Desuperheaters are a critical component used in the management of steam from power generation sources to industrial uses of steam. Kiekens delivers Desuperheaters for all kind of steam applications that experience wide variations in load.

Desuperheaters as solution for steam applications

How does a steam desuperheater work?

Desuperheaters are used to control and condition the temperature of superheated steam, which is crucial for many industrial processes. The definition of a desuperheater is a piece of equipment that atomizes and evaporates the cooling supply liquid, usually water, into droplets in the smallest size possible.  The aim of this process is to reduce the temperature of the incoming gas, usually steam, within the shortest possible distance and time to the required and controlled level. 

What is steam?

Steam is water in the gas phase, which is formed when water boils. Steam is invisible; however, “steam” often refers to wet steam, the visible mist or aerosol of water droplets that is formed when water vapour condenses. At lower pressures, such as in the upper atmosphere or at the top of high mountains, water boils at a lower temperature than the nominal 100 °C (212 °F) at atmospheric  pressure. When heated further, for instance in a boiler superheater, wet steam becomes superheated steam.

What is superheated steam?

Superheated steam is steam that is heated above the previously mentioned boiling temperature of 100 °C (212 °F) at atmospheric pressure. This process involves a few different steps. When additional heat content is added to boiling water in liquid condition, the heat content (enthalpy) of the water further increases. The temperature of the water, however, maintains a constant value until all liquid has been evaporated to a gaseous state. At this point, a condition of so-called saturated steam is reached. When additional heat content is added to saturated steam, a condition of superheated steam is obtained. Superheated steam is also known as “dry” steam, as it contains no water droplets. It does contain, however, a lot of energy. The stored energy in dry steam makes it highly efficient to transport it  to a point where the energy is required.

Diagram for steam

What is desuperheated steam?

Desuperheated steam is superheated steam that is cooled or attemperated by a Desuperheater. If the temperature of superheated steam is not constant, processes are difficult to control, particularly when there are wide variations in load and pressure. A Desuperheater lowers or controls the temperature of superheated steam by injecting a calculated amount of water droplets into the flow stream. As a result, the saturated steam can be used more effectively for heating and the transfer of heat in all kinds of industrial processes. 

What are the advantages of desuperheated steam?

Desuperheated steam, with its precise temperature, improves the quality of industrial products or processes, protects downstream equipment from heat related damage and can reduce unscheduled downtime of the production line or improve the efficiency of the downstream heat exchanger. Furthermore, the precisely controlled temperature allows you to start your chemical processes more efficiently.

In a back pressure turbine the desuperheated steam temperature is a function of steam load to the turbine. When the steam temperature gets too high, usually at low turbine loads, desuperheated steam can be used to assure proper operation of the downstream installed process equipment.

Frequently, this downstream equipment consists of heat exchangers that are preferably fed with steam at a temperature as close as possible to the saturation temperature.

What is the true desuperheated steam temperature?

It is advised, by temperature control suppliers, to always measure “dry” steam. This means that all injected cooling water should be evaporated in the desuperheated steam. If the specified desuperheated steam must be at saturation temperature, it is advised to keep temperature around 5 °C above saturation temperature to prevent water droplets or wet steam hitting the temperature controller. If water droplets or not completely mixed wet steam hit the wall of a temperature controller, it will not give a true reading. The system may start to get unstable, as the temperature is not rightly controlled. The distance from the water injection point to the temperature sensing device is critical. Steam and water should be properly mixed and evaporated. The shorter the distance, the more efficient the desuperheater.

What is an attemperator?

The majority of high pressure boilers are using one, two or even three superheaters. The temperature in the superheaters tends to fluctuate as a result of many operational parameters. These parameters include things like excess air, feed water supply temperature, the type of fuel used, and other operational issues like start-up. Of course, the variation in the steam flow demand also plays an important part. Since high efficiency boilers and furnaces operate with temperatures as close as possible to maximum allowable temperatures dictated by the construction materials, the protection of the superheaters – in particular the second and third one – is essential. 

Such protection is necessary to prevent serious erosion problems and damage to the materials of the superheater and downstream steam piping. For this purpose, a desuperheater is typically installed between the primary and secondary superheater. In this case, the desuperheater is often referred to as an attemperator. It is named an attemperator because it tempers the high steam temperature to an accurate temperature. Functionally, a desuperheater and attemperator are the same, i.e. they remove the superheat content of the steam with the help of water. But in this set-up it is even more crucial to install a reliable and accurate desuperheater. The ASME Boiler I: Rules for Construction of Power Boilers, however, only mention a desuperheater arrangement between superheater or reheater, not attemperator. 

Difference between a desuperheater and an attemperator?

The term attemperator is generally used in places where it is crucial that the final steam temperature reaches an accurate temperature. Every attemperator is a desuperheater, but not every desuperheater can be used as an attemperator.  

The in-line Kiekens Venturi is the best available boiler attemperator solution for use in the HP inter stage, reheat inter stage and final stage application. The absence of moving wear and tear parts makes it very reliable. Only minimized maintenance is required during its lifetime. Due to its working principle it is highly accurate, even at extreme high demands of water injection (up to 40%). Furthermore, a controlled and full evaporation of the cooling water is guaranteed without the need for a so-called thermal liner or thermal sleeve to protect the downstream piping. 

When is steam pressure and temperature control required?

Superheated steam coming out of the power boiler is generated at high pressure and temperature, and transported as thermal energy for a lot of systems in the plant. Depending on plant demand in refineries, (petro) chemical plants and power plants, these high pressures and temperatures must be reduced to desuperheated steam in order to assure stable, accurate and safe operation. 

The Kiekens desuperheaters can be combined with a separate pressure control valve (PCV) and (separate) temperature control valve (TCV) to reduce both the pressure and temperature in a so-called pressure reducing desuperheating system (PRDS). Desuperheater process calculations can be made to select the best suitable desuperheater and desuperheater valves in the PRDS. 

Following examples of pressures reducing systems can be found in the plant:

  • EHP (90-120 bar) to HP (40-60 bar) level;
  • HP (40-60 bar) to MP (15-25 bar) level;
  • MP (15-25 bar) to LP (4-8 bar) level;
  • LP (4-8 bar) to LLP (1-3 bar) level;
  • EHP (90-120 bar) to MP (15-25 bar) level;
  • HP (40-60 bar) to LP (4-8 bar) level.

When selecting a desuperheater it must be taken into account that this is not a standalone device. It is an important part of a system, and as such it has to be designed and sized in relation to the other system components and system parameters. Even the best desuperheater will not or insufficiently perform without correct sized control valves, the correct sensing- and control elements, a correct sizing of the connecting steam lines, sufficient straight pipe length up- and downstream of the unit, and eventual precautions against excessive noise.

Based on your specifications, Kiekens-DSH can perform all necessary calculations described above and supply a complete pressure reducing desuperheating system. 

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