The development of the chimney
Whereas originally the fire burned open in the room, an outlet for the smoke which arose was created with the chimney open at the top. The beginning of chimney technology made a reasonably tolerable inhabitance of the heated rooms possible, frequently the only rooms for cooking, living and sleeping. Chimney development is therefore to be seen in close relation to developments in heating technology. Today, flue gas systems must be matched to each other so that unacceptable conditions or hazards cannot even materialize. They have the function of leading noxious components of the flue gases, hazardous to human health, over rooftop level where they can be dispersed in the atmosphere.
Changes to fuels as well as continuous development of firing installations demanded continuous changes in chimney technology.
Single wall masonry chimneys
The predominant design in domestic housing chimney construction was for a long time the single wall masonry chimney. For reasons of simpler and faster assembly, this construction was replaced by the single wall chimney of prefabricated parts.
Double wall system
The increasing use of oil-fired systems however called for acid resistance as well as structural integrity and resistance to fire. This led to the double wall chimney. The inside pipe of fire clay met the requirement for acid resistance, the external sheath structural integrity and both components together, the fire resistance.
Single and double wall chimneys are still approved today and described in relevant technical regulations. As far as modern energy-efficient firing installations are concerned however they are practically obsolete.
The energy crises of the 1970s resulted in further development of firing installations. They finally resulted in heating boilers with improved energy efficiency and lower flue gas temperatures entering the market. This trend resulted in development of the three wall chimney.
Three wall chimney
In this system the inside tube of fire clay was covered with a special layer of heat insulation. It ensures that the low temperature flue gases in the chimney do not cool too much and are safely dispersed over the rooftop. In doing so the heat insulation layer must ensure that the movement of the internal wall necessary to absorb heat expansion is possible.
The latest stage of development has been reached with the dampproof chimney with integral combustion air intake. With it, firing installations not dependent on ambient air can be directly connected to the combustion air intake and flue gas exhaust. As a universal system it covers a sector of application which extends from condensing technology to large-scale systems.
Fire prevention of the chimney
Chimneys are especially heavily stressed components. This is the crux of the matter. They must be structurally integral, resistant to heat, flue gas and soot fires inside the chimney. Please observe all locally applicable regulations and directives.
Pressure and flow conditions
In every firing installation the components - boiler, connecting piece, chimney - must be carefully matched to each other. Only then are proper operating conditions ensured. Faults with matching of the components can result in operating failures of the firing installation e.g. incomplete combustion and soot formation in the firing installation and flue, with the danger of an uncontrolled soot fire.
The pressure conditions in the firing installation are influenced by the flue draft, the design of the firing installation itself and the configuration of the burner fan.
Pressure boilers must be tight so that flue gases cannot enter the installation room. Firing installations which operate as natural draft boilers only at negative pressure do not need to be completely tight. When they are not tight, ambient air can flow into the installation but flue gas cannot escape. Natural draft boilers are also manufactured extremely tight these days because defective tightness increases the losses at standstill and impairs the efficiency of the system.
Through appropriate dimensioning of the flue and the connecting piece as well as exact matching to the conditions of the heat generator, it must be ensured that the lift required to extract the flow volume of flue gas is achieved. As mentioned initially, we recommend having the configuration checked by the chimney manufacturer or heating specialist. Of course the chimney sweep or responsible authority is also available for consultation.
Temperature conditions in the chimney
On its way from the firing installation via the connecting piece and the flue, the flue gas cools down. The heat losses of the flue gases depend for the most part on the following criteria:
- Heat insulation of the chimney
- Height of the chimney
- Internal surface of the flue
- Flow speed of the flue gas
When burning hydrogenous fuels, steam is formed. This must be taken off to the outdoors so that no water damage occurs to the chimney. As for air, flue gas can only absorb a limited quantity of steam. This quantity decreases with sinking temperatures. If the flue gas cools too much so that the dewpoint is undercut, condensation is formed in the connecting piece or in the flue. The flue gas temperatures of the heat generators can be significantly reduced, if condensation of the steam in flue systems unaffected by moisture is allowed. In this case certification of a sufficiently high flue gas temperature at the entry to the flue, to avoid undercutting of the dewpoint and moisture penetration, is no longer necessary. The requirement for compliance with pressure conditions remains.
Certification of safe usability
After fitting of the complete firing installation, flue gas systems, firing installation and ventilation equipment are to be inspected by the chimney sweep or responsible authority and safe usability certified.
For this section, comprehensive information from the website of Schiedel Kaminsysteme GmbH is used. www.schiedel.com