Steps to Designing a Hot Air System – Part Two of Four

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 and 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. Be sure to read the first article which covers Defining the Problem before continuing.
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In this article we will look at the next step – Gathering Process Information. This critical step builds on the problem definition by quantifying the important characteristics and constraints of the application.

Step 2 – Gather as much process information as possible

​You cannot expect to make good equipment decisions without good information. This section is by no means exhaustive; while it’s a good starting point, your process may have other details you need to account for. If you are unsure if a detail is important, we recommend including it in your report. 

1. Temperature

There are a number of areas that need to be considered here. The most obvious is the operating temperature or temperatures. For example, does a chamber need to be maintained at a set temperature? Or does a part need to be heated to a target temperature?
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There also may be limits to the temperature such as an upper limit above which there will be damage to the materials. For instance, plastics may begin to degrade or sustained elevated temperatures may affect the heat treatment of a metal.

2. Speed of process

​The speed of the process relates to a more important variable when applying process heat – time. Time is a critical variable in the design of a process heat system. The heat requirements of the system depend directly on the time available. Using bad estimates will likely lead to under or over estimating the heat requirements.

3. Geometry

​Like speed, the geometry involved in a process directly impacts the amount of energy required. Relevant geometry includes:

• Size of a part to be heated
• Size of a chamber to be kept at temperature
• Length/diameter of any pipes or nozzles the hot air will pass through

 
Using an inaccurate estimate of these geometries will skew the thermal calculation and cause the system to be under- or over-specified. 

4. Precision

There is always a temptation to put very tight tolerance requirements on a process. However, it’s important to consider two things:

Precision costs money
It is generally accurate to say that the greater the desired precision, the more complex and expensive the solution. In order to attain tighter tolerances more sophisticated equipment is required. The heaters will have to be controlled via an external temperature controller and thermocouples to ensure the temperature is being measured and controlled at precisely the right location. If the area being heated is large, it is likely that multiple heaters/thermocouples and controllers would be required. At the other end of the spectrum, if the required precision is very loose it is possible that the simplest heater without any electronics or temperature control will be adequate. If the temperature needs to be set but the tolerances are not extremely tight, a heater with on-board power electronics that control temperature or the power output of the heater is normally a cost effective and flexible solution.

The capability of the technology
It is important to keep in mind the capabilities of hot air technology. If your process requires precision to within fractions of a degree it is unlikely that hot air is the correct technology choice. It is important not to unnecessarily eliminate solutions by specifying tolerances that are artificially constrained.
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By not giving the precision requirements due thought you may be adding unnecessary cost or complexity to your solution.

5. Other physical constraints

Sometimes there are things in the process that must be worked around. This is especially true when modifying an existing system. Examples include:

• Interference with another piece of machinery
• Requirements for maintenance
• Ambient temperature
• Width of conveyor

 
It is by thoroughly addressing Step 2 that we can avoid a “garbage in – garbage out” situation. Take the time to gather as much of the necessary details up front to avoid wasting time and money later. Having this information will also allow you to speak confidently to any suppliers about your requirements.

Need help with this? Give us a call at 1-888-438-6324 or email info@stanmech.com.