Application of heat pipe exchangers in industrial waste heat recovery projects

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A heat pipe is a heat transfer component with high thermal conductivity; it transfers heat through the evaporation and condensation of a working fluid within a completely sealed vacuum tube. It boasts a high level of thermal conductivity, good isothermality, the ability to vary the heat transfer area on both the hot and cold sides, the capability for heat transfer over long distances, and temperature control – among other advantages. The disadvantage is that its antioxidant and high-temperature resistance are poor. This drawback can be addressed by installing a set of ceramic heat exchangers at the front; ceramic heat exchangers effectively solve the problems of high-temperature resistance and corrosion resistance. Heat exchangers made of heat pipes possess advantages such as high heat transfer efficiency, compact structure, low fluid resistance, and ease in controlling dew point corrosion. It is now widely used in industries such as metallurgy, chemicals, petroleum refining, boilers, ceramics, transportation, light textiles, and machinery as an energy-saving device for waste heat recovery and the utilization of thermal energy in industrial processes, achieving significant economic benefits. Waste heat refers to the energy that remains unused in energy utilization equipment under certain economic and technical conditions; it is, in other words, excess or discarded energy. It includes seven types: waste heat from high-temperature exhaust gases, cooling media, waste steam and wastewater, high-temperature products and slag, chemical reactions, combustible exhaust gases, liquids, and wastes, as well as the residual pressure of high-pressure fluids. According to surveys, the total amount of waste heat available in various industries accounts for approximately 17% to 67% of their total fuel consumption. Of this total, about 60% can be recovered and utilized.

I. Application areas for waste heat recovery and waste heat power generation:
1. In the chemical and petrochemical industries:
(1) Waste heat recovery from upstream and downstream gas in small-scale ammonia synthesis.
(2) Waste heat recovery from upstream and downstream gas in medium-scale ammonia synthesis.
(3) Waste heat recovery from the combustion of ammonia stripping gas.
(4) Waste heat recovery from the flue gas of the first-stage ammonia synthesis furnace.
(5) Waste heat recovery from the second-stage conversion furnace in ammonia production facilities with an annual capacity of 300,000 tons.
2. In the sulfuric acid industry:
(1) Waste heat recovery from the boiling layer of the boiling roasting furnaces used in sulfuric acid production. A factory producing 100,000 tons of sulfuric acid per year can recover 55,000 tons of steam.
(2) Waste heat recovery from the high-temperature SO2 gas emitted during the boiling process. A factory producing 100,000 tons of sulfuric acid per year can recover 105,000 tons of steam, with a potential power generation value of around 6 million yuan.
3. In hydrochloric acid and nitric acid production processes: Similar to case (2).
4. In the petrochemical industry:
(1) Waste heat recovery from hydrocarbon pyrolysis furnaces (operating at temperatures of approximately 750–900°C).
(2) Waste heat recovery from ethylbenzene dehydrogenation reactors.
(3) Waste heat recovery from cyclohexanol dehydrogenation reactors.
(4) Waste heat recovery from catalytic cracking regeneration heat exchangers.
(5) Waste heat recovery from various other heating furnaces.
5. In the building materials industry:
(1) Waste heat recovery from the hot air furnaces used in kaolin spray drying.
(2) Waste heat recovery from glass furnaces.
(3) Waste heat recovery from cement furnaces.
(4) Waste heat recovery from various ceramic firing furnaces and tunnel kilns.
6. In the metallurgical industry:
(1) Waste heat recovery from continuous heating and soaking furnaces in steel production.
(2) Waste heat recovery from billet heating furnaces.
(3) Waste heat recovery from wire annealing furnaces.
(4) Waste heat recovery from sintering machines. For example, an 180 m² sintering machine can recover 10–22 tons of steam per hour.

II. Waste heat recovery technologies and methods:
1. Separated heat pipe waste heat recovery technology.
2. Integrated heat pipe waste heat recovery technology.
3. Waste heat recovery methods: Different designs and approaches for waste heat recovery can yield significantly different results, meaning the amount of waste heat or steam recovered can vary greatly.

Working principle of heat pipe waste heat recovery devices: Superconducting heat pipes are the main heat transfer elements in such devices, and they differ fundamentally from ordinary heat exchangers. The heat exchange efficiency of heat pipe waste heat recovery devices can exceed 98%, a level that ordinary heat exchangers cannot achieve. These devices are also smaller in size, accounting for only 1/3 of those of ordinary heat exchangers. Their working principle is as follows: The left side serves as the flue gas passage, while the right side is for clean air (water or other media). A partition separates the two sections to prevent interference. High-temperature flue gas flows through the left passage; as it does so, it contacts the heat pipes. When the temperature of the flue gas exceeds 30°C, the heat pipes become active and transfer heat to the right side. At this point, the left side of the heat pipes absorbs heat, causing the temperature of the high-temperature flue gas to drop. The heat is then absorbed by the heat pipes and transferred to the right side. Clean air (water or other media) at normal temperature flows through the right passage under the action of a blower, thereby contacting the heat pipes. This causes the right side of the heat pipes to release heat, heating the clean air (water or other media). As the air flows through the heat pipes, its temperature rises. Waste heat recovery devices composed of multiple heat pipes are installed at the flue outlets of boilers or furnaces, where they absorb heat from the flue gas and transfer it rapidly to the other end, reducing the flue gas temperature close to the dew point and thus minimizing heat loss. The heated clean air can be used to dry materials or supplied back into boilers and furnaces for reuse, thereby improving the thermal efficiency of these systems and reducing fuel consumption, achieving energy savings.

The application of heat pipe heat exchangers in flue gas waste heat recovery mainly focuses on recovering waste heat at medium and low temperatures. Further research is needed to improve high-temperature heat pipes, reduce their costs, and enhance their reliability, so as to expand their use in high-temperature waste heat recovery scenarios and increase their competitiveness compared to other types of efficient heat exchangers. This article is excerpted from Borne Heat Exchanger Mall: http://www.360bhe.cn/news/1199.html