Once you know the volume of air and the static pressure of the system to be cooled, you can determine the fan specifications for your product.

Transcription

1 COOLING FAN REQUIREMENTS CALCULATIONS Determining System Impedance Determining the actual airflow produced by a fan mounted in an enclosure is much more difficult than calculating the airflow required. Obstructions in the airflow path cause static pressure within the enclosure, referred to as system impedance (expressed in static pressure as a function of flow in CFM). To maximize airflow, any resistance should be minimized, except for baffles that may be necessary to direct the airflow. A typical system impedance curve for electronic equipment follows what is called the "square law". This law states that static pressure changes as a square function of changes in the CFM. The chart to the right displays typical impedance curves. Static pressure through complex systems cannot be easily arrived at by calculation. The experimental method of finding airflow through an enclosure is very accurate, but costly, time consuming and cumbersome. In practice, empirical methods are normally used to estimate airflow resistance. Experience shows that: An empty enclosure usually reduces airflow by 5 to 20%. A densely packed enclosure reduces airflow by 60% or more. Most enclosures have a static pressure between 0.05 and 0.15 in H2O. Once you know the volume of air and the static pressure of the system to be cooled, you can determine the fan specifications for your product.

2 Required Air Flow DC or AC Power Voltage Speed Life Expectancy EMI/RFI Heat Dissipation Auto-restart Acoustic Noise This guide will help you determine the best cooling sol for your product. Intake or Exhaust Forced air-cooling for packaged electronic enclosures can be achieved by either evacuation (with fan on exhaust side) or pressurizing the enclosure with a fan on the intake side. This choice should be made early in the design process. Although both theoretically use the same volume of air to dissipate the heat, they have different effects on placement of components within the enclosure. When using an exhaust fan, the air distribution inside the cabinet is flexible and heat from the fan itself is not dissipated into the cabinet. Evacuation has the disadvantage of reducing the pressure within the enclosure, so airborne dust is drawn in through all the vents and cracks in the enclosure. Filtering of the fan on the exhaust side is extremely difficult. A further benefit is that the enclosure is slightly pressurized so that dust is not drawn into the enclosure from the surrounding environment. The disadvantage of intake fans is that filters must be changed frequently to eliminate dust accumulation. A

3 clogged filter can severely restrict airflow, causing elevated temperatures in an enclosure that may be more of a problem than the dust itself. Another disadvantage of a fan that pressurizes the system is that air dissipated by the fan motor can slightly warm the incoming air. This can reduce the air's cooling effect. Components that have the most critical cooling requirements should be placed closest to the air inlets. High temperature components should be placed closest to the air outlets. If exclusion of dust is required, it is better to use a fan that pulls air into the enclosure. In this configuration, a filter at the fan inlet can remove dust from the incoming air. Air that is drawn into the fan flows in a continuous, non-turbulent movement called laminar flow, which allows for a uniformly distributed airflow velocity in the enclosure. This is important in eliminating stagnant air and hot spots. Air exhausted from the fan is turbulent. Heat dissipation in a turbulent airflow can be up to twice that of a laminar flow with the same volumetric flow rate, except that the turbulent airflow region near a fan exhaust is normally limited. Developing a well-defined airflow path through the whole enclosure is essential to minimizing airflow waste. Vents should be at least 50 percent larger than the fan openings themselves. Care must also be

4 taken to eliminate air recirculation in a fan, as over ninety percent of the airflow can be lost. Baffles may be used to eliminate re-circulation of the same air since an airflow path will always take the path of least resistance (Figure Above). Subassemblies and components within the enclosure should be positioned to direct the airflow to places that require cooling. Component placement should always be considered in order to take advantage of natural convection; for example, placing warm components above cool components. Avoid placing large components so that they shield smaller components from the flow of air. Use baffles, where necessary, to direct the airflow to critical hot spots. Smaller systems usually use axial cooling fans, where airflow is perpendicular to the fan blades. The airflow required to dissipate the heat generated can either be obtained by calculation or from a graph. This airflow requirement will depend on the heat generated within the enclosure and the maximum temperature rise permitted. When estimating the power dissipated within a system, use a worst-case estimate for a fully loaded system to allow for the possibility of future changes and additions of heat generating subsystems. In many applications, using an intake fan rather than an exhaust fan can double or triple the life of the fan. The heated air passing over an exhaust fan stresses the fan's bearings much more than the 25 C air flowing over an intake fan. This reduction in

5 temperature provides a dramatic increase on fan life, as seen in the JMC Life Expectancy Curve (Chart Above). Power Parameters In the past, the higher cost of DC fans led to the almost exclusive use of AC fans. Today the price differential between the two has disappeared and DC fans have many advantages compared to AC fans. For example, DC fans typically have a longer life and consume almost 60% less power. Brushless DC fans are usually available in four nominal voltages: 5V, 12V, 24V, and 48V. If the system has a regulated power supply with one of these voltages, then a brushless DC fan may be utilized. This fan will provide performance required, without the input variables that plague AC fans. The speed and airflow of a typical DC fan is proportional to the voltage supplied. Therefore, a single product may be utilized in different applications by adjusting the supply voltage to provide the desired airflow. The voltage range for satisfactory operation depends on the individual fan design. Brushless DC fans do not draw constant currents. The choice of the power source, along with the addition of other peripheral devices, will affect the type and number of DC fans and their motor current characteristics. Throughout blade rotation (particularly at commutation), the current will fluctuate from minimum to maximum. The wave form and level of ripple current will vary significantly between fans and motor designs, making specifications in narrow terms difficult. An understanding of the power source limitations and how they may be impacted by various brushless DC fans early in the design phase will help

6 prevent problems and allow maximum system flexibility. What is Locked Rotor? A locked rotor is a type of sensor output that measures when the fan has completely stopped or locked. It sends an alarm signal, at either high or low voltage when the rotor locks. If the fan starts spinning again, the alarm signal condition will stop. What is Tach Output? A tach output or "sensor output" indicates the speed of the fan at different operating levels. Its purpose is to identify when the fan drops below a certain RPM, and to identify a potential problem with airflow. A tach output fan will always have at least 3 wire leads.

7 Variable Speed Fan in an Electronics Enclosure A variable speed fan is automaticall y controlled by the fan circuit, which changes speed as the temperature changes to provide optimum air flow at all times. Two options are available: 1. A remotely mounted thermistor on a third lead wire (thermistor not provided). 2. A thermistor mounted on the fan PCB (thermistor provided). With the three lead wire fan, varying the voltage on the third lead from 0 to 6 volts can also control speed variation. Variable and Two Speed Fan Information Specially designed variable-speed and two-speed fans can be used to significantly reduce noise generation and power consumption. With standard single speed fans, the DC fan rpm and air output can be controlled by varying

8 the input voltage or by switching between power supply circuits. However, these methods limit the range of low speed operation; and they may involve switching the motor IC on and off when changing speeds. Fan motor damage or malfunction is possible. To overcome these problems, two speed fan motors and variable speed fans are available from JMC. These specially designed JMC fan motors offer a wide variation of fan rpm while keeping the supply voltage constant. Two speed fans can be used in many cooling applications. When extra heat is generated in an application, the fan can switch from idle mode to high-speed operation for extra cooling.

9 Important: Bearing type and temperature matter The life and reliability of all DC fans, regardless of manufacturer, depends on a combination of voltage, frequency, ambient temperature, and airflow restrictions. The normal failure mode is in the bearing system, which is usually related to the total temperature that the bearings are subjected to, although other factors may apply. In general, there is not much difference in life between sleeve and ball bearings when the total temperature is relatively low. As this total temperature increases, however, ball bearings will give a progressively longer life than sleeve bearings.

10 be taken separately. Equations and Variables: Maximum static pressure and maximum air volume measurements must Maximum Static Pressure Measurement: When the nozzle is closed, the pressure in chamber A will reach a maximum. The pressure difference P s represents the maximum static pressure achievable by the fan. Maximum Air Volume Measurement: The nozzle is opened and the auxiliary blower is used to lower the pressure in chamber A to Ps = 0. The maximum air volume can then be calculated using Pn, D and the air volume equation below. Q represents the maximum achievable airflow with the fan in free air. Heat Transfer is represented by: Q = 3.16W / T f or Q = 1.76W / T c Where: Q = Airflow required in CFM (cubic feet per minute) W = Heat dissipated in watts T c = Temperature rise above inlet temp C T f = Temperature rise above inlet temp F Using the chart to the left, one can estimate the airflow required. The vertical axis represents the heat to be removed and the horizontal axis represents the airflow. Notice that both axes are logarithmic. The sloping lines define the

11 temperature rise in C. First find the sloping line that represents the permitted rise, and then find the point on this line that corresponds to the heat to be removed. The horizontal position of this point shows the required airflow. For example, 35 CFM of airflow is required for a system that dissipates 200W, which results in a 10 C temperature rise. Noise has no effect on a fan's cooling performance,

12 but it makes a big difference to the people working nearby. Audible noise originates from several sources, some of which can be controlled by the enclosure designer. Others are a result of a fan manufacturer's design. Read below to find out more about the causes of fan noise and what you can do to minimize it. Causes of Fan Noise Noise emanating from axial fans is a function of many variables and causes: Whirl pooling: This is a broadband noise source generated by air separation from the blade surface and trailing edge. It can be partially controlled by good blade profile design, proper pitch angle and notched or serrated trailing blade edges. Turbulence: Also a broadband noise, generated within the airflow stream itself. Inlet and outlet disturbances, sharp edges and bends in the airflow will cause increased turbulence and noise. Speed: Speed of rotation is a major contributor to fan noise. Fan load: Noise varies with the system load; fans are generally quieter when operated near their peak efficiency. Structure vibration: This can be caused by the components and mechanism within the fan, such as residual unbalance, bearings, rotor to stator eccentricity, and by motor mounting. Cooling fans are basically motors and should be treated as such when mounted. System disturbance: System disturbances are the biggest cause of fan noise. When a fan is designed for low noise operation, it can be very sensitive to inlet and outlet disturbances caused by card guides, brackets, capacitors, transformers, cables, finger guards, filter assemblies, walls or panels, etc. Trial and error, combined with common sense and intuition, are often the best tools for determining fan selection and component placement for low noise operation. How to Minimize Fan Noise

13 The following design actions will help produce a system with minimal fan noise: Reduce system impedance at the inlet and outlet ports. If a large part of the fan's flow potential is used up by the impedance of the inlet and outlet, a larger, faster, noisier fan will be required. Avoid obstructions to the airflow, especially in the critical inlet and outlet areas. When turbulent air enters the fan, noise is increased by as much as 10 db, usually in a discrete tone form that is particularly annoying. Use a larger, slower fan rather than a faster, smaller version whenever possible. Often, this solution will produce less noise for the same airflow. Relax the temperature rise limits where possible, to reduce the airflow required. This will allow you to use a smaller, slower fan that produces less noise. Isolate the fan to avoid vibration transmission. Because fans operate at a low frequency and are light in weight, vibration isolators must be soft and flexible. Since the transmission is dependent on the system, trial and error is the best approach to a quiet system/fan interaction. In systems that require 20 CFM or less, cabinet vibration is the predominant source of noise, and isolation of the fan is the only practical solution. Cooling fan noise is expressed in decibels (dba). The dba rating is determined directly by a sound level meter equipped with a filtering system which de-emphasizes both the low and high frequency portions of the audible spectrum. This measurement is recorded at a distance of 1 meter from the source.