Steam screw expander, not a steam turbine..

Steam turbines work by converting the energy of high pressure steam into mechanical energy to turn an electrical generator or other piece of equipment. Steam turbines are generally designed for superheated steam applications. Now however there is new technology specifically designed for saturated steam.

The basis for this new technology is a screw compressor in reverse. Many will be familiar with a screw compressor, two screws turn and effectively compress the medium between them. The screw expander works in reverse, exhausting lower pressure steam out of it. The screw expander emerged from research work carried out at City University in London in the 1990’s. This work has been further developed into a working unit which is now available to buy in the marketplace by Heliex Power Limited. The expanding rotor design required considerable research so there is close to pure rolling contact between the moving rotors. The process fluid going through the machine effectively lubricates the moving parts. 

It is typical for steam to be generated at a higher pressure and for it to be used at a lower pressure. Therefore the pressure of the steam is reduced near the point of use by using a pressure reducing valve, or now potentially with a screw expander in parallel, normally taking all of the steam load.

The reason for generating at high pressure and using at a lower pressure can be summarised below:

  • Most steam boilers work better at higher pressures. This ensures that wet steam is not produced and steam is generated in the most economical way. Reducing the pressure of a boiler can typically lead to a reduction of its output.
  • The maximum allowable working pressure (MAWP) of the item of steam equipment, is lower than the pressure of the steam boiler.
  • Distributing steam at higher pressure has the advantage of using smaller steam mains. This is due to the volume required being smaller the higher the pressure.
  • By reducing steam to a lower pressure at the point of use will reduce the percentage of flash steam produced in the condensate system after the piece of steam using equipment. Reducing flash steam being produced will save energy, water and therefore cost.

Potentially the screw expander can be installed in parallel with the pressure reducing valve station and used to generate electrical power. The screw expander is used to turn the electrical generator. Thought must be given to the use of power, preferably locally on the same site as the steam system. Even if power is not going to be fed back into the national grid a G59 panel and consultation with the local power company is required (UK customers). This is a similar to the steps required if power is generated through small solar or wind projects. By generating power in this way C02 financial savings can result. Heliex power are claiming payback in the region of 3 years for suitable applications.

Other potential uses include waste heat applications where steam can be generated which can then be put through the screw expander.

We look forward to following this interesting technology.

Posted by Admin on 07/09/2014


Comments

Interesting bit of kit,  a screw compressor in reverse, so simple. Why didnt I think of that! Be interested to see what actual kw's they can get out

Big Al - Wed 17 Sep 2014 - 11:16

The total energy available will be the enthalpy dierence between the inlet pressure and outlet pressure by the mass low rate i.e 1,000 kg/hour of steam being reduced in pressure from 13 Bar to 3 Bar would have potentially 14 kW of energy available, I wonder what the efficiency of the expander would be in converting this to electrical energy?

kierang - Wed 24 Sep 2014 - 09:44

Interesting post Kieran, your spot on with your steam tables. I looked at a turbine driving a pump years ago and and tried to do the energy balance and it didnt add up with the quote the turbine company provided. What they added into their calculations was that the steam came out at a different i.e lower dryness. I guess that it may be similar with the expander. I did some rough calcs based on above and think you could potentially have about 30-40 kw of energy available with a slight lowering of dryness fraction, if it is the same as a turbine. Then you would have the losses converting this to electrical energy. On an overall payback I guess you would need to take into account the effect of this reduced dryness fraction out. Maybe a flow rate larger than 1000 kg/hr would be needed too.

 

Hot cross bun - Wed 24 Sep 2014 - 12:54


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