Choosing the correct blower is a fundamental step in designing a functional and efficient system. We’ve covered the basics of Regenerative versus Centrifugal Blowers and Understanding Blowers as part of System, in this article we’ll show how to read a blower curve and use that information to specify the best blower for your project.
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Most blower suppliers use similar types of specifications to describe the blower function. Below is an example of a typical blower spec sheet.

Air knives are used in many industries to remove unwanted materials from processes. This can range from blowing water off of bottles in a bottling facility to removing cooling fluid in a metal rolling plant to removing crumbs from a bakery conveyor system. While the use of air knives for blow-off is widespread, so are the errors in setup that cause ineffective or inefficient operation. In this article we will review some of the fundamentals of implementing an effective blow-off system.

One issue that seems to cause universal confusion when designing a blower-based system is understanding the differences between flow, velocity, and pressure and knowing when each metric is important. This article investigates this topic with a focus on how they relate to each other in applications with industrial blowers.

First, let’s define each term:

• Flow is a measure of air output in terms of volume per unit of time. The common units are litres per minute, cubic feet per minute (CFM), etc.
• Velocity refers to how fast the air is moving in distance per unit of time. The common units are feet per second, metres per second, etc.
• ​Pressure is the measure of force applied on an area. The common units for pressure are pounds per square inch (PSI), Pascals (Newtons per square metre), etc. There are also some traditional measures such as inches of water or inches of mercury which are defined as the pressure exerted by a column of water (or mercury) of 1 inch height.

A common misconception we encounter while working with customers to develop process heat systems is that the airflow transitioning from a small space to a larger one will expand to fill any volume. While this is true (to a point) for compressed air, which undergoes a rapid volume expansion as it transitions from a high pressure environment (80-100PSI) to an atmospheric environment, it is not true for blower-based systems where this pressure change does not exist.

Airflow from a blower-based system will always follow the path of least resistance; this means air will continue to travel straight away from an outlet unless forced to change course. There may be some spreading effect from the moving air traveling through still air and some rise if the air is hot; however, for practical purposes this effect is negligible. When designing a blower-based system, one must make specific design choices to equalize flow and avoid uneven heating patterns.

Here are a few problematic examples we see again and again:

• Nozzles that are overly large for their heater outlet and airflow
• Nozzles with multiple strategic outlets, but no systems to control/equalize flow
• Heaters that outlet directly into an open heat tunnel or heat chamber

​In order for the air to diffuse as needed, it must be influenced and directed. This is most commonly done by strategically: restricting the outlet, installing deflectors and baffles within the system, or combining both of these solutions.

​Inside every Leister hot air tool is a heating element composed of a ceramic honeycomb supported resistance wire filament (See below). Electric resistance wire heaters work on the principle that when electrical current passes through a conductor heat is generated, and the amount of heat generated is related to the resistance of the conductor. 

Some resistance wire heaters are able to function without airflow because they have been designed in such a way that they will not reach temperatures above their safe operating limits. Examples include: a toaster, some ovens, wrap heaters, some space heaters, etc.
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However, Leister heaters are designed to operate at very high temperatures—most are designed to heat airflows up to 650°C, and some up to 900°C—and as a result require airflow at all times. To reach these high air temperatures, the heating element must be capable of reaching even higher temperatures. Without adequate airflow, the element will heat up unchecked and will exceed safe limits, leading to the destruction of the element. 

Although blowers are commonly used in manufacturing, it can be difficult to find good sources of information on the different types of blowers and how to choose the appropriate one. The purpose of this article series is to give a good, basic understanding of the different types of blowers and provide you with the technical information required to make a good decision for your application.

Recommended Reading

• Part 1: Impeller-based Blowers
• Regenerative versus Centrifugal Blowers
• Part 2: Positive Displacement Blowers
• Part 3: Blower Comparison Chart
• Flow, Velocity, and Pressure
• Part 4: Specialty Blowers
• Part 5: Important Design Parameters for Choosing a Blower

​In order to make the information in the previous articles more tangible let’s look at a number of scenarios and the thought process that might go into selecting a blower for each one.

Although blowers are commonly used in manufacturing, it can be difficult to find good sources of information on the different types of blowers and how to choose the appropriate one. The purpose of this article series is to give a good, basic understanding of the different types of blowers and provide you with the technical information required to make a good decision for your application.
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Recommended Reading

• Part 1: Impeller-based Blowers
• Regenerative versus Centrifugal Blowers
• Part 2: Positive Displacement Blowers
• Part 3: Blower Comparison Chart
• Flow, Velocity, and Pressure

• Part 4: Specialty Blowers

The first step in blower selection is to understand the application requirements. Below are some of the questions you should ask yourself:

• ​Is this a pressure or a vacuum application?
• What air volume/flow rate is required?
• How much back pressure will the system apply on the blower?
• Are there any space constraints for the size of the blower?
• What is my budget?

These types of questions should narrow the focus to one or two types of blowers. As with any engineering problem, there is not always a single solution and there can be some overlap in the characteristics of two different blowers. For a refresher on the most common types of blowers available, see Part 1: Impeller-based Blowers and Part 2: Positive Displacement Blowers. 

While Laser IR Thermometers are an extremely common tool, they are entirely ineffective for measuring the outlet temperature of an air heater. To understand why, we must first understand how this tool works. Laser IR Thermometers measure the surface temperature of an object by measuring the thermal energy emitted by the target. Knowing the amount of thermal energy discharged and the emissivity of an object’s surface, the object's temperature can be determined by the device.

When measuring a heater’s output air temperature, the largest issue with these tools is that they measure surface temperatures. As the heated air is transparent, the measurement will always be the surface temperature of a nozzle or a component of the heater housing; and frequently it will be an exterior surface. These items will always be cooler than the heated air, often by a significantly larger margin than the user would expect. To accurately measure the output temperature of an air heater you must measure air temperature and laser IR thermometers are incapable of doing so.

Although blowers are commonly used in manufacturing, it can be difficult to find good sources of information on the different types of blowers and how to choose the appropriate one. The purpose of this article series is to give a good, basic understanding of the different types of blowers and provide you with the technical information required to make a good decision for your application.
​
Recommended Reading

• Part 1: Impeller-based Blowers
• Regenerative versus Centrifugal Blowers
• Part 2: Positive Displacement Blowers
• Part 3: Blower Comparison Chart
• Flow, Velocity, and Pressure

At its most basic level a blower is a tool that draws in air at an inlet and pushes air out as a steady stream at the outlet. Blowers can largely be classified into two categories: impeller based and positive displacement. Impeller based blowers have fins that radiate outwards from a rotating central axis. Positive displacement blowers use a mechanism of filling and emptying chambers at the inlet and outlet, respectively, to create flow. The fundamental difference between the two is that impeller based blowers have output flow that varies with pressure whereas positive displacement blowers have a more constant output flow less affected by changes in operating pressure.

​When an environment is unusually harsh—such as explosive, corrosive, or hot environments—a normal blower design must be adapted in order to operate safely.