Various types of Evaporators and their working principle

 Various types of Evaporators and their working principle

1) Vertical Falling Film Evaporator

The process fluid to be vaporized is feed to the evaporator at the top of the tube sheet. A suitable distribution unit is nessecary in order to achieve an even liquid distribution. It is paramount the for this process minimal wetting rate is achieved.In general the process liquid flows at the tue inside. But there are also applications where the liquid is applied on the tube outside.

 Due to gravity the liquid forms a continious liquid film with hydrodynamic characteristics which are determined by the Fil Reynods and Kapitza number.In general steam is used as a heating source. The steam condenses on the tube outside of the tube and flows also gravity controlled to the bottom of the unit

Inside the tubes the two phase flow is separated by means of a separator at the bottom of the unit.
In multieffect falling film evaporators the vapour produced is used in order to heat a so called second effect on a lower temperature and pressure level. This is possible due to the very low required meant temperature difference between heating medium and process liquid. In seawater desalination up to 12 effects are possible in order to evapoate most efficently.


 In the picture on the left side it is shown how the heat e.g. by means of condensation, is transfered through the wall to the process liquid to be evapoarated.

2) Horizontal film evaporator

In case of horizontal film evaporation the liquid to be vaporised is distributed evenly on the top of a horizontal tube bundle. For this purpose perforated plates and special designed jet nozzles are applied. A minimum recirculation rate is required in order to maintain a minimum wetting rate. The liquid chraracteristics between the tubes are determined by the Film reynods number and Kapitza number. The fluid can be encounter between the tubes as droplets, jet or continious sheet. Due to the liquid imingment the heat transfer in horizontal units is in general higher compared to vertical evaporators

 

 

3) Atmospheric Forced Draft Evaporator

Where the quantity of wastewater is small and difficult to treat, atmospheric forced draft evaporator is the ideal choice. Various models from 100 to 5000 Liters per day capacities are standard.

Advantages and Features

  • Low capital cost
  • Ease of operation and maintenance
  • No addition of chemicals
  • Very small quantity of sludge
  • Can handle difficult to treat wastewaters

Working Principle

 

 Atmospheric evaporator consists of Feed Pump, Evaporator Tank with Electrical or Diesel Heater, Circulation Pump, Blower, Mixing tube, Mist Eliminator and Humid air Outlet. The wastewater from the wastewater tank is pumped by the feed Pump to the evaporator tank. Electric or Diesel heater heats the effluent in the evaporator tank. The hot effluent is pumped into the Mixing Tube, through which air is blown from atmosphere by the blower. The air and wastewater are thoroughly mixed in the mixing tube and passes to the evaporator tank, before getting out through the Mist eliminator and humid air outlet. Electrical controls limits the operation of feed pump, circulation pump and blower depends on the levels in the evaporator tank and wastewater tank. Control system with automatic ON/OFF, safety shut off and switch gears with control panel are provided with the system.

Applications


  • Concentration of Drag Outs in electroplating
  • Machine Coolants
  • Rinse waters
  • Tumbling and vibratory solutions
  • Die cast solutions
  • Photographic solutions
  • RO / UF Concentrates
  • Phosphatizing compounds
  • Steam cleaner water
  • Pressure washer effluents
  • Auto Garage effluents
  • Ink and chemicals wash up

4) Low Temperature Vacuum Evaporator

The low temperature vacuum evaporator vaporizes water at lower temperature around 40 Deg. C than normal 100 Deg. C due to vacuum. Hence vacuum evaporators are useful for
- recovery of heat sensitive chemicals such as cyanide plating baths,
- recovery of chemicals sensitive to air oxidation such as
  cyanide plating bath and Stannous tin bath
- recovery of solutions containing volatile components


Working Principle

 

The low temperature vacuum evaporator mainly consists of an evaporator tank 2, evaporator heat exchanger 4, process water circulation pump 3, refrigerant compressor 8, condenser heat exchanger 7, distillate tank 11, vacuum producing venture 13, and distillate circulating pump 12. The feed water enters the evaporator tank 2, and the feed water is circulated by the pump 3 to evaporator heat exchanger 4, where the feed water is heated by high pressure refrigerant from the refrigerant compressor 8. The hot water flashes as water vapor into the evaporator tank at the top. This water vapor passes through the mist eliminator 16 and the condenser heat exchanger 7. The refrigerant from the evaporator heat exchanger passes through air cooler and expansion valve and becomes cool liquid. The cool refrigerant cools the hot water vapor to distillate. The distillate is collected in the distillate tank. The distillate is circulated by the distillate circulation pump through a venture, which produces vacuum in the evaporator tank. The vacuum produced by the venturi in the evaporation tank causes process water boils between 40 to 50 Deg. C. The concentrated process water from the evaporator tank and the distillate from the distillate tank are automatically drained by the electrical controls. All necessary controls and switch gears are part of the system.

Advantages and features

  • independent of solution heating requirements
  • less or no air pollution
  • can handle heat sensitive compounds
  • carbonates do not build up scales as with other evaporators

Application

  • concentration of liquids from finishing processes
  • concentration of oily emulsions
  • concentration of wash baths
  • recovery of metals from rinse waters (chromium, nickel, copper)
  • concentration of contaminated baths from degreasing and pickling
  • recovery of tempering salts
  • purification of wash waters
  • recovery of gold from the residue of the treatment process
  • concentration of working liquids in die casting units a (detaching   product, glycol, lubricants)
  • recovery of food additives from different industrialprocess liquids
  • recovery of proteins from whey
  • recovery of solvents utilized for extraction process
  • concentration of phosphoric degreasing baths
  • recovery of vulcanizing salts

5) Mechanical Vapor Recompression

 MVR (Mechanical Vapor Recompression) and Multiple Effect Evaporators for large volumes and energy economy.
MVR is very energy efficient, since the latent heat of vaporization is fully utilized through vapor recompression and condensation. Typical layout of a MVR Evaporator is shown in figure.

Advantages and application

The major advantage of MVR is the energy economy. Typical MVR energy requirement is 0.05 to 0.15 kwh per kg of water evaporated. Water evaporation for a Triple Effect Evaporator to 120 kcal for a six effect evaporator. As the operating cost of MVR is low, large flow system favors its application in all sectors of industry and also desalination of sea and brackish water.


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