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npaisnel
03-02-2002, 11:51 AM
I am in the process of building a still to produce ethanol for fuel purposes.
The vapour cooling part of the still consists of approximately 15 ft of 5/16 inch pipe wound in a loose coil about 50mm external diameter in side a 67mm copper pipe. Water is passed through the coil, and the ethanol vapour rises up around and through the coil, condenses out and drips back down to a collecting funnel and outlet pipe.

What I would like to do is replace the water flow with the cooling side of a domestic refrigerator. I will have to first de gas the unit I wish to use, silver solder up the new pipe work and get the unit re-gassed.
Question is, how to regulate the amount of cooling at the coil so it does not get too cold and ice up to a solid block. This would also be bad, because the very cold ethanol returning to the reflux colomn would upset the temperature balance of the reflux packing.
Could the speed of the pump motor be varied, using phase angle controller/light dimmer type circuit? The cooling needs to be stable, IE pulses of heating/cooling is not desirable, a constant controlled amount of flow of refigerant through the coil is required.
Fianlly can you see any othe r problems? or any other advice

The design of the still unit I am using can be found at
http://hades.itl.net/~npaisnel/alcohol/design.pdf
and the cooling coil
http://hades.itl.net/~npaisnel/alcohol/cooling coil.pdf
although I am using a greater daimeter colomn and copper not glass

Cheers

Neil

frank
03-02-2002, 04:13 PM
Neil

The only way to have control over what you are trying to do is to replace the cooling water with temperature controlled water. If you try to use refrigerant liquid as the cooling medium then you may have numerous problems with controlling temperatures especially if your mass flow of ethanol varies or indeed it's temperature varies.

What are your mass flow rates and the temperatures you are trying to achieve?

npaisnel
03-02-2002, 05:09 PM
Heat input to the boiler 3kW, vapour temp 78.5deg C, to cool down to 60-65degC

Have not done any calculations as regard actual mass flow rates.

I was hoping to get away from using water, and have the system, electric only. If I have to re circulate and cool water, it is far easier just to use continous water flow as I am using at the
moment in a more simple still design

Thanks for reply

Neil

npaisnel
03-02-2002, 06:12 PM
I have not built the new fractionating tower, coil, and boiler yet.

The standard design as on the pdf files on my first post, uses a flow of cooling water, from memory about 10 litre/min, temp rise at the moment unknown. I was hoping to get rid of water cooling, and just use cooling from a second hand, re filled domestic refrigrator, with re wound coil.
Heat to be removed from the system will be variable. The boiler will have 2 x 3kW heat elements, two running initially to bring the boiler (200litre) up to temp and then a single 3kW element running through phase angle controller to keep vapour temp below the condensor coil at 78deg C. Once this temp is reached it will stabilise until virtually all the ethanol is removed from the wash. Until the thing is up and running volume of vapour is unknown.

It will be a system that will be run under supervision at all times, so a totally manual control of the cooling is OK. I had hoped varying compressor speed would do this, but did not know.

This is for a small home/hobby project and is not a commercial build .

Andy
03-02-2002, 09:29 PM
Hi, Neil looks interesting. With regards to the cooling do you need refrigeration to provide the cooling, would mains water run through the heat exchanger and then to waste not do? As I recall 13l/min would cool the heads of a 150Hp compressor, so dependant on final product temp would mains water not do and it would be a lot simpilar to control.
As Marc said a few calculations would be in order.
Regards. Andy.

frank
03-02-2002, 10:20 PM
Based on your figures of 15ft of 5/16 copper tube we can calculate that the surface area of the tube is

A = Pi x D x L

Where
15ft =4.57m
5/16bore = 0.01m o.d.

so A = 3.141592654 x 0.01 x 4.57 = 0.143m2

The inlet temp of the vapour into the shell is stated @ 78.5deg C and the outlet temp is required @ 60deg C.
Assuming the inlet temperature of the cooling water entering the tube is 6deg C and leaving @ 35deg C we can calculate that the LMTD would equate to 44.61deg C.

The U value of copper is 390w/m2.K

So: the heat transfer would be Q = U A dT

Q= 390 x 0.143 x 44.61 = 248W

Trying to convert a domestic fridge rated at or about 500W to do this load directly would take some engineering! but it would be possible to create a secondary water tank cooled by the fridge circuit with a little pipework evaporator submerged in the tank and then a small domestic circulator feeding the tube/shell exchanger. A hand control valve would be required in the secondary circuit to set up the cooling water mass flow rate and an immersion stat in the tank to control the fridge circuit.

Gary
04-02-2002, 04:19 AM
Why not use an EPR valve?

npaisnel
05-02-2002, 12:57 PM
My knowledge of theory and calculations regarding refrigeration goes back to my agricultural college days 12 years ago, and physics lessons some 20 years ago, so understand the calclations you are putting forward, but remembering enough of the theory to have bee able to do them my self is another thing entirely. So thanks for the figures.

As for an EPR valve, I will need help on that . What is it?

If I use water as the only coolant, as is the standard method for all stills, I am using water from a borehole / pressure tanks system, with a fairly stable temp of around the stated temp in your calcs of (6deg C).

I am assuming from what you are all saying that just reducing the speed of the pump/compressor unit to slow the speed of the coolant flow will not work.

As I mentioned earlier, I can carry on using the traditional water flow cooling , but was hoping for something that got away from needing water, so the whole system would be self contained, electric powered, withouth the need for secondary water cooling tanks etc.

Thanks again to all of you for your time

Neil

Frosty
05-02-2002, 07:46 PM
Golly gosh Frank! So, it looks like you might have learnt something after all then on our HNC course! Only jokin Frank.........LOL

Frosty

frank
07-02-2002, 08:31 PM
Frosty mate

Does this mean that you have checked my calcs for the parallel flow heat exchanger???

Are they correct ? - if they are then I'll have to thank the guy that did them for me - Ha Ha !!:D

Any chance of a job ?

Frosty
07-02-2002, 09:03 PM
Yes Frank, we have plenty of jobs for you mate? The **** house toilet cistern (trap 1 to be precise) is leaking...you can start on that......make sure you clean the bowl as well......with a toothbrush!!!!!!

No Frank, enough of this joviality! For you mate...go to the top....CEO - you can start Monday...after you have given yourself your notice, Hee Hee

Gary
08-02-2002, 06:26 AM
As for an EPR valve, I will need help on that . What is it?

An Evaporator Pressure Regulating valve. It goes in the suction line and holds the evaporator coil at a steady pressure, and therefore temperature.

The downside is that the compressor will probably run in a vacuum, and may overheat. This being a small compressor, a fan blowing on it may be enough to keep it from cooking it's oil.

npaisnel
08-02-2002, 09:44 PM
I think for what i need it is all getting far too copmlicated, It will just have to be back to the water flow from the mains.

Thanks for the info anyway chaps

Neil

Gary
09-02-2002, 12:22 AM
Be sure to run the water counterflow or crossflow, rather than parallel flow. This makes a big difference in heat transfer efficiency.

And good luck.