Chemical Cleaning and Efficiency Improvement

By: Aquachem  05/12/2011
Keywords: Water Treatment, Heat Transfer, cooling towers

AquaChem provides a specialised service to chemically remove unwanted deposits from boilers, heat exchangers, heating and chilled systems plus reactor vessels.

Since 1997 we have successfully used chemical and non-chemical solutions for cleaning systems and in this time have gained considerable expertise in this field.

Why chemically clean?

The build up of scale and other deposits in boilers, heat exchangers, cooling towers and system pipework reduces efficiency of the equipment. In the case of steam boilers this can cause the boiler to be dangerous to operate. On-line chemical cleaning or in exceptional circumstances acid cleaning may have to be considered in order to remove these the deposits as a method of restoring the heat transfer efficiency.

Calcium, magnesium and silica scales which build up on boiler heat exchange surfaces act as an insulator which results in an increase in temperature of the boiler metal. Unless treated or removed the scale will cause the metal to overheat resulting in blisters and eventually tube or furnace failure. The effect on the boiler efficiency can be seen from the following table.

Thickness of Boiler
Deposit mm

Fuel Wasted







When should a system be acid cleaned ?

Boilers - usually, with boilers the insurance inspector will fail a boiler if it considered to be in a dangerous condition and will not allow the boiler to operate until cleaned satisfactorily. For higher pressure boilers mainly chemical cleaning may also be carried out to restore heat transfer efficiency and this decision may be taken by maintenance personnel in conjunction with their water treatment company.

Heat Exchangers and Pipework – if the system suffers from blockages, restricted flow and increased pressure drop caused by poor or inadequate water treatment this will result in a reduction in heat transfer efficiency. Chemical cleaning including acid cleaning must be carried out before the pipework is blocked completely otherwise the chemical solution cannot be circulated throughout the system.

Cooling Towers - scale build up in a cooling tower can encourage the growth of Legionella bacteria and since it is a requirement of the UK HSE that systems are clean. In addition scale formation can lead to a dramatic increase in the energy costs required to operate cooling towers. Where heavy scale build-up has occurred chemical cleaning may have to be considered and any heat exchanger associated with the cooling system will also require chemical cleaning.

Evaporative Condensers – as for cooling towers scale build up on the coils of an evaporative condenser can encourage the growth of Legionella bacteria. These coils generally cool refrigeration gases or in some cases closed cooling water circuits. Scale build-up will result in reduced in cooling efficiency so the refrigeration plant will also be inefficient because the gas is not cooled correctly. Where heavy scale build-up has occurred on the coils chemical cleaning may have to be considered to comply with UK HSE requirements and to reduce the running costs associated with operation of Evaporative Condensers.

What is involved in acid cleaning ?

Chemical cleaning involves the circulation of acid at low concentration around the equipment that requires scale removal. The acid used will depend on the type of deposit and the equipment material that is being descaled and typically are hydrochloric, sulphamic, citric, formic, hydroflouric and phosphoric.

The procedure uses specialised equipment designed for chemical cleaning and in outline this as follows:

  • A sample of the deposit is analysed to establish the most suitable acid to be used.
  • The equipment to be cleaned is isolated to ensure that the cleaning solution cannot escape and contaminate other equipment, pipes or water courses.
  • Pumps are connected for recirculation.
  • The equipment is filled with acid to the required strength.
  • Corrosion inhibitor is added to prevent acid attack on any exposed metal.
  • The solution is monitored for acid strength and further acid added if required.
  • The procedure is stopped when the acid strength remains constant which indicates that no more scale removal is taking place or by visual examination if this is possible.
  • The acid cleaning solution is removed to drain and neutralised at the same time.
  • The equipment is refilled with water with a small amount of alkaline solution to neutralise any residual acid.

What can be done to prevent scale formation ?

Scale deposition can usually be prevented by awell designed and controlled water treatment programme.

Systems such as steam generation, cooling towers and evaporative condensers which are supplied with hard water generally have some form of external water treatment equipment such as a water softener or reverse osmosis system to remove the hardness from the water before it enters the equipment. This is the only long term satisfactory method of ensuring scale free conditions and will be successful as long as the equipment is maintained and monitored correctly. See our Plant & Equipment section for further information on this type of plant.

If external water treatment equipment is not used, then scale formation can be expected, but the amount will depend upon the levels of hardness in the incoming water, the quantity of water being used and the temperature at which the system operates. Some types of systems where scale can be a problem include:

  • Calorifiers
  • Combi ovens
  • Dishwashers
  • Extrusion systems
  • Heat exchangers
  • Hot water cylinders
  • Hot process tanks
  • Humidifiers
  • Injection moulding machines
  • Metal working systems
  • Showers
  • Water stills

Keywords: Cooling Tower, cooling towers, Heat Transfer, Heating And Chilled Systems, Water Treatment,

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