Industrial waste-heat recovery

Find rejected heat

Common sources include refrigeration condensers, compressor cooling, warm effluent, dryer exhaust, condensate, cooling water and ventilation air. A heat balance should record temperature, flow, contaminants, distance and availability.

Use the hierarchy

First reduce avoidable heat demand. Then reuse heat directly where temperature and timing permit. Apply a heat pump when a useful sink sits above the source temperature. Storage may help when source and sink do not coincide exactly.

Estimate a heat stream from measurements

For a liquid stream, thermal power is mass flow multiplied by specific heat capacity and usable temperature change. The usable change stops at the point where process, fouling, freezing or exchanger-approach constraints intervene. Air and vapour streams require humidity and latent heat to be handled correctly.

Do not multiply a peak measurement by annual hours. Build a duration profile from logged flow and temperature data and align it with the sink schedule.

Capture temperature before it is destroyed

Cooling a stream through a tower or mixing hot and cold effluent can remove exergy before recovery is considered. Source segregation and exchanger placement can raise the available temperature and reduce compressor lift. The process modification may deliver more value than selecting a higher-temperature machine.

Recoverable is not the same as usable

A large theoretical heat flow can have little value if it is too cold, remote, dirty or available only during shutdown. Conversely, a modest but steady warm stream beside a year-round hot-water demand can support a strong project.

Build a heat-source register

A useful register names the process, stream, temperature profile, flow, contaminants, operating schedule, distance and current rejection method. It should distinguish measured data from design values. Cooling-tower nameplates and refrigeration capacity do not prove how much heat is available at a useful temperature during production.

Sources should be logged through representative production and cleaning cycles. Short samples can miss seasonal cooling loads, batch transitions and stoppages. A source that looks continuous from monthly electricity use may cycle sharply within each hour.

Use direct recovery as the reference case

If a hot stream can preheat a colder stream through a heat exchanger, direct recovery normally uses less electricity and less equipment than a heat pump. The heat-pump case should therefore be compared with the best practical direct-recovery arrangement rather than with doing nothing.

Direct recovery and heat pumping can also work in sequence. Direct exchange can capture the higher-temperature portion first, after which a heat pump upgrades the remaining lower-grade heat. The temperature profiles must be modelled together to avoid counting the same heat twice.

Dirty sources change the design

Effluent, exhaust and food-process streams may foul or corrode an evaporator. Intermediate loops, cleanable plate exchangers, filters and material selection protect the refrigerant package but add temperature loss and pumping power.

Design performance should include the expected fouled condition and cleaning interval. An excellent clean exchanger approach that degrades after a week can produce a poor annual result.