9.3 Surface type evaporator
The flow diagram for a typical single-stage evaporator is shown in Fig 3.67 and the associated evaporator assembly in Fig 3.68. The principle involved is similar to that described for the vertical-tube type evaporator.
However, the natural circulation is replaced by a small pump which ensures circulation to the heating chamber. Various heat exchangers are also added to increase the thermal efficiency of the process.
Treated feedwater is pumped successively through four heaters/coolers, a preheat and a vent vessel to an evaporator. After vaporisation in the evaporator, it is condensed into distillate and delivered to the system discharge point by a distillate pump.
During its passage from the feedwater pump to the evaporator, the temperature of the feedwater is raised by heat exchange, first by distillate from the condenser, secondly by condensate from the evaporator and, thirdly, by bled-steam from the turbine. Feedwater is also used (by the second cooler in the line) to cool non-condensable gases vented from the vent vessel, evaporator and condenser.
Gases are vented from the feedwater in a vent vessel, positioned between the preheater and the evaporator, after which it enters the vapour chamber of the evaporator. It is then circulated to the heating chamber of the evaporator by a circulation pump. The feedwater is vaporised by heat exchange with bled-steam from the turbine. The vapour rises, re-enters the vapour chamber and then passes to the evaporator vapour condenser.
Bled-steam condensate from the heating chamber, which includes that from the preheater, passes through two of the heaters/coolers where it exchanges heat with the incoming feedwater as already described.
The drain from the heater/coolers is taken to the condenser flash vessel.
A vacuum system connected through the incondensable gas cooler to the evaporator, vapour condenser and vent vessel; draws gases, together with some vapour, from these vessels and discharges to atmosphere.
The system maintains a vacuum at the back end of the evaporator plant which provides the necessary temperature variation through the evaporator for efficient heat transfer.
The vapour chamber of the evaporator is connected to a blowdown system which allows a calibrated flow of water to be pumped out of the vapour chamber. This fixed flow rate maintains the correct body concentration when the evaporator is on-load.
The purified water vapour from the evaporator is condensed in the condenser by heat exchange with condensate from the turbine. It is then drawn from the condenser by a distillate pump and pumped through a cooler/heater to the system discharge point. The distillate raises the temperature of the incoming feed-water as already described. After leaving the heater/ cooler the temperature, pressure and conductivity of the distillate are checked.