How much is the typical coal fired boiler efficiency?
Probably the best operating boiler can achieve ~80% or +/-2%.
Historically, boiler efficiency has been limited due to the minimum temperature allowed for the auxiliary equipment. Heat lost up the stack was in exchange for keeping the flue gas temperature above the water vapor dew point to protect the air heater or economizer from acid corrosion. If water vapor was allowed to condense out, rapid deterioration, due to acid corrosion, of the outlet duct and stack would also occur.
The contribution of dry flue gas is ~9-12% & ~5-8% due to presence of humidity & water in combustion products. The reason is that the flue gas temperature (generally 170 - 200°C) is limited by dew point to avoid condensation of downstream exchangers.
With the development of CHX™ condensing heat exchangers, boiler efficiency can now approach 90%. Approximately 1% gain in boiler efficiency can be expected for every 20° C reduction in flue gas stack temperature. Therefore if we can lower it to ~90°C or so, we can practically recover more heat with total thermal efficiency increase from 80% to ~88% or so.
In the CHX™ condensing heat exchanger, all gas wetted surfaces are covered with DuPont Teflon~. The Teflon covered heat exchanger surfaces are impervious to all acids normally resulting from the combustion of fossil fuels. This allows the flue gas to be cooled to below the water vapor dew point with no subsequent corrosion of the heat exchanger surfaces. If this heat is not recovered it will account for a boiler's second largest thermodynamic loss.
CHX™ condensing heat exchangers use a single gas pass to remove both sensible and latent heat from the flue gas. The flue gas enters the Teflon covered heat exchanger through a carbon steel inlet plenum at the top and flows downward across the horizontal banks of heat exchanger tubes and exits the heat exchanger through an FRP outlet plenum on the bottom of the heat exchanger. The cold water flows through the heat exchanger tubing. For optimum heat recovery, the heat sink fluid flows countercurrent to the flue gas.
As the flue gas temperature reaches the water vapor dew point at the tube surfaces, condensation occurs. Due to the hydrophobic nature of Teflon, droplets of condensate form and fall as a constant rain over the tube array. This provides two important advantages. It enhances the latent heat transfer and at the same time keeps the tube surface clean.
The modules are manufactured in a number of different sizes. The variety of module sizes and the modular construction allow the CHX condensing heat exchanger design to be optimized for each application.
The CHX™ tubing, in water cooled applications, is 1.125 in. O.D. alloy C70600 covered with a 0.015 in. thick extruded layer of FEP Teflon. The inside surfaces of the heat exchanger shell are covered with a 0.060 in. thick sheet of PTFE Teflon. During fabrication, the tubes are pushed through extruded tube seals in the Teflon covered tube sheet to form a resilient Teflon to Teflon seal. This ensures that all heat exchanger surfaces exposed to the flue gas are protected against acid corrosion. Tube connections are made outside of the heat exchanger shell.
To protect the Teflon, maximum flue gas inlet temperature is limited to 260° C. The maximum water inlet pressure and the maximum water outlet temperature are 150 psig and 120° F respectively. CHX™ heat exchangers are installed as passive systems in order to assure that these limitations will always be met.
The CHX heat exchanger can also be used to heat air. The materials of construction and the maximum operating parameters vary somewhat from above.
The most common application for a CHX™ condensing heat exchangers is the recovery of waste heat to preheat boiler make-up water. Preheating make-up water can increase boiler efficiency 3-5% or more. The heat recovered by a CHX™ condensing heat exchanger can offset much of the extraction steam required by a low pressure feed water heater or deaerator. This offset will reduce fuel consumption while maintaining a fixed net steam output, or when required, it can increase the net steam output by maintaining the same fixed fuel consumption.
Heating make-up water is not the only heat recovery application for a CHX™ condensing heat exchanger. CHX™ units can have a number of other uses in the plant environment. Applications range from building heat to heating process streams in food processing chemical plants, and various pulp and paper applications.
In one actual installation, a midsized industrial plant has been saving an average of $1,000 per day for the past 10 years in energy costs by heating process water with boiler flue gas. The passively installed system utilizes 160,000 tph of 333° F glue gas to heat 550 gpm of process water from 90° F to 136 ° F. The flue gas is cooled to 125 °F. The additional heat recovery has in effect increased the capacity of the plant without requiring the purchase of another boiler. This CHX™ heat recovery system paid for itself in less than 25 months.
CHX™ units can also heat water or process streams indirectly. When a process steam is incompatible with the CHX™ unit design, water or other liquid heat sinks can be circulated in a closed loop through a CHX™ condensing heat exchanger. A closed loop system can be used to heat process streams that are abrasive, corrosive or have a pressure higher than the CHX™ unit design pressure. A closed loop system can also be used for flue gas reheat or in some cases to cool flue gas to a lower temperature where required.
For the past 16 years CHX™ condensing heat exchangers have successfully demonstrated their ability to operate below the acid and water vapor dew point to recover low level heat from fossil fueled boilers, HRSG'S and process dryers. While a majority of CHX™ heat exchanger installations have been retrofit applications, there have been several cases where they were included in the heat balance design for new construction or plant expansion.
Based on our experience, the most efficient use for CHX™ condensing exchangers in the future will be for new construction or plant expansion when the customer and their A&E company engineers recognize in the design phase that there will be a continuous requirement to heat a large volume of cold water for a specific use. When the condensing heat exchanger is an integral part of the total project heat balance design it provides the opportunity to maximize the use of the heat recovered to the benefit of the total system heat balance. Another advantage is that the installation cost is typically lower than the cost to interface with existing equipment in a retrofit application.
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