Air Cooler Recirculation-Winterization
Air-cooled heat exchangers, aka air-coolers (or fin-fan by the Americans), are essentially giant radiators and are used in industry to cool hot process fluids using ambient air temperature – not unlike blowing on hot soup.
The greatest demand for air-coolers is during the summer when air temperatures are hottest, and thus they are sized much larger than normally necessary to sufficiently cool the process fluid during the colder days.
However, in winter the air-temperatures can be quite cold, and the air-coolers become too effective, so the risk becomes over-cooling the process fluid. This is especially troublesome when water is present in fluid but is also a concern for heavy-oils and other fluids that become too viscous (thick and slow to flow). Saturated gases can form hydrate crystals.
In this picture (right), we can see a typical air-cooler that is designed for maximum cooling and no regard for over-cooling. Glycol (anti-freeze) processes do not require winterization, as the fluid will not freeze or thicken significantly.
The effect of both hot and cold air temperatures on the performance of air-coolers can be compared to your car during seasonal extremes – older cars that are not in as-new condition tend to overheat during the summer, and they also might have issues warming up when it’s -30C out.
The solution to over-cooling (during winter) is to re-circulate the warmer air that is normally dumped to atmosphere during summer conditions. In the picture (left), louvres are added in three key places:
- At the air inlet (sides), to control incoming `cold’ air.
- At the air outlet (top), to control exiting `warm’ air.
- In the recirc duct (middle), to control the amount of air recirculating from top to bottom.
Air temperature sensors (and sometimes process sensors also) are used by client’s control system to determine when and how much to actuate the three sets of louvres.
In the summer, inlet and outlet louvres will be wide open, and the recirculation louver closed shut.
In the coldest winter nights, the inlet and outlet louvres will be almost completely closed, and the re-circulation louvres completely open. 2% Heat loss will still occur through air leakage from louvers, as well as heat-conduction through the panels and louvres themselves. Some customers use louver blankets and spray foam insulated panels as additional protections.