This 360° clam shell heater has a ceramic emitter.
How does using radiant heat reduce costs and optimize processing in wire, cable and tubing applications? Wire and tubing applications present processing challenges:
- The product runs continuously.
- The material being processed has a relatively small size.
- The material is sensitive to both temperature and touch.
Wire and tubing also are exposed to a range of operations, including drying, printing, striping, curing, preheating, heat shrinking and sintering. Manufacturers want to maximize output while controlling energy costs and minimizing capital investment in terms of both the direct cost of the oven and the indirect cost of floor space the oven requires.
3 Methods of Heat Transfer
Metal sheath emitters can be used in a keyhole oven.
To understand the benefits of radiant energy in this application, it is important to understand basic heat transfer. There are three methods by which heat is transferred: conduction, convection and radiant. Conduction heats through the physical contact between the heat source and the target. Convection transfers heat through the movement of heated air around the product. And, radiant heats through the transmission of electromagnetic waves that energize the atoms on any surface they hit.
Because conduction heaters must be in physical contact with the product, they will not work for wire and tubing.
Convection ovens require an energy source, an air circulation system and, if gas is used as the fuel, a ventilating system. Also, because air is a relatively poor conductor of heat, convection ovens have long startup periods and require insulation to prevent heat losses to the surrounding atmosphere. The additional insulation also means convection ovens are slow to cool, and if the line stops or slows due to breakdown or changeovers, the product could be overheated if the oven is heated beyond the product’s maximum temperature. The primary advantage to convection heat is that the heat energy can be conducted deeper into the part. This often requires a longer dwell time in the oven, which means wire and tubing processors may need to slow the line speed or increase the oven length.
Radiant, or infrared, heat does not heat the surrounding air and does not become heat until it is absorbed by the surface it contacts. Any object with a temperature above absolute zero emits radiant energy in the form of electromagnetic waves. The electromagnetic waves are measured primarily by their frequency and length (microns). The lengths of the waves are directly correlated to the temperature of the heat source (emitter) and are broken into three categories:
- Short (3,000 to 6,800°F [1,649 to 3,760°C]).
- Medium (500 to 2,999°F [260 to 1,648°C]).
- Long (0 to 499°F [-18 to 259°C]).
As temperature increases, the length of the waves get shorter and the frequency increases. This range can be plotted using Planck’s curve, and peak energy is calculated using Wien’s Law of Displacement.
This data can be used to guide the choice of emitter. Many materials are forgiving and can absorb radiant energy over a range of temperatures. As an example, an emitter operating at 600°F (316°C) produces waves at 7 µm and 1,000°F (538°C) at 3.5 µm. Plastic tubing and plastic-like materials receive at both wavelengths but the 600°F is gentler or softer on the plastic. Radiant ovens may allow for better control of the process and to maximize output.
Table 1. Infrared Heater Comparison Chart
Radiant's Cost Effectiveness
To illustrate the energy efficiency of radiant heat, we need to go through the Stefan-Boltzman equation, which calculates the total amount of energy in watts generated by a heater at any given temperature. From this calculation, we can see that the output in watts of a radiant heater increases to the fourth power for every degree in temperature rise. For example, an increase in temperature of 5°F in the heater converts into an increase of 625 W radiated.
Convection heaters increase output only on a one-to-one ratio. Radiant ovens have a faster warmup time that is, depending on the emitter used, sometimes measured in seconds, meaning less energy is used getting ready to run. Once the product is running, exponentially more energy input into the system (Stefan-Boltzman) gets transmitted onto the product, reducing energy costs.
A radiant oven typically is smaller than a convection oven, and it does not need an air circulation system or additional insulation.
There are several important factors to the successful design of a radiant oven for a given application. The most important is the emissivity of the material bring processed. Emissivity is a measure of how much radiant energy can actually be absorbed by the material as a percentage of the energy generated: the higher the emissivity, the more energy that is absorbed. Absorption rates are measured against a theoretical “black body” which absorbs 100 percent of the energy transmitted and has an emissivity of 1. Below is a chart of emissivity rating for some common materials.
The percentage of energy that is not absorbed is either reflected from the target or passes through (transmission) the material. Clearly, material that is shiny or polished reflects most of the waves and has a low emissivity while darker, or oxidized parts absorb the energy and have higher emissivity.
Emissivity is also a measure of the percentage of energy transmitted from the heat source. The key operating characteristics for radiant emitters are shown in table 1.
An optimized oven design will array the heaters to direct as much radiant energy onto the product as possible.
In addition to emissivity, any oven design will need to incorporate:
- Temperature change in the material (Final Temperature – Starting Temperature).
- Size and weight of the wire or tubing.
- Specific heat of the material.
From this information, the oven designer can calculate the approximate wattage required. Once that is determined and based on the watt density capabilities (watts per square inch of surface area) of the selected emitter, we will know how many heaters are required, which in turn will dictate the size of the oven. The optimized oven design will array the heaters to direct as much radiant energy onto the product as possible.
Since wire and tubing products tend to be round, the heaters should be arrayed in a circular fashion around the product. The design will also minimize the distance from the emitters to the product. The emitters should be backed by polished-steel reflectors so that any energy waves transmitted through the product, reflected off the product surface or simply missing the target will be redirected back toward the product, maximizing energy utilization.
To summarize, radiant energy allows for flexibility in the design of your oven by better matching the heat source to the processing requirements of the wire, cable or tugging being processed. Radiant heaters efficiently transmit energy input into the system into the product and have fast startup times, thereby reducing energy costs. Finally, radiant ovens tend to be physically smaller, reducing the indirect cost of floor space and the direct capital cost of the oven.