Thermal & Hydraulic Design
Thermal selection must deliver the temperature programme within the pressure-drop limits on both circuits.
Thermal & hydraulic design
Temperature programme
Heat duty follows flow rate and enthalpy change. Counter-current arrangements can support a close temperature approach; the actual flow scheme determines the effective temperature driving force and LMTD correction.
Area versus pressure drop
Surface area, overall heat-transfer coefficient and temperature difference are balanced with channel velocity and allowable pressure drop. Fouling allowance, thermal margin and part-load conditions are considered during selection.
What determines the overall coefficient
Q = U × A × F × ΔTlm
The overall coefficient combines resistance on both fluid sides, conduction through the plate and deposits on each surface. Area cannot compensate for an unsuitable flow distribution or an unrealistic fouling allowance. The LMTD correction follows the real pass arrangement; counter-current and co-current layouts produce different temperature profiles.
Establish the heat balance
For a single-phase stream, duty follows mass flow and the enthalpy change between inlet and outlet. When heat capacity varies with temperature, use properties appropriate to the range. For condensation or boiling, include latent duty and any sensible heating, cooling, superheating or subcooling. Check that the two specified streams form a consistent heat balance.
Allocate the pressure-drop budget
The available pressure drop must cover connections, distribution regions and heat-transfer channels. Increasing velocity may improve the film coefficient but also increases hydraulic resistance. A nozzle or pass arrangement that is acceptable for liquid service may not suit a low-density gas or a changing two-phase volume.
Review margins and off-design conditions
Thermal margin and fouling allowance should follow the process specification rather than an arbitrary percentage. Adding plates can reduce velocity and change the behaviour at low load. Evaluate start-up, turndown, control range and the expected cleaning interval alongside the nominal duty.
Selection deliverables
Agree the thermal duty, pressure loss for each circuit, flow arrangement, materials and design conditions before fixing the model. Final equipment ratings and dimensions belong on the approved project datasheet and drawing, rather than being inferred from the connection diameter.
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