1.Deaerator.
2.Low-Pressure Heater.
3.High-Pressure Heater.
4.PRESSURE CLASSIFICATION
5.Zero Discharge of RO Reject

Deaerator

Why Gases need to be Removed from Boiler feed Water Oxygen is the main cause of corrosion in hot well tanks, feed lines, feed pumps and boilers. If carbon-di-oxide is also present then the pH will be low, the water will tend to be acidic and the rate of corrosion will be increased. Typically the corrosion is of the pitting type where, although the metal loss may not be great, deep penetration can occur in a short period.

Elimination of the dissolved oxygen may be achieved by Mechanical or chemical methods but the former being economical.

The essential requirements to reduce corrosion are to maintain the feed water at a pH for not less than 8.5 to 9, the lowest level at which carbon-di-oxide is absent, and to remove all traces of oxygen. The return of condensate from the plant will have a significant impact on boiler feed water because less feed water treatment is required.

Water exposed to air can become saturated with oxygen and the concentration will vary with temperature, the higher the temperature the lower the oxygen content.

The first step in feed water treatment is to heat the water to drive off the oxygen. Typically a boiler feed tank should be operating at boiling point. This leaves oxygen content of around 7 ppb.

The Deaeration Principle

Using Henry's law of partial pressures, the principle behind deaeration can be explained as follows: The quantity of a gas dissolved in a given quantity of liquid is directly proportional to its partial pressure surrounding the liquid. Therefore, by reducing the partial pressure of the unwanted gasses in the surrounding atmosphere, the gasses are diminished. These partial pressures are reduced by spraying the liquid into a countercurrent flow of steam. The steam, which is free of non-condensable gasses, is the liquid's new atmosphere and Henry's law prevails. Using steam is advantageous in that the solubility of a gas in a liquid decreases with an increase in the temperature of that liquid. The liquid is sprayed in thin films in order to increase the surface area of the liquid in contact

Low pressure / High pressure Heaters

A feedwater heater is a heat exchanger designed to preheat boiler feedwater by means of condensing steam extracted (or “bled”) from a steam turbine. The heaters discussed here are classified as closed, since the tubeside fluid remains enclosed by the tubes and channel, and does not mix with the condensate, as is the case with open feedwater heaters. They are unfired since the heat transfer within the vessel does not occur by
means of combustion, but by convection and condensation.  The steam extraction process in a closed feedwater is referred to as uncontrolled extraction. The flow rate of steam into a feedwater heater is not limited by the amount of available steam (as opposed to a surface condenser, for instance). The shellside operating pressure in a feedwater heater is determined by the pressure of the steam supplied to it, not by the amount of heat transfer surface.

The heating process by means of extraction steam is referred to as being regenerative. The feedwater heaters are an integral portion of the power plant thermodynamic cycle. Normally, there are multiple stages of feedwater heating. Each stage corresponds to a turbine extraction point. These extraction points occur at various stages of the expansion of steam through the turbines. The presence of the heaters in the cycle enhances the thermal efficiency of the powerplant; the greater the number of extraction stages, the lower the amount of thermal energy required to generate a given amount of electrical energy. A beneficial by-product of the energy extracted by the heaters is the reduced rate of rejection of energy to the environment.

PRESSURE CLASSIFICATION

Low Pressure Heater: A heater located (with regard to feedwater flow) between the condensate pump and either the boiler feed pump or, if present, an intermediate pressure (booster) pump. It normally extracts steam from the low pressure turbine.

High Pressure Heater: A heater located downstream of the boiler feed pump. Typically, the tubeside design pressure is at least 1500 psig, and the steam source is the high pressure turbine.

Intermediate Pressure Heater: (if present). A heater located between the booster pump and the boiler feed pump. Usually the tubeside pressure is within 200-300 psi of the low pressure heaters, and the steam is extracted from an intermediate pressure turbine.

Zero Discharge of RO Reject:

Accelerated evaporation ponds are especially useful in pollution control applications by removing water from toxic or otherwise harmful water solutions, by, in effect “spray drying the contaminating materials.  The resulting non-sprayable sludge can then be allowed to dry completely and / or be removed to a proper disposal.

These ponds are designed for evaporating water, the same water is continuously re-sprayed until the bulk of the water has been evaporated.  The efficiency of the evaporation pond hinges on a proper pond location and layout, pond construction, spray nozzles, spraying pressure and water temperature.

Evaporation pond technology is practiced primarily in the middle east and to a lesser extent in arid regions of Australia.  At this time, it is probably the most widespread method of brine disposal from inland-based desalination facilities.  This disposal system is especially effective in regions like India, where climatic conditions are favorable for steady and relatively rapid evaporation rates.

The area needed for the evaporation ponds is directly proportional to volume of reject water and inversely proportional to the evaporation rate.  This new technology involves periodic circulation of pond brine, designed to increase the effective evaporation surface area.  This results in enhanced evaporation, which of course also depends on wind speed and direction in addition to relative humidity.  Existing literature indicates that application of evaporation ponds is a relatively simple and straight forward method of brine disposal.  Capital cost arise primarily from acquisition of land.

But it occupies lesser area by 5 to 10 times compared to conventional system and the evaporation rate is 90 – 95%.  The operation cost is 10 times cheaper than the thermal evaporator.