Why Economizer in Boiler Feedwater Preheating Reduces Fuel Consumption and Lowers Exhaust Temperature

Heat production via combustion in boilers is not fully utilized as the heat carrier leaves the system after it passes along the main heating sections, in particular in the form of hot flue gas, which still has some valuable power. An economizer is one type of device used in a boiler system to recover some of the residual energy from the hot flue gas. The feedwater gets heated from the transferred recovered energy in the preheating unit before the water is directed into the steam generation section.

Through economizer in boiler flue gas heat recovery, the boiler recasts the exhaust flue gas heat at the expense of feed water pre-heating which is a major factor for thermal efficiency as well. This heat recovery in flue gas not only reduces the exhaust gas loss in terms of useful heat but also improves thermal efficiency significantly by lowering exhaust gas temperature.

What Makes The Economizer Work?

There is one major thing that makes the working and design of boiler economizer very simple.

The feed water goes in through the tubes and in between, flue gas which has been heated is flowing over the outside of such tube system. Heat conducts in the process from one side (gas side) to the other side of the tube wall (water side).

As a result, entering water temperature to the boiler is higher.

This economizer in boiler feedwater preheating process reduces the amount of additional energy required to bring the water to steam-generation conditions.

On the other side, economizer in boiler flue gas heat recovery lowers the temperature of exhaust gas before it moves toward the stack.

The two processes happen at the same time, which makes the economizer an important link between the boiler’s water circuit and flue-gas path.

Why Feedwater Preheating Matters

Feedwater temperature directly affects the amount of energy the boiler must supply.

The system is expected to bring in heat to raise the level of relative cold waters that are supplied to the boiler. However, this duty of heating to some extent comes from the recovery of flue-gassing instead of fresh fuel through the economizer in boiler feedwater preheating.

If a boiler has very cold feed water that enters at a very low level from outside then boiler fuel consumption is higher as additional heat is needed to raise the level of the water temperature to boiler temperature. Therefore, economizer in boiler feedwater preheating reduces boiler fuel consumption by preheating part of the heat coming from flue gases rather than fresh fuel. So this approach increases the overall heat balance of the boiler.

Such benefits of economizer in boiler feedwater preheating that are continuously running industrial boilers can accumulate over thousands of hours of operation. In practice, these are some of the variables such as boiler output, feed water conditions, fuel type, and operating load that determine the actual improvement to some extent.

The Role of Flue Gas

Flue gas contains most of the recoverable heat in a boiler system. Therefore, it is one of the prime targets for heat recovery.

Through economizer in boiler flue gas heat recovery application a part of this last energy is captured before the gas leaves the system so the exhaust is not just being treated as it is. In other words, the boiler can reuse some of that energy after all.

But one needs to note that engineers are not to take a shot at recapturing every possible degree of heat just to push heat recovery to the limit.

If a too significant drop in flue-gas temperature is the case, there can be condensation as well as the possibility of corrosion and fouling depending on the type of fuel and composition of the gas. In a nutshell, economizer in boiler flue gas heat recovery is about getting the right heat recovery level that the system can manage safely.

What Factors Influence Economizer Performance?

Heat-transfer tubes are a regular choice for an economizer in a boiler system depending on the required water flow, gas flow, temperature difference and pressure conditions.

Such features as tube diameter, wall thickness, material, layout of the tube in a bundle as well as, of course, heat-transfer area are all the determinants of performance.

One thing that the engineers should make calculations for in case of economizer in boiler feedwater preheating is their water-side heat demand and allowable pressure drop. A system where the resistance is so much that the pumping power increases and the efficiency of recovering heat is reduced is quite a practical way of looking at the benefits of heat recovery.

The other side on the gas, economizer in boiler flue gas heat recovery has to take into account gas velocity, ash content, fouling potential and required frequency of cleaning.

This is the way that Sunwen considers all these factors together with each other when developing economizer solution rather than relying on selecting tube dimensions purely from standard configurations.

Typical Running Challenges

Deposit problems such as fouling is a big threat.

A build-up of particulate or deposit from ash on the gas-side heat transfer surface is quite a regular event.

Covering by deposit layer over a certain part reduces the effectiveness of heat transfer and increases the gas-side resistance as well.

Another problem that comes up related to water is scaling of water.

Tubes within which scale forms will have their economizer in boiler feedwater preheating efficiency affected because the thermal resistance increases between water and the tube wall.

Hence, the importance of maintenance goes hand in hand with the importance of the boiler operation thermal performance. Frequent, well-planned inspections of tubes, and cleaning ensure that economizer in boiler flue gas heat recovery stays at a satisfactory level of operational performance.

Fundamentals for Buyers’ Evaluation

A person or company purchasing an economizer in boiler will be wise to check a list of parameters relating to boiler operations before a decision is made.

These are:

· Boiler capacity and steam output,

· Feed water inlet temperature,

· Feed water outlet temperature that is required (for the sake of design),

· Flue-gas temperature before it enters the economizer,

· type of fuel and ash characteristics,

· Water pressure and flow,

· Gas-side (exhaust) pressure drop,

· Tube material and thickness of wall,

· Frequency/extent of needed cleaning and inspection activities.

These information is necessary for engineers to be able to see whether or not the proposed economizer in boiler feedwater preheating fits the actual boiler and, at the same time, define a economizer in boiler flue gas heat recovery system that allows recovery of significant energy while preventing any major damage that results in excessive pressure drop or corrosion risk.

Why Economizer Design Should Match the Boiler

Different boilers require different economizers. Therefore, there is no one-size-fits-all economizer for boilers.

For example, a coal-fired boiler could present completely different issues of fouling than a gas-fired unit. On the other hand, a high-pressure industrial boiler will probably use different tube specifications compared to a smaller steam system.

Hence, a boiler-specific economizer in boiler feedwater preheating design should be the basis, whereas boiler-specific economizer in boiler flue gas heat recovery design should be considered as the other hand.

At Sunwen, we take the whole heat-transfer system into account during the design phase. As a result, a suitable compromise is made between heat recovery, pressure drop, and material durability, and maintenance as far as possible.

Summary

Main function of Economizer in boiler is recovering useful heat, i.e. heat which is still in flue gas after it leaves the main heating sections, and transferring this recovered heat to feedwater of theboiler. The system increases feedwater temperature preheating through economizer in boiler feedwater preheating before the feedwater is directed into main boiler section. At the other side, through this economizer in boiler flue gas heat recovery, useful heat is reduced which is leaving via exhaust system.As with most things, the final efficiency gain largely depends on factors such as proper tube design, control of temperature, fouling management, material qualities as well as operating conditions. An economizer that is able to combine all these factors properly and in turn convert unused flue-gas energy to useful heat and thus make boilers more efficient is definitely worth the ‍‌cost.