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Industrial process heating can take many forms and be used for many applications. One such application is an industrial oven, which itself also can come in many forms. Among them are:

  • Conveyor ovens, which are useful for heating large amounts of product.
  • Hot air ovens, which use dry heat and are suited for moisture removal.
  • Batch ovens, which are useful in many laboratory settings.

Common oven applications include drying, curing, laminating, coating and heating.

With all of these varied applications, it is important to choose the oven type that best corresponds to the specific application. One feature that can exist within a hot air oven is the use of recirculated hot air.

Recirculated hot air ovens offer benefits beyond the features provided by standard hot air ovens. If an oven manufacturer wants to save costs, increase energy efficiency and achieve consistent temperature gradients, recirculated hot air ovens should be considered. Recirculated hot air ovens work well on the same types of applications as other hot air oven types. In this article, we will review how these applications work as well as the benefits of using hot air recirculation in industrial ovens.


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Recirculated hot air is suitable for shrink tunnels and helps to minimize heat losses. Photo credit: Leister Technologies (Click on the image to enlarge.)


Hot Air Oven Overview

One of the most common oven forms is the hot air oven, in which an electric air heater works in tandem with an industrial air source. Typically, in a hot air oven, there will be an insulated base structure that is used to contain heat while allowing for insertion and removal of the object that is being heated. An electric heat source then will be connected to this oven structure. Often these electric heaters will be flanged or mounted directly onto the oven for maximum efficiency. Attached to the inlet of this heater will be an air source, usually in the form of a blower. This air source will flow air into the electric air heater, where the air will be heated. From the heater, the hot air will flow into the oven and heat up the air in the structure. This hot air in the oven is used to heat up whichever object is used in the application.

In these industrial ovens, many features can be added to improve efficiency. One such feature is hot air recirculation. The difference with a recirculated hot air oven is that the hot air from the oven is recirculated back into the inlet of the blower. This contrasts with the room temperature air that is fed into a regular hot air oven. In a recirculated hot air oven, there is typically an exhaust exiting the oven. From this exhaust, this air is routed back to the inlet of the blower. Because the hot air is recirculated, the initial temperature going into the heater is lower. The smaller the difference in temperature between the inlet air on the heater and the outlet air on the heater, the lower the energy that is needed to heat up the air. Hot air recirculation results in lower energy output because there is a lower temperature difference between the heater inlet and desired temperature inside the oven.


03 PH 0322 Leister Recirculated Hot Air Ovens Microsoft Teams

The difference with a recirculated hot air oven is that the hot air from the oven is recirculated back into the inlet of the blower. Photo credit: Leister Technologies (Click on the image to enlarge.)


Why Choose to Use Recirculated Hot Air Ovens?

While there are many benefits to using recirculated hot air ovens, this lower energy output of the heater is the primary benefit. Because there is a lower energy output, there is a difference in the necessary electric power used by the heater. This decreased energy usage results in a greener carbon footprint and a lower electricity bill. Depending on how optimized your oven recirculation is and its usage time, this can result in thousands of dollars of saved costs annually. This is especially important as current trends show a continuation in the rise of energy costs in the future.

As an example, consider an oven in which you want to heat up 150 cfm from a room temperature of 70°F (21°C) to a desired oven temperature of 900°F (482°C). Without recirculation, this results in a change in temperature of 830°F (461°C). Now, consider if the hot air was recycled and the recycled inlet temperature was changed to 450°F (232°C). This would instead result in a change in temperature of 450°F (250°C). Because the change and temperature and power usage are directly proportional, the ratios of the change and temperature will be equal to the ratio of the power. Comparing these numbers, it can be calculated that a roughly 46 percent savings in energy will be achieved in this scenario. This, in turn, would result in a 46 percent savings in energy costs.


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While there are many benefits to using recirculated hot air ovens, the lower energy output of the heater is the primary benefit. Photo credit: Leister Technologies (Click on the image to enlarge.)


Compared with gas-burning ovens, electric hot air ovens have a benefit in that they have the capability to regulate oven temperature more accurately in a closed-loop system. In closed-loop temperature control, there is a feedback control that allows for a consistent temperature, regardless of disturbances. This works because of a thermocouple that is inserted inside of the oven, which measures the oven’s internal temperature. This measurement then is fed to a control system, usually in the form of a PLC or temperature controller, where it is compared to the setpoint temperature. This control system then will tell the heater whether to increase or decrease power based on the measured temperature. If the measured temperature was below the set point temperature, the control system would tell the heater to increase power, while, if the measured temperature was above the set point temperature, the control system would tell the heater to decrease power. This helps keep the temperature inside the oven consistent with the setpoint temperature.

When creating this closed-loop temperature control system in an industrial oven, thermocouple placement must be taken into consideration. Ovens may be very large inside and the air temperature may vary throughout the interior of the oven. In the oven, the thermocouple will only measure the temperature at the specific point where it is placed. This may not be an issue if the accurate temperature is only important at one point in the oven or if the minor variation of the temperature gradient within the oven is not impactful to the heating process. If a more uniform heat distribution within the oven is important, multiple heaters with multiple strategically placed thermocouples could mitigate these variations in the temperature gradient.

One downside of recirculated hot air ovens is that the initial capital cost of the oven may be higher than that of an ordinary hot air oven. In a recirculated hot air oven, you need to have a blower that can withstand hot air flowing through and an electric air heater that can tolerate hot air on the inlet side. Heaters and blowers that can handle this hot recycled air tend to be slightly more expensive than those that cannot. Additionally, for peak efficiency, insulated hosing going into and out of the blower is recommended in order to retain as much heat as possible. This also is an increased cost over ordinary hosing. While the initial cost may be slightly higher, it is almost always made up by the savings in energy costs.

Peak efficiency is achieved in a recirculated hot air oven when there is as little heat loss as possible. The goal is to set up the system in a way that will prevent these heat losses. There are a number of actions that can be taken to mitigate these heat losses in the system. One such example is ensuring that the oven, hosing and other parts of the system are properly insulated. Insulating your oven with material that is resistant to heat losses will help make your system more efficient. Using accessories such as high temperature seals and flanges will also help prevent heat losses. Taking actions to ease these heat losses will help to maintain that high recycled inlet air temperature. This will, in turn, result in lower energy requirements to heat the recycled air back to the desired oven temperature.


05 PH 0322 Leister Recirculated Hot Air Ovens Microsoft Teams

Hot air recirculation results in a lower energy output because there is a lower temperature difference between the heater inlet and desired temperature inside the oven. Photo credit: Leister Technologies (Click on the image to enlarge.)


In summary, industrial process heating is a vital part of industrial manufacturing in almost all industries and can come in many forms depending on the application’s needs. When designing process heating applications, engineers should consider methods that maximize efficiency. When using an industrial oven for a process heating application, one way in which efficiency can be maximized is using recirculated hot airflow. As explained here, there are numerous ways in which recirculating hot air helps to achieve peak efficiency in an industrial oven. With the lower necessary energy output, cost savings and more consistent temperatures gradients, recirculated hot air should be considered by end users for their industrial ovens.

With the number of companies that specialize in ovens and hot air, end users will have many options for their industrial oven needs, allowing them to achieve a process that can maximize efficiency and achieve the application results that they desire.