When designers work on an air-cooled heat exchanger thermal design, they commonly select face velocity in the design as a convenient means of specifying the airside flow rate.
Face velocity is defined as the average air velocity at the inlet face of the bundle. Typically, face velocity is based on standard conditions [21 °C (70 °F) and 101.3 kPa (14.696 psia)], and design face velocities vary from 2 – 4 m/s. Xace calculates
Face area = Tube length × Bundle width
Face velocity = Volumetric airside flow rate / Face area
HTRI has not conducted research to determine optimal face velocities for design. To address this open issue, members of the Communication Committee – Houston, TX, USA (CC-Houston) recently completed a parametric study of factors which affect face velocity selection, including
- bundle geometry (tube diameter, pitch, fin geometry)
- power consumption
- fan selection
Based on this study, CC-Houston members propose the following general recommendations:
- Maintain face velocities greater than 1.5 m/s (300 fpm) because process control becomes more difficult at lower velocities.
- Keep driver power less than 45 kW (60 hp) to avoid gear drive transmission.
In addition, they provide specific face velocity ranges for different numbers of tuberows, as shown in Table I.
|No. of tuberows||Face velocity, m/s||Typical design value, m/s|
|3||2.9 – 4.5||3.7|
|4||2.6 – 4.0||3.3|
|5||2.5 – 3.8||3.2|
|6||2.3 – 3.5||2.9|
|7||2.1 – 3.3||2.7|
|8||2.0 – 3.1||2.6|
|10||1.9 – 2.8||2.3|
|12||1.7 – 2.6||2.2|
CC-Houston attached the complete review report of Recommending Face Velocity Ranges to the May 23, 2012, meeting minutes.
Note: The proposed face velocities in Table I are a good starting point for designers. The “best” face velocity really depends upon the specifications and constraints of each application, as outlined in CC-Houston’s report.