Heat exchanger pressure drop is the difference between inlet and outlet pressure as fluid moves through an exchanger. Tubes, baffles, nozzles, bends, and friction all resist flow. The goal is to meet the heat duty without exceeding the pressure available from the system.
Pressure drop in heat exchangers must be checked separately for both streams. What is acceptable for cooling water may be unsuitable for oil, gas, or steam.
What Is Heat Exchanger Pressure Drop—and Is It Good or Bad?
At its simplest:
ΔP = inlet pressure − outlet pressure
However, three values should not be confused:
- Allowable pressure drop: the maximum loss the system can accept on one side of the exchanger.
- Calculated pressure drop: the loss predicted for a proposed design at stated operating conditions.
- Measured pressure drop: the difference observed across installed equipment.
A higher pressure drop does not guarantee better heat transfer. More velocity may improve turbulence, but also raises pumping demand, erosion, and vibration risk. Low pressure drop may be intentional or accompany low velocity, poor distribution, or insufficient flow.
The target is a design that delivers the required thermal performance within the allowable pressure drop and other operating limits.
What Determines Pressure Drop on the Shell Side and Tube Side?
The two flow paths create resistance differently. Tube-side pressure drop depends on tube diameter, length, roughness, passes, return heads, nozzles, flow, and fluid properties. More passes or less flow area usually add resistance. Tube inserts can improve mixing but add pressure loss; see heat exchanger turbulators.
Shell-side pressure drop depends on shell diameter, tube layout and pitch, baffle type, cut and spacing, nozzles, and leakage or bypass paths. Closer baffle spacing can strengthen crossflow, but normally increases resistance and may affect vibration. See heat exchanger baffles.
Fluid properties affect both sides. Temperature can change viscosity, especially for oil. Higher flow normally raises velocity and pressure loss. Gas and two-phase services need additional review because pressure changes may also affect density or phase behavior.
Fluid allocation must also balance pressure, fouling, corrosion, cleaning, and phase. See shell side vs tube side fluid allocation.
Why Does Pressure Drop Change During Operation?
Compare an exchanger with its design data or earlier readings at similar flow and temperature. A single reading can mislead.
A gradual rise at similar flow may indicate scale, deposits, plugged tubes, or debris in baffle windows. A blocked strainer, partly closed valve, or restricted pipe can look similar, so check the system before blaming the exchanger.
A sudden change may follow a flow adjustment, valve movement, instrument problem, or changed fluid properties. A low reading may result from lower flow, an open bypass, internal damage, or measurement error. It does not prove oversizing.
Record temperatures, pressures, flow rates, load, valve positions, filter condition, and recent changes before acting. See common heat exchanger problems and heat exchanger cleaning methods for further checks.
How Is Allowable Pressure Drop Used in Heat Exchanger Design?
State the allowable pressure drop for each side before finalizing the thermal and hydraulic design. The calculated result is then checked against that limit along with heat duty, outlet temperatures, velocity, fouling, cleaning, erosion, vibration, and size.
There is no universal acceptable pressure drop in a heat exchanger. A fixed psi, bar, or kPa rule can mislead because pump head, downstream requirements, fluid properties, and operating margins differ.
Reducing pressure drop may require more flow area, fewer passes, larger tubes, wider baffle spacing, another shell arrangement, or larger nozzles. Each affects cost, heat transfer, cleaning, or installation. More velocity may allow a compact unit only when pressure margin and mechanical risks are acceptable.
For an existing unit, a heat exchanger pressure drop calculation must use its geometry and operating data. Outside dimensions alone cannot confirm hydraulic or thermal performance. See the required inputs in our heat exchanger sizing guide.
Need a Heat Exchanger Designed Around Your Pressure-Drop Limit?
JED manufactures custom shell and tube heat exchangers for new and replacement projects. We can work from an approved drawing or review a configuration from confirmed operating data.
Send the media, normal and maximum flow, temperatures, pressures, allowable pressure drop for both sides, materials, installation limits, and drawings. For replacement, add current readings, dimensions, photos, maintenance history, and the reason for replacement.
FAQ About Heat Exchanger Pressure Drop
Can you review a replacement exchanger without the original pressure-drop calculation?
Yes. A drawing, datasheet, dimensions, operating data, and current readings can provide a starting point. Required duty and allowable pressure drop must still be confirmed.
Can pressure drop be changed while keeping the same connections and installation dimensions?
Sometimes. Tube size, pass arrangement, bundle layout, or baffles may offer options, but fixed nozzles and dimensions limit changes. Thermal duty, velocity, cleaning, and vibration still require review.
Should I provide normal flow rate, maximum flow rate, or both?
Provide both. Normal flow supports routine evaluation; maximum flow helps check the highest expected pressure drop and operating margin.
Does a hydrostatic pressure test verify operating pressure drop?
No. A hydrostatic test checks pressure integrity. Operating pressure drop is created by fluid flow and must be evaluated at the relevant flow rate and properties.
