Student Project - Organic Rankine Cycle

[uc-orc]

Hi All.
I am a mechanical engineering Student from Christchurch, New Zealand.
My team of four has been given the project of designing and building a small scale organic rankine cycle converter - a project we have been finding difficult as inexperienced students.
This requires using a heat source to evaporate a refrigerant (r134a), which spins an expander producing electricity. The refrigerant is then condensed, pumped up to pressure, and the cycle begins.

The refrigeration engineers we have spoken to so far have been incredibly helpful, so I am hoping that I can tap into an even larger source of knowledge here :)

Technical Summary:
Refrigerant – r134a. This will be reaching a temperature of approximately 80° C at a pressure of 21 bar. The flow rate is 0.13 kg/s (around 0.1 L/s)

Heat Source - We are using 250+°C exhaust gases. We have had a fin and tube heat evaporator built for this capable of extracting 25 kW.

Turbine - This extracts the work from the expanded refrigerant. We are using a reversed scroll compressor (from the AC of a car).

Condenser – Basic coil and drum with water cooling. This will condense the liquid at a pressure of 7 bar and temperature of 30° C (5° subcooled). The condensate drips into a receiver.

Pump – Hydraulic pump with electric motor. (A compressor is not involved at this system). It was difficult finding a pump that can produce a pressure of 21 bar with such a low flow rate. Any other suggestions for pump types would be very much appreciated 

We will also have a variety of pressure and temperature gauges as we need to get some idea of whats happening within the system. There will also be a suction accumulator and pressure relief valve after the evaporator, a check valve before the pump and sight glasses placed around the system to check the state of the refrigerant.

Any technical help or insight you can provide will be thoroughly appreciated.
Thanks for taking the time to read :)

[mad fridgie]

Ask Prof Williamson, if they have cryogenic pump you can use.
Other wise you could look at a small gear pump, but you need to be aware of possible cavitation issues.
I would look at sub cooling after the receiver (the leg to the pump)
Because you have 250C heat available I would look at lifting your evap SST.

[uc-orc]

Thanks for the advice mad fridgie :)
We are worried about cavitation in the pump, so plan to subcool by 5° in the condenser and have the receiver a meter above the pump to add head. Would we be able to successfully sub cool before the receiver or does it have to be after?
We are using lubricant for the expander (I think its called PAG 40 or something). We are worried that it will accumulate in the evaporator as the liquid refrigerant evaporates, does anyone know what to expect here?
Thanks again.

[mad fridgie]

Thanks for the advice mad fridgie :)
We are worried about cavitation in the pump, so plan to subcool by 5° in the condenser and have the receiver a meter above the pump to add head. Would we be able to successfully sub cool before the receiver or does it have to be after?
We are using lubricant for the expander (I think its called PAG 40 or something). We are worried that it will accumulate in the evaporator as the liquid refrigerant evaporates, does anyone know what to expect here?
Thanks again.

There is quite some argument on sub cooling and receivers. (the only ways to ensure high levels of sub cooling, are after the receiver or get rid of the receiver and partially flood the condenser.

Oil return should not be an issue, at these temps. How ever in your calcs you should allow a little oil filming on you heat exchange transfer surfaces. (you have number of these experts at Cant Uni)

Look at your drawing you have a number items within the liquid line, which cause pressure drops (practically means sub cooling reduction)