Re: Semidiesel hot bulb sprayer

Hi Tim, Those clips from the manual (why is it that's the last place I look?) are both interesting. I agree that it reads as if *a bit* of fuel should wet the sides of the bulb even in the hot running position - in which case we'd be talking about a minimum spray cone angle of around 22 degrees. Regarding spindle/nozzle matching - yes, that agrees with those comments from boatmen that I previously posted indicating that nozzles were replaced from time to time with no evidence that the spindle came as some sort of matched pair.

I guess a sort of performance/design envelope is beginning to emerge:

1. A hard to guess at hole size, but perhaps 500 um would be a fair stab
2. Internal angles matching the spindle, and with a close fit to the spindle
3. As has been discussed, an external 'contersink' around the jet, I would suggest (from that hot bulb sectional drawing I did) a cone angle of 110 Degrees - that should just wet the end of the copper bolt
4. A hot running cone angle as above

On a related point I didn't realise how bad the non-return valves on the fuel pump were until I put a pressure gauge on the outlet. Watching the needle going up and down it time with the hand pump was somewhat depressing. I've stripped them and am a bit dubious as to whether I could ever get a really good seal on the steel ball bearing/brass seat. For the moment I've put a diesel compatible o-ring in between ball and seat, and the pump is now holding pressure really well. Obviously I'm worried about lifetime (bits of o-ring clogging up the sprayer and so on...). Any thoughts or comments?

Cheers

Tim N

Wales (remove the spamtrap to rely direct)

Are you remembering, re the min.cone angle, that the book recommends opening the sprayer no more than 1/4 turn, which at 26 tpi is about 10 thou?

I knew a man who had a few new ones, probably dead by now though........

If the steel ball is in good order, if you can get the seat refaced to a fairly sharp corner & then tap the ball onto the seat with a small hammer it should improve things.

Cheers Tim

Tim Leech Dutton Dry-Dock

Traditional & Modern canal craft repairs

(Snipped)

Tim, On the minimum spray cone angle and how much the spindle has to rotate to achieve it (ie can it be achieved with the available 90 Deg rotation) , it's a real case of suck it and see, leaving a trail of failed sprayer tips behind if one's unlucky!! As far as I can understand it, for a optimum spin dle/nozzle combination (ie all the angles mate, things are snug, and so on) then the cone angle of the wide cone is set by the external counter sink, and the narrow (ie hot) cone angle is set much less reliably, by:

i) Hole length (the shorter the hole, the faster the rate of rotation of the emerging liquid jet, and the wider the cone angle under the influence of centrifugal forces.)

ii) Hole diameter (the narrower the hole diameter the faster the velocity of the emerging jet and the less important the contibution of the radial forces described above.

iii) What I'm not sure about is how this trades off against degree of atomisation within the cone. I would imagine that a 20 Deg cone with finer droplets in it (inasmuch as this can be achieved) is preferble to a 20 Deg cone with far fewer coarse droplets. My instincts say that a narrow high velocity jet will tend to break up better than a wider lower velocity jet.

Combining the three I seem to be heading towards a narrowish hole (< 500 um?, maybe even as low as that radical 300 um I showed on that drawing) which is kept as short as possible (100 - 200um?) to try and get the cone angle up to 20 Deg. The only real way is try outs: make a nozzle and then slowly shorten the hole (which is tricky with a countersink feature - the old oilstone trick is no good!) whilst testing it with the spindle backed off to 90 Deg for cone divergence.

If you want, when you are getting to that stage, I could bring my test rig round (it's portable, all it needs is some mains and an environment where we can spray some diesel without choking!) and you could use it to try and 'home in' on the magic optimum geometry.

Regarding non return valves, thanks for that. I'll have a try and if I am unsuccessful I still have the o-ring option.

Cheers

Tim N