than from the inception of local boiling. This simplification has two justifications. First, the empirical expres-... 

DNB EU — axial location at which DNB occurs for uniform heat flux (in.), starting from inception of local boiling... 

Bernath, L., 1960, A Theory of Local Boiling Burnout and Its Application to Existing Data, Chem Eng. Prog, Symp. Ser. 56(30) 95- 116. (2)... 

Dilution of cone, sulfuric acid by water is vigorously exothermic and must be effected by adding acid to water to avoid local boiling. Mixtures of sulfuric acid and excess snow form powerful freezing mixtures. Fuming sulfuric acid (oleum, containing sulfur trioxide) reacts violently with water. 

Experimentally, several precautions must be taken if reliable Raman data are to be obtained from solution studies. Firstly, the instrumental slit-width should be appreciably smaller than the half-width of the band to be studied. This means that slits wider than 2 cm-1 are to be avoided. Secondly, photolytic decomposition of the sample and local boiling of the solvent have also to be avoided. Careful choice of laser frequencies, use of a low incident power and, if necessary, sample spinning are indicated. The need for a relatively high solute concentration usually means that there is little choice of solvent. Particularly for coloured samples the presence of a vestigal resonance Raman effect must be tested by measurements with a variety of... 

If a gas pipe is closed-in, internal convection provides only a modest cooling effect, with little effect on the rate of temperature rise. However, for liquids, local boiling may occur that causes the internal pressure to increase and may rupture the pipe if not relieved. 

Sulphuric acid, approximately 1.5 M - slowly with stirring, add 80 ml sulphuric acid, approximately 98% m/m H SO, to about 800 ml water in a 2-1 beaker Note sulphuric acid is highly corrosive and generates heat when diluted standing the beaker in a sink with a few centimetres of cold water before adding the acid will reduce any likelihood of localized boiling. Wear PPE for this step.) Cool and dilute to 1 I. 

One additional feature of upward vertical flow with heat transfer needs to be mentioned. If subcooled liquid is present, local boiling may occur, that is, vapor bubbles may be formed at the wall which will col-... 

Wright (W3) has recently measured local boiling coefficients in vertical downflow systems, which have the advantage of being free of hydrostatic-head effects. His results were correlated reasonably well by equations of the type of (91) or (92),... 

Caution. Pouring the sulfonation mixture into ice generates a high amount of heat and should be done carefully to avoid splashing and local boiling ... 

Cavitation corrosion is caused by repeated pounding resulting from rapid collapse of vapor bubbles. On a metal surface where there is a violent flow of water, such as that of a ship s propeller, the pressure at some area may be reduced to where localized boiling forms bubbles of vapor. At another site, these bubbles suddenly collapse. 

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