Heat Exchanger Construction, Balances and Operation

Separate the two fluids and follow the heat

A heat exchanger transfers energy between streams. In an indirect exchanger, a metal boundary separates the fluids. In a direct-contact heater, such as a deaerator, the streams mix. Paper 4A requires the theory and operation of heat exchangers as well as the three heat-transfer modes. Knowing only the word “convection” does not explain a leaking tube, a waterlogged heater or a falling outlet temperature.

A shell-and-tube exchanger has a pressure shell surrounding a bundle of tubes. Tube sheets secure the tube ends and separate shell-side fluid from tube-side fluid. Headers or channel heads distribute tube-side flow. Baffles support tubes and direct shell-side flow across the bundle, improving heat transfer while increasing flow resistance. A fixed-tube-sheet design constrains differential expansion; U-tubes and floating-head arrangements accommodate it in different ways. Tube material, cleaning access, thermal stress and allowable pressure determine the suitable construction.

A plate exchanger uses thin, shaped plates to form alternating passages for the two streams. Large area relative to size and turbulent passages can produce compact equipment. Gasketed plates can be separated for inspection; brazed or welded constructions have different access and service limitations. Gasket compatibility, pressure/temperature ratings and blockage risk matter. A plate exchanger is not automatically suitable for any steam pressure simply because another plate unit serves hot water.

Flow arrangements and thermal resistance

In parallel flow, both streams enter at the same end and travel in the same overall direction. In counterflow, they enter at opposite ends. Counterflow generally maintains a more useful temperature difference along the length and can cool the hot stream below the cold stream’s outlet temperature, while still respecting local heat flow from hot to cold. Neither arrangement allows the cold stream to exceed the entering hot-stream temperature without another energy source.

The approximate heat-transfer rate is Q̇ = U A ΔTmean. The overall coefficient U incorporates fluid-film, metal-wall and fouling resistances. Increasing area or temperature difference can increase duty, but design limits and available flows constrain the result. Fouling lowers U; a dirty unit may require a larger temperature difference for the same duty. Pressure drop and duty should therefore be trended together, rather than diagnosing performance from one outlet temperature.

Check an energy balance

For steady heating of liquid water with negligible loss, use Q̇ = ṁ c ΔT. If a stream flows at 2 kg/s and warms from 20°C to 60°C, with supplied c = 4.18 kJ/(kg·K), duty is 2 × 4.18 × 40 = 334.4 kW. If the heating steam releases 2,200 kJ/kg between inlet steam and outlet condensate, required steam flow is 334.4 / 2,200 = 0.152 kg/s, or about 547 kg/h. The enthalpy difference must match the actual steam and condensate states; it is not always equal to atmospheric latent heat.

A disagreement between hot-side and cold-side balances can indicate heat loss, inaccurate instruments, unsteady inventory or leakage. Do not conclude “leaking tube” from balance mismatch alone. Check units, sensor locations and whether both readings represent the same steady period. A changing tank level means the control volume is storing mass and energy.

Steam-side drainage and air removal

Condensing steam must reach the heat-transfer surface and condensate must leave it. Air or other non-condensable gas can form an insulating film, reducing performance even when total indicated pressure appears adequate. Air vents remove gas according to the exchanger design. A trap removes condensate while limiting live-steam loss. If the steam-space pressure falls below condensate-return backpressure, the trap may stall; condensate accumulates and heating becomes unstable. More steam pressure is not a universal remedy for a return-system problem.

Waterlogging can produce poor temperature control, corrosion and water hammer. Compare load, steam pressure, return pressure, trap condition and vent condition. A vacuum breaker, where specified, admits air as steam condenses during shutdown and can protect against unwanted vacuum effects. It is a designed device with a defined purpose, not permission to loosen a fitting to admit air.

Operation and inspection

Before placing an exchanger in service, verify the approved piping arrangement, relief protection, vent/drain routing, instrument condition and receiving-system readiness. Follow the maker’s sequence to establish circulation and warm the unit gradually. Rapid differential heating can strain tubes, joints and gaskets. During operation, trend flows, inlet/outlet temperatures, pressure drop and evidence of cross-contamination. Leakage generally travels from the higher-pressure side to the lower-pressure side, so identify both pressures before evaluating the consequence.

For maintenance, isolate and depressurize both sides, account for trapped liquid expansion and verify the isolation. Apply confined-space requirements where entry is involved. Cleaning method must suit the metal, gasket and deposit; aggressive chemicals or mechanical tools can damage the boundary. After authorized work, the inspection and return-to-service process must establish integrity and correct line-up before operation resumes.

Comparison aid

Source notes

Official scope and technical references checked 2026-10-17: Reference 1; Reference 2. Archived DOE handbooks are used for physical principles, not current Canadian legal requirements.