MODERN INDUSTRIAL PACKAGE TYPE BOILER or NESTLER BOILER
Nestler Boiler Construction and Working:
Package boilers represent the bulk of fire tube boilers manufactured today. They are modified scotch-type boiler units, engineered, built and fire tested before shipment.
Packaged boilers burn any type of fuel namely, solid, liquid or gas or a combination of these fuels.
Figure illustrates the features of Nestler Boiler.
Nestler boiler is an oil-fired smoke tube boiler. Fuel oil used is light diesel or viscous furnace oil. But viscous furnace oil is usually preheated to bring about more fluidity.
The boiler consists of a combustion tube 'CT', smoke tubes 'ST' and tube passes 'TP' (Fig). All these are connected within the end plates and boiler shell itself to reduce radiation losses.
The fuel burns at the front end and flue gases move towards rear end of the shell. From there through tube passes they travel towards front end. Once again tube passes direct them to move up and traverse back. Thus flue gases travel over a good length of tubing and impart large amount of heat to the surrounding water. The flue gases finally escape through the chimney.
Water treatment is highly essential for the water used in this boiler. Various mountings such as pressure gauge, stop valve, blow off valve, water gauge, anti-priming pipe, fusible plug etc., are fitted to the boiler as usual.
The entire boiler is mounted on a steel frame.
Specifications of Nestler boiler are :
• hp : 105:
• working pressure : 0.7 bar.
• Shell size : 1.6 m diameter x 3.48 m length.
• No. of tubes : 94.
• Size of tube : 63.5 mm
• No. of passes : 3;
• Steam space : 1.47 m3
• water space : 2.25 m3;
• other features: oil firing units have low fuel consumption: better efficiency, silent operation; better heat transfer due to high flame temperature.
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COMPARISON BETWEEN FIRE TUBE (SMOKE TUBE) AND WATER TUBE BOILERS :
Fire Tube Boilers :
They are also called smoke tube boilers. Hot gases from the furnace flow through the tubes while water surrounds the nest of tubes. Tubes may be vertical or horizontal. They have large water storage capacity. Rate of steam production is limited to 9000 kg/hr and operating pressure range is 17 to 25 bar. Operating efficiency is about 80%.
Low initial cost, simplicity, free circulation of water, evaporation of large quantities of steam, more accessible to repairs, easy maintenance and so on.
Comparatively bigger in size and time rate of evaporation is low.
Ex : Simple vertical, Cochran, Cornish, Lancashire, Scotch, Locomotive Boilers.
Water Tube Boilers :
Water to be evaporated is circulated inside the tubes while hot gases and flames flow over them. Surrounding hot gases conduct heat to the water inside the tubes. Tubes may be placed inclined to produce syphonic flow of water through them. These boilers can operate upto a pressure of 200 bar at 540°C. Evaporation rate is as high as 500,000 ku/hr. The overall efficiency is about 92%.
High mass rate of production due to limited water circulation, higher operating pressures, best suited for power plants and marine applications, high overall efficiency, smaller in size for a given output, quick starting.
High initial cost, complicated construction, formation of scales if hard water is used inside the tubes leading to caustic embrittlement.
Ex. : Babcock and Wilcox, Yarrow, Stirling. Benson, La Mont, Loeffler Boilers