Condensing coefficient

Here, if Z is expressed in moles of collisions per square centimeter per second, r is in moles per square centimeter. We assume the condensation coefficient to be unity, that is, that all molecules that hit the surface stick to it. At very low Q values, F as given by Eq. XVII-3 is of the order expected just on the basis that the gas phase continues uniformly up to the surface so that the net surface concentration (e.g., F2 in Eq. XI-24) is essentially zero. This is the situation... 

Explain on what basis the condensation coefficient, c, might be expected to depend on cos 6, where 6 is the angle from the normal of an incident beam of particles (note Ref. 122). 

If the fraction of sites occupied is 0, and the fraction of bare sites is 0q (so that 00 + 1 = 0 then the rate of condensation on unit area of surface is OikOo where p is the pressure and k is a constant given by the kinetic theory of gases (k = jL/(MRT) ) a, is the condensation coefficient, i.e. the fraction of incident molecules which actually condense on a surface. The evaporation of an adsorbed molecule from the surface is essentially an activated process in which the energy of activation may be equated to the isosteric heat of adsorption 4,. The rate of evaporation from unit area of surface is therefore equal to... 

N tubes in a vertical row where the total condensate flows smoothly from one tube to the one beneath it, without splashing, and still in laminar flow on the tube, the mean condensing coefficient h i for the entire row of N tubes is related to the condensing coefficient for the top tube hi by... 

Another correladou for vapor-shear-coutrolled condensation is the Boyko-Kruzhiliu correlation [Jnt. J. Heat Ma.s.s Transfer, 10, 361 (1967)], which gives the mean condensing coefficient for a stream between inlet quality ac and outlet quality ay, hD,... 

With tube side condensation, coefficients are generally lower than for comparable shell side condensers. This phenomenon is attributed to (1) lower coolant velocities outside the tubes than are possible with tube side cooling, and (2) increased film thicknesses, namely, film resistance inside the tubes. 

Air or inerts can cause up to 50 percent reduction in condensation coefficients. 

The charts of Chen are also useful for solving the equations for condensing coefficients however, the correction for the effect of multiple fluid stream is not included. Therefore, the results should he conservative. 

Summer area required (not making any correction for change in water film coefficient or condensing coefficient) ... 

Figure 10-76. Condensation down-flow in vertical tubes. Note = average value of condensing coefficient between two points G = con-...

Note The value of hj, is not correct for the condensing coefficient as described in step 6 of this outline. In this case, the error is small, but that is not necessarily true for other situations. 

Determine condensing coefficient tube loading. Figure 10-67A. 

It is to be noted that only the steam-condensing coefficient will change (lower tube loading, increasing hjo). Because an arbitrary maximum value was used for coefficients, the overall U will not change nor will the At. Therefore, only the available area of the new sized unit needs to be checked against the previously calculated required area. For a 12-in. shell with 32 tubes available ... 

D,.g = an equivalent diameter of a finned tube for calculation of condensing coefficient, ft. 

Two flow models are used to estimate the mean condensation coefficient in horizontal tubes stratified flow, Figure 12.45a, and annular flow, Figure 12.45. The stratified flow model represents the limiting condition at low condensate and vapour rates, and the annular model the condition at high vapour and low condensate rates. For the stratified flow model, the condensate film coefficient can be estimated from the Nusselt equation, applying a suitable correction for the reduction in the coefficient caused by... 

Estimate tube wall temperature, Tw assume condensing coefficient of 1500 W/m2oC, Mean temperature... 

Take steam condensing coefficient as 8000 W/m2oC, fouling coefficient 5000 W/m2oC butane fouling coefficient, essentially clean, 10,000 W/m2oC. 

For condensation inside horizontal tubes, the Nusselt Equation can be applied with a correction for the reduction in condensing coefficient caused by the accumulation of condensation. The correction usually applied is 0.8. No correction for the number of tubes is required. Thus, for condensation inside horizontal tubes ... 

Particular care needs to be adopted if a vapor to be condensed has noncondensable gases present. Here the vapor diffuses through the gas to the cold surface where it condenses. But as the condensation proceeds, the concentration of the noncondensable gas increases, which increases the diffusional resistance and decreases the condensing coefficient. To take this into account requires complex models, which is outside the scope of this text. 

Ec is a condensation coefficient. Schrage (S2) has shown that the absolute rate of vaporization is given by... 

---