Steps to Designing a Hot Air System – Part Three 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 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 and second articles which cover Defining the Problem and Gathering Information before continuing.
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In this article we will look at the next step – System Design through Calculations, Experiments and Simulation. The results of this critical step determines the specifications of the equipment that should be installed for the application.

Step 3 – System Design through Calculation, Experiments and Simulation

​Now that the relevant process data has been collected, we can complete initial flow and thermal calculations for equipment sizing. Although actual air flow and heat transfer can be very complex, there are a number of calculations that can be done to estimate the energy requirements and approximate required air flow of a system. 

1. Flow and Thermal Calculations

In some cases where either a process is quite simple or where having a ballpark value of the required specifications of the hot air system is sufficient, there are simple flow and thermal calculations that can be performed. This will give a designer an estimate of heater power and blower output required.

Flow Calculations
In some applications—such as heat tunnels or ovens—a cavity of some sort must be filled with hot air. In these cases, the air flow of your system must be sufficient to not only fill the cavity but allow the air to be exchanged several times per minute (we recommend ten exchanges of air per minute) to ensure there is adequate heat transfer and consistent temperature.

To calculate the required air flow needed, simply calculate the volume of the cavity to be filled and multiply by ten. This will give you a value for flow in some units of volume per unit of time such as litres per minute or cubic feet per minute (CFM).

Once you’ve determined that the necessary air flow to fill the space with an adequate number of exchanges, you must check the thermodynamic calculations to be sure that there is enough energy available to heat the calculated air volume to the desired temperature.
 
Heat Calculations
When selecting a process heater for industrial/manufacturing applications, one of the most common mistakes we see is selecting a heater based on its temperature capabilities rather than its power rating. Temperature can be important but is generally a dependent variable.

When we "heat" something, we are really adding energy to a system in order to make the atoms move faster. Energy is expressed in the SI unit joules (other common units include BTU, calories, and kilowatt hours). Power is simply the amount of energy per unit time, or joules per second, and its SI unit is the watt. Therefore, the watt rating of a heater tells us how many joules of energy we can put into a system per second.

There are many factors that will impact the heat requirements of any process. Below are some of the design criteria you should consider. In any given application some of these criteria will be relevant and some will not (or will have such a small affect as to be negligible). For details on the calculations please refer to the article – The Basics of Heat Calculations.
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1. The heat required to raise the material to the required temperature.
   Every material has a property know as specific heat. This is the amount of energy required to raise the temperature of a given mass of the material by one degree Celcius. Specific heats of most common materials can be found in data tables such as this one.
2. The heat required to change materials from one state to another.
   In processes where the material needs to change state from a solid to a liquid or from a liquid to a gas, the amount of heat must be calculated from the latent heat for that particular material and the mass of the material. Like specific heat, latent heat is also a material property that can be found in material data table.
3. The heat required to compensate for thermal losses in the system.
   It is very difficult to build a heat system where no energy is lost to the surrounding environment. The amount lost depends on several factors including: thermal conductivity of the wall and insulation materials of the system, temperature differential between inside and outside of the system, and the thickness and surface area of the walls, etc.

Once you’ve completed the above calculations according to your process requirements, you have an idea of the size of heater you will require. If you find that the number of watts required is large, you should look at the factors in the calculations to see if there is an opportunity to mitigate. For example, if losses to the environment are causing a large increase in the requirements, look for ways to minimizes losses through insulation, process redesign, heat recirculation, etc. 

2. Do physical testing if possible

​Physical testing is an extremely valuable tool for confirming some of the parameters of the process heat system. There are times when even simple testing can be done that will be helpful. For example, at STANMECH for many applications our first step is using a simple hand held hot air gun to determine things like the activation temperature of a material and a sense of the cycle time. Understandably it can be impossible to model many applications fully but the process can often be broken down into smaller parts that can be tested, which will increase confidence in the design.
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When talking to suppliers, such as STANMECH, ask if they provide physical testing as part of the equipment selection process. If they do, take advantage of the offer.

3. Simulation

In processes that are too complicated to do the flow and thermal calculations and too expensive to do physical experiments, there is the option to do computational modeling. Using specialized software, a complex system can be modeled and virtual experiments performed until a final design is determined. Unless you have the expertise to do this type of modeling, you will likely have to find a third party who can perform it for you.

Computational modeling does not replace physical testing. Testing on the final design should be performed if possible to verify the results. For more information on computation modeling please see our article on what is it and how it can help you.
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Of all the design steps discussed thus far in this series, this is the one which you may have to go outside of your organization for help. At STANMECH we can help with some or all of the design process and ensure you get the system that works for you.