What a heat recovery exchanger actually does
A waste heat recovery heat exchanger transfers energy from a hot exhaust stream into a useful sink — feedwater, combustion air, supply air or a process loop — without mixing the two fluids. The measure of success is not peak efficiency on paper, but how much fuel or steam you stop buying over a year of real operation.
The data you need before sizing
Send your engineer these six items:
- Exhaust composition — gas or air, moisture, dust, solvent or corrosive components. This drives material and cleaning access.
- Flow rate at operating conditions (not standard conditions).
- Inlet temperature and the lowest temperature you can safely reach.
- The cold-side fluid, its flow rate and inlet temperature.
- Allowable pressure drop on the exhaust side — the fan often sets this limit.
- Duty cycle — hours per year, and whether the load is steady or batch.
Sizing logic in plain terms
The available duty is the exhaust mass flow times its heat capacity times the temperature drop you are allowed to take. The heat transfer area then follows from the overall coefficient and the LMTD (log mean temperature difference). A common mistake is chasing a very low stack temperature: below the acid dew point, condensate attacks the cold end and the unit fails early. A good design leaves margin there.
Fouling decides long-term performance
Dust, lint and oil mist build a thermal resistance that no catalogue coefficient accounts for. Specify accessible cleaning, realistic fin spacing for dirty streams, and a fouling factor from the start. Otherwise the unit performs well for a month and then quietly stops paying back.
From duty to payback
Annual saving ≈ recovered duty × operating hours × fuel cost ÷ boiler efficiency. Compare that with installed cost and maintenance. Recovery projects on lines running multiple shifts commonly justify themselves far faster than equipment replacement projects.
FAQ
Q: Can an existing line be retrofitted? Usually yes, if there is space in the duct or near the stack. We size around the actual installation envelope.
Q: What if the exhaust temperature varies? Give us the range and the hours at each level; we size for the weighted duty, not a single peak reading.