Electrically heated thermal fluid systems are extremely useful, but the user should understand what’s “inside the box” when specifying and purchasing this equipment.

Continuing my series on specifying a thermal fluid system, in this installment, I will discuss the specification of electric thermal fluid systems. Electric thermal fluid heaters serve several useful purposes for process heating applications:

• They can be obtained with smaller thermal ratings than the smallest fired heater, making them ideal for smaller processes.
  
• They are compact and do not have to be located so that a smoke stack can vent flue gas.
  
• They can exhibit extremely deep turndown ratios, allowing them to supply fluid at very consistent temperatures over a wide range of heat demands.
  
• The equipment can be less expensive per BTU delivered than fired heaters.
  

So why aren’t all thermal fluid heaters electric? The answer is the fuel cost, which in this case is electricity. The price of fossil fuel varies over time and the cost of electricity varies geographically, but, in general, it can cost two to three times as much to heat thermal fluid with electricity as it does to heat it with gas or oil.

When preparing to purchase a new electric heater, it is important to understand the details of construction as well as the general types of equipment available.

Heater Types

For purposes of this discussion, I will look at two major types of electric heaters: portable (also called cabinet) heaters and circulation heaters.

Cabinet Heaters. These are the smallest heaters generally available. In my experience, I’ve encountered cabinet heaters as small as 6 kW (~20,500 BTU/hr) and as large as 50 W (~170,000 BTU/hr). There are many manufacturers of these smaller heaters, so there may be larger and smaller heaters available, but I have not seen them in my practice.

The representative system in figure 1 shows the compact design of a cabinet heater. The enclosed cabinet contains the heating element, pump, expansion tank, controls and all necessary valving for heater operation. Cabinet heaters also are available with cooling options that can be used to maintain constant temperature in processes that require cooling as well as heating.

Cabinet heaters are self-contained and ready to run right out of the crate. This is truly “plug and play” technology. Once on site, users simply connect electric power and the thermal fluid connections, and the system is ready to run. These smaller heaters are often seen in services such as plastic extruder service, die and platen press service, laboratory equipment heating and pilot equipment heating.

Because of the large number of manufacturers, there is a wide assortment of capabilities in these units. Careful evaluation of manufacturers’ offerings is important if the user wishes to get a unit that performs well for long periods of time. A specification that defines the requirements for watt density of the heating elements, maximum temperature rating, controls, materials of construction and pump type is important in getting a good value for the money spent. This is one instance where the lowest price may not be the best value.

Choices of thermal fluid are also important for cabinet heaters. The proximity of the expansion tank to the hot piping can allow the expansion tank to run hot and cause fluid oxidation. Low cost “hot oils” may not perform acceptably in these heaters. A fluid with more resistance to oxidation generally will be the “value” decision in cabinet heaters.

Circulation Heaters. Circulation heaters can have capacities as small as the smallest cabinet heaters but also can be very large. I personally have worked with one system that was almost 1 MW (3,400,000 BTU/hr!), but this is an exception.

Figure 2 shows two circulation heaters supplied as complete systems, including heating element and controls, along with the pump and expansion tank. By contrast, figure 3 shows a circulation heater system supplied with a control panel but not the pump or expansion tank. This configuration allows system customization.

Circulation heaters generally are not offered as standard or stock systems but are constructed when ordered. This allows the buyer to specify any special features that may be desired to optimize the system performance.

As the figures illustrate, circulation heater systems can be specified and purchased as integrated systems - complete with pump(s), expansion tank and all valves - or as  heater-only units with electric heat exchangers and temperature control systems. Both variants have their advantages. The integrated systems offer the convenience of the smaller cabinet systems and can offer the advantages of more robust components, higher temperature ratings or special control features. The heater-only systems offer a considerable level of freedom to customize the system. Heater-only systems often are used when the arrangement of the equipment to be heated does not allow a compact, skid-mounted system.

As with cabinet heaters, the user is well advised to develop a detailed specification for the equipment to be purchased. The owner also is advised that using the manufacturer’s proposal as the specification carries the risk of purchasing a system with less than optimum performance.

Specifications

There are a large number of electric thermal fluid systems, and those systems exhibit a wide range of capabilities. Of course, the owner always wishes to purchase a system that will deliver the required performance and not be burdened with reliability issues. To achieve this, a number of issues should be understood in order to specify a system that is adequate for the application.

Thermal Fluid. As stated in previous articles, the choice of thermal fluid is pivotal in selecting the thermal fluid system components. The owner should select his fluid based on his process requirements. This selection should be transmitted to the equipment manufacturer, so he can use the fluid properties to match his equipment to the process requirements.

The Electric Heat Exchanger. The heat exchanger is the heart of the electric heater system. Heating elements are mounted in a flange and inserted into one end of a metal container. Fluid circulates through the container and picks up heat from the heating elements. The buyer should be aware of several critical concerns when ordering an electric heat exchanger - or an entire system.

• Heat Duty. The duty is simply the amount of heat per hour that needs to be delivered to the thermal fluid and the process. One kilowatt hour is equal to approximately 3,414 BTU/hr. The amount of available excess heat is up to the owner, but most will specify from 1.25 to 1.5 times the running load to allow for heatup and extra demand.
  
• Watt Density. Heating elements are rated by how many watts per square inch of heating element are delivered to the process. This is the most critical fact to know and to specify when purchasing an electric heat exchanger. It costs the manufacturer little more to manufacture a heating element with a high watt density as a low watt density. Manufacturers can save money, therefore, by manufacturing higher watt density units, which equates to a smaller package size, and sell equipment at a lower cost and still meet the total wattage requirement. High watt densities, however, result in high fluid film temperatures, which can result in thermal degradation of the fluid. Smaller cabinet heaters sometimes have watt densities of up to 24 W/in². Larger circulation heaters may have much lower watt densities, depending on the process temperature and the thermal fluid chosen.
  
• Baffles. The heating elements should have baffles to help direct the fluid over all parts of the element equally. This will ensure that that no areas of low or no flow exist inside the heating element, which would result in extreme fluid degradation.
  
• Flange. The electric heat exchanger is part of the system piping and should conform to the system’s piping specification. Here again, the manufacturer can save money by using a lower rated flange, such as ANSI Class 150 lb, but a higher rated flange, like ANSI Class 300 lb, may be more appropriate to prevent fluid leaks.
  
• Electrical Junction Box. The electrical junction box mounts on the flange of the heating element. The junction box needs to meet the electrical classification of the area in which the unit is installed. It also needs to be sufficiently robust to withstand the temperature that will be conducted by the heating element ends. On very high temperature units, the junction box may be installed with an air gap between the flange and the electrical box to help reduce the amount of heat transmitted to the terminals.

• Wiring. The possibility of elevated temperatures in the electrical junction box always exists. To combat this, wiring in the terminals should have high temperature insulation capable of withstanding 300 or even 400°F (~150 or even 200°C).
  
• Heat Exchanger Arrangement. It is important to understand how the heat exchanger is to be mounted. Units mounted vertically, as is typical in many cabinet heaters, take up less room but position the junction box directly over the heat exchanger, increasing the heat transmitted to the junction box and electrical terminals.
  

Piping Materials. The piping materials inside an electric thermal fluid system should be the same as the piping materials used in the process. Threaded pipe should be avoided as it presents the possibility of leaks. Also, smaller systems sometimes utilize copper tubing or brass valves in their systems. Copper (and any copper-containing alloy) should be avoided in thermal fluid systems. Copper is a reactive metal and will act to catalyze the chemical degradation of the fluid.

Pump. The specifying engineer should evaluate the pump provided with a system. Many smaller systems have gear pumps while larger systems have centrifugal pumps. The pump is often the most visible component of a thermal fluid system because it requires the most maintenance. Evaluate the maximum temperature rating of the pump and seal provided and compare it with the fluid temperature required by the process.

A manufacturer’s “standard” pump may sometimes be an inexpensive unit that will perform the duty required but may give trouble down the road. It is worth considering upgrading the pump, if available, to have a robust, reliable piece of equipment that will last. When I design systems, the available pump choices (or lack of choice) often drive me to purchase a circulation heater without a pump and specify a robust pump to be installed in the field.

Expansion Tank. When specifying a heater, always tell the potential vendor the total system volume. That value will be used in determining the proper size for the expansion tank.

Ideally, expansion tanks will be piped to allow full fluid fluid through the system during startup and later have only a single expansion leg when the system is at temperature.

Fluid properties and operating temperature will aid in determining whether the expansion tank is to have a nitrogen-blanketing system (Never a bad idea!) and whether the expansion tank must be pressurized.

Controls. The temperature controls should include, as a minimum:

• Fluid temperature controller.
  
• Fluid high temperature alarm and shutdown.
  
• Low flow alarm and shutdown.
  
• Low expansion tank level alarm and shutdown.
  

Some manufacturers of the electric heat exchanger offer a feature that places a temperature element in contact with the heating element. This feature allows you to sense high surface temperatures that may occur in the event of low fluid flow or a bubble in the system. Not all systems have - or require - this feature, but it is worth evaluating during the specification stage.

If the manufacturer offers a control system contained in a small PLC, make sure that the code for the program is supplied as part of the manufacturer’s documentation. Also, be sure the plant has, or has access to, the proper software to interface with the PLC for troubleshooting purposes. It is not a good idea to alter the manufacturer’s program. If you do, you’re on your own with regard to heater performance and operability, and you will probably void all warranties.

Electrically heated thermal fluid systems can be a cost-effective choice as a thermal fluid system, but in order to purchase a system that provides optimum performance and robust reliability, it pays to understand the system features and construction. Careful specification and evaluation up-front can offer benefits for the life of the system.

Sidebar
Specifying a Thermal Fluid System: A 6-Part Series

Use the links at the bottom of the page to continue reading this six-part series on specifying a thermal fluid heating system. You've just finished:

Part 3: Electric Thermal Fluids Heaters

Other parts in this series include:

Part 1: Choosing the Thermal Fluid
Part 2: Fired Thermal Fluid Heaters
Part 4: Thermal Fluid Pumps
Part 5: The Expansion Tank
Part 6: Piping Materials, Valves and Insulation

Links

• J.G. Hudson & Associates
• Jay Hudson, P.E.
• Specifying a Thermal Fluid System: Part 1
• Specifying a Thermal Fluid System: Part 2
• Specifying a Thermal Fluid System: Part 3
• Specifying a Thermal Fluid System: Part 4
• Specifying a Thermal Fluid System: Part 5
• Specifying a Thermal Fluid System: Part 6