Recirculating heat systems capture and reuse excess heated air that would otherwise be vented to the atmosphere, improving system efficiency and lowering power consumption. Because the heat is recirculated, the intake air stream through the blower and heater is at a much higher temperature than in a typical process heat system. Non-specialized heaters and blowers are designed with the assumption that the intake air is at or near room temperature, the higher inlet temperatures associated with recirculating systems will damage these tools. Specialized equipment is available that is designed to withstand the high inlet temperatures of recirculating systems. In this article we discuss these specialized pieces of equipment and how they differ from the equipment used in non-recirculating process heat systems.
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.
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. 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.
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. 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.
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. At STANMECH we have been approached by customers who believe they know exactly what they need.
Customers go to the website and pick out a tool without going through the full design process. The best case scenario is that the customer wastes a small amount of money and time selecting and buying the wrong equipment. The worst case scenario is that the customer designs, builds, commissions a complete processing line, spends a great deal of money and time and end up in a situation where it doesn’t work, they’re in a corner, and much more money and time must be spent to correct the problem. The purpose of this series of articles is to keep the reader from going down the wrong path when in most cases it can be avoided by more thinking more carefully about the hot air system they are designing. In this article we are discussing the first step in designing a hot air system (or any system) – carefully defining the problem. It is tempting to skip right to a solution or jump into analysis without defining the problem you are trying to solve. Properly defining your problem will help to get to the appropriate solution and helps you work with third parties, such as equipment suppliers, more effectively.
Hot air systems, at their most basic, combine two components—a stream of air supplied by a blower/compressor and heat generated from a heating element—to produce hot air. Sometimes these two components are supplied by a single tool, other times they are supplied by two separate tools. The goal is to increase the temperature of the stream of air and to use this air for a task. Understanding how air flow and temperature relate to each other is helpful when choosing air and heat sources for your system to ensure your system will be able to do the work required.
Raising the temperature of air requires energy; the amount of energy required depends on the volume of air and the magnitude of the temperature increase. Air heaters are rated by power in watts or kilowatts which specify the energy that the element is capable of applying per unit of time. See our article on The Basics of Heat Calculations for further explanation. There is an inverse relationship between air flow and temperature. For example, if the air flow over the element increases then one of two things happens: to maintain a constant temperature the element must increase its power output or the temperature of the output air will be lowered. By far the most common mistake we see is confusion between the concepts of temperature and heat. The terms are often used in conversation as if they are the same thing. This can have big implications when choosing a heater or designing a hot air system.
Heat and temperature are related. When heat is added to a material, the temperature of the material increases. The amount the temperature increases depends on the material and the amount of heat energy applied. Let’s define the two concepts in greater depth: Heat is form of energy. It can be transferred from one body to another. It can be created by, or transformed into, other forms of energy. As heat is a form of energy it can be used to accomplish useful work and it is measured in units of energy (SI units: Joules). Temperature is the measurement of hot or cold. It is a measurable quality of a substance. Temperature cannot be “added” to a material and cannot do work. When an object’s temperature decreases, the object has lost heat energy to its surroundings; when its temperature increases, it has absorbed heat energy from its surroundings. Or – Why heaters come in different sizes
Every application requiring hot air has its own design requirements. Ideally, these requirements will guide the designer to the right heater for the job. Unfortunately, many times design decisions are made based on price or with a “bigger is better” mentality.
Looking at Leister’s line of air heaters you may notice that while the heaters range in power from 550W to 16kW they all have a maximum operating temperature of 650°C. We’ve written previously about the difference between temperature and heat and why the power rating of the heater is the critical specification when sizing a heater. Or – Why a HOTWIND might not be the tool for you
Here at STANMECH, one of our most common customer requests is for the HOTWIND hot air blower. The HOTWIND is a well-designed combination heater/blower and it works extremely well in the right application. The attraction is obvious: it's everything you need in a compact package, it is capable of reaching the target temperature you require, and it looks more affordable because it’s only one unit rather than two.
However, the HOTWIND is often chosen for the wrong reasons. This type of tool incorporates a specific blower and a specific heater; unless the application lends itself to that exact blower and that exact heater size the tool is simply wrong for the application. This is true of all combination hot air blowers not just the HOTWIND. The practical application of blower curves
How a blower will function once it is connected to other components is often misunderstood, which can result in the selection of the wrong tool for an application. In previous articles we have discussed the differences between centrifugal and regenerative blowers, how to interpret blower curves, and the differences between flow, velocity, and pressure. This article expands on these topics with a practical case that shows how these concepts interconnect.
When looking at a list of specifications for a blower you will find values for the Maximum Volumetric Flow Rate and Maximum Operating Pressure. While this is good information to have when selecting a blower, in most cases, it is not nearly enough information to predict the blower’s performance once it is part of a system. Pressure Drop: A Key Piece of (Often) Missing Information
When designing a blower-based system, it is important to understand the concept of pressure drop, how it affects blower performance, and how system design affects it. Pressure drop is the difference in pressure from one point in a system to another. When sizing a blower for a system, the most important pressure drop to consider is from the outlet of the blower to the end of a system (usually where it vents to the atmosphere). This is often referred to as the back pressure on the blower, or the operating pressure of the blower.
|