Steam oven
A steam oven is, in the narrower sense, an oven that is heated with steam.
Technical principle
A steam oven is an oven in which the heat is transferred from the boiler room to the baking chamber by means of steam in a closed system, the Perkins pipes introduced by Loftus Perkins . These are tubes which, like heat pipes, are partially filled with water and which are closed at both ends. The pipes protrude approx. 15 to 30 cm into a boiler room, where they are heated with a flame generated by wood, coal, heating oil or gas. This creates water vapor in the pipes (up to 300 ° C), which leads the heat from the heating chamber into the baking chamber and thus heats it up. Perkins tubes, which are installed in the lower area of the baking chamber, form the lower heat, those in the upper area form the upper heat. The top and bottom heat cannot be regulated individually. The heating creates an overpressure in the Perkins tubes, which are closed on both sides, so that the boiling point of the water increases. By releasing the energy to the baking chamber, the steam condenses and runs back in the pipe to the boiler room. It is important that the pipes have a slight gradient towards the boiler room in order to ensure an undisturbed cycle (heating - evaporation - heat emission - condensation - renewed heating). If the part of the pipe protruding into the boiler room overheats (e.g. because the condensate does not run back or has been heated too much), the pipe can explode due to overpressure, which used to happen in bakeries and has a negative impact on the image of the steam oven to this day.
Steam ovens with Perkins tubes represented a revolution in oven construction in the 19th century, as the heating chamber and baking chamber were separated for the first time. It was also possible to reheat during baking and thus bake continuously. This was not possible with the old German wood-burning oven, as the baking chamber and heating chamber were identical. So you first had to heat up the baking chamber and store the energy in the vaulted baking chamber made of firebricks. After removing the burn, it was then possible to bake in the same room at a falling temperature. If the stored heat was used up after the end of the baking process, it had to be reheated.
Annular tube furnace
A further development of the Perkins oven is the ring tube oven , which is still manufactured in Luxembourg, France and Italy today. A closed, rectangular outer ring is made from thick-walled, seamlessly drawn pipe, into which cross pipes are welded. Depending on the depth of the oven, the rings are arranged one behind the other so that the cross tubes form the top and bottom heat of the baking chambers. The lower part of the rings, some of which are filled with water, is heated by hot combustion gases (oil, gas or wood). As with the Perkins oven, a water-steam mixture with a high temperature (up to 300 ° C) is created, which rises and brings the energy to the baking chamber via the cross tubes. When the energy is transferred to the baking chamber, condensation occurs and the liquid water runs back down to be heated up again. Here, too, there is a cycle, in contrast to the Perkins furnace, there is only one direction of flow in the pipes, rising steam and returning condensate do not meet. This makes the oven much safer to operate; overheating could only occur if the oven's temperature control fails (the burner heats up), which in turn can be ruled out by a safety temperature limiter that switches off the burner directly in the event of excess temperature.
The main advantage of the ring tube steam oven is that the circulation of the heat transfer medium (water) takes place "by itself" in a hermetically sealed system without additional components such as pumps or valves. This allows a long service life, apart from the burner and steam generator, no maintenance is required and the oven is extremely quiet. Ring-tube steam ovens are manufactured using both a pure steel construction and a masonry design. With the latter, the flue gas duct is bricked up and finally the outer rings are embedded in cement, so that a baking muffle with a high thermal mass is created, which can store more energy and has significantly less temperature drop when the oven is loaded. Overall, the ring-tube steam oven is said to be energy-saving, as the exhaust gas temperatures can be kept low and no electrical consumers such as pumps or fans are required. Characteristic for this type of oven is generally a sluggish temperature behavior, which can also be disadvantageous, especially when initially heating up and when changing the baking temperatures. The important manufacturers of these ring-tube steam ovens are still Bongard and Fringand in France, Hein in Luxembourg and Mondial Forni, Tagliavini and Logiudice in Italy.
From a technical point of view, an oven in which you want to bake larger quantities in a shorter time requires two separate rooms, the heating chamber and the baking chamber, which are connected by a medium for heat transfer, we speak of indirectly heated ovens. Back then, water was the best thing to do. B. also with the steam engine. In contrast to the steam engine, the ring-tube steam oven is still justified today. Air, flue gas and thermal oil are widely used as additional heat carriers today. On the other hand, there are also directly heated electric ovens that do not require a heat transfer medium, as the heating elements in the baking chamber are above and below the baked goods and thus have the advantage of adjustable top and bottom heat. They are mainly used in pastry shops.
history
The appearance of the steam oven marked a change in the history of the oven. Until the middle of the 19th century, traditional ovens were used, in which oven stones were heated by a fire on which the bread was then baked. Such ovens were found in many houses and also in the local commercial bakeries. However, they were not suitable for mass production in order to effectively ensure supply in the fast-growing cities of the time.
The urgent need made a realignment necessary. The first large-scale bakery was founded in Berlin in 1844, a Frenchman invented a dough kneading machine a short time later, and a new type of steam oven was presented in England.
Around 1900 a development began in which old bakeries were converted into modern steam bakeries. The name Dampfbäckerei guaranteed the customer that bread was produced using the latest technology and consistently high quality. Many bakery foundations in the first decades of the 20th century therefore included the term steam bakery as part of the name, a term that is still seen sporadically in Austria today. It was only in the middle of the 20th century that the large bakeries were converted so that the ovens were used with conveyor belts or with pull-out stoves, but these ovens were no longer heated with steam, but mainly with heating oil or gas.
Web links
- Baker Perkins in the Bakery Industry (English) - history of the steam oven from Perkins (partly also Pfleiderer)
Individual evidence
- ^ Baker Perkins Historical Society: The history of AM Perkins & Son, London (English).
- ↑ "History of Bread" , DieBackStube.de, Version 2006.