Vapour velocity

A high vapour velocity upwards tends to increase the thickness of the film and thus reduce h though the film may sometimes be disrupted mechanically as a result of the formation of small waves. For the downward flow of vapour, TEN Bosch<5 9 has shown that h increases considerably at high vapour velocities and may increase to two or three times the value given by the Nusselt equation. It must be remembered that when a large... 

Under conditions of high vapour velocity Carpenter and Colburn 9 have shown that turbulence may occur with low values of the Reynolds number, in the range 200-400. When the vapour velocity is high, there will be an appreciable drag on the condensate him and the expression obtained for the heat transfer coefficient is difficult to manage. 

Carpenter and Colburn have put forward a simple correlation of their results for condensation at varying vapour velocities on the inner surface of a vertical tube which takes the form ... 

Carpenter, E.F. and Colburn, A.P. Proceedings of the General Discussion on Heat Transfer (September 1951) 20. The effect of vapour velocity on condensation inside tubes (Inst, of Mech. Eng. and Am. Soc. Mecli. Eng.). 

Actual residence time = vessel length/vapour velocity... 

Increasing the vessel diameter will have also changed the vapour velocity and the height above the liquid surface. The liquid separation will still be satisfactory as the velocity, and hence the residence time, is inversely proportional to the diameter squared, whereas the distance the droplets have to fall is directly proportional to the diameter. 

The principal factor that determines the column diameter is the vapour flow-rate. The vapour velocity must be below that which would cause excessive liquid entrainment or a high-pressure drop. The equation given below, which is based on the well-known Souders and Brown equation, Lowenstein (1961), can be used to estimate the maximum allowable superficial vapour velocity, and hence the column area and diameter,... 

The flooding condition fixes the upper limit of vapour velocity. A high vapour velocity is needed for high plate efficiencies, and the velocity will normally be between 70 to... 

The lower limit of the operating range occurs when liquid leakage through the plate holes becomes excessive. This is known as the weep point. The vapour velocity at the weep point is the minimum value for stable operation. The hole area must be chosen so that at the lowest operating rate the vapour flow velocity is still well above the weep point. 

Several correlations have been proposed for predicting the vapour velocity at the weep point see Chase (1967). That given by Eduljee (1959) is one of the simplest to use, and has been shown to be reliable. 

Un Vapour velocity based on net cross-sectional area LT-1... 

Vapour velocity (vapour flowing alone in tube)... 

In partial condensation it is usually better to put the condensing stream on the shell-side, and to select a baffle spacing that will maintain high vapour velocities, and therefore high sensible-heat-transfer coefficients. 

A common practice is to calculate the pressure drop using the methods for single-phase flow and apply a factor to allow for the change in vapour velocity. For total condensation, Frank (1978) suggests taking the pressure drop as 40 per cent of the value based on the inlet vapour conditions Kern (1950) suggests a factor of 50 per cent. 

The freeboard between the liquid level and shell should be at least 0.25 m. To avoid excessive entrainment, the maximum vapour velocity uv (m/s) at the liquid surface should be less than that given by the expression ... 

A continuous fractionating column is required to separate a mixture containing 0.695 mole fraction //-heptane (C7H16) and 0.305 mole fraction n-octane (C8H18) into products of 99 mole per cent purity. The column is to operate at 101.3 kN/m2 with a vapour velocity of 0.6 m/s. The feed is all liquid at its boiling-point, and this is supplied to the column at 1.25 kg/s. The boiling-point at the top of the column may be taken as 372 K, and the equilibrium data are ... 

It is desired to concentrate a mixture of ethyl alcohol and water from 40 mole per cent to 70 mole per cent alcohol. A continuous fractionating column, 1.2 m in diameter with 10 plates is available. It is known that the optimum superficial vapour velocity in the column at atmosphere pressure is 1 m/s, giving an overall plate efficiency of 50 per cent. 

It has been found by Carey(54), Carey el al. 55> and Souders and Brown(56) that the vapour velocity is a prime factor in determining the diameter of a column. Kirschbaum,57) using an equimolar mixture of ethanol and water on a 400 mm diameter plate containing 15 bubble caps, obtained the following results ... 

The extent of entrainment of the liquid by the vapour rising over a plate has been studied by many workers. The entrainment has been found to vary with the vapour velocity in the slot or perforation, and the spacing used. Strang 60-1, using an air-water system, found that entrainment was small until a critical vapour velocity was reached, above which it increased rapidly. Similar results from Peavy and Baker 6 11 and Colburn 62 have shown the effect on tray efficiency, which is not seriously affected until the entrainment exceeds 0.1 kmol of liquid per kmol of vapour. The entrainment on sieve trays is discussed in Section 11.10.4. 

---