Inline open-coil resistive air heaters satisfy a gamut of process heating applications, including parts curing, sterilization, drying, cutting, melting, heat staking and de-soldering. These heaters are suitable for processes that use inert gases such as air or nitrogen as the thermal fluid. Several styles of heaters are available to be matched to the process requirements, depending on the type of air supply and flow rate. These systems require few components, are clean and require little, if any, maintenance once in place.
Though the systems do not seem complex, there are numerous factors to consider before beginning the design of an inline open-coil resistive air heater. These include air supply type, heater style selection, process temperature control considerations, and safety device concerns. This tutorial explains the various parts and how they work in a system.
Air Supply. Airflow to an open coil heater is as important as oil in an engine. It provides the thermal fluid, which allows heat to be transferred from the resistive coil. A regenerative blower can be used for large volumes of air (up to 2,000 scfm). In the case of lower airflows, or where higher pressures must be maintained, compressed air is used. Several options exist for processes that require high flow rates where no space is available for a bulky blower. Venturi-style air inputs used in conjunction with compressed air can produce an output flow rate up to sixteen times higher than their flow input.
Prior to selecting the size of the heater, attention must be paid to the intended air source. This step in the design process plays a key role in determining the style of heater required. As a matter of fact, the type of air source can have a tremendous effect on the final wattage of the heater. For instance, in processes utilizing regenerative blowers, the temperature of the input air can be up to 120°F (49°C). This is important to note and relates to the ΔT portion of the equation used to determine the heater wattage (see web-exclusive sidebar for more on sizing).
Heater. The heart of any system, the heater can be configured either for large airflow rates with low pressure drops or for low flow rates in high pressure systems. In cases where it is possible or likely that direct contact with the heater might occur, heater design options are available to include an extra air passage to keep the exterior relatively cool to the touch. These cool-to-the-touch heaters are offered with several inlet and exhaust fittings to suit most applications.
Resistance coils are manufactured from iron, chromium, aluminum or nickel chromium alloy wires. The coils typically are supported using either mica or ceramic cores. They are electrically connected for single- or three-phase applications in a configuration that will minimize the watt density (W/in2). Watt density is a measure of power per square inch of surface area of the coils. This number usually is a good predictor of heater life and, for this purpose, is limited to the lowest value possible when designing the core of the heater.
For high flow applications, heaters are designed for minimum pressure drop. This is accomplished by winding the coils around a mica core. The open design minimizes resistance to airflow. Heaters using this design can handle airflows up to 2,000 scfm and are normally available in wattages up to 75 kW. Because these heaters are designed for high flows, the pressure ratings are moderately low.
On the opposite end of the spectrum are heaters constructed for high-pressure systems and lower flow rates. These heaters are typically designed to be placed in systems rated up to 120 psig. This style uses either a mica or ceramic core and is available in wattages up to 12.5 kW. The more compact design reduces the maximum airflow to about 100 scfm and increases the pressure drop due to its inherent resistance to airflow.
The heater exhaust temperature can be accurately predicted if the input variables are fixed. However, as is often the case, the output temperature may be required to be held to a precise tolerance. In situations like this, many controllers are offered to meet the requirements of any system.
Controller. The function of the process controller is to provide the user with the ability to maintain accurate exhaust temperatures based on temperature feedback to the processor. The range of controller packages gives the system designer flexibility in precision vs. cost decisions. In situations where precise exhaust temperature tolerances do not have to be maintained, a controller may not be needed.
After the heater type is selected and air source identified, it is important to determine how tightly the final temperature of the system must be controlled. In cases where the final temperature does not need to be maintained to very close tolerances, an open-loop control system is often ideal. In this type of system, the controller may be as simple as an on/off switch. In other situations, the power output may be adjusted with a phase-angle control or a pulse-width-modulated control (PWM). Care should be taken in this type of system to ensure that the heater is always receiving an adequate supply of air. As a matter of fact, unstable air pressure normally is the greatest contributor to unwanted changes in exhaust temperatures.
In some systems, an open-loop control will not maintain an adequate tolerance on the exhaust temperature of the heater. Usually some type of temperature feedback is imperative for precise system control. If the process requires stable temperatures, there are many closed-loop options available to provide tight control. These types of systems utilize thermocouples, thermistors or RTDs for feedback to a microcontroller, which in turn pulses a solid-state relay (SSR) or a silicon-controlled rectifier (SCR). Most microcontrollers use either proportional integrative and derivative (PID) control or proportional derivative (PD) control. Some models allow fuzzy logic control and timer, ramp and soak functions. These controllers have the capability of maintaining process setpoints to within 1 or 2°F when coupled with a properly sized heater.
With all of this said, several areas should be given particularly careful attention. When using a closed loop style controller it is imperative that the controller receive feedback from the point where system output occurs. If it is impossible to get feedback from this location, the system will have to be adequately insulated downstream of the heater to minimize heat loss unless the exhaust temperature of the heater is high enough to compensate for the losses.
Safety Devices. Even with good process control of the system, it is advisable to invest in some basic safety devices. They can be installed easily and will prove to be valuable assets for protecting the integrity of the operation. These low cost items often save the user the expense of lost production due to equipment failure.
An assortment of devices is available to protect inline heaters. For example, a thermal cutoff can be placed either directly in the heater circuit or as a pilot duty signal to a relay or a contactor to open the circuit in the event of an overtemperature condition. High-limit thermocouples are another option that can be installed for protection from overheating and possible damage to the resistance coils. Finally, there are flow switches available that can be placed upstream from the heater to ensure that proper airflow is maintained. If all of these precautions are taken, an inline open coil heater can provide the user with many years of dependable service and little downtime in the operation.
Understanding the uses, capabilities and functionality of open-coil air heater systems will help users implement a clean, cost effective and reliable source of heat for almost any process or application.