The concept of the Kordina named after its inventor Zsigmond Kordina (see below), is described on page 153 of Wardale’s book “The Red Devil and Other Tales from the Age of Steam”, its function being to:
“expand the release steam to low pressure and high velocity at the point where the exhaust flows from the two cylinders join, preventing backflow of release steam into the opposite cylinder which occurred whenever the flow are of the exhaust passage junction was much larger than the blast nozzle tip. This was normally the case with First Generation Steam locomotives and showed as a jump on the back pressure line of the indicator diagrams at approximately mid-stroke position, the most unfavourable point.”
Wardale adds the interesting observation that:
“Ideally, some of the great amount of energy in the release steam could be used to create an ejector pump action on the exhaust from the opposite cylinder, lowering the back pressure in this cylinder, even to below atmospheric pressure."
In the case of Wardale’s Red Devil, he describes its exhaust manifold being formed such that
“the exhaust passages from each side first branched into those for the front and rear chimneys respectively, and at the very top, the dividing walls separating the exhaust steam flows from the right and left cylinders ended. At this point the combined flow section was equal to the total exit section at the blast nozzles and this was the essence of the Kordina.”
Plate 16 from Wardale's book is copied below, showing the arrangement of the Red Devil's Kordinas:
In the case of the 5AT, in FDC 12 line 301 Wardale describes the Kordina as "... essentially the reduction of the total flow area of the exhaust steam passages from right and left cylinders to that of the total blast nozzle tip area at the point where these passages combine below the blast pipe", and he illustrates this in Fig 12.5 (copied below), showing Kordina to form the upper end of the exhaust manifold in such a way that the (circular) passage from one cylinder fits inside the passage from the other cylinder, forming an annulus around the first, each passage having exactly the same cross-sectional area, each being the same as the total blast nozzle tip area.
Richard Coleby has incorporated this detail into his 3D "Solidworks" drawing of the 5AT cylinder block as illustrated below:
In his book, Wardale states that Zsigmond Kordina was a Russian-born engineer, however he must have spent some of his life working in Hungary since his name is mentioned in a paper titled "The Role of Engineers and Milestones of Industrial Development in Hungary" as follows:
MVAG started locomotive manufacturing in 1873. The 1000th locomotive displayed at the Millennium Exhibition in 1896 is a proof of boost. With the leadership of Zsigmond Kordina, an internationally recognised locomotive designer team was working in MVAG at that time. The locomotives manufactured here could run at the speed of 100 km/h in 1900. Recognition of locomotive manufacturing was well demonstrated at the Paris World Exhibition in 1900: MVAGs twin-cylinder express locomotive was awarded a Grand Prix.
In his book "Compound Locomotives" [pp 37-38; Atlantic Publishers, UK, 1994; ISBN 090689961 3], John van Riemsdyk mentions Kordina as being the original designer of a numerous and long-lived fleet of tandem-compound 4-4-0 locomotives built for the Hungarian State Railways between 1890 and 1904.