Energy recovery from exhaust gases using Heat Pipe technology Energy recovery from industrial flue and exhaust gases can provide large amounts of valuable energy savings, together with significant reductions in CO 2 and Carbon emissions. However, in the past this opportunity has been largely unexploited due to technological barriers and the lack of a simple, reliable solution capable of recovering this wasted heat, which would provide a realistic return on investment. Industrial exhaust gases can be dirty and corrosive, and can contain particles that are abrasive or will build up and coat the surfaces of an exhaust gas system. In this type of application traditional heat exchangers can easily become clogged or damaged due to the high thermal stresses generated. Breakthrough developments in the design of heat pipes and the techniques used to manufacture them mean that heat pipes can now be used as the core heat exchange component for exhaust gas energy recovery heat exchangers in industrial applications. The use of heat pipes will overcome the problems previously associated with traditional heat exchanger designs. Long Service Life Robust construction, manufactured from a variety of sturdy materials to suite the application. Flexible heat exchanger design allows for the free thermal expansion and contraction of individual components. The heat pipes operate at a constant and uniform temperature across their length. This avoids thermal stress and avoids cold spots where gas condensation could occur. Flexibility The simple, sturdy heat pipe based heat exchanger construction allows units to be applied in difficult or even hostile environments, where traditional heat exchanger designs could not be considered. The ability to add or remove individual heat pipes from a heat exchanger allows for precise optimisation of a system. Reliability Each heat pipe within a heat exchanger operates independently of the others. Therefore if there is a fault with an individual pipe, it will have little effect on the overall performance of the heat exchanger. The faulty pipe can just be replaced during the next scheduled maintenance period. This unit is saving 2.4mW per year from recovered exhaust heat Low Operating Costs The payback period on an initial investment is often very short. And the solution will continue to reduce energy costs and emissions long after the initial investment is recovered. Pressure drop through the heat exchanger is low, so extra fan power is rarely required. Individual heat pipes in a heat exchanger are very easy to access and clean. If required, heat exchangers can be designed with self-cleaning mechanisms.
heat pipe heat exchanger What is a heat pipe? Construction A heat pipe is a metal tube, sealed at both ends with a vacuum inside, filled with a small quantity of fluid. The fluid used depends upon the required working temperature range of the heat pipe. The working temperature range with a water fill is 80ºC to 320ºC which will meet over 90% of application requirements. Other temperature ranges can be addressed by a variety of other liquid fills upon request. Heated fluid out Separator plate Exhaust in Heat pipes Exhaust out Heat pipe area in fluid Seal Cool fluid in The metal used for the heat pipe construction depends upon the application needs. In industrial applications the most common materials used are: Copper Carbon steel Stainless steel (AISI 304 and AISI 316) Aluminium Heat pipe heat exchanger construction Heat pipe area in exhaust Many other construction materials are also available to meet the needs of a wide range of applications. Thermal features As a result of the high quality construction of the heat pipe and in particular its high integrity vacuum, heat transfer between the hot stream and the cold stream is virtually instantaneous. Air at 60 C For example, where water is used as the working fluid, it will boil at 0ºC and will transfer energy very rapidly to the cold end of the heat pipe. Another very important thermal characteristic of the heat pipe is its isothermal operation; operating with a uniform temperature along its whole length: Very little thermal stress along the length of the pipe. Separation plate 320 C Isothermal temperature along heat pipe 190 C No cold spots occur where condensation could take place, eliminating the risk of any corrosion. Heat pipe Isothermal operation
Operation The lower end of the heat pipe is installed in a hot stream and the top end is fitted into a colder stream. The hot and the cold streams are separated by a separation plate, into which the heat pipe is fitted, with a seal between the hot and the cold sides. The part of the heat pipe immersed in the hot stream absorbs heat, causing the liquid inside the pipe to evaporate. The evaporated liquid (steam) then travels to the top of the heat pipe. When this steam reaches the top of the heat pipe it gives up its enthalpy of evaporation to the cold stream, heating up the cold stream and causing the steam to condense back into a liquid. The liquid then flows back to the bottom of the heat pipe and the cycle will continue as long as there is a temperature difference between the hot stream and the cold stream. Due to the difference in density of the working fluid in its liquid and vapour phases a natural circulation cycle operates inside the heat pipe. It is not necessary for the heat pipe to be mounted vertically for this cycle to operate effectively. It will operate perfectly, even when the heat pipe is installed as little as 4º from the horizontal. Heat pipe heat exchangers The mechanical and thermal properties of heat pipes with their many advantages make them particularly suitable as the heat conductors in heat exchangers suitable for energy recovery applications that can t be addressed by traditional heat exchanger designs. In particular, energy recovery from high temperature, corrosive or contaminated exhaust streams. Heat pipe based energy recovery heat exchangers can be designed for most heating applications, with the majority addressing one of the following: Exhaust Gas Air Heating Exhaust Gas Water Heating Exhaust Gas Steam Generation Many application demands can be met from a modularised standard range of heat pipe heat exchangers, where standard cartridges of heat pipes are installed into a standard heat exchanger casing, suitable for a particular application. Where it is necessary, individual designs can be made to suite the individual needs of a particular application. Exhaust gas energy recovery installations will usually have one or more of three elements. Vacuum tube Heat out Separation plate Condensation Steam Heat exchanger This where the energy is extracted from the hot exhaust gas and transferred to the cold stream (water, air etc.) to heat it up. Heat in Heat pipe operation
spiraxsarco.com Diverter System A system of by-pass duct work around the heat exchanger together with diverter valves. To facilitate maintenance or cleaning of the heat pipe heat exchanger. As part of the overall thermal design of the system where some of the exhaust gas flow is diverted around the heat exchanger during normal operation. To divert exhaust flow around the heat exchanger in the event of there being a problem with the heat exchanger. This will ensure that a process can continue to operate safely. Control systems Controls are required to ensure the heated fluid or gas is heated to the required temperature. They will be required to ensure the system operates safely and will also shut down safely if there is a problem. If a diverter system is incorporated into the system design, control logic will be required to operate the diverter valves, ensuring they open and close when needed and in the correct sequence. The control system can also incorporate Alarms Communications Self-diagnostics Heat pipe cartridge Exhaust Heat exchanger casing Standard modularised gas to liquid HPHE unit
heat pipe heat exchanger Case studies The following are examples of where heat pipe based energy recovery systems have been used to recover energy. Customer Heat source Application Energy recovered Payback period Steel casting company in the Czech Republic 450ºC from melting furnaces Heating HVAC services water 560kW 6 months Ceramic tile manufacturing company in India 316ºC from tile kilns Pre-heating the air entering the tile kilns 610 kw 16 months Automotive parts manufacturer in the USA 400ºC from an aluminium furnace Pre-heating the air entering the aluminium furnace 530kW 16 months Portable power provider in Botswana 325ºC from heavy fuel oil boilers Pre-heating heavy fuel oil 120kW 3 months Oil & gas well head thermal oxidiser manufacturer in Canada Exhaust gas at 350ºC from a diesel oil fuelled burner Pre-heating burner combustion air 1,840kW 5 months
spiraxsarco.com Applications Heat pipe based energy recovery systems can be used wherever there is a source of heat available in the exhaust from a combustion process, and there is a use for the energy recovered. They are particularly suited to difficult applications where traditional designs of heat exchanger would not be suitable. Industry Typical heat sources Applications Metals Food Petrochemical Construction materials Power Melting and holding furnaces Smelters Sintering machines Cooking ovens Vacuum pumps Incinerators Crackers Thermal oxidisers Fertiliser plants Cement plants Glass furnaces Ceramics furnaces Brick works Turbines Boilers Diesel generators Absorption cooling Absorption cooling Pre-heat boiler feed water Pre-heat fuel oils Contaminated exhaust streams Heat pipe heat exchangers are really the only choice when an exhaust contains particulates or suspensions that could settle onto the heat exchange surface. The construction of the heat exchanger allows the heat pipe surfaces to be cleaned easily and quickly with very little system down-time. The heat pipes can be constructed from a wide variety of materials, resistant to the majority of corrosive elements often found in industrial exhaust streams Pressure drop Compared to other heat recovery solutions available, heat pipe based solutions have extremely low pressure drops across them. This is particularly significant where size or weight is a consideration, as this low pressure drop is achieved with systems that are also much smaller and lighter than traditional systems. Waste processing Incinerators Built in redundancy Within the heat exchanger each heat pipe is an individual heat exchange unit. The heat pipes are very Institutions Boilers Pre-heat boiler feed water Absorption cooling robust, designed to operate in aggressive industrial environments. However, even if one or two heat pipes fail, this will have a very small effect on the overall performance of the heat exchanger. Increased reliability Heat pipes do not rely upon thin metal surfaces for effective heat transfer and therefore can be constructed from robust materials that offer increased resistance to erosion. In the heat exchanger, heat pipes are free to expand and contract within the heat exchanger casing, causing minimal thermal stresses in the overall construction. The isothermal operation of the heat pipe ensures that cold condensation spots do not form, eliminating the possibility of low temperature corrosion. SB-P211-01 CH Issue 1