Comparison of Water and Smoke tube boilers

Advantages of the water tube boiler over the smoke tube design

Thin Shell formulae-
Longitudinal stress = Pressure x diameter / 2 x thickness of wall

Conduction formulae-
Heat conducted = Difference in tempo x area x time / thickness of material

Hence, for increased pressure ( in search of greater Carnot cycle efficiency for the plant), whilst maintaining reasonable wall thickness and hence heat conduction, there must be a reduction in diameter to prevent excessive stress.

• Advantages
  1. Saving in weight-
     Relevant weight ratio of a scotch boiler to a water tube boiler for the same heating surface area is 3:1 at normal water level
  2. Higher pressures and temperatures possible
     With reference to the thin shell formula, small diameter tubes with reduced wall thickness' are able to withstand higher internal pressures without exceeding maximum stress levels
  3. Increased plant efficiency
     Carnot Cycle efficiency = T1 - T2 / T1

     This means that with increased pressure within the boiler and increase temperature T1 is increased, T2 being set by the sea water temperature and hence cannot be altered, thus increasing plant efficiency.

  4. Increase heat conductivity
     The heat conducted is proportional to the temperature gradient, area and time. It is inversely proportional to the material thickness. It can be seen that the relatively thin wall numerous tubes in a watertube design offer greater conductivity and therefore allow higher firing rates per unit size.
  5. Greater mechanical flexibility
     Scotch boilers are very sensitive to changing load, this is due to its poor natural circulation resulting in mechanical straining and grooving
  6. Reduced warm up time
     Due to the above scotch boilers can take a typical 20 hrs to bring on line, for a water tube boiler this is reduced to 3 to 4 hrs. slopping of the tubes by a minmum of 15^o aids the natural circulation
  7. Saving in space
     Due to the increased surface area, improved natural circulation very high evaporation rates are possible. Due to the large quantities of water required in a scotch boiler and due to the increased wall shell thickness' required for increased diameters there is a limitation to the maximum size ( thus evaporation rate ) that can be achieved. Therefore multi smoke tube unit installations are required in plants that can be served by a single watertube unit.
  8. Increased safety
     Due to the reduction in the water capacity and due to the smaller diameter tubes the possibility of serious damage via a ruptured is much reduced when compared to the catastrophic failures of scotch boilers. Overheated furnace tube failures would empty the entire contents of the boiler near instantaneously into the engine room . Limited tube diameters and protected drum surfaces mean failure of tubes releases a flow of steam dependant on the tube diameter \\
  9. Improved furnace shape
     Improved furnace shape allows for more efficient combustion of the fuel, this also allows the use of reduced excess air
  10. Reduced tube size and wall thickness
      The reduced tube size allows the tubes to be more easily bent and expanded and bell mouthed. Thinner tube material allows for rapid raising of steam increasing the rate of heat transfer from combustion.
• Disadvantages
  1. Reduced water content
     This means that a sophisticated means of controlling the water level is required, in the event of feed pump failure only a short period is allowed before the level in the drum becomes critical
  2. Increases pressures and temperatures
     The increased temperatures means that conditioned for the reduced tube wall material are more arduous. Accurate water chemistry is necessary with even small amounts of corrosion/scaling causing large problems