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Industrial Processes

Targets set for boiler companies

   
 
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A proposed EU directive to set up an efficiency rating system for gas fired boilers alarmed the industry, which realised that different test centres could not test boilers with the high level of reproducibility required by the rating system.
Testing centres and boiler manufacturers therefore launched a Measurements And Testing (MAT) project to develop and improve testing methods so that the maximum difference between laboratories measuring the same boiler would be less than 2.5% (reproducibility). The rating system of the directive is based on 3% (absolute) steps in efficiency.
The partners successfully harmonised procedures in different test laboratories, allowing laboratories to streamline testing and cut costs. Some of the procedures are also being incorporated in European standards.
The partners have continued their collaboration to further improve testing procedures.

A sense of panic descended on producers of gas and fuel oil fired boilers when the European Commission proposed to introduce a rating system for such appliances based on measurements of 3% steps in efficiency. The reason was that a 1991 comparison of efficiency measurements by different European laboratories, on the same boiler circulated among them, showed that measurements could not be made with a reproducibility of better than 4.5%.
This held out the prospect of incorrect labelling of boilers assessed under the rating system.
To avoid being unable to meet the requirements of the rating system, gas suppliers, testing laboratories and boiler manufacturer companies began a project with the support of the European Commission's Measurements And Testing (MAT) programme to establish test methods that could be used to assess boiler efficiencies with a reproducibility of less than 2.5%.

Testing time

The partners, coordinated by the Danish Gas Technology Centre, investigated a number of aspects of boiler operation and testing. These included the problem of determining when a boiler has reached stable operation, the effects of ambient conditions, the way that the waste heat from a boiler's water pump affects the efficiency readings and the influence of burning gases with different calorific values. For example, a laboratory can sometimes find it difficult to determine whether a boiler has reached operating stability so that measurements can be made. The MAT partners therefore developed a way of establishing that stability had been reached by statistically comparing efficiency measurements of a boiler made during four successive periods.
Because boilers are tested under different ambient conditions, the partners also developed correction formulae that take into account the influence of ambient humidity, temperature and pressure. Traditionally, only few laboratories can conduct tests at the standard specified atmospheric conditions, although in some cases, such as when boilers are operated at part load, the correction can be more than 1%. This is a significant level in terms of overall test accuracy.

Adding up influences

Tests have traditionally not taken into account the heat from pumps and other electrical components in boilers, with the result that efficiency figures have been artificially high. In a typical 20 kW boiler, the pump might contribute about 100 W to the total heat production, and the relative effect is greater when a boiler runs at part-load. As a practical solution to the problem, the project partners developed a universal model that gives reasonable results based on 10 common pumps. A laboratory can therefore easily take into account the effect of a pump without having to carry out expensive testing of the pump. Theoretical and practical studies of the influence of burning gases of different calorific values in test boilers showed that the effects were too low to worry about.
The partners also realised that they used different formulae to calculate heat output and flue gas losses of boilers under test. Yet when they developed harmonised formulae the results were little different from the results obtained using the laboratories' own approaches. This showed that the different calculations were not the reason for the poor reproducibility of test results.

Common test results

The project has led to the development of correction formulae that can be conveniently applied by all laboratories to produce common test results throughout Europe.
The partners accept that implementing all of their findings will cost laboratories some money, typically to write correction algorithms. However, they stress that overall costs will fall because thanks to stability criteria, a boiler will now have to be tested only once, rather than the several times previously needed to overcome instability problems. Any investment is therefore likely to be recovered in six to 12 months, say the partners. The new approach gives a reproducibility of less than 2.5% at full boiler load. This is much better than the 4.5% previously achieved by the laboratories.

Reducing uncertainty

The work has also identified new sources of systematic uncertainty, including the drift found in instruments, particularly water and gas flow meters. Although drift can be minimised by regular calibration, laboratories tend to have different frequency of calibrating instruments.
The harmonisation of calibration methods across Europe is hampered by the influence of national standards organisations, but the partners believe that greater cooperation between them and the exchange of information about instrumentation and calibration will lead to more common approaches and therefore even more consistent test results. For example, meters of a certain type might be calibrated once a year.
Good practice documents have already been produced, and the results of the MAT project have been passed to the European standardisation organisation, CEN. Some features, such as correction formulae for condensing boilers, have already been adopted in standards. The partners also consider that an important result of the MAT project has been their decision to continue cooperating. In particular, they intend to work together to solve the problems when boilers are operating at part load. In this regard, the project has opened up an interesting area for further study, through boosting the contact between organisations that would not otherwise have worked together.

 

 

Project Title:  
Full and part load efficiency measurements for boilers

Programmes:
Industrial and Materials Technologies (BRITE-EURAM/CRAFT/SMT)

Contract Reference: MAT1-CT92-0009

Cordis DatabaseFor more information on this project,
go to the CORDIS Database Record

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