Development in field testing for air inlet filter

Paul Boughton

By reducing fouling, efficient filters with low pressure drop generate big cost savings. Jim Benson reports.

Modern gas turbines are highly sensitive to even the smallest particles. In order to achieve low maintenance costs and a consistently high efficiency rating, choosing the right filter considering specific site conditions becomes critical.

Today's air filtration marketplace offers a wide array of products with different characteristic such as filter type, media, efficiency, construction or materials. In addition, the environmental conditions of each site vary such as: humidity, temperature, precipitation and air contaminants. These variations increase the difficulty for turbine operators to differentiate manufacturers' claims.

The industry has developed common filter classification and laboratory test standards, such as ASHRAE 52.2 or EN: 779 and EN: 1822, for determining how an air filter should be measured and classified. Test laboratory are performed in optimal conditions to reduce uncertainty and give the most accurate rating. Although these organisations continuously work on improving their tests, optimal lab conditions differ from real-life. Laboratory tests don't allow the users to predict how a filter is going to perform on sites. As well, when users have to compare test results, the numbers are frequently questioned or debated if the duration of the test period, dust type, air flow or another parameter has varied from test to test.[Page Break]

Field testing is trying to minimise some of these gaps. Various solutions have been developed such as air sampling, dust sampling and the latest being the introduction to the market of mobile test rigs.

On-site field testing

Optimising the air inlet performance with data from on site testing is relatively unknown among gas turbine owners even though it can have great impact on the inlet system performance and turbine output.

Air sampling has been mostly use at an early stage to design the inlet system. By understanding size distribution, concentration and chemical content of particles in ambient air, the air inlet manufacturer can determine the optimal filtration stages for specific environment. The self contained systems are easily shipped to site and assembled by the user, who can do a few samplings are over a short period of time, and return for analysis at the lab within the same week.

Dust or soil sampling are usually use to troubleshoot difficult sites where air sampling was not performed in the first place. By evaluating the particle size and chemistry of the dust on a used filter, combined with information on site conditions and current filter performance such as water washing frequency, it is possible to formulate a recommendation on filter efficiency or media type.[Page Break]

Mobile test rigs allow a more in depth analysis of filters performance in-situ and their evolution over time. These on-site evaluations follow ASHRAE Guideline 26-2008 as reference to select the right test equipment, test set-up and data interpretation. It is important to realise that field measurements generally result in larger uncertainties than laboratory measurements since air pollution and site conditions vary over time. Thus, field tests should not be use as a filter performance rating method but rather for performance comparison between filters.

For example, placed in the immediate vicinity of a power plant or like, a mobile test rig could be used to:

- Analyse the ambient air quality.

- Determine whether the turbine is currently equipped with adequate filtration to handle these contaminants; and

- Further determine the best type and level of filtration for the turbine by simultaneously testing several filter combinations.[Page Break]

Mobile test rigs

Mobile test rigs are usually built inside a shipping container or a trailer that can be easily transported or towed to a user site. The units contain multiple ducts that may be fitted with pre-filters and final filters, box or canister type. Each duct has its own independent fan and control system and operates at the same airflow, simulating the actual inlet velocity of the site. All air intakes are located as close as possible so the sample air is as homogenous as possible for all test ducts.

During the test, air filters are monitored and analysed for contaminants removal performance, energy consumption and mechanical integrity. The control system monitors the resistance to airflow across the filters. Flow meters are installed in each duct and maintain constant flow with the variable frequency drive on each fan motor. The particle counter efficiency test system operates automatically at set times, sampling up and downstream contaminates. By running multiple data sets, it gives a clear picture of the filter performance over time. All data are recorded during the entire test period, usually three to six months, giving to the operator valuable historical information for future filter comparisons.[Page Break]

The measured parameters include:

- Ambient dust concentration.

- Airflow.

- Filter resistance to airflow (pressure drop).

- Filter efficiency by particle count/size.

- Temperature.

- Relative humidity.

The latest versions developed for the gas turbine market carry sophisticated analysis equipment. In order to avoid having staff on site monitoring the test, the unit is equipped for wireless data collection. The unit can be controlled by remote access allowing data to be retrieved from the test site in real-time. They are also built to operate in hazardous environment (NEC or ATEX) commonly found on power generation or refinery sites.

The results allow the operator to make the most effective filter selection, minimising total life cycle cost with the right product for its site.

With all data necessary for an in-depth analysis, the mobile test rig makes it easier to differentiate between products and achieve low operating and maintenance costs and a consistently high efficiency rating.

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Jim Benson is Senior Product Engineer, Camfil Farr Power Systems, Québec, Canada.

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