Machine Design

Air movers and coolers

Air movers can be generally classified as fans or blowers. Fans are usually axial fans or propellers that produce low-pressure air movement. Blowers, on the other hand, are usually centrifugal devices that deliver air at a higher pressure, usually for a process such as a reaction, as well as cooling. For a discussion of heat exchangers, see Fluid Power Components.

When fans are used to cool enclosures, they either draw air through the cabinet, or move it through an air-to-air heat exchanger. Although heat exchangers do not mix inside with outside air, they do pass heat to the environment. However, the technique isolates outside dirt and debris from sensitive components within the heating system.

An airtight cabinet without a fan can dissipate heat but may require a large enclosure for a minimum temperature rise. Adding a fan improves the cooling process over convection and radiation alone by up to tenfold.

Air filtration: When a fan is required for enclosure cooling, a washable air filter at the incoming air stream is recommended. When using air filters, a plenum must also be used to separate the fan from the filter and to maintain even, effective airflow. If a plenum is not used, the filter is placed against the fan reducing filter efficiency because 90% of the airflow passes through the tips of the fan blade.

Fan location:Carefully positioning the fan in the cabinet can avoid some common cooling pitfalls. First, placing a hot component, such as a transformer, in the incoming airflow may seem good for the transformer but subjects other components to unwanted heat. If possible, hot devices should be located near the exhaust so that their heat dissipates directly to the outside.

Also, because axial fans have relatively low static pressure, their exhaust grill is often sized too small. To be safe, the size of the opening should be about 1.5 times the fan area.

Manufacturers recommend placing the fan at the air inlet for three reasons. First, maintaining a positive pressure within the enclosure helps keep dust and dirt out when the cabinet is unsealed or frequently opened. Second, a blowing fan produces turbulence, which improves heat transfer within the cabinet. Lastly, fan life is prolonged when in the path of coolest air.

Fan sizing:In most cases, the relationship between pressure and flow is described by P =KQ2 where P = static pressure drop,= air density, K = load factor, and Q = airflow rate. Typical performance curves reveal the relationship between fan pressure and flow rate. Because flow rate is inversely proportional to resistance, decreasing resistance allows use of a smaller, less-expensive fan. Also, when multiple fans are used in an enclosure, they should be the same size.

Fan size can be determined using a graphical approach. First, estimate the amount of heat, in watts, that must be dissipated. Then, on the accompanying fan selection graph, select a fan based on the amount of temperature rise over ambient allowed within the enclosure.

Another technique for fan sizing calculates required flow rate for a given temperature rise. The minimum airflow is found from Q = fW/(Tout - Tin) where Q = flow rate, cfm; f = flow factor and is the product of air density and specific heat; W = internal heat generated, W; Tout = exhaust air temperature, °F; Tin = ambient air temperature, °F. At sea level, f = 3.1; at 5,000 ft, f is about 3.6.

System impedance:Resistance to airflow within an enclosure (impedance) is expressed as static pressure, which is a function of flow rate. For most applications, impedance is estimated with P =KQn where n = turbulence factor (usually 2).

In complex systems, impedance may be found most easily by measuring the static pressure with a candidate fan. Using the estimated impedance value, the fan's static pressure versus airflow graph determines whether the fan is suited for the application.

Another way to cool electronics with air is by using vortex tubes. Though they move less air volume than fans and blowers, they can lower air temperature below ambient without using compressed gas or refrigerants. The processed air typically feeds through a manifold, directed to a location within the enclosure.

Vortex tubes have been used for decades, but their principle of operation is still debated. Regardless of the physics, good designs have evolved over the years and many sizes have been put to work in countless applications. They typically run from 400 to 1,500 Btu/hr at about 20°F differential temperature. To get an idea of capacity, consider that one 400 Btu/hr vortex tube cools a 1 X 3 X 4-ft machine-tool control box.

Shop air typically is available to power vortex tubes so separate motors and air compressors are not needed. A filter cleans the incoming compressed air that passes through a solenoid valve before entering the vortex cooler. In some applications, a thermostat in the cabinet throttles the solenoid valve to maintain internal temperature within a limited range. The temperature and volume of air emitted by the vortex tube is a function of the supply pressure and air volume rated in scfm.

The compressed air enters a generator stage in the vortex tube that forces the air to spin approximately 1 million rpm and travel toward the hot-air control valve along the interior walls. Part of this air passes through the hot-air needle valve to the atmosphere. The balance of the air moves back through the center of the sonic-velocity airstream at a slower speed and gives up its heat. Thus, the high-velocity air picks up the heat and exits through the exhaust (hot-air control valve), while the cool air travels out the cold outlet port to the enclosure.

The cold volume of air is a percentage of the total air released. A low cold fraction (less than 50%) produces the lowest temperatures but at a lower airflow than that at cold fractions near 60 to 70%.

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