Question

A triple-effect forward-feed evaporator is used to concentrate a liquid which has marginal elevation in boiling point. The temperature of the stream to the first effect is 125°C, and the boiling point of the solution within third effect is 45°C. Find out at what temperatures the fluid boils in the Ι and ΙΙ effects. The overall heat transfer coefficients are 2250W/m2 in the Ι-effect, 1800W/m2 in the ΙΙ-effect and 1300W/m2 in the ΙΙΙ-effect respectively.

Answer 1

Given: Temperature of stream in I effect, T_s=125⁰C

Boiling point of solution in III effect, T^'₃=45⁰C

Overall heat transfer coefficient in I effect, U₁=2250 W/m²

Overall heat transfer coefficient in II effect, U₂=1800 W/m²

Overall heat transfer coefficient in III effect, U₃=1300 W/m²

Assumption: Area of all three evaporator atills are equal.

The total temperature drop, T is

where T_s is the Temperature in first still

T^'₃ is the Boiling temperature in third still

Temperature drop across first effect, T₁, is obtained using the expression

where U₁, U₂ and U₃ is the Overall heat transfer coefficient in the first, second and third still, respectively

Temperature drop across second effect, T₂, is

Temperature drop across third effect, T₃, is

Boiling point temperature in the I effect, T^'₁, is

Boiling point temperature in the II effect, T^'₂, is

Therefore, in first still the fluid boils at T^'₁=104.9⁰C and in second still the fluid boils at T₂^'=79.78⁰C.