Case Study: Thermal Sludge Drying


Leading Manufacturer of Thermal Sludge Drying Equipment


  • Fan wheels for modular sludge drying plants
  • maximum energy efficiency
  • optimal engine utilization


unhoused fan wheels of the punker D-Series


one fan wheel type for all plants

Project Overview

Our client is part of a group of companies from the environmental sector. In 2008, the client approached punker with the challenge of supplying suitable fan impellers for new types of modular thermal sludge drying plants that make a significant contribution to maximizing the energy efficiency of the entire plant. punker supplies highly efficient fan impellers of the Dclassic SWL series with a diameter of 1,000 mm. Flexible wheel widths do justice to the modular character of the plant and ensure optimum utilization of the motors used.

Thermal Sludge Drying with punker Dclassic SWL

Our Client

Our client is part of a stock listed group in the environmental sector. Their mission revolves around developing access to drinking water, protecting resources and ecosystems, optimizing the use of resources and producing new resources. The group’s self-declared goal is to become the worldwide leader in environmental services.

The group’s main activities include management solutions for the water cycle, recycling & waste recovery, and solutions for water treatment. Since 2008, the company has been purchasing air handling solutions from punker for its thermal sludge drying plants, which are used worldwide.


The goal of our customer was to develop thermal sludge drying plants with maximum energy efficiency. The customer approached punker with the challenge of supplying suitable fan impellers for the modularly designed plants, which make a significant contribution to the energy efficiency of the entire plant. In addition to a high static efficiency of the fan impellers, this also required exact matching of the fan impellers to the different operating points resulting from the modularity of the systems and the motors used, which were to be optimally utilized in terms of overall system efficiency.


What is thermal sludge drying?

Sewage sludge is dried to minimize its quantity and thus transport and disposal costs. Dried sludge is easy to store and handle so that it can be disposed of or recycled in a variety of ways. In addition to being used as fertilizer, dried sludge can also be used as fuel for energy production, with a calorific value comparable to that of lignite.

The limitations of mechanical sludge drying sometimes make thermal drying by water evaporation necessary. The company has developed a range of sludge drying processes that allow the sludge to be used for a variety of applications as fertilizers, by-products for composting or as fuel for incinerators.

Benefits of sludge drying:

    • By removing the water from the sludge through evaporation, the volume and weight of the sludge is reduced, which facilitates storage and transport.
    • Drying stabilizes the sludge, reducing odor and improving hygiene.

By evaporating the water, the final product can be used use in a composting plant. This means that the sludge can be recycled for use in agriculture.


The punker application engineers drew up a detailed requirement profile for the thermal sludge drying plant and determined backward-curved fan impellers of the R63D series (predecessor of Dclassic SWL) as the optimal air-technical solution. With a maximum static efficiency of 75%, these make a significant contribution to the energy efficiency of the entire plant, which is being used in a project for drying bio-solids in the watershed area of a large metropolitan are in the USA. With 34 wheels with a diameter of 1,000 mm and different widths, all operating ranges of the modular plant are covered and at the same time the motors used are optimally utilized. Due to the special suitability of the D-Series for free-running operation (without spiral housing), they can be flexibly used in existing box-shaped plant designs. punker supplies all fan impellers with inlet nozzles specially adapted to them, which are a prerequisite for optimum air performance.

The client’s system is a thermal sludge dryer that operates at low temperatures. While other manufacturers’ sludge dryers usually operate with hot air up to 180°C, this belt dryer requires only 90° C hot air. This low temperature requirement, the used heat recovery and the use of punker fan wheels make our client’s sludge dryers extremely energy efficient. Energy consumption is reduced, greenhouse gas emissions are lowered and odour formation is controlled by closed air circulation.

punker Dclassic SWL

With Dclassic SWL, punker offers the optimal fan wheel for unhoused applications that require a static pressure increase of 1500 to 2500 Pa | 6 to 10 in. WG (AMCA Class II). Thanks to the welded steel design, Dclassic can be used for high rotational speeds up to 75 m/s | 246 fps.

Your benefit: Very high ventilation system performance with 75 % static efficiency (83 % total efficiency) in unhoused applications.

punker WEB | Fan Wheel Selection

puncat WEB lets you select fan wheels directly from our website. Display fan wheels that meet your application requirements by entering the desired duty point , speed and wheel diameter range. You can specify your search by choosing industry suitability, fan wheel type and fan wheel series.

Select the optimal fan wheel here.

Your Project with punker

Are you also planning an innovative product that will revolutionize the market? Rely on the support of the punker R&D team. Our design engineers will work with you to find the perfect air movement solution for your project.

Do you already have a detailed view of your new product and can provide information on air performance, drive and design specifications? Then simply send us these details using the technical questionnaire below and our team will immediately define suitable solutions for you from the broad punker portfolio.

Or are you looking for a sparring partner who will work with you to develop ideas for a new or optimized series generation and accompany you all the way to series production? Then simply contact our application engineers by e-mail at

Technical Questionnaire




    Your eMail address*:




    Attention: Frequently all operating details are either not known or only inaccurately known. You must therefore as a manufacturer make sure by suitable tests that the correct fan wheel has been selected. When the following questions are answered as comprehensive as possible, we can submit a proposal as quickly and as defi nite as possible. Please give us further details on a supplementary sheet. This questionnaire on no account replaces a technical specifi cation (TSP).

    1. General Information

    1.1. Fan application (type of unit and appliance, rail application, compressor, etc.)

    1.2. Are there any special requirements on safety conditions? (e.g. explosion protection, danger of accident at wheel failure, etc.)


    Please state special requirements:

    1.3. Medium to be handled and its state (e.g. smoke gas with t max = 300° C, dust laden air or similar)??

    2. Air Duty

    2.1. Volume flow
    V' = m³/hcfm

    2.2. Static pressure increase
    Δpfa = PainWG

    2.3. single flow/double flow?

    2.4. Density of the medium
    rho = kg/m³lb/ft³

    2.5. Max. media temperature
    t = °C°F

    2.6. Max. A-rated sound power level
    Lw = dB(A)

    2.7. Other technical details (e.g. max. flow speed, special curve steepness, etc.)?

    3. Drive

    3.1 Is the speed variable?


    minimum ; maximum rpm
    fixed speed rpm

    3.2. Direct motor drive

    n = rpm

    3.3. Is a maximum motor power specified?


    maximum motor power Pw = kWhp

    3.4 Operating mode?

    S1 - IEC 60034-1other operating mode

    other operating mode =

    4. Dimensions and Constructional Execution of the Fan Wheel

    4.1. Can a standard fan housing be used?


    max. dimensions of spiral housing:

    4.2. Are certain main wheel dimensions defined?

    main wheel dimensions:

    4.3. Hub bore
    Ø d = mm, fit
    with/without keyway?


    4.4. Hub model and arrangement (e.g. inside the wheel, outside, etc.)

    4.5. Material of wheel body


    other material:

    4.6. Additional surface protection


    other surface protection:

    4.7. Additional specifications

    5. Demand, Pricing

    5.1 You would like to request an offer for:

    Sample wheels: pcs.

    (later) batch: pcs/year

    Lot size pcs/order

    5.2 planned start of production TT/MM/JJJJ