Industrial Heating Methods, Convection or Infrared?

This article compares two industrial heating methods: infrared and convection to determine the benefits of each in terms of cost and time spent.

  • While it is always important to consider the cost and time requirements or a new heating system. Top of mind should be how effective that heating method is at reaching your product goals. Saving time and money on a particular heating method is no good if the method being used produces defects in the product being heated. Therefore, it is imperative to look at the effectiveness of the heater you are considering.

    A Wattco Infrared HeaterIn this article, we will look at the benefits and possible disadvantages of the quartz infrared heater and the gas convection heating methods. Of course, both are effective; however, neither are effective for every heating scenario.
     
    There are many variables to consider when looking at different heating methods, including:
  • The amount of product to be heated
  • Whether or not the product needs to remain stationary
  • The amount of time available to heat the product

The Size of the Heating System and the Production Rate

In starting an analysis, determining the heat load is the first equation to consider. Each part’s weight, as well as the total weight processed within an hour must be calculated in terms of British Thermal Units (BTUs) per hour or BTU/h, so that the equation used to determine the size of the heating system would be BTUs equal Weight per hour times specific heat times the temperature change. BTU/hr = (mass/hr) * Specific heat * Delta Temperature. For fluids with many components, the total BTU/hr is the sum of the BTU’s for each component.

Methods to Convey Parts

How parts or products are loaded, held, and conveyed past or into a heat source is what must be decided next. Good airflow or unobstructed transfer of heat in an infrared heating system is important for successful heating. Either the heater or the product must be rotated, if neither is possible, then the heater controls must be regulated to avoid overheating particular locations in either heating system.
 
Highly successful use of an infrared heater will allow for either the heater or product to be rotated to permit the heat’s exposure on the part to an average rate instead of one area of the product or part receiving too much exposure. Too much heat to one area could produce warping or damage to the product in that area.

The Principle Difference between Infrared and Convection Heating Method

An infrared heater is produces radiating heat; whereas, convection is produced by hot air. When a heater or product is heated by an infrared source, the heater requires a direct line of sight to the product. The distance or space between the part or product and the heater will decide the rate with which the product is heated.
 
A lesser-known factor in infrared heating is how the products’ or color affects the heating rate. Emissivity is the measure of the part’s capability to absorb or emit energy when compared to that of a black object. The emissivity of the product must be measured to determine its heat-up rate.
 
Infrared energy heating could be the most efficient and effective source to heat products. However, proper temperature control must be used to ensure that the product is not overheated. Similar to how a black car can be hotter than the air around it on a sunny day, an object heated by infrared can be hotter than the air around it. To correct for this, ensure that you are measuring the temperature of the object itself and not the air around it. 
 
In hot air or convection heating, the position and shape of the product are less important due to the distribution of heat by the air’s movement. Hot air is circulated from various angles encircling the product at the same rate, like an oven set to a consistent temperature with highly accelerated heat passing around the parts. In this scenario, the parts absorb the heat uniformly reducing the chance of overheating in spots.  
 
A frequent argument against convection heat is depending on the mass or size of the object to be heated it is oftentimes slower than other methods. Another factor with gas convection is the dissipation or exhausting of combustion by-products to prevent unsafe conditions in the heated chamber.
 

Time is the critical difference in these heating methods, only when time is a factor do the differences significantly manifest.    

Heating Time Requirements

When the length of time to heat a part or product has not been established previously, a heat profile must be conducted on the part or product. Data loggers and thermocouples can be used in this process. Thermocouples are mounted onto the parts in critical locations and record temperature data during heating.
 
To determine which is the more efficient heating method, data must be logged for each respective heater. Temperatures and times may vary for convection heaters during curing, whereas infrared heating will show a higher temperature on the surface throughout the heating cycle.
 
Another important note is that maximum temperatures of the parts or products during heating will never exceed the maximum temperature setting of the oven. As you will see below, this is not the case with infrared heaters. 
 
In comparison, the profile of a product heated by infrared shows a substantially higher surface temperature, as well as on the interior thermocouples.
·         An infrared heated cure may reach a higher temperature during the post-heat stage, which could be appropriate for some processes. However, in some other processes that uneven heat could damage certain products being heated.
·         A convection heated product or part might take a longer time to arrive at the necessary temperature dependent on its mass, yet it is known that temperature will never be exceeded in any portion of the product heated and all parts will be thoroughly heated given appropriate time.
 

Therefore, when convection heating is chosen, ensure adequate time is given to thoroughly heat the product and when infrared heating is chosen beware of overheating and hot spots. When curing coatings, beware of overheating and uneven heating to avert defective parts.        

Heating Methods: Advantages vs Disadvantages

There are benefits and hindrances in using either method, the trick is to measure heat times while avoiding overheating or spot heating. Consider the need to cure coatings and the effects of each method on this process.
 
Use of an electric infrared heater instead of a gas convection oven eliminates the need to exhaust gas-burnt byproducts from the heating chamber, so that expense is also eliminated.
 
While comparing costs, the average electric power rate for usage during infrared heat versus the amount of gas consumed will depend on operating times, operating temperatures, and the product’s weight, size, and quantity. The cost of electric power or kilowatts per hour used versus the cost of gas used are compared to determine which method is more cost effective for power consumption during heating of parts and products.
 

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