The Usage Polymers In Heat Exchanging Materials: Analytical Essay

Abstract

Polymers represent a good alternative option for current metallic heat exchangers. That is due to their low weight ,cost, and volumetric-thermal-capacity. Polymers also considered as stable anti-corrosive material. Furthermore, fouling is less likely to stick on polymer surface compared to metals. However, polymers thermal conductivity is way less that metals (more than 100 times). Several researches in the literature were dedicated to exploit the properties of polymers by several ways. The solutions reviewed here are: use polymeric coating for metals, using composites, reduce the polymer wall thickness, apply micro/nano surface finishing, and integrate two different discrete materials. among the studied cases, it was noticed that only few researches designed for hydraulic performance. however, from thermal performance point of view. There are several proven cases of superior polemer performance compared to metallic heat exchanger.

Introduction

Heat transfer basics

Heat exchangers are used to transfer heat from one medium to another, usually with a barrier between the materials. In this study, we are interested in transferring heat between fluids. Thus, the heat is supposed to pass from 1- the 1st fluid medium, 2- the heat exchanger material, 3-the 2nd fluid material. So, the performance of the heat exchanger depends on how these materials are interacting with each other, and what are the materials properties. It actually follows the following equation (under steady stat conditions).

Where Q is the overall heat transfer (watts), R is the thermal resistance, and is the logarithmic mean temperature difference between the two fluid medium.

To insure the maximum heat exchanger performance we have to insure the lowest possible value of the thermal resistance R, so here are the main parameters that affects the value of R

1. Fluid to barrier interaction is supposed to be optimized, in order to increase the rate of heat transfer from the fluid-solid interface.
2. high thermal conductivity for the heat exchanger material,
3. low thickness for the heat exchanger barrier,
4. maximum surface area between the barrier and the adjacent fluid.

Polymers as heat exchangers

Why polymers

There are several reasons that make polymers better options, but the main reasons are the five listed below.

1. Polymers are known for being very stable components, thus they offer corrosive resistance material.
2. It also provide an anti-fouling medium, so we will be less worried that heat exchanger performance reduced over long time, nor that the heat exchanger channels will be closed due to the formation of fouling. And thus we will be able to design a smaller fluid channels.
3. Low volumetric thermal capacity material: when the heat exchanger operate at the beginning, it go through transient conditions, which means that it is required to heat/cool the heat exchanger material. Low heat capacity means less energy waist/gained for this operation. For a highly intermitted temperature heat exchanger, this is a very important feature.
4. Low weight, which make them a potentially preferred option for vehicles, as they reduce the fuel consumption. As well as many other applications.
5. Low cost material.

For a better understanding of the properties Table #1 shows comparison between metallic and polymers in general.

material

Corrosion resistance

Thermal conductivity

Thermal capacity

(volumetric)

Cost

Density

Aluminum

Poor

222

2.50

3.47

2719

Copper

Good

387.6

3.38

6.2

8978

Stainless Steel

Good

16.3

3.93

3.51

8030

Brass

Good

109

3.51

3.7

8730

Titanium

Very Good

7.4

2.64

56.5

4850

Polyethylene

Excellent

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0.33-0.48

1.6-1.8

1.0-1.7

925-959

ABS

Excellent

0.1

2.19

1.5

1070

Polypropylene

Excellent

0.14

1.7

1.6

946

Challenges

1- Thermal conductivity, as known that polymers are known for low thermal conductivity, actually they are sometimes used as insulators. Comparing the thermal conductivity of polymers with that of metals we will notice that the thermal conductivity of metals is 100 times of the thermal conductivity of a polymer (Chen, Su, Reay, et al., 2016) However, this is not the only parameter that controls the overall performance of the heat exchanger (as depicted in Section ‎1.1).

2- Durability: due to the different mechanical stresses, it is less durable than available metallic option.

Researches:

Researches in this field are trying to maximize the performance of polymer heat exchangers. The main control variables for this goal are the materials that are being used and the design shape of the heat exchanger.

Composites

the usage of composites allow to enhance the material thermal conductivity and other properties.

According to (Hinze et al., 2017) The high volumetric heat capacity of metals limits the performance of low-temperature absorption chiller, polyamide 6 integrated with expanded graphite particles enhanced the performance of the absorption chiller. The experiment were carried on by changing the volume percentage of the graphite particles. The proposed material lowered the thermal capacity. Though the obtained thermal resistance was much higher than that of conventional aluminum heat exchanger ( 35.2 to 173.7 times), the performance of the system enhanced as a result of 30~34% reduction in heat capacitance. However, this result is with the assumption that surface-to-volume ration was just as metallic aluminum heat exchanger.

Similar job was done by (Cevallos, Bar-cohen and Deisenroth, 2016) where they utilized a polymer composite based heat exchanger for HVAC system. They used staggered tube lines (known as webbed tube), their composite composed of Nylon 12 filled with pitch-based carbon fibers. The heat exchanger were manufactured by injection molding[footnoteRef:1]. Based on the obtained results, the additions of carbon fibers allowed a performance enhancement over all polymer heat exchanger performance by a factor of 1.65 as a result of increasing the conductivity. However, the performance they got was way less than typical conventional aluminum HX. [1: Refer to section ‎3.1]

Polyaimid 66 filled with metals

Thin walled -all polymer- heat exchanger.

The Idea of thin-walled heat exchangers is to minimize the wall thickness of the fluid barrier so that the thermal resistance drops. This method result in a reduction over the thermal resistance.

In the research paper (Arie et al., 2017), additive manufacturing technique is utilized. HDPE material was used as heat exchanger material. With a wall thickness measured in the microscale, the obtained similar results of typical aluminum heat exchangers, the pressure drop was also similar. They also tested it for the pressure, where it started to fail at a pressure difference of 2 bars.

Thicker wall were designed by (Han et al., 2019) Acrylonitrile Butadiene Styrene ABS material were chosen with a wall thickness of about 300 μm. The design of their proposed heat exchanger was a modified version of regular tube array heat exchanger, as they replace the circular cross section of the tubes by a droplet cross section. A regular aluminum tube array heat exchanger was considered as a baseline. The proposed heat exchanger cost is lowered, while the thermal and hydraulic performance were 92% , and 163% of their baseline, respectively.

Polypropylene hollow fibers were used by (Astrouski, Raudensky and Krásny, 2016) to replace aluminum radiators in cars. The fibers were fabricated by extrusion[footnoteRef:2] with a two different outer diameters 0.6 and 0.8 mm with a wall thickness of 60 ,and 80μm respectively. these fibers can handle relatively very high pressure of 50 bars, and a temperature of 80. Selecting the smaller diameter increases the number of required fibers which will increase the pressure drop on the liquid side. While using the larger diameter will increase the pressure drop on the air side. However from the thermal performance point of view, hollow fibers overall thermal conductivity was superior to the metallic conventional heat exchanger. [2: Refer to section ‎3.1]

Polypropylene hollow fibers were studied again by (Chen, Su, Aydin, et al., 2016) for water to water application. The fiber outer diameter used was 0.55mm with a wall thickness of 0.1mm. bundle of this fiber was installed inside a tube to form a counter flow heat exchanger. They studied deferent diameters for the outer tube, and got a heat exchanger with a performance of 2-8 times better than their reference metallic counter flow heat exchanger. Comparing this result with the previous one studied by (Astrouski, Raudensky and Krásny, 2016), won’t be fair, as they both studied two different media, with different conditions.

Polymer and metals as two discrete materials

Integrating the polymers with the metals was another side of literature studies. Such an idea is using polymer as a coating a very thin coating, just like micro thick walled heat exchanger idea, thin coating result in very small thermal resistance while allowing us getting the advantages of polymer heat exchanger (anti-fouling, and anti-corrosion). Such HX keep the robust support. A nano-scale coat is applied to a metallic heat exchanger. Poly oligo ethylene glycol methacrylate POEGMA, was used for this purpose by (Ege et al., 2019). The polymer here was produced using addition polymerization.

Another idea was proposed by (Arie et al., 2020) They produced a polymeric cross-flow heat exchanger with ABS material integrate with aluminum wires. These wires are in contact with both liquid and gas side of the heat exchanger-figure1-. The Heat exchanger with both materials was 3D printed using Fused Filament Fabrication process (FFF). Compared to all-metallic heat exchanger, a performance enhancement of 25% was obtained, in addition to the lower weight and coast.

Figure 1: aluminum wires integrated with ABS heat exchanger channels. (Arie et al., 2020)

Composite and microchannel and nanochannels

Using micro and nano channels allow increasing the surface area of the heat exchanger. Which in return reduces the thermal resistance. However, using this method will also result in high head losses.

Further enhancement over the performance of the polymer heat exchanger for aluminum- polypropylene composite were studied by (Sun et al., 2017) , the enhancement was due to facilitate a microchannel heat exchanger. Which was fabricated according to isothermal hot embossing in solid-like state (IHESS)[footnoteRef:3]. They claimed that their proposed heat exchanger was slightly superior to the conventional Aluminum heat exchanger. [3: Refer to section ‎3.2]

There are various several research in Nano/microstructure were found in the literature for solid to air heat sinks.

It should be noted that there are many researchers studied the performance of solid to air heat sinks however they were not reviewed here as heat sinks is out of this project scope.

Polymer fabrication methods:

In the researches we found several methods for fabricating the polymers, this section highlights the most important methods.

Typical polymer fabrication

Injection molding (Rosato, Dominick V., Rosato, Donald V., 2012) is one of the most common method for polymer industry, in which the polymer is heated above the melting temperature, and then injected to a mold, this process can accept thermoplastic and thermosetting polymers.

Polymer extrusion is another very common method for fabricating the polymers, thermoplastics/thermoplastic composites are melt, then pushed through either a die or a nozzle. Extruding polymers through a die means that they will have a uniform shape. Die extrusion. While using a nozzle usually exist for 3D printing, where the polymer is formed layer by layer.(Pierre G. Lafleur, 2014)

[bookmark: _Ref26403430][bookmark: _Toc26907082]Nano- micro structure fabrication

One of the main methods for micro tube manufacturing is the isothermal hot embossion in solid like state (IHSS) (Sun, 2018). This method allows fabrication of micro to nano scale polymer. The method is composed of five main processes. Set the polymer inside the required mold, heat the polymer with the addition of pressure, cool the polymer inside the mold, and finally remove the polymer for the mold. We notice that we require thermo plastic material for this process to be done.

3d printing also allow the development of a micro-scale printing.

Discussion and conclusion

Using polymers for heat exchanger add several required properties such as anti-fouling and anti-corrosive properties. However, polymer thermal conductivity is very low. In the literature, several ideas were proposed to overcome this issue. One method is using polymer based composites. But using composites alone were not enough boosting the performance to the level of aluminum heat exchanger.

Rather, Some researchers investigated combining two discrete materials. such as coating the metallic heat exchanger with polymers to minimize the fouling. Penetrate the all polymer heat exchanger with metallic wires for such case the performance of the system further enhanced to slightly better than metallic heat exchanger.

Micro and nano surface finishing was also investigated and resulted in heat exchanger that slightly exceed the performance of aluminum heat exchanger.

One last method was reviewed in this report is to use a very thin polymeric wall thickness. Several material were studied. Including High Density Polyethylene, ABS, and Polypropylene, according to the reviewed result, polypropylene hollow fibers was the best performance heat exchanger among all other heat exchangers. But in general comparing all of the mentioned heat exchangers with each other won’t be fair. That because each research studied different medias with different conditions.

References

1. Arie, M. A. et al. (2017) ‘Experimental characterization of heat transfer in an additively manufactured polymer heat exchanger’, Applied Thermal Engineering. Elsevier Ltd, 113, pp. 575–584. doi: 10.1016/j.applthermaleng.2016.11.030.
2. Arie, M. A. et al. (2020) ‘An additively manufactured novel polymer composite heat exchanger for dry cooling applications’, International Journal of Heat and Mass Transfer. Elsevier Ltd, 147, p. 118889. doi: 10.1016/j.ijheatmasstransfer.2019.118889.
3. Astrouski, I., Raudensky, M. and Krásny, I. (2016) ‘Polymeric hollow fiber heat exchanger as an automotive radiator’, 108, pp. 798–803. doi: 10.1016/j.applthermaleng.2016.07.181.
4. Cevallos, J., Bar-cohen, A. and Deisenroth, D. C. (2016) ‘Thermal performance of a polymer composite webbed-tube heat exchanger’, International Journal of Heat and Mass Transfer. Elsevier Ltd, 98, pp. 845–856. doi: 10.1016/j.ijheatmasstransfer.2016.03.075.
5. Chen, X., Su, Y., Aydin, D., et al. (2016) ‘Experimental investigations of polymer hollow fibre heat exchangers for building heat recovery application’, Energy & Buildings. Elsevier B.V., 125, pp. 99–108. doi: 10.1016/j.enbuild.2016.04.083.
6. Chen, X., Su, Y., Reay, D., et al. (2016) ‘Recent research developments in polymer heat exchangers – A review’, Renewable and Sustainable Energy Reviews. Elsevier, 60, pp. 1367–1386. doi: 10.1016/j.rser.2016.03.024.
7. Ege, J. et al. (2019) ‘Progress in Organic Coatings Evaluation of Surface-initiated Polymer brush as Anti-scaling Coating for Plate Heat Exchangers’, Progress in Organic Coatings. Elsevier, 136(July), p. 105196. doi: 10.1016/j.porgcoat.2019.06.042.
8. Han, U. et al. (2019) ‘Development and design optimization of novel polymer heat exchanger using the multi-objective genetic algorithm’, International Journal of Heat and Mass Transfer. Elsevier Ltd, 144, p. 118589. doi: 10.1016/j.ijheatmasstransfer.2019.118589.
9. Hinze, M. et al. (2017) ‘Reduction of the heat capacity in low-temperature adsorption chillers using thermally conductive polymers as heat exchangers material’, Energy Conversion and Management. Elsevier Ltd, 145, pp. 378–385. doi: 10.1016/j.enconman.2017.05.011.
10. Pierre G. Lafleur, B. V. (2014) Polymer Extrusion. John Wiley & Sons.
11. Rosato, Dominick V., Rosato, Donald V., R. M. G. (2012) Injection Molding Handbook. 3rd edn. Springer Science & Business Media. doi: 10.1007/978-1-4615-4597-2.
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