On the surface, designing an effective hot air system can seem like a simple exercise. However there is an underlying complexity which, when ignored, can result in wasted time and money. Designing a system without being aware of these complexities can often lead the incorrect conclusion regarding which tools should be used for an application.
Regularly, customers go to the STANMECH website and pick out a tool without going through the full design process. The best case scenario is that the customer lucks into selecting the correct tool. The worst case scenario is that the customer designs, builds, and commissions a complete processing line, spending a great deal of money and time to end up in a situation where it does not work. At this point they find themselves in a corner must spend more money and time to correct the problem.
The purpose of this series of articles is to keep the reader from going down the wrong path which, in most cases, can be avoided by thinking more carefully about the hot air system they are designing. Before continuing, be sure to read the first article which covers Defining the Problem, second which covers Gathering Process Information, and third which covers System Design before continuing.
In this article we will look at the next step – Selecting Process Equipment. Only after the first three steps are completed should we begin selecting equipment.
Step 4 - Selecting process equipment
At this point we should have most of the required information in order to choose equipment. There may still be some unknowns, which is less-than-ideal, but there are times when the information will be incomplete. From our testing and calculations the kilowatts, air flow, and temperature requirements of the systems should be established. This, along with the other application details/information, will allow us to make good equipment decisions.
1. Type & size of heater (Open Coil and Fully Supported Coil Heaters)
Not every type of heater will work for every application. In order to get the best possible results for your application it is important to select the correct type of heater. A good place to start is to check the specifications (maximum operating temperature, maximum operating pressure, etc.) for your heater to make sure that they line up with your application requirements.
Open coil heaters are best for applications that require high volumetric flow at relatively low temperatures (usually less than 400°C). Their open structure allows for large volumes of air to pass through with relatively little pressure drop. However, the ceramics used to support the element typically break down at lower temperatures than those used in non-open coil heaters; and the element has less support which can be problematic at higher temperature.
Fully supported element heaters (i.e. honeycomb heating element) are best suited for applications that require lower airflows at higher temperatures. That being said, this style of heater can be used for low and high temperature applications, as well as low and high volumetric flow applications provided that the air source can generate the required airflow at a high enough pressure to overcome the resistance inherent to fully supported heating elements.
For heat recirculation applications, the electrical connection points are usually stood off from the at heater body. Be sure to check both the heater’s maximum rated temperature as well as the maximum rated inlet temperature.
Once you have established the correct type of heater, we can focus on the size required. The most important value is the power rating (typically in kW). This value dictates how much air can be heated to the temperature required by your process. It is not enough for the heater to be capable of reaching the temperature required by your process, it must be able to reach that temperature at the necessary volume for your process. Volume air flow and temperature are inversely related; with constant power an increase in air flow will yield a lower output temperature. See our article on the Relationship between Air Flow and Temperature for more details.
Occasionally it may be necessary to use more than one heater for your application. If this is the case, it is usually to meet a high power requirement or to more evenly distribute the heat in your system.
2. Type & size of blower (Centrifugal & Regenerative blowers)
As with heaters, not every type of blower will work for every application. In order to get the best possible results for your application it is important to select the correct type of blower. A good place to start is to check the specifications (maximum operating pressure, maximum volumetric flow rate, etc.) for you blower to make sure that they line up with your application requirements. Be sure to look at the blower curve to ensure it will provide the performance you require. See our article on Interpreting Blower/Fan Curves for more information.
Centrifugal blowers are typically best for applications which require high volume flow rates but do not have much back pressure exerted on the system. Typical sources of back pressure are: constrictive nozzles and passage ways, long lengths of pipe or hose, serpentine flow paths, etc. Centrifugal blowers are known for having higher flow rates (at similar power ratings), however these flow rates drop off quickly as they encounter back pressure. See our article on Understanding Blowers as part of a System for more information about back pressure.
Regenerative blowers are typically best for medium pressure applications requiring lower- to medium-sized air flows. These applications will contain some or all of the back pressure sources listed above. For example, a regenerative blower with a lower maximum rated volumetric flow rate than a centrifugal blower will often provide more volume air flow through a fully supported element heater, because it is more capable of pushing the air through the resistance that is inherent to this style of heater. See our article on the Differences between Regenerative and Centrifugal Blowers for more information.
Some applications may require high volume air flows at medium pressures. For these applications it may be necessary to use a more specialty blower, such as a “high speed centrifugal blower”.
The size of the blower required (typically in kW) should be determined by referring to the blower curves and determining which blower will meet the performance requirements of the application. It is sometimes necessary to use more than one blower for an application. This can be a bit trickier and more nuanced than with heater selection. STANMECH is happy to help you with this process.
3. All-in-one hot air blowers
For applications where there is both a small power and small flow requirement, a combined heater/blower may be appropriate. These tools are attractive because of they are ‘all-in-one’ but they should only be selected after the thermal and flow requirements establish they will be adequate for the application. See our article on All-in-one Tools for more information
4. Temperature control
Article 2 of this series discusses determining the level of precision required for the process. This becomes a factor when establishing how temperature will be controlled from the heater. Heaters come with different levels of sophistication which will determine their applicability for a process. Below, ordered from lowest to highest, is an outline of the levels of sophistication for temperature control:
If the application requires more than one heater there are a few control options depending on the level of precision required and the system design. A single thermocouple and controller can be used to control multiple heaters simultaneously, a thermocouple and controller for each heater can be used to control tools individually, or a combination may be used if precision is more important in some areas than others.
5. Nozzle/air knife/heat tunnel, etc.
Designing a hot air system is not simply about the choice of a heater and blower. How the hot air interacts with the target can determine the success or failure of the system.
Some applications, such as drying paint, require a diffuse, soft heat to avoid unwanted effects such as ripples in the dried surface. Others may require a focused jet of hot air, such as when deflashing plastic parts. An air knife can be used to create a fast moving sheet of hot air, this is useful for applications such as flashing forming/stamping oil off a sheet. A heat tunnel or oven would be used when a part needs to be submersed in hot air, such as when the goal is to warm an object or shrink packaging materials around a part.
More details on nozzle design can be found in the article Three Things to Consider when Choosing a Nozzle.
Final Testing and Calibration
In an ideal world the equipment you choose for your process will work perfectly right out of the box. In reality there will be some final testing and fine tuning required to optimize the performance. Leave yourself some time and budget at the end of the project for the unexpected.
Designing a good system requires patience and expertise. Don’t be tempted to jump straight to the answer without doing all of the steps in between. If you need help finding the system that works for your process, call STANMECH to see how we can help.