An evaporator is a device, majorly a type of heat exchanger, used in an industrial process to separate the liquid, mainly water, from a chemical compound into gas or water vapor.
Evaporator are commonly used in food industry to concentrate the food by evaporating the water content from the food.
At first, the chemical compound is fed to the evaporator. It then passes across a heat source that the compound into a gaseous form(vapor).After this, the remaining part of the compound is condensed and fed into a second evaporator if it is a multi-effect evaporator system. If it is a single evaporator system, the condensate(condensed material)is removed.
Evaporators are used in several industries like: ·Food·Pharmaceutical·Petrochemical·Paper and pulp·Waste management ·Textile
There are several types of evaporators available in the market, depending on their use, we can choose from the comprehensive list mentioned below:·Natural/forces circulation evaporator·Falling film evaporator·Rising film(long tube vertical)evaporator·Short tube evaporator·Climbing and falling-film plate evaporator·Multi-effect evaporator·Agitated thin-film evaporator·Vertical short tube evaporator·Kettle evaporator·Scraped /Wide surface evaporator·Centri therm evaporator·Gasketed plate evaporator·Robert type natural circulation evaporator·Flash evaporator·Compact evaporator
1.1. Thermal considerations:
Tube size, tube arrangement, and tube material:The tube diameter, tube arrangement, and tube length are determined by trial and error calculations whereas, the tube materials are selected based on the corrosiveness of the compound and working situations:
Heat transfer coefficients: The heat transfer coefficient on the shell side usually is very high compared with the liquid side. The later heat transfer coefficient controls the rate of heat transfer. The overall coefficient is calculated from the performance data of an operating evaporator of the same type and processing the same solution.
Boiling point elevation(BPE):When the boiling point of the concentrated liquor is higher than that of pure solvent(or water), it is know as boiling point elevation(BPE). Boiling point of solvent is higher when other component is added. BPE(Boiling Point Elevation)occurs because the vapor pressure is less than that of the pure solvent at the same temperature.
Note: Selection of suitable evaporator: The selection of the most suitable evaporator type depends on several factors. Mainly these are:(i)throughput ,(ii)viscosity of the solution(and its increase during evaporation),(iii)nature of the product and solvent(such as heat sensitivity and corrosiveness),(iv)fouling characteristics and ,(v)foaming characteristics.
2.2. Mechanical considerations:
Operating temperature and pressure: The operating temperature is the temperature that is maintained for the specific operation of the matal vessel suitably selected during design.
Operating pressure: The operating pressure is the pressure at the top of the vessel at which it normally operates.It should be lower than design pressure or set pressure of any pressure relieving device.
Design temperature and pressure: The design temperature is generally the sum of the maximum allowable pressure and the static head of the fluid in the pressure vessel. The design temperature is the temperature which corresponds to design pressure.
Maximum allowable working pressure(MAWP):The maximum allowable working pressure is the maximum permissible pressure at the top of the vessel in its normal operating position at a specific temperature, usually the design temperature.
Thermal expansion:Thermal expansion between various parts of equipment has a significant effect on the mechanical design. Thermal expansion may also determine how tubes are fixed to the tube sheet.
Specific heat of feed is constant for all temperature and concentrations.
Overall heat transfer coefficient remains constant through the operation of the evaporator.
The evaporator is operated at a steady state.