Figure 1. The A-frame air-cooled condenser has downflow and upflow reflux zones.
Understanding process conditions and potential problems can influence design decisions that improve operation. At the other end of the spectrum, troubleshooting exchanger performance requires a working knowledge of the design attributes. At our research and technology center, Heat Transfer Research Inc. uses industrial-scale equipment as well as computational fluid dynamics (CFD) and other simulation capabilities to study the impact of heat exchanger design attributes and operational problems on performance.
This article discusses the benefits of a design change in the shape of a tube in an air-cooled condensing system. It also highlights how the study of heat exchanger performance trends can help pinpoint operational problems and identify design improvements..
Benefits of Noncircular Tube Geometries
Power plants use A-frame air-cooled condensers to cool exhaust steam from the low pressure stage of a steam turbine. An A-frame air-cooled condenser, whether mounted on the ground or on columns, offers users several advantages over the horizontal tube arrangement typical for API 661 applications. First, the higher duty per unit of ground area with inclined tubes reduces the required footprint. Second, tube inclination facilitates condensate drainage, resulting in higher condensate loading in each tube without generating excessive pressure drop (figure 1).
Figure 2. This diagram shows the effect of tube geometry on airflow.
Just as with other heat exchangers, the tube geometry in air coolers greatly impacts a unit’s efficiency and performance. A-frame heat exchangers have finned elliptical, oval, flat or circular tubes. While the circular tube geometry is easier to manufacture and can withstand higher pressures for a given wall thickness, it also increases the air-side pressure drop and leads to a non-uniformity in the mean temperature difference across tube rows.
Noncircular tube geometries modify the air-side flow in a manner similar to the wings of an aircraft or the streamlined contours of a sports car. As shown in figure 2, the noncircular geometry:
- Delays boundary layer separation.
- Reduces the size of wakes.
- Reduces form drag, thereby reducing the power consumed by the fans.
The air-side heat transfer rate increases because the boundary layer now contacts a greater portion of the finned area, reducing the number of tube rows required for a given duty and providing a more uniform temperature difference between the air and the process fluid. A uniform mean temperature difference across all tube rows reduces condensate subcooling and the occurrence of freezing in the first tube row. Additionally, elliptical or flattened tubes increase the condensate loading with low two-phase pressure drop on the tube side. The net result of these effects is a better thermal design.
Other factors such as cleaning and fouling potential often dictate the final choice of tube geometry for A-frame applications. However, noncircular tubes offer some benefits that circular tubes do not.
Using a Heat Exchanger Thermal Flow Loop
Figure 3. A heat exchanger thermal flow loop is shown in this schematic. Analyzing thermal performance during the design stage is important to help determine final design criteria.
Combining applied research with practice offers plant engineers and designers of heat exchanger systems the best understanding of underlying processes and potential improvements.
Plant engineers often are faced with reconciling computer simulations of different operating conditions with plant operating data. Invariably, the simulations and actual plant data differ, and the engineer must determine if the difference is due to plant instrumentation, the accuracy of the simulation predictions, or degradation of plant equipment.
HTRI recently designed and constructed a heat exchanger thermal flow loop for use in training of plant engineers. Using the heat exchanger thermal flow loop in conjunction with HTRI’s simulation software provides a method to answer design and operation questions. In 2011 we plan to open training sessions with the demonstration loop.
The demonstration loop consists of three heat exchangers (figure 3). Exchanger 1 (E1) heats test fluid in the hot loop using an electric heater. The hot fluid flows from E1 to Exchanger 2 (E2), where the heat is rejected to the cold loop inside E2. In the cold loop, the heated fluid travels from E2 to Exchanger 3 (E3) where it rejects heat to room air. The flow demonstration loop uses industrial instruments to monitor operating heat exchanger performance.