Industrial equipment and facilities such as boilers, incinerators, chillers, process plants and steel mills generate heat when used. But how many of us are aware that such heat sources also generate "waste heat"? Waste heat is heat discharged from a process into the environment even though the heat can be tapped for some useful purposes. It presents great opportunities for thermal energy recovery.

Heat recovery can take many different forms. In general terms, it can be classified into direct recovery, indirect recovery and secondary recovery. Direct recovery refers to the use of flue gas, which is a combination of carbon dioxide, nitrogen and oxygen generated by the combustion process in an incinerator or boiler, to preheat or dry products directly. Indirect recovery takes place when flue gas is used to preheat combustion air or fuel. Secondary recovery utilises the waste heat to preheat an external medium or to generate power. Fuel savings is the most obvious benefit of waste heat recovery and this is the key motivating factor for the companies to invest in equipment to tap their waste energy. Other than that, a reduction in fuel usage also results in less emission of pollutants, which include carbon monoxide, hydrogen sulphide and sulphur dioxide, to the environment. Moreover, emission of carbon dioxide, which is a green house gas, will also be reduced.

Waste Heat Recovery in Boiler Systems

This simple concept of waste heat recovery could be illustrated by looking at boiler systems (see Figure 1 above). Feed water enters the boiler to produce process steam. The energy for the steam production normally comes from the combustion of diesel or fuel oil, which produces an exhaust with a temperature of about 220 - 270 °C. For very old boilers, the temperature of the exhaust could even be higher. The exhaust flue gas, which still contains a substantial amount of waste heat, is discharged directly to the atmosphere through the chimney systems. As a result, precious energy is wasted.

The energy from the exhaust flue gas could be tapped through the use of heat exchangers, as shown in Figure 2 below.

In this case, the exhaust flue gas is used to preheat the feed water and/or the combustion air. Consequently, much less fuel is required to produce the same amount of steam. For example, a 8 tonnes/hr boiler in which the flue gas flow rate is 9,700 Nm³/h would result in approximately 22,000 L savings per year in fuel oil or diesel consumption for every 10 °C recovery of the exhaust gas temperature. This would be translated to more than US$5,000 in actual dollar savings annually¹.

The dew point of the flue gas is normally in the range 110 -140 °C, depending on the sulphur content in the fuel used. Therefore, for most applications, the temperature of the exhaust fumes could theoretically be reduced by at least 70 °C before low temperature corrosion occurs. This results in potential savings of more than US$35,000 a year.

1 Assuming that the calorific value, density and price of the fuel is 43.6 MJ/kg, 970 kg/m3 and US$0.24 per litre respectively.

Problems with Conventional Heat Exchangers in Waste Heat Recovery

However, most industrial boilers that are less than 10 tonnes/hr are normally not equipped with heat exchangers to recover the waste heat. This is because there are many maintenance problems associated with low temperature corrosion if conventional heat exchangers are used for such applications.

Low temperature corrosion refers to the corrosion at the tubing walls of the heat exchangers as the gas film temperature at the wall is below the dew point of the flue gas. This is because the gas side heat transfer coefficient is relatively low. This results in a huge temperature gradient between the tubing wall and the flue gas. As a result, the flue gas temperature must always be maintained at around 200 °C if conventional heat exchangers are used.

Due to the above, it is not economically viable to install waste heat recovery systems for smaller boilers in the past. For larger boilers that uses conventional heat exchangers for heat recovery, the amount of waste heat released to the environment is still significant.

Heat Pipe Heat Exchangers

The corrosion and maintenance problems faced by conventional heat exchangers in heat recovery systems could be addressed using the heat pipe heat transfer technology. In terms of industrial applications, this is still a relatively novel technology. Early efforts were directed toward space applications; however, due to the high cost and the rapid rise in demand in energy, the commercialisation and application of heat pipe heat exchangers in other industries have become more widespread in recent years.

Figure 3 above shows a schematic diagram of a heat pipe heat exchanger. Heat pipes are simply pipes that contain a working fluid. They are sealed at both ends after a vacuum is created above the working fluid.

The heat exchanger is divided into two ends: evaporating and condensing ends. The hot medium flows through the evaporating end and evaporates the working fluid. The vapour rises up the heat pipes, and condenses at the condensing end such that energy is transferred to the cool medium. As such, heat pipes operate at almost constant temperature.

The phase change heat transfer in heat pipes gives them very high thermal conductance. In fact, the conductivity of heat pipes is about 100 times higher than copper.

Moreover, the ratio of the heat transfer area between the evaporating and condensing ends of the heat exchanger could be easily manipulated by adjusting the height of, and adding fins to, the evaporating and condensing ends.

Advantages of Heat Pipe Heat Exchangers

Due to the high thermal conductivity of heat pipes, their simple construction with no mechanical moving parts and the ease of manipulating the heat transfer area, heat pipe heat exchangers offer many advantages compared to conventional heat exchangers. These are listed in the Table 1 below.

One key advantage is that the temperature gradient between the wall of a heat pipe and the flue gas is not drastic. Therefore, the heat exchanger is extremely tolerant to low temperature corrosion and the effluent flue gas temperature could be as low as 140 °C. This makes this kind of heat exchangers extremely economically viable for waste heat recovery applications.

Conclusions

The heat pipe heat exchanger offers many advantages compared to a conventional heat exchanger. It allows for large quantities of heat to be transported through a small cross-sectional area over a considerable distance with no additional power input to the system. Further more, design and manufacturing simplicity, small end-to-end temperature drops, and the ability to control and transport high heat rates at various temperature levels are unique features of heat pipes. These features make it extremely appropriate for waste heat recovery applications.

Waste heat recovery not only helps to protect the environment, but will also result in substantial cost savings for the company. Results from case studies have shown that the payback period for such investments using the heat pipe heat transfer technology is within two years.