Dr. Arvind Thekdi, an Energy Expert for the U.S. Department of Energy, routinely conducts energy assessments to improve energy efficiency of process heating systems at industrial plants. During the assessments, he often encounters questions that indicate confusion about how process heating systems operate. In this article, Dr. Thekdi provides some basic information about process heating systems, and offers solutions for reducing heat losses to increase efficiency.

Question: What is process heating?
Answer: Process heating is a critical step of manufacturing operations, used to heat materials in production of metals such as steel and aluminum, and non-metals such as glass, cement, rubber, plastic, petroleum products and ceramics. Heat is applied to raise temperature of solids, liquids or gases in heating equipment such as furnaces, process heaters, melters, ovens or dryers. The heating process softens, melts or evaporates the materials, and may promote chemical reactions, molecular rearrangements or breakdown of molecules of the materials being heated. Heat required in process heating equipment comes from fuels such as natural gas, fuel oil, coal and other energy sources such as electricity or steam.

Q: What are the different types of process heating systems?
A: The three most common process heating systems are: fuel-based, electricity-based and steam-based systems.

In fuel-based process heating, heat is generated by the combustion of solid, liquid or gaseous fuels, and transferred either directly or indirectly to the material. The combustion gases are either in contact with the material (direct heating) or are confined and separated from the material (indirect heating; e.g., radiant burner tube, radiant panel, muffle). Examples of fuel-based process heating equipment include furnaces, ovens, heaters, kilns and melters.
Electric-based systems (electro-technologies) use electric currents or electromagnetic fields to heat materials. Direct heating methods generate heat within the material by either: 1) passing an electrical current through the material, 2) inducing an electrical current (eddy current) into the material, or 3) by exciting atoms/molecules within the material with electromagnetic radiation (e.g. microwave). Indirect heating methods use one of these three methods to heat a heating element or susceptor which transfers the heat either by conduction, convection, radiation or a combination of these to the material. Examples of electric-based heating systems include induction heating and melting, electric arc furnaces, infrared ovens and vacuum furnaces.

Steam-based systems use steam to supply heat to the materials directly or indirectly. Direct steam heating systems inject steam into liquids or gases. Indirect systems use a heat exchanger in which steam is cooled and condensed in tubes; the heated tubes supply heat to the liquids and gases. Steam offers several advantages in process heating operations, such as high heat capacity, ease of transport, low toxicity and cost, and can be generated by a variety of by-product fuels. Examples of steam-heated systems include distillation columns, water or air heating, paper drying, and humidification.

Hybrid systems use a combination of process heat systems using different energy sources. An example is a paper drying process that combines an electric-based infrared technology with a fuel-based dryer.

Q: What is the most common area of efficiency improvement for process heating systems?
A: Reduction or elimination of heat losses is the most important consideration in reducing energy use for process heating equipment. In cases where it is impossible to reduce the losses, consider recovering part of the energy lost and using it within the process itself or for other useful purposes. Several heat recovery methods exist that can be used within the heating system itself, within the plant or converted into easily transportable energy such as electricity.

Efficiency of heating equipment is measured by the ratio of the amount of heat used by the material being heated to the amount of energy supplied to the heating equipment. For example, if heating equipment uses 10 MMBtu/hr heat and the load or material uses or receives 6 MMBtu/hr heat during the heating process, thermal efficiency of the process is considered 60 percent. The heat not used by the material is lost through the system.

Heat loss depends on many factors, such as the type of heat supply system used, equipment design, operations, and maintenance of the equipment. Here are some common areas of heat loss, and steps to take to reduce or recover the heat loss.


Area of Heat Loss

Steps to Reduce Heat Loss

Heat in flue or exhaust gases from the heating system. This includes heat content of total mass of flue gases, including air leakage into the system through opening: make-up air; moisture in flue gases or other sources.

  • Reduce excess air used for fuel combustion in burners.
  • Control and minimize the amount of make-up air in ovens, dryers, etc., while following the safety guidelines.
  • Minimize air leakage by reducing size and number of openings and controlling pressure in oven or furnace.
  • Recover heat by using heat recovery devices such as a recuperator for combustion air preheating, economizer for feed water heating, hot water production, steam generation, charge drying or preheating.
  • Employ heat cascading: use high-temperature exhaust gases to supply heat to lower temperature heating processes.

Heating equipment walls or outside surfaces

  • Use proper type and thickness of insulation for furnace/oven walls.
  • Repair and maintain insulation and refractories used for the walls and doors.

Material handling equipment such as conveyor, belts, trays, fixtures, etc.

  • Minimize the weight of fixtures, trays, baskets used for material handling.
  • When possible, return the conveyor belt, fixtures as hot as possible.

Heat stored in refractories, insulation or other materials used for the equipment itself when it is heated from lower (usually ambient temperature) to the operating temperature

  • Avoid cooling of walls of ovens or furnaces by using them continuously.
  • Keep doors closed and adjust the stack damper to prevent cold air draft through the stack when the furnace is not used or it is on hold condition.

Cooling system air or water, if used

  • Avoid use of water-cooled parts in furnace
  • If cooling is necessary, insulate the water or air-cooled parts.

Openings by thermal radiation

  • Reduce openings, cracks, holes in the heating equipment walls.
  • Minimize door openings during charging and discharging the load or charge material.

Heat content of special atmosphere or reaction gases introduced in the system or released from the material being heated when these gases or vapors are discharged at high temperature from the heating equipment.

  • Minimize the use of process atmosphere or reaction gases used in furnaces. Maintain seals and reduce atmosphere/gases leakage.
  • Discharge the vapors or gases produced by drying or reactions at minimum possible temperature to reduce heat carried out by these gases.

Other losses and solutions

  • Operate the furnace or ovens at or close to the design capacity.
  • Control and minimize use of auxiliary heating systems such as flame curtains or while following the safety guidelines.
  • Minimize idling time for the furnace and ovens.
  • Conduct regular inspection and energy assessment for the large energy use equipment.

 

Estimate heat losses in your system by using ITP's Process Heating Assessment and Survey (PHAST) software tool. PHAST also allows the user to estimate reduction in these losses by energy saving measures for each of these areas of heat losses.

Additional Resources
For more information on how to improve your plant's process heating system efficiency, please see the following ITP resources.

If you have questions or comments about this column, contact Dr. Arvind Thekdi at: athekdi@e3minc.com.

Photo of Dr. Arvind Thekdi.


Dr. Arvind Thekdi has more than 40 years of experience in R&D and design of industrial process heating systems. During his career, he has worked for industrial heating equipment supply companies and government organizations, and conducted more than 50 process heating energy assessments in all major industrial sectors. Dr. Thekdi has helped to develop several energy efficiency software tools, including DOE’s Process Heating Assessment Tool (PHAST). In addition to his role as a DOE Process Heating Energy Expert, he is an instructor for DOE's process heating end user and Qualified Specialist training classes. Dr. Thekdi has published more than 50 technical papers and contributed to two books on combustion and process heating, and holds 15 U.S. and foreign patents related to high temperature processes and equipment. Dr. Thekdi received a M.S. degree from Indian Institute of Science, and Ph.D. from Pennsylvania State University.