Details of the technology

Back to main narrative Plumbing matters The piping The containers (Büchsen)
The employees Operations Manual Passing-through ("Durchfahrt") mail Operational troubles

Plumbing matters

In its simplest form a Pneumatic Post system consists of a single tube linking two points or "stations". Inside this tube, closely fitting cylindrical containers are either blown along by compressed air or drawn along by a vacuum, and by placing messages inside the containers, a self-contained communications system is produced. In a more sophisticated form, two tubes would link the individual Pneumatic Post Stations, thus enabling two-way traffic to be maintained: one tube would be under pressure and in the other a vacuum would be maintained. Vienna had only a single pipe between stations, and depended on strict adherence to a complicated timetable. Hajek writing in 1933 outlines a proposal "to bring the system up to modern standards as exemplified at Berlin" which would involve doubling the pipes; this was rejected as too expensive.

In the book "K.k. Telegraphen Central Station, Wien 1874", there is a "fusebox diagram"; a small piece is shown here and the complete diagram is in the maps section. It provides useful insight into possible operation modes of the original system.

The first type of apparatus used was the "Felbinger-Crespin Bronze Cannon", impressive in appearance but inconvenient to install and to operate. Later Scharfenberg and Felbinger invented better (and more compact) devices; but the "1892 Apparatus" described below was far superior to all the others. There was yet another complication – there were four operational situations, each requiring different valves and different procedures: end-of- pipe or intermediate; with or without local supplies of air & vacuum. The operation of the apparatus was indeed complex.


The next picture shows a typical "Felbinger" intermediate Pneumatic Station. Incoming cylinders arrived in the large chamber, and were retrieved through the rectangular hatch; outgoing ones were inserted through the fitting at the top of the curved pipe. Note the complex arrangement of 2- & 3-way valves, which the operator had to manipulate in the correct sequence to send or receive the cylinders. With an intermediate station, many of the incoming cylinders required forwarding to the next station (a label on each cylinder said which station it was for), and with this design of apparatus this involved taking the cylinder round to the other side. "Through traffic" was not possible.


For more pictures of this device see Vienna Technical Museum.

A simpler and smaller apparatus was later devised, called the "1892 model": engineering drawings and a picture of an end-station version are next, and the method of interconnection of three of them is in a diagram below that. Although more modern types of apparatus were invented, the Pneumatic Post was never able to afford them. Indeed, Hajek observes that the Börse was still using a Felbinger-Crespin Cannon in 1933!

The "1892 model" (end-of-line version).

Method of interconnection of three of the "1892 model".

The piping

To supply and maintain the required pneumatic pressure, power stations and 20m3 air reservoirs were needed. The network of 1875 had a length of 14km: 11.8km was used to send the mail; the rest was service piping supplying 40%-60% vacuum and 1-1½ats compressed air from the 20HP steam-powered machine unit in the Central Telegraph Office to two receivers in the Laurenz building in the Fleischmarkt. In the list of Pneumatic Post Offices, a column indicates the "machine house" for each station. By 1932, a total of five machine units (by now electrically-powered), plus five air-receiver stations, was required to supply the 44 Pneumatic Post Offices then in operation.

The pipes measured 74±2mm OD x 65±1mm ID and were made of 5m lengths of wrought iron (later steel). The ends were threaded then screwed into 20mm thick 150mm dia 4-bolt flanges, which had spigots to ensure correct alignment; the non-contact face of the flange was then brazed to the pipe. They were proof-tested at 25ats, given two coats of paint, tarred, wrapped in jute, and covered in powdered chalk; then laid at approx. 1m depth under the streets of Vienna. The minimum bend radius was 8m.

In this file you can read a detailed technical description, in German from an unknown but evidently long work, of the manufacture and assembly of the pipework. See also the last page of Ehlich's book.

The containers (Büchsen)

The mail was carried through the pipes in cylindrical containers (called Büchse, plural Büchsen). Pneumatic post stationery had to be small and thin, and folded to fit in the cans. The folding was on the vertical axis, twice, so that when the item is opened out it has three vertical fold marks; often the second fold's marks do not show. Pneumatic envelopes were sometimes rolled instead. These creases are helpful in identifying non-Pneumatic Post stationery which has actually passed through the Pneumatic Post, such as express letters sent from Vienna to other places and incoming express mail put into the Pneumatic Post for speedy delivery within the city.

Registered mail (allowed from 1.8.1925) was carried in special (reinforced & secured) Büchse dedicated to working between two specific offices, whose numbers were on a rotatable metal disk on the top. These could be on separate lines, requiring a transfer en route (eg from Favoriten to Westbahnhof). If the registered item had to be transferred between Büchsen, however, it was recorded in a list at the transfer station.

Normal Büchsen were marked in some way (piece of paper?) to indicate their destination. Somehow they also carried their source station number so that they could be returned, which was done even if they were empty. [The descriptions in the Dienstanweisung remind one of the children's game where you pass something round and try not to be left with it when the music stops!]

Details, diagrams and photos of Büchsen are here.

The employees Operations Manual

The employees had every aspect of their duties prescribed in an Operations Manual entitled "Dienstanweisung für den Rohrpostverkehrsdienst". The 10th edition (1932) has two parts: (1) "Behandlung der mit der Rohrpost zu befördernden Sendungen" [the regulations for dealing with the mail - suited to the counter clerks] (2) "Vorschriften für den Rohrpostzugsverkehr und den Rohrpostapparatdienst" [the mechanics of operating the apparatus, with diagrams & maps - suited to the equipment operators]. Both parts are full of specimens of the registers etc. that had to be meticulously kept.

When it was required to send mail only to an adjacent Pneumatic Post Station the leather-capped container would be inserted into the tube by itself. When there were containers for transmission to more than one station, these would be linked up according to destination and a driver placed at the trailing end, thus making up a "pneumatic train" consisting generally of up to 10 steel cans, with leather caps. Drivers (Treiber) were generally shorter than the containers and were made of wood or solid metal; they were thus heavier and had an additional leather washer in the form of a tail-piece (Manschette), thus providing extra contact with the tube. They travelled through the pipes at about 50km/h. The despatch and arrival of each train was signalled between the stations by bells. Extra trains, eg to return empty containers to their source, were carefully fitted in to spare slots in the timetable: the mail schedule could not be delayed!

A dedicated telegraphing system was provided through wires laid beside the pipes. This report of the Vienna Electrical Exhibition in 1883 describes the telegraph system in detail, and the sixth paragraph (shown in a bigger font) explains the special arrangements for the pneumatic system.

A photograph of an early installation at the Central Telegraph Office is on the wall of the Technical Museum in Vienna. For a discussion, see the Telegrams section.

Passing-through ("Durchfahrt") mail

"Durchfahrt" mail is mail from A to C which had to pass through B because that's the way the pipe ran. The plumbing is such that "through traffic", where the station is physically bypassed, is not possible. You need to see the drawings and photos for this, and apply engineering principles not philatelic. That means that any one line was either working or closed; it was not possible to omit intermediate stops on, say, Sundays. The analogy with the Vienna Underground is not 100%. Hence, mail from A to C via B has to be handled at B. This was done in one or other or both of the following ways.

Possibility 'a'. Each incoming cylinder is taken out of the apparatus at B and examined. If it is not for B it is passed to the despatch side and sent onwards in a minute or two.

A typical train timetable (**) shows a brief (0.5 to 1.5 minute) stop at each station. What this means in practice is that the train arrives - it can consist of 15 Büchse - then the receiving chamber is isolated from the pipe, the compressed-air/vacuum vented off, and the door opened. Each Büchse is taken out of the apparatus and examined. If it is not for B it is passed to the despatch side, combined with any Büchsen from station B itself, assembled in station order to make up the outgoing train, the last Büchse fitted with the leather to make it a driver, and sent onwards by loading it in order into the outward-going pipe (driver last!) then manipulating the valves. In other words, the "train" has to be disassembled and re-formed at every station!

** The timetables are in "Post- usw Dienst im Lokalpost-Rayon von Wien: Feb 1892". Picking "line 1", the 'station stops' from Centrale are 45 seconds at Fleischmarkt; 60 at Kärntnerring; 45 at Wieden etc. This map shows the system in 1892, with the direction of travel round each loop indicated.

Possibility 'b'. Each incoming cylinder is taken out of the apparatus and opened. It will contain bundles of mail, each bundle bearing the number of the station it is for. If a bundle is not for B it is passed to the Büchse-filling person and sent onwards by the next train (typically in 20 minutes). There isn't time to load and despatch a Büchse by the train a bundle arrived on.

We do not see how this bundle-sorting can be avoided, because each station could have items for all the other 52 stations; there are 3 trains an hour each with up to 15 Büchsen; there could therefore be a delay of 100 minutes at each intermediate station in forwarding an item; the delays are too long. What they could have done, we suppose, is to have a separate Büchse for each station on their own line, and put all items for other lines into one or more Büchsen for the station at which they would be transferred. So looking at the above map, mail from Centrale for Hundsturm, Meidling and Westbahnhof would all be put into a Büchse for Gumpendorf. At Fleischmarkt, Kärntnerring, and Wieden it would be examined and sent onwards unopened. At Gumpendorf it would be opened, and the bundles sorted into three separate Büchsen for the next train on that line.

Occasionally one finds a used item of Pneumatic Post stationery which appears never to have been inside a container as it bears no trace of having been folded: these are possibly philatelic items, although it is perhaps evidence that the Pneumatic Post system has been used simply for the express delivery service it provided, the sender being prepared to pay the extra Pneumatic Post fee in order to benefit from the prompt attention given to Pneumatic Post mail. Or, the item was used because its imprinted rate happened to equal that required for some other service: eg, this letter-card to London "found in the (red) letter box", where the 30Kr overpaid the postage of 25Kr. Or, the item was folded or rolled, and has been ironed flat to "improve" its appearance before sale!

Operational troubles

The last Chief of the Viennese Pneumatic Post, Bezirksbauführer Ing. Walther Turner, wrote a private memoir "The Stadtrohrpost in Vienna 1875-1956" (it is in typescript, and must have been Gestetnered or similar). It has a chapter on the most common causes of breakdown, such as loose leather caps, containers for registered letters falling apart, over-heavy trains, greatly worn and thus porous driver sleeves, defective pipes, and condensed water which sometimes even froze.

If cans stuck, the first remedy was to apply full vacuum to one side and full compressed air to the other. If that failed, a train composed only of solid drivers was sent down, the impact of which normally dislodged the blocked cans. Condensed water was removed with a can carrying a sponge, or a driver with a horse-hair brush. Jams & blockages would if possible be cleared by applying alternate pressure and vacuum; if that failed a 1kg iron driver was sent down, using a 5ats (75psig) portable compressor borrowed from the Vienna City Works Department!

If all else failed, the section was isolated with wooden stoppers; the operator then blew a puff of air into the tube and used a stop watch to time the "echo" in seconds. Taking the speed of sound in air as 330m/sec, the round-trip distance is 330 times the stop watch reading, and the distance to the blockage is half that. Using large scale street maps (1:360) the location of the blockage was determined, and the street dug up. A postcard would be sent - by pneumatic mail of course! - to advise the Vienna Town Hall of the work. See the Portofreie Rohrpost section for examples.

Back to main narrative Plumbing matters The piping The containers (Büchsen)
The employees Operations Manual Passing-through ("Durchfahrt") mail Operational troubles