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Any process that has to concentrate (purify) a stream by eliminating water or another solvent utilises evaporation. The evaporation process consumes a lot of energy. Thus any approach to designing an evaporator system should consider heat recovery and the total utility use in the final design. The system design must take product qualities into account as well; many might change during evaporation, particularly in a Food and Beverage application.
Many different industries or processes employ evaporators, including:
- Chemical Purification
- Food and Beverage
- Pharmaceutical
- Agricultural Chemical
- Pulp and Paper
- Gas and Oil
- Fuel Ethanol
1 Working Principle of the Evaporator
Each chemical species has a different boiling point and evaporates at a specific temperature; this principle is utilized in an evaporator for purification purposes. The chemical ingredient is initially put into the evaporator, then heated and transforms into a gaseous state (vapor). In a multi-effect evaporator system, the residual portion of the chemical is condensed and fed into another evaporator.
2 Evaporators Types
Evaporators come in many shapes and sizes and are selected based on technical requirements. Some prominent evaporator types are given below:
a) Natural circulation evaporator
b) Thin-film evaporators
c) Long tube vertical falling film evaporator
d) Long tube vertical raising film evaporator
e) Plate type evaporator
f) Horizontal tube evaporators
g) Inclined tube evaporators
h) Compact evaporators
i) Flash evaporators
Figure 1: Schematic of long tube vertical falling film continuous evaporator
3 Different Factors in Evaporator Design Process
The best evaporator type selection is dependent on several variables. These include, but are not limited to, i) throughput, (ii) solution viscosity, (iii) product and solvent properties (e.g., corrosiveness, heat sensitivity, etc.), (iv) fouling attributes, and (v) foaming properties.
Some other important factors are as follow:
3.1 Thermal Considerations
Tube materials, tube arrangement, tube size: tube arrangement, length and diameter are determined by the hit-and-trial approach, whereas the tube material is selected based on the corrosive effects of the chemical/mixture under process and working scenarios.
Coefficients of heat transfer: heat transmission to the shell from the liquid side is often much higher. The rate of heat transmission is determined by a coefficient known as the latent heat transfer coefficient.
3.2 Mechanical Considerations
Operating temperature: temperature sustained for the specific metal vessel operation.
Operating pressure: the pressure that ordinarily exists at the vessel top. It should be less than any pressure-relieving device’s design or set pressure.
Maximum Allowable Working Pressure (MAWP): The MAWP is the highest pressure at which equipment can be used safely. A vessel’s MAWP is its maximum allowed pressure in its normal working state at a certain temperature, often the design temperature.
Thermal expansion: The mechanical design of equipment is substantially impacted by thermal expansion between various elements. How tubes are attached to the tube sheet may also depend on thermal expansion.
4 Evaporator Design Considerations
General assumptions for designing an evaporator:
- For all concentrations and temperatures, the feed’s specific heat remains constant.
- The overall heat transfer coefficient stays constant.
- The evaporator run in a steady-state environment.
- Each effect produces vapors that are devoid of solute.
- Heat transfer area for every effect is almost equal.
4.1 Steps in the Calculation of Evaporator Design
Heat transmission, vapor-liquid separation, and effective energy use are the three main factors of evaporator design.
The following calculations are for a Natural Circulation Evaporator, although they can change depending on the column type.
a) The number of required tubes (N)
No. of tubes = Heating surface / π x Effective Length Mean x Diameter
where,
Effective tube length = Tube length – 2(Tube plate thickness) – 2(Tube allowance for expansion)
b) Tube plate and Downtake diameter
The area occupied by tubes in tube plate:
Tube plate area for tubes only (AT) = (Tube Pitch2 x 0.866 x Total Tubes / Proportionality factor) x %allowance.
Down take diameter:
- Single downtake diameter = Tube plate dia for tubes x % downtake on tube plate.
- Central downtake diameter = √ [ 4/π x (Area of the single downtake – Total area of peripheral down take)].
Final tube plate diameter = √ [ 4/π x (Area of the Tube plate for tubes + Downtake area)]
c) Diameter required for vapor entry and Calendria in radial steam/vapor entry
- Diameter of the Vapor Inlet:
Each steam entry diameter= √ [ (4/π) x (Area for the vapor entry / No. of vapour entries)]
- Calendria diameter at steam entry
Diameter of calendria at the point of radial steam entry = width of the steam entry + final tube plate diameter.
d) Vapor outlet pipe diameter
Vapor outlet pipe diameter = √ [vapor volume / (velocity of vapor x 0.785)]
e) Diameter of the condensate line
Each condensate line diameter = √ (Volume of the condensate / (velocity of condensate x 0.785).
f) Noxious gases connections
Each non-condensable gases line diameter = √ (Total area of non-condensable gases /0.785 x no. of points)
5 References
- Adib, T. A. (2012). Thermal evaporator design. Handbook of Food Process Design. Berlin, Heidelberg: Springer, 460-88.
- Minton, P. E. (1986). Handbook of evaporation technology. William Andrew.
- Kakaç, S. (Ed.). (1991). Boilers, evaporators, and condensers. John Wiley & Sons.
