Selection criteria of Power Boiler

1.Cost

2.Efficiency

3.Appropriate Load Response

4.High Turndown/full modulation

5.Maintenance

6.Waterside treatment
Efficiency

•Radiation and Convection Losses 2 - 4%

•Heat Loss in Flue Gases, 18%

•Heat Loss in Blow down, up to 4%

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•Result

Energy Output [Boiler efficiency] @ 75-77%

•The greatest relative source is the exhaust gases from the process of combustion, which can decrease efficiency by 18% or more. The next area of loss pertains to the heat that is radiated from the boiler, whether operating or in standby mode, which can result in losses up to 4%. Too frequent blow down cycles can cause as much as 3% or more in heat loss

Quick Load Response

•this should not be confused with cold start-up when the boiler has been off line and no pressure exists within the vessel. Frequently a boiler is selected based on how quickly it will cycle on from a cold start, meaning no pressure on the boiler and internal water temperature is at ambient. While low water volume boilers may be brought to steam pressure in a very short period of time, rarely are steam loads a batch process where steam boilers cycle off and cool down before the next steam demand. Changing steam loads require that a boiler follow these fast load changes without an excessive change in steam quality or drop in steam pressure

High Turndown

•the boiler must be equipped with a burner that can consistently achieve modulation from minimum to maximum firing rate with consistent fuel/air ratio as the steam demand changes throughout the daily demand. A high turndown burner will minimize on/off cycling [efficiency loss] and maintain steam flow and quality regardless of system demand. For example, a 100 HP boiler with 10:1 turndown can operate with a minimum output of 10 HP, while still maintaining high efficiency. Some “on/off” boiler manufacturers try to approximate modulating high turndown boilers through “modular” configurations but cannot operate efficiently where variable steam loads occur. Consequently, one should ascertain whether the selected boiler has the ability to operate efficiently at minimum or mid-fire input rate without on/off cycling or excessive excess air.

Fireside/Waterside cleanliness

•Fireside/Waterside cleanliness contributes to effective heat transfer. A design that offers ease of access for inspection and cleaning will contribute to favorable efficiency. Generally, low water volume boilers require intense water treatment, and variables in the treatment program can lead to rapid scaling and a significant decrease in heat transfer efficiency. Likewise, if the fireside is difficult to access, inspect and clean, it will become an area that is too often neglected, which results in a decreased energy transfer.

Maintenance

•proper maintenance includes bottom blow down, surface blow down, water conditioning, control functionality, fuel burning components, water feed equipment and control. Thus, equipment design contributes to this function being easily performed. Is the burner easy to access at a normal work level? Is it easy to open and inspect without having to disassemble part or all of the major components? How frequently must the boiler be subjected to bottom or surface blow down? The greater the frequency of blow down, the greater the heat losses. How easily can a failed/compromised tube be replaced? If a tube replacement cannot be done at a reasonable cost, the entire heat exchanger may have to be replaced instead, which increases the life cycle cost of the boiler.

Water Quality

•While low-water volume, low-mass boilers may produce steam quickly, the water quality must be exceptional to avoid scaling and sludge buildup. For example a 100 Hp boiler with a generous heating surface and water volume is less prone to scaling and corrosion compared to a unit that has half the heating surfaces and a third of the water volume. Each square foot of the heating surface must evaporate 11.5 pounds of steam. If Boiler A has 500 square feet of heating surface and Boiler B has 250 square feet, Boiler B will have a greater occurrence of solids concentration due to its smaller heating surface. A boiler with a small heating surface requires more frequent blow down to maintain proper cycles of concentration and increased chemical make-up.