How to sizing an Archimedean Screw turbine with HPP-Design

What is an Archimedean Screw Generator (ASG)?

An ASG is a positive displacement machine, which consists of a rotor in the shape of an Archimedean screw rotating in a semicircular trough. By filling the buckets of the screw, incoming water provides a tangential force, thus a torque, on the shaft of the turbine.

Thanks to its robust design, cheap construction, good efficiency, and tolerance to floating object transit, the ASG is a good solution for low heads and medium discharges.

When to use an ASG?

An ASG can be used for heads ranging from 1 to 5 m and discharges from 0.5 to 7 m³/s, as depicted by the red line in the picture below. Multiple ASGs are commonly used in side-by-side arrangement in order to increase the plant discharge, up to 30 m³/s (green area in the picture below).

How to select an Archimedean Screw Generator in HPP-design?

Just create a new sizing, enter a value of net head [H] and maximum discharge [Q] suitable for ASGs (e.g. H=3m, Q=5 m³/s), click “create sizing” and select the icon of the Archimedean Screw.

Why it is not possible to select the ASG icon?

Check the values of net head [H] and maximum discharge [Q] you entered, probably they are out of the range of the ASG (H = 1-5 m and Q = 0.5-30 m³/s)

Why there is more than one possible solution when I select ASG icon?

Because the required discharge can be achieved with different numbers of generators. More generators require more space and the cost of the plant is normally higher, nevertheless the single generator will be smaller and easier to transport, plant efficiency higher at partial loads and the maintenance easier.

What is the “Suggested configuration”?

Is the solution that allows minimizing the number of generator in the plant, thus the width and the total cost.

Why is there a limit in the maximum discharge of the single Archimedean screw?

Because of construction and transportation limits. Moreover, the lower the available head, the lower the maxim discharge per turbine due to aspect ratio constraints.

Once I have selected the turbine on the list, are there other possible options?

Yes, on the sizing-detail page, you can choose between a fixed-speed and a variable-speed regulation of the turbine. You will see a change in the part load efficiency on the graph.

Which turbine should I choose if both Kaplan and ASG are available?

What are pro and cons of an ASG as compared to a Kaplan turbine?

For heads between 2 and 7m, both a Kaplan turbine and an ASG are available. The first one has a slightly higher efficiency and smaller dimensions. On the other side, ASGs are frequently cheaper (lower CAPEX and OPEX), easier to inspect and allow the transit of debris without the needing of an automatic trash rack.

Through HPP-Design you can compare multiple solutions, check the features of each machine and ask for a quotation!

What is the expected efficiency of an ASG?

Despite its simple construction, an ASG is able to achieve hydraulic efficiencies over 80%. Main losses are related to hydraulic frictions, turbulence at intake and discharge section and water leakages between the screw and the through. Provided that water velocity in the screw is one order of magnitude lower then in reaction turbines, friction losses and kinetic energy loss at discharge are low.

How to regulate an ASG?

An ASG is rather different from traditional turbines, such as Kaplan, Francis or Pelton, since it is a positive displacement machine. Thus, it is regulated without the need of adjustable blades or gates. The screw self-adapt to the decreasing flowrate through a lower filling of the buckets and a lower water level at intake section.

A speed regulation through an inverter system allows achieving higher efficiencies at partial loads by keeping a constant level at intake section. 

Is an ASG reliable?

Although ASG is a relative newcomer to the hydro world, having only arrived on the scene over the last 25 years, they have been around for many centuries as pumps where tens-of-thousands unit have been installed worldwide, particularly in sewage treatment works. The same manufacturers that dominate the pump market are now the main suppliers into the hydropower market as well, providing reliable machines with very low operative costs.

HPP- Design will help you in this choice, providing accurate sizing data and energy calculation for every type of turbine!

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Doubts or questions? Read our [FAQ]

How to choose a hydroelectric turbine, a specific speed question!

The two main data needed to design a turbine are the maximum flow rate and the net head associated to this flow. However, these two values do not allow to uniquely define the type of turbine that must be designed; the designer is required to choose a third fundamental parameter: the number of revolutions.

Then it will be possible to define what we call the turbine specific speed.

But what is the specific speed? Leaving aside the theory about mechanical similarities, let's briefly explain what it is.

The specific speed of a turbine is defined as:

Where

    Q    maximum discharge     [m^3/s]

    H    net head  [m]

    g     gravity  (normal 9.806[m/s^2])

angular velocity of turbine [rad/s] calculated as:  

This parameter relates to each other the characteristic data of the turbine, and has the peculiarity of identifying families of geometrically similar machines (and then to scale them), having very different powers and sizes, but with one fundamental common characteristic: they are part of the only family of turbines that can process the available head and the discharge with the highest performance possible.

With simple calculations you can use the characteristic data of the turbine (H, Q and n) to find a value of specific speed  that uniquely identifies the most efficient type of turbine and its main dimensions. All this thanks to the experience gained in the design of more than a century of hydropower construction and excellent theoretical studies that have enabled the development of the theory of similarity at the basis of the specific speed calculations.

This does not mean that all turbines manufacturers build the same equipment once given the same typical number. After choosing the type of turbine and the main dimensions defined, each manufacturer has developed its own geometry and parameters that differentiate the turbines in terms of operation, reliability, cost and efficiency. Over the years, various design schools have proposed many definitions of the specific speed. The UNI-ISO has tried to standardize them into a single parameter, but in fact the definitions remain different.

In HPP-Design we use the parameter k, seen above, and nq

In the picture, every value of typical number of machine k (or nq) corresponds to a very specific type of turbine, exception made for some overlapping areas in which there is no univocal choice and the experience has shown that they can adapt well to different types of turbines (Pelton / Francis and Francis / Kaplan).

The question that arises is: how is this value really used?

Known the head and discharge, a preliminary study is made to define the appropriate number of revolution, taking into account some possible rotation speeds, and then calculating the relative specific speed which corresponds to a well-defined design solution. The possible solutions are then compared, in terms of performance, cavitation behaviour, main dimensions, etc.  and finally the choice is made for the solution that best suits the specific project. Once the rotation speed is fixed, starting from the specific speed it is possible to choose the type of turbine and start the detailed design.

Hpp-design is the tool that helps the designer to make a preliminary choice by comparing these elements directly into the chosen page. Try it out!

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