RefluxRatio = coolantTemperatureDifferenceAcrossRefluxCondenser/coolantDifferenceAcrossBothCondensers
Here is a copy of the calculations from the first post:
I've instrumented my still with four thermometers:three on the coolant lines, and one on the boiler. Everything has been insulated excessively. I've input all data into an Excell spreadsheet, which I don't seem to be able to include in this post. If you would like a copy, please PM me.Maritimer wrote: Suppose we run the boiler at 1000 watts = 238.8 cal/sec.
This will be converted to vapour which will be condensed, heating the coolant with this 1000 watts.
If the coolant is flowing at 50 ml/sec = 50 gm/sec for water, it will be heated at the rate of (238.8 cal/sec)/(50 gm/sec) = 4.776 cal/gm.
1 calorie is the heat required to raise 1 gram of water 1*C, so the coolant will rise 4.776*C.
Does the volume of the condenser matter? Suppose we have two condensers, one of 100 ml volume, and the other at 400 ml volume. At 50 ml/sec coolant flow, the 100 ml condenser is flushed in 2 seconds, and the 400 ml is flushed in 8 seconds.
The 100 ml condenser is heating up at 238.8 cal/sec, but it takes 2 seconds to flush the condenser, so the temperature rise is (238.8 cal/sec)/(100 gm/2 sec) = 4.776 cal/gm = 4.776*C for water.
The 400 ml condenser is the same: (238.8 cal/sec)/(400 gm/8 sec) = 4.776 cal/gm = 4.776*C.
So, the temperature rise is proportional to the vapour power, and inversely proportional to the coolant flow rate:
tempDiff = vapourPower/coolantFlowRate [(cal/sec)/(gm/sec) = cal/gm = *C]
And this is proportional to the mass of condensate because the mass vapourized is the boilerPower/LHV: (238.8 cal/sec) / (220.8 cal/gm) = 1.082 gm/sec for pure ethanol, for example.
So,
tempDiff = Constant X rateOfCondensateProduction [the Constant will depend on the vapour composition] With the same flow rate in the two condensers, the ratio of the temperture rises is then proportional to the rate of masses produced by the condensates:
tempDiffOfRefluxCondenser / tempDiffOfProductCondenser = massOfReflux/massOfProduct [the Constants cancel, and the rate i.e., 1/sec cancel]
And ultimately,
RR = (mass of reflux condensate)/(total mass of vapour from boiler)
RR = tempDiffOfRefluxCondenser / (tempDiffOfRefluxCondenser + tempDiffOfProductCondenser )
M
During the experiment, I varied boiler power and coolant flow rate. The calulated values of RR were all over the place, but were centered around 0.822. When I realized that what I was seeing was noise, I took readings for 22 minutes, every minute. The average was 0.835 with maximum of 0.900 and minimum of 0.765.
Now the question arises: What is the source of the noise?
There are two possibilities:
1) The boiler power controller uses random cycle powering to generate powers less than maximum.
To check if the power pulsing is the cause, the four 1000 W elements of the boiler can be arranged in a series-parallel circuit to provide 1000 W, or a single element can be used. If the noise disappears, the noise source will have been identified.
2) The condensation process is inherantly noisey. I suspect this more than the power supply. As the vapour condenses, it will create a vacuum, pulling air and more vapour into the void. The condensation causes a sudden heating of the condenser which is transferred to the coolant. The noise is a reflection of the turbulance in the condenser.
In the next experiment, temperature sensors will be glued onto copper tubing which will be inserted into the coolant lines. This way, the real-time temperature can be seen on an oscilloscope. The solution might be to build analogue filters (or to use digital averaging) to find the constant component of the temperture.
M