[ 07.02.2022 ]

Axial fans are the most popular type. They get their name from the fact that the airflow they generate runs parallel to the axis of rotation of the impeller. The main purpose is to provide airflow cooling for semiconductor devices located inside the device cage. This is achieved by installing a fan outside or inside the enclosure, whereby cool air is blown in from outside or heated gas is evacuated from the enclosure. Axial (axial) fans are designed to create large airflows.

Basic concepts about the laws of ventilation in cooling systems

The fans create an airflow that overcomes the resistance of the environment and propagates through the semiconductors to be cooled. The fresh, cooler air blows over the electronic components, taking the excess heat with it. Air can only move through the enclosure space if there is a pressure difference at the start and end of the section along which it travels. The picture is similar to that seen in electrical engineering: a current can only flow through a section of a circuit, which has some resistance, if there is a potential difference at the ends of this circuit.

The design features such as motor power, speed, impeller diameter, number of blades, blade geometry, etc., determine differences in the behaviour of fans at varying pressure and airflow rates. For example, axial fans, for example, usually have minimal dimensions and are therefore fitted with relatively small motors. This means that their speed is dependent on the load and can vary significantly. The same fan placed in different housings (in different media) will show different flow rates. (And vice versa: very different fans operating in the same environment can have the same output.).

When designing an air cooling system for semiconductors, selecting a suitable fan is one of the important tasks. Leading manufacturers of ventilation equipment, such as Panasonic, recommend the following sequence of actions:

• The first step is to identify the complete set of cooling system requirements that need to be met to ensure an efficient system that will provide the desired operating conditions for maximum performance and trouble-free operation of all system components.

Three factors are critical:

1. the amount of heat to be removed from the frame;
2. the heat transfer, expressed in watts in relation to the change in temperature;
3. the amount of airflow required to remove excess heat, expressed in m3/min or CFM (cubic feet per minute).

Since the performance of all fans specified in the technical data is determined at zero static pressure, the choice of unit for a closed cage must be made from appliances with high flow rates.

Forced-air cooling for compact electronic enclosures can be achieved either by evacuating the air (with a fan on the discharge side) or by blowing it out (with a fan on the inlet side).

Exhaust ventilation has the disadvantage that, due to the reduced pressure within the enclosure, airborne dust is drawn into the enclosure through all openings and crevices. Air filtration is not possible in this case. The delivery fan can be installed with a filter in the inlet, the exhaust fan cannot. The excess pressure created inside the casing prevents dust from entering through the slots. Another disadvantage of positive displacement ventilation is the slight additional heating of the intake air by the engine. This can reduce the effect of the air cooling system.

The advantages of placing the fan on the intake side:

• Positive (increased) pressure is maintained in the enclosure, preventing the ingress of dust from outside.
• The increased turbulence of the flow inside the system increases the overall heat dissipation at the outlet.
• Fan uptime is increased because the motor runs in the inlet flow at low temperatures and in comfortable conditions. It must be said that on the duration.

Advantages of placing the fan on the discharge side:

• Better airflow control by placing the air inlets close to heat sources.
• Reduced noise levels due to the fact that the outlet is usually located away from the user, on the remote side of the enclosure.
• The heat generated by the fan motor does not affect the operation of the semiconductors, and is dissipated away from the equipment operator (does not discomfort him).

Remember that the efficiency of the cooling system depends on:

• the volume and speed of the air flow (the higher these values, the better the cooling system);
• the temperature difference between the incoming and outgoing air (cooler air should be supplied and more heat should be dissipated);
• the bigger the surface area to be cooled (the bigger the surface area, the more efficient the convection heat transfer and heat dissipation and the better the cooling system);
• thermal conductivity and heat capacity of the material (use radiators made of copper or aluminium alloys)
• air resistance (try not to obstruct the airflow, avoid installing baffles);
• the layout of the components and the presence of dead spaces;
• the motor capacity: even if the resistance of the medium is high, a powerful fan will not overheat and will last longer.

Main features of the fans

RPM

Revolutions Per Minute or number of revolutions per minute is one of the main characteristics that stands for how it translates. The higher the RPM, the faster the fan rotates. The characteristic is undoubtedly an important one; it can affect both the noise generated by the sound of air movement or the motor (depending on its quality) and the performance of the fan (not always), namely the amount of air the fan can blow through itself.

CFM

Cubic Feet per Minute or cubic feet per minute is the volume of air that a fan passes through per unit of time (per minute). The higher the value, the greater the volume that passes through the fan, which gives us more performance.

Fan size

Fan size is another important consideration when selecting a fan. A larger fan is not always more efficient than one that is smaller, but what is certain is that at lower revolutions per minute, larger fans will pass more or equal volumes of air through and will have less noise. Fan sizes come in the standard 120mm, 140mm, 200mm as well as smaller 135mm, 138mm etc.

Air pressure

As we said above, air pressure fans are divided into two main types: airflow-oriented and high static pressure. The image below, using arrows, shows how both types of fan work. The high static pressure fan is shown on the left. It is oriented to create a dense airflow in the centre of the fan. Whereas a conventional fan creates an airflow that spreads out in all directions.

Fans with high static pressure are usually used when some resistance must be overcome after the fan. These resistances can be all sorts of filters installed in enclosures, dense metal mesh or the fins of water-cooled radiators (depending on their density).

Bearing types

Different types of bearings are used in fans. The type itself influences both the longevity of the fan and the noise it generates. For example, hub-mounted fans may have a shorter life, but emit less noise than those that use ball bearings. Consider each type of bearing.

• Sleeve bearing

Fan designs with plain bearings are inexpensive, rugged and simple, which has led to their widespread use in most fans. The robust design ensures that they can operate in many difficult conditions, and their simplicity means that they are less prone to malfunctions. Another advantage of the design of fans with plain bearings is that they make less noise when running, allowing them to be used extensively in quieter environments such as offices.

The central shaft of a sleeve bearing fan is encased in a sleeve-like casing with a lubricating oil to facilitate rotation. The sleeve protects the shaft and keeps the rotor in the correct position by maintaining clearance between the rotor and stator. A hydrodynamic plain bearing is a bearing which also has a sealed cavity with grease.

A polyoxymethylene plain bearing is a bearing whose shaft is filled with polyoxymethylene to increase the sliding coefficient. A plain bearing with a screw thread on the bush, which allows the lubricant to be retained on its surface due to its construction.

A magnetically centred plain bearing in which the clearance is achieved by means of magnetic fields, resulting in a magnetic levitation effect.

• Ball bearing (rolling bearing)

Ball bearing fan designs are designed to eliminate some of the disadvantages of sleeve bearing fans. They are generally less susceptible to wear and can be operated in any position and at higher temperatures. However, ball bearing fans are more complicated and expensive than sleeve bearing designs and are also less robust. As a result, shocks can have a big impact on the overall performance of a ball bearing fan. They also create more noise in their operation.

Ball bearing fan designs use a ring of balls around the shaft to solve the problems of uneven wear and rotor run-out. Most fan motor designs have two bearings, one in front of the other, and these bearings are usually separated by springs. The bearings provide less friction than the bushes and the springs can help with any tilting of the fan that might cause unbalance of the rotor. If the springs are arranged around the entire length of the shaft, the unit can be used at any angle without wear and friction, making the design more reliable.

Among the rolling bearings are a double bearing with less vibration and a hydrodynamic rolling bearing in which the cavity is filled with grease and sealed, helping to reduce noise and increase reliability.

Noise generated

Fans, as much as we would like them to, do make noise. Another question is what level of noise is considered optimal for us personally?

Of course the noise level is individual, some people can hear every rustle very well, others cannot. Fans themselves make air movement noise at high speeds. In addition to air noise, a motor or ball bearing noise may be heard. Some fans make an unpleasant noise when they vary in rotation, and there are those that only make noise at certain revolutions, these can simply be limited by the number of revolutions per minute at a level that is comfortable for us.

The key to quiet cooling is the unique Panasonic hydrodynamic bearing (UHDB).

Non-contact rotation ensures low noise levels

Even with prolonged use of Panasonic fans, noise does not increase.

Panasonic's main product lines:

Model	UDQF56C1	UDQF5GH	UDQF4GH	UDQF4EH	UDQFHEH	UDQFZFH
Fan type	Axial type	Axial type	Axial type	Axial type	Axial type	Radial type
Frame	Plastic	Plastic with grill	Plastic with grill	Plastic	Aluminum	Plastic and Aluminum
Dimensions WxDxH (mm)	50x50x15	50x50x10	40x40x10	40x40x10	30x30x12	40x40x7.7
Speed (r/min)	5000	3900	3600	5100	3400	4600
Air flow (m3/min)	0.290	0.229	0.100	0.163	0.037	0.030
Static pressure (Pa)	36	16	13	25	6	27
Rated voltage (V)	13.5	5.0	5.0	5.0	5.0	4.5
Rated current (mA)	70	110	40	90	30	70
Operating temperature (°C)	−40 to +95*1	−30 to +85	−30 to +85	−30 to +85	−30 to +75	−30 to +85

 

ASFP DC fan motors

Frame	Plastic
Dimensions	40 x 40 x 10mm, 60 x 60 x 25mm, 80 x 80 x 25mm, 92 x 92 x 25mm, 120 x 120 x 25mm 120 x 120 x 25mm
Rated operating voltages	5V DC, 12V DC, 24V DC, (48V DC, ASFN1*B** series only)

 

Key points:

• Fan blockage protection;
• Reverse polarity, overvoltage protection;
• Long lifetime;
• Quiet operation;
• Optionally available with lock sensor (alarm in case fan is blocked).

ASEP AC fan motors

Frame	Aluminum alloy die-casting
Dimensions	60 x 60 x 30mm, 80 x 80 x 25mm, 80 x 80 x 38mm, 92 x 92 x 25mm, 120 x 120 x 25mm, 120 x 120 x 38mm, 150 x 172 x 38mm
Rated operating voltages	100V AC, 115V AC, 200V AC, 230V AC, 240V AC

 

Key points:

• Universal 50/60Hz applications;
• 3 rotation speeds per model;
• Long lifetime;
• Quiet operation;
• Optional: Connecting via cable (length 300mm) or Faston terminal.

ASFN DC fan motors

Frame	plastic
Dimensions	120 x 120 x 38mm
Rated operating voltages	12V DC, 24V DC, (48V DC, ASFN1*B** series only)

 

Key points:

• Fan blockage protection;
• Reverse polarity, overvoltage protection;
• Long lifetime;
• Quiet operation;
• Optionally available with lock sensor (alarm in case fan is blocked).

ASEN AC fan motors

Frame	aluminum alloy die-casting
Dimensions	60 x 60 x 30mm, 80 x 80 x 25mm, 80 x 80 x 38mm, 92 x 92 x 25mm, 120 x 120 x 25mm, 120 x 120 x 38mm, 150 x 172 x 38mm
Rated operating voltages	100V AC, 115V AC, 200V AC, 230V AC, 240V AC

 

Key points:

• Universal 50/60Hz applications;
• 3 rotation speeds per model;
• Long lifetime;
• Quiet operation;
• Optional: Connecting via cable (length 300mm) or Faston terminal.