Delta and Wye are a common point of misunderstanding and cause for confusion when understanding and calculating how 3-phase power is used. It is important to understand the difference between the two when they are used with respect to heater design. The three phases of power are created at power plants, such as hydroelectric, nuclear, or coal, where power is generated. The power plant’s turbine generators which create the electricity have three separate coils that align with the generator poles to create the three different phases. As the generator armature makes its way around the 360° circle it causes three AC waves that are 120° out of phase with each other. In the United States of America, it is common for the AC electricity to change directions 60 times per second hence the 60 Hz we see on data plate labels and for electrical components. As electricity is transmitted, it is stepped down via a transformer to a usable amplitude, such as 120V, 240V, 480V, etc. 3-Phase power would use all three distribution lines whereas single-phase power could use only one (like 120V L1-N) or two distribution lines (like 240V L1-L2).
The Differences
The electrical load used in the heater can be wired either in a Delta or Wye configuration depending on the heater design. A Delta configuration, typically used up to 277VAC, is a circuit in which three loads are connected in a single “triangle” with each load representing a side of the triangle. A Wye configuration, used above 277VAC, is a circuit in which three loads are connected in a Y with each load representing a leg of the Y. Both configurations offer unique attributes which we can use to our advantage to create the optimal heater design.
It is worth noting that both wiring styles react differently when a coil fails. When a coil fails on a three-coil Delta, the heater will drop to 66.7% of its original capacity. If two coils failed, the third can still produce heat. On the other hand, if a single coil fails on a three-coil Wye, the heater will drop to 50% of its original capacity, and the heater will not produce heat if two coils fail.
Each Type of Wiring Configuration (Delta and Wye) Has Unique Benefits. Hopefully, this gives you a better understanding of the two types of configurations and how we use them in our heaters to produce the optimal heater design.
Regenerative blowers are portable, low-pressure air sources that are ideal for use with many Tutco SureHeat Air Heaters. They provide inexpensive, clean, oil free air, and can be easily mounted on equipment.
A blower must be able to provide your desired amount of flow even with the restrictions caused by the heater, plumbing, and manifolding. These restrictions can reduce the amount of flow to your process, and, if severe enough, can cause the blower motor to overheat and become damaged.
Selecting the right blower is easy. Just follow the simple steps below:
Step 1: Determine Heater Back Pressure The amount of back pressure caused by the heater is found by looking at the dotted lines on the heater Performance Curve found in the Operating Instructions. The amount of back pressure in PSI or Inches of Water (27.7 inches of Water = 1 P.S.I.) depends on the heater operating temperature and desired airflow, Standard Cubic Feet per Hour (SCFH).
Step 2: Determine Blower Capability Blower manufacturers provide performance curves for various size blowers. The maximum output of the blower is based on zero exit restrictions (zero pressure). In the example below, this is a 27 CFM-rated blower. Note how the blower output (CFM) decreases with increasing back pressure (Inches of Water). Also note that blower output is lower at 50 Hz operation.
Using the blower curve, verify that the blower chosen can generate sufficient flow (for your process), for your expected back pressure.
Remember, additional restrictions can add to the back pressure seen by the blower. Therefore it is better to choose a slightly larger blower, rather than a smaller one, like in our example. Excess flow can be diverted upstream of the heater using a bleed valve, or you can buy a variable speed blower which is easy to adjust, but is more expensive.
Heaters with openings smaller than 1” NPT are not recommended for use with blowers. The SureHeat JET, SureHeat MAX, Serpentine VI, 2-1/2” Inline, and Flanged Inline are suitable for use with blowers.
Example (reference charts below):
A manufacturing process using a Serpentine VI heater #F040292 with housing #F057088 requires 10 SCFM (600 SCFH) of flow at 1000°F.
Step 1: Determine Heater Backpressure At this flow and temperature, the heater will produce approximately 20” H2O (0.72 PSI) of backpressure.
Step 2: Determine Blower Capability At this back pressure value, the blower will output approx. 15.0 CFM, sufficient to meet our 10 CFM requirement.
Proposition 65 (commonly referred to as Prop 65) is regulation originally passed in 1986 by the citizens of California and its purpose is to alert consumers in California of items which may pose a risk of cancer or birth defects. The State of California maintains a list of these substances and new substances are added periodically. This regulation primarily affects retail businesses but also affects other businesses that may deal directly with consumers and producers of consumer products. It is not illegal to have a listed substance in a product but business owners and product manufacturers have an obligation to label the item according to the new Prop 65 regulations, which are effective August 30th, 2018.
Why are we labeling our products?
Although we do not sell products directly to consumers, many of our customers do and many sell directly to consumers in California. TUTCO products may contain Nickel, Cadmium, and Lead which are on the Prop 65 list of substances. Nickel is found in our resistance wire and some of our electrical components. Cadmium and Lead are also found in some of our electrical components.
Some customers may not be aware of the labeling requirements set forth in Prop 65 regulation so, to assist our customers, we are placing a Prop 65 Warning label on either the product or the box / container to alert you that labeling of your products may be required. You will also see the warning statement on packing slips, invoices, quotations, and on our website. These warnings will only be placed on products containing Prop 65 substances.
What do you as our customer need to do?
If your company is knowingly and intentionally placing products containing Prop 65 substances in the State of California, then you must place a Prop 65 Warning label on your product and work with your retailer or customer on how the warning will be displayed or communicated to the citizens of California. If you do not label products entering into the California consumer market, then you become a high-risk target for special interest groups and individuals that might take legal action for violation of Prop 65.
What is TUTCO doing going forward?
TUTCO will continue to work in collaboration with our customers on ensuring communication of substance information is made available for products in the California consumer market or other affected markets. If you have any questions regarding any governmental, safety, or environmental regulation, we encourage you to contact our Customer Service or Sales group for assistance.
Watt density is a useful measure when considering the various types of heating elements available. TUTCO-Farnam Custom Products manufactures open coil heating elements which are generally used to heat a gas flow, such as air or nitrogen. We are also manufacturers of surface heaters that heat by conduction. This discussion will pertain specifically to watt density and open coil elements.
What is watt density?
Watt density is defined as the heating element power divided by the actively heated surface area of the element. If you were to pick up a heat transfer textbook, power/surface area is identified as the heat flux, but the term watt density has found common usage within the industry. In the United States, the units commonly used are [Watts]/[inches]². In other countries, [Watts]/[mm]² is often seen.
Watt Density = Power / Surface Area = Heat Flux
Below is a mental picture I use when thinking about watt density. For a given area, a higher watt density will be more “dense” with wattage.
Why is watt density useful?
Watt density is a useful measure for predicting relative heating element temperature when comparing different alternatives. The operating temperature of the element is an important consideration for heat transfer efficiency and life.
There are three modes of heat transfer, conduction, convection, and radiation. TUTCO-Farnam Custom Products’ open coil elements heat the air stream via forced convection. Forced convection takes place when a gas flow passes over a surface with a temperature different than the gas flow. The fundamental equation for convection heat transfer is Newton’s Law of Cooling.
Q/A = h(ΔT) where;
Q = Power [Watt]
h = convection coefficient [Watt/( in.² · °F)]
h = f (V), function primarily of air velocity
A = surface area [in.²]
ΔT = temperature difference between surface and gas flow [°F]
ΔT = ( Tsurface – Tair)
The left side of this equation is watt density. On the right side of the equation, we have convection coefficient (h) multiplied by the temperature difference between surface and air flow (ΔT). The convection coefficient is primarily dependent on the velocity of the air flow. Unfortunately, the convection coefficient can be a bit complicated to calculate. The phenomenon’s complexity can be readily inferred from the numerous empirical equations used to calculate this quantity. In addition, in actual practice, the air velocities across a heating element are seldom uniform which further complicates calculations as well.
Example of use
However, with this equation, we can still derive some good information with a minimum of effort. For example, I am a design engineer tasked with revising a product that uses a heater/blower combination. The existing product has a 100 CFM blower, and a 500W 120V CB heater with a watt density of 30 Watt/ in.². Marketing has determined that our company can gain more market share if a 1000W heater is used like their competitor’s. A 1000W 120V CB heater has a watt density of 45 Watt/ in.². I want to keep the same blower if at all possible. With the same blower, I can assume the air velocity is essentially the same, and therefore, (h) is the same for both cases. I also know that Tair the inlet air is room temperature whether I’m using a 500W or 1000W element. Knowing that I can instantly infer the 1000W element wire surface temperature, Tsurface, will operate at a higher temperature.
We can take this a step further with a minimum of math and information to estimate, “How much hotter?”. I don’t know what the coil surface temperature, Tsurface, of the existing 500W is. But if I take an educated guess, I can still estimate and get a feel for how much hotter the 1000W element will operate. I’m guessing the coil temperature, Tsur1, is somewhere between 600°F and 1000°F.
Q/A = h( Tsurface – Tair)
Case 1, 500W30 Watt/ in.² = h( Tsur1 – 70 °F)
If Tsur1 = 600°F
If Tsur1 = 1000°F
Case 2, 1000W
45 Watt/ in.² = h( Tsur1 – 70 °F)
Tsur2 = 865 °F, a 44% increase
Tsur2 = 1465 °F, a 47% increase
Since Tair is generally small relative to Tsurface, it becomes apparent an initial estimate in temperature increase can be had simply from the ratio of watt densities.
(Q/A)2 / (Q/A)1 ≈ Tsur2 / Tsur1
Conclusion
The watt density of an open coil element quickly gives one a feel for how it is going to operate. TUTCO-Farnam Custom Products generally limits the watt density for forced convection applications to 60 Watt/ in.². However, if the air velocity is high, or the air is directed directly on the coils, such as the Tutco-Farnam Custom Products Heat Torch family, watt densities as high as 110 Watt/ in.² are possible.
By following a few rules-of-thumb you can determine the wattage requirement for your application.
Calculating the wattage requirements to heat a system is a straightforward process as long as all the parameters of heat energy flowing in and out of a system are considered. Heat requirements that must be considered are:
Initial heat for the startup of the system, usually from ambient temperature to the desired processing temperature.
Heat losses to the environment due to conduction, convection, and radiation.
Heating of material being processed during operation.
Heating of material flowing through the process such as a liquid that will be heated and pumped to be used elsewhere.
The losses due to phase changes of materials, either during initial heat-up or while processing (melting a solid to a liquid or boiling a liquid to a gas).
Luckily, such precision is usually not required since temperature controllers are commonly employed in most heating applications, meaning that a quick calculation can be used to get you up and running.
Before beginning the calculations, it is important to realize the distinction between energy and power and their relationship to wattage requirements.
In metric units, power is measured in watts (W) and energy is measured in watt-hours (W x hr).
In imperial units, power is measured in British Thermal Units per hour (BTU / hr) and energy is measured in BTUs. A BTU is the amount of energy required to heat 1 pound of water by 1°F (specifically from 39° to 40°F).
Think of power as the rate at which energy is used. A 60 watt light bulb uses 60 watt-hours of energy in one hour.
It also should be noted that the difference between the starting temperature and the desired final application temperature is commonly referred to as delta T (ΔT). If a process is started at room temperature, say 72°F, and the application process temperature is 500°F, then ΔT is 500°F – 72°F, or 428°F.
Below are some good guidelines for heating different materials in different situations.
To calculate the wattage requirement to heat steel, use the following equation:
Watts = 0.05 x Lbs of Steel x ΔT (in °F) / Heat-Up Time (in hrs)
Example: To heat 50 lbs of steel by 250°F in 1 hour; .05 x 50 x 250 / 1 = 625 Watts. Using the same example, reaching temperature in 15 minutes (0.25 hrs); .05 x 50 x 250 / .25 = 2,500 Watts. This equation is suitable for mild and stainless steel. If you are heating a different material than steel, you can replace the 0.05 in the equation above with the following coefficients:
Brass: 0.053
Aluminum: 0.018
Copper: 0.056
To calculate the wattage requirement to heat water in a tank, use the following equation:
Watts = 3.1 x Gallons x ΔT (in °F) / Heat-Up Time (in hrs)
Example: To heat 20 gallons of water by 100°F in 30 mins (0.5 hrs); 3.1 x 20 x 100 / 0.5 = 12,400 Watts
To calculate the wattage requirement to heat flowing water, use the following equation:
Watts = 165 x Gallons Per Minute X ΔT (in °F)
Example: To heat water flowing at 2 gallons per minute by 45°F; 165 x 2 x 45 = 14,850 Watts
To calculate the wattage requirement to heat oil in a tank, use the following equation:
Watts = 1.35 x Gallons x ΔT (in °F) / Heat-Up Time (in hrs)
Example: To heat 5 gallons of oil by 300°F in 15 mins (0.25 hrs); 1.35 x 5 x 300 / 0.25 = 8,100 Watts
There are a variety of other equations that can be used to provide good estimates of the power requirement to heat substances in varying situations. The numbers calculated from these formulas do account for typical losses in most applications and will provide a number that is more than adequate to get your process to temperature in the time required.
For more complicated heating applications, the Tutco engineering team can provide support to help with your calculations and provide solutions for your heating needs.
Sometimes, when all the design work is done, the heating element is too fragile for either the assembly process, or field operation. This can occur when making a low wattage heater that uses a high voltage. In these cases the element needs to be more robust but the design won’t allow it. One solution is to use a diode in series with the heating element. The diode will reduce the applied energy to the heater for any given voltage and allow a larger lower resistance heating element to be used. Naturally, this is not applicable to DC-powered heaters.
How it works:
A diode blocks half of each cycle in the incoming AC voltage wave. This means that there is no power dissipated or applied during those half cycles. So while the applied voltage remains the same as the application requirements, the power reaching the heating element is cut in half. As a result, a larger tougher element with half the resistance is now useable to meet the application wattage.
Engineering Details:
Vrms is the application supply voltage – (Vpk = Vrms * 1.414)
Vrms’ is the volts RMS applied with a diode
W is the application wattage
W’ is the wattage with a diode
R is the originally calculated resistance
R’ is the required resistance to get the same wattage with a diode
Neglecting the .7v diode junction drop for simplicity, we get the following change in voltage applied to the heating element:
So, R’ will have to be half of the original design R to get the same wattage with a diode.
Design notes:
The diode must be able to withstand the peak reverse voltage for the life of the heater. Any diode selected should probably have a Reverse Breakdown Voltage (Vbr) of at least twice the(not the RMS ) of the application voltage. The diode must be able to withstand the peak heater current, for the diode that’s the rated Forward Current or If. Therefore If must be greater than Ipk, where Ipk = Vpk / R’.
Here’s the Catch:
First, once the heater is assembled you can’t check the coil resistance through the diode. If you need to check it you must find a way to test that doesn’t include the diode in the circuit. Second, the calculations shown above seem simple enough but you cannot confirm them with a handheld meter. After contacting a manufacturer of True RMS meters, they verified that no handheld meter will produce accurate readings with half the waveform missing since they cannot do the actual integration of the waveform. So you have to trust the calculations or rely on an oscilloscope to measure the results.
Example:
The photos below illustrate the use of an oscilloscope and the problems with a handheld meter:
Variac setting showing 30 Vac
Rectified waveform:
Peak voltage shown in upper right (30Vac * 1.414 = 42.4 vac).
The scope can integrate the waveform to produce an accurate RMS result:
RMS voltage is shown in the upper right
((42.4 – 0.7) / 2) = 20.85
True RMS meter reading Vac and Vdc
The readings are completely inaccurate.
In Conclusion:
While there are certainly some testing difficulties to overcome when using a diode, it is an easy way to make a fragile heating element more robust.
In the heating industry, we are often asked what is the difference between a distributed wattage heater and a zoned heater. These various construction techniques can be applied to many different types of conduction heaters such as cartridge heaters, ceramic and mica strip heaters, and band heaters. Almost any heater that uses a wound coil can use one of these construction methods of controlling watt density. Before beginning, some definitions should be discussed.
Definitions
Coil: A resistance wire (usually Nichrome) that is wound around a round mandrel or ceramic core to hold the shape. When a coil is wound, it is usually done in a uniform manner.
Close Wound: If all the turns of a wound coil touch, this is called a close wound coil.
Open Coil: If the close wound coil is held by its ends and stretched, it becomes an open wound coil. Coils can also be initially wound as open coils if there is space between each turn of the coil.
Pitch: The distance between two turns of an open coil is the pitch.
Watt Density: The wattage produced by a heater divided by the surface area of the heater. Common units of measure are W/in² and W/cm².
Distributed Wattage Heater
What makes a distributed wattage heater? Distributed wattage is when a coil winding’s pitch is not consistent throughout its entire length. Some parts of the winding are more tightly packed with turns than other sections, to “force” the heat profile to be more concentrated in some areas of the heater than another. The non-uniform pitched coil will have sections of closely wound coils and sections of loosely wound coils. These methods of non-uniform stretching or non-uniform winding of the coil pitch allow the heater manufacturer to increase or decrease the watt density of the heater in certain sections. Because the coil winding in some of the sections will be closer together than other sections, the watt density will be greater in those sections. These higher watt density heater sections will be hotter than the other sections. Certain sections of the heater can be now be made to run hotter than other sections. Only one set of power leads is used in this configuration, as the entire heater turns on as one unit.
Application of Distributed Wattage Heaters
A distributed wattage heater allows the end-user to have higher heat where needed. This method of distributing heat is especially beneficial if only a small section of the heater is providing the majority of the heating or in applications where heat is needed over a long length and there tends to be temperature loss or drop-off near the heater ends. Distributed wattage heaters are often used in platens and sealing bar applications where heat loss compensation near the ends of the heater is important.
Zoned Heater
Zoned heating is where a specific section of the heater has its own “dedicated” wound coil with uniform pitch and its own set of power leads for each particular section of a heater. The internal coil construction within this particular type of heater has sections or zones of dedicated uniformly wound coils. The key word in this construction style is “dedicated”. The customer can literally control each section of the heater individually. The heated area is very specific and controllable. For example, a 2-zone heater will have 4 power leads coming out of it; a pair of leads for each zone. Sometimes a common wire may be used; in this 2-zone example, the number of leads can be reduced because each zoned coil shares a common lead. A heater like this would have 3 power leads.
Application of Zoned Wattage Heater
Zoned heaters can be used in any application distributed wattage heaters are used but in most cases, this would probably be overkill and not justify the added expense of the zoned heater. Zoned heaters are best used in applications requiring a significant level of heat control over the length of the heater. Zoned cartridge heaters can have various sections literally turned off entirely while only a fraction of the entire heater is energized. These heaters are commonly used in heater bar applications where heat control at precise locations is an important factor.
As the name implies, duct heaters are generally designed to be installed into ducting. They are usually installed through the side wall to cause the air in the duct to be heated as it flows around and through the open-coil elements.
Duct heaters made by TUTCO-Farnam are not for HVAC use. They are for industrial type applications and are not built to the standards required for the typical residential HVAC. For HVAC applications go to www.tutco.com/duct-heaters/.
Sizes and Shapes
At TUTCO-Farnam the standard shape is square, but rectangular duct heaters are common as custom-built units. Round duct heaters are also possible for unique applications.
Our pre-designed square duct heaters are offered in 6”, 12”, 22” & 36” sizes.
Circuit Types
Single-phase and three-phase duct heaters are available in voltages up to 600V.
We make individual duct heaters from less than 1kW up to 75kW.
A duct heater may be configured in stages, if desired. Each stage is a ‘stand-alone’ circuit. Multiple stages may be used for different reasons via a dual controller, if desired.
Each stage may be powered up separately to achieve various levels of heat output.
Each stage may be wired independently to minimize the amp load, which allows smaller gauge cables. They may still be operated in unison.
Watt Density
Watt Density of the heater coil is the watts per square inch of the surface area of the coil wire.
Watt density is not normally specified by the user, however, one may specify a ‘not to exceed’ limit on the watt density. Watt density can range as high as 90 W/In² but lower numbers are preferred. A watt density of 50 W/In² is very conservative. Generally, the life of the coil is shortened as the watt density increases.
Min/Max Flow Rates
The minimum and maximum flow rates are calculations based on the velocity of the air and the area of the duct.
We suggest a minimum of 200 fpm of air flow, but more is better, if possible. Without adequate airflow most duct heaters are apt to fail due to coil overheating and result in breakage.
The airflow is required to keep the coils from overheating.
We suggest a maximum of 7000 fpm of air flow, primarily to avoid deflection of the coils due to the ‘wind resistance’. Of course, a light gauge coil is much more susceptible to deflection than a heavy gauge coil would be. Light gauge coils are avoided whenever possible for this reason.
Installation Concerns
The primary concern with installation of a duct heater is the orientation of the heater with respect to the axis of the coils. The coil axis must be horizontal. That means the termination plate (face, or front) of the heater is mounted to a vertical side of the duct.
Horizontal mounting assures the coils are adequately supported by the ceramic bushings. If the heater were to be mounted vertically, the coils could sag down to the point of shorting the coil wraps together. They could also drop out of the “bottom” bushing and short to the ducting.
Should the application require multiple heaters, avoid mounting them closely side-by-side. Instead, leave at least a few inches between them. This allows the air to mix or blend before it encounters the next bank of heater coils.
When a thermocouple is to be mounted downstream, it should be mounted away from the heater for the same reasons as above. Thermocouples should also be long enough to reach well into the duct, away from the less-mixed (and cooler) air near the walls.
Summary
Your TUTCO-Farnam duct heater will have a long life when it’s properly installed and operated.
Thermodynamics, specifically heat transfer, is used throughout our daily lives, but not always thought of. A common practice of cooking breakfast would be one simple example. You place one type of media, your frying pan, onto a hot surface and apply the “heat”, which is your source of energy, to cook the food. The heat transfer that is occurring between the higher temperature stove-top and the cooler frying pan is a great practical application. It is also a simple example of the second law of thermodynamics, “Heat cannot, of itself, pass from a lower temperature to a higher temperature.” Thus, for heat transfer to occur, we can state that a temperature difference must exist between two mediums.
As we further study heat transfer and any thermal system, we will need to consider the three types of heat transfer, which are conduction, convection, and radiation. In many cases, you can have two or even all three sources of heat transfer happening simultaneously. As each form of heat transfer is briefly discussed I will list an example in the following paragraphs.
In order to fully understand how conduction, convection, and radiation are affected, you must consider the rate at which a certain medium will affect your system design. The rate at which energy or heat is absorbed or dissipated is dictated by the thermal conductivity of a material or combination of materials, temperature difference, area of the surfaces, and mass of the combined components. By varying the previously mentioned attributes, one can increase or decrease the speed and efficiency of a thermal system.
Now let’s look at a short study of each type of heat transfer. The first form is conduction. Conduction is a thermal process that occurs between two surfaces in contact with each other, where a temperature gradient exists, or even in one material that has a temperature gradient between two planes. If we use a simple experiment of a uniform bar of cross-section A, perfectly insulated on all sides except at the ends; where heat can only flow in the ‘x’ direction (see Fig. 1). If the bar is maintained at t1 on one end and t2 at the other end, Q (BTU/Hr (BTU = British Thermal Units, HR = Hours) will be transferred steadily from the entry, at location 1 to the exit at location 2. The rate of heat flow (heat flux) is directly proportional to the cross-sectional area and temperature difference from point to point of the bar. You may want to compare the cross-sectional relationship of the bar to how water flows through a pipe. The larger diameter of the pipe, the more flow of water (energy) it can transfer. If we now determine how the length of the bar will affect the heat transfer rate, we double the length (2L). It is found that the heat transfer rate is cut in half, which demonstrates that the heat transfer is inversely proportional to the length of the bar.
Fig. 1
Equation 1 shows mathematically the relationship of all the factors, where the proportionality constant, k, is a property of the material called thermal conductivity. The negative sign has been included in equation 1 to indicate a positive heat flow. The conductivity, k, is usually a function of temperature, but for moderate temperatures and temperature differences, it can be considered a constant.
Equation 1: Reference bibliography
Example: A plane wall constructed of solid iron with thermal conductivity 70 W/m°C, thickness 50 mm and with surface area 1 m by 1 m, temperature 150°C on one side and 80°C on the other.
The second form of heat transfer is convection. Convection is the transfer of heat through the motion of a liquid or gas relative to the body of material. There are two types of convection, forced convection, and natural convection. If the motion of the fluid is caused by the different densities initiated by the different temperatures in various locations of a fluid, it is known as natural convection. If the motion of the fluid is caused by an external force, such as a fan or blower in air heating then it is considered forced convection. With natural convection, the minor temperature differences in a fluid can cause heat transfer. For example, a room in your house could have small temperature differences from an outside wall to an interior wall. Those hot and cold particles coming into contact with the wall will collide and cause a transfer of energy. The equation for Newton’s law of cooling helps explain how basic convection is mathematically represented (see Equation 2). It is much more in-depth to explain forced convection, so that could be covered in future articles.
Equation 2: Reference Bibliography
Q = heat-transfer rate (BTU/hr)
A = heat-transfer area (FT2)
∆T = temperature difference between the surface and the bulk of the fluid away from the surface (°F)
h = coefficient of heat transfer (BTU/hr – ft² – °F)
Example: Fluid flows over a plane surface 1 m by 1 m with a bulk temperature of 50°C. The temperature of the surface is 20°C. The convective heat transfer coefficient is 2,000 W/m2°C.
The third and final form of heat transfer is radiation. Radiant heat transfer differs from the other forms. Radiation does not require any medium to transfer heat. Radiant heat transfer is similar to the “electromagnetic phenomenon” similar to light, x-rays, and radio waves. In this case, a transfer of heat occurs when the absorption of energy is greater than what is radiating from the same body. A body that absorbs all radiation and does not radiate any heat energy itself is considered a “blackbody.” The small amount of heat that is reflected is considered the body’s reflectivity, the amount of heat absorbed is known as absorptivity, and the effectiveness of as a thermal radiator is known as emissivity. The radiant heat transfer rate is shown in equation 4.
Equation 4
σ = Stefan-Boltzmann constant = 0.173 x 10 – 8 BTU/hr – ft2 – °R4 (in SI – 5.669 x 10-8 Watts/m2 – °K4)
Fe = emissivity factor
FA = Geometric factor to allow for the average solid angle through which one surface “sees” the other
Example: Radiation from the surface of the Sun If the surface temperature of the sun is 5800 K and if we assume that the sun can be regarded as a black body the radiation energy per unit time can be expressed by modifying (1) like
q / A = σ T4
= 5.6703 10-8 (W/m2K4) (5800 (K))4
= 6.42 107 (W/m2)
All three of the previously mentioned heat transfer factors must be considered when sizing a heater for any application. If a band, cartridge, or a strip heater, is selected, all of these elements work on the same design principles. The system can be insulated to improve efficiency during operation and controlled to more accurately provide heat. The final power requirements and efficiencies will depend on a well-designed system that eliminates heat loss and offers close control. A good rule of thumb, after the initial requirements are determined, that a designer uses a 25% service factor or a 1.25 multiplier for the wattage output of the system.
Bibliography Equation 1, 2, 3 and Table 1 from “Thermodynamics and Heat Power”, Sixth Edition by Irving Granet and Maurice Bluestein, Copywrite 2000, Published by Prentice-Hall, Inc., Upper Saddle River, New Jersey 07458
Footnotes 1 See page 581, from “Thermodynamics and Heat Power”, Sixth Edition by Irving Granet and Maurice Bluestein, Copy-write 2000, Published by Prentice-Hall, Inc., Upper Saddle River, New Jersey 07458
When designing an electric heating system for industrial processes, many factors must be addressed. These factors routinely include required power, location, ducting, air source, and controls, but circuit protection is often overlooked. Proper circuit protection is of utmost importance not only for safety but also to avoid costly downtime and repairs to the system. Electrical branch type and size of protection are important considerations when adding circuit protection to a heating system. Depending on the type of controls employed, the speed of operation can also be a factor.
There are two common primary types of overcurrent circuit protection: circuit breakers and fuses. The type of protection required depends on the type of control system used. A simple convection heater system, using only a heater and blower or fan, can be effectively protected with a circuit breaker. However, more complicated systems that add controls tend to need more complex protection.
Proper sizing of the circuit protection is the first step. The current value of the protection should be 125% of the maximum continuous amperage drawn on the circuit. This sizing will eliminate any false tripping or open fuses in the system.
Example: A heater rated for 10kW at 480V 3PH is being installed in a drying system. The heater will be operated at full power with no control over its output. It will be coupled with a 6 HP regenerative blower in the same circuit that is rated for an airflow of 200 CFM at 5.2A maximum load at 480V 3PH 60Hz. What size circuit breaker or fuse would be adequate for branch circuit protection of this system?
First, let’s find the line current on the heater:
Where: A = Line Current, W = Total Heater Wattage, V = Line Volts
Since the blower is rated at 5.2A, the continuous load of this circuit is 17.2A. 125% of 17.2A is 21.5A and so this circuit will require a 25A common trip circuit breaker. If the load is a delta circuit, a three-pole circuit breaker should suffice. However, if the load has a neutral connection, then a four-pole circuit breaker should be employed to ensure a positive disconnect of the neutral wire.
If the above heater system included a temperature controller to regulate the power to the heater, it would be wise to verify the means employed to fire the power circuit. In most modern heater controls, a PID temperature controller receives feedback from a temperature sensor at the exhaust end of the heater. This temperature controller then produces a control output that turns a solid-state device on and off depending on the control state. The solid-state device then delivers power to the heater.
Solid-state devices for heater applications generally are either a silicon-controlled rectifier (SCR) or a solid-state relay (SSR). Both of these devices are effective at handling the power loads required in a heater circuit. Although these devices are ruggedized for industrial applications, they are very sensitive to overcurrent and short circuit conditions. Since they tend to fail closed, it is extremely important to ensure they are protected.
There are several types of overcurrent protection devices on the market targeting semiconductors. When protecting these types of devices, the first step should be to determine the I2T rating requirement of the semiconductor. I2T is the amount of energy required to clear the electrical fault. Generally, a circuit breaker will not react quickly enough to save the semiconductor device and should be avoided. There are two types of semiconductor fuses on the market; gR-type and aR-type. The aR-type fuse is most suitable for this type of application since they are faster acting than the gR-type fuses. The aR-type fuse is often labeled as ultra-fast or ultra-rapid. Each wire in the branch circuit supplying the temperature controller should have an aR-type fuse installed. Calculations for sizing of the protection devices are the same as provided in the previous example.
Although this information has been carefully considered, always consult local and national electric codes when installing new electrical equipment.
