Feedwater Heater and Deaerator

 

I. Feedwater heater

A. What’s feedwater heater?

     Components in which fluid exiting the condenser is preheated by extracted steam from turbines  

 

B. Why feedwater heater?

o    To reduce thermal shock in boiler

    Thermal shock is a force that is caused by difference of expansion of solid (usually metals) because of  large difference of the temperature on its part

1. Causes bending and cracking of the solid. Can be prevented/mitigated by 
2. Reducing temperature difference (Applicable in this system)
3. Using high conductivity materials (Applicable, but costly)
4. Forcing the temperature increase/decrease to become gradual (not feasible

To reduce boiler load (except for startup phase)  = By preheating the  working fluid, fuel consumption is reduced 

     Disadvantages :

1. More components, more initial cost, and more maintenance cost 
2.  Steam bled from turbine is used to preheat the fluid instead of producing work, which in turn, decreasing the work output
3. Adding components through which the fluid flows increase the loss by friction and heat loss to surrounding

Key consideration: Optimization of advantages 

C. What are the variations of feedwater heater?

  

 Open Feedwater heater

           Steam bled from turbine is allowed to mix with saturated water output of condenser.  

 

      Advantages

1.  Faster heat rate between steam and saturated water, because of direct contact of both fluids 
2. Direct contact heat exchanger is generally less expensive compared to other type (especially S&T HE), and easier maintenance process

     Disadvantages

1.  Because of the pressure difference in all three state of turbine, condenser, and boiler, additional pump is required for each feedwater, which means, additional initial cost and maintenance cost

Closed Feedwater Heater

 Steam bled from turbine does not mix with saturated water output of condenser. In practice, shell and tube heat exchanger is commonly used

      Advantage 

1.  Fewer pumps required (only necessary in case of forward fed feedwater heaters). Because the non-direct contact heat exchanger, pressure difference between turbine and condenser is irrelevant
2. Technology of S&T HE, which is commonly used in this case, is already mature, and still continue to develop

     Disadvantage

1. Utilizing a steam trap valve, a throttling device that only allows liquid to pass. While cheaper and needs less maintenance than a pump, this device produce more irreversibility while producing no work. 
2. HE used in this system (especially S&T type) is expensive, and the maintenance is not as easy as its direct contact counterpart  

Steam Trap

    When the steam enter the system (ex : closed feed water)  and heated the water inside pipe of CFW it will condenses and produce condensate and reduce heat contact area, we need remove it and need the valve to prevent leakage of steam

Steam traps are a type of automatic valve that filters out condensate (i.e. condensed steam) and non-condensable gases such as air without letting steam escape.

II. Deaerator

What's deaerator?

A device that is used to dissolved gasses from working fluid?

 

What’s the importance of using deaerator?

      It helps to prevent corrosion by removing, mainly, dissolved oxygen (main corrosive agent) and CO2 (secondary corrosive agent, by carbonic acid) Oxygen is removed until at most 7ppb (0.005 cm3/L). CO2 is usually completely removed)

How's deaerator work? 

    By heating the working fluid. Rate of dissolved oxygen released from the water is proportional to the temperature. At saturation temperature at a given pressure, almost all oxygen is released

                By adding oxygen scavenger (sodium sulfide (N2SO3) to Sodium sulfate (N2SO4))

Sodium sulfide is more reactive to oxygen compared to water. One of the disadvantages of this method is the sedimentation of sodium sulfate at the bottom of de-aerated water storage tank.

What are the variations of deaerator?

·         A. Tray type (Cascade Type Deaerator)

This type includes a vertical domed deaeration section mounted on top of horizontal cylindrical vessel which acts as de-aerated water storage tank.

The purpose of the cascaded trays within the deaeration section is similar to fill in cooling tower, which is to increase the heat transfer of water, which in turn, increasing the heat transfer rate. Boiler feedwater enters the deaeration section on top of the device; while heating steam enters through submerge pipes in the de-aerated water storage tank. As mentioned above, solubility of oxygen in the water decreases with increase of temperature.

B. Spray type

   The spray type deaerator consists only of a vessel that serves both as deaeration section and as de-aerated water storage tank.

 

In this type, instead of using trays to increase the heat transfer area, sprinkler nozzle is used. The main goal is still to increase the temperature of the boiler feedwater.

In both of these deaerator types, some things that need to be considered for choosing are :  

• Price. Generally, tray type deaerator is more expensive and harder to design compared to spray type deaerator 
• ·Efficiency. Efficiency of deaerator can be calculated by the same method of heat exchanger efficiency.  This depends on, among other thing, the size of the deaerator.

Some engineers also added vent condenser at the exit of the removed gasses, to filter some steam/water that might escape the deaeration process. This addition, of course, reduce the need of makeup water. 

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