Why aerodynamic design of filter houses is crucial to gas turbines

gas turbine
Gas turbine. Image by photosoup on 123rf

The aerodynamics of gas turbine (GT) inlet filter houses is often an area that is not fully considered in the initial design and layout of power plants.

Plant designers tend to start with how to fit the equipment into the space they have. However, if aerodynamics are not thought about at the beginning of a project, the chances are that the performance of the GT, especially for faster / compact operating intake systems, will not be optimised – and small margins will make a big difference to overall plant performance.

By Kate Taylor, principal engineer and Marcus Walters, head of engineering for the Parker Hannifin Gas Turbine Filtration Division

So, what are the issues and what do they mean for efficiency, power output, reliability, and maintenance?

Kate Taylor

A good inlet filtration solution requires more than looking at the performance of individual filters. It uses multiple filters and filtration stages and almost certainly incorporates weather protection along with other systems such as evaporative or coil cooling. These multiple systems and components need to work together, and the design needs to be optimised to ensure GTs are both protected and operations optimised.

A uniform airflow through the filter bank is essential to minimise the pressure loss, and to ensure the filter efficiency and filter life are optimised. The overall design of the filter house should encourage an even distribution of flow across the whole filter array.

If the flow is not optimised, the operation of the filters and the GT itself will be compromised. For example, if a filter house is designed with a bank of 500 filters, an uneven airflow may mean only 400 of them are being utilised correctly, with some running too fast and others barely being used at all. This will result in shorter filter life, higher pressure loss, and potentially lower filtration efficiency.

The size and position of a GT inlet filter house are important. The position of a filter house will influence how the air flows.

Marcus Walters

Have environmental and other features of the installation been considered? To balance the layout and airflow through the system, the inlet position is important. The wrong elevation, sharp angles, transitions that are not smooth, or proximity to walls or other obstructions, will all influence how the air flows. Poor transition design will also increase pressure loss.

Has the prevailing wind been considered in the design? Wind will carry contaminants such as dust or sea spray with it.

Is the inlet open to sea spray, emissions from cooling towers, or other equipment or infrastructure that will increase exposure to particulates that are harmful to GT performance? What elevation is the inlet house at? If low down, systems will pull up dust from the ground and suck it into the filters.

Even if filters are working reasonably well, a poorly designed cooling system will reduce the benefit this expensive piece of equipment can offer. It will increase the risk of water carry over, impacting differential pressure across the system and the life of the gas turbine. It may also result in the loss of the augmented power the system was installed to produce, and uneven temperature distribution downstream will reduce the stall margin of the GT.

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Inappropriate design of turning vanes will negate much of the savings in pressure losses they have been installed to achieve. An incorrectly placed anti-icing system may have the potential to melt filters. Ultimately, there are many aspects of inlet house design that can have a serious impact on overall plant performance.

Good design practice

A lot can go wrong in inlet filter house design, but how can these challenges be overcome?

The answer lies in experience combined with an understanding of the underlying flow physics and good Computational Fluid Dynamics (CFD) simulation.

Sometimes, fundamental issues can be obvious to experts, but CFD modelling shows what the problems are and can help operators or EPCs understand the issues.

It provides answers to how multiple components will interact and the effectiveness of the system configuration. It is also a valuable tool to balance cost and performance, enabling the simulation of different options to produce a solution that is optimised in every aspect of its design.

In one example, when a CFD simulation was carried out of the ducting downstream of a filter house for an offshore intake system it became clear that the inner radius of the 90-degree bend was too small – causing a large region of separated flow (image above left).

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While design guidelines exist for sizing elbows, there is not always the space available to avoid flow separation entirely. Running the CFD model with different bend radii helped to determine how large an increase was necessary to reduce the size of this region and limit any resulting downstream flow distortion to acceptable levels.

Where the filter house includes an inlet bleed heating system, determining the number, position and spacing of the bleed heat nozzles is critical to a successful design.

Images courtesy of Parker Hannifin

In the example shown above, the CFD model predicts that the outer column of nozzles is positioned too close to the side wall of the filter house – hot bleed flow is entrained by a vortex from the edge of weather hood (as shown by the streamlines) and impinges on the filter house structure.

Simply considering the nozzle flow without the interaction between that and the weather hood wake would not have identified this potential problem.

Both the flows illustrated are difficult to simulate accurately, requiring some skill to choose appropriate modelling parameters and interpret the results.

As power plants try to make use of increasingly faster, more compact intake systems, sometimes site engineers who are used to running big filter houses very slowly may not consider the impact of aerodynamic design.

Parker has long experience in the design of power plant GT inlet houses and has learned many lessons over the years through its close association with major GT OEMs. The ability to model the performance of a GT inlet house requires such knowledge and experience to ensure the air being modelled is representative of the installation.

Ultimately, expert CFD modelling gives insight into how GT inlet houses, and, consequently, GTs themselves, will perform in the unique conditions of a particular installation.

It validates a solution before it reaches the site when it will be difficult to modify and adjust to deal with any problems experienced in a live application.

By taking the time to consider the aerodynamics of the system upfront, power plants can fully harness the benefits of more efficient GTs, extend system life and availability, and reduce maintenance overheads

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