Retrofitting Mechanical Draft Fans to Optimize System Performance

Mechanical draft fans are used exclusively in power generation to move air and gas from one point to another. They create draft in a process system so that fluid media can be induced, forced, and boosted. Among all draft fans, centrifugal designs are the most common in the power industry. They are robust in construction and resilient in operation.

Capital investment, as well as the downtime generation loss to replace these machines with a brand new installation can be substantial. An economical alternative, which can save both money and time, is to retrofit an existing fan. This article discusses possible options and offers some strategies to maximize available resources for retrofitting these fans.

New Demands on Old Technology

Most U.S. power plant fans have been in operation for some time. Many were installed in the 1960s and 1970s, and are still in use after 40 or 50 years. They were sized and selected for certain flow requirements with little margin for future modifications. As policies have changed and populations have boomed, the demands placed on plants have also been altered.

Some specific reasons that might prompt a plant to retrofit its existing fan are new environmental regulations, an emissions control system upgrade (such as converting from an electrostatic precipitator to a fabric filter), a boiler fuel conversion (such as changing from bituminous coal to Powder River Basin coal), the fan is being scheduled for retirement, or changed system demand. Due to the high cost associated with a new installation, retrofitting the existing fan is usually preferred.

Furthermore, addressing any one of these issues will usually result in new system requirements for fans, such as a change in air/gas temperature, gas composition, suction/supply pressure needs, volume requirements, or power consumption due to changing the air/gas characteristics. This means that any fan retrofit is an engineering project in its own right.

The goal of any retrofit is to minimize modifications to the existing fan while maximizing the fan’s flow and pressure capability as it relates to current system demands. Ideally, this is accomplished utilizing the existing casing without modifying the foundation. Goals for the project will normally include identifying the lowest capital cost, staying within the current motor horsepower with no new electrical infrastructure, and making minor or no modifications to the existing fan, foundation footprint, ductwork tie-ins, and related fan components.

However, the ultimate solution may require modification to one or more of the following: fan wheel, fan casing, drive motor, ductwork tie-ins, and even the foundation in some cases. When retrofitting, it’s also advisable to make use of a computational fluid dynamics (CFD) study for validation purposes. This computer based computational process cuts down on modeling costs and saves precious time. Actual model tests can also be performed with a geometrically similar fan, if budget and time allow.

There are several techniques that can be used to accomplish a successful retrofit. Among them, “optimization” and “right-sizing” are the ones most commonly used. Two short case studies illustrate both of these approaches.

Optimization

A utility plant in the Midwest was unable to meet peak load during the summer months. The draft fan was being operated at or above the maximum motor amperage load. A flow performance test and subsequent CFD study confirmed that the inlet cone velocity was not properly diffused. The fluid media inside the blade passages were separating and as a result the fan was not performing efficiently. A new, high-efficiency, optimized rotor and inlet cone were installed. The existing motor was reused. The plant met load demands successfully with horsepower to spare. Table 1 and vector plots in Figures 10 and 11 demonstrate before and after results.

Table 1. Draft fan motor amperage at various generating loads. Motor current was reduced using the optimization technique. Courtesy: ProcessBarron

10. Pre-installation velocity vectors. Flow separation is evident in the images. Courtesy: ProcessBarron

11. Post-installation velocity vectors. Flow separation was reduced utilizing the optimization technique. Courtesy: ProcessBarron

Right-Sizing

The draft fan at another utility plant in the Midwest was experiencing vibrations and generating a lot of noise, which concerned plant management. A comprehensive flow performance test was performed. It was determined that the fan was oversized for the application. A redesigned, “right-sized” rotor was installed using the existing shaft and drive system. A minor modification was made to the casing.

Post-installation analysis revealed no vibrations. The acoustics issues had also been resolved. The synergies of the right-sizing process also produced an added benefit in the form of energy savings (Table 2) resulting in a rapid return on investment.

Table 2. Before and after draft fan motor amperage. Motor current was reduced using the right-sizing technique. Courtesy: ProcessBarron

Evaluating Solutions

A retrofit fan design requires a detailed and careful review of existing system performance and operational data. The most effective strategy is a total system approach: Analyze each component; isolate critical paths; and provide a practical engineering solution. When retrofitting a power plant fan, engineers should evaluate the entire draft system; perform a field inspection; establish baseline flow parameters; establish baseline geometry; calculate future system requirements; study interactions between fan characteristics and system demands; customize a retrofit fan to suit the projected system requirements; predict future fan performance; perform mechanical, metallurgical, and structural (civil) studies; validate theoretical basis with a CFD study; and perform a geometrically similar fan model test (depending on time and budget).

The most critical path through a draft system is the draft fan. The key to a successful retrofit is the pragmatic application of a sound theoretical basis. Determining a solution requires comprehensive study and a clear understanding of the problem it is meant to address. Understanding the dynamics of draft machines and their interactions with systems is extremely important. Experience and access to resources on the subject matter are other key ingredients for achieving success in the retrofit process. ■

— Nurul “Moni” Talukder, PE ([email protected]) is chief engineer in the Air & Gas Handling Group at ProcessBarron.