Sizing a shell and tube heat exchanger: energy balance, LMTD and margin

Sizing means finding, for a specific thermal duty, the size that transfers the required heat, stays within the permissible pressure drop and remains inside the operating limits. This guide shows the calculation steps - the same ones our configurator performs on the server. Where the calculation model ends, our team does the sizing, for example with a pressure-drop limit on an oil, glycol or steam side.

How do you set up the energy balance?

Each side without a phase change obeys the same relation:

Q = m · cp · dT

where Q is the heat duty, m the mass flow, cp the specific heat capacity and dT the temperature change between inlet and outlet. Because the duty on both sides has to be equal, it is enough to state some of the values - the rest follows from the balance. Water carries around 4.19 kJ/(kg·K), more than twice as much heat per kilogram and kelvin as a typical oil at about 2 kJ/(kg·K); the oil side therefore needs correspondingly more mass flow or a wider temperature change for the same duty. Where a medium condenses, the enthalpy difference between steam at the inlet and condensate at the outlet takes the place of cp · dT.

What does the log mean temperature difference tell you?

The driving temperature difference changes along the length of the unit. For the area calculation what counts is its logarithmic mean (LMTD) of the differences at both ends. The smaller the LMTD, the more area the same duty needs - temperatures that converge closely make units grow quickly. Where the temperatures cross, counterflow arrangement helps; where the gap falls to a few kelvin, the operating point needs a critical review.

How do you get from area to size?

The required area follows from Q = U · A · LMTD, where the overall heat transfer coefficient U depends on both boundary layers and on the tube wall - and therefore strongly on the media and mass flows. Instead of estimating U from tables, our sizing checks every size in the catalogue against the operating point using measured transfer characteristics and discards whatever violates the duty, the pressure drop or the temperature limit of the range; operating pressure and material are fixed in the quote. What remains are concrete proposals with a stated margin, calculated for the number of units in parallel that you set in the configurator (one unit if left blank).

What role does pressure drop play?

For liquids, more flow improves heat transfer but also raises the pressure drop. A permissible pressure drop per side therefore belongs in every sound enquiry: it helps decide which sizes qualify, and it prevents a unit that fits thermally but overwhelms the pump. The configurator checks this limit automatically on a water side; with a pressure-drop limit on an oil, glycol or steam side, our team does the sizing.