Circulation Heaters Information
Circulation heaters are used primarily to heat moving, flowing, or circulating fluid streams. Fluid flows through the heater, which transfers heat to the fluid stream. Any liquid or gas is generally acceptable for use with a circulation heater. Often, circulation heaters are used as stock tank heaters, asphalt heaters, livestock tank heaters, circulation tank heaters, dry well heaters, and septic tank heaters.
Configuration
Circulation heaters can be configured to provide direct heating of process fluids or may be used for indirect heating of processes and systems.
Direct Heating
When used for direct heating applications the process fluid circulates directly through the heater. This can be done by either installing the heater inline, when a constant supply of heated medium is required or in a side arm configuration where the heated medium is fed into a reservoir.
Inline heating allows for a constant supply of heated fluids. This configuration does not use a reservoir and the volume of fluid available is limited by the size of the circulation heater.
Image Credit: Vulcanic
Side arm heating is useful when a large volume of fluid needs to be heated. The heater is mounted alongside a tank or vessel. The inlet and outlet of the heater feed and supply fluids from this reservoir. In some applications several heaters are mounted in a side arm configuration, allowing for rapid heat transfer. Once the medium is brought up to a desired temperature only one or two circulation heaters are left running in order to maintain the desired temperature level.
Image Credit: Baptistry Heaters Direct
Indirect Heating
When used for indirect heating applications, a thermal fluid is circulated through a heat exchanger or other piping system to radiate thermal energy into a system. Circulation heaters can also be used in ovens, molding operations, extrusion lines, dryers as well as other applications where an indirect heat source can be used to control temperature.
Indirect Temperature Control of Extruders -- Video Credit: heatinc1
Applications
It is important to select a heater that is designed for its intended application. The viscosity, specific heat, flash temperature and corrosive properties of the fluid to be circulated through the heater may limit the watt density, recommended maximum temperature or require specific materials of construction. Common applications for circulation heaters include the heating of gases or vapors, clean water, process waters, high-purity waters, lightweight oils and degreasing solutions, heavy weight oils, medium weight oils, mild corrosive solutions, severe corrosive solutions, caustic solutions, and liquid paraffin.
| APPLICATION | 1 | 2 | 3 | 4 | 5 |
|---|---|---|---|---|---|
| SOLUTION OR HEATER TYPE | ALKALINE OR ACID CONTENT (EST. % BY VOLUME) | SHEATH MATERIAL | WATT DENSITY (W/IN2) | VESSEL MATERIAL | |
|
Water |
Clean Water |
pH6 to pH8 (Neutral) |
Copper |
45 - 100 |
Galvanized Steel |
| Mild Solutions | Process Water and Very Weak Solutions Weak Solutions Demineralized, De-ionized or Pure Water | ph5 to pH9 (2 - 3%) 5 - 6%— | INCOLOY® | 45 - 86 45 - 7545 - 75 | Stainless Steel |
| Corrosive & High Viscous Solutions | Mildly Corrosive Solutions More Severe Corrosive Solutions Severely Corrosive Solutions | 5 -15% 10 - 25% 30 - 60% | INCOLOY® | 20 - 2520 - 2510 - 20 | Stainless Steel |
| Oil | Low Viscosity Oils Medium Viscosity Oils High Viscosity Oils (Fuel Oil) | —— | Steel | 20 - 25 10 - 205 - 15 | Steel |
| Air, Gases & Steam | Medium Temperatures to 750°F High Temperatures to 1400°F |
—— |
INCOLOY® | 20 - 25 10 - 20 | Steel; Stainless Steel |
Image Credit: Chromalox
Heating Elements
Industrial circulation heaters are typically powered by electricity and use either flanged or screw plug immersion heating elements. The use of immersion heating elements allows the user to replace or exchange heating elements without disrupting the entire system.
Flanged immersion heaters are typically equipped with ANSI flanges. They are installed by bolting the unit to a matching flange welded to a vessel wall. ANSI flanges are specified by a nominal pipe size and pressure class rating. Common pressure classes for ANSI flanges are: 150, 300, 400, 600, 900, 1500 and 2500 psi.
Screw plug immersion heaters are equipped with a tapered pipe fitting that screw into a threaded well. Tapered threads have a concentric thread diameter and are used for sealing without gaskets. Tapered pipe fittings specified by ANSI/ASME adhere to national pipe thread (NPT) standards.
Selection Criteria
Important parameters to consider when specifying circulation heaters include performance specifications, applications and mounting criteria.
Performance Specifications
Performance specifications for immersion heaters include maximum operating temperature, AC voltage, heating capacity and watt density.
Maximum operating (sheath) temperature is the highest temperature that the heater's sheath (or protective cover) may reach. This is not the maximum temperature a heated substance may reach.
AC voltage is the minimum alternating current (AC) volts required to operate the cartridge heater.
Heating Capacity, measured in kilowatts (kW), is the wattage that a circulation heater can deliver.
Image Credit: Spirax Sarco
Where:
 |
= | Mean heat transfer rate (kW) |
 |
= | Mean secondary fluid flowrate (kg/s) |
| cp | = | Specific heat capacity of the secondary fluid (kJ/kg K) or (kJ/kg°C) |
| ΔT | = | Temperature rise of the secondary fluid (K or °C) |
Watt density is a good measure of how quickly circulation heaters can transfer heat to a heated surface.
High-watt density immersion heaters should not be used with extremely viscous materials, explosives, or substances that are not well circulated because the risk of fire or scorching increases as fluid viscosity and heater-watt density increase.
Mounting Criteria
When installing or mounting a circulation heater the primary specifications to consider are the type of connections, the flange or thread diameter used for the heating element, and the inlet and outlet separation distance.
Flange Mount / (ANSI) Flanges — Circulation heaters are equipped with ANSI flanges. Immersion heating elements are installed by bolting the unit to a matching flange welded to the vessel wall. ANSI flanges are specified by a nominal pipe size and pressure class rating. Common pressure classes for ANSI flanges are: 150, 300, 400, 600, 900, 1500 and 2500 psi.
Screw Plug / NPT Threaded — Circulation heaters are equipped with national pipe threads (NPT). Immersion heating elements are installed with tapered pipe fittings that screw into a threaded well. Tapered threads have a concentric thread diameter and are used for sealing without gaskets. Tapered pipe fittings specified by ANSI/ASME adhere to national pipe thread (NPT) standards.
Flange or Thread Diameter — The diameter of the mounting flange or pipe thread is a nominal size describing the bore of the flange or diameter of a pipe thread. A circulation heater will generally have identical inlet and outlet diameters.
Inlet and Outlet Separation — The inlet and outlet separation distance is measured on center between the return or supply and discharge ports.
Resources
Chromalox - Circulation and Immersion Heaters
ASB Heating Elements, Ltd. - Circulation Heaters