Fans and Forced Convection in Process Cooling
A good understanding of process cooling and the available options can save time and money.
Factories, foundries and industrial plant facilities can be quite hot, mainly due to the heat-producing equipment used within their walls. In many cases, this ever-present heat is required and is used for drying, curing, casting, molding and other processes. These high temperatures, over time, can potentially lead to the premature failure and downtime of necessary — even critical — equipment, impacting schedules and costing money, which is why the topic of process cooling is so important to understand.
Heat Sources and Consequences
Heat is used in many industrial settings. Metal is cast and plastics are molded. Extrusion uses hot dies to accomplish the forming of materials. Some metals are heat treated prior to being joined by welding. Rubbers, composites and adhesives need to be cured as do many types of coatings. Here, high heat is used to accelerate the process of cross-linking.
Textiles, chemicals and food processing also use heat in one way or another, from drying inks and dyes to baking bread and cooking meat. Equipment that uses and produces heat is found throughout manufacturing, so the widespread need to manage and remove this heat is paramount. Metals, plastics and most coatings must cool before they are moved to the next processing step. In many cases, the same is true for the equipment being used. Welders, dies, ovens and rollers all need to be kept cool to maintain process stability and avoid breakdowns.
Here is a look at several options for cooling industrial equipment, starting with a reminder of the physics involved.
First, remember that heat always flows away from its source. So, when a piece of metal tube is heated, for instance, the thermal energy flows down to the cooler end. The rate of flow is proportional to the temperature difference: heat moves faster when the difference is larger.
Second, there are two basic mechanisms of heat flow: conduction and convection. Conduction is when heat moves through a solid material. Think of it as the kinetic energy of the atoms passing the energy along.
Convection is associated with fluids and is really a buoyancy phenomenon. As a gas or liquid gets warmer, it becomes less dense, rising to let cooler fluid take its place.
Third, when a liquid boils, its characteristics change and become a vapor. This type of energy can be used as a cooling method. It is generally called evaporative cooling.
Process Cooling Options
There are many ways to remove heat from equipment. Leaving cabinet doors open is generally not one of them because this can be dangerous, not very effective and can allow contaminants such as dirt, dust and humidity to get inside. Here are some better options:
Heat Sinks. These allow heat to flow from the source, but ultimately they rely on convection to take it away.
Heat Pipes. Consisting of a sealed copper tube filled with fluid, you will see these in some computer equipment. The heat of the electronics boils the liquid, and it rises away from the heat source, where it can cool and condense. This method relies on moving the thermal energy to a cooler location close by. If the factory area also is hot, they may not be very effective.
Water Cooling. Water is pumped around the heat source, where it is warmed up. Next, it flows through a heat exchanger, where a large surface area and convection carry the heat away, lowering the water temperature.
Air Conditioning. This is essentially a convective cooling process where the temperature difference increases so that heat flow is subsequently increased. Air-conditioning systems can be expensive to install and operating costs can be high.
Natural Convection. This may be effective in a large open space, but it is much less likely to be effective — if at all — in an enclosed cabinet. Besides, ambient air can be humid and dusty, and so the open factory air may not be an option for cooling electronic equipment.
Forced Convection. This means increasing the air velocity and is the wind-chill effect at work. As the air moves faster, it is able to remove more heat at a faster pace. Vortex coolers are an interesting option for spot or local cooling, but using compressed air to create a stream of cool air makes them expensive to run. This is why fans are a more widely accepted method for creating forced convection.
Axial and Centrifugal Fans
An axial fan’s blade looks similar to the propeller of an airplane or a boat. The blades are mounted on a shaft aligned with the direction of airflow, and the airfoil section of blades pulls the air through. These are good for moving large volumes of air but are less effective when there is significant static pressure.
Static pressure — or more simply, resistance to motion — is an issue with ducting and cabinets. A duct with a small cross-section offers a lot of resistance to air movement because of the impact of friction against the walls. Likewise, airflow in and out of electrical cabinets is restricted by filters and louvers.
Centrifugal fans, more commonly known as blowers, come in many sizes and forms. Essentially, they consist of a wheel similar to those used on the old paddle steamers. The rotation of this wheel throws air outward and is replaced with air drawn in through the center. One point to note is that the incoming air changes directions in a centrifugal fan application, which will determine how the fan should be installed. Blower-type fans usually are the preferred choice when there is significant back pressure in the ductwork or cabinet.
It is essential to consider the worst case or highest air volume required when designing an air-handling system. Inevitably, this means specifying a blower that is big enough for extreme conditions but oversized for 90 percent or more of its service requirements. The result may be excess energy consumption.
There are two ways around this: Use a blower fan that can run at variable speeds or use multiple smaller fans.
Speed control once depended on having either a gearbox or a motor wound for two speeds. In recent years though, variable-frequency drives (VFD) have emerged to provide much greater control over motor speed. Their additional upfront cost may be warranted if energy costs are a concern.
Velocity or CFM (cubic feet per minute) Control. An alternative to varying the speed of the fan motor is to put dampers in the ductwork. These can be complex louvers or simple butterfly-type shutters. They reduce airflow by reducing the cross-sectional area of the duct. The downside is that they increase back pressure, so they tend to be less efficient.
Depending on the application and environment, fan noise can be an issue. In cases where higher noise levels are not acceptable, it may be necessary to locate the blower fans on a rooftop where workers will not be disturbed.
The source and especially the temperature of the incoming air should be considered. In most cases, filters should be used to remove potentially harmful particulates. Also, if the incoming air is at or near the same temperature as the equipment to be cooled, it is unlikely to be very effective.
Choose the proper cooling method. In industrial environments, temperature control is essential for reliability and uptime. Cooling can be provided in many ways. While each has advantages in specific applications, the wrong type of cooling can be detrimental to the product or equipment. Especially in these situations, forced convection with blower-type fans can be efficient and cost effective.