Blowers are typically used as air movers in pneumatic transport systems. Such blowers are compressors that are sucking air at certain pressure (most of the time atmospheric) and are rejecting it at higher pressure conditions. During compression, part of the energy spent is converted in heat, thus the air at blower outlet is significantly hotter than at suction conditions. Temperatures can reach 50-60 celsius, sometimes higher, which may be causing a problem for the process downstream. For example, star valve should be design for working in hot conditions which may be detrimental to their performance, or simply the material conveyed cannot handle such temperatures leading to melting for example. In such cases, it is desirable to cool down the air flow thanks to a heat exchanger positionned right after the blower, caller an after cooler.
Aftercoolers are actually heat exchangers and can either use a liquid media as coolant, or simply air.
Such aftercoolers are usually run with water, refrigerated or chilled. It is an efficient design to control the temperature but presents drawbacks and their operation must be carefully controlled :
- Cold water can create condensation in or out of the aftercooler which can lead to detrimental consequences (if water is carried away in the pipe it can lead to blockages, among other potential consequences)
- In case of leakage of the heat exchanger, water will enter the process pipes
- The process control must be quite fine to ensure a good temperature control
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The coolant here is just air. The atercooler is equipped with a fan that forces air through the heat exchanger. It avoids the previous drawbacks in terms of risks related to condensation or leakage, but in return provides a less efficient cooling, dependent on the ambient conditions. One way around this issue is to have the blower and the aftercooler in an air conditionned room, to have constant cooling air conditions.
In order to be efficient, aftercoolers are made of small tubes that increase the heat exchange area. However, such tubes can be subject to fouling with time, especially if the air supply is not very clean or if some blowback from the line occur. It is therefore key for safety to have the proper instrumentation to monitor the performance of the cooler :
- Pressure drop across the heat exchanger : the system must be stopped and cleaned to remove build-up when the pressure drop reaches its maximum limit
- Temperature before and after the heat exchanger to verify the cooling efficiency and control the cooling
Note that an aftercooler generates an additional pressure drop to be considered in the design of the pneumatic transport line.
Having a check valve to prevent blow back in the heat exchanger may also be considered.