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Boiler Economizer |HVAC Economizers

The heat carried away through the flue gases from a boiler is very high. This flue gases thermal energy utilized to heat the feed water. Feedwater temperature increases with heat transfer from flue gases. “Therefore, a device use to heat the feed water with the help of exhaust flue gases heat before leaving the chimney called Economizer.”

Boiler Economizer also defines as:

“An economizer is a feedwater heater which receives heat from the flue gases and transfers this heat to feed water before entering the boiler.” It is a heat recovery device. Through increasing feed water temperature with the help of an economizer, the amount of fuel required to produce steam from water also reduces. Therefore, it reduces fuel consumption and improves the economy of the boiler.

By raising the temperature of feed water to about 60C, boiler efficiency may increase by 1%. It uses in power plant industry and Heating Ventilation and Air Conditioning (HVAC) systems.

Boiler Economizers:

In a boiler, an economizer installed in the way of exhaust flue gases near the chimney. To understand the working of this device, we will discuss this topic with the help of a diagram. An economizer consists of a large number of vertical tubes. These vertical tubes made with cast iron or steel. The size of the tubes varies with the capacity of the boiler. Generally,  2.75m in length, 11.4cm internal diameter and 11.5mm thick tubes used for economizers. Boiler feedwater enters the economizer from the right bottom of the below diagram. Flue gas passes outside the tubes and water flowing inside the tubes. This feed water moves upward and gains heat from the exhaust flue gases. After gaining heat from flue gases, the temperature raises significantly. In the diagram, clearly we can see at the top left, feedwater leaving after gaining the heat.

Boiler Economizer Diagram

Blow off cock also installed at the opposite side of the feedwater inlet. Blow off cock installed to remove mud or sediment depositions from the bottom boxes. The safety valve also installed at the top right side. The deposition of soot in the vertical tubes reduces economizer efficiency. Therefore, it necessary to removes soot from vertical tubes. For this purpose, scraper provides with each tube. Scrapers of two adjoining tubes grouped together and coupled with rods and chains to the adjacent group of scrapers. The chain passes over the pulley. With the help of chain and pulley combination, the group of scrapers balances the adjacent group of scrapers. The pulley further connected with the worm wheel. This worm wheel drives with the worm on the longitudinal shaft. After reaching the scrapers on the bottom or top of the vertical tubes, scrapers reverse automatically. These kept in continuous motion during the economizer in use. The speed of scraper is about 46m/h.

The temperature of feed water should not less than 350C. Otherwise, corrosion occurs in the vertical tubes due to moisture present in the flue gases. An economizer is more useful in thermal power plants where fuel consumption is very high.

Types of Economizers:

Following are the main types of economizers:

  • Steaming
  • Non-steaming
  • Straight tube or bent tube
  • Independent or integral

Steaming Economizers used in large steam power plants. In this type, almost 5 to 7% feed water changes into steam during passage through the economizer.

Non-steaming Economizer heats the feed water to within 75% of the saturation temperature in the boiler

Straight tube or bent tube economizer depends on the design and working conditions

Independent or integral economizers installed outside the boiler house at a suitable place


  • An efficient economizer should have the following properties:
  • The minimum heat transfer surface
  • Flue gases to feed water heat transfer surface maximum
  • Have uniform feedwater flow to avoid the steam formation
  • Flue gas side and waterside pressure drop should be minimum
  • Prevent dust collection and easily cleanable
  • The vertical tube has sufficient margin to expand and contract without any strain on joints


  • 15 to 20% fuel saving
  • It increases the boiler steam raising capacity because it reduces the conversion of water into steam time in the boiler
  • Prevent formation of scales in the boiler water tubes, because the maximum deposition of scaling due to feed water occurs in the economizer
  • Since the feedwater entering the boiler is hot. Therefore, strains due to unequal expansion minimized

HVAC Economizers in Rooftop:

Nowadays Economizer plays an important role in Heating Ventilation and Air Conditioning (HVAC) cycle. To maintain building temperature, we need proper ventilation and cooling/ heating systems. For this purpose air conditions and air handling units are used. These units have heavy electric loads and increase the operating cost of HVAC units. If you are interested to save energy in terms of electricity then the HVAC economizer is much suitable for this purpose.

“HVAC economizer is a device that uses cool air from outside of a building to cool or maintain the temperature inside the building with maximum energy saving.”

How Economizer Save Energy:

Almost 60% of electricity (energy) saved with the help of this device. This device is not suitable for those countries that have a hot and humid climate. It is used as an airside type or waterside type. In the airside type, it uses fresh cool air from outside the building to cool the building inside. On the other hand, natural cooled air used to cool the water (cooling tower). HVAC economizer consists of return air dampers, supply air dampers, motorized actuators, heating and cooling coils, temperature sensors and logic controls. The economizer reduces the compressor operating time. Logic controller and outside temperature sensor decide the compressor and economizer operation. Motorized actuators receive the signal from logic control to open and shut the outside air dampers. When the outside conditions suitable (temperature and humidity in the required limit), outside air damper open. The logic controller also shuts the air conditioning compressor and reduces electric charges. If the outside temperature is too hot and did not provide cooling then the logic controller opens the return air damper and also starts the compressor.

Therefore, controlling of compressor operating time reduces the electric consumption and provides major energy-saving factor in HVAC. In winter season when outside air humidity high, outside air damper, close and allow only 10 to 20% fresh airs depending on the building ventilation.

HVAC Economizer Diagram
HVAC Economizer


The installation of the HVAC economizer in the rooftop is very easy. These devices have moderate sizes and no need for additional structural or mechanical work. So, if you are still using the HVAC unit without an economizer, then install this device. Energy-saving and money-saving both are the major advantages of this device.

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