Tubular and process heaters is the catch-all category of heaters. They are general purpose devices used to generate heat as part of a manufacturing or industrial process. They
can be installed inside of ducts to heat air or gases or put inside a tank as part of chemical manufacturing. Immersion heaters, screw plug heaters, and flanged heaters are all examples
of process heaters.
Principles of Heating
Heating is a complex process governed by the principles of thermodynamics, fluid mechanics, and heat transfer. Heat transfer, in particular, is useful knowledge as it will help
inform important aspects of heater selection and system design.
Heat transfer is the movement of heat from one area (or object) to another, always from areas of high concentration (hot) to areas of low concentration (cool). There are three
methods of heat transfer:
Conduction
Conduction is the method of heat transfer by way of direct contact. The classic example is heating one end of a metal rod. After a period of time the heat will travel through the
rod to the cooler end. Conduction heating is influenced by the ease of which heat travels through a material, its thermal conductivity, as well as the amount of mass available to absorb
the heat energy.
Convection
Convection is best explained by the saying, “heat rises.” Heat causes liquids or gases to expand which results in lower weight per unit volume. The lower weight increases buoyancy of
the heated fluid causing it to rise. As it rises cooler, heavier fluid fills the void. As the new, cooler fluid is also heated it too rises thus continuing the cycle and creating
enough movement to form a convective current capable of moving large amount of heat. Convection is influenced by the fluid's permissible watt densities, thermal conductivity and
expansion coefficient.
Radiance
Radiant heating relies upon electromagnetic waves to radiate heat onto an object without any need for direct contact. While all electromagnetic waves will "heat," the infrared band of
the spectrum contains the most useful heating energy. Important factors in radiant heating are the distance between object and heat source, and how well the object absorbs radiant energy.
Gases absorb radiant energy poorly so are largely unaffected by radiant heating. Dark surfaces absorb radiant heat better than light, or reflective surfaces.
How Heaters Work
Electric heaters work on the principle of resistive heating in which an electrical current passing through a conductor creates heat. According to Joule’s First Law, the heat produced by an
electric current is equal to the product of the resistance of the conductor, the square of the current, and the time for which it flows.
Heating systems generally require precise instruments to reach and maintain appropriate temperatures and to control the large current load need to create heat from electricity. Systems
usually have the following components:
Power Supply + Heater + Temperature Controller + Temperature Sensor + Load Handling Device
Generally speaking, the heater takes electrical power and converts it to heat. The temperature controller switches the heater on or off after comparing the pre-set target temperature
with the actual process temperature supplied by the temperature sensor. Temperature controllers, however, are unable to switch the high current loads required from most heaters. For
all but the smallest heaters, the system must include a load handling device such as a power controller, SSR (solid state relay), SCR (silicon-controlled rectifier), or mechanical relay.
These electronic switching devices are able to handle large current and act upon the orders of the temperature controller.
Calculating Required Heat Energy
The first step in selecting and electric heater is determining the amount of heat required to do the job asked of it. The total heat energy, in either kWH or BTU, required to satisfy the
system needs will be whichever of the following values is larger:
- Heat required for start-up
- Heat required to maintain the desired temperature
Calculating either of these values requires a fair bit of knowledge. A somewhat simplified method can be used for a quick estimate:
Heat required for start-up:
- Start-up watts = A + C + 2/3 L + safety factor
Heat required to maintain the desired temperature:
- Operating watts = B + D + L + safety factor
Variables:
- A = Watts required to raise the process temperature to the operating point, within the time desired
- B = Watts required to maintain the process temperature during the working cycle
- Calculating A & B: (lbs x Cp x °F) ÷ (hrs x 3.412)
- lbs = weight of material
- Cp = specific heat of material (BTU/lb x °F)
- °F = temperature rise
- hrs = start-up or cycle time
- C = Watts required to melt or vaporize load material during start-up period
- D = Watts required to melt or vaporize load material during working cycle
- Calculating C & D: (lbs x BTU/lb) ÷ (hrs x 3.412)
- lbs = weight of material
- BTU/lb = heat of fusion or vaporization
- hrs = start-up or cycle time
- L = Watts lost from surfaces by conduction, radiation, and convection
- Calculating L: (k x ft2 x °F) ÷ (in. x 3.412)
- k = thermal conductivity (BTU x in./[ ft2 x °F x hr])
- ft2 = surface area
- °F = temperature differential to ambient
- in. = thickness of material (inches)
- Safety factor is normally 10 to 35 percent based on application
Watt density, described as watts/in2 of heater surface, is an important consideration in designing heating systems. Watt density does not describe the total power of the system
but rather the extent to which power is concentrated on the heater surface. For example, a 1000 watt system with a heater having 100 in2 of surface area has a watt density of 10 watts/
in2. A 1000 watt system with a heater having 50 in2 of surface area has a watt density of 20 watt/ in2. Both systems have the same power but their watt densities
are much different. This is especially important with regard to material compatibility. All materials have suggested maximum watt densities beyond which the material will burn.
Things to Consider When Selecting a Tubular or Process Heater:
- What material do you want to heat?
- What is the maximum watt density for the material?
- How much of the material (mass) is there to heat?
- What is the available voltage and phase?
- What is the starting process (cold) temperature?
- What is the final/operating process temperature?
- What is the target heat up time?
If you have any questions regarding tubular and process heaters please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com or calling 1-800-884-4967.
