Metal Heat Exchangers for Industrial Furnaces

Release time:

May 13,2026

source:

Metal Heat Exchangers

During the writing of this book, heat exchangers built with silicon carbide tube bricks and fewer joints were under construction. These tube bricks are pressed together by gravity, and their performance is not yet fully understood.

II. Metal Heat Exchangers

If air leakage is not allowed, the heat exchanger must be made of metal. From the end of the 19th century to the beginning of the 20th century, many metal heat exchangers were made of cast iron, and their joints leaked air. Modern metal heat exchangers are welded, sometimes connected with screws.

At high temperatures, metals are oxidized or combined with sulfur, causing them to soften and creep. When large tensile and compressive stresses act alternately and exceed the yield limit, fatigue occurs, resulting in cracking and damage. Therefore, the strength and service life of a metal heat exchanger depend on the maximum working temperature of the metal, the metal composition, and the structure of the heat exchanger, which should allow metal expansion and contraction without producing excessive stress.

The formation of iron oxide scale on heat exchanger walls is undesirable, because thick oxide scale reduces heat transfer and lowers metal strength. Therefore, metal materials that do not oxidize easily or can form a protective film should be selected. Data on the oxidation rate, tensile strength, and creep strength of various alloy materials can be found in the first volume.

When the working temperature of the metal is very high, valuable alloy materials containing tungsten and/or other advanced components are required. Carbon is an alloying element that improves creep strength but reduces plasticity.

Metal heat exchangers are rarely manufactured by a plant itself as part of the furnace. They are specialized equipment developed by manufacturers based on experience. When designing a heat exchanger, the following points must be considered: local overheating, correct selection of alloy materials, excessive creep, expansion stress exceeding the yield limit, and corrosion caused by furnace waste gas.

A brief introduction to some successfully used heat exchangers is meaningful for both furnace builders and users.

The earliest metal heat exchanger consisted of several suspended tubes. It was installed below the preheating section of an inclined-bottom continuous heating furnace. Combustion products flowed downward inside the tubes. The sharp ends at the bottom of the tubes were inserted into a sand seal for expansion and sealing. However, the sand seal did not perform this function very well. Air flowed upward between the horizontal baffles outside the tubes.

This type of heat exchanger structure, although it was the predecessor of the well-known and widely used heat exchanger, has now been eliminated. In newer heat exchangers, waste gas flows upward through several tubes, while air generally flows downward between horizontal baffles. Either the top plate of the tubes or the bottom plate of the tubes is fixed. When the expansion of the tubes is greater than that of the heat exchanger shell, this expansion difference is absorbed by an expansion ring, as shown in Figure 146. In addition, to absorb the expansion difference between individual tubes, each tube has its own expansion joint.

At the bottom, the highest-temperature waste gas meets the highest-temperature air, so the tube wall temperature is highest at this position. To reduce the damage caused by high temperature to the tubes, the lower parts of the tubes are made of heat-resistant alloy materials and adopt the structure shown in Figure 146.

Cold air flows downward through the heat exchanger in two paths. Most of it flows between the horizontal baffles outside the tubes, while a small portion flows along the central large tube into the chamber above the bottom plate to cool the bottom plate and the lower part of the heat exchanger. Hot air with a lower preheating temperature flows upward along the tubes around the chamber and then mixes with the high-temperature hot air.

The bottom plate that fixes the tubes is directly exposed to radiation from a thick layer of high-temperature waste gas and the refractory brick wall. To prevent damage to the bottom plate, it must be protected with a layer of refractory material. This refractory material is fixed by hooks welded to the bottom plate and tapered expanded tube ends. The tapered expanded tube ends also have another important function. Compared with sharp tube ends, they can reduce pressure loss. The tapered tube ends are not cooled and therefore gradually burn away. After that, the refractory material becomes the flow path guiding the waste gas, while the refractory material remains fixed in place by the hooks.

The average service life of the heat exchanger shown in Figure 146 is long. Abnormal operating conditions, such as an air preheating temperature higher than 700°C, very incomplete combustion before the heat exchanger, and the presence of a corrosive atmosphere, will shorten its service life. If combustion proceeds so slowly for uniform heating of the workpiece that combustion is still incomplete when the combustion products enter the heat exchanger, high temperature will be produced at vortices or mixing zones due to secondary combustion. Many soaking furnaces are built with various natural refractory materials; some refractory materials release gases at high temperatures that corrode alloy steel.

The heat exchanger shown in Figure 146 is widely used in many countries. Under extremely severe operating conditions, as described in the previous paragraph, it is used together with a radiation heat exchanger. The issue of combined use will be introduced in the section “Radiation Heat Exchangers.”

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Main Circuits and Power Supplies of Medium Frequency Induction Furnaces

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Main Circuits and Power Supplies of Medium Frequency Induction Furnaces