Superheaters
Reason for superheating steam
The maximum efficiency possible for a plant is given by the Carnot cycle and
can be calculated using the formula
Efficiency = T1- T2/ T1
Where T1 is the maximum temperature in a cycle ( kelvin ), and
T2 is the minimum temperature in a cycle. For the steam plant these equate to
blr outlet temperature and the exhaust temperature of the turbine.
Hence, to increase final temperatures at boiler outlet conditions
either; the boiler pressure can be increased, or the degree of superheat can be
increased. Boiler pressure increase is ultimately limited by the scantling
requirements,more importantly however, the energy stored within the steam is
little increased due to the reduction in the latent heat.
Increasing the degree of Superheat not only increases the
temperature but also greatly increases the heat energy stored within contained
another advantage would be that the onset of condensation through the
turbine would be delayed. However this increases the specific volume which
would require excessively large plant. Also there would be insufficient pressure
drop for efficient expansion through the turbine. There would also be little
allowance for feed heating.
There is therefore a combination of increased Pressure and
Superheat to give the increased efficiency potential allied with practical design
parameters.
Limit of Superheat
Superheated steam, having a lower specific heat capacity then water does not
conduct heat away as efficiently as in water cooled tubes, and hence the tube
metal surface temperature is higher.
This has led to the external superheat design and parallel steam
flows in an effort to keep metal temperatures within limits
For mild steel, upto 455oC superheat is possible; for higher temperatures, up
to 560oC the use of chrome molybdenum steels is required. The use of special
alloy steels introduces manufacturing and welding difficulties.
It can be seen that there is a requirement for some form of superheat
temperature control
Positioning of the superheater
Integral (FW D-type)
This design suffered from heavy slagging of the tubes, particularly
the superheater bank, caused by the vanadium bearing ash of the increasingly
poorer quality fuel blends.
This ash caused a heavy bonding slag deposit which often bridged
the gap between the tubes. This slagging attached to the hot surfaces of the
superheater support tube led to wastage and failure.
Increasing slagging would eventually lead to blockage and hence reduced gas
path with increased gas velocities over the smaller number of tubes, this led to
overheating and failure.
Access for cleaning was limited, this and the problems outlined above led to
the external superheater design
External (FW ESD III)
In this position the superheater was protected from the radiant
heat of the flame and with roof firing complete combustion was ensured within
the furnace space with no flame impingement, this allied to reduced gas
temperatures meant that condiitons for the superheater bank was less
arduous.
The positioning of the superheater banks allowed for easier inspection and
cleaning. More effective sootblowing could also be employed.
With the positioning of the bank external meant that the surface area of the
nest had to increased to give the same heating effect.
Mounting of the tubes in the athwartships direction allowed for a simpler
mounting arrangement
The secondary superheater, mounted below that of the primary superheater
was of the parallel flow type, this ensured that the lower temperature
attemperated steam was in the tubes in the highest temperature zone. In
modern Radiant Heat boilers it is common to mount the primary superheater
below that of the secondary and use parallel flow throughout; this ensure
adequate cooling throughout.
Designs of Superheater banks and mounting
arrangements
U-Tubes
Use limited to the integral positioning fort he superheat bank, the modern
method is to hang the tubes vertically, this prevents the sagging that can occur
with the tubes in the horizontal.
The tubes were supported by a support plate which hung off a
special increased diameter water cooler tube called the support tube. As the
supports were situate in a high temperature zone they were susceptible to
failure.
Division plates were welded into the headers, these allowed the
steam to make many passes increasing the efficiency of the bank. Small hole
were formed in these plates to allow for proper drainage, failure of these plates
caused short circuiting, overheating and subsequent failures. Failure of a single
tube, although possible leading to a restriction in the flow meant that the
heating surface was reduced by only a small amount.
The superheater inlet and outlet flange were mounted on the same side.
External (melesco type)
In this design there are no baffles fitted inside the header, instead the steam
makes a multipass over the gas by way of the many limbs or bends of each
tube. The disadvantage of this system is that if a tube should fail then a
significant reduction in heating surface would occur.Simpler, more reliable
support methods are possible allied to the easier access and sootblowing
arrangement.This type of superheater has the advantage that the number of
expanded or welded joints are reduced.
With this design the initial passes are made of Chrome
Molybdenum steel. a transition piece attaches this to the mild steel passes.
The inlet header is made out of mild steel and the outlet an alloy steel.
Methods of attachment
Expanded
Only used in superheaters fort temperatures upto 450 oC Tube ends must be
cleaned and degreased and then drifted and roller expanded into the hole, the
end of the tube must be projecting by at least 6mm. The bell mouth must have
an increase of diameter of 1mm per 25mm plus an additional 1.5mm.
It is important that the tube enter perpendicular into the head, a seal will be
assured if the contact length is greater than 10mm, if it is not possible to enter
perpendicularly then the contact length should be increased to 13mm.
For larger diameter pipes then grooved seats are used.
Welded
Welding gives advantages over expanding in that access to the internal side of
the header is not so important and so the number of handhole doors can be
much reduced eliminating a source of possible leakage. welding also generally
provides a more reliable seal.
The disadvantage is that heat treatment following welding is
required.
The purpose of the backing ring fitted to the conventional
attachment method is to prevent the weld metal breaking through into the
tube
The Melric joint offers the following advantages over the conventional
method;
Dispenses with butt joints and internal backing rings
Allows for maximum access for welding
The joints can be annealed locally by electric heat muff or torch
according to manufacturers recommendtions
The stub bosses can be readily blanked off externally in the event of
tube failure and so do not require the access to the header internal
side
