VAPORIZER IN PROCESS INDUSTRY

Vaporizers are heat exchangers which are specially designed to supply latent heat of vaporization to the fluid. In some cases it can also preheat the fluid then this section of vaporizers will be called upon preheating zone and the other section in which latent heat is supplied; is known as vaporization zone but he whole assembly will be called upon a vaporizer.

Vaporizers are called upon to fulfill the multitude of latent-heat services which are not a part of evaporative or distillation process.

There are two principal types of tubular vaporizing equipment used in industry: Boilers and Vaporizing Exchangers. Boilers are directly fired tubular apparatus, which primarily convert fuel energy into latent heat of vaporization. Vaporizing Exchangers are unfired and convert latent or sensible heat of one fluid into the latent heat of vaporization of another. If a vaporizing exchanger is used for the evaporation of water or an aqueous solution, it is now fairly conventional to call it an Evaporator, if used to supply the heat requirements at the bottom of a distilling column, whether the vapor formed be stream or not, it is a Reboiler ; when not used for the formation of steam and not a part of a distillation process, a vaporizing exchanger is simply called a vaporizer.So any unfired exchanger in which one fluid undergoes vaporization and which is not a part of an evaporation or distillation process is a vaporizer

The commonest type of vaporizer is the ordinary horizontal 1-2 exchanger or one of its modifications, and vaporization may occur in the shell or in the tubes. If steam is the heating medium, the corrosive action of air in the hot condensate usually makes it advantageous to carry out the vaporization in the shell.

In the case of vaporizer, however, operation is often at high pressure, and it is usually too expensive to provide disengagement space in the shell m, since the inclusion of disengagement space at high pressures correspondingly increases the shell thickness. For this reason vaporizers are not usually designed for internal disengagement. Instead some external means. Such as an inexpensive welded drum, is connected to the vaporizer where in the entrained liquid is separated from the vapor.

When a 1-2 exchanger is used as a vaporizer, it is filled with tubes and cannot be adapted for blowdown, since all the feed to a vaporizer is usually of value and a rejection as blowdown is prohibitive. If the feed were completely vaporized in the vaporizer, it would emerge as a vapor and any dirt which was originally present would be left behind on the tube surface over which total vaporization of occurred, fouling it rapidly, If the 1-2 exchanger (vaporization) were over-designed, that is, if it contained too much surface, disengagement would have to occur on the tubes and due to the excess surface the vapor would superheat above its saturation temperature.

 The feed to a vaporizer should not be vaporized completely. The value of this rule is apparent. If less that 100 percent to the feed is vaporized in 1-2 exchangers, the residual liquid can be counted on to prevent the accumulation of dirt directly on the surface of the heating element. A maximum of about 80 percent vaporization appears to provide favorable operation in 1-2 exchanges, although higher percentages may be obtained in vessels having interval disengagement space.

Forced and Natural – circulation Vaporizer. When liquid is fed to is fed by forced circulation. The circuit consists of a 1-2 exchanger serving as the vaporizer and a disengaging drum from which the unvaporized liquid is withdrawn and recombined with fresh feed. The generated vapor is removed form the top of the drum.

The vaporized may also be connected with a disengaging drum without the use of a reticulating pump. This scheme is natural circulation. It requires that the disengaging drum be elevated above the vaporizer. The advantages of forced circulation or natural circulation are in part economics and a part dictated by space. The forced-circulation arrangement requires the use of a pump with its continuous operating cost and fixed charges. As with forced-circulation evaporators, the rate of feed recirculation can be controlled very closely. If the installation is small, then use of a pump preferable. If a natural-circulation arrangement is used pump and stuffing box problems are eliminated but considerably more headroom must be provided and recirculation rates cannot be controlled so readily.

The vaporization of a cold liquid coming from storage, the liquid may not be at its boiling point and may require preheating to the boiling point. Since the shell of a forced-circulation vaporizer is essentially the same as any other 1-2 exchangers, the preheating can be done in the same shell as the vaporization. If the period of performance of a vaporizer is to be measured by a single overall dirt factor, it is necessary to divide the shell surface into two successive zones, one for preheating and one for vaporization.

 The true temperature difference is the weighted temperature difference for the two zones, and the clean coefficient is the weighted clean coefficient.

Vaporizers tend to accumulate dirt, and for his reason higher circulation rates and large dirt factors will often be desirable. Preference should be given to the use of square pitch and a removable tube bundle. Although it may reduce the possibility of using a 1-2 vaporizing exchanger for other services, the baffle spacing can be increased or staggered form inlet to outlet to reduce the pressure drop of the fluid vaporizing in the shell.