The hunt for the vacuum
Regular readers will recognise that we have been searching for the fabled 20+ inches of vacuum that our fellow stem boaters claim to achieve…. We, meanwhile, have only ever seen 5 to 10 inches at best 😦
Well, over the last few weeks, I think we have been edging towards an answer, and I think there is some valuable information for other steam plant operators, that can be gleaned from our experiences…
One thing we had noted was that if you reduced the speed of the Worthington Simpson duplex pump, (acting as cooling water circulating pump for the condenser) then the vacuum started to disappear. Secondly, it became clear that the water being pumped over the side (after passing through the condenser) was often warm and sometimes hot.
Also there seemed to be something of a “sweet spot”, where more cooling water resulted in no more (and possibly less) vacuum.
While out with John Maltby (ex-chief engineer and friend, from the SBA) he suggested that:
- A) maybe the condenser was too small (surface area)
- B) maybe we would be better with an electric circulating pump
- C) it also occurred to me that we had never calibrated the vacuum gauge, so we might be chasing shadows!
Some Testing and Computation
If the condenser was too small, then having to additionally condense the steam exhausted from the circulating pump would aggravate matters. So, a first test was to divert the exhaust from the pump over the side (puffing mode), and see if with less steam to condense the vacuum increased…
It didn’t. BUT, when you closed the throttle and stopped the engine, the vacuum remained! This suggested that the search for leaks in the exhaust/condenser circuit could be considered finished! …AND it suggested that the condenser was not at it’s limit (from a surface area perspective), as reducing the steam to be condensed had not increased the vacuum.
So, the next step was to replace the Worthington Simpson duplex pump with an electric pump…and Mr Maltby provided some very valuable advice from an old text book. This entirely reworked my thinking, as I had previously imagined a very small amount of cooling water would do the job – wrong!!! His words:
“Just been doing a bit of research, cooling surface area of condenser should be about 0.8 sqft per IHP. This is of course approximate. Weight of cooling water required to condense weight of steam (assuming heat exchanger is fairly efficient) at 10 decC and 26 in vac is 15.4… So, your boiler should evap about 240 lb/hr (at 8 lb/hr/ft sq h.s.) So, water required is is 3696 Lbs/hr or 61.6 Lb/minute, or 28 Kg (litres) per minute.”
This is a lot of water! (1.5 jerry cans a minute!)
The secondary effect of the steam circulating pump is that it uses steam we would prefer to be using for propulsion, and, as people have subsequently pointed out, this type of pump is very inefficient, as it has no inlet steam cut-off….
So, the pump is bad because:
- It’s heavy about 50kg
- It uses a lot of steam, placing a heavier load on the boiler
- It makes the condenser work harder condensing it’s own exhaust…
… Its only merits are that it’s entertaining to watch and 100% reliable!
So, we purchased a nice Chinese 12v impeller pump rated at 30litres per minute, and fitted this temporarily with it’s own ex-landrover battery (which had been retired weak) running in dead-loss mode!. When the pump was plumbed in , we finally saw an improvement; a reasonably-maintainable 13″of Hg vacuum at moderate cruising speed. RESULT!!
Also, it was obvious, that the burner and boiler were doing better at keeping up with demand!
However, when we opened up the engine, the exhausting cooling water became too hot to keep your hand in the flow, and the vacuum gradually sank to approximately zero…
So, we are going to need a bigger pump!
But the GPS said we were making over 6 knots!!, we’re getting closer to hull speed.
A Bit of Serendipity
As a final note to this story: Due to pilot error, we managed to almost drain Befur’s batteries while being a “tourist attraction” at the Windermere Jetty.
This prevented the burner from starting when we came to leave (potential embarrassment) … but, with a bit of borrowed wire and crock clips (thanks to John Winn), we popped the circulating pump’s battery in parallel and saved the day!
…and then while sailing back to the berth, we realised we had over 15″ Hg of vacuum – a result we had never seen, and we realised that as the circulating pump was now being charged by the engine, the voltage was higher an so the circulating pump was pumping more water and providing more vacuum!!!
- We need a larger electric pump (plan to order a Jabsco centrifugal pump, rated at 120L/m for about £250!!!)
- We have acquired a much bigger Alfa-Laval plate condenser that we will jemmy into the boat over the winter….
- We probably should arrange a back-up battery (service & spare) for similar emergencies
- We really should calibrate the gauge!
- We need to find buyers for:
1: an almost brand new 30l/m electric pump
2: a restored Worthington Simpson duplex steam pump
3: all the bags of sand we will be able to take out, that were ballasting the above mentioned pump!
Finally, we have just returned from the SBA’s 50th Anniversary Rally on Windermere, a fabulous week, and we will write this up in the next few whiles…..