Engineering task and calculation objective
The PLRE module assesses warm-air recirculation at air-cooled heat exchangers. In air coolers and dry cooling installations, part of the heated exhaust air can be drawn back into the intake cross-section – for example under unfavourable wind directions, insufficient free installation height, or closely spaced cooler bays. The resulting rise in cooling-air inlet temperature reduces the driving temperature difference and thus the transferable heat duty.
Starting from the free height of the heat exchanger, the geometry of the air guiding device (height and radius) and the inlet flow cross-section, the buoyancy of the warm exhaust air is determined from the density difference between the air inlet and the air outlet. The air volume flow and the outlet velocity then show how reliably the warm-air plume is kept away from the intake zone. As a result, the module compares the heat duty without recirculation against the heat duty with recirculation and reports the loss of performance.
Engineers who need to calculate recirculation effects use this estimate above all when planning the layout of air coolers in plant engineering: it shows early on whether an air guiding device (diffuser, stack) has to be raised or the distance to neighbouring structures increased, before costly performance shortfalls appear in operation.
Calculation workflow
- Capture geometry and installation layout: The free height of the heat exchanger, the height of the air guiding device, the radius of the air guiding device and the inlet flow cross-section are entered as the geometric boundary conditions of the installation.
- Determine the air density conditions: From the temperatures at the inlet and outlet, the air densities at inlet and outlet are obtained; their density difference is the driving quantity for the thermal buoyancy of the warm-air plume.
- Calculate the discharge momentum: The air volume flow at the outlet and the outlet cross-section yield the air discharge velocity. Together with the buoyancy, it determines how far the exhaust air is carried above the intake plane.
- Evaluate the recirculation fraction: From the ratio of discharge momentum, buoyancy and installation geometry, the fraction of warm air drawn back into the intake and hence the increased effective intake temperature are estimated.
- Report the performance comparison: Using the effectiveness of the heat exchanger, the heat duty without recirculation and the heat duty with recirculation are calculated; their difference is reported as the loss of performance due to recirculation.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Height of heat exchanger | H | m |
| Height of air guiding device | HW | m |
| Radius of air guiding device | W | m |
| Cross section area of flow | F | m² |
| Density of air at inlet | ρe | kg/m³ |
| Density of air at outlet | ρa | kg/m³ |
| Density difference | Δρ | kg/m³ |
| Volume flow of air at outlet | Va | m³/s |
| Flow velocity of air at outlet | va | m/s |
| Heat duty without recirculation | Qist | kW |
Calculated results
| Quantity | Symbol | Unit |
|---|---|---|
| Efficiency | eR | - |
| Heat duty with recirculation | Qist;R | kW |
| Loss of performance due to recirculation | Rezirkulation | % |
Frequently asked questions
When is the risk of recirculation at air coolers particularly high?
Critical conditions are low discharge velocities (weak momentum of the exhaust plume), small temperature or density differences between exhaust and ambient air, low air guiding devices, and crosswinds that push the plume into the intake zone. Several cooler bays installed close together also feed warm air into each other's intakes.
Why does recirculation reduce the heat duty so strongly?
The recirculated warm air raises the cooling-air inlet temperature. Since the transferred duty is proportional to the driving temperature difference between the process medium and the cooling air, even a rise of a few kelvin in intake temperature translates directly into a performance shortfall for tightly sized coolers – in the summer design case, the design point may then no longer be reached.
Which design countermeasures can be evaluated with the module?
Above all, raising the air guiding device (discharge stack/diffuser) and increasing the discharge velocity: both increase the momentum and discharge height of the warm-air plume. Via the geometry inputs, the effect of a changed free height or a different inlet flow cross-section can be read off directly in the performance comparison.