Normally, most users are used to select cooling fan or blower by referring maximum flow rate, maximum static pressure and rotational speed data in product specification. It’s not a complete thinking, because the fan is always working inside the system. Y.S. TECH has to highly recommend you to focus on demanded operating point works in fan performance curve profile, do not only refer the maximum point or rotational speed.
Meanwhile, a cooling requirement should not only regard flow rate or static pressure, but two key factors of power consumption and acoustic noise. However, these critical factors are trade-off, so how do we select a right fan to meet with thermal solution. We are going to illustrate you some methods how to select a right fan in the following content. And then we will discuss other important technical topics including Life (L10), RoHS and Application Note.
- Ask five questions before choosing a fan
- Choose a right fan & blower to meet your thermal inquiry
- Choose a solution with Low Noise
- Management Regulations for the Environment-Related Substances
- Cooling Fan Life Expectancy：L10 and MTTF
- Cooling Fan and Blower Application Notices
- Convrsion Tables and Equations
- Wave Form of IC Function
Here are five questions of thermal inquiry we need to verify at first. That include:
1.Watt：How many watts would you need to dissipate？
2.Air Impedance：What is your system air-impedance？
3.Noise：What is acoustic noise specification you need？
4.Temperature Gradient：What is your design of ΔT？
5.Dimension：What is fan dimension you need？
The effects of heat transfer include Conduction, Convection and Radiation. Most heat transfer by conduction and radiation effects that concern about system mechanism. For example, a good chassis or heatsink design is more helpful for thermal solutions. Convection effects contain free convection and forced convection. Cooling fan and blower are the major effect for force convection and always increase entire thermal solutions more efficient. Illustration 1 shows you a normal system of heat dissipation status. Pcon&rad presents heat dissipation on free convection and Radiation transfer. Psys presents total power consumption of system. Pfan presents forced convection that need cooling fan or blower to dissipate. ΔT presents Temperature GradientΔT = T2 - T1, T2 is the thermal spec of critical parts with margin tolerance. T1 presents Ambient Temperature.
Illustration 1:System Heat transferred
Then we can estimate Flow Rate inquiry by followed equations
By incorporating conversion factors, specific heat and density of sea level air we can summarize above equations as Qeff that called effective flow rate. Qeff ask fan’s operating point need to be met. In another words, Fan Operating Point should over or equal to Qeff.
Fan Operating Point is an intersection point by fan performance curve (flow rate/ static pressure) and system air impedance curve. It’s caused by different system or components placement and form factors. Normally, It is measured by static pressure, △Pi . Its formula may show as below:
2 shows two fans performance curve and system air impedance curve.
Even B’s maximum flow rate is higher than A’s but the Qop_A better than Qop_B. Both Qop_A and Qop_B are Operating Points.
Illustration 2: Fan Performance Comparison
If Qop_B ＜ Qeff ＜ Qop_A, Then we can say Fan A is a proper choice for this thermal solution. So that why we emphasize that focus on fan performance profile rather than on maximum flow rate or static pressure or rotational speed.
As aforementioned, the flow rate, static pressure and acoustic noise are always trade-off. It is very difficult to think over these factors at the same time. Meanwhile, a lot of troubles are caused by improper applications too. For example, fan mounted to chassis improperly may cause vibration and flow disturbance, and then got higher acoustic noise. Here are some key points regarding to low noise design for your reference:
1.A proper system air impedance design
Higher system air impedance needs a higher static pressure fan, but It accompanies with higher noise. Give an enough space to your critical parts and place them at flow path as possible. But it is a tough work to get space for thermal solutions in a slim and light. However, we recommend you to measure you system air impedance and collect enough parameters to know your △Pi. Normally, most specialized fan manufactures will support you to measure it by Air Chamber.
2.Choose a proper fan that base on Qeff
We have illustrated you a method to figure out a right fan for Qeff, and then you should consider about power consumption and acoustic noise. Which one is the first priority? To evaluate these two parameters under the same Qeff base is Y.S. TECH’s recommendations.
3.Review a fit mechanism design between fan and application system
Vibration and Flow Disturbance always cause resonance and get higher acoustic noise. A proper fan mounting and flow field design may decrease acoustic noise. For example, mounting with a rubber cap on high-speed fan model will decrease vibration resonance. Review your design to make sure there is enough margin space (over 1.5mm) at flow inlet/outlet side and no any stuff to disturb flow filed.
4.Advance fan speed control by your thermal profile
Thermal profile is similar to fan performance profile. Normally, different function will need to dissipate different power consumption. Then we can modulate fan rpm and ask fan working on a proper rpm by different system function. The most popular advanced fan speed controls are PWM control (Pulse Width Modulation), Thermostat control (NTC, thermistor) and both of them.
5.Sound Quality analysis
In cooling fan industry, we are always focused on Sound Pressure only in the past but there is no longer sufficient to us because even though legal regulations have lead to a reduction of noise limits, the tendency of people to feel disturbed by noise is increasing. A sole reduction of noise levels is thus not sufficient to reduce the annoyance due to noise to a degree noticeable by human beings. This is due to the fact that the subjective human aural perception is often disregarded. However, the judgment of a sound event involves a wide range of different parameters forming into the total hearing impression. So we are not only concern about sound pressure but also 「Sound Quality」.
The examination methods are based on the idea of correct recording and describing the noise exposure from the acoustic environment in a way that reflects what humans subjectively perceive. In order to record this entire perception, physical aspects as well as psychoacoustic characteristics of hearing and cognitive aspects must be considered. The main focus of psychoacoustics is the subjective aural perception by human beings. The goal is objectively describing this subjective perception. Psychoacoustic measuring methods account for the actual hearing impression, as opposed to conventional measuring methods that only record the sound level in the form of the equivalent continuous sound level.
Y.S. TECH introduced the「Sound Quality Analysis System」of HEAD ACOUSTIC in German. Those include an Artificial Head and analysis tool and also the most popular measuring and analysis system in automotive industry. Its parameters are include「SPL, sound pressure level」、「Tonality」、「Sone」and「Modulation」.
The measuring method and standards are as below：
1.ANECHOIC Room Noise Measurement System.
2.Digital Head Measurement System, 16-bits version.
3.SQlab III, Mobile Multi-channel Analysis System.
4.Specifications:ISO 3744, ISO 3745, ISO 7779,CNS 6753, JIS 8346
5.Background Noise: < 17dB(A)
Fan reliability can be evaluated in several ways. The data for a life test can be plotted as a cumulative distribution that shows the total fraction of fans failing up to any operating time. Fig. 1 is a sample of cumulative distribution, which was stopped at 8,400 hours after 18 out of 48 fans had failed.
Figure 1：Sample cumulative distribution function, Weibull vs.
Empirical with 95% confidence bands
(Reference：IBM Corp. , May 1996, Vol.2, No.2, Electronics Cooling)
Some vendors provide life expectancy data to customers based on the exponential assumption. However, life test data does not support the use of the exponential distribution. Nevertheless the past experimentation fitting has shown that the Weibull distribution provides a good fit to real fan life data. The Cumulative Distribution Function ,F(t) of Weibull distribution is a below：
Normally, L10 was introduced a life expectancy parameters by fan vendors. That means the tenth percentiles under an assumed life distribution such as the Weibull.
Sometimes vendors will also quote the Mean Time To Failure (MTTF) then we need to figure out the correlation between L10 and MTTF by following equations:
After we have verified the correlation between L10 and MTTF, we also need to know how long should a sample size be tested to determine with 90% confidence level that L10 greater than or equal to expectancy value at a test temperature without failure (x = 0). Here we introduce the Poisson Distribution to estimate.
Normally on the condition of 90% confidence level and 0 failure then Br;c = 2.303.
Then we introduce Takes Martin Marietta Model to estimate Life at different environment stress.
Then we can define the Required Test Time (t) with zero failure is as below:
I.Test Conditions and definitions
Most specifications of cooling fan are measured after 5 minutes rotating in an ambient of 25℃/ 65% RH The operating voltage and temperature were defined after fan rotating continually at rated voltage. Starting Voltage was defined on power on/off condition and Rotational Speed was defined on full speed by its rated value. Except for the feature of the Lock Rotor Protection specifically stated, Y.S. Tech highly suggests not to stop the impellers of the working fans such interruption will cause adverse effect.
Noise Level is different from abnormal noise. We estimate noise level by equation when noise level is lower than background noise (17dB). L10 of Life test is a deductive value under statistical method and it is different from product warranty.
Please be cautious when fan is being exercised or handled. Applying pressure to the impeller, handling the fan by lead wire, or dropping the fans to the production platform is resulting in damage. Fan is to be stored in a dry/cool place. High levels of humidity are harmful to products. If fan was stocked at an ambient temperature under 5℃ and over 24 hrs. Please stock fans to an ambient temperature over 20℃ and remained over 24 hrs before using. All specifications include abnormal noise have to be measured after 30 minute running.
The correct polarity, Positive (+) and Negative (-), has to be clearly identified before connecting the fan to the power. Be aware of the connection with reverse polarity may lead to damage since no effective protection can be introduced against such errors.
With exception of suitability of some particular designs, any failure and problems regarding safety of the product caused by the introduction of powder, droplets of water or encroachment of insert in the hub are not guaranteed. It is also not well suited for corrosive environments that include liquids, gases, or matters.
A written request should be submitted to Y.S. Tech prior to approval if abnormality and deviation from specification is required. Meanwhile, send abnormal samples to Y.S. TECH for more detail analysis is necessary.
Please be cautious. Y.S. Tech is not responsible for any excess resonance, vibration and subsequent noise caused by incorrect mounting of fans. Take necessary precaution handling fans when in operation. Finger guards are recommended to prevent personal injury. To avoid any unstable power, an “over 4.7μF” capacitor has definitely be connected to fan externally whatever multiple fans are applied in parallel.
1. For RoHS, Decabromobiphenyl ether (DecaBDE) in polymeric applications is exempted by Commission Decision of 13 Oct. 2005 amending Directive 2002/95/EC notified under document 2005/717/EC.
2. PBBEs=PBDEs=Polybrominated Diphenyl Ethers=PBDOs=PBBOs.
●Standards for Preconditioning and Measurement
Typical pre-conditioning methods are as follows:
1.Incineration under the existence of sulfuric acid.
2.A pressurized acid decomposition method done in a sealed container.
(A microwave decomposition method such as EN 13346:2000 and EPA 3052:1996)
3.An acid decomposition method under the existence of sulfuric acid or hydrogen-peroxide water.
(e.g. EPA3050B Rev.2: 1996).
4.A wet decomposition method under the existence of sulfuric acid, nitric acid, or hydrogen-peroxide water.(e.g. BS EN 1122:2001)
5.If precipitates (insoluble matter) are produced, dissolve them totally by taking some means.
(e.g. alkali dissolution)
6. US EPA 3540C or 3550 for organic or organic compounds substances.
II. Measurement methods
Typical measurement methods are as follows,
1.Inductively Coupled-Plasma-Atomic (Optical) Emission Spectroscopy(ICP-AES, ICP-OES)(e.g. EN ISO11885:1998)
2.Atomic Absorption Spectroscopy (AAS) (e.g. EN ISO5961: 1995).
3.Inductively Coupled-Plasma Mass Spectroscopy (ICP-MS)
4.Gas Chromatography Mass Spectroscopy (GC-MS) for organic or organic compounds substances.
5.If a combination of a pre-conditioning method and a measurement method can guarantee that the lower determination limit of cadmium is less than 5 ppm, the combination is also applicable.
6.Any one of the measurement methods above (except AAS) enables you to analyze.cadmium and lead simultaneously.
Environmental Concern & Keep Improving
I. Air Flow Rate
II. Static Pressure
III. System Allowable Temperature Rise
IV. Acoustic Noise
According to above equations, it is very clear the acoustic noise level will reduce 6 dB when the distance doubled. Comparatively, the noise level will also increase 6 dB when distance shorten by half.