Box clever and keep cool

Robin Cook, fan specialist at Aerco, looks at the issues involved with cooling modern enclosures

 

The fact that discrete electronic components are becoming more efficient and therefore generate less heat is of no consolation to the engineer responsible for heat management of electronic enclosures. Under competitive pressure, designers continue on a relentless quest to cram more capability into smaller areas thus creating more heat and less space – the worst possible scenario for the heat management engineer.
In addition, modern electronic systems are becoming ever more complex and the cost of failure due to overheating is prohibitively expensive. There are numerous methods of removing unwanted heat such as Peltier thermoelectric coolers, heat pipes and water cooling modules. However, in the vast majority of cases the ubiquitous equipment fan, in one of its many guises, remains the chosen method.
The same but different
Ranging from small instrument cases, through desk-top enclosures and studio desks up to large, free-standing cabinets, enclosures come in all shapes and sizes, each one with its own unique characteristics in terms of cooling requirement and shape. Operating environments are equally diverse varying from the benign, air-conditioned, temperature controlled, clean rooms to harsh and hostile high temperature environments such as manufacturing plants.
The right place
Cooling fans need to be located within the enclosure and in the best possible position to provide maximum cooling effect. There is nothing more frustrating from a thermal management point of view than to see an enclosure that has the cooling device bolted on to the outside of the enclosure. This is a sure sign that cooling was an afterthought, a process that is unlikely to achieve the most effective solution.
The fan must be located in the best position to ensure the optimum air path and it may be that more than one fan is required to deal with overall cooling and specific hot-spots. Whatever the final layout, it is highly recommended to draw ambient air into the enclosure and exhaust warm air. This means that lower temperature air passes over the bearing of the fan thus optimising its life.
Don’t yield to the pressure
To choose the right fan for the job requires accurate calculation and interpretation from data provided by the fan manufacturer and it is a relatively simple job to calculate the air volume required from the formula: Airflow (l/s)/1.2 x ∆t (ºC) = P(W).
However, airflow is always quoted in free air and it must be remembered that the system pressure, the pressure drop across the enclosure caused by the various components and restrictions to airflow, has a vital bearing on fan performance.
It is possible to measure pressure drop by using a venturi tube and anemometer but this is a fairly complicated task and may require the assistance of the fan manufacturer or a specialist laboratory. In the real world the task can be accomplished by intelligent and experienced interpretation of the fan performance curve from which the airflow that the fan will achieve at any given pressure will establish what is known as the ‘duty-point’ of the fan.
The right type
Choosing the right fan for the job really is horses for courses and will depend on the outcome of the deliberations described earlier. Although AC fans still have their place for some applications the superior efficiency, flexibility and controllability of DC fans mean they will be chosen in the vast majority of cases.
In general terms, axial fans give high volume but do not work well against high system pressure; blowers provide less volume but work well at higher pressures while air wheels deliver both high volumes and high pressures. At Aerco we supply axial, flat axial, centrifugal blowers, single and double inlet, skeleton fans, cylindrical air wheels and backward curved impellers. The list is too long to discuss at length here but an experienced cooling specialist will establish the best choice very quickly by analysing the space available, the air volume required and the anticipated system pressure.
The intelligent approach
Most modern, DC fans provide signal wires that allow monitoring and control of fan speed meaning that the system temperature can be controlled within strict parameters. These signals can be used to control, with a high degree of accuracy, the cooling rates within the enclosure, the noise levels of the fans and also provide audible or visual signals to identify potentially damaging reduction of fan speed or, in extreme cases, fan failure.
The right accessory makes all the difference
We have said that every component in the housing will increase system pressure but it is worth noting that the use of a purpose-built wire finger guard creates less restriction than a pattern of punched holes in the housing. The sharp edges of a punched hole create harmonics in the airflow causing turbulence and greater resistance thus affecting both the inlet and exhaust flow of the fan.
While on the subject of accessories, due care must be taken to consider access to the enclosure for maintenance and fan replacement and also the judicious use of filter assemblies will ensure they are in keeping with the style of the cabinet.
Think the whole package
Other aspects to be considered are the environmental conditions and the required IP rating of the fans and it is very important that the projected life of the fan matches that of the system to be cooled. In general terms, sleeve bearings are less expensive and quieter than ball bearings; ball bearings provide maximum life while newly developed hybrid bearings provide an affective compromise.
It is a simple fact of life that the earlier cooling is considered in the design cycle the more effective and efficient the overall system will function and the less likely that those costly down-times will occur.