Taking the heat out of control panel design

In modern control cabinets, components are tend to be more compact. As a result, ‘hot spots’ - areas in which heat-generating components can create stresses on the adjoining components - is getting more prevalent. If components are mounted very close together, then any air flow will be restricted. If these components cannot be separated for design reasons and there is a reduced airflow, then a local temperature of over 45C is possible. For components within the ‘hot spot’ or within close proximity, premature ageing can occur.

Cooling options:

1. No cooling and reliant on the external ambient temperature to maintain temperature stability
2. Forced air cooling, incorporating fans/blowers which are often mounted either top, side or front of cabinet
3. Air conditioning units, mounted similarly.

Whichever method is adopted, issues arise with effective air circulation and consequent component cooling. Manufacturers of active and passive automation components will always state a de-rating factor in their technical data sheets. An example is a standard 5A (120 watt), 24V power supply. Beyond 50C, the device will de-rate at 4W/C. Therefore at 60C, for example, the unit will only effectively run at 3.3A (80W). Not only will long term exposure to higher temperatures de-rate the component, but this will also cause premature component failure.

Similarly, according to manufacturer's data the typical life expectancy of an electrolytic capacitor (an integral part of all control equipment), which at 40C can be expected to last around 10 years, at an increased temperature of 60C (not uncommon within modern control cabinets), this is dramatically reduced to only 30 months.

Analysis of current control panel build methods reveals:

• 19% of equipment has air short circuits - those areas where colder circulated air and component heated air are cancelled out therefore preventing an effective air circulation and consequent internal cabinet temperature reduction
• 93% of layouts do not have a ‘targeted’ cold air supply installed
• In 64% of layouts, the warm components are not in the air flow between the inlet and outlet of the cooling
• 18% of layouts have obstructions between the inlet and outlet of cooling.

The preferable approach is to remove all front-mounted cable channels by moving the cable to the rear of the panel; this will enable more efficient use of air flow and reduce the internal temperature of the cabinet. Also, as there is now free space at the rear of the control panel, a chimney effect will be created, causing air to flow in a more natural fashion, making better use of any air cooling that is available. A supplementary benefit is an increase in component mounting surface area of approximately 30%, enabling the components to be spaced further apart.

Individual cabinets are preferable to multiple connected cabinets. Here, heat can dissipate into the surroundings via all free surfaces through convection and radiation.

With free cooling, the heat dissipation released within the control cabinet should be from bottom to top and areas of high density temperature generation avoided. The common method of mounting power supplies for example is at the top of the cabinet. But, as the cool air starts at the bottom of the cabinet, then this is the more sensible place to mount them.

There is a tendency to automatically install an air conditioning unit as soon as the cabinet becomes over populated. But in many cases, an air conditioning unit is not required - a simple an internal air blower mounted to the top of the frame (forced air cooling) will facilitate a better airflow and a chimney effect can be created.