![]() I don’t know what the coil surface temperature, T surface, of the existing 500W is. We can take this a step further with a minimum of math and information to estimate, “How much hotter?”. Knowing that I can instantly infer the 1000W element wire surface temperature, T surface, will operate at a higher temperature. I also know that T air the inlet air is room temperature whether I’m using a 500W or 1000W element. With the same blower, I can assume the air velocity is essentially the same, and therefore, (h) is the same for both cases. I want to keep the same blower if at all possible. A 1000W 120V CB heater has a watt density of 45 Watt/ in.². ![]() Marketing has determined that our company can gain more market share if a 1000W heater is used like their competitor’s. The existing product has a 100 CFM blower, and a 500W 120V CB heater with a watt density of 30 Watt/ in.². For example, I am a design engineer tasked with revising a product that uses a heater/blower combination. However, with this equation, we can still derive some good information with a minimum of effort. In addition, in actual practice, the air velocities across a heating element are seldom uniform which further complicates calculations as well. The phenomenon’s complexity can be readily inferred from the numerous empirical equations used to calculate this quantity. Unfortunately, the convection coefficient can be a bit complicated to calculate. The convection coefficient is primarily dependent on the velocity of the air flow. On the right side of the equation, we have convection coefficient (h) multiplied by the temperature difference between surface and air flow (ΔT). The left side of this equation is watt density. ΔT = temperature difference between surface and gas flow H = f (V), function primarily of air velocity The fundamental equation for convection heat transfer is Newton’s Law of Cooling. Forced convection takes place when a gas flow passes over a surface with a temperature different than the gas flow. TUTCO-Farnam Custom Products’ open coil elements heat the air stream via forced convection. There are three modes of heat transfer, conduction, convection, and radiation. The operating temperature of the element is an important consideration for heat transfer efficiency and life. Watt density is a useful measure for predicting relative heating element temperature when comparing different alternatives. For a given area, a higher watt density will be more “dense” with wattage. Watt Density = Power / Surface Area = Heat Fluxīelow is a mental picture I use when thinking about watt density. In the United States, the units commonly used are /². 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. Watt density is defined as the heating element power divided by the actively heated surface area of the element. This discussion will pertain specifically to watt density and open coil elements. We are also manufacturers of surface heaters that heat by conduction. TUTCO-Farnam Custom Products manufactures open coil heating elements which are generally used to heat a gas flow, such as air or nitrogen. Watt density is a useful measure when considering the various types of heating elements available.
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