Convection is a type of heat transfer between two bodies that do not have contact with one another but instead transfer heat via contact with a gas or liquid. The use of a fan inside of an industrial furnace to produce airflow over a heated load inside the heating chamber is one of example of convection, allowing for increased heat transfer.
The same principal applies during the cooling cycle. The hot load will release heat quicker when cooling air is sufficient and properly delivered, reducing cooling time.
Heat-treat processes that use convection heating and/or cooling rely on the mass of the air to transfer the heat to (or from) the product being processed. Convection heat transfer is used in industrial equipment such as ovens, process coolers, refrigeration systems, electronics and heat exchangers. The ability to move more air makes the processes more effective.
To describe how fast heat is transferred using air, we look at the convection heat-transfer coefficient (BTU/hour/feet2 • ºF). This factor can vary greatly depending on the air velocity and the materials being processed. For example, the heat-treat coefficient for a low-velocity laminar airflow over cast iron is between 1.0-2.0, whereas the heat-transfer coefficient of a high-velocity heated airflow over aluminum is upwards of 35. Aluminum will accept heat literally as fast as the circulated air can deliver it. By using increased convection and heat input, heat transfer to part will be optimized, reducing the cycle time of the furnace.
Since the density of air is dependent on its temperature, the ability of air to transfer heat is also affected by the temperature of the air. This is shown by the following formula:
ρt = ρStd • (T + 460) / 530
ρt = density at temperature
ρStd = density at standard conditions (0.075 pounds/CF)
T = Temperature of the air
Therefore, air heated to a temperature of 650ºC (1200ºF) has a density of about one-third of that of room-temperature air, making the hot air only one-third as effective at transferring heat. At elevated temperatures, this becomes more and more significant and is important to remember when calculating the effectiveness of a heat-treat system. It is also important to note that since the weight of the air is reduced at higher temperatures, the furnace system requires less energy to move the air, which can be advantageous when selecting a motor for industrial blowers.
Elevation is another factor to consider (Fig. 1). Because air is less dense at higher elevations, it also transfers heat less effectively. At a location with an elevation of 7,000 feet above sea level, the density of air is only 80% of those at standard conditions. As a result, the heat transfer will be reduced and the heating or cooling process will be 20-25% less effective. This must also be considered when designing or selecting equipment because it impacts the design of air chillers, fume ventilation systems, cooling systems, burners, boilers and ovens.
One of the determining factors when designing an oven is the air recirculation rate, which is measured in CFM (cubic feet per minute) and can also be expressed in “air changes per minute” (CPM). The CPM reflects the number of times per minute all the air is completely recirculated throughout the heating chamber of the furnace.
A high-performance age oven should deliver approximately 40-60 CPM. This is in contrast to general-use ovens, which are often 5 CPM (or 2,800 CFM in the above example). Considering that a 28,000-CFM oven operating at 350˚F will circulate 82,300 pounds of air per hour versus 8,230 pounds per hour for a 2,800-CFM oven, it is understandable why airflow has such a big impact on oven performance.