Generating more with less ! Duct design optimization for efficient power plant

Chaitanya Rane | Monday, December 18, 2017 | Comments

Efficiency is money | Every Power Industry in the World !

Every power generating industry in the world deals with conversion of Energy from one form to another. Efficiency plays a vital role in this process. It reflects directly on the cost of power production and thus an efficient system is what every power industry looks forward to.

Economisers

Valve trim shape and characteristics

Layout of Boiler, Economiser, Flue Gas Duct and other Components

Economisers are one such component that are instrumental in improving Thermal Power Plant Efficiency. Economisers use the waste flue gases from the boiler furnace to preheat the water entering into the boiler. Ducts are used to transport the flue gases from the furnace to the economiser. There is a rise in pressure drop across such ducts which affects the air flow into the economiser. This in turn reflects on the efficiency of the power plant.

So how do we solve this issue ?

The major cause of pressure losses in ducts and pipes are sudden changes in direction of flow and friction losses due to the surface of the duct. I made an attempt here to bring a gradual change in direction by using fillets and reducing the length of the duct by reducing the bend angle. Lets see how that worked out for me !

A Square Duct is used to connect the Boiler and the Economiser. The installation constraints and the position of the components demand the use of a duct with minimum 2 bends. Pressure losses are clearly evident due to the presence of these bends. The experiment shall aim at reducing this pressure loss and ensure the flow is uniform as it enters the Economiser.

Valve trim shape and characteristics

Flow across a Straight Duct of 8.5m length

*Note: All Pressure values are Gauge Pressures in Pascal.

The air enters the duct with a mass flow rate of 1.4 kg/s. A Static pressure of 0 Pa is considered at the outlet. The pressure loss across a straight duct of 8.5m shall be considered as the reference value.

A comparison will be made between the pressure losses across the design modification and the reference. An increase in pressure drop within 20% of the reference value shall be considered as a good design. I have used Generic Simulation App by simulationHub for the CFD analysis and calculation of pressure drop across the ducts.

Design Modifications

Valve trim shape and characteristics

Generic Simulation to my Rescue !

For my experiment I had to simulate the air flow in all the designs mentioned above. A standard CFD Simulation has to go through CAD Modelling, Fluid Domain Extraction, Meshing, Simulation Run and Post Processing ! Performing these tasks for all the 5 Designs can be very tedious.

No Worries ! Generic Simulation by simulationHub comes to my rescue here. A task which should have taken days is now merely completed in hours. Generic Simulation makes it very designer friendly and takes the complexity of a CFD Simulation out of my experiment. To be honest, it has taken me longer to write this blog than it took to complete the simulations ! Within hours I was ready with the results and could analyse the problem at hand.

Time for the Results !

Flow across the Duct

The Velocity Contours show that there is a significant flow seperation in case of the duct with 900 bends.

This flow seperation is reduced when the angle of bend is dropped to 63.400.

As the bend angle is dropped the total length of the duct also reduces from 8.5m to 7m.

Also the variation of velocity across the length of the duct has dropped

The Velocity Contours show that there is a significant flow seperation in case of the duct with 900 bends.

This flow seperation is reduced when the angle of bend is dropped to 63.400.

As the bend angle is dropped the total length of the duct also reduces from 8.5m to 7m.

Also the variation of velocity across the length of the duct has dropped

With the introduction of fillets , the flow seperation becomes negligible and there is less recirculation of flow.

As we increase the fillet radius from 0.5m to 0.75m the total length of the duct drops from 6.84m to 6.77m.

The velocity variation further drops making the flow more uniform along the length of the Duct.

With less recirculation and more uniform flow along the length of the duct shall prevent loss of energy.

With the introduction of fillets , the flow seperation becomes negligible and there is less recirculation of flow.

As we increase the fillet radius from 0.5m to 0.75m the total length of the duct drops from 6.84m to 6.77m.

The velocity variation further drops making the flow more uniform along the length of the Duct.

With less recirculation and more uniform flow along the length of the duct shall prevent loss of energy.

Flow at the Entrance of the Economiser

The Velocity Contour at the Entrance of the Economiser (Duct Outlet) shows a distinct variation in flow pattern.

The duct with 900 bends has its flow concentrated only in the upper region.

As we reduce the bend to 63.400, the flow is seen well distributed in the upper half of the duct cross section.

But there is lack of flow in the lower region of the duct outlet.

The Velocity Contour at the Entrance of the Economiser (Duct Outlet) shows a distinct variation in flow pattern.

The duct with 900 bends has its flow concentrated only in the upper region.

As we reduce the bend to 63.400, the flow is seen well distributed in the upper half of the duct cross section.

But there is lack of flow in the lower region of the duct outlet.

With the introduction of fillets, the flow is seen uniformly distributed across the entire cross-section of the duct.

As the flow recirculation is reduced significantly, sufficient flow is seen across the duct outlet.

As we increase the fillet radius from 0.5m to 0.75m, the flow covers lower region of the duct as well.

This ensures uniform air flow to the Economiser.

With the introduction of fillets, the flow is seen uniformly distributed across the entire cross-section of the duct.

As the flow recirculation is reduced significantly, sufficient flow is seen across the duct outlet.

As we increase the fillet radius from 0.5m to 0.75m, the flow covers lower region of the duct as well.

This ensures uniform air flow to the Economiser.

The above comparisons show us that there is a qualitative improvement in the flow with every design modification. A more uniform flow shall enhance the efficiency of the economiser.

Pressure Drop Across the Ducts

Valve trim shape and characteristics

Flow at the Entrance of the Economiser

The Duct with 900 bends has a pressure drop of 37.2929 Pa which is 964.5 % more than that of a straight pipe. Such a design will result in a substantial energy loss which is not suitable for the Economiser.

With the reduction of bend angle and use of fillets a significant decline in pressure drop is seen. Design D with 63.40 bend angle and fillet radius of 0.75m, has 4.1458 Pa pressure drop which is only 18.33 % more than that of a straight pipe.

The Duct with 900 bends has a pressure drop of 37.2929 Pa which is 964.5 % more than that of a straight pipe. Such a design will result in a substantial energy loss which is not suitable for the Economiser.

With the reduction of bend angle and use of fillets a significant decline in pressure drop is seen. Design D with 63.40 bend angle and fillet radius of 0.75m, has 4.1458 Pa pressure drop which is only 18.33 % more than that of a straight pipe.

The reduction in length and gradual change in direction certainly improves the the Pressure Drop across the duct. With minimum energy lost in the duct and an uniform air flow into the economiser will contribute to make the system more efficient.

This will reflect in the cost of energy production and shall be beneficial to the power plant.

Chaitanya Rane - simulationHub Team

Chaitanya Rane

Chaitanya is a Jr. Member of the Technical Staff at simulationHub. He is interested in the fields physics and mathematics and is exploring the domains like fluid mechanics, computational fluid dynamics, and aerodynamics. He has worked on projects including automobile intake-exhaust system design and vehicular chassis optimization. He is also an enthusiast for exploring and unveiling the depths of fields like CFD, FEA, computational mechanics and upcoming ventures like Data Science and AI. He holds a B.E in Mechanical Engineering from the University of Pune.

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