It has always been a priority for manufacturers to maintain seamless production. However, higher energy costs, new regulations and increased concern about environmental responsibility are challenging the old ways of thinking. New focuses on initiatives such as sustainability and lean manufacturing — topics that were not on the tongues of production managers years ago — are common in the manufacturing lexicon.
Process cooling can be an energy- and time-intensive part of production, especially if the cooling solution involves chillers or cold-storage areas. As business priorities shift and waste reduction becomes as important as increasing output, the days of energy-intensive refrigerated air to cool items on a production line surely will get another look. In some cases, alternative technologies will replace refrigerated air.
Companies looking for energy-efficient alternatives for process cooling can start by considering the products that likely already exist in their plants — like those used to cool their employees. Airflow from overhead and directional fans is less expensive than refrigeration, and it can be effective for cooling products to room temperature.
Conventional wisdom says that higher velocity air will cool products fastest, and this is correct for the most part. However, at the same time, fan affinity laws dictate that higher air speeds require more energy. So, finding the lowest air speed that meets the process cooling requirements often yields the most energy-efficient solution.
Each production line is different, and each requires a cooling solution tailored for its needs. Facilities should enlist the help of a company with trained application engineers who can make site visits, model 3D spaces and run equations using empirical data and analytical solutions to determine how many and what type of fans are needed, and where they should be placed for optimal cooling.
Why Airflow Works for Process Cooling
The cooling effect of airflow is instinctive. When you are about to slurp a spoonful of hot soup, you blow on it to cool it down. Everyone knows it works even if they do not know why. Yet, a fan cannot reduce the temperature of a room, and you cannot cool an 85°F (29°C) product by blowing 85°F air on it. So how does fan cooling work for process cooling? A quick refresher explains on the physics involved.
For starters, there is no cold, only heat, which is a measure of molecular kinetic energy. Cold is simply the lack of heat. According to the laws of physics, heat takes the path of least resistance — from high energy to low energy — and transfers among different types of materials, whether solids, liquids or gases, at different rates until an equilibrium is achieved.
Humans feel variances in ambient temperature because air molecules hold kinetic energy. Temperature is relative — to humans, 100°F (38°C) is pretty hot, but to a brick coming out of a 2000°F (1093°C) kiln, for example, it is positively freezing. If left sitting in a 100°F room, the 2000°F brick will slowly cool as its heat transfers into the surrounding, cooler air through a process known as natural convection.
Airflow from fans creates forced convection, which speeds up cooling. The speed up occurs because every object develops a boundary layer of slow-moving, warmer-than-ambient air around it. This warm, stagnant air of the boundary layer acts as an insulator, slowing the rate of heat loss.
Fans cool both people and objects by reducing the thickness of the boundary layer, mixing it with ambient air and allowing for more rapid transfer of heat. As conventional wisdom suggests, the faster the air speed, the quicker the heat transfer; however, there is a point of diminishing return.
High or Low Speed Airflow for Industrial Cooling?
There are many factors to consider when determining which cooling method is right for any given process. When selecting fans for energy efficiency, process managers and engineers should quickly identify that point of diminishing return. In short, they should seek to find the slowest air speed needed to achieve the desired resulting temperature in the correct amount of time.
Here is an example. When cooling people, about 200 ft/min of airflow is an ideal middle-ground between personal cooling and fan energy use. A high speed fan that blows air at 600 ft/min using 750 W of power can provide 11°F (6.1°C) of perceived cooling. A high efficiency fan blowing 200 ft/min over the occupant provides 8 or 9°F (4.4 or 5°C) of cooling effect using just 30 W. Are a few degrees of perceived cooling enough of a benefit to justify a 2,500 percent increase in energy use?
Process cooling generally requires higher air velocity than comfort cooling. Though the formula works a bit differently for cooling objects, the curve looks similar. The eventual point of diminishing return means that moderate air speed across an object (rather than the maximum possible amount of air movement) could strike the best balance between cooling and energy use. Airflow only can reduce a boundary layer so much, and materials have different rates of heat transfer. Once the maximum rate is reached, additional air speed will not make a significant difference.
Finding the right fans for the job can be intense work. Variables to consider include:
- The heat-transfer coefficient (the calculation of which can be exceedingly complicated in and of itself).
- The surface area of the object.
- The time it takes for the object to reach the next step on a manufacturing line.
- The maximum desirable temperature of the object.
- The energy use of the cooling method.
When seeking a solution, engineers trained on selecting and sizing fans can help you find an effective process cooling fan solution that provides the lowest energy costs.
Forced Convection Added to Industrial Refrigeration Systems
Another common option for air-based process cooling is refrigeration. While fans cannot reduce the temperature of the air they are moving, refrigeration units can, which makes them seem like the only sensible solution at first glance. At times, however, fans cool items to room temperature much faster than air conditioning alone.
Combining fans and air conditioning may seem to be the best solution. In some applications, however, this solution provides only a marginal increase in cooling speed. This is why a cooling solution tailored to each production line is important.
Consider the following hypothetical example. A metal fabricating company needed to cool sheets of stainless steel from 350 to 100°F (176 to 37°C) so that workers could safely handle them. The company considered three options:
- Airflow from fans in an 85°F (29°C) warehouse.
- Storing the sheets in a 40°F (4°C) walk-in refrigerator.
- Adding airflow from fans in the 40°F (4°C) walk-in refrigerator.
In the first option, presuming an air speed of 150 ft/min by using fans in the 85°F (29°C) warehouse, the metal plates would reach 100°F (37°C) in about 40 minutes. In the second option, by relying on natural convection in the 40°F (4°C) refrigerator, the plates would take more than an hour to cool. In the third option, using a combination of 150 ft/min of airflow and 40°F (4°C) air provides the fastest cooling at about 35 minutes. However, that option is also the most energy-intensive. It uses five times the wattage of the fans alone yet produces only a five-minute reduction in cooling time.
Using Natural Convection for Industrial Cooling
Natural convection, in which products are left to cool on their own in ambient conditions, is cheap but slow. In many applications, using forced convection with fans can save money over natural convection.
A real-world example can illustrate this. Impulse Manufacturing, a Georgia metal fabricator, relied on natural convection to cool metal components on a production line. However, the parts typically were 165°F (74°C) when they reached associates — too hot to handle.
To keep output high and avoid line shutdowns, managers tested airflow cooling with a single fan. It did not work. The company considered installing a longer conveyor line so the components would have more time to cool. The conveyor-line upgrade would have cost $40,000 to $50,000, plus the energy needed to run it.
Impulse’s engineers instead worked with a manufacturer of industrial fans to develop a better forced convection solution. The engineers designed a more direct airflow path on the parts at a slightly higher speed. This was found to be ideal for dropping the temperature of the objects 65°F (36°C) within 80’ of conveyor.
After installing eight fans and testing several layouts, the company found a setup that maximized cooling. Ultimately, for Impulse Manufacturing, this solution avoided the conveyor addition and increased output by 20 percent.
“With the added fans, we no longer have to leave gaps. We can load the line at 100 percent, as it was intended to run,” said Clay Reiser, general manager.
In conclusion, remember that each process has unique characteristics that need to be taken into account. For process cooling, consider fans as well as refrigeration and line upgrades. Overhead or directional fans are effective and provide an energy-efficient air cooling solution for getting products to room temperature quickly. To find the best cooling method, do not be afraid to test different layouts, fan speeds and fan sizes. To eliminate the disruption of trial and error, enlist the help of thermal and process engineers who can calculate the cost and benefit of airflow.