Holzwarth gas turbine

The Holzwarth gas turbine is a form of explosion, or constant volume, gas turbine in which combustion takes place cyclically in a combustion chamber closed off by valves. The Holzwarth gas turbine is named after its developer Dr Hans Holzwarth (1877-1953) who designed several prototype engines used for testing and experimental service in Germany and Switzerland between 1908 and 1943.

Design and Development
At the start of the 20th century, designers across Europe were working on the development of the constant flow gas turbine. The biggest problem for these early pioneers was the design of the compressor. The low efficiency of the turbo compressors available at the time meant that the compressor consumed all the energy supplied by the turbine. The only large-scale gas turbine that ran during this period was the 1906 Armengaud-Lemale machine which could sustain its own compression but was too inefficient to do useful work. Holzwarth's design avoided this problem by having combustion take place cyclically in combustion chambers closed off by valves. Since the pressure rise took place in the sealed combustion chamber there was no need for a compressor.

Holzwarth developed the theoretical concept for his gas turbine from 1905, while he was an employee of Hooven-Owens-Rentschler in the USA. Holzwarth returned to Germany in 1908 where he designed and built a 25 hp machine while working for Thyssen & Co. This first machine was manufactured and tested at Körting's workshop in Hanover and is now on display at the Deutsches Museum in Munich.

From 1909, Holzwarth worked with Brown, Boveri & Cie to build a market-ready 1000 hp gas turbine however output and efficiency were below expectations and Brown Boveri withdrew from the project in 1912.

Holzwarth continued to work on his design while he was employed as chief engineer for gas turbines at Thyssen. In 1923 a prototype machine was delivered to the Prussian state railway where it was used to drive a 350 kW generator for several years. During this period tests were carried out using coal dust fuel which was found to burn acceptably in the combustion chamber but produced exhaust particles which damaged turbine blades. In 1927, Hans Holzwarth left Thyssen & Co and founded his own company, ''Holzwarth Gasturbinen GmbH. ''

In 1927 Aurel Stodola tested a 500 kW oil fired Holzwarth gas turbine and found that only 8% of the fuel's energy was transformed into mechanical energy''. '' Observations regarding the loss of waste heat to the cooling water led jacket led to the development, by Brown Boveri, of the commercially successful Velox boilers. which in turn led to the development of the first modern industrial gas turbines.

From 1928, Holzwarth once again collaborated with Brown Boveri to build a version of his gas turbine with two sets of combustion chambers connected in series. These two-stage machines used a compressor driven by a stream turbine which was fed from the evaporation of water from the cooling water jacket. In 1933, a two-stage machine driving a 2000 kW generator was installed at the Thyssen steelworks in Hamborn where it was initially operated with fuel oil and later with blast furnace gas.

The last Holzwarth gas turbine was an experimental 5000 kW machine built by Brown Boveri's Mannheim factory in 1938 for the Hamborn steelworks. Fuel for combustion was blast-furnace gas compressed to about 6 bar. The gas turbine had hydraulically operated valves working at 60-100 cycles per minute. The unit was only infrequently run and was not part of the steelwork's regular equipment. In 1943, the gas turbine was damaged during an Allied bombing raid after which no further test runs were made. After World War II ended, interest in the Holzwarth design declined and no further units were built.

Operation
The explosion chamber (A) is filled intermittently with a rich mixture supplied from the gas chamber (C) and the air chamber (B). The mixture is ignited by spark after which the explosion of the mixture causes and increase in pressure throwing open the nozzle valve (F), allowing the compressed gases to flow through the nozzle (G) to the turbine wheel (H) on which the work is to be performed. While passing through the nozzle, the gases are expanded to the pressure of the exhaust (J). The nozzle valve (F) is kept open by fresh air throughout the expansion and subsequent scavenging and cooling.

When the expansion has been completed, air is blown, or drawn, in at a slight pressure through the valve (D). This scavenging air throws any residual gases left in the combustion chamber. through the nozzle, into the exhaust, after which the nozzle valve and the air valve (D) are positively closed. At this point the combustion chamber (A) is filled with pure, relatively cold air, into which the pure fuel (gas or atomized oil) is blown through the valve (E), thus forming the explosive mixture which is ignited by a spark. In order to make the impulses imparted to the turbine wheel more uniform, several combustion chambers working alternately are arranged in a circle around the turbine wheel (H).

In early Holzwarth machines, gas and air were supplied at a relatively low pressure around 0.1-1 bar in later models a gas compressor was used to supply fuel at pressures up to 6 bar. The gas compressor was driven by a steam turbine fed from the evaporation of water from the gas turbines cooling jacket which required the addition of a surface condenser package.

The two Holzwarth gas turbines built by Brown Boveri after 1928 used two stages of combustion chambers and a “two stoke” version of the cycle where gas admission and exhaust took place simultaneously. The first stage comprised the final charging of the combustion chamber, the explosion, the delivery of heat and energy in the steam generator and gas turbine. The second stage comprised the scavenging and the pre-charging. The residual exhaust gases escaped through an economiser to the atmosphere.

Surviving example
This first prototype Holzwarth gas turbine is on display at the Deutsches Museum in Munich.