Waste Heat Recovery | Common Sources of Waste Heat | Energy Conservation in Plants


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Document Category Engineering
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A Waste heat stream is a flow of energy which is being discarded to the environment.
Waste heat is available from many sources and it can take many different forms such as, building/ equipment heat loss, process exhaust streams, process cooling fluids (air, water,…) and unnecessary heated product in a process.

Waste Heat Recovery | Common Sources of Waste Heat | Energy Conservation in Plants

  • Air conditioning exhaust from compressor unit exchanger
  • Commercial refrigeration exhaust from compressor heat exchanger
  • Beverage sterilizing, distilling
  • Baking, drying, curing ovens
  • Aluminum melting furnace
  • Steel billet reheating furnaces
  • Glass melting furnace
  • Hydrogen plants
  • Solid waste incinerators
  • Fume incinerators
  • Steam boiler exhaust
  • Cement/limestone kiln
  • Reciprocating engine exhaust/gas turbine exhaust
  • Asphalt paving mix
  • Heat treating furnace
  • Drying and baking oven
  • Catalytic crackers
  • Annealing furnace cooling systems
  • Cooling water from:
  • Furnace doors
  • Bearing
  • Welding machines
  • Injection molding machines
  • Annealing furnaces
  • Air conditioners
  • Forming dies
  • Air compressors
  • Pumps
  • Internal combustion engines

To determine the energy content of the waste heat streams, measures are often needed, e.g., flow meters, thermocouples or infrared thermometers, flue gas analysis equipment.

Operating hours should also be monitored to perform energy balance. Waste heat survey worksheet is useful in identifying, listing, and comparing different sources of waste heat in a plant.

Basically, any process or equipment where energy is being added for heating purposes is a candidate for a waste heat recovery application.
Advantages of waste heat recovery include; fuel saving, pollution control and obviously process energy consumption improvement. Waste heat recovery projects fall normally under the medium to high cost categories. The potential of heat recovery project should only be considered after improving the efficiency of the thermal equipment as much as practically possible. A methodical Approach for the optimum use of waste heat may be summarized in the following steps:

  • Identify potential sources of waste heat.
  • Evaluate the heat content of the waste heat sources.
  • Determine possible applications for waste heat.
  • Match waste heat sources to the correct application.
  • Select a possible method/equipment configuration for heat recovery.
  • Evaluate energy and cost savings potential of waste heat.
  • Develop a budget cost estimate for implementation.
  • Evaluate return on investment.
  • Perform detailed study and analysis.
  • Implement the heat recovery project.

First rule is to stay within the equipment or process, to ensure common operating times and demands, minimize distances and heat losses, e.g., boiler exhaust gases can be used to preheat feed water or combustion air or makeup fluid. If application in the same process or equipment is not possible, then it should be as close as physically possible to the point of recovery hours. The application should have the same operating hours. The application should be selected so that equipment costs are minimized. For example compressor cooling water can be used directly as boiler feed water.

Energy and cost savings must be estimated before going any further. In addition to the information in the work sheet, more information, e.g., load changes, temperature and variations with time, seasonal variations in operation conditions, should be obtained.

Further, chemical analysis and gas moisture content measurements should be performed for hot gas streams. Knowing the energy content in the waste heat stream, the total energy recovered can be calculated if the efficiency of the heat recovery equipment is known (if not known estimate at about 65-70%). Using the prices of fuel that the recovered heat will replace, cost savings can be calculated.

Equipment costs include, cost estimate of heat exchangers (obtain budget quotations from vendors) or use information from previous projects of a similar type. Planning costs include, in-house labor hours and engineering consultant fees.

Installation costs may reach significant fraction of the equipment cost depending on the size and complexity of the equipment. Maintenance costs include, regular cleaning which is especially important in many heat recovery systems and also replacement of worn parts.

Operating costs include, additional auxiliary equipment that may consume energy thereby adding to the operating costs. Production downtime is not mandatory, however, if production stops during installation, start-up cost shall also be included besides some other costs such as space preparation costs in case of very large projects.

With a monetary value of potential energy savings and an estimate of the cost of the heat recovery equipment the financial attractiveness of the project can be quickly evaluated.
With initial estimate of payback period < 4 years, the project can proceed to the next step.

With payback period from 4-6 years, a reconsideration of the options and factors affecting the savings is always warranted.
With payback period > 6 years, the project is normally not attractive to pursue unless there are other significant benefits, e.g., process improvement, productivity increase, environmental consideration, or labor saving.

Once the initial financial attractiveness has been determined more detailed analysis is required. Detailed information on flow rates, pressures, temperature and composition of waste streams should be provided to the vendors to get detailed specification sheets and detailed costs. The vendor’s design should then be checked to ensure equipment will meet the required heat duty. Installation details, ducting, controls and peripheral equipment are sized, and a final design is developed. Based on that, total cost estimate is revised. A detailed financial analysis may be completed, including initial costs, operation and maintenance costs, and energy savings. An overall return on investment can be calculated before the final approval of the project.

One more note on the tasks required for the implementation of energy projects is the testing and inspecting of equipment before delivery and the careful planning for installation to minimize disruption of the process.

Testing and monitoring after installation may last several months. Measurements of energy savings will also help determine the overall success of the project and provide guidelines for future project.

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