Attemperator Vs Desuperheater

Attemperator Vs Desuperheater

The basic function of Attemperator and Desuperheater is same that is to desuperheat the steam or reducing the steam temperature by spraying water in the flowing steam. Then what is the difference between them. Let us discuss that in brief.

The meaning of the word attemper is to Regulate. Hence, attemperator means regulator. The Attemperator is used to regulate the steam temperature at the outlet of Final Superheater. It is used in Boiler application and is located between primary & secondary superheater and between secondary and tertiary superheater. It is installed to regulate the final temperature as per the project requirement.

Desuperheaters are generally combined with Pressure reducing stations. It is located at the downstream of pressure reducing valve and are used for process applications and for turbine bypass.

Following are the differences between them:

• Attemperator is used based on the boiler requirements and hence covers wide range of steam parameters whereas Desuperheaters are located after pressure reduction and hence it is used in Medium or low pressure steam.

• Attemperator deals with superheated steam at both upstream and downstream of it. The temperature regulation in Attemperator needs to be very precise whereas in desuperheater 5-7 degrees above saturation temperature in outlet steam is acceptable.

• The spraying water quality in attemperator is very important as the steam is fed to Turbine hence the spray water should essentially be Boiler feed water with a strainer installed before Temperature control valve whereas for desuperheater both boiler feed water or condensate can be used based on the requirement.

Pressure Reducing and Desuperheating Station (PRDS)

When there is requirement of lower pressure steam, than what is generated, by various equipments / processes, PRDS are used so as to fulfill the requirement. PRDS is used for steam conditioning by reducing the pressure & temperature of steam. It is a combination of control valve for pressure reduction and an atomising / spraying nozzle for desuperheating. The outlet steam should be 5 to 7 degrees higher than saturation temperature or else there may be danger of supplying wet steam.

Types of PRDS

Type- 1 : Split PRDS

In this type of PRDS, desuperheating is done external to the pressure reduction. Cooling water is sprayed downstream of pressure control valve through a separate desuperheater unit after the pressure reduction of steam is completely done. In this type of PRDS, pressure reducing station can have a manual bypass valve for pressure reduction with a common desuperheater unit.

Type-2 : Combined PRDS

This is an integral type PRDS which means the pressure control action and water spray take place in the same unit. This type of control valve consists of an inlet & outlet process connection and a nozzle for the entry of the spraying water. The cooling water enters through the valve & sprays water in the low pressure zone. If any bypass is required in this type of PRDS then the bypass valve has to be the same control valve. Manual bypass will not do as it will again require a separate Desuperheater for the bypass line. 

Spray Water

As the spray water is mixed with the steam to desuperheat the steam, the spray water quality should be same as that of steam. Hence, either Boiler feed water or Steam condensate shall be used as spray water. Spray water is tapped either from Boiler Feed Pump or Condensate Extraction pump based on the pressure & temperature requirments. For high pressure steam, boiler feed water is used whereas for low pressure steam, condensate is used.

Spray water Pressure requirement (This data needs to be confirmed from PRDS Vendor. However for preliminary calculations / specifications, following can be used.)

For Type-1

Spray water pressure = Steam pressure + 5-7 bar

For Type-2

Top Entry   : Pw = [(Pi + Po)/2] + 7 Bar

Bottom Entry through stem  : Pw = Po + 7 Bar

Bottom Entry through nozzle: Pw = (Pi / 2) + 7 Bar

PRDS Calculations

Inlet Steam Flow rate = Mi , Outlet Steam Flow rate = Mo

Spray water Flow rate = Mw

Inlet Steam Enthalpy = Hi , Outlet Steam Enthalpy = Ho

Spray water Enthalpy = Hw

Enthalpies are to be found from Steam table based on the Inlet & outlet operating parameters

Mi + Mw = Mo ………………………………. Eqn. 1

Mi x Hi + Mw x Hw = Mo x Ho ………….. Eqn. 2

Using the above two equations, any two unknown parameters can be evaluated. Either inlet or outlet steam flow rate is always known. The spray water flow rate requirement can be calculated through these equations. An example below for better understanding.

Inlet Steam Conditions;

Pressure = 42 kg/cm2 (g), Temp = 380°C

Hi = 755.74 Kcal/Kg

Outlet Steam Conditions;

Pressure = 4 kg/cm2 (g), Temp = 156°C (Atleast 5° above saturation temp.)

Mo = 30 TPH, Ho = 658.68 Kcal/kg

Spray water Conditions;

Temp. = 60°C, Hw = 60 Kcal/kg

Steam inlet flow rate & Spray water flow rate needs to be calculated.

Mi + Mw = Mo

Mi + Mw = 30 ……………………………………… Eqn. 1

Mi x Hi + Mw x Hw = Mo x Ho

Mi x 755.74 + Mw x 60 = 30 x 658.68 ………. Eqn. 2

Solving both the equations,

Mi = 25.8 TPH & Mw = 4.2 TPH

Layout Requirements

1. When there is space constraint, Type-2 PRDS is a better option as it consumes lesser space than Type-1. 

2. If PRDS is a continuous process then Type-1 is cost effective as it can have a manual bypass valve.

3. Minimum straight length required at the outlet of PRDS is 4-5 mtrs. It varies from manufacturer to manufacturer.

4. Minimum distance of temperature sensor from the point of water injection shall be 12 m.

Riser and Downcomer Concept

The above scheme is called natural circulation. The natural circulation is one of the oldest principles for steam/water circulation in boilers.

Subcooled FW enters the drum, mixes with the steam & water mixture inside the drum and attains saturation temperature instantly. Downcomers carry the resultant cooled water to the bottom of the evaporator tubes. As this water goes through evaporator tubes, it picks up its latent heat progressively from the hot flue gases and starts boiling to form steam. However, at the lower levels, the pressure increases due to static head, which increases the Boiling Point and hence steam formation takes time. The circulation happens by itself/naturally due to the density differences between the water in downcomers and Water-steam mixture in risers. The external Risers carry the water-steam mixture to the steam drum. Being at the topmost elevation, major steam formation takes place in steam drum. This steam is continuously separated in the drum by the steam separators.

The quantity of mixture flowing through the system is determined by Circulation Ratio.

Circulation ratio of high pressure boilers are in the range of 6 t o 8. Reciprocal of Circulation ratio is dryness fraction. Hence, the dryness fractions in riser tubes are in the range of 1/8 to 1/6 i.e. 0. 125 to 0.167. So, the steam quantity in risers is approximately 12% to 17%.

However, at start -up, the circulation ratio is very high as the system is completely filled with cold water. Hence, to be conservative, downcomers, Boiler banks as well as risers are designed for saturated water.

Following are different forms of Boiling in Evaporator tubes.

About Me & Plant Engineering

Hi There!!!

I am Sumana, a mechanical Engineer with approx. 15 years of experience in the field of plant design engineering. Plant design engineering includes basic design engineering of plant and detailed engineering. Here in this blog, I would like to share my experiences & knowledge with you. Before starting, let me explain in brief, basic design engineering & detail engineering for you.

Basic Design Engineering

Once the concept has been evaluated or selected and the technical scope of plant is determined, Basic engineering comes into picture. All expected costs and expenses are evaluated in basic engineering.

Basic engineering starts with a basic flow diagram which represents the process engineering procedure of the plant. The input, chemicals, reactants & output are defined. Then they are assigned to their corresponding equipments for storage or reaction or for any other process. The types of equipments and sequence of basic process operations are determined. Basic flow diagram is the main skeleton of the project and generally is created by the Owner itself.

Then the next level is to create the Process Flow Diagram (PFD) based on basic flow diagram. All essential component requirements like Pumps, compressors, heat exchangers, tanks, working/standby philosophy, the services requirement like water, steam, air etc. are decided at this level. The major heat and mass balance are done at this level.

The preparation of Major Equipments' specifications for the RFQs can be prepared after this level i.e based on PFDs. PFD helps for the estimation / Budget of the project.

The next higher & the final step is to prepare Process and Instrumentation diagram (P&ID) which shows each & every process detail, process requirements, equipment details, piping connections, instrument requirements and control philosophy. The detailed heat & mass balance calculations, equipment sizing, pipe sizing, pressure drop calculations are done at this stage. The equipment specifications, piping material specifications, instrument specifications are finalised at this stage.

Detail Engineering

The biggest contribution to the project is made by detailed engineering. The important activities during detailed engineering are selection of Equipments / finalisation of vendors, preparation & finalisation of Equipment layouts,  Optimisation, Piping layouts, Pressure vessel / tank design & detail engineering, finalising Structure / platform requirements, Structural detail engineering, pipe detailing, support detailing, E&I technology etc.

Detail engineering is highly multidisciplinary activities taht involves many specialists to complete each packages in detail. The multidisciplinary activities are interdependent and are also carried out parallalely which require lots of to & fro communications / discussions. Hence, good co-ordination, interdisciplinary communications & technical understanding are few of the important aspects for carrying out detail engineering of the Plant.

Another important task of detail engineering team is to solve all the problems that client come across during the erection of the plant which may include some interface issues, material unavailability etc.

Similarly the basic engineering team needs to support during the commissioning stage.