I. What Energy Conservation Really Says
The first law of thermodynamics is the most plain energy conservation in engineering: the energy entering a system equals the energy leaving the system plus the change in the system's internal energy. For a steady-state operating heat exchanger, the system itself stores no energy, so "heat absorbed by the cold fluid = heat released by the hot fluid." Written as an equation:
Q_absorbed = Q_released, i.e., m_cold·c_cold·(T_cold_out − T_cold_in) = m_hot·c_hot·(T_hot_in − T_hot_out)
where m is the mass flow rate and c is the specific heat capacity. This equation is the "general ledger" of the entire waste heat recovery design—for any heat exchange scheme, settle this account first to know how much heat can be recovered.
II. Where Industrial Waste Heat Is Lost
In industries such as chemical, textile printing and dyeing, food processing, and metallurgy, a considerable portion of energy ultimately escapes as low-temperature flue gas, cooling water, and exhaust air. Take a stenter as an example: exhaust above 200°C discharged directly carries sensible heat that, calculated at 6,000 operating hours per year, often equals hundreds of tons of standard coal. This heat has not "disappeared"—it has simply flowed out of your process system.
- Sensible heat of flue gas/exhaust: High temperature and large flow, the primary recovery target;
- Process wastewater/cooling water waste heat: Medium temperature, suitable for preheating feedwater or space heating;
- Equipment surface heat dissipation: Dispersed and low-grade, usually addressed first by insulation.
III. How a Heat Exchanger "Moves" Heat
A heat exchanger itself does not generate heat; it only acts as a mover: it lets the high-temperature fluid hand its heat to the low-temperature fluid without mixing the two fluids. An air-to-air plate heat exchanger moves the heat from drying exhaust into the make-up air, saving the new energy needed to heat the make-up air; an air-to-water heat exchanger moves flue gas waste heat into the hot water system. As long as the energy-conservation account balances, the recovered heat equals the fuel not burned.
IV. Three Conservation Checkpoints in Design
When making a scheme, it is recommended to check item by item:
- Hot-end balance: Whether the heat released by the cooling of the hot fluid equals your expected recovery amount;
- Cold-end balance: Whether the heat needed to raise the cold fluid's temperature can be met by the hot end;
- Air-leakage/heat-loss: The actual recovery amount must deduct the heat exchanger shell heat dissipation and possible cross-contamination losses.
Many cases of "theoretically recoverable 30%" ultimately deliver only 20%—the gap is exactly these items not accounted for. Treat the first law as a red line, and the scheme will not be overstated.