Heat can be moved in three ways: By conduction, where heat is transferred by physical contact; by radiation, where heat is transferred over a distance via electromagnetic waves; and by convection. Convection is simply a combination of conduction and physical movement, where the material in contact with the heat source is fluid. Where the fluid contacts the heat source, it absorbs energy through conduction and its density changes as it gains heat. Because fluids are able to move freely, this now warmer and less dense area of fluid rises and physically carries its new heat energy away from the heat source.
This article will review the basic principles behind heat transfer. It will consider examples of each and discuss ways of optimizing heat transfer efficiency and, hopefully, help you save energy and reduce costs.
Conduction heaters transfer heat through physical contact. A common example is a band heater, which consists of a metal housing, an insulating material and a resistive heating element. Band heaters often are used on plastic molding machines. The band is secured to the barrel or nozzle and transmits heat directly into the metal, which is used to keep the plastic inside molten.
The amount and rate of heat transfer are determined by several factors. The most integral factors are the properties of the materials in contact with one another, the difference in temperature, the amount of heat being generated by the element, and the quality of contact.
To optimize the efficiency of a conduction heater, several things must be taken into account. The first consideration should be the materials. Different materials have different thermal conductivity (k) values. These values are measured in watts per meter Kelvin (W/m.K), which can be described as the amount of thermal energy in watts that can travel through one meter of a material. Thermal conductivity can range in solids from values as low as 0.05 W/(m.K) for paper all the way to 401 W/(m.K) for copper. The higher the thermal conductivity of both the heater material and the material it is in contact with, the easier heat will flow through them and between them.
Once the proper materials have been chosen, the next most influential item to consider when trying to maximize heat transfer efficiency is the quality of contact. If the heater does not fit tightly, air gaps or voids in the areas where the materials are in contact can significantly decrease heat transfer efficiency.
Convection heaters employ convective currents in a fluid to aid in the heating process. Convection heaters can be employed to heat a fluid (air or liquid), which in turn is used to heat a desired piece.
A good example of this would be a standard blow dryer. Air is accelerated over the heating coil, which heats the air rapidly. The air then is directed at the wet hair, essentially making the air just a vessel for the heat needed to dry the hair.
Another common example is an immersion heater. Immersion heaters consist of heating elements encased in tubular metal housings, which are designed to be immersed in a liquid, often water. They deliver heat to the water through conduction, which in turns creates convective currents in the water, carrying away heated water and delivering cooler water to the surface of the heating element.
In general, immersion heaters are considered highly efficient because all of the heat generated by the element goes into the fluid in which it is immersed. However, given the movement of the fluid and the rate at which it moves, preventing heat loss becomes critical to the overall process efficiency.
More variables are involved when trying to improve the efficiency of convection heaters than there are for conduction heaters. In addition, the specific variables depend upon the type of convection heater and the application.
As an example, if an inline air heater is considered, the designer must contemplate many things in order to improve efficiency. In addition to rather obvious concerns such as insulating the outside of the heater and the pipe carrying the hot air, the designer must consider process-specific variables. These include:
• The amount of time the air is in contact with the heating element.
• The flow characteristics of the air.
• The composition of the air.
• The original temperature of the air.
• The amount of surface area of the heating element.
• The material used to construct the heating element.
To maximize the efficiency in this type of convection heater, the designer would need to find the optimal time of contact between the air and the element for that specific application. Too much time could take more heat from the heating element than is required by the application; too little time would not extract enough heat and could slow down the process.
The flow characteristics of air also factor into how long the air comes into contact with the element as well as how evenly the air is heated. In many air heaters, the convective current induced is enough to cause the air to flow in a turbulent manner, which helps ensure that all of the air is heated evenly and that the maximum amount of heat is extracted by the air. Depending on the application, turbulent flow usually is more efficient than laminar flow. Also, maximizing the surface area of the heating element allows it to contact more air at any given time, which raises the rate at which the air heats up. Depending upon the materials utilized, a shorter heater with a larger surface area of heating element may be less expensive.
The composition and properties of the fluid used in convection current also have a significant impact on the efficiency and overall operation of the heating process. A good example of this is the simple difference between heating dry air and saturated, humid air. Dry air has a specific heat of 1,005 J/kg.K while saturated humid air at 77°F (25°C) has a specific heat of 1,086 J/kg.K. This 8 percent difference in specific heat can be substantial enough to warrant consideration.
Radiant heat consists of electromagnetic waves of varying lengths and frequencies. Typically, it is used in processes where there can be no physical contact between the heater and the material being heated. Examples include infrared heating for specialized paint drying, static-free situations and drying lines in which the material being heated is moving.
Factors that influence the efficiency of radiant heating include the emissivity of both the source (emitter) and the target. The higher the emissivity, the more efficiently the emitter will generate electromagnetic waves and the target will absorb the waves. A black body (a theoretical object) has an emissivity of 1 while highly reflective materials like polished steel have an emissivity of 0.07.
It should be noted that while radiant heat — unlike convection and conduction — does not need a medium to transfer heat, the medium through which radiant heat is transmitted should have a low emissivity. For example, humid air has a higher emissivity value than dry air and will decrease the amount of radiation that actually reaches the desired target.
The second driver for radiant energy is the emitter temperature. Logically, the hotter the emitter, the more energy is generated. Utilizing the Stefan-Boltzman equation, you can calculate the amount of energy being generated; then, using Wien’s displacement law, you can calculate a peak wavelength for a given temperature. Because materials do not absorb all wavelengths of radiant energy equally, selecting emitters that produce wavelengths absorbed by the material being processed can help optimize efficiency.
Every application has unique heating requirements. Maximizing the transfer of heat energy from the source to the target is a balancing act between the ability of the target to absorb heat, the source’s ability to generate energy and the medium through which the energy is being transmitted.
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