Flash Vapour Calculation | Flash Vapour Recovery Vessel Design Calculation

Condensate flash vapour recovery system calculation with example

What is Flash Vapour:

The liquid suddenly passing from high pressure to low pressure condition then produces a spontaneous evaporation without any external heat energy it is called “flash” or flash vapour.

The flash vapour recovery is one of the important energy conservation system in all process industries. Simple we can say that flash pot is one of the energy conservation device.

The flash pot having compartments with different pressures. The comportment connected to subsequent bodies of evaporator set.

The high temperature water suddenly pass from high pressure compartment to lower pressure compartment. It thus produces a spontaneous evaporation or “flash”, which takes place almost explosively by reason of the physical incompatibility of the conditions of pressure and temperature. The quantity of water evaporated will correspond exactly to the quantity of heat given up.

The main steps in the flash vapour recovery vessel system installation.

  • Collection of the data. Find the absolute pressure of each compartment.
  • Inlet and outlet water temperature with quantity for each compartment.
  • Calculate the flash vapour generation quantity in each compartment.
  • Find the ascending and descending branch lines dia of each compartment.
  • Calculate siphon height from one compartment to next compartment.
  • Finally calculate compartment area required for the flash vapour and flash vapour line dia.

In this article calculation explained by the taking with example

Here consider flash vapour system designed for 5000TCD crushing capacity plant. The flash pot for 1st , 2nd & 3rd vapours of  quintuple evaporator set.

Generally, for 1st effect condensate purpose design separate flash pot or use PHE for heat recovery from that condensate water.

Find the inlet water temperature with quantity of the each compartment.

Formula for finding the evaporator body outlet condensate temperature.

Temperature of condensates

It is generally assumed that the condensates leaving a calendria are at the temperature of the vapour used in that calendria. Actually the condensates cool down slightly in running along the tubes, and their temperature may be given approximately.

Tc = Tv – 0.4 ( Tv – Tj )

Tc= temperature of the condensates leaving the calendria.

Tv = temperature of the vapour used for heating the calendria.

Tj = temperature of the juice in the vessel.

Consider  1st body calendria Absolute Pressure = 1.2 + 1.03323 = 2.23323 Kg/cm2

5 th body vapour pressure = 0.17674 Kg/cm2 (630 mm of Hg).

  ΔP Pressure difference in absolute Pressure  In absolute Temperature in oC. Latent heat in Kcal/kg ( λ ) Total heat in Kcal/kg Specific volume  in M3/kg
Exhust 2.23323   123.1 524 647 0.81
1st body 0.4524 1.7808 115.9 529 645 1.01
2nd body 0.4319 1.3489 107.6 534 642 1.31
3rd body 0.4113 0.9376 97.2 541 638 1.84
4th body 0.3907 0.5469 83.1 549 632 3.04
5th body 0.3702 0.1767 57 565 622 8.76

For more information about calculation of pressure drop across the Multiple Effect Evaporator go through the below link

Pressure drop in Triple, Quadruple and Quintuple effect evaporator bodies with online calculator

Calculation for the  1st Compartment :

Water inlet temperature – Ti = 115.9 –  0.4 ( 115.9 – 107.6) = 115.9 – 3.32 = 112.58 oC

Water outlet temperature  To = 107.6 oC

Condensate inlet Water quantity to the compartment (Q ) = 70 T/hr (As per the vapour bleeding calculation).

Specific heat of the vapour ( Cp ) = 1 Kcal/ kg/ oC

Flash Vapour generation = Q x Cp x ΔT / λ   =70 x 1 x ( 112.58 – 107.6) / 534 =  0.653 T/hr = 0.653 x 1000 / 3600 = 0.1814 kg/sec = 0.1814 x 1.01 = 0.2366 M3/sec

Calculation for the  2nd Compartment :

Water inlet temperature – Ti = 107.6 –  0.4 (107.6 – 97.2) = 107.6 – 4.16 = 103.44 oC

Water outlet temperature  To = 97.2 oC

Condensate inlet Water quantity to the compartment (Q ) = 70 + 63 = 133 T/hr (As per the vapour bleeding calculation).

Specific heat of the vapour ( Cp ) = 1 Kcal/ kg/ oC

Flash Vapour generation = Q x Cp x ΔT / λ   =133 x 1 x (103.44 – 97.2) / 541 =  1.535 T/hr = 1.535 *1000/3600 = 0.42639 kg/ sec = 0.42639 x 1.84 = 0.784 M3 /sec


Calculation for the  3rd Compartment :

Water inlet temperature – Ti = 97.2 –  0.4 ( 97.2 – 83.1) =97.2 – 5.64 = 91.56 oC

Water outlet temperature  To = 83.1 oC

Condensate inlet Water quantity to the compartment (Q ) = 70+63+31 = 164 T/hr (As per the vapour bleeding calculation).

Specific heat of the vapour ( Cp ) = 1 Kcal/ kg/ oC

Flash Vapour generation = Q x Cp x ΔT / λ   =164 x 1 x ( 91.56 – 83.1) / 549 =  2.526 T/hr = 2.526 x 1000/3600 = 0.70166 kg/m3 = 0.70166 x 3.036 = 2.13 M3/sec.

Calculation of area required for flashing

The required area for flashing depend upon the flash vapour velocity at the time of generation. The Souders – Brown equation is used for calculating the maximum vapour velocity.

Velocity of flash vapour in flash tank(m/sec) = Umax = C x [ (ρ L – ρV)/ ρV]0.5

Water Density = ρ L in kg/M3

Flash vapour Density = ρ V in kg/M3

C = The value of the constant C is a measure of the droplet size that will be carried over and depends on the degree of separation of liquid and vapor required. According to Cane sugar engineering by Peter Rein , coefficient C recommended 0.01 m/s.

Flash Vapour Calculation | Flash Vapour Recovery Vessel Design Calculation | Flash cigar calculation in sugar industry

Description UOM 1st comp. 2nd comp. 3rd comp. Remarks
Water inlet temperature ( Ti ) oC 112.36 102.92 90.5 Tc = Tv – 0.4 ( Tv – Tj )
Inlet water quantity (Q ) T/hr 70.00 133.00 164.00 As per the vapour bleeding calculation
Outlet water temperature ( To ) oC 107.2 96.5 81.5 From the above table
latent heat of vapour ( λ) Kcal/kg 534 541 550 From the above table
Specific volume ( µ ) m3/kg 1.322 1.883 3.222 From the steam table
Flash vapour Quantity ( Qv ) T/hr 0.676 1.578 2.682  Q x ( Ti – To) / λ
M3/sec 0.24823 0.82519 2.39995 Qv / µ
 Pressure difference in absolute (ΔP ) bar 0.4404 0.4195 0.3985 From the above table
Density of water (ρ L ) kg/m3 953 960 969 From the steam table
Density of vapour ( ρ V ) kg/m3 0.751 0.541 0.329 From the steam table
Velocity of vapour in comp. ( Umax) m/sec 0.3561 0.4211 0.5426 Umax = C x [ (ρ L – ρV)/ ρV]0.5
Velocity of vapour ( Vv ) m/sec 32.5 35 40 From Handbook of Cane sugar Engineering by Hugot
Compartment area required ( A ) m2 0.697 1.959 4.423 Qv / Vv
1.046 2.939 6.634 Taken 50% extra to avoid fluctuation in the body
Take Dia of the flash pot ( W ) mtr 2.000 2.000 2.000 Assume as per the site condition
Compartment width (H = A/ W) mtr 1.000 1.470 3.317 Width to be take minimum 1mtr to arrange inlet and outlet lines
flash vapour line dia mtr 0.108 0.190 0.303 Qv = (0.785 x D2) x  Vv and consider vapour 20% extra
mm 125 200 300
Velocity of the siphon descending  line m/sec 0.8 0.8 0.8 From Handbook of Cane sugar Engineering by Hugot
Velocity of the siphon ascending line m/sec 0.6 0.6 0.6 From Handbook of Cane sugar Engineering by Hugot
Condensate descending pipe dia mm 200 250 275 Fundamental formula       Q = AV
Condensate ascending pipe dia mm 225 300 325 Fundamental formula      Q = AV
Siphon height mtrs 4.77 4.51 4.25 10.33 x ΔP / Density of the water
Flash vapour connection 2nd vapour 3rd vapour 4th vapour

Some Important considerations in flash pot design:

  1. Flash pot is kept minimum 3 to 3.3mts from the common condensate pump for free removal of water and maintain NPSHa value for the pump.
  2. Suction and delivery lines of the common condensate pump to be equalize with lost compartment of the flash pot. ( equalizing connection to be take from the top of the compartment).
  3. Evaporator condensate to be connect to Descending line of the compartment. ( Evaporator 2nd body condensate line to be connect 1st compartment siphon descending line. Evaporator3rd body condensate line to be connect 1st compartment outlet line mean 2nd compartment descending line. Evaporator 4th body condensate line to be connect 2nd compartment outlet line mean 3rd compartment descending line.)
  4. Pans and juice heater condensates to be connect descending branch lines of the flash pot as per the bleed vapour.
  5. The inlet water line provide center of the compartment and it is to be provide throughout length of the compartment. The opening of the water to the compartment from the perforated holes only. The perforated holes area to be provide 150% of the cross section area of the in water line. The perforated holes prove only bottom portion of the pipe line.
  6. The outlet water line elevation to be provide 100 to 200mm below the inlet line bottom level.
  7. Generally ascending line provides in the center of the descending line. So here                                                   Descending line cross sectional =  Cross sectional area of center line ( ascending line) + Required cross section area of the water of the descending.
  8. Sight and light glasses to be provide in the middle of the each compartments in opposite direction.

Flash Vapour Calculation | Flash Vapour Recovery Vessel Design Calculation | Flash cigar calculation in sugar industry

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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 .

8 thoughts on “Flash Vapour Calculation | Flash Vapour Recovery Vessel Design Calculation

    Bharatkumar Magdum

    (October 3, 2017 - 6:28 am)

    Thanks madam, it is very important & valuable calculation for steam economy in sugar industry

    antonia plascencia silva

    (October 8, 2017 - 2:23 am)

    me gustaria contar con informacion laboro en la indistria azucarera y me interresa su pagina


    Emmidisetty gopi

    (March 12, 2018 - 10:14 pm)

    Pl sale as cd for calculation purpose

      siva alluri

      (March 13, 2018 - 2:22 pm)

      Mr. Gopi
      This website purely non commercial. All calculators providing in this website without any sale.
      Only motive of this website to educate the all technologist to share his knowledge.
      Go through the below link. In this provide list of online calculators


    (March 24, 2018 - 8:51 am)

    I shall require calculations for the condensate flash /sigar tank for 2500 TCD.sugar plant as working for quadruple set as both working with quintuple set .

      siva alluri

      (March 25, 2018 - 5:22 pm)

      Mr. Ashok Kale
      Read this article carefully. Definitely it will give the answer for your question.

    Bharatkumar Magdum

    (May 17, 2018 - 8:19 am)

    Hallo madam
    Plz give the one of the drawing of flash recovery system.

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