Overview
Anyplace there is a temperature differential between exhaust and entry streams, there is the potential for heat recovery. The higher the differential and the larger the flow, the higher the potential for savings.
Low Temperature
Low Temperature is roughly defined as operating temperatures below 1,000F. These applications include:
- Boiler Economizers to recover flue gas temperature to pre-heat boiler make-up water
- Air-to-Air Heat Exchangers to reduce the cost of building space heating and cooling by transferring BTUs without mixing the air streams
- Enthalpy wheels to exchange both heat and moisture and some mixing of the air streams
- Reciprocating engines in cogen applications to produce hot water and low pressure steam
High Temperature
High temperature applications are going to mainly produce steam for power production and high-pressure industrial processes or be self-contained in recuperated burner systems.
- Gas Turbines with steam waste heat boilers on their exhaust
- Kilns used to produce high pressure steam off their exhaust
- Waste incinerators use to produce steam at municipal refuse plants
- Heat Treating furnaces with recuperated burners
- Glass Melting furnaces with recuperated burners
- Catalytic Oxidizers that reduce air emissions
General Guidelines
- There must be a concurrent availability of waste energy and a place to use it
- The concurrent need must take place a LOT of hours per year
- The concurrent needs must take place in or near the same proximity from a limited number of sources (otherwise, high installation costs)
- The value of the recovered energy must exceed the costs to recover it, or there is never a payback
To determine the savings potential:
- Estimate the BTUs in the waste stream based on its volume and temperature
- Estimate the BTUs that could be used by the Need
- Estimate the number of hours of concurrent availability
- Estimate the Efficiency of the recovery technology (manufacturer’s sources)
- Calculate the Savings Potential = (Available x Efficiency x Hours) <cannot exceed> Need
- Estimate the Installation Costs
- Estimate the Operating Costs
- Payback = (Installation Costs + Operating Costs) / Savings
Examples:
- A 500 hp process boiler in a food processing plant runs year round; large volumes of hot water are consumed in the process, in a steady flow that matches boiler run hours. Sounds like a good application for a boiler stack economizer to pre-heat the incoming water.
- A 100 hp space heat boiler supplies a closed-loop hot water heating system, in a moderate climate. Probably not much savings potential.
- An industrial facility running 24/7 has a 500 kW genset running in a peak-shaving mode. They have a couple wash lines located in different corners of the facility that run about 1/2 the time. Probably not a great application due to high installation costs and limited availability of thermal source.
- A controlled atmosphere heat treat furnace is using standard radiant tube burners in a continuous 1,500F process. Consider recuperated, ceramic tube radiant burners, as a furnace retrofit to reduce operating and maintenance costs.
More Information
Waste Heat Recovery High Temp Air
Source: Bob Fegan 3/02; rev 1/2004; revised 9/2005;