Air preheaters: cast iron, glass and hybrid

By-Cast specialises in the design, production, and maintenance of cast-iron (DEKA) and glass heat exchangers. The heat exchangers designed and built by By-Cast are largely used in industrial boilers and fired heaters to preheat the combustion air before mixing it with fuel; this explains why the exchangers are generally called air preheaters. These heat-exchangers transfer heat from flue gases in combustion processes to the combustion air.

In doing so they considerably increase the efficiency of the installation and recuperate energy that would otherwise be lost. Originally designed to withstand the harsh conditions of coal-fired boilers in power plants, our air preheaters have gained a proven track record of quality and durability in many processes, including industrial boilers firing fuel such as coal, gas, and oil, but also with less conventional fuels such as wood chips and municipal waste.

Cast iron

We offer three types of cast-iron tubes with respect to the ratio of heat-transfer surfaces between air (inside tube) and flue-gas side (outside) to achieve an acceptable Minimum Metal Temperature to avoid corrosion.

HT (A/B): Finned surface inside and outside > A maximal heat-exchanging surface
LT(B): Finned surface outside and ribbed inside > Higher metal temperature
LLT(B): Finned surface outside and flat inside > Higher metal temperature when one approaches the dew point

Each tube type is available in its 1^st- , 2^nd-, and 3^rd- generation configuration. The 3^rd generation is the most recent and efficient (HTB, LTB, and LLTB).

The DEKA cast-iron material has properties that make the DEKA air preheater the optimal choice for fuels with a high acid and dust content. We use gray cast iron where carbon is in the form of unbonded lamellae. This carbon, being inert to most acids harmful to metals, gives gray cast iron the following unique properties:

High heat conductivity
Corrosion resistance
Elasticity comparable with steel springs
Wear resistance

This combination of properties gives a DEKA air preheater an unparalleled operational life span. When used according to design specifications, our air preheaters last on average more than 20 years. Our solutions thus assure an unbeatable total cost of ownership (TCO) over the whole life cycle.

Glass

By-Cast supplies glass heat exchangers which can either be used as a units on their own or to preheat the air entering a cast-iron DEKA air preheater. (See hybrid.)

For this application, By-Cast uses borosilicate glass tubes supported by PTFE-lined tube sheets. These tubes have excellent resistance to acids. Furthermore, they have a high thermal capacity and high resistance to thermal shock. Although glass is brittle, the borosilicate coating gives the glass tubes a high elasticity. The glass tubes can also be washed regularly during operation; this constitutes a great advantage for maintenance.

Hybrid

The metal temperature of the cast-iron tubes must always be kept higher than the dew point of the flue gas. For safety reasons we recommend keeping the metal temperature 10°C higher than the dew point. This restriction severely limits how much heat can be recovered when acid flue gases are involved. By first sending the air through a glass air preheater, it is possible to recover as much of the waste heat as possible without any risk of condensation or corrosion in the cast-iron air preheater.

A long working life

By-Cast is widely recognised as global market leader in cast iron air preheaters. Our solutions have an unbeatable total cost of ownership (TCO) over the whole life cycle. A DEKA air preheater will last up to thirty years with proper use! Due to the modular construction of a DEKA air preheater it is also possible to replace tubes or modules on site.

Construction method

An absolute added value from By-Cast: we are the inventor of the proven DEKA concept. The heat-transfer surface is made of rectangular, finned, cast-iron tubes. These tubes are stacked vertically alongside each other to form rows, then the various horizontal rows on top of each other form a single module. A fiber-rope compound gasket between the tubes accommodates the differential expansion of the various materials of construction and ensures tightness. The quality of our sealing system allows us to guarantee a leakage of less than 0.4%. Depending on the specific project requirements, modules are stacked on top of each other to create a multi-pass arrangement. In most applications the flue gas passes vertically downward over the tubes, while the air flows cross-counter flow into the tubes. Numerous other arrangements are possible to accommodate specific site constraints.

Since tubes are attached to each other or to the structural steel of the module by bolts, they can be replaced individually, thus avoiding the replacement of an entire module or the complete air preheater. This constitutes a major advantage over our competitors.

We are usually asked to design air preheaters for new installations of fired heaters, boilers, or incinerators, but By-Cast also often supplies DEKA units for retrofitting application of existing installations. Our advanced engineering approach and modular construction will guarantee an optimal solution to your specific challenges.

Want to know more about our cast iron, glass or hybrid air preheaters? Contact one of our staff members. Contact us