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Understanding and Predicting Impacts of Fouling on Enhanced Condenser Tubes

Understanding and Predicting Impacts of Fouling on Enhanced Condenser Tubes

From ASHRAE Journal Newsletter, August 24, 2021

Fouling could affect equipment efficiency, so it’s important to understanding and accurately predicting the negative impacts of fouling on enhanced tube heat transfer. ASHRAE RP-1677, Measurement and Prediction of Waterside Fouling Performance of Internally Enhanced Condenser Tubes Used in Cooling Tower Applications, investigated the effects of tube geometries, water quality and water velocity on the fouling process. A generalized fouling prediction model suitable for project application was also developed in this study.

The project’s principal investigator, Xinlei Wang, Ph.D., and co-investigator, Chao Shen, Ph.D., talked with ASHRAE Journal about the research.

Why was it important to explore this topic now?

In the condenser, enhanced tubes transfer heat to cooling tower water from a cooling tower loop. The circulating water typically contains high amounts of mineral ions, such as calcium and magnesium, that are deposited on the internal surface of enhanced tubes over long periods of time, facilitating fouling. This build-up reduces the heat transfer performance of enhanced tubes and increases water-side pressure drop. Thus, enhanced tubes used in water cooling towers unavoidably face the issue of fouling. The current practice to account for fouling in design is to include a fouling resistance factor in calculations and to oversize the heat exchanger based on this value. However, the recommended factors are based upon smooth tubes or were set by experience, years before enhanced tubes were used. Previous research has suggested that fouling increases in enhanced tubes and may negate the benefits of the enhancements.

Fouling deposition is a slow process, and many fouling studies have been conducted using accelerated fouling test that uses a high foulant concentration. Real field data of fouling in cooling tower water was severely lacking for both factual insight into this area and for the development of a reliable fouling model to predict fouling resistance. Prediction models of fouling in cooling tower water are very useful for the designers and users in the refrigeration and air-conditioning industry. Due to the lack of long-term fouling tests, most fouling prediction models were developed on the basis of data collected from accelerated fouling experiments, and thus are likely of limited use for practical operating systems. Long-term (non-accelerated) fouling tests are required to analyze the combined fouling of enhanced tubes used in cooling tower water systems. Some improved fouling models developed and confirmed based on extended field test data are needed for a wide-range use.

This study, in which an actual (non-accelerated) fouling measurement system was built in a laboratory setting, is important to address those research needs and challenges. The results will benefit the ASHRAE community by offering a set of accurate, reliable, and robust experimental data; will facilitate the development of fouling prediction methods for real application; and will offer essential information for further revisions of existing AHRI/ASHRAE standards.

What lessons, facts and/or guidance can an engineer working in the field take away from this research?

    • Enhanced tubes always showed higher heat transfer performance, compared to the plain inside tube in any conditions.
    • One could not find an enhanced tube that always had the highest heat transfer performance in any fouling conditions. Therefore, the selection of an enhanced tube should be based on the heat transfer performance, the fouling characteristics and the practical working condition.
    • The fouling performance of 3-dimensional enhanced tubes was not absolutely superior or inferior to the 2-dimensional tubes.
    • In general, for the same heat transfer tube, the higher the fouling potential of water, the greater the asymptotic fouling resistance. Thus, some measures should be taken to reduce the fouling potential of cooling tower water in the industry.
    • The maximum difference of asymptotic fouling resistance caused by tube geometry were much lower than the average difference caused by water quality, suggesting the influence of water quality on the fouling process was much bigger than the tube geometry. A small change in the Langelier Saturation Index (LSI) had significant impact on the fouling deposition process. Therefore, LSI of cooling tower water should be controlled tightly in an actual cooling tower water system.
    • The time when the fouling began to deposit rapidly on the surface of enhanced tubes was different at the different water velocities. The greater the velocity was, the earlier fouling started to deposit. A higher water velocity caused a smaller asymptotic fouling resistance, except for the plain tube. Therefore, in practical operation, the cooling tower water velocity should be increased to achieve lower fouling.

How can this research further the industry's knowledge on this topic?

Detailed mechanisms of the effects of tube geometries, water velocity and tube geometries on the fouling process for enhanced tubes are revealed from this research. One generalized fouling thermal resistance prediction model based on long-term fouling data is developed, which reflects the influences of three types of operational variables: water quality, water velocity and geometry of enhanced tubes. The generalized fouling prediction model is recommended for the application in the HVAC&R industry due to its suitability and accuracy in practical project applications. An empirical fouling model was established. The fouling prediction model is more suitable for the application in real project systems in comparison to existing ones, which could provide more accurate fouling data for designers and users of heat exchangers in the HVAC&R industry.

Were there any surprises or unforeseen challenges for you when preparing this research?

Due to the high fouling potential of test water (much higher than the typical cooling tower water), the fouling continuously accumulated into the cooling tower water tank, resulting in frequent failure of the cooling tower water pumps. The switch of on-off-on of the water pump caused the fall off of fouling from the internal surface of the test tube during the test around the 60th and 80th day, which caused delay of the project.