The decision from which follows all the other design choices is the size and capacity of the Vacuum Ejector (Jet Pump).
I've decided to use a bronze Penberthy GL-1 Jet Pump operating at 60 psi and which has a vacuum pipe of 1" nominal inside diameter. (see pages 8, 9 of the link)
This is why:
Extracted from the Penberthy Jet Pump Technical Data manual, this is the sequence of activities to determine the best item for the job.
For steps 2 & 3, have a look at the table below (also extracted from the Penberthy Jet Pump Technical Data manual (click it to make it readable)):
|Jet Pump Selection Chart|
|Conversion of table figures for non-1.5" Jet Pumps|
Two columns to the right of my highlight, is a figure of 3.7 for a GL-1.25 jet pump. Although the figure is fairly close to the 3 minutes/100 cu ft required, the GL-1.25 uses 2.9% of the boiler capacity (See Vacuum Braking (2) Design (1)). Whilst 2.9% is OK (135 lbs/hour), a GL-1 uses only 76 lbs/hour or 1.6% of boiler capacity. With these figures, it's worth having a look again at the original requirements.
(The lbs/hour figures come from the right hand column of the first table multiplied by the capacity factor for the size of jet pump in the second table. The first table is normalised for a 1.5" jet pump but converted for other sizes by the capacity factor to avoid having to produce a separate table for each jet pump size, e.g. a GL-1.5 at 60 psi working to give 10" Hg abs. vacuum uses 221 lbs/hour. A smaller GL-1 uses 0.344 x 221 = 76 lbs/hour - only 1.6% of the boiler's 4600 lbs/hour capacity).
Requirement 1 was for four Mk1 carriages, i.e. 16 cu ft of evacuation space in 30 seconds. In fact, 16 cu ft only needs to be evacuated fully when the system has been full of air. Once evacuated and the brakes applied, because the carriage vacuum cylinders are still partly evacuated, the full 16 cu ft volume does not need to be evacuated. Hence the requirement can be relaxed to some extent.
Requirement 1 assumed four Mk1 carriages. At Midsomer Norton, it is very unlikely that as many as four would be involved and two carriages or occasionally three would be nearer the mark.
Thus if two carriages are involved, a smaller GL-1 should be able to evacuate 8 cu ft in 30 seconds and reduce the steam consumption accordingly.
Arguably this is cheating; however, without an original requirement figure, it is very difficult to know what to aim for. Having gone round the calculation loop once, the implications of the requirement have now become apparent and show that there is a good pragmatic case for using a smaller Ejector - particularly if it is being run continuously.
Compared with the Penberthy No. 3 size water lifting ejector having a 1" suction pipe size, the GL-1 has the same suction pipe size but is specially designed for sucking air and not water. So, intuitively, the GL-1 is a similar size to the No. 3 type used on other Sentinels but should perform more efficiently.
Recalculating the figures for two Mk1 carriages with a GL-1 Jet Pump gives a time of around 0.5 minutes to evacuate two carriages whilst using 7.6 gallons of water per hour.
Well! That's the theory anyway.
These Penberthy Jet Pumps are imported by Jenex Ltd of Great Yarmouth, UK. I'm very grateful to Mark Collins of Jenex for helping me to order the right item.