by RC Al » Fri May 15, 2020 12:08 am
I cant say if making all the flow areas equal makes a cap work better, but on face value it seems a good approach. If you could work out the flow resistance/ pressure drop that would occur with the slot size and number vs bath depth vs the kw input, you could then find the perfect matching pipes, but at our level it comes back to the generalisations on slot size/spacing already mentioned to get in the ball park and feeding that seemingly restricted flow area with evenly spaced pipes or making it all the same with big slots. Havent seen anyone saying their big slot caps didn't work, but quantifying efficiency is next to impossible for us
Why dose this matter? Every restriction causes some back pressure to the boiler, the higher the boiler pressure, the higher the boiling point of the the contents, the bp's of ethanol and water are very different, but the effects of pressure are not proportional to each other and it works against us, making the bp's closer and thereby harder to separate. I have the idea that a lower boiler temp would help seperate fractions better too, but i havent seen anything to support that idea
Making the riser and cap envelope similar sizes seems intuitive, but in my opinion making the envelope a bit larger counteracts laminar flow effects from the donut shaped passage, you could even go as far as dropping the riser down to dn32 @ 700mm and not be too stressed as the main holdup is the slots (unless you go taller than 14mm, see next)
With the min recommend distance of 4-5mm between the slots, with a 2mm slot, into a 159 circumference cap = 26 slots @5mm high, 260mm plus 52 per mm of additional slot height.
The wider the slots, the larger the bubbles, reducing the amount of vapor/bath interaction, reducing the efficiency of the plate, making this style of large single cap more suited to making a flavoured product. Going taller slots is mitigated somewhat by more bath depth, but will still contribute to bubble size and reduce efficiency.
The cup on the downcommer needs to be at least the bath depth to create the hydraulic lock
Remember you can make any sized cap or pipe either with sheet or by taking a slot out of a larger pipe, A 56mm cap would suit the dn40 better and gain 3-4 slots
There is no need to make the riser/cap that tall unless you want to run a really deep bath. The riser can even be a bit shorter than the downcommer, the pressure/flow is sufficient to stop any reversal, the shorter the pathway, the less resistance, less pressure
The saw tooth pattern is a method to pass solids, frequently used with a clear space below the cap, haven't seen anyone worry about that on a home still in a while, more an industrial thing