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ATS Liquid Cooling Ebook Select Technical Articles on Liquid Cooling and Its Various Roles in the Thermal Management of Electronics

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ATS Liquid Cooling Ebook Select Technical Articles on Liquid Cooling and Its Various Roles in the Thermal Management of Electronics ATS Liquid Cooling eBook Select Technical Articles on Liquid Cooling and its Various Roles in the Thermal Management of Electronics 89-27 Access Road, Norwood, MA 02062 | T: 781.769. 2800 F: 781.769.9979 | www.qats.com © Advanced Thermal Solutions, Inc. 1 Table of Contents Calculating The Loads For A Liquid Cooling System 2 Heat Transfer Analysis of Compact Heat Exchangers Using Standard Curves 4 What Fluids Can Be Used With Liquid Cold Plates in Electronics Cooling Systems 7 Cold Plates and Recirculating Chillers for Liquid Cooling Systems 10 Measuring The Thermal Resistance of Microchannel Cold Plates 13 © Advanced Thermal Solutions, Inc. 2 Calculating The Loads For A Liquid Cooling System This article presents basic equations for liquid cooling and Where: provides numerical examples on how to calculate the loads Tmod = surface temperature of the device being cooled in a typical liquid cooling system. When exploring the use qL= power dissipated by the device of liquid cooling for thermal management, calculations Rcp = thermal resistance of the cold plate and TIM are needed to predict its performance. While it is often Tw1 = water temperature entering the cold plate assumed that a liquid coolant itself dissipates heat from a In an open loop cooling system, the value of Tw1 is known component to the ambient, this is not the case. A closed and controlled by the supply system. Calculating Tw1 in a loop liquid cooling system requires a liquid-to-air heat closed loop system is more involved. This is because the exchanger. Because of its structure, several equations water temperature is both rising from the power transferred must be calculated to fully understand the performance and from the device and dropping while transferring heat to the behavior of a liquid cooled system. ambient in the heat exchanger. For this article we consider a liquid cooling system as a The temperature rise of the water due to the device power closed loop system with three major components: cold is given by: plate, heat exchanger and pump. The cold plate is typically L Tw2 - Tw1 = q (2) made from aluminum or copper, and is attached to the Cw device being cooled. The plate usually has internal fins Where: which transfer heat to the coolant flowing through them. Tw2 = temperature of the water exiting the cold plate and This fluid moves from the cold plate to a heat exchanger entering the heat exchanger where its heat is transferred to the ambient air via forced Cw = heat capacity rate of water convection. The final part of the cooling loop is the pump, The heat capacity rate is found by multiplying the mass flow which drives the fluid through the loop. rate and the specific heat of water. Equation 1, below, was derived to predict the final Once the liquid enters the heat exchanger it transfers heat temperature of the device being cooled [1]: into the air. The amount of heat transfer, at steady state, is equal to the heat produced by the component. Tmod = qL • Rcp + Tw1 (3) qL = ε • Cmin (TW2 − Ta ) Where: Ta = temperature of the air entering the heat exchanger ε = effectiveness of the heat exchanger Cmin = heat capacity rate of water (Cw) or air (Ca), whichever is smaller Next, we solve Equations 2 and 3 for Tw1 1 1 ) Ta (4) Tw1 = qL ( – + ε • Cmin Cw Finally, we combine Equations 1 and 4 to solve for Tmod 1 1 ) ) Ta Tmod = qL (Rcp + ( – + (5) Figure1. Closed Loop Liquid Cooling System [1] . ε • Cmin Cw © Advanced Thermal Solutions, Inc. 3 Several factors must be known to effectively use this mechanical concerns for the heat exchanger dictate that equation as a design guide. First, we must determine the it should be considered first. The cold plate and pump cold plate’s thermal resistance. This can be obtained from are typically easier to optimize. For example, if a heat a computational fluid dynamic (CFD) analysis of the cold exchanger’s size is limited, a copper micro fin cold plate plate using software such as CFDesign or Flotherm. It is and a high volume pump may be needed. If a sufficiently- also important to understand the manufacturing limits before sized heat exchanger is available, a lower cost cold plate starting the mechanical design. Typically a cold plate’s design is driven by these limits, which control the minimum width of its internal channels and fins. A cold plate’s thermal resistance is usually in the range of 0.05 to 0.25°C/W, depending on size and material. Next, the performance of the heat exchanger must be specified. A heat exchanger’s performance is the product of its efficiency (ε) and the total specific capacity rate flowing through it (Cmin). The performance specifies, in units of W/°C, the amount of heat removed from the heat exchanger at a given difference between the water and air temperatures. A typical heat exchanger used for electronics cooling, with a 120 mm fan, provides Figure 2. Device Temperature as a Function of Cold Plate Thermal Resistance. performance values of 11 to 20 W/°C [3]. As an example, consider the design of a liquid-cooled loop and pump may be used with sufficient results. for cooling an Intel Xeon X5492 processor. In summary, liquid cooling loops are an important part of Where: the electronics industry, and their use will continue into the qL= 150 W future. T = 63°C mod, Max It is important to understand the impact on performance T = 25°C a of all three major parts of liquid cooling loops (cold plate, M = 0.032 L/s (0.5 gpm) w heat exchanger and pump) to ensure an acceptable level of Rcp = unknown performance at the lowest cost. It is also critical to address Heat exchanger performance = 16.7 W/°C the design of the heat exchanger as it can be the limiting Equation 5 is used to solve for the device temperature factor in many systems. (Tmod) as a function of cold plate resistance. Figure 2 shows that to cool an X5492 processor in a 25°C environment a References: cold plate resistance of less than 0.18°C/W is required. 1. Simons, R., Estimating Temperatures in a Water-to-Air Figure 2 illustrates the basic steps needed to properly Hybrid Cooling System, Electronics Cooling, May 2002. design a liquid cooling loop. It also shows the importance of 2. Iyengar, M. and Garimella, S., Thermal Optimization and characterizing all the components within a system. The cold plate, heat exchanger, and pump must all be selected and Design for Manufacturability of Liquid-Air Hybrid Cooling designed as a group to ensure acceptable performance. Systems, Electronics Cooling, August 2008. The heat exchanger is often the limiting factor in a compact 3. Whitenack, K., Demystifying Cold Plates, Electronic system due to its relatively large size. Packaging and other Products, May 2003. © Advanced Thermal Solutions, Inc. 4 Heat Transfer Analysis of Compact Heat Exchangers Using Standard Curves Introduction In previous issues of Qpedia Thermal eMagazine [1,2], we Where, covered some fundamental and practical aspects of heat The Stanton number is defined as: exchangers. In this issue, we will concentrate on compact Nu heat exchangers and how to use standard curves for heat St = RePr transfer analysis. Compact heat exchangers are a class of heat exchangers which features at least one of the Nu, Re and Pr are Nusselt, Reynolds and Prandtl numbers, fluids, on one side, as a gas. They generally have a very respectively. large heat transfer area per unit volume (>700 m2/m3) After considering all of the constraints and identifying [3]. Most of the applications of compact heat exchangers the heat exchanger type, the engineer must look at the are in the area of electronics cooling, because space for a graphs and match the heat exchanger configuration to the cooling device is typically at a premium. Generally, the heat available data, if it is available, and extract the information exchangers used in electronics cooling are of the air-to- liquid type. The heat transfer coefficient on the air side is from there. significantly smaller than the liquid side, hence increasing The following example shows how to use curves to the surface area on the air side can compensate for the solve a heat exchanger problem: low value of the heat transfer coefficient. Compact heat Consider a circular tube-circular fin compact heat exchangers can come in different shapes and forms [3]. exchanger as shown in figure 1. They can be made as: The liquid water transfers the heat from hot components • Flat tubes (liquid) – continuous plate (air) through the aluminum tube. Ambient air is blown through • Round tube (liquid) – continuous flat plate (air) the fins attached to the tubes to remove the heat from hot • Round tube (liquid) – round disks (air) fluid. Calculating the overall heat transfer coefficient on the • Flat plate (liquid) – folded fin (air) single pass or multi-pass air side: Flow passages are generally very small (hydraulic diameter The geometrical dimensions of this heat exchanger and Dh < 5 mm) and the flow is considered laminar. Kays other information are as follows: and London [4] have compiled a large volume of data for Tube outside diameter = 9.65 mm different heat exchanger configurations. The data plotted is Fin pitch = 343 per m in the form of the Colburn factor (JH) and friction factor as -3m a function of Reynolds number (Re), as shown in figure 1. Flow passage hydraulic diameter = 3.929×10 2/3 -3 The Colburn JH factor is defined as: J H = StPr Fin thickness = 0.46×10 m © Advanced Thermal Solutions, Inc.
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