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Optimizing Heat Transfer

Featured article in June 2022 Process Heating Emagazine
Authored by Eric Pink, Sales Director & Product Development for Process Heat at Durex Industries

Optimizing Heat Transfer via Effective Heater Configuration

No matter what type of fluid is being heated or the fluid flow conditions, it is important to understand and consider the specific application conditions.

Electric immersion heaters and circulation heaters using tubular heating elements are used in industrial applications — from heating gases and liquids to supercritical fluids and molten materials. Because of the myriad application situations, particular attention must be paid to the process conditions, properly factoring them into the sizing and configuration of heaters.

The goal of every application is to find the most economical configuration that will perform successfully. To effectively do that, we must maximize the watt density — often measured in watts per square inch — on the heater elements. The more power that can be gotten into a smaller package, the lower the cost. It is important to do that without damaging the fluid or overheating the heater elements, however.

Electric immersion heaters and circulation heaters using tubular heating elements are used in a variety of applications in industry. It is important not to damage the fluid or overheat the heater elements.

To optimize the heater configuration, it is important to start with good application and fluid property information. This includes:

Fluid properties such as specific heat capacity, density, thermal conductivity and viscosity.
Minimum and maximum mass (or volumetric) flow rate.
Minimum and maximum operating pressure (along with maximum allowable pressure drop in flow applications).
Process (or inlet and outlet) temperature.

For stagnant fluid conditions — typically tank heating of liquids — electric process heater manufacturers often refer to watt density charts (table 1). These charts provide recommended maximum watt density values that work well in most applications. This greatly aids in applying electric tank heaters with confidence.

TABLE 1 - Watt Density Chart

Liquid	Max. Operating Temps °F (°C)	pH Level and/or Concentration	Recommended Sheath Material	Typical Flange Material	Max. Watt Density W/in²(W/cm²)
Clean, Potable Water	212 (100)	Neutral (pH 6 to 8)	Copper	Brass	60-90 (9.5-14)
Process Water & Weak Solution	212 (100)	pH 5 to 9; 2-3%	Incoloy®, 304SS	Steel or 304SS	48 (7.5)
Demineralized, Deionized Water	212 (100)	Neutral	304SS or 316SS (passivated)	304SS or 316SS	60 (9.5)
Weak (Acidic) Solutions*	210 (100)	Up to 5-6%	Incoloy®, 316SS	Steel	48 (7.5)
Mildly Corrosive (Acidic) Solutions*	180 (80)	5-15%	Incoloy®, 316SS	304SS or 316SS	20-30 (3.5)
Severely Corrosive (Acidic) Solutions*	180 (80)	>15%	Incoloy®, titanium, Teflon®	304SS or other	15-16 (2.5)
Caustic Soda 10%	200 (100)	pH 14; 10%	Incoloy®	Steel	20-23 (3.5)
Caustic Soda 50%	250 (120)	pH 14; 50%	Incoloy®	Steel	16 (2.5)
Fuel Oil 1 & 2; Kerosene Lube Oil SAE 10 to 30	200-250 (95-120)	-	Steel	Steel	20-23 (3.5)
Fuel Oil 4 & 5; Lube Oil SAE 40 to 50	200-250 (95-120)	-	Steel	Steel	15-16 (2.5)
Fuel Oil 6 & Bunker C	160 (70)	-	Steel	Steel	6-8 (1.2)
Hydraulic Oil	100 (40)	-	Steel	Steel	15-16 (2.5)
Ethylene Glycol	300 (150)	-	Steel	Steel	23-30 (3.5-4.5)
Glycerin	500 (260)	-	Incoloy®	Steel	8-10 (1.5)
Molten Salt Bath	800 (425)	-	Steel, 321SS, Monel®	Steel	23-30 (3.5-4.5)
Paraffin Wax	150 (65)	-	Steel or 304SS	Steel	15-16 (2.5)
Molasses	100 (40)	-	304SS or 316SS	304SS	4-5 (0.7)

Poor heating results can occur if you blindly apply these values, however. Typical errors in application occur when the watt density is not de-rated (lowered) for liquids with high viscosity in cold temperatures. If the heater is needed to heat the cold viscous fluid yet rated for use in the hot condition, the heater elements may not be able to properly shed their heat to the liquid. This may cause higher element temperatures and overheating of the liquid, leading to premature breakdown (in heavy oils, for example). In such cases, lowering the watt density value by half or two-thirds can help prevent any issues.

The situation is somewhat similar in very hot liquid application conditions. In such applications, often the liquid will have low viscosity. While low viscosity is good for heat transfer, a relatively high watt density heater generally requires a large temperature differential (delta T) to “push” the heat energy into the liquid. As the heater element temperature rises, the liquid film temperature — the portion of the fluid in close physical contact with the heater elements — may go above the recommended limits. This can lead to a premature fluid breakdown, filter clogging, equipment damage, etc. Significantly lowering the watt density can help prevent these conditions from occurring.

For example, consider an application with a medium-weight oil (for example, SAE 30 oil) that has a viscosity of 90 cSt at 100°F (38°C). When the temperature falls, say to 32°F (0°C), viscosity increases to approximately 1300 cSt. In frigid temperatures, warming the oil before starting the engine — to gently raise the temperature and decrease the viscosity of the oil — would be the goal. So, although a typical heater watt density for this type of oil is 15 or 16 W/in², in such an application, a good approach would be to de-rate the heater to 12 or 8 W/in². The heater size may increase, but the heating application is more likely to be successful long-term.

The best thing to do in any application, if possible or practical, is to run trials to determine what specific watt density rating works for the specific application conditions. For instance, a 16 W/in² heater might work just fine in highly viscous conditions. When confirmation testing is not available, it is best to reference watt density information and then make adjustments to ensure a higher probability of success.

Durex Industries Gas Seal Heater A gas seal heater is used to illustrate a typical heater for proces heating.

Other Conditions That Affect Watt Density

What about flowing fluids? Can you apply a recommended watt density meant for stagnant tank liquid heating to flowing liquid? Does the flow rate make much of a difference? And if it does, under what conditions?

To answer these questions, let’s look at an example using a 50/50 mixture of ethylene glycol and water, which is commonly used as a heat transfer fluid. Most heater manufacturers would recommend a maximum watt density of 30 W/in².

The maximum allowable film temperature of this fluid is normally around 375°F (191°C). If the application temperature in a tank is relatively low, then there is no problem to apply this. If the tank process temperature is, say, 350°F (177°C), then a high watt density would need to be de-rated to avoid the heater elements pushing the fluid beyond the maximum film temperature.

How does this compare to a flowing application? Let us assume a 4" circulation heater with six heater elements heats 50 gal/min with an outlet process temperature of 250°F (121°C), a heater rating of 50 kW, and watt density of 30 W/in². After running calculations, the film temperature is estimated at approximately 300°F (149°C). The overall length of the heater is around 10', which might be too long in some instances. Can the heater length be shortened by using a more aggressive (higher) watt density without overheating the fluid?

In the calculations, the heater manufacturer suggests 45 W/in². The result is an approximately 330°F (165°C) film temperature and a 2.5’ reduction in the overall heater length. In this case, yes, it is possible to be aggressive and go above the recommended watt density. Such a change also results in cost savings due to a shorter heater.

Dropping the flow velocity and turbulence across the tubular heater elements can lower the heat transfer coefficient.

What about a slower flow condition? Let’s slow down the flow in this same heater to 10 gal/min and keep everything else constant. The estimated film temperature jumps significantly, to just below 375°F (191°C) — right up against the maximum. The heater manufacturer probably would want to de-rate this to 25 or 23 W/in². While the heater will get a few feet longer, the more conservative design can help avoid overheating situations.

Why the large increase in film temperature? The drop in flow from 50 gal/min to 10 gal/min significantly dropped the flow velocity and turbulence across the tubular heater elements. This results in the heater temperature rising because it cannot shed heat energy as quickly in the lower heat transfer conditions. The element temperature rises until enough heat energy can be “forced” into the liquid with a high temperature difference (high delta T).

The point here is that, in general, the watt density charts for stagnant flow can be used as a good starting point for flow applications. They cannot be applied blindly without risking performance problems, however. If there is a high flow velocity through the heater, then there should be no problem. If there is a slower, lazier flow rate, then seek the advice of a good heater vendor to analyze the situation and look at ways to improve flow velocity and enhance heat transfer to ensure a successful application.

With respect to gases, we shift the focus to the heater elements themselves but still do look at gas film temperatures if heating hydrocarbons. Gases typically will not be harmed by high temperatures, so here the limitations are often more around the heater element temperature.

For example, heater manufacturers often site 20 or 23 W/in² as a recommendation for air heating. Let’s put that to the test. Let’s stay with the 4" circulation heater, six heater elements, 50 kW and use a watt density of 23 W/in². Let’s assume 60°F (15°C) inlet and 250°F (121°C) process outlet. At an airflow rate of 500 scfm, the watt density graphs indicate that the heater element temperature will be 650 to 700°F (343 to 371°C). That is perfectly fine for heater elements, which, depending on the sheath material, can operate at or above 1400°F (760°C).

What impact will reducing the amount of liquid flow by 80 percent (100 scfm) have on heater element temperature? Looking at the graphs, the heater element temperature will now be at approximately 1150°F (621°C) or so. Same heater. Same gas. Same watt density. Significantly different flow conditions. Significantly different results!

In summary, regardless of what type of fluid is being heated and the fluid flow conditions, it is important to understand and consider the specific application conditions. Fluid properties affect the ability of the fluid to transfer heat from the heater elements to the liquid or gas. Use watt density charts and graphs as good starting points for proper heater applications but make sure to consider and evaluate the specific application conditions. This will allow proper analysis and provide superior heater performance results.

ABOUT DUREX INDUSTRIES

Durex Industries is a vertically integrated, lean enterprise advancing the state of the art in industrial electrical heating, temperature sensing and control. Headquartered in Cary, Illinois, Durex Industries serves the Analytical Instrumentation, Medical Device, Semiconductor, Photovoltaic, Foodservice, Plastics, Packaging, Process Heating, and general industrial markets.

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