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

Concept and formulas for Wetting number or Wetting rate or 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 perimeter.

Wetting rate or wetting factor in Evaporators-sugarprocesstech

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 assure complete wetting of the tubes and avoiding 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) providing partition plates in the middle of the top tube plate and bottom saucer for separation of the body into two parts to achieve maximum syrup for each tube

wetting rate calculation:

Wetting \ rate \ in \ kg/hr/tube= \frac{ Juice \ Flow \ Rate \ in \ kg/hr}{No. \ of \ tubes}

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 all this 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 tube remains dry at any moment.

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

Wetting \ rate \ in \ kg/hr/tube \ = \frac{Q}{n \times \pi \times d}

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

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 ENGINERING 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

The constant wetting of the heating tubes with juice is of great importance for a 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.

W=Q x L  /H x 100 x D (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 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 author Falling Film evaporator Wetting factor to be maintained minimum 2000 Lt/hr/m. According to that we can calculate capacity of circulation pump.

Online Calculator to estimate circulation pump capacity in Falling Film Evaporator

Click Here


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Post Author: siva alluri

The aim of this Blog "sugarprocesstech" is Providing basic to advance knowledge in sugar process industry and providing maximum calculation regarding capacity and equipment design online calculators .

20 thoughts on “Wetting rate | factor | in Robert, kestner & Falling Film Evaporators

    Sushil Kumar Singh

    (June 1, 2017 - 4:16 am)

    Please White in more détail.

    Suryakant Takawane

    (June 1, 2017 - 5:32 am)

    Good techenical information

    R S kulkarni

    (July 20, 2017 - 5:59 am)

    Dear sir
    Robert evaporator wetting information is very good.however one thing is not clear you have mentioned automation/arrangment is not required. to maintain wetting of tubed. This sounds to me contradictory to you said above.kindly clear the meaning of it.l am not able to interpret clearly
    Sugar sugar

      siva alluri

      (July 20, 2017 - 3:47 pm)

      Thank you very much for your valuable comment sir,
      In this I this my opinion is if we have provided correct wetting rate for Robert and Kestner evaporators than we can run evaporators with or without control system. In case of FFE evaporator compulsory need control systems. Its my personal opinion tell me about your opinion.

      The “www.sugarprocesstech.com” invites from all technologists to share your knowledge, achievements in your working organization and new developments and technologies in sugar industry and its concerned units. It is very much helpful to show your identity to the world at the same time it will helpful to another technologist to enhance their insight and enhance great execution in there working.


      (October 11, 2021 - 6:14 am)

      What’s the evaporation rate of FFE (kg/hr/m2)?

        siva alluri

        (January 2, 2022 - 8:46 am)

        Its depends upon the number of effects.

    R S kulkarni

    (July 22, 2017 - 4:16 am)

    Dear sir
    I got your point. My concern was about Robert evaporator only.You mean to say maintain wetting number for Robert bodies 40 ltr/tube/hr. I agree for it. Shah and pickok has presented their work on Google.They have also found this figure.
    How ever evaporator automation is required to maintain steady operation without concentrating in some body having more heating surface.Hence pl recommend evaporator automation.
    Ugar sugar

      siva alluri

      (July 23, 2017 - 5:12 pm)

      Thank you for expressing your opinion, I am also agree with you
      According to your interest I requested Mr. Kulkarni submit one article from sugar industry technologies.

    ramesh shivaji bawane

    (May 11, 2018 - 8:30 am)

    nice information on FFE wetting rate calculation

    ramesh shivaji bawane

    (May 11, 2018 - 8:34 am)

    please mention regarding how to calculate pump with their pump head for FFE in co operate with the wetting rate

      siva alluri

      (May 13, 2018 - 4:43 pm)

      Pump head calculated in three basic concepts
      1. Pump shaft to FFE inlet elevation.
      2. Head loss due pipe line fittings and its length
      3 FFE inside pressure
      All these parameters to be convert into MWC then it will gives required pump head
      Go through the below link for more information regarding to calculation of Head loss due to pipe line length in pump suction and delivery

    Deepak Raut

    (August 31, 2018 - 6:45 am)

    The information on wetting rate is really useful.


    (February 24, 2019 - 5:31 am)

    Dear sir
    FFE,s evaporator wetting information is very good plz calculation of Circulation pump in detail.

      siva alluri

      (February 24, 2019 - 5:17 pm)

      it is very simple
      For example
      No. of tubes of FFE = 2600
      ID of the tube = 32 mm
      Wetting facotor – 2000 lt/hr/m
      then circulation pump capacity = 2000 x ∏ x 0.032 x 2600 = 522496 Lt/hr ~ 530 m3/hr

    Ajay Singh

    (March 15, 2019 - 8:58 am)

    Plz give the calculation of FFE juice film thickness and retaintion time


    (April 12, 2020 - 3:11 am)

    For any falling film evaporator only wetting rate can not be the criteria for design or calculation. Overall heat transfer coefficient also plays crucial roll in the calculation. For Robert evaporators dessin coefficient gives an approximate estimation whereas there is no such Coefficient yet established for falling film evaporator so my question is what should be e recommended overall heat transfer coefficient for sugar FFE.

      siva alluri

      (April 26, 2020 - 12:03 pm)

      Wetting factor is not a only key parameter for design. But it play a key role for design circulation pump capacity and avoid jaming of tubes, getting good evaporation rate.


        (August 6, 2020 - 8:03 am)

        dear sir,
        All parameters remaining same , if optimum wetting rate is maintained in any type of evaporators, there is definite improvement in OHTC .There are many articles to prove the same. There is sufficient proof of the same in literature. There is also adequate literature for OHTC figures for robert and FFE type eveporators.

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