Simulating air-cooler header and fan performance informs physical testing plans

Salem A. Bouhairie, Senior Project Engineer, Research

Both tubeside and airside behavior affect the overall performance of an air cooler. Two concerns with air-cooled heat exchanger (ACHE) operation are two-phase process-side maldistribution in headers and misrated airside fan performance.

At HTRI’s Research & Technology Center (RTC), we are developing plans to construct a header box for advanced tubeside analysis. In addition, we want to identify differences between the actual and rated air delivery of the Air-Cooled Unit (ACU) fan. Integral to these test plans are computational fluid dynamics (CFD) simulations, in concert with Xace® software and experimental tests at the RTC, to understand these two performance limitations.

Eventually, we can develop guidelines to help others resolve or troubleshoot these tubeside and airside issues.

CFD study of air-water header box flow

ACHEs in the gas, oil, and power industries often process two-phase fluids. Two-phase maldistribution in headers negatively affects the output products and can cause tube pullout, as indicated in API 661 [1]. HTRI has completed three-dimensional CFD simulations of the hydraulics in the inlet header of an experimental air-water ACHE. Figure 1 shows a CFD simulation of the vertical distribution of an air-water mixture in the tuberows. Figure 2 provides insight on the lateral mixture distribution, which Xace does not report.

Figure 1. Contours of water volume fraction through four rows of single-pass ACHE inlet header
Figure 2. Lateral distribution of liquid in air-water mixture through four rows of single-pass ACHE inlet header

Guided by these predictions, HTRI plans to design and build a scaled-down model of the header and tube bundle for adiabatic air-water testing in the Multipurpose Visualization Unit (MVU). With this approach, we can make recommendations for effective header design and later modify Xace to report a two-phase flow maldistribution parameter for the header [2].

CFD study of airside fan performance

Most ACHEs are heat transfer limited on the air side, making the fan the most important component because it affects tubeside product quality. Fan rating software can predict airside performance, but some end users have reported that actual fan operations differ significantly from rated predictions.

Fortunately, we can use CFD to analyze the performance of the ACU fan (Figure 3) and compare those CFD results with the fan vendor’s rating. Figure 4 shows a CFD simulation of the velocities generated in the ACU fan in a section of wind tunnel, built according to the ANSI/AMCA 210 [3] standard.

Figure 3. Fan installed in the ACU
Figure 4. CFD simulation of the ACU fan with velocity vectors

Carefully validated CFD simulations can reveal performance limitations in ACHE headers and fan output. Moreover, the results of these simulations allow HTRI to provide adjustment factors or warnings for Xace calculations of header maldistribution or fan flow.

To learn more about HTRI’s CFD capabilities, email contracts@htri.net.

References

  1. API STD 661: Petroleum, Petrochemical, and Natural Gas Industries—Air-cooled Heat Exchangers, 7th ed., American Petroleum Institute, Washington, DC (2013).
  2. M. Rezasoltani, Air-cooler header pressure drop and flow maldistribution, AC-18, Heat Transfer Research, Inc., Navasota, TX (2017).
  3. ANSI/AMCA 210-99: Laboratory Methods of Testing Fans for Aerodynamic Performance Rating, Air Movement and Control Association International, Inc., Washington, DC (1999).