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Watertube Boilers

In watertube design boilers the water runs though tubes surrounded by combustion gases. Watertube boilers are used on the very high and very low spectrum of the boiler market. These are identified by their respective horsepower ranges as:

• Utility — 100000 MBH and up
• Industrial — 30000 to 100000 MBH
• Commercial — Up to 30000 MBH

Watertube boilers are characterized by smaller diameter tubes, which are longer, in bent, curved or straight configurations. Many of these boilers have serpentine or tubes configured in a spiral. These tubes are often headered in parallel to make-up a complete wall or panel of heat absorbing watertubes. For this reason, the heat absorption rate per unit of furnace volume or furnace wall area is relatively high as compared to firetube boilers.

All high pressure boilers above 300# are of watertube design due to the fact that tubing of a given diameter and wall thickness can withstand a much higher internal pressure than external pressure. In the case of the firetube boiler, the tubes have to withstand the water pressure from the outside of the tube and are thus limited in their ability to function in very high pressure situations.

Most failures in watertube boilers come from split or burned tubes, which can result in a collapsing of the watertube walls.

As most watertube boilers have very small steaming areas and water content, they are very sensitive to changes in demand and water level. They require close attention, more sophisticated control systems, demand good consistent maintenance and water treatment.

There are several different types of watertube boilers encountered in our markets, they are:

Copper Watertube | Steel Watertube | Steel Bent Dry Base | Steel Bent Wet Base

Job-Built Boiler Designs

If you encounter a situation where an old firetube boiler needs to be replaced with a new firetube boiler, but cannot get the new boiler in without significant structural work to the existing building, try a “job-built” boiler.
Job-built boilers are boilers which are fully constructed at the factory then cut into 36-inch or less pieces. These pieces have been planned from their conception for this task and arrive marked with clear “match points” for easy reassembly on the job site. However, these boilers require code certified welders and welding methods to insure their safety and credibility of operation.

Steel Bent Wet Base Characteristics

Prime Duty: Hydronic heating 30# to 160# WWP. hospitals, small commercial/industrial and process.
Firing Modes: Forced draft firing only.

Comments:

  • Economical boiler where high hydrostatic pressure over 30# WWP are required.
  • Entire combustion chamber and all flue gas passages backed with tangential water tubes.
  • Boiler good for hydronic applications, however, low water content, low disengaging area and low steam volume makes it difficult to maintain steady water levels with fluctuating steam loads.
  • For steam applications, modulating feedwater system required.
  • Good longevity, reasonable cost.

Steel Watertube Characteristics

Prime Duty: Hydronic heating systems, apartments, office buildings, small commercial/industrial buildings.
Comments:

  • Economic where high hydrostatic pressure over 30# WWP are required.
  • Boiler not affected by thermal shock, however is subject to flue gas condensation caused by low inlet and/or operating temperatures.
  • Atmospheric firing. Pressure fired not true forced draft.
  • Large refractory surfaces not water-backed cause “hot spots.”
  • Heavy insulation of jackets required.
  • Cannot be used for process steam due to low water content.
  • Medium cost/low longevity.

Steel Bent Dry Base Characteristics

Prime Duty: Domestic hot water and hydronic heating for apartments, office buildings, small commercial/industrial.
Comments:

  • Economic where high hydrostatic pressure over 30# WWP are required.
  • Boiler not affected by thermal shock, however is subject to flue gas condensation caused by low inlet and/or operating temperatures.
  • Atmospheric firing. Pressure fired not true forced draft.
  • Large refractory surfaces not water-backed cause “hot spots.”
  • Heavy insulation of jackets required.
  • Cannot be used for process steam due to low water content.
  • Medium cost/medium longevity.

Copper Watertube Characteristics

Prime Duty: Domestic hot water, pool heaters, hospitals, health clubs, motels, small industrial plants, laundries, restaurants and car washes.

Comments:

  • Quick recovery
  • Low cost
  • Low longevity
  • Consult manufacturer’s data regarding minimum water circulation rates and pressure drops.
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