You’ll remember I was talking in my last blog I was looking at how to best configure cooling in a rack for reliability, and so far we’d reviewed equipment operating temperatures, and the proposed ambient temperature range of the environment it would be in, which in our scenario for a challenge, we had calculated some equipment with a 0°C delta, to ambient. So, we’ll now take a look about what we can do to address this thermal challenge.

Step Two: Getting the cool air in…
Given that the worst case thermal operating temperature is equal to the upper limit of the equipment manufacturers recommendations we must provide the equipment with air as close to the ambient temperature as possible, however having identified that there will be no change in hardware or active cooling and that there is also an acoustic limit, we know that strategies such as moving elevated volumes of air through the cabinet would not be suitable in this instance.

In order to resolve the issue the cabinet would require either no door, or be fitted with a vented door which creates little or no attenuation to airflow and so allows all equipment to receive air directly or indirectly from the ambient environment through the shortest and most direct path possible and with minimal pressure drop.

Tip : If needed raise the cabinet on a plinth to increase the flow of air entering the cabinet base, this will assist with the chimney effect created and help mitigate stagnant air pockets.

This may seem counter intuitive when used in conjunction with a fan lid (exhaust and vented rear door) however the air will always follow the path of least resistance, therefore creating a low pressure zone internally at the at the exhaust, will naturally draw air from the surrounding area, even though the rack has a vented rear door a portion of the air be derived from elsewhere in the cabinet, the more readily this can enter at the base of the rack the greater the flow will be vertically within the cabinet (even if vented doors are being utilised).

Step Three:  Managing the waste heat

In order to ensure maximum reliability we need to not only make sure that cool air can get in, but also that the cabinet provides a managed airflow that ensures there are no hotspots and that any  recirculation of warm air is minimised and constrained to non-critical areas.

In order to achieve this we should consider the maximum electrical load of the cabinet to be the approximate thermal load we are to displace, however our example includes an acoustic requirement also which for our example limits the performance and number of fans which can be fitted as an exhaust to the cabinet.

It is common to have acoustic limits with equipment of this type and having to limit the assistance for removing thermal waste from a commercial cabinet (no custom fans) leaves only one option. Maximising the opportunity for air to exit the cabinet naturally, in this case utilising a high flow and supporting this with exhaust fans specified to ensure there is a continuous flow of air through the cabinet and so reduce the opportunities for recirculation and hotspots. Careful consideration should be made in the balance between the amount of ventilation provided by the door and the fans assisting the internal air column as in some configurations this can lead to a reduction of low in the lower cabinet and so elevate the thermal delta within the cabinet air column.

That’s all for this blog, but in my next one, I’ll be taking a close look at how we can assess the reliability of a rack and its equipment, and highlight mathematically the impact of the cooling practices you’ve just read all about.