ORC project with compressor working as a expander

[agiraias]

Hi everybody!
I´m project engineer. We are working in a new ORC (Organic RAnkine Cycle) project and our porpouse is to recover thermal power from a low temperature source. The inlet conditios in the expander (is not defined yet) are: T=82ºC and P=8,3 bar. The oulet conditions are different depending if we are in winter or summer resort (T=33ºC and T=14.5ºC). Working fluid is a refrigerant; R245fa

We are looking for a machine to expand the fluid and generate electricity. After reading some literature we know that compressors in a reverse mode are used in those applications, that is; screw compressor as a screw expander, scroll compressor as a scroll expander, air motors,etc. We are trying to choose the most suitable machine for our application. Could someone tell me what is the best one for this application? Could someone tell me what kind of compressors can be used as an expander?

And the last question, does anybody now manufacturers of Screw EXPANDER and Scroll EXPANDERs?
Thank you in advance

Best regards

 

[dcasto]

Screw won't work because it either requires a sealing oil or has a high bypass rate if a dry screw is used (ie 50% or the fluid would just act like it went through a valve). I don'tknow about a scroll, but I'd think it would have similar problems.

You also need to set the outlet pressure too to help select what the equipment is going to look like and how much energy you can get back. Also, is it two phase flow? or sat vapor or liquid?

 

[agiraias]

Thank for your prompt replay dcasto.
I have some questions, what "high bypass rate" means? I don´t understood what do you want to say.
The outlet pressures are 1 bar or 2 bar depending if we are in winter or in summer conditions. We are talking about a global efficiency of 11 percent in winter conditions (best ones), assuming 70% of issoentropic efficiency in the expander. Do you know a solution to this "expander"? For us is the most difficult part of the project.
Regars

 

[Rotator]

I have come across turbo-expanders used in natural gas plant applications - try contacting

http://www.mafi-trench.com.

They should be able to help you or suggest other mfrs if they aren't into this application.

 

[dcasto]

screw and scroll compressors do not have a seal between the moving metal surfaces like a piston compressor has with its sealing rings. There is always a gap between the rotors in the compressors that allow gas to escape inside the compressor case from the high pressure end to the low pressure end. To compensate, screw compessors pump an oil into the gas. The oil sticks to the metal parts and kind of act like a seal, keeping the gas from bypassing.

Ge Rotoflow (

http://www.gepower.com/businesses/ge_oilandgas/en/our_brands/rotoflow.htm)

and Texas Turbines (

http://www.txturbine.com)

can also do the job.

I estimate the expander will give you 5Kj/mole of energy at 1 bar and 3.5 Kj/mol at 2 bar. The outlet temperature will 30 C at 1 bar and 45 C at 2 bar.

 

[agiraias]

Thanks for your replay dcasto. So the principal function of the oil is acting like a seal, but it works also as a refrigerant I suppose. I read some information about screw compressors and "dry ones" exist, that is; without oil injection, do you know this technology?
I don´t understand the concept of "keeping the gas of bypassing", could you explain me that?
And the last question, what is the range of efficiency in screw compressors?
Thanks for all
regards

 

[zdas04]

Dry screws pre-dated flooded screws by several decades. See faq1036-911 for a discussion of the differences.

In an oil-flooded screw, the oil has three functions: (1) seal between the rotor and the case to reduce bypassed flow; (2) lubricate the metal-to-metal parts; and (3) remove the heat of compression.

David

 

[dcasto]

The expanders mentioned above would be about 80 to 85% efficient. A screw compressor is about 50% efficient as a compressor and I can't imagine how bad as a expander. The company I worked for did an experiment a long time ago. They took an old steam turbine and used it as a expander in natural gas service. They measured a 40% effiency. Not bad for an experiment.

After you read about screws and see a video, you'll see what I mean about bypassing. Kobelco makes big dry types. We use mostly Sullaire and Frick oil types in my business.

 

[agiraias]

First of all, thank you very much for all.Dcasto , there are no movies in the links or I didn´t find them, so I still don´t know the concept of bypassing. The rest of information helped me a lot, so, thanks again.
What about air motors? how do they operate? and what about their efficiency? could be another possibility, I think. I contacted with a manufacturer but he didn´t give any technical data about efficiency, operating mode, coupling to the alternator, etc. So before going on with this proposal I would like to know your opinion.

Thanks again

 

[dcasto]

IR makes air motors, there are a couple of others. Most have a MAOP of 7 bars. Most models can run on hydrocarbons.
go here to see curves. you can generate random ac and convert to dc then back to exact frequency ac.

http://www.irtools.com/products/airmotors/index.asp

 

[crjones]

You might try Dr Stosic at City University in London. They were doing work on expanders. He may be able to direct you to a manufacturer that can help you.

 

[dcasto]

cme across this topic on the engine & fuel engineering. Lysholm screw-type expander. I googled it and there are patents but I found no one selling such a device.

 

[agiraias]

Hi everybody again!! thanks for all your answers, they have been really useful for me. We are still deciding the expander, this state of the project is lengthening too much. My question is; Does anybody know the operating mode of the vane compressors? Could you tell me a useful web side for that information?
Thanks again

 

[LimaBerge]

Agiraias how much water do you have at that temperature? Where is it located? Are you wanting to sell the power generated ack to the grid or just use it to suppliment your own?