Clarifiers Flash tank design formulas with online calculation sheet

Juice Clarifier Flash Tank construction parameters with online calculator

About Flash tank

1) Clarifiers are normally preceded by a flash tank. This is a simple cylindrical tank located just above and ahead of the clarifier, with a flue open to the atmosphere. Nowadays according to energy conservation, the flue is also used for filter juice or clear juice heating. General in flash tank design juice enters in a tangential direction from the juice heaters. In some flash tank designs  juice entry will be giving into center of flash tank. In this article discussed tangential entry-type flash tank design.

2) It is important to heat the juice to above the boiling point and allow it to flash. This enables any air in the juice to be removed and ensures that the juice runs at a constant temperature to the clarifier. In the process any air entrapped in particles of bagasse or bagacillo also escapes, enabling these particles to settle and not carry over with the clarified juice.

3) The extent of superheating required does not seem to be critical. It is generally thought that a control set point 2-3 ^oC above boiling point should be used so that even with some excursions around the set point, the juice will always be delivered superheated to the flash tank. Temperatures above this are deemed to be unnecessary and only increase the amount of steam used in the factory.

4) Temperatures much higher than design could lead to excessive flash that the flash tank may not be able to handle, leading to entrainment and loss of sugar from the flash tank. Generally, the temperature should never exceed 105⁰C.

Flash tank design formulas :

The diameter of the flash tank should be based on the maximum liquid flow rate that the flash tank will receive. The amount of flash at maximum flow rate needs to be computed, and the diameter chosen to prevent entrainment of liquid. This requires an estimate of the maximum allowable upward velocity the in flash tank. The Souders-Brown equation for the design velocity is used;

U_max = C x [ (ρ _L – ρ_V)/ ρ_V]^0.5

The value of the maximum velocity U_max is given in m/s. 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 Perry (1963), for a decontamination factor (kg vapor/kg liquid) of 10000, C should have the value 0.0060 m/s,

Required Data:

1. Flash tank Juice inlet Quantity to the flash tank =       M   in T/hr
2. Juice Inlet Temperature = Ti   in ^oC
3. Juice outlet Temperature = To in ^oC
4. Specific heat of the juice = C_p in kcal/kg
5. Juice Density = ρ _L in kg/M³
6. Flash vapour Density = ρ _V in kg/M³
7. Juice Inlet velocity to the flash tank = Vi in m/sec (Juice inlet velocity to flash tank having 1.5 to 1.8 m/sec)

8. Juice outlet velocity to the flash tank = Vo in m/sec (Juice outlet velocity from flash tank generally having 0.6 to 1 m/sec)

9. Flash vapour velocity in vent pipe = Vv    in m/sec (Flash vapour velocity in vent pipe having 4 to 10 m/sec)

10. Latent heat of the Flash Vapour = λ   in Kcal/Kg
11. Specific Volume of the flash tank = S_p in M³/Kg
12. Coefficient = 0.006 m/sec (Note : Refer to Cane Sugar Engineering by Peter Rein)
13. Specific volume and latent of the flash vapour are to be taken from a steam table. Online Steam Table For Saturated Steam (Note : Latent heat was taken at inlet juice temperature and specific heat value was taken for the average juice inlet and outlet temperature.)

Calculation Part:

• Velocity of flash vapour in flash tank(m/sec) = U_max = C x [ (ρ _L – ρ_V)/ ρ_V]^0.5
• Flash vapour Quantity (T/hr) =  [ M x C_p x (Ti-To)]/ λ
• Flash vapour Qty. (M³/Sec) = Q
• Flash Tank Dia = By using general formula Q = AV (here velocity taken U_max)
• Flash Tank Juice inlet Dia = By using general formula Q = AV( here velocity to take Vi)
• Flash tank Vapour Vent Pipe Dia = By using general formula Q = AV( here velocity to take Vv)
• Flash tank juice outlet Dia = By using general formula Q =AV( here velocity to take Vo)
• The flash tank juice outlet line size is particularly important, It should be sized to be self-venting, at the maximum liquid flow rate. So the dia to be cross checked by the formula D > 1.116 (Q)^0.4       Here D is dia of the juice outlet pipe and Q is the juice quantity in M³/sec.
• The inlet of the juice to be provided in tangential direction.
• In the side of the flash tank, it is better to provide partition plates, at just below of the inlet of the juice to reduce the velocity and steady outlet flow of the juice.
• The bottom cone angle of the flash tank is generally 30 to 35deg and the top of the flash tank having flat type or minimum cone angle is about 15 to 20deg.
• The cone of the flash tank should be positioned at the minimum height above the clarifier that will allow juice outlet from a short stub with 90° bend and the appropriate length of horizontal feed pipe to the clarifier. The juice pipe from the flash tank to the clarifier must include a drain line,
• The height of the flash is 1 to 1.5 times on dia for general flash tank designs.

Flash Tank Design Calculation Sheet

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