Heat Exchanger Problems – Part 3September 15, 2014
Hot Air Recirculation in Air Cooled Heat Exchangers
Air cooled heat exchangers provide cooling by passing ambient air over a tube bundle. As the air passes over the bundle it is heated and so the air leaving the heat exchanger is at a higher temperature than ambient. If the hot exit air is allowed to recirculate around the cooler it will raise the effective air inlet temperature and therefore reduce the cooling capacity of the heat exchanger.
In an installation involving multiple air cooled heat exchangers, two types of hot air recirculation are possible. Hot air leaving a heat exchanger can either recirculate around the same heat exchanger, or it can travel to the entrance of another heat exchanger.
Hot air recirculation is a potential cause of performance problems, and must therefore be addressed in the plant design. Attention must be paid to the design of individual heat exchangers and to the overall layout of the plant if both types of recirculation are to be successfully avoided.
Why Does Hot Air Recirculation Happen?
The hot air leaving the cooler is travelling vertically upwards and is less dense than the surrounding air (and therefore buoyant). Recirculation requires some of this hot air to travel downwards in order to re-enter the cooler (see Figure 1). For this to occur, the air pressure at the sides of the cooler must be less than the air pressure at the top of the cooler. This pressure difference is the driving force for recirculation.
When air is moving, its pressure is a function of its velocity (the Bernoulli principle). The higher the velocity, the lower the pressure. A pressure difference which allows hot air recirculation is therefore created when the velocity of air approaching the sides of the cooler (Vapp) is somewhat higher than the velocity of the air leaving the top of the cooler (Vexit). This gives rise to the most basic rule for avoiding hot air recirculation, which is that the air approach velocity should be less than the air exit velocity.
Forced Draft vs. Induced Draft
The relationship between the approach velocity and the exit velocity provides an understanding of why hot air recirculation is less likely in an induced draft cooler than in a forced draft cooler (see Figure 2). Both types have essentially the same approach velocity, but the induced draft design will have a substantially higher exit velocity. The exit velocity for induced draft is the velocity leaving the fans, whereas for the forced draft arrangement it is the bundle face velocity.
However, although the induced draft arrangement is always preferred in terms of hot air recirculation, it does suffer from some disadvantages. The rotating machinery is less accessible and therefore harder to maintain than in the forced draft arrangement, and the fan handles hot air rather than cold air. For these reasons, forced draft arrangements are common despite the risk of hot air recirculation.
Wind increases the risk of hot air recirculation in a number of ways. Firstly, the plume of hot air rising from the air cooler exit is pushed sideways. This makes recirculation on the downwind side of the source cooler more likely, and also gives the hot air the potential to travel to another air cooled heat exchanger. Secondly, the wind will create a low pressure area on the downwind side of the source heat exchanger which will encourage air movement in the downward direction (see Figure 3).
Cross winds can have other damaging effects on air cooled exchangers. For example, the fans on the upwind side of the cooler can be affected by unstable air flows caused by the wind interacting with the cooler structure (see Figure 3).
When Does Hot Air Recirculation Affect Performance?
Some air coolers are more likely than others to suffer performance problems as a result of hot air recirculation. Factors which are likely to make a cooler particularly vulnerable to impaired performance are:
• Forced draft arrangements
• Restricted approach areas
• Close temperature approach (e.g. process outlet temperature requirement <10K above maximum ambient air temperature)
• Significant wind speeds in summer months
An example of a restricted approach area is where a cooler is positioned in the middle of a large bank of air coolers (see Figure 4). Compared to a stand-alone cooler, such an arrangement significantly reduces the air approach area and therefore leads to relatively high approach velocities.
Designing to Avoid Hot Air Recirculation Problems
Where the above factors are present, the designer must take action to ensure that the performance of the heat exchanger is adequate under these challenging conditions. Other than switching to an induced draft arrangement, the simplest action to take is to increase the ground clearance (Hgc in Figure 1) to achieve the required approach velocity (taking account of any restrictions in approach area).
One difficulty with this approach is that, for large installations, the ground clearance required may be quite large. This then exposes the cooler to greater wind velocities (since wind velocity tends to increase with elevation) and therefore to more significant wind effects.
In such cases, mesh screens can be combined with increased ground clearance to protect large forced draft air coolers from the undesirable effects of cross winds. Such screens are mounted on the sides of the air cooler structure and provide stable air conditions in the fan entrance area (see Figure 5). The screens also ensure a greater physical separation between the exit and approach air streams, such that the risk of recirculation is reduced.
Where justified, motorised screens can be combined with a weather monitoring station to provide automatic optimisation of screen position with wind strength and direction. On hot, calm days the screens can be completely retracted to maximise air flow to the cooler.
Plant Layouts for Multiple Air Cooler Installations
As shown in Figure 3, hot air can travel from the exit of one cooler to the inlet of another. It is therefore important that the overall plant layout minimises the risk of such hot air migration to vulnerable coolers. This requires consideration of the orientation of coolers to the prevailing summer wind direction, relative cooler positions and elevations, mixing of forced and induced draft coolers and the influence of local topology. The presence of other sources of hot air (e.g. engine exhausts) should also be considered.