Friction sensibility

In a submerged-tube FC evaporator, all heat is imparted as sensible heat, resulting in a temperature rise of the circulating hquor that reduces the overall temperature difference available for heat transfer. Temperature rise, tube proportions, tube velocity, and head requirements on the circulating pump all influence the selec tion of circulation rate. Head requirements are frequently difficult to estimate since they consist not only of the usual friction, entrance and contraction, and elevation losses when the return to the flash chamber is above the liquid level but also of increased friction losses due to flashing in the return line and vortex losses in the flash chamber. Circulation is sometimes limited by vapor in the pump suction hne. This may be drawn in as a result of inadequate vapor-liquid separation or may come from vortices near the pump suction connection to the body or may be formed in the line itself by short circuiting from heater outlet to pump inlet of liquor that has not flashed completely to equilibrium at the pressure in the vapor head. 

This represents the fraction of the total available head between points A and B, which represents the sensible heating zone. This neglects liquid friction in the sensible zone and assumes the liquid level in the distillation column is maintained even with the top of the tubesheet. Equation 10-170 then gives the fractional tube length devoted to sensible heating. Refer to Figure 10-110 and note that ... 

It will be assumed that the liquid level is maintained even with the top tubesheet. Neglecting inlet line friction, the sensible heating zone length may be estimated from Equation 10-176 ... 

Reib-Empfindlichkeit friction sensitivity sensibility au frottement... 

The effect of strain amplitude is most pronounced in compounds containing reinforcing fillers and can result in a reduction in shear modulus of as much as a factor of 4 when going from a very small strain to about 10%. This is due to breakdown of filler structure which is associated with energy losses that cause a peak in the tan8 value. It was because of this that earlier British and international standards called for tests to be made at 2 and 10% shear strain, a sensible recommendation that has been overlooked in the present version. Turner6 produces an interesting model based on frictional elements to explain this behaviour. 

If the liquid resting on a solid is volatile, it can distribute itself along the surface of the solid through the vapour thus Hardy6 found that only those acids and alcohols which have a sensible vapour pressure distribute themselves along the surface, from a drop in one spot, so as to lubricate it in any reasonable time. It does not appear to be necessary, for this kind of spreading to occur to some extent, that the contact angle should be zero it was found that the coefficient of friction of paraffin wax was... 

Osmotic Pinch Effect Feed is pumped into the membrane train, and as it flows through the membrane array, sensible pressure is lost due to friction effects. Simultaneously, as water permeates, leaving salts behind, osmotic pressure increases. There is no known practical alternative to having the lowest pressure and the highest salt concentration occur simultaneously at the exit of the train, the point where AP - AH is minimized. This point is known as the osmotic pinch, and it is the point backward from which hydraulic design takes place. A corollary factor is that the permeate produced at the pinch is of the lowest quality anywhere in the array. Commonly, this permeate is below the required quality, so the usual practice is to design around average-permeate quality, not incremental quality. A 1 MPa overpressure at the pinch is preferred, but the minimum brine pressure tolerable is 1.1 times n. 

Liquid temperatures in the tubes of an LTV evaporator are far from uniform and are difficult to predict. At the lower end, the liquid is usually not boiling, and the liquor picks up heat as sensible heat. Since entering liquid velocities are usually very low, true heat-transfer coefficients are low in this nonboiling zone. At some point up the tube, the liquid starts to boil, and from that point on the liquid temperature decreases because of the reduction in static, friction, and acceleration heads until the vapor-liquid mixture reaches the top of the tubes at substantially vapor-head temperature. Thus the true temperature difference in the boiling zone is always less than the total temperature difference as measured from steam and vapor-head temperatures. 

The calculation of the resistance coefficients can be accomplished in the frame of the Monin-Obukhov similarity theory (Monin and Yaglom, 1971). The genuine flux quantities are the friction velocity u and the scale functions 0 and referring to temperature and humidity. The turbulent momentum flux f, the sensible heat flux the mass flux from evaporation and condensation and the corresponding latent heat flux are... 

In addition to friction, which is often confused with adhesion, there are several other phenomena which give attraetions between bodies. These phenomena can therefore mix up the issues of moleeular adhesion. As Newton wrote the attractions of Gravity, Magnetism and Electricity reach to very sensible distanees, and so have been observed by vulgar Eyes, and there may be others whieh reaeh to so small distances as hitherto escape observation. ... 

So far, we have considered neutral conditions in the surface layer. In cases with light to moderate wind speeds, the effects of surface heating and cooling also become rel-evant. Consequently, the virtual sensible heat flux w9m at the surface needs to be considered as an important variable as well [because it impacts on the buoyancy term in Eq. (3)]. The combination of surface heat flux with friction velocity defines a characteristic length scale L, given by... 

---