Boiler economizer is a heat exchanger that heats a fluid usually water to a temperature slightly below the boiling point. This heat exchanger is called economizer because it can use the enthalpy of fluid flow which is hot but not enough for boiler use and thus recovers more useful enthalpy and enhances boiler efficiency, so the economizer uses exhaust gas energy from the boiler to pre-heat cold water inlet or boiler feed water.
Economizer or water pre-heater is a thermal exchanger of water tube type that can be made from simple or finned tube. Economizer can increase boiler efficiency up to ten percent by reducing the temperature of the chimney. Overall, 20 ° C reduction in flue temperature of in boiler is equivalent to one percent efficiency increase.
In fire tube boilers, flue temperature is between 180 and 300 ° C, which theoretically can reduce to the dew point using the economizer. Dew point is the temperature at which water vapor in the smoke is condensed and becomes liquid droplets. Dew point temperature depends on the type of fuel. At this temperature, carbon dioxide and sulfur dioxide gases due to combustion are dissolved in water and form acid that will cause very quick corrosion.
Dew point for natural gas is 6 ° C, and for diesel, it is 85 ° C. However, it is recommended that the economizer design take place on the grounds smoke temperature is not reduced below 120 ° C for corrosion protection of the chimney.
The most important point of difference in the application of the control system in the boiler economizer is a water tube and fire tube. In water boilers tube, water flow and smoke have a permanent basis and in economizer is constantly doing water and smoke heat transfer, while in fire tube boilers, burner is controlled with the pressure switches or transmitters, and depending on the steam consumption, for a while it reaches minimum flame or even turns off. Moreover, the amount of water entering the boiler is set with water level inside the boiler and in case of using the level switch, water entering the boiler is interrupted for a few minutes.
Pump and burner work time have no direct proportion meaning that, it is possible that boiler water level is reduced but not the pressure. In this case, the feed pump starts to work and injects water into the boiler, while in economizer no smoke passes, and economizer does not work. If the reverse happens, the pressure drop, but the boiler water level is high, the burner starts to work and makes standing water in the economizer tubes into steam, resulting in increased economizer pressure and valve operates. To solve the above problem, water should flow in economizer tubes when the burner is working, even if the boiler water does not flow. Therefore, if the de-aerator system is installed, the de-aerator tank is used. Water in the de-aerator storage tank is sent to de-aerator by a circulation pump and again returns to the reservoir. Circulation pump starts only when the burner is on. The disadvantage of this design is that the water cannot be heated over 100 ° C since the de-aerator are usually atmospheric. Moreover, if the boiler returning water is composed mostly of condensate return, before entering the economizer, it will have a high temperature, which will minimize economizer performance in practice. The best state is use of a pressurized tank or de-aerator, so that the water can be preheated up to the maximum possible.
To ensure the lack of vapor production in economizer, the smoke entry path must be equipped with a bypass damper circuit, so that if the water in the economizer outlet is close to the saturation temperature, the bypass damper is opened and the smoke exits the contact with economizer tubes. To design heat level of economizer, important points must be noted: first point is boiler load of. As we know when the boiler works with 50% of its nominal load, smoke discharge is also half, but smoke temperature reduces too, so the heat level required in 50 percent of the load would be more than 100 percent.
The second point is the use of finned pipes. Seven fins per inch for gas fuel, five fins per inch for diesel fuel, and without fin for oil are recommended. The third point is attention to the allowable pressure drop of smoke based on burner fan. This means that pressure drop created by economizer should not be such that burner fan cannot overcome it and the torch shut it down. The heat recovery from boiler smoke depends on factors such as design of boiler and smoke emissions temperature, amount of excess air, return water temperature, fuel type, and also the quality of economizer control.
In general, in economics any investment whose return on investment is less than 66 months is known as very attractive, return on investment of less than 18 months as attractive, and for more than two years needs to be assessed. With this definition, we can say that the return on investment for gas-fired boilers over 7 tons is less than 18 months and for oil-fired boilers, it is less than 6 months. Thus, they are seen as attractive and very attractive investment and using economizer for them is recommended.