On generating boilers as well as in heat exchange tubes, a scale deposit might occur on heat transfer surfaces. This deposit occurs when soluble matter reacts in the boiler water to form a layer of material on the waterside of the boiler exchange tubes.
Scale creates a problem because it typically possesses a thermal conductivity with an order of magnitude less than the corresponding value for bare steel. When a deposit of a certain thickness and given composition is formed on the heat exchange surface, the heat transfer through surfaces is reduced as a function of the scale thickness. Even small deposits might thus serve as an effective heat insulator and consequently reduce heat transfer. The result is overheating of boiler tube metal, tube failures and loss of energy efficiency. By removing the deposit, operators can easily save on energy use and on the annual operating costs.
Fuel waste due to boiler scale may be 2 % for water-tube boilers and up to 5 % in fire-tube boilers.
At boiler level, a regular removal of this scale deposit can produce substantial energy savings.
By treating feed-water to prevent scale deposits, the use of chemicals may increase.
Whether scale deposits need to be removed can be ascertained by a simple visual inspection during maintenance. As a rule of thumb, maintenance several times per year may be effective for appliances at high pressure ( 50 bar). For appliances at low pressure ( 2 bar) annual maintenance is recommended.
It is possible to avoid deposits by improving the water quality (e.g. by switching to soft water or demineralised water). An acid treatment for deposit removal has to be carefully as sessed, particularly for high pressure steam boilers.
在定期維護保養時以目視檢查方式可以決定結垢是否需要清除。一般規則是高操作壓力的裝置(50 bar)每年會有幾次的維保安排；至於低壓裝置(2 bar)則維持每年一次維保。
Depends on the method used, and other factors, such as raw feed-water chemistry, boiler type, etc. Payback in fuel savings, increased reliability of the steam system and increased operating life of the boiler system ( giving savings on lost production time and capital costs) are all achievable.
See examples, in Annex 7.10.1.
Energy Efficiency (2009) 3.2.6