Double Pipe Heat Exchanger Annulus Calculator
Model hairpin double-pipe heat exchangers: calculate annular hydraulic and equivalent diameters, Dittus-Boelter heat transfer coefficients, and annular pressure drop.
Pipe Sizing & Hairpin Dimensions
Annular Fluid Properties
Annular Pressure Drop & Velocity
Hydraulic Dimensions
Hairpin Design Suitability
Recommended Tools & Equipment
Tested hardware and components for high reliability
Frequently Asked Questions
What is a double-pipe (hairpin) heat exchanger and when is it preferred?
A double-pipe heat exchanger consists of one pipe positioned concentrically inside another. It is ideal for small heat duties (surface areas under 200 sq ft), high-pressure services (where a thick shell would be cost-prohibitive), and processes with extreme temperature crosses requiring true counter-current flow.
Why are there two different diameters (Dh and De) for the annulus?
The hydraulic diameter (Dh = Di2 - Do1) is based on the wetted perimeter that causes fluid friction (both inner tube OD and outer pipe ID). The equivalent diameter (De) is based only on the heat-transfer surface area through which thermal energy conducts (the inner pipe OD).
How does annular velocity affect heat transfer versus pumping power?
Heat transfer coefficient scales roughly with velocity to the 0.8 power (ho ~ V^0.8), while pressure drop scales with velocity squared (ΔP ~ V^2). Narrowing the annulus boosts heat transfer but rapidly multiplies pumping pressure drop.
When are longitudinally finned inner tubes used in hairpin exchangers?
When the fluid flowing in the annulus has a much lower heat transfer coefficient than the fluid inside the inner tube (e.g. heavy fuel oil or viscous hydrocarbon in the annulus, high-pressure steam in the tube), longitudinal fins welded along the outer surface of the inner pipe equalize thermal resistance.