Superheaters

 Superheaters

1. The superheater is a heat exchanger whose function is to superheat the wet or saturated steam upto desired temperature by transfer of heat energy.
2. The superheaters are either located in the passage of hot flue gases or directly over the furnace depending upon its design and application.
3. In modern steam power plants, the superheating and reheating of steam is done in the range of 440°C to 650°C.
4. The 40% of total heat generated in a modern boiler is utilized by the superheaters.
5. The advantages of superheating and reheating of steam are : (a) It improves the overall efficiency of the plant. (b) It reduces the moisture content in the last stages of turbine.
6. It helps in preventing the erosion of blades and corrosion of various components of steam turbine.
7. The superheaters are made of special high strength steels in case of superheating is done above 450°C but less costly carbon steel can be used for temperatures below 450°C.

Classification of superheaters :

These are classified based on method of heat transfer and their location as follows :

1. Convective superheaters (CSH)
2. Radiant superheater (RSH)
3. Combined convective and radiant superheaters which are generally pendent type (PSH).

Convective superheaters are often called as primary superheaters in which the steam from boiler drum flows into superheater and heat is transferred from flue gases as shown in Figure A.

Figure A

By energy balance :

These are usually located ahead of economizer in the convective zone of the boiler furnace.

The superheated steam from convective superheater is supplied to radiant superheated (RSH) located in the radiant zone of the furnace.

Thus, the heat is transferred to steam mainly by radiation.

Steam from radiant superheater is supplied to desuperheaters (for temperature control) and then to combined convective and radiant superheater (PSH), called as mixed flow or secondary superheaters.

These are usually pendant type superheaters and the temperature of steam obtained is controlled upto the desired value.

The heat transfer from gas to steam in three types of superheaters are shown in Figure B on (T-S) diagram. 

[Desuperheater : It is located between primary and secondary superheaters to reduce the quality of steam by spraying highly pure water either directly into the steam or on outside of the chamber containing steam . It ensures to get desired temperature from pendant superheater.]

2. Design considerations : The heat transfer rate in three types of superheaters are as follows (Refer Figure B)

Figure B

where,

σ = Stefan Boltzmann constant (5.67 x 10^-8W/m²K)

A_t = Surface area of superheater exposed to flame.

F_fw = Shape factor between flame and wall surface of tube

T_f = Flame temperature

T_w = Surface temperature of tubes

a = Fraction of energy transferred by convection 

(ΔT_m) = Logarithmic mean temperature difference (depends on parallel, counter flow, mixed flow of steam and gases) 

U_o = Overall heat transfer coefficient.

(i) Convective superheaters (CSH) : Following points may be observed with regard to

(a) U_o is low since convective heat transfer coefficient h_gas and h_steam are low. However, m_s, m_g and mass flow rate of fuel will all increase with increase in load, the value of U_o increases.

(b) Mass flow rate of steam, m_s depends on number of superheater coils, velocity of steam, V_s  and diameter of tube. Following are the considerations :

(i) Velocity of steam is kept higher from 10 m/s to 20 m/s with decrease in boiler pressure . Higher velocities increase heat transfer rates.

(ii) Diameter of tube is kept between 5 cm to 7.5 cm . However, pressure losses decrease with increase in diameter of tube.

(iii) Use of higher gas velocity increases heat transfer rate. Simultaneously, depending on ash content in fuel, the use of high gas velocities increases crosion rates (since erosion « v) and the pressure losses (< K.E.). Increased gas velocity will therefore will also increase the requirement of fan power.

Allowable velocity of gases is kept between 8 m /s to 16 m/s . Higher velocities are used with fuels having lower ash content in fuel.

(ii) Radiant superheaters (RSH) :

The heat transfer is proportional to T⁴ ∵ T_f >>> T_w) (Refer Equation (ii) above). Therefore, these superheaters are able to absorb high amount of radiant heat.

In convective superheaters the temperature of steam at exit increases with increase in load due to increased mass flow rate of fuel and temperature of flue gases. Whereas in case of radiant superheaters, the exit temperature of steam tends to decrease with increase in load since T_f is not much affected and m_s increases. response of various super heaters.

Thus the combined effect of convective and radiant superheraters in series is to give almost constant temperatures of superheated steam as shown in Figure C.

Figure C

The steam velocity in radiant superheaters is kept in range of 7-8 m/s.

(iii) Pendant superheaters (PSH) : These receive heat both by convection and radiation and they are used for applications needing high temperature of superheated steam. A pendant or mixed flow type of superheater is shown in Figure D. These superheaters are hung from above. They have firm structural support but the disadvantage is that the flow of steam is blocked due to condensation of steam after a cold shunt down. Thus, these needs a slow restart to purge water which gets accumulated at the bottom of superheater coil from previous run. Velocity of steam is kept in the range of 15 to 20 m/s.

Figure D

3. Type of coils / platens for radiant and pendent superheaters :

Radiant and pendant type of superheaters can be used in the form of coils or platens.

Figure E

The coil type superheaters are shown in Figure E. The coils may be single, double with one tube or double coil with double tube.

These heat exchangers provide very large surface area per unit volume it double coil is used.

Figure F

The platen type of superheaters are shown in Figure F which may be triangular or rectangular.