There are numerous reasons to seek an alternative radiant-tube heating system in an existing furnace and there are many radiant-tube systems options from different suppliers. To pick out the best possible solution requires a detailed analysis of the existing furnace, sound knowledge of both the combustion equipment and process and the creativity to find an economic solution that does not compromise the technical performance.
The most common radiant-tube designs on the market are shown in Fig. 1. Straight, U- and single-ended tubes are made of both high-alloy metals and ceramics, the latter predominantly reaction-bonded SiSiC. The market share of ceramic radiant tubes is increasing quickly. Ceramic tubes are brittle, but are very heat resistant and stable with regard to high temperatures and thermal shock, and have good strength even at high temperatures. Radiant-tube design and performance are discussed in more detail in a previous article [1].The following examples of retrofit projects illustrate a variety of options and solutions. Some of the approaches can apply to new equipment as well.
Retrofit projects
Furnace retrofit projects rarely are carried out by selecting components from a catalog alone, but instead are done with close cooperation between the customer and supplier. Retrofit projects are carried out by furnace manufacturers, combustion-equipment suppliers, engineering companies or a combination of these, depending on the size and complexity of the project.
The starting point for a retrofit usually comprises analyzing the existing equipment, defining the goals and investigating the options. Often, an analysis of the existing equipment brings a number of problems to the surface and, in some cases, a solution can be found with minor modifications of the equipment. In most cases, it is very useful to perform a complete heat balance of the process. Information such as production rate, fuel consumption, furnace geometry, zone temperatures and output and other details are very helpful to get a good understanding of the current situation. After discussing the goals and improvements to be achieved, a hypothetical heat balance can be carried out again to see the conditions after a potential retrofit.
Figure 2 shows the results of a heat balance of a continuous furnace. The red line indicates increasing strip temperature. Heating occurs in the first four zones, and the fifth zone is a soaking zone. The blue line shows the net heat to the load, indicating that the burners in the first two zones are firing 100% and cutting back in zones three to five. The green line shows the radiant-tube temperature, which is rising until the end of zone two. Then the radiant-tube temperature becomes the limiting factor, and the radiant-tube capacity is reduced accordingly. From such a calculation, the limiting factors (for the production rate, for example) can be determined.
A more important consideration is tube arrangement. The temperature around the radiant tubes is not homogeneous; a detail that often is neglected. This is especially true when tubes are positioned too close together, where the tube side facing the wall is considerably hotter than the side facing the load (Fig. 3). These conditions can be examined using special purpose programs that are available from some furnace and burner manufacturers. For a more detailed study of the furnace, computational fluid dynamics (CFD) software can be used (Fig. 4). These general-purpose software packages allow for a complete three-dimensional (3-D) simulation of the furnace. However, these programs require a lot of experience, and, depending on the complexity of the task, a lot of computer power and time. A lot of progress has been achieved regarding user friendliness, and it is certain that these programs will play an increasing role in the future.
Besides the general layout, questions about burner control, flame safety, piping and other issues have to be discussed. New communication standards like the serial field bus Profibus (Profibus International, Karlsruhe, Germany) enable adding functionality without the need for thousands of wires around a furnace.
Converting straight-through tubes
This example illustrates a conversion process of a furnace having straight-through tubes installed originally. The main reason for retrofitting the furnace was to obtain the same design capacity of the furnace. The furnace originally achieved its design capacity, but the production rate decreased over the years mainly for two reasons: burners out of adjustment and tubes coming out of service (end of tube life).
When planning the retrofit, other aspects also had to be considered including increasing the efficiency, reducing atmosphere losses and adding flame safety. Another requirement was to keep downtime required for the retrofit to minimum. Evaluating several options and considering all the technical, operational and economic requirements led to the following solution.
The existing radiant tubes would be reused to avoid major structural work on the furnace and to keep investment low. As shown in Fig. 5, the design principle was changed from straight-through to a recirculating single-ended tube design by inserting ceramic inner tube segments and plugging up one end of the radiant tube. By eliminating the second opening per tube in the furnace, the tube was able to expand freely, and the furnace was made completely tight.
Energy savings in the range of 30%, compared with the original design specifications, were achieved by using self-recuperative burners. Actual savings were substantially higher because many burners could not be set to proper ratio before the retrofit.
The control system was changed from a proportional zone-control system to a sequential pulse-firing system. Then, each radiant-tube system was equipped with a gas and air solenoid valve, ignition electrode, flame safety and burner-control unit. Besides having a better temperature accuracy, temperature uniformity and lower NOx emissions, the on/off operation of the burners avoids shutting down an entire zone if one flame detector is detecting a flame failure. The control system now allows for cooling the zone by simply opening just the combustion air valves of individual burners. The burner adjustment was facilitated because each burner can be adjusted individually, simply using a manometer to check for differential pressure from air and fuel orifice plates and the use of an exhaust gas analyzer during the initial start up.
The required downtime was kept short by delivering all the burners on site already prepiped and tested. Gas, air and exhaust piping had to be modified to obtain constant pressure at every burner. The original blowers could be used because pressure level and pressure characteristics met the specifications for the new system.
After the retrofit, the design capacity was reached easily and stayed consistent over time. Tube life was largely extended, mainly through tube temperature uniformity and avoiding tube stress by letting the tube freely expand inside the furnace. Another benefit was reduced temperatures around the furnace having lower exhaust temperatures.
Replacement of W-tubes with P-tubes
Insufficient equipment life and high maintenance were reasons to develop a concept for replacing W-tubes in a furnace. After judging various options, P-tubes having a diameter of more than 10 in. (254 mm) were selected (Fig. 6). Besides having a large cross section for internal recirculation, the tubes have a much higher structural strength compared with small diameter tubes. Changes in the control concept were minor because the original tubes were high/low controlled. A combustion air blower and piping was added to change the system from a pull (negative pressure in the exhaust system, no combustion air blower), to a push-pull (negative pressure in the exhaust and positive pressure in the combustion air) system.
Ceramic, single-ended radiant tubes replace alloy tubes
This example shows how a retrofit increased production of a strip line well beyond the original specifications. The task was to investigate whether the production rate of a furnace could be increased without extending the furnace length. To achieve the goal, the strip must be heated in an even shorter heating zone length, because the cooling rate could not be increased due to metallurgical reasons. The furnace was originally equipped with high-efficiency self-recuperative burners, operated in on/off mode and with alloy single-ended radiant tubes.
Thorough studies were performed on what net heat input and what zone temperatures were required to achieve the required heat transfer to the low-emissivity strip. Calculations showed that the goal could be achieved by replacing the alloy tubes with ceramic SiSiC tubes.
The inner tubes were already ceramic and could be reused. The burners had to be equipped with ceramic recuperators, which can withstand the higher exhaust temperatures. To fight increasing NOx emissions due to higher tube and air preheat temperature, the burner could be operated in flameless oxidation (FLOX(r) a registered trademark of WS GmbH, Germany) mode. Several test tubes were installed throughout the furnace to refute any doubt whether the tubes would hold up in the case of a strip breakage, and after a certain period, the project was approved.
Because a similar radiant tube design and control concept was installed prior to the retrofit, the changes on the furnace shell, piping system, blowers and furnace control system were minor, keeping the required downtime very short (Fig. 7).
Using the data from the analysis, the higher production goal could be reached almost immediately after the retrofit. The specific energy consumption remained about the same because a similar combustion air-preheat system was used. Slightly lower efficiency (available heat) of the radiant tube systems due to higher exhaust temperatures was offset by lower specific wall losses of the furnace. The NOx emissions could be lowered despite the higher furnace zone temperatures using the FLOX(r) technology. The cost savings by avoiding an extension of the furnace was substantial.
Conclusion
During the past several years, the potential of new burner designs, radiant-tube concepts and radiant-tube materials were successfully demonstrated in the field. In the future, when rising energy prices demand even more efficient systems, thoroughly designed regenerative systems, which currently are being tested, will be available. System design and the study of retrofit solutions will increasingly depend on more sophisticated mathematical tools that can predict the performance.