Wetting rate | Wetting factor | in Robert, kestner & Falling Film Evaporators

Concept and formulas for Wetting number / Wetting rate / wetting factor  in Evaporators

Wetting rate : Sufficient wetting rate of the heating surface by liquid is extremely important for trouble-free operation. Apart from the heating tube dimensions, it is very important to choose a suitable distribution device to achieve full wetting. Due to the low liquid content and high flow velocity in the heating tubes, the residence time of the product is very short. This not only reduces heat injury to the juice but also scaling intensity is reduced. Wetting rate is also known as juice loading per unit area of the perimeter.

Wetting rate in Robert type evaporators:

Flow of juice or syrup for each tube:  When the calculation of a multiple effect has been completed with the dimensions of vessels and their tubes, and the number of tubes, it is advisable to verify the flow of juice or syrup for each tube. Generally, for regular operation, it is necessary to maintain 40 kg/h  of juice or syrup per tube, calculated on the quantity entering the vessel. This rate is easily attained for the first vessel, but not for the later ones. Where it is not reached, it is necessary to provide for recycling juice between the outlet and the inlet of the vessel, in order to ensure complete wetting of the tubes and avoid periods of operation with partly dry tubes at times of low juice flow. The rate of 40 kg/hr/tube is considered as optimal, and the recycling pump is calculated on this quantity.

In some designers for later bodies (especially for 3rd effect) provide partition plates in the middle of the top tube plate and bottom saucer for the separation of the body into two parts to achieve maximum syrup for each tube

Wetting Rate Calculation in Robert’s Body:

Wetting rate for Kestner evaporators:

Kestner evaporators work better if the flow per tube (51 mm tubes) is kept above 80 kg/h (Rein and Love 1995). The rate of fouling is also reduced flow rate, leading to the theory that low flow rates per tube can lead to partial dry-out of the heating surface. In some cases a recirculation line is installed on Kestner to achieve these rates However, in Robert and Kestner and evaporators, there is no need for any control systems to keep the tubes wetted.

Wetting rate for Falling Film Evaporators:

In Falling Film Evaporators require uniform and adequate wetting of the tubes and so need the complication of juice recirculation and a distribution system feeding equal quantities of juice to every tube. An emergency water feed to the evaporator is also required, in case the juice supply fails; the absence of adequate feed even for a short time can lead to severe fouling of the tubes. The main problem, with a descending-film evaporator, is the distribution of juice on the upper tube plate. It is necessary that all tubes from this plate, or from one section of the tube plate, should receive the required volume of juice so that the juice is distributed only in a thin layer around the tube, and that no section of the tube remains dry at any moment.

As per E. Hugot, the wetting of the tubes by the juice is calculated as

where

• • Q is the weight of juice in kg/hr,
  • n is number of tubes, and
  • d is the diameter of the tube in meters.

The optimum value varies from 500 to 600 kg/hr/m of circumference of tubes, to a maximum of 1200 to 1500 kg/hr/m.

As per CANE SUGAR ENGINEERING by Peter rein

The falling film tubular evaporator has to have a minimum flow rate through each tube, and needs a pump-around system to recirculate liquid. This requires a level of liquid held in the sump of the vessel and a control system to ensure that the required liquid flow to the distributor is achieved. The falling film plate evaporators need a similar system, although the wetting rate required is lower.

Generally 8-16 L/cm/hr in a falling film tubular evaporator in the beet industry and for falling film tubular evaporators in the cane industry, a much higher wetting rate of 12 to 15 L/cm/hr is recommended. Good juice distribution is the key to good performance from falling film evaporators.

Wetting rate calculation in FFE

The constant wetting of the heating tubes with juice is of great importance for the trouble-free operation of a falling film evaporator. The wetting is indicated by the wetting rate W (l/h.cm). Wetting rate sometimes can also  defined as Kg/ meter-sec.

(l/h.cm)

W=Q x L  /H x 100 x D (l/h.cm)

or

(l/h.cm)

W= Q /  D x π x n x ID (l/h.cm)

• • W = Wetting rate (l/h.cm)
  • Q = juice flow at the bottom of the tube (kg/ h)
  • L = tube length (m)
  • H = heating surface (m² )
  • D = juice density (kg/ l)
  • n= no of tubes
  • ID = ID of the tube in cm.

According to the author Falling Film evaporator Wetting factor is to be maintained minimum 2000 Lt/hr/m. According to that, we can calculate the capacity of the circulation pump.

Online Calculator to estimate circulation pump capacity in Falling Film Evaporator

Click Here

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