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Miscellaneous Boiler Types, Economisers and Superheaters : International site for Spirax Sarco
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An explanation of specialist boiler types and other specialist features.
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You are here: Home Resources Steam Engineering Tutorials The Boiler House Miscellaneous Boiler Types, Economisers and Superheaters
Miscellaneous Boiler Types, Economisers and Superheaters
Steam generators
In many applications:
The amount of steam required is too small to warrant a shell boiler, i.e. Less than 1 000 kg / h.
The small process requiring steam operates on a day shift only, meaning that the plant would be started every morning and shut down every night.
The capital cost of a conventional shell boiler would adversely affect the economic viability of t he process.
The level of expertise on site, as far as boilers are concerned, is not as high as would be required on a larger steam system.
To meet these specific demands two types of boiler have been developed.
Coil boilerThese are a 'once through' type of water tube boiler, and referred to in some regulations as, 'boilers with no discernible water level'.
Fig. 3.4.1
Coil boiler
Water supply to the boiler will usually be at 10 to 15% above the steaming rate to:
Ensure that all the water is not evaporated, thus ensuring that superheated steam is not produced.
Provide a vehicle for the feedwater TDS to be carried through. If this vehicle was not available, the salts in the feedwater would be deposited on the insides of the tubes
The Boiler House
Introduction
Shell Boilers
Water-tube Boilers
Miscellaneous Boiler Types, Economisers and
Superheaters
Boiler Ratings
Boiler Efficiency and Combustion
Boiler Fittings and Mountings
Steam Headers and Off-takes
Water Treatment, Storage and Blowdown for
Steam Boilers
Water for the Boiler
The Feedtank and Feedwater Conditioning
Controlling TDS in the Boiler Water
Heat Recovery from Boiler Blowdown (TDS control
only)
Bottom Blowdown
Water Levels in Steam Boilers
Methods of Detecting Water Level in Steam
Boilers
Automatic Level Control Systems
Water Level Alarms
Installation of Level Controls
Testing Requirements in the Boiler HousePressurised Deaerators
Steam Accumulators
Related Content
Steam Tables
A comprehensive set of steam tables is
available here.
The Steam and Condensate Loop
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and impair heat transfer, leading to over heating and eventually to tube failure. Clearly, a separator is an essential component of this type of boiler to remove this
contaminated water.
Being of the water tube type, they can produce steam at very high pressures.
Typical applications for steam generators and coil boilers include laundries and garment manufacture, where the demand is small and the rate of change in load is slow.
Vertical tubeless packaged steam boilerVarious models are available with outputs in the range 50 to 1 000 kg/h, and pressures up to 10 bar g. Boiler heights vary typically from 1.7 m to 2.4 m for outputs of about 100
kg/h to 1 000 kg/h respectively.
A cross section of the design is shown in Figure 3.4.2. Note the downward path of the flame, and the swirling action. The heat path is reversed at the bottom of the boiler and the
hot gases rise, releasing heat to the fins.
Also note the small quantity of water in the boiler. This allows the boiler to be brought up to operating temperature very quickly, typically 15 minutes. However, this small quantity
of water means that only a small amount of energy is stored in the boiler, consequently it is not easily able to cope with sudden and maintained changes in load. If the load
change occurs faster than the boiler can respond, then the pressure inside the boiler will drop and ultimately the boiler will prime with feedwater. This is aggravated by the small
water surface area, which gives high steam release velocities. However, the path of the steam is vertically up and away from the water surface as opposed to horizontally over the
water surface (as in a shell boiler), and this minimises the effect.
Fig. 3.4.2
Vertical tubeless packaged steam boiler
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Economisers
The flue gases, having passed through the main boiler and the superheater, will still be hot. The energy in these flue gases can be used to improve the thermal efficiency of the
boiler. To achieve this the flue gases are passed through an economiser.
Fig. 3.4.3
A shell boiler with an economiser
The economiser is a heat exchanger through which the feedwater is pumped. The feedwater thus arrives in the boiler at a higher temperature than would be the case if no
economiser was fitted. Less energy is then required to raise the steam. Alternatively, if the same quantity of energy is supplied, then more steam is raised. This results in a higher
efficiency. In broad terms a 10C increase in feedwater temperature will give an efficiency improvement of 2%.
Note:
Because the economiser is on the high-pressure side of the feedpump, feedwater temperatures in excess of 100C are possible. The boiler water level controls should be
of the 'modulating' type, (i.e. not 'on-off') to ensure a continuous flow of feedwater through the heat exchanger.
The heat exchanger should not be so large that:
The flue gases are cooled below their dew point, as the resulting liquor may be acidic and corrosive.
The feedwater boils in the heat exchanger.
Superheaters
Whatever type of boiler is used, steam will leave the water at its surface and pass into the steam space. Steam formed above the water surface in a shell boiler is always
saturated and cannot become superheated in the boiler shell, as it is constantly in contact with the water surface.
If superheated steam is required, the saturated steam must pass through a superheater. This is simply a heat exchanger where additional heat is added to the saturated steam.
In water-tube boilers, the superheater may be an additional pendant suspended in the furnace area where the hot gases will provide the degree of superheat required (see Figure
3.4.4). In other cases, for example in CHP schemes where the gas turbine exhaust gases are relatively cool, a separately fired superheater may be needed to provide the
additional heat.
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Fig. 3.4.4
A water tube boiler with a superheater
If accurate control of the degree of superheat is required, as would be the case if the steam is to be used to drive turbines, then an attemperator (desuperheater) is fitted. This is a
device installed after the superheater, which injects water into the superheated steam to reduce its temperature.
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