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After Cooler / heat exchanger in pneumatic conveying

Cooling heat exchanger at blower outlet

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Section summary
1. What is the purpose of an aftercooler ?
2. What is an aftercooler after a blower ?
3. Types of aftercooler
4. Important design considerations

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1. What is the purpose of an aftercooler ?

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 much 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.

2. What is an aftercooler on a compressor or blower ?

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.

3. Types of aftercoolers

Aftercoolers are actually heat exchangers and can either use a liquid media as coolant, or simply air.

Liquid media aftercoolers

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

The simplest design for an heat exchanger using a liquid coolant, is a shell-tube heat exchanger. In this design, many tubes are enclosed within a shell. The conveying air is circulating in the tube while the coolant, typically water, is on the shell side. One drawback of this kind of design is that the heat exchanger can be quite large to achieve a defined duty as the heat exchange on the air side is quite low.

Figure 1 : principle of shell and tube heat exchanger

An alternative design consists in using finned tubes to increase the heat exchange area and thus perform the same duty as a shell & tube heat exchanger but within a much smaller volume. For this kind of exchanger, tubes are equipped with fins and the position of the air and the coolant is reversed : the air goes through the shell and contacts the fins, while the coolant, typically water, is inside the tubes. Note that the pressure drop through the exchanger must be monitored as fins can be prone to fouling.

Figure 2 : principle of extended surface heat exchanger

Air Aftercooler

The coolant here is just air. The aftercooler is equipped with a fan that forces air through the heat exchanger. The inside of the heat exchanger is made of aluminium which is fined on both sides in order to allow for a large heat exchange area within a relatively small volume. It avoids the previous drawbacks in terms of risks related to 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.

Figure 3 : principle of an air-air forced draft heat exchanger for pneumatic conveying

4. Important Design considerations

In order to be efficient, aftercoolers are made of small finned 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.

The other key design consideration to have in mind is the risk of condensation when the air is cooled down in the exchanger. The Engineer designing the system must check that the temperature within the exchanger is not lower than the dew point of the air. If it is the case, the air must be dehumidified. It can be achieved by supplying conditioned air to the blower, or by having 2 heat exchangers in series : the 1st one cools the air and traps the condensed water, feeding a dry but saturated air to a second exchanger that will this time heat up the air to lower the relative humidity and decrease the dew point.