Providing Sufficient Cooling - Electronic Cooling and Sensible heat
Modern living is becoming increasingly reliant on information and communication technology (ICT) using microchips and semiconductors to compute and execute data flow. As data processing becomes faster and computing devices become smaller the amount of power used per unit of space needs consideration in order to maintain the integrity and reliability of operation.
Power losses caused by inefficiencies and resistance occur in the electronic components that make up ICT equipment and these power losses lead to heat being generated. The amount of heat produced will depend on the activity level of the components and the dry heat that is generated is known as the sensible heat load. The amount of sensible heat power produced will be expressed as Watts and the temperature that the component reaches will depend on its mass, its thermal capacity, the thermal capacity of adjacent components and materials and the surrounding air.
For example, an electronic device with an inherent power dissipation inside a totally enclosed cabinet will cause the air temperature within the cabinet to rise. There will eventually become a point where the components that make up the device inside the cabinet cannot absorb any further heat energy and the temperature will stabilise. Further heat energy dissipated by the equipment will be absorbed by the surrounding air, be conducted through the cabinet panels out of the cabinet and into the ambient air surrounding the cabinet by convection. The system will reach equilibrium and the temperature inside the cabinet will depend on the amount of heat leaving the cabinet due to the thermal conductivity of the panels and the surrounding ambient temperature.
In the majority of cases, electronic components and devices are installed in an open enclosure that allows external ambient air to come into direct contact with the surfaces inside the enclosure. Providing that the external ambient temperature is cooler than the surface of the components inside the enclosure, cooling will take place. It is also common in densely packed ICT equipment for a fan to be installed to force air into the enclosure. Forcing air into an enclosure increases the cooling effect on the components by increasing the exposure to ambient air.
The amount of heat dissipated by the fan will depend on the volume of air provided, the difference in temperature between the ambient air and the stabilised temperature of the air inside the enclosure, the density of the air and its specific heat capacity. From this information, we can calculate the amount of air required to maintain a stable temperature inside an enclosure given the amount of heat that is being dissipated inside the box.
Calculation of Volume flow required to dissipate sensible heat load:
Example: If we have an electronic enclosure with a combined power dissipation (heat loss) of 1kW, the maximum allowable temperature inside the enclosure of +40oC, and designing for worst case conditions an external ambient temperature of +35oC (temperature difference of 5oK) the calculation becomes:
V (m3/hr) = 1kW x 3600
1.2kg/m3 x 1.005kJ/kg.K x (40-35)oK
V = 597 m3/hr = 165 Litres / sec
There are a number of variations on the above formula below that reduce the number of terms involved. Using the power dissipation of the equipment, the difference in temperature between the inside and the outside of the enclosure and the density of the air passing over the warm surfaces, an accurate volume flow rate can be calculated to ensure sufficient cooling is provided.
Resistance to Flow
The air that surrounds us in normal everyday life contains small particles of airborne dust. When combined with wind and thermal airs the smaller lighter particles of dust and pollen become airborne and can become carried into homes and buildings. This dust can be drawn into equipment that is being forced cooled by fans which if allowed to build up in sufficient quantities, can affect the operation of the equipment.
To prevent this happening we can use a filter design that blocks the passage of dust/particulates whilst allowing the air to pass. The method in which dust is arrested can vary and as the filters gradually clog up with dust, the free area available for the air to pass decreases, which increases the resistance to flow. As the resistance of the filter increases the pressure required to force the air through the filter also increases.
It is important that when we select a fan to force cool electronic enclosures we take into account the resistance to flow of filters or louvres that are required to prevent ingress of unwanted particulates and moisture
Mounting considerations – Position of the fan in the system
Where we are using a fan to force cool an electronics enclosure, an axial fan is a common selection for this type of application. The combination of high volume flow versus lower pressure in the enclosure make an axial fan the most appropriate fan type with respect to high efficiency and quiet operation.
However, the position of an axial fan in the application will also affect its operating conditions and the cooling effect it will provide for the electronics inside the enclosure.
In the example below, the choices in where to position a fan to cool an enclosure containing electronics that are dissipating a heat load is shown…
The components of an axial fan are designed to promote a smooth laminar flow of air on to the impeller blade. A laminar inlet flow maximises the efficiency of moving the air from the suction to the exhaust side of the impeller blade. On the exhaust side of the impeller blade the flow becomes random and turbulent if flow straightening devices are not used.
Placing the fan at the inlet and blowing into the enclosure is one option to consider. We can use the turbulent exhaust of the fan to spread the air around the enclosure. This will ensure that all surfaces of the components within the enclosure will encounter some of the cool supply air. Being turbulent the random air path will create more pressure in the system as the air makes its way towards the exhaust. More pressure in the system will create more noise from the fan and require more power to deliver the required airflow.
On the other hand, we could place the fan at the exhaust which will change the system characteristic. The air will follow the path of least resistance which means that some of the components will experience less cooling. The pressure in the system will be lower resulting in quieter operation and lower power consumption. The fan will also see a higher operation temperature as the air passing over the motor will have an increased temperature caused by the dissipation of heat from the components inside the enclosure.
The amount of airflow required to remove the heat lost by an electrical or electronic load can be simply calculated using the sensible heat load calculation. The most appropriate fan selection will be the one which meets the flow requirement, whilst overcoming the resistance to flow, and providing an air path that will achieve the desired cooling characteristic within the enclosure.