What Is a Steam Boiler Economizer? Working, Benefits, and Types
This seemingly simple concept can have important implications for the operation and efficiency of a large-scale boiler.
By preheating the feed water, an economizer reduces the demand on the boiler; the energy required to produce a given quantity of steam will be less than if the feed water were cold.
The result can be a reduction in fuel consumption and a boost in boiler efficiency.
Economizers are a common feature on boilers associated with large-scale, continuous steam generation such as power plants, process plants, chemical processing, and other industrial facilities.
In this article, we'll explore "what is a steam boiler economizer", how it works, its benefits, types of economizers, and the implications on boiler efficiency.
What Is a Steam Boiler Economizer?
So, "what is a steam boiler economizer" in simple terms?
As its name suggests, an economizer economizes... That is, it saves resources.
Specifically, it enables the recovery of energy from hot flue gases to preheat boiler feed water.
A typical boiler burns fuel to produce heat.
This thermal energy can be transferred to water to produce steam.
In the process, the hot combustion gases that exit the boiler still have ample energy.
Instead of releasing this waste heat into the atmosphere, consider using an economizer to recover some of this energy.
Hot flue gases flow over tubes containing relatively cold feed water.
The energy in the gases is transferred through the tube walls, heating the water.
Ultimately, the hotter feed water that enters the boiler has less energy demand on the boiler to turn it into steam.
Simple Example
Let's imagine a boiler that normally starts with feed water at 70° C.
An economizer preheats the water to 110° C (or some other temperature, depending on the design).
The boiler doesn't have to work as hard to bring the feed water to whatever temperature is required for steam generation.
The fuel savings opportunity depends on the boiler pressure, feed water temperature, flue gas temperature, economizer design, and boiler operating conditions.
How Does a Steam Boiler Economizer Work?
Let's go over the process that takes place within a boiler and associated economizer.
Step 1: Fuel Is Burned
The boiler burns natural gas, oil, biomass, or other fuel to produce thermal energy.
Step 2: Steam Is Generated
The heat from the fuel is transferred to water, turning it into steam.
Step 3: Hot Flue Gas Leaves the Boiler
The hot combustion gases have transferred much of their energy to the water.
However, they are still hot enough to be useful for preheating boiler feed water.
Step 4: The Economizer Recovers Heat
The hot flue gases are routed so that they flow across the surface of tubes containing feed water.
The energy in the gases is transferred to the water within the tubes.
Step 5: Preheated Feed Water Enters the Boiler
The feed water that has been preheated by the economizer now enters the boiler.
Due to the preheating, the boiler doesn't need to consume as much fuel to raise the water temperature and produce steam.
This process takes place in what is known as a waste heat recovery or flue gas heat recovery system.
Why Is a Boiler Economizer Important?
One important reason to know "what is a steam boiler economizer" is because it makes a boiler much more efficient.
Boilers are not 100% efficient.
That is, not all of the energy in the fuel is transferred to the water as heat.
Some of this thermal energy escapes in the stack gases that exit up the chimney or through an exhaust pipe.
An economizer can recover some of this "waste" heat and put it back into the system to preheat the feed water.
The benefits can be lower fuel consumption, improved boiler efficiency, reduced operating costs, reduced stack temperature, reduced greenhouse gas emissions per unit of steam produced, and improvement in overall energy utilization.
Feedwater preheating by the economizer can be beneficial for other reasons as well, such as improved feedwater conditions.
For boilers with large steam production demands, even a marginal improvement in efficiency can result in significant savings.
Main Components of a Steam Boiler Economizer
Now that we understand "what is a steam boiler economizer", let's review some of its main components.
Heat-Transfer Tubes
The tubes carry the feed water; they also provide the surface area across which heat is transferred from the gases to the water.
Tube material and design depend on temperature, pressure, corrosion potential, and other factors.
Headers
These are the manifolds that connect tubes containing feed water.
They also serve to collect the heated water before sending it back to the boiler.
Flue-Gas Passage
The economizer is located in the path of exhaust gases so that the hot flue gases flow across the surface of the tubes containing feed water.
Support Structure
The economizer needs some structural supports and connections to handle pressure, weight, expansion, and other forces.
Soot-Blowing or Cleaning Equipment
Depending on the fuel source and the conditions, an economizer might need periodic cleaning or soot blowing to keep it operating at peak efficiency.
Types of Steam Boiler Economizers
There are a variety of economizers, each with its own characteristics and applications.
Bare-Tube Economizers
These are economizers in which the tubes do not have any fins or other protrusions.
They are relatively simple and effective for applications with clean fuel and minimal sooting.
Finned-Tube Economizers
They involve the addition of fins or other surface extensions on the tubes.
The increase in surface area improves the heat-transfer capability of the economizer.
However, the device can be made more compact than bare-tube economizers.
Cast-Iron Economizers
Cast-iron economizers are made of cast iron and have been used in a number of boiler applications.
They are not used as often today, due to the availability of other materials and technologies.
Cast-iron economizers find their applications in specific areas, depending on the pressure, temperature, and other operating conditions.
Modern boilers often use steel or other materials.
What Is a Steam Boiler Economizer Used For?
The application of a steam boiler economizer can vary from one industry and boiler to the next.
Economizers tend to be used most frequently in power plants, chemical plants, food processing, pharmaceutical, textile, paper, refinery, large commercial boiler systems, and similar settings.
Any facility that has significant steam demand and has suitable economizer conditions can potentially benefit significantly from the use of an economizer.
Example: Food Processing Plant
A food-processing facility uses a large natural gas boiler to provide steam for its operations.
The exhaust gas from the boiler is at a high temperature.
Rather than simply losing all of that heat up the stack, the facility purchases and installs an economizer.
This allows the transfer of energy from the hot exhaust gases to the feed water, resulting in a reduction in fuel consumption.
The amount of fuel reduction can be significant over the thousands of hours per year that the boiler is in operation.
Economizer Vs. Air Preheater
A natural question related to "what is a steam boiler economizer": what is the difference between an economizer and an air preheater?
Both devices recover heat from the flue gas.
However, an economizer transfers heat to the feed water, whereas an air preheater transfers it to the combustion air that will be delivered to the furnace.
If you are using the hot exhaust gases to preheat water or another fluid that will enter the boiler, then you have an economizer.
If you are using the hot exhaust gases to preheat the air that will be used to combust the fuel in the boiler, then you have an air preheater.
Some large-scale boiler systems use both an air preheater and an economizer.
How Much Can an Economizer Improve Efficiency?
The amount of improvement depends on the type of steam boiler economizer used in the system.
Each boiler has its own set of characteristics, and no single rule of thumb will apply to every situation.
The potential fuel reduction depends on the boiler size, the fuel type, the flue gas temperature, the feed water temperature, the steam pressure, the boiler load, and a number of other considerations.
As a general rule of thumb, if a boiler is large and operates at a high load, an economizer will provide significant benefits.
A small-scale, intermittent-use boiler might yield only a marginal improvement in efficiency.
A complete engineering analysis is necessary to determine if and what type of economizer is appropriate for a given boiler.
What Is a Steam Boiler Economizer and How Does It Save Fuel?
At the most fundamental level, "what is a steam boiler economizer" is a question about fuel savings.
Let's consider a simple example.
Let's say that a boiler needs to produce 10,000 kg of steam per hour.
If the feed water temperature is relatively low, the boiler will have to add a lot of heat to the water to generate the necessary quantity of steam.
Now, imagine that an economizer is installed that uses waste heat from the flue gas to make the feed water hotter before it arrives at the boiler.
The boiler now has to add relatively less heat to the water to generate the same amount of steam.
If the boiler consumes less fuel to generate the same amount of steam, its fuel economy has been improved.
The fuel savings would need to be determined based on actual operating conditions.
It is not enough to make general observations about boiler size or capacity.
Advantages of Installing an Economizer
Now that we understand "what is a steam boiler economizer", let's look at some of the advantages of installing one.
Improved Thermal Efficiency
Recovery of heat normally lost in the stack gases can improve the thermal efficiency of the boiler.
Reduced Fuel Consumption
Since less heat is needed from the fuel to generate a given quantity of steam, fuel consumption can be reduced.
Reduced Operating Costs
With less fuel being consumed, the cost associated with producing the steam can be reduced.
Lower Stack Temperature
The economizer makes it possible to lower the temperature of the exhaust gases that exit the stack.
Reduced Emissions
Since less fuel is being burned to generate a given quantity of steam, the total carbon emissions, as well as other emissions from the combustion process, will be reduced.
Better Use of Waste Heat
The waste heat that would otherwise be lost can be used to increase the feed water temperature.
Potential Challenges and Limitations
Although a boiler economizer can often provide important benefits, the economizer itself has to be designed to handle fuel conditions, water quality, and other aspects of the system.
Corrosion
If the temperature of the flue gases is lower than the acid dew point or water dew point, condensation can occur.
Depending on the fuel, corrosive compounds may form that will degrade the tubes.
Fouling
Ashes and soot can build up on the inner surfaces of the tubes, reducing the heat transfer capability of the economizer.
Pressure Drop
The economizer adds resistance to the water circuit within the boiler.
Excessive pressure drop can have adverse impacts on the performance of the system.
Thermal Stress
Changes in temperature will impose mechanical stresses on the economizer.
Maintenance Requirements
The economizer may require regular inspections and cleaning to ensure that it continues to operate at peak efficiency.
Because of these potential limitations and challenges, economizer design has to consider fuel specifications, flue gas composition, water quality, and other factors.
How to Maintain a Steam Boiler Economizer
Now that we understand "what is a steam boiler economizer" and its advantages, let's briefly review some of the maintenance considerations.
Some examples of maintenance include:
Checking for corrosion in tubes
Checking for leaks
Monitoring the flue gas temperature
Measuring pressure drop
Cleaning fouling deposits
Checking the water chemistry
Inspecting supports
Monitoring heat transfer
A rise in stack temperature can be an indication of reduced heat transfer, which points to a fouling problem or other issue.
Changes in feed water temperature or pressure drop can also indicate a problem with the economizer.
By monitoring the performance of the economizer on a regular basis, plant personnel can identify potential problems before they result in expensive outages and downtime.
Example: Comparing Boiler Operation With and Without an Economizer
Let's imagine two identical boilers that are used to generate the same amount of steam.
The first boiler, Boiler A, has no economizer.
Feed water enters the boiler at a relatively low temperature, and the hot flue gases exit the stack at a high temperature.
The second boiler, Boiler B, has an economizer.
The economizer transfers some of the heat from the flue gases to the feed water.
Rather than allowing this waste heat to escape up the stack, it is used to preheat the feed water entering the boiler.
Both boilers produce the same amount of steam, but Boiler B is able to do so with less fuel.
This is the primary advantage of an economizer: the ability to recover waste heat that would otherwise be lost.
Frequently Asked Questions
What is the main purpose of a boiler economizer?
The main purpose of an economizer is to recover heat from hot flue gases and to use this heat to preheat boiler feed water.
Is an economizer a boiler?
No, an economizer is not a boiler.
An economizer is a heat recovery device.
A boiler normally generates heat by burning fuel, and the heat is then used to generate steam.
An economizer doesn't generate steam; it only transfers heat.
Does an economizer reduce fuel consumption?
Yes.
By using waste heat to preheat the feed water, an economizer reduces the amount of heat that must be generated by the boiler.
Where is an economizer installed?
An economizer is typically installed in the flue gas circuit within the boiler.
It is located downstream of the main heat transfer surfaces and upstream of the stack or other exhaust equipment.
Is an economizer suitable for every boiler?
Depending on the scale of operation, boiler operating hours, fuel type, flue gas temperature, feed water conditions, and other factors, an economizer might not be appropriate for a given boiler.
Conclusion
So, "what is a steam boiler economizer", and why does it matter?
As we've discussed, this relatively simple device enables the recovery of heat normally lost in the exhaust gases.
This means more efficient steam generation using less fuel.
For example, a boiler that runs continuously can use an economizer to make use of the large amount of energy in the exhaust gases.
The fuel savings can be significant, especially when a boiler operates for thousands of hours a year.
Ultimately, it is all about recovering waste heat and putting it back to work.
With proper design, monitoring, water treatment, and maintenance considerations, an economizer can be a very valuable component within a steam-generating system.
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