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Heat Exchangers

4 min read

Parent
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Equipment Design
Vessels and exchangers
Current
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Heat Exchangers
Tube-in-tube, coil

A heat exchanger moves heat from one fluid to another through a wall without mixing them. The simplest is a tube inside a tube with the two fluids flowing in opposite directions (counterflow), which is the most efficient layout and is the only one that can warm the cold fluid above the hot fluid’s outlet temperature; the other is a coil in a tank. Size it from the heat duty, Q = mass flow × specific heat × temperature change, and the mean temperature difference.

Part of Industrial Chemistry

Tube-in-tube and coil exchangers for cooling reactions and recovering heat, with the sizing arithmetic.

The Carrier’s Two Designs

Tube-in-tube. A smaller tube runs inside a larger tube. Hot fluid flows in the inner tube, cool fluid in the outer tube in the opposite direction, and heat passes through the inner wall. The carrier calls the counterflow arrangement the most efficient.

Coil-in-tank. A long tube coiled inside a tank; one fluid flows through the coil, the other fills the tank. The carrier calls it simple to build and effective for moderate heat exchange.

The corpus’s Condenser Coil page treats the coil-condenser case, and Distillation Apparatus sizes the cooling water for a still. Why exchangers matter in the first place is Heat Management.

What You Need

  • Tubing of a material that resists the process fluid (see Reaction Vessels) and conducts heat; copper conducts well but is attacked by ammonia.
  • A cooling water supply with known temperature.
  • Thermometers on all four connections.
  • Pressure on the right side. Decide which way a leak should go and set the pressures accordingly.
  • Valves to isolate and drain.

Method: Sizing An Exchanger

Step 1: Compute the duty, Q = m × c × ΔT for the stream you know best.

Step 2: Fix the end temperatures and compute the mean temperature difference.

Step 3: Divide: area = Q ÷ (U × mean ΔT), where U is the overall heat-transfer coefficient, which you must measure or estimate from a similar working exchanger.

Step 4: Convert area to tube length, area per metre = π × tube diameter.

Worked Example: Counterflow Versus Parallel

A hot stream of water enters at 90 °C and leaves at 40 °C. A cold stream enters at 20 °C and is to leave at 60 °C. Flow of the hot stream is 0.05 kg/s (illustrative); specific heat 4.18 kJ per kg per °C (recalled).

  1. Duty: Q = 0.05 × 4.18 × (90 − 40) = 0.05 × 4.18 × 50 = 10.5 kW.
  2. Counterflow temperature differences at the two ends: hot inlet against cold outlet = 90 − 60 = 30 °C; hot outlet against cold inlet = 40 − 20 = 20 °C. The log-mean difference = (30 − 20) ÷ ln(30 ÷ 20) = 10 ÷ 0.405 = 24.7 °C.
  3. Parallel flow with the same end temperatures would need hot inlet against cold inlet = 90 − 20 = 70 °C and hot outlet against cold outlet = 40 − 60 = −20 °C. A negative difference means heat would flow the wrong way at the outlet end: parallel flow cannot reach this duty. Counterflow can.
  4. Area at an assumed U of 800 W per m² per °C (illustrative, to be measured): A = 10,500 ÷ (800 × 24.7) = 0.53 m².
  5. For tube of 25 mm outer diameter, area per metre = π × 0.025 = 0.0785 m². Length = 0.53 ÷ 0.0785 = 6.8 m of tube, wound as a coil or doubled back.

With a smaller mean temperature difference the exchanger must be proportionally larger: halving the difference doubles the area.

Common Mistakes

  • Flowing the streams the same way when a large temperature approach is needed.
  • Fouling: scale and sludge raise resistance; plan for cleaning.
  • Letting an acid cooler leak into the cooling water, which is a hazard for sulfuric acid in particular (large heat and corrosive mist).
  • Using brass or copper with ammonia.
  • Forgetting to vent air from the coolant side.

Safety

Hot fluids, steam and cross-leaks

A leak between streams can bring a hot or corrosive fluid into cooling water that someone then uses. Pressure-test, put the higher pressure on the side where a leak is harmless, and never route cooling water that serves a reactor to a drinking source. Hot pipework burns: insulate or guard it. See Safety and Waste.

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