ATS Releases New Line of Tube-to-Fin, Liquid-to-Air Heat Exchangers

Advanced Thermal Solutions, Inc. (ATS) has introduced a new line of tube-to-fin, liquid-to-air heat exchangers that “push the boundaries of the technology with the industry’s highest density fins.” These new heat exchangers, available with or without fans, come in seven different sizes and 49 different options and are part of the array of liquid cooling products that ATS offers.

Heat Exchangers

ATS has released a new line of tube-to-fin, liquid-to-air heat exchangers that boast the industry’s highest density fins. (Advanced Thermal Solutions, Inc.)

ATS heat exchangers maximize heat transfer from fluid to air, which allows liquid to be cooled to lower temperatures than other heat exchangers on the market. The fins and tubes are made of copper and stainless steel and are suitable for a variety of different liquids, including water, dielectric fluids and custom designed heat transfer fluids.

Read the full product release announcement at https://www.qats.com/News-Room/Press-Releases-Content/1183.aspx.

ATS heat exchangers can be used in a variety of applications including laser cooling, cooling medical equipment and imaging devices, compressor cooling, semiconductor processing, HVAC, food and beverage processing, and other liquid cooling applications.

The following table shows the different heat transfer capacities and dimensions of the different heat exchangers that ATS has released:

Heat Exchangers

The heat exchangers have silver-solder brazed joints and have been internally cleaned and externally coated for corrosion protection. They are available with or without fans.

Watch the short video below to learn more:

For more information about Advanced Thermal Solutions, Inc. thermal management consulting and design services, visit www.qats.com or contact ATS at 781.769.2800 or ats-hq@qats.com.

Case Study: Designing Air-to-Air Heat Exchanger With Heat Pipes

Advanced Thermal Solutions, Inc. (ATS) engineers were tasked by a client to design an air-to-air, aluminum heat exchanger with multiple copper heat pipes that could meet high power demands (more than 400W) with a thermal resistance requirement of 0.046°C/W and could withstand a wide range of ambient temperatures from -40°C to 60°C. Also, the separation between the heat pipe’s evaporator and condenser sections needed to be air tight.

Heat Exchanger

ATS engineers were tasked with designing an air-to-air heat exchanger with heat pipes that would fit inside an enclosure. (Advanced Thermal Solutions, Inc.)

Using analytical modeling, ATS engineers calculated the system pressure drop from the heat pipe to the fin block to the flow turn and also the thermal performance of the fins in ducted flow to determine the proper amount of fins to avoid over pressurizing the fans, while at the same time meeting the thermal resistance demands of the system. It was calculated that a maximum of 14 fins per inch could be used, while the overall size was well within the client’s requirements.

Challenge: To design an air-to-air heat exchanger that could handle high power demands of more than 400W and specific requirements on thermal resistance (0.046°C/W).

Chips/Components: Electronics junction box that requires internal air cooling.

Analysis: ATS engineers conducted analysis of the pressure drop across the system from the heat pipe to the fin block to the flow turn section, as well as analyzing the thermal performance of the entire heat exchanger. This analysis included calculating the ducted flow, heat transfer coefficient, and the fin and heat pipe resistance of the exchanger. The analysis also explored the difference between designs with copper and with aluminum fins.

Design Data: The data showed that thermal resistance and pressure drop of the CFD model were within 16% of the analytical model. The thermal performance of the heat exchanger with heat pipes was 0.044°C/W, meeting the client’s requirements.

Solution: The ATS design was optimized for four heat pipes and a suggestion was made to enhance the heat exchanger by using copper fins, rather than aluminum, because of a higher fin efficiency and lower thermal resistance.

Net Result: The customer was supplied with a production design of a heat exchanger block with heat pipes that could fit into the enclosure and provide the necessary forced convection cooling to maintain the proper temperature for the system. ATS also supplied the heat exchangers from the prototype stage to production.

For more information about Advanced Thermal Solutions, Inc. thermal management consulting and design services, visit www.qats.com or contact ATS at 781.769.2800 or ats-hq@qats.com.

How is a Heat Exchanger Used in Liquid Cooling

A heat exchanger is a device that transfers heat from a fluid (liquid or gas) to pass to a second fluid without the two fluids mixing or coming into direct contact. Heat exchangers are commonly used in liquid cooling systems to dissipate heat from a fluid that has passed over a cold plate attached to the heat-producing component. The cool fluid is pumped through the system and back across the cold plate.

Heat Exchanger

An example of a standard liquid cooling loop using a heat exchanger to transfer heat from the liquid to the ambient. (Advanced Thermal Solutions, Inc.)

Heat exchangers are designed to maximize the surface area of the wall between the two fluids, while minimizing the resistance to fluid flow through the exchanger. The addition of fins or corrugations in one or both directions increases the surface area and increases the heat transfer capacity of the heat exchanger.

There are several types of liquid-to-air heat exchangers.

In a shell and tube heat exchanger, one fluid flows through a series of metal tubes and the second fluid is pumped through a shell that surrounds them. The fluid flow can be either parallel (flowing in the same direction), counterflow (flowing in opposite directions) or crossflow (flows are perpendicular to each other).

Tube-to-fin heat exchangers (as shown in the GIF above) use fins surrounding two tubes that carry the fluids. The fins increase the surface area and maximize heat transfer to the ambient. Some finned tube heat exchangers use natural convection and other can include fans to increase the airflow and heat transfer capacity.

Plate and frame heat exchangers have two rectangular end members holding together a series of metal plates with holes in each corner to allow the liquids to pass through. Each of the plates has a gasket to seal the plates and arrange the flow of the fluids between the plates. Brazed plate heat exchangers avoid the potential for leakage by brazing the plates together. Plate and frame heat exchangers are commonly used in food processing.

Common applications for heat exchangers include telecommunications, process cooling, power electronics, medical device and medical imaging, automotive, industrial, and HVAC.

Watch the video below to learn more:

For more information about Advanced Thermal Solutions, Inc. thermal management consulting and design services, visit www.qats.com or contact ATS at 781.769.2800 or ats-hq@qats.com.

Industry Developments: Heat Exchangers for Electronics Cooling

By Norman Quesnel, Senior Member of Marketing Staff
Advanced Thermal Solutions, Inc.

(This article will be featured in an upcoming issue of Qpedia Thermal e-Magazine, an online publication dedicated to the thermal management of electronics. To get the current issue or to look through the archives, visit http://www.qats.com/Qpedia-Thermal-eMagazine. To read other stories from Norman Quesnel, visit https://www.qats.com/cms/?s=norman+quesnel.)

Heat exchangers are thermal management tools that are widely used across a variety of industries. Their basic function is to remove heat from designated locations by transferring it into a fluid. Inside the heat exchanger, the heat from this fluid passes to a second fluid without the fluids mixing or coming into direct contact. The original fluid, now cooled, returns to the assigned area to begin the heat transfer process again.

The fluids referred to above can be gases (e.g. air), or liquids (e.g. water or dielectric fluids), and they don’t have to be symmetrical. Therefore, heat exchangers can be air-to-air, liquid-to-air, or liquid-to-liquid. Typically, fans and/or pumps are used to keep these heat transfer medium in motion and heat pipes may be added to increase heat transfer capabilities.

Figure 1 shows a basic heat exchanger schematic. A hot fluid (red) flows through a container filled with a cold fluid (blue) but the two fluids are not in direct contact.

Heat Exchangers

Figure 1. In a Simple Heat Exchanger Heat Transfers from the Hot (Red) Fluid to the Cold (Blue) Fluid, and the Cooler After Fluid Re-Circulates to Retrieve More Heat. [1]

One example of a common heat exchanger is the internal combustion engine under the hood of most cars. A fluid (in this case, liquid coolant) circulates through radiator coils while another fluid (air) flows past these coils. The air flow lowers the liquid coolant’s temperature and heats the incoming air.

Applied to electronics enclosures, heat exchangers draw heated air from a cabinet, cool it, and then return the cooled air to the cabinet. These heat exchangers should be designed to provide adequate cooling for expected worst case conditions. Typically, those conditions occur when the ambient is the highest and when electrical loads through the enclosure are very high. Under typical conditions, heat exchangers can cool cabinet interiors to within 5°F above the ambient air temperature outside the enclosure.

Air-to-Air

Air-to-air heat exchangers have no loops, liquids or pumps. Their heat dissipation capabilities are moderate. Common applications are in indoor or outdoor telecommunications cabinetry or in manufacturing facilities that don’t have a lot of dust or debris.

Air-to-air heat exchangers provide moderate to good cooling performance. They don’t allow outside air to enter or mix with the air inside the enclosure. This protects the enclosure’s contents from possible contamination by dirt or dust, which could damage sensitive electronics and electrical devices and cause malfunctions.

An example of higher performance, air-to-air heat exchangers is the Aavid Thermacore HX series. These heat exchangers feature rows of heat pipes that add effective, two-phase heat absorbing properties when moving hot air away from a cooling area. The liquid inside the heat pipes turns to vapor. This transition occurs inside a hot cabinet. (See Figure 2)

The vapor travels to the other end of the heat pipe, which is outside the cabinet. Here it is cooled by a fan, transitions back to liquid form, and cycles back inside the cabinet environment.

Heat Exchangers

Figure 2. An Air-to-Air Heat Exchanger with Heat Pipes Extending Inside (top) and Outside (bottom) a Cabinet. Internal Heat is Transferred Outside the Enclosure. (Aavid Themacore) [1]

Other air-to-air heat exchangers feature impingement cooling functionality that can provide better performance than using heat pipes. Aavid Thermacore’s HXi Impingement core technology uses a folded fin core that separates the enclosure inside and outside. A set of inside fans draws in the hotter, inside air and blows it toward the fin core. This inside impingement efficiently transfers the heat to the fin core. Similarly, a set of outside fans draws in the cooler, ambient air and blows it toward the outer side of the fin core removing the waste heat. See Figure 3 below.

Heat Exchangers

Figure 3. Air-to-Air Heat Exchangers with Double-Sided Impingement Cooling Technology Can Move Twice the Heat Load of Conventional Exchangers. (Aavid Themacore) [3]

Liquid-to-Air

In some electronic cabinets, high power components can’t be cooled by circulating air alone or the external ambient air temperature is not cool enough to allow an air-to-air heat exchanger to solve the problem unaided. In these applications, liquid-to-air heat exchangers provide additional cooling to maintain proper cabinet temperatures.

For example, in a situation where heat is collected through a liquid-cooled cold plate attached directly to high power components. Even with the cold plate, the ambient air external to the cabinet is not cool enough to maintain the internal cabinet temperature at an acceptable or required level. Here, a liquid coolant in an active liquid-to-air heat exchanger can be used to cool the enclosure.

Heat Exchangers

Figure 4. Tube-to-Fin, Liquid-to-Air Heat Exchangers Provide High-Performance Thermal Transfer. [4] (Advanced Thermal Solutions, Inc.)

Advanced Thermal Solutions, Inc. (ATS) tube-to-fin, liquid-to-air heat exchangers have the industry’s highest density of fins. This maximizes heat transfer from liquid to air, allowing the liquid to be cooled to lower temperatures than other exchangers can achieve. All tubes and fins are made of copper and stainless steel to accommodate a wide choice of fluids.

Available with or without fans, ATS heat exchangers are available in a range of sizes and heat transfer capacities up to 250W per 1°C difference between inlet liquid and inlet air temperatures. They can be used in a wide variety of automotive, industrial, HVAC, electronics and medical applications. [4]

Heat Exchanger

Figure 5. Small, Light-Weight Liquid-to-Liquid Heat Exchanger Provides Efficient Cooling Performance. [5]

Lytron’s liquid-to-liquid heat exchangers are only 10-20% the size and weight of conventional shell-and-tube designs. Their internal counter-flow design features stainless steel sheets stamped with a herringbone pattern of grooves, stacked in alternating directions to form separate flow channels for the two liquid streams. This efficient design allows 90% of the material to be used for heat transfer. Copper-brazed and nickel-brazed versions provide compatibility with a wide range of fluids. [5]

Nanofluids

The development of nanomaterials has made it possible to structure a new type of heat transfer fluid formed by suspending nanoparticles (particles with a diameter lower than 100nm). A mixture of nanoparticles suspended in a base liquid is called a nanofluid. The choice of base fluid depends on the heat transfer properties required of the nanofluid. Water is widely used as the base fluid. Experimental data indicates that particle size, volume fraction and properties of the nanoparticles influence the heat transfer characteristics of nanofluids. [5]

When compared to conventional liquids, nanofluids have many advantages such as higher thermal conductivity, better flow, and the pressure drop induced is very small. They can also prevent sedimentation and provide higher surface area. From various research, it has been found that adding even very small amounts of nanoparticles to the base fluid can significantly enhance thermal conductivity.

Heat Exchangers

Figure 6. 3D Design of Curved Tube Heat Exchanger. Increased Turbulence and Velocity Increases Heat Transfer Rate. [6]

A recent paper by Fredric et al. proposes a theoretical heat exchanger with curved tubes and with nanofluids as the coolant. Nanofluids in place of regular water provide improved thermal conductivity due to the increased surface area. The heat transfer rate is further improved using curved tubes in place of straight tubes because the used of curved tubes increases the turbulence and fluid velocity, which helps increase the heat transfer rate. [6]

References
1. Advanced Thermal Solutions, Inc., https://www.qats.com/Products/Liquid-Cooling/Heat-Exchangers.
2. Aavid Thermacore, http://www.thermacore.com/documents/system-level-cooling-products.pdf.
3. Aavid Thermacore, http://www.thermacore.com/products/air-to-air-heat-exchangers.aspx.
4. Advanced Thermal Solutions, https://www.qats.com/Products/Liquid-Cooling/Heat-Exchangers.
5. Kannan, S., Vekatamuni, T. and Vijayasarathi, P., “Enhancement of Heat Transfer Rate in Heat Exchanger Using Nanofluids,” Intl Journal of Research, September 2014.
6. Fredric, F., Afzal, M. and Sikkandar, M., “A Review on Shell & Tube Heat Exchanger Using Nanofluids for Enhancement of Thermal Conductivity,” Intl. Journal of Innovative Research in Science, Engineering and Technology, March 2017.

For more information about Advanced Thermal Solutions, Inc. thermal management consulting and design services, visit www.qats.com or contact ATS at 781.769.2800 or ats-hq@qats.com.

Fin Optimization in Heat Sinks and Heat Exchangers

(This article was featured in an issue of Qpedia Thermal e-Magazine, an online publication produced by Advanced Thermal Solutions, Inc. (ATS) dedicated to the thermal management of electronics. To get the current issue or to look through the archives, visit http://www.qats.com/Qpedia-Thermal-eMagazine.)

In electronics cooling, often separately managed Thermal/Mechanical (TM) and Software/Electrical (SE) engineering teams are finding themselves facing common challenges, as they are being driven towards similar business goals, such as product differentiation, company growth and profitability.

More so than ever today, these teams are being directed to find ways to increase component performance, particularly on highly populated boards within complex systems, at an acceptable cost of manufacturing. They are also discovering that their goals are being held back by governing specifications, environmental conditions, mechanical limitations and budget restrictions.

Heat Exchangers

Closeup of fin array on an ATS tube-to-fin heat exchanger. (Advanced Thermal Solutions, Inc.)

TM’s design thermal solutions based on airflow, envelope size, power dissipation, etc. and migrate (as expected) to the lower cost “standard solutions” whenever possible. If adequate margin is not met, reliability implications are more apparent as engineers will have to optimize solutions. This is because, in most cases, the form factor, layout, boundary conditions, etc. are set.

Thermal solutions become the gatekeeper, and in some cases, the determining factor in product deployment.

Many leading companies design their products by using technologies that will sustain long product life cycles for increased market share and brand awareness. As products are refined through the design cycle, thermal solutions may have to be optimized and this requires many investigations to be undertaken.

As the electronics industry continues to use components dissipating more and more power, new heat sink solutions must be able to accommodate large heat fluxes while keeping the same spatial dimensions [1]. Finned heat sinks and heat exchangers are largely employed in many engineering fields, and this demand spurs researchers into devising and testing new geometries for the heat sinks.

Engineers constantly try to develop new designs to enhance the performance of heat exchangers. One such effort is the design of the wavy fins to enhance the surface area.

Figure 1 shows a close up view of an extrusion type thermal solution where the profile has a feature of undulated fins. In general, a wavy fin heat sink should perform better under natural and forced convection due to the increased surface area created by the fins. This feature can easily be manufactured with a die. The “waviness” can be adjusted to increase surface area resulting in a positive impact on thermal performance.

Heat Exchangers

Figure 1. Close-Up View of Simply Wavy Fin Geometry [1]

Theoretical models have been devised to find the pressure drop and the heat transfer from wavy fin geometries. Figure 2 shows the schematic of a wavy fin.

Heat Exchangers

Figure 2. Schematic of a Wavy Fin Geometry [2]

In this figure, the fins are assumed to have a sinusoidal geometry where

λ = Wave length (m)
H = channel width (m)
S = channel height
2A = twice the amplitude of the wave

The shape of the curve is assumed to be:

The length of the curve can be found from the following equation:

Shah and London [3] came up with the following equation for the friction and Nusselt number in channels:

Where,
F = fanning friction factor
aspect ratio

The same equation applies for a wavy fin based on the correct length:

The Nusselt number for the straight fins and wavy fins is the same as long as the correct surface area is used:

The above equations are for the low Reynolds number.

For high Reynolds number Shapiro et. al [4] derived the following equations:

Where,
Dh = hydraulic diameter (m)
Reynolds number based on hydraulic diameter
L = half length of the channel (Le/2)
Pr = prandtl number
Dh = 2SH/(S+H)

The combined asymptotic for the friction and Nusselt number is as follows:

Figure 3 compares the results of the above analytical equations with the results from Kays and London [5]. In the graph, the Colburn j factor is shown and is defined as:

The results show that the experimental values of Shah and London are within 20% band of the values obtained from the above relations. The data is for the fin type 11.44-3/8W.

Heat Exchangers

Figure 3. f and j Values as a Function of Reynolds Number.[2]

Marthinuss et al. [6] reviewed published data for air-cooled heat sinks, primarily from Compact Heat Exchangers by Kays et al [5] and concluded that for identical fin arrays consisting of circular and rectangular passages, including circular tubes, tube banks, straight fins, louvered fins, strip or lanced offset fins, wavy fins and pin fins, the optimum heat sink is a compromise among heat transfer, pressure drop, volume, weight and cost.

Figure 4 shows that if the goal is to get a higher value of heat transfer per unit of pressure drop, the straight fin is the best. Figure 5 shows that when heat transfer per unit height is of concern pin fin is the best.

Heat Exchangers

Figure 4. Profile Comparisons Based on Heat Transfer/Pressure Drop. [6]

Figure 5. Profile Comparisons Based on Heat Transfer/Volume. [6]

Sikka et al. [7] performed experiments on heat sinks with different fin geometries. Figure 6 shows 3 different categories of heat sinks tested. The conventional fins, such as straight and pin fins, are shown in (a); (b) shows the fluted fins and (c) shows the wavy fin design. The tests were done for both horizontal and vertical direction of air flow at natural convection and low Reynolds number forced flow. Table 1 shows the dimensional values of each of these heat sinks.

The last column shows the values of At/Ab (total surface area/base surface area).

Figure 6. (a) Traditional Fins, (b) Fluted Fins, (c) Wavy Fins. [7]

Table 1. Geometries and Dimensions of the Heat Sinks. [7]

The values of the Nusselt number were reported based on the following relation:

Figure 7 shows that for natural convection in the horizontal direction, the pin fin has the best performance. The fluted fins have, in general, a better performance compared to longitudinal fins. The lower graph in figure 7 shows that the wavy fins are essentially the same as the longitudinal fins.

Figure 7. Nusselt Number As a Function of Rayleigh Number for Natural Convection-Horizontal Direction. [7]

Figure 8 shows the natural convection cases for the vertical direction. The figure shows that heat transfer decreases for the pin fin, but increases for the plate fin. The pin fin still is better than the plate fin, but the difference is only 4-6%. Figure 8 also shows that the cross cut heat sink has the best performance. The bottom figure in 8 confirms that the wavy fins do not have much better heat transfer compared to plate fins.

Figure 8. Nusselt Number as a Function of Rayleigh Number for Natural Convection-Vertical Direction. [7]

Figure 9 shows the Nusselt number for forced convection over a horizontal plate as a function of Reynolds number. This figure indicates that, for very low Reynolds numbers, the cross fin is better than the pin fin; but, around Re = 2000, the situation reverses and the pin fin gets better than the cross cut heat sink. For low Reynolds numbers, the longitudinal pins are better than the wavy fins; but, at higher Reynolds numbers, the performance of the wavy fins gets better by almost 12-18%.

Figure 9. Nusselt Number as a Function of Reynolds Number for Forced Convection-Horizontal Direction. [7]

Figure 10 provides the Nusselt numbers for the vertical direction for forced flow. In comparing the results with the horizontal direction, the results are almost the same, with the difference being that the wavy fin heat sinks perform better than the plate fin heat sinks, by about 14-20%.

Figure 10. Nusselt Number as a Function of Reynolds Number for Forced Convection-Vertical Direction.[7]

The results presented in this article strengthen our understanding about how heat exchangers and heat sinks can be made more compact and efficient. The results show that the design of the fin field is still an issue and much remains to be investigated for optimization, depending on the conditions and application.

Further empirical testing is warranted for the evaluation of the effects of wavy fin heat sinks, as fine meshing and a high degree of confidence is not easily obtained through simulating these profiles using commercial CFD tools.

References:

1. Lorenzini, M., “Performance Evaluation of a Wavy-Fin Heat Sink for Power Electronics” Applied Thermal Engineering, 2007.
2. Awad, M., Muzychka, S., “Models for pressure drop and heat transfer in air cooled compact wavy fin heat exchangers”, Journal of Enhanced Heat Transfer, 18(3):191-207(2011).
3. Shah, R., London, A., “Advances in heat transfer, suppl. 1, laminar forced flow convection in ducts”, New York, Academic press, 1978
4. Shapiro, A., Sigel, R., Kline, S., “Friction factor in the laminar entry region of a smooth tube,” Proc., 2nd V.S.Nat. Congress of applied mechanics, PP. 733-741, 1954.
5. Kays, M., London,L., “Compact Heat Exchangers”, Third Edition, McGraw-Hill, 1984.
6. Marthinuss, E., Hall, G., “Air cooled compact heat exchanger design for electronics cooling”, Electronics cooling magazine, Feb 1st, 2004
7. Sikka, K., Torrance, K., Scholler, U., Salanova, I., “Heat sinks with fluted and wavy fins in natural and low-velocity forced convection”, IEEE, Intersoceity Conference, 2000.

For more information about Advanced Thermal Solutions, Inc. thermal management consulting and design services, visit www.qats.com or contact ATS at 781.769.2800 or ats-hq@qats.com.