The inert nature of PTFE (Teflon) has naturally endured it to applications involving corrosive chemicals. Not only does PTFE stay non-reactive to almost all chemicals (notable exceptions being sodium and alkalis at elevated temperatures), it also exhibits its properties all the way up to temperatures of 250°C. This seemingly invincible nature allows fabricators and systems designers to incorporate PTFE without worrying which chemicals may or may not present within a system.
Many chemical baths are designed to hold a host of different chemicals. The need to uniformly heat or cool these chemicals is met by the use of heat exchangers. In its most basic design, the heat exchanger consists of two adaptors, connected to one another by many lengths of tubes. The adaptors are round blocks with multiple holes through them, each meant to house one end of tube. The adaptor in turn is connected with a device that pumps out fluids, which then pass through the tube and out the other adaptor. The fluid may be heated or cooled to accordingly heat or cool the chemical bath, respectively. The tubes – which may be many meters in length – are submerged in the chemical bath, allowing for the heat transfer to take place between the chemical and the fluid within the tubes. The size, length and quantities of these tubes will define the volume of fluids than can be passed through the heat exchanger. Furthermore, the wall thickness of the tubes use will add to the efficiency of the heat transfer.
PTFE heat exchangers follow this design, with both the adaptors and the tubes being made from pure virgin PTFE. However, the fabrication process can be tricky. For one, as PTFE does not easily join even itself, the fusing of the tubes with the adaptor needs to be done in one of two ways:
1. Bonding – the outer diameter of the tube is chemically treated as are the inner holes of the adaptor. Bonding can be done using an industrial grade adhesive, which would also need to show resistance to the chemicals in the bath. The advantage of bonding is that it is much easier to fabricate such an assembly. The disadvantage is that the bond may not hold in the face of high temperatures and in the event that an unexpected chemical enters the mix
2. Welding – welding PTFE is very tricky and is as much an art as it is a science. PTFE welding can only be done if specially modified grades of PTFE are used, which allow themselves to be welding. Grades such as Chemour’s NXT and Inoflon’s M490 are examples of modified grades that can be used to make the adaptors. The tubes too need to be made accordingly. Modified grades of tubing are needed to ensure that both the tube and the adaptors are able to fuse with one another under high-temperature conditions
Aside from temperature and chemical resistance, it is also vital that the heat exchanger assembly is able to withstand the pressure build-up due to the passage of fluid. In the event of higher pressures, both the fused/bonded assembly as well as the tube itself would need to be capable of holding the same. Because of this, the wall thickness of the PTFE plays a dual role. On the one hand, because PTFE is a poor conductor, a lower wall thickness ensures that the heat transfer is more efficient. However, a thinner tube means more complications in bonding and/or welding, while also a lower capacity to withstand higher pressures.
Apart from the use of PTFE in the heat exchanger assembly, expanded PTFE (ePTFE) also finds significant use in this application. Like virgin PTFE, ePTFE also exhibits superior chemical and heat resistant properties. At the same time, the sealing properties of ePTFE ensure that it forms an effective gasket material in any portion of the assembly that may require clamping and effective sealing for the fluids.
Overall, it seems unavoidable that for certain chemical baths, PTFE is the only viable option for a heat exchanger assembly. With the proper fabrication and right materials, it offers a very effective, durable, and versatile solution for all industries where heat exchangers are needed.