How to choose a good mechanical expander?

Complete opposite from compressors, the expanders reduce the level of signals below a threshold. When it comes to pipe manufacturing, numerous mechanical expanders can be found in the market which may confuse buyers. So, this article will help you learn more about the mechanical expander and have a better picture of your ideal mechanical expander.

The Content list:

1/ What is the mechanical expander?

2/ Considered parameters for choosing a good mechanical expander

What is the mechanical expander?

Mechanical expander consists of cylinder-piston assembly, cone, and dies. Piston reciprocates in the boom. Boom acts as a casing of the piston to protect it. Over the piston on the other side, there is an attachment of a cone that can easily assemble and dis-assemble from the piston.

Cone is having an angular cross-section that is nearer to 4 degrees or it can be made according to the requirement. Dies are attached to the cone with the help of liners. Liners facilitate die to reciprocate inside and outside. Grooves are provided on the cone where liners can be attached easily with the help of bolts. Liners are generally having a T cross-section over which dies can easily move forward and backward.

Hydraulic oil is supplied from the reservoir to the pump. The pump supplies hydraulic oil to the cylinder via a control valve, by which the piston can move forward and backward in the cylinder. An expander can expand the pipe from 1 mm to few centimeters. Hydraulic energy is converted to mechanical energy to perform the task in the case of an expander.

Considered parameters for choosing a good mechanical expander

To perform the final choice of the optimal mechanical expander, a good balance between several factors has to be found. Roberto Capata and Fabio Pantano listed several considered parameters for the choice in their article published in 2020 as the following:

1/ Efficiency: Overall isentropic efficiency as described in each dedicated paragraph. Both scroll and screw mechanical expanders have shown interesting efficiencies.

2/ Machinery volume: Represents machine overall encumbrance. From this point of view, the turbine shows the optimal characteristics again. The screw expander has the greatest volume, while the scroll machine offers a good compromise between volume and efficiency.

3/ MTBF (Mean Time Between Failure): this aspect considering all the moving parts. The scroll and the screw mechanical expander present just a few moving parts, which can either be in contact (if good lubrication is expected) or built with a clearance that does not decrease the machine efficiency and guarantees the absence of contact, thus reducing the wear damage. Moreover, the low rotational speed of the volumetric machines implies less stress on the bearings, with longer maintenance intervals and increasing their reliability.

4/ Lubrication: While the turbine does not present any issues related to lubrication, the issues related to the screw motor can be relatively easily dealt with by choosing an unsynchronized configuration working with a good lubrication fluid. Both scroll and rotary vane expanders, to guarantee good reliability and longer life, should not be oil‐free, thus an oil circuit has to be expected.

5/ EM coupling: The simplicity of connection to an electric generator mostly depends on the rotational speed. In this case, the turbine is the least favorable.

6/ PLE (Partial Load Efficiency): IFR turbines' low flexibility makes their use undesirable in partial load applications. On the contrary, volumetric expanders show great flexibility and can operate smoothly under unsteady conditions. This is one of the main reasons that make those machines appealing to bottoming ORC waste heat recovery processes. The screw motor, in particular, has shown in literature excellent behavior under unsteady conditions.

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