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Cooling Systems. Part 3: Complex Approach to Computer Cooling

September 18, 2002



The problem of effective cooling of high-performance computer systems has been on everybody's lips for a long time already and it adds troubles both for experts, enthusiasts and average users. The situation is aggravated by the fact that many PC assemblers simply "ignore" (probably for the sake of a higher income) a complex approach to cooling of a whole computer system: most part of produced computers are pressed into too tight and hot boxes deprived of any effective means of internal ventilation. It's not crucial for budget systems, but correct and reliable operation of a high-performance computer filling in such conditions is questionable. 

Last time we took a deep look at operation of fans and their key technical parameters. Today we are coming back to these devices: we are going to use their performance curves in practice and look at the ways of objective estimation of effectiveness of cooling devices for computer cases. 

Initial conditions

Apart from arranging internal components and satisfying users' aesthetic needs every system enclosure is in change of efficient removal of heat generated by the filling, including a power supply unit. Almost every computer component requires its own climatic conditions. The most severe requirements come from modern processors of Intel and AMD: the air temperature on the "input" of a fan of a processor's cooler mustn't exceed 35-40°C. Other components (mainboard, video card, hard drives, DVD-ROM/CD-RW drives etc.) are less exacting, but they all are in the same "hold" and willingly support desires of the latter. 

The problem of maintaning the optimal internal temperature gets harder to tackle: the overall heat "capacity" of computers keeps on growing (heat development of modern complex systems on the Athlon XP or Pentium 4 can reach 250-300 W), and no steps of thermal optimization of typical configurations of ATX cases can be noticed. Some advanced users try to optimize cooling systems themselves using the cut-and-try method which not always brings desirable results. However, there is a simper and much more reliable technique that allows us to estimate objectively effectiveness of a given cooling system and, if necessary, to optimize the system or get a new better system enclosure. 

This technique is based on a simple semiempirical formula 

Q = 1.76*P/(Ti - To), where (1)
P - full thermal power of a computer system,
Ti - temperature inside the system enclosure,
To - temperature on the input of the box (ambient, indoors temperature),
Q - performance of the complex cooling system. 

This correlation shows what performance a cooling system must have to remove required thermal power at given temperatures inside and outside a computer unit. Note that it accounts only for convective heat transfer (i.e. with an air flow). Other types of heat transfer - thermal conductivity (i.e. via contacts of internal devices and panels of the enclosure) and radiant heat exchange (via electromagnetic radiation) are not taken into account. However, these two types do not have a noticeable contribution (it doesn't exceed 2-5% of the overall heat development), that is why P can be considered an entire thermal power of a system. 

Well, let's take an average statistical configuration of a high-performance computer, write down values of thermal power developed by its components and put them into the Table 1. 

Table 1. Thermal power of computer components
Component Thermal power, W
AMD Athlon XP 2000+ (Intel Pentium 4 2 GHz)  65
VIA KT333 based mainboard (Intel i845E)  25
DDR DRAM, 512 MB 10
Nvidia GeForce 4 video card 20
IDE 40-60 GB hard drives, 7200 rpm, 2 pcs 15
DVD-ROM drive 5
CD-RW drive 5
Multimedia card/ sound card (5.1 channel) 5
Overall power of the components 150
Thermal power of a standard power supply unit with passive PFC (efficiency - 0.75) 50
Total 200

So, let's set the "input" temperature equal to 25°C and the desirable internal temperature to 35°. The sought value of performance of a cooling system is approximately equal to 35 CFM. If you are going to put all components into a fan-less case, 25-30 CFM as a rated performance of the internal fan of the power supply unit will be the maximum that can be reached, which is still not enough for the comfortable conditions. Besides, as we found out last time, real performance of a fan in certain conditions is much lower than the rated one. As a result, we can face a problem of impossible maintenance of either comfortable internal temperature or even a permissible one. 

System impedance

System impedance is used for quantitative description of resistive effect a given computer system and its components have on an air flow. This aerodynamic characteristic is based on the following formula: 
P = K*Qn, where (2)
K - system constant,
Q - fan performance,
n - turbulent factor (1 <= n <=2, n = 1 in case of a laminar flow regime, n = 2 in case of a turbulent flow),
P - system impedance. 

System impedance is calculated in mm or inches of water (like static pressure) and shows pressure of an air flow at a given volume rate in a given system. An exact shape of an impedance curve can be found only in lab conditions, with a defined system constant and turbulent factor. But in most cases this can be brought to a linear dependence 

P = k*Q, (3)
where dimensional constant k can be found in a reference source (later on we will see several values of the constant for typical configurations of ATX cases). 

The system impedance is important from the practical standpoint: if you draw the impedance curve using experimental or reference data and superpose it with a characteristic curve of a fan you can define a real performance of the fan in a given system. 

Let's take as an example the IN-WIN IW-S508 case (without additional fans), put in here the above mentioned configuration based on the Athlon XP and install the CWT-420ATX12 power supply unit with a non-standard powerful fan ADDA AD0812HB-A70GL rotating at 3100 rpm. The impedance of such system can be expressed as P = 0.085*Q. Now let's draw a final system impedance curve and superpose it with the fan's performance curve to get a working point of the fan, i.e. its real performance in these conditions. 




Figure 1 System impedance, fan's performance curves and 
working points

On Figure 1 the curve I corresponds to the impedance of our system, the curve H to the fan's performance curve, and the point of their intersection (point A) to the working point of the fan. As you can see, a real performance level of even such a powerful fan is far from required 35 CFM (it's just 18 CFM). Taking into account that typical power units of 250-300 W usually have quite slow fans rotating at 2000-2500 rpm and having 25-30 CFM (their characteristic curves correspond to curves L and M), the flow rate in such systems (points B and C) will be around 10-14 CFM. An internal temperature thus can easily reach the dangerous limit of 55°C, which is unfavorable not only for a processor but for other components as well (primarily, hard drives and video cards). It's naive to hope for a reliable and correct operation of a high-performance system in such an oven! 

So, a typical fanless enclosure can't serve as a comfortable house for a high-performance computer system. The upper limit for such cases is 100-115 W of thermal power which usually corresponds to heat generated by budget or highly integrated systems used for office tasks. For systems that generate over 115 W fanless enclosures are not a match, they are even dangerous. 

Attention. The above things concerned top rear mounted power supply cases (TRMPS). Core logic mounted power supply cases (CLMPS) usually have a higher system impedance. Therefore, the actual performance of fans in the CLMPS cases will be lower. The maximum thermal power such cases are able to cope with is within 75-100 W. Be careful!

So, what should we do to provide the required climatic conditions for the computer stuffing? The way out is just one - take a system enclosure equipped with additional cooling devices. 

Additional fans

A wide range of system enclosures have special seats for fans on the front/rear panels. As we found out, a standard fan installed into a power supply unit is sufficient to remove only 115 W of thermal power. But with a hotter system we can't do without additional coolers. Well, they are installed right in the above mentioned seats. 

Will such fans be able to become an effective cooling complex and provide acceptable climatic conditions for computer components? Let's see! We are using again the IW-S508 case and the Athlon XP based system, plus one additional fan (the same ADDA AD0812HB-A70GL) into an appropriate seat on the rear panel. Here is Figure 2. 




Fig 2. Performance curves of different complex cooling systems

What brings the rear fan? First of all, at the expense of redistribution of air flows inside the case the overall system impedance falls down considerably, now we can write it down as P = 0.054*Q (curve I on fig.2). Secondly, the average volume air flowrate gets much higher (curve RF demonstrates the total performance of the power supply unit's fan and the additional fan). Thus, the actual performance of the general cooling system reaches 33-34 CFM (point A), which is very close to the required 35 CFM and enough to maintain a comfortable internal temperature. 

Now let's see if we add one more fan onto the front panel. Unfortunately, it doesn't bring much benefit. The impedance of the system remains the same (even a little bit greater), the overall performance of the cooling complex consisting of three fans (one in the power supply unit and two inside the case) grows up by just 4-5 CFM (fig.2, curve RFF and point B, the impedance curve is the same). 

So, the behavior of our system shows that the rear fan of the rated performance of 39 CFM and an impeller's speed of 3000 rpm is a necessary and sufficient condition for effective removal of 200 W of heat and for maintaining the internal temperature within 35°C. One more fan installed onto the front panel and having the same characteristics provides an insignificant performance boost for the cooling complex, which is even unnecessary. 

But power supply units of ordinary system enclosures come with rather weak fans, that is why an actual performance of an integrated cooling system will be lower in the same conditions. For example, the IW-S508 unit with an additional rear 39 CFM fan equipped with average statistical power unit of 250-300 W (like Jou Jye Electronic AP-3-1 or PowerMan FSP300-60BT/60BTV) comes with a volume air flowrate not higher than 28-30 CFM. And to remove 175-200 W from such system we should use not only a rear fan but also a front one of the rated output of 39-41 CFM. 

So, two additional fans installed inside are able (with appropriate characteristics) to cool down modern high-performance computer systems and provide a comfortable internal temperature when a computer generates about 200-225 W in all. However, you should also take into account that a good performance characteristic goes along with a higher noise level; for some users it's better to accept the fact that the computer is overly hot than to suffer from noise. 

Bearing this in mind look at the typical constants k for several versions of implementation of ATX cases obtained in the experiments (the data are given in Table 2). 

Table 2. Approximate values of the dimensional constant k in the formula 3 for TRMPS cases with an additional rear fan
System type Constant k, mmH2O/CFM
Total case size below 40 l, standard power supply unit LFL1 0.07
AFL2 0.08
HFL3 0.11
Total case size is 45 l, standard power supply unit LFL 0.05
AFL 0.06
HFL 0.08
Total case size is 50 l, standard power supply unit LFL 0.04
AFL 0.05
HFL 0.07
Total case size over 55 l, standard power supply unit LFL 0.04
AFL 0.04
HFL 0.05
1LFL - low filling level (AGP, 1 PCI, 1 5.25" bay, 2 3.5" bays are used).
2AFL - average filling level (AGP, 2-3 PCI or other buses, 2-3 5.25" bays, 2 3.5" bays are used).
3HFL - high filling level (AGP, 4-5 PCI or other busses, 3-4 5.25" bays, all available 3.5" bays are used).

Well, with the Table 2 you can draw a system impedance curve for typical cases. You are to choose a reference case which is the closest to yours in volume and internal configuration and use the respective k in the formula 3. This constant can be changed within +-5%, if volume of your case is a bit greater or smaller than the reference values. 

Now about the fan performance curves. Unfortunately, it's not always easy to get such characteristic (it's even hopeless for noname models). However, there is a simple solution! For a wide range of fans measuring 80x80x25 mm and rotating at 1500-3000 rpm a real dependence of air flow static pressure on its volume rate (which is actually the desirable performance curve) can be approximated using the following correlation: 

P = Pmax - m*Q, where  (4)
Pmax - maximum (rated) static fan's pressure,
Q - fan's performance,
m - dimensional multiplier, m = 0.12 (mmH2O/CFM),
P - static pressure. 

To draw this line it's necessary to know only a rated fan's performance (Qmax). One point of the line is certainly (0, Qmax). And you know how to find the other (Pmax, 0). 

When one additional rear fan is installed, the performance curve of the cooling complex (power supply unit's fan plus rear fan) is based on the following formula 

P1f = Prf, max - m1f*(Qps + 0.45*Qrf), where (5)
Prf, max - maximum static pressure of rear fan,
m1f - dimensional multiplier,
Qps - performance of power supply unit's fan,
Qrf - performance of rear fan,
P1f - static pressure of cooling complex. 

The resultant line defined by the formula (5) is as easy to draw as for formula 4: you have to mark the extreme points (Pmax, rf, 0) and (0, Q1f,max = Qps, max + 0.45*Qrf, max). 

Finally, if a front fan joins the rear one the performance curve is based on the following correlation 

P2f = Prf, max + 0.10*Pff, max - m2f*(Qps + 0.45*Qrf + 0.16*Qff), where (6)
Prf, max - maximum static pressure of rear fan,
Pff, max - maximum static pressure of front fan,
m2f - dimensional multiplier,
Qps - performance of power supply unit's fan,
Qrf - performance of rear fan,
Qff - performance of front fan,
P2f - static pressure of cooling complex. 

The end points of the line defined from the correlation (6) are not difficult to calculate, like in case of the correlation (5). 

So, now, we can draw lines of the system impedance and performance characteristic of a cooling complex and find out an actual performance of the complex (the point of intersection can be found from the diagram or combined equations) and compare it with our requirements for a comfortable internal temperature. 

Well, let's call it a day, and next time we will go into details about thermo grease (and other heat-conducting interfaces), and take a glimpse at their physical and chemical properties and functional performance. 
 
 

Vitaly Krinitsin (vit@ixbt.com)
 

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