What is An Economizer? | How does An Economizer Work?

The economizer is one of the most important parts of the boiler, power plant, and HVAC system. An economizer is a heat exchanger that transfers heat from the flue gases to another mode (usually the boiler feed water), although other flows such as make-up water can also be used. It improves the efficiency and performance of the system. This article mainly explains the economizer function, working, parts, and applications.

What is an Economizer?

An economiser (UK spelling) or economizer (US spelling) is a mechanical device designed to decrease the consumption of energy by preheating the fluid. In simple words, an economizer is part of the boiler that uses waste heat to heat the incoming water.


The economizers are used for different applications such as engines, power plants, HVAC, and boilers. In short, an economizer works as a heat exchanger.

The economizers in the boilers are used to increase the energy efficiency of the system.

Thermal power plant economizers are utilized to economize the electricity generation process.

The recovered energy is utilized to preheat the feed water of the boiler which is finally converted into superheated steam. This means it basically collects the waste heat and applies it where you need it, saving a lot of fuel consumption and economize the processes.

Working of Boiler Economizer

The boiler economizer works on the heat transfer principle. In general, there is a heat transfer from high temperature to low temperature.

function of economizer

In the boiler, the water to be preheated is cold, and the exhaust or flue gases are hot at the boiler outlet. Therefore, this temperature difference between the flue gases and the water contributes to increasing the temperature of the feed water.

According to the mode of operation, the economizer may be intended either as a water tube type or a smoke tube type.

In the water tube boiler, the water flows inside the tubes, and flue gas moves outside the tubes. However, in the smoke tube type, the water flows outside the tubes while the flue gases flow inside the tube.

Types of Economizers

The economizer has the following major types:

  1. Non-condensing Economizer
  2. Condensing Economiser

1) Non-condensing economizer

These types of economizers are used to keep the flue gas temperature higher than the condensing temperature of the flue gases to stop corrosion of the flue gas ducting. This is especially vital for fuels that contain sulfur.

Sulfur compounds can form in the flue gas, which increases the overall dew point of the flue gas and can form highly corrosive sulfuric acid on condensation.

non condensing economizer

These are the most commonly used economizers. The non-condensing economizer is a heat exchanger coil (usually finned) installed in the flue tube at the boiler exit. They have the ability to improve the overall efficiency by 2-4% of the system.

Advantages of Non-Condensing Economizer

  1. It has the capability to decrease the fuel consumption requirements of the boiler by shifting heat from the waste flue gas to the boiler feedwater.
  2. It improves overall efficiency by 2-4%.
  3. They are intended to keep the flue gas temperature higher than the condensing temperature of the flue gases to stop corrosion of the flue gas ducting.

2) Condensing Economizer

These economizers help to improve the recovery process of the waste heat by cooling the flue gases lower than the dew point (approximately 135°F) of natural gas combustion products. The condensing economizer recovers both latent heat from the flue gas by condensing water vapor and sensible heat from the flue gases.

Consequently, condensing economizers must have high resistance to acid corrosion, and if the heat exchanger parts are not kept clean, they will become less efficient over time. The Emvertec CEECON series of the economizer prevents this by utilizing an anti-corrosion structure and an automatic wash lance cleaning system in the biomass plant.

Advantages of Condensing Economiser

  1. The condensing economizer improves the thermal efficiency of the power plant.
  2. They improve the electrical output of the producing plant.
  3. These economizers decrease the fuel consumption of the plant.
  4. They have the ability to transfer heat for external processes.
  5. They decrease the moisture content of exhaust gas leaving the stack.
  6. Emvertec’s CEECON series condensing economizer goes one step further by ‘polishing’ flue gases to reduce the plant’s environmental footprint and further reduce pollutant levels after traditional flue gas pollutant removal systems. It protects the downstream system from acid corrosion and provides high-quality heat.

What is the Efficiency of the Boiler Economizer?

The position of the economizer in the chimney highly matters when deciding whether an economizer is suitable for a boiler system.

In order to ensure maximum heat recovery during the process, you must install the economizer as close as possible to the furnace breach. This arrangement will assist in preventing heat loss and cooling.

The table given below shows the boiler efficiency comparison:

Combustion Efficiency at 4% Excess OxygenStack Gas Temperature
 Boiler 78% to 83%350F to 355F
 Boiler with Condensing Economizer 92% to 95% 80F to 150F
 Boiler with Standard Economizer 84% to 86% 250F to 300F

How to improve the efficiency and ventilation of the HVAC economizer?

The economizer in the HVAC system is mainly employed to help the system switch to “free-cooling” mode as frequently as possible. This is a cost-saving mode. This mode pumps cool outdoor air into the room with little or no mechanical interference to reduce refrigerant and energy consumption costs.

For example, you work in an office building. The outdoor temperature of the office building is 57°F, but your office may get a slight sunburn due to the sun shining through the office windows, and 46 colleagues nearby are in the range of about 101°F. Instead of turning down the thermostat, open the windows and let in fresh air to cool the office room. This is a good idea behind Free Cooling.

To permit the outdoor air, a typical economizer is fitted with some sort of damper that opens as free cooling is possible; otherwise, it stays closed. You may control these dampers through various methods. The most common method is barometric, which opens the damper when the pressure difference between the outer and inner sides is within a specific range.

The second advantage of free cooling in the commercial HVAC system is shorter run times, which extend the life of the equipment.

Read More: Latest HAVC maintenance cost

Difference Between Condensing Economizer and non-condensing Economizer

  1. The non-condensing economizer is employed to keep the flue gas temperature higher than the condensing temperature of the flue gases to stop corrosion of the flue gas ducting. The condensing economizers help to improve the recovery process of the waste heat by cooling the flue gases lower than the dew point (approximately 135°F) of natural gas combustion products.
  2. The non-condensing boilers have higher operating temperatures than the condensing boilers.
  3. The combustion air for the condensing burner is taken directly from the outside, unlike non-condensing systems that take air from inside the building.
  4. The condensing boiler is more efficient (at least 25%) than the non-condensing boiler. The condensing economizer achieves the desired efficiency by preheating the cold water entering the boiler with the waste heat from the flue gases, extracting more heat from the outside than from the inside air.

Applications of Economizer

The economizers are most commonly used for the following applications:

1) Stirling Engine

The economizers are used in Stirling engines. The economizer stores heat from the engine’s hot part as it passes to the cold end and releases it into the cooled air as it returns to the hot end. This invention improved the Stirling engine’s efficiency.

Read More: Types and working of Stirling Engine

2) HVAC System

The HVAC system of a building can save energy in the building by using an air-side economizer and utilizing cold outdoor air to cool the interior air of the room.

As the outdoor air temperature becomes lower than the recirculated air temperature, the cooling with the outdoor air is more energy efficient than the cooling with recirculated air.

If the outdoor air is cool and dry enough (weather dependent), the enthalpy of the air is acceptable, and no further adjustment is required; this part of the air-side economizer control scheme is referred to as free cooling.

3) Steam Power Plant

In the steam power plant, the economizer recovers the exit heat from the boiler flue gases and transfers it to the boiler feed water.

4) Power Plant

The function of the economizer in a thermal power plant is to recover some of the heat stolen from the chimney to heat the boiler feed water.

5) Refrigeration

The economizers are most frequently utilized for industrial refrigeration that requires vapor compression refrigeration. Systems with economizers are designed to do some of the freezing work at high pressure. In this case, the gas compressor has very high efficiency.

Read More: Working of Refrigeration Cycle

Advantages of Economizer

  1. The economizer is employed to recover more heat from the exhaust gases, which is not possible with a normal air preheater.
  2. Due to the high price of the fuel, all power plants want to improve boiler efficiency. Therefore, an economizer is utilized to improve the boiler efficiency and save additional heat.
  3. Idle power plants require large amounts of water to cool flue gases prior to desulfurization. This is lessened by utilizing an economizer.
  4. If the steam-air preheater requires steam, the efficiency of the power plant decreases.

FAQ Section

What is an economizer in a refrigeration system?

An economizer is a heat exchanger that utilizes a small part of the total refrigerant flows from a condenser to lower the temperature of the remaining refrigerant flow. As the liquid refrigerant converts into the vapor form, it transfers into the compressor at a medium pressure level.

Why economiser is used in the boiler?

Boiler economizers are used to increase the efficiency of steam boiler systems. The economizer works as a heat exchanger.

What is the function of the economizer in the refrigeration system?

An economizer is a simple unit with complex controls. It continuously notices the outside air temperature. As long as the outside air is cool enough to replace the cold air generated by the compressor, the economizer automatically transfers outside air into the freezer/fridge.

What is the function of the economizer?

The main function of the economizer is to extract the heat from the exhaust gases and decrease the consumption of energy by preheating the fluid.

What are the types of economizers?

  1. Condensing economizer
  2. Non-condensing economizer

How to calculate boiler economizer efficiency?

The following formula is used to calculate the efficiency of the boiler economizer:

Economizer efficiency = (heat absorbed, Btu/h)/(heat available, Btu/h)

Read More
  1. Working of HVAC System
  2. Working of Refrigeration Cycle
  3. Working and Part of Boilers

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