Three levels of arrangement

When a plate heat exchanger does not meet its duty, the instinct is to add plates. Often the better answer is to change how the flow is arranged, because arrangement affects the three things that actually decide whether a unit works — the mean temperature difference, the pressure drop, and how evenly the flow is shared between channels.

"Flow arrangement" means three different things at three different scales, and they are frequently confused:

• Pass arrangement inside the frame — how many times the stream traverses the pack, set by divider plates and blind channels.
• Unit-level arrangement — whether several exchangers are piped in series or in parallel.
• Connection layout — which corner each port occupies, and therefore how the flow enters and leaves the pack.

Single pass, counter-current: the default

A single-pass plate pack is close to ideal counter-current flow. Hot and cold streams enter at opposite ends and travel in opposite directions through alternate channels, so the terminal temperature differences at the two ends can be adjusted almost independently by the plate count. This is the arrangement that makes close-approach duty possible in a plate unit, and it should be the starting point for every selection.

It is worth being explicit about why it is the default: a single pass gives the highest effective LMTD for a given set of terminal temperatures, the lowest pressure drop per unit of area, and the most uniform flow distribution. Almost every deviation from it trades one of those three away for a benefit that has to be earned.

Series and parallel at unit level

Two identical units can be piped two ways, and the choice is decided by which constraint is binding.

In series, the whole flow passes through the first unit and then the second, on both sides. Total area doubles, total pressure drop roughly doubles or more, and the effective temperature profile is stretched across both units. Series is how you reach a closer approach than a single frame can deliver, or how you handle a duty where the plate count would otherwise produce an unmanageably long frame. It is also how you add capacity to an existing installation without changing the piping footprint.

In parallel, the flow splits between the units. Each unit sees half the flow and delivers half the duty at roughly the same approach; total pressure drop is essentially that of one unit. Parallel is the answer when the duty is large and the pressure drop budget is the constraint, or when redundancy is required so one unit can be isolated for cleaning while the plant keeps running.

The mistake is to assume that series always improves performance. It improves the approach, at the cost of pressure drop. If the pump cannot deliver the additional head, the series installation will simply run at reduced flow, and the duty will be missed in a way that looks like a fouling problem.

What series does to the numbers

Consider a duty that needs 30 m2. One frame of 30 m2 may be a tall unit with a long plate pack. Two frames of 15 m2 in series give the same area but a different hydraulic picture: the velocity in each channel is unchanged (same flow, same channel count per unit), the pressure drop is roughly doubled because the fluid now passes through two packs, and the temperature profile is spread over four terminal differences instead of two.

For a close-approach duty this last effect is the point. Spreading the duty across two units lets each unit operate with a comfortable terminal difference while the pair as a whole achieves the close approach. It also introduces an intermediate temperature that must be checked — the hot stream leaves unit one at a temperature that becomes the inlet to unit two, and if that intermediate point falls where one of the fluids becomes unusually viscous or begins to deposit, the design has a problem the single unit would not have had.

Asymmetric arrangements are common in practice: series on one side, parallel on the other. District heating substations often pipe the primary in series through two units while the secondary is parallel, because the primary has the pressure drop available and the secondary does not. These arrangements work, but they must be drawn and checked rather than assumed.

Multi-pass within one frame

A multi-pass pack uses divider plates to turn the flow around inside the frame, so a stream traverses the pack more than once. In a shell-and-tube exchanger multi-pass is routine because it raises velocity in the tubes. In a plate unit it is far less common, for a straightforward reason: the single-pass pack is already counter-current, so a second pass does not improve the mean temperature difference — it usually degrades it, because the arrangement becomes partly co-current and the LMTD correction factor falls below unity.

Multi-pass is used when the flow rate on one side is very low relative to the other — for example a small flow that would otherwise trickle through a wide parallel set of channels without reaching a useful velocity. In that specific case, forcing the stream through fewer channels at higher velocity can raise the coefficient enough to justify the loss in effective LMTD. It is a targeted fix for a flow-rate mismatch, not a general performance upgrade.

Port and connection layout

The four ports of a plate unit occupy the corners of the plate. Which corner carries which stream determines whether the pack behaves as a clean counter-current arrangement or as something closer to cross-flow, and it also determines how uniformly the flow distributes across the width of the plates.

In a Z arrangement the two connections for a given side sit at diagonally opposite corners, so the stream enters at one end and leaves at the other. This is the standard counter-current layout and gives the most even distribution across the pack.

In a U arrangement both connections for a side are at the same end. It simplifies piping in tight skids and is used where the pipe route matters more than the last few percent of performance. The cost is that the flow has to turn within the pack, distribution across the plate width is less even, and the effective mean temperature difference is slightly lower. For a large-approach duty the difference is negligible; for a close-approach duty it is not.

There are also layouts where the two streams flow in the same direction — co-current — which are chosen deliberately when the duty requires a controlled outlet temperature or when one stream is heat-sensitive. Co-current flow caps the cold outlet at the hot outlet temperature by construction, which is occasionally exactly what the process wants.

Matching arrangement to duty

Common mistakes

Sizing on area alone. Two arrangements can have identical area and completely different performance. The selection sheet should state the arrangement, not just the square metres.

Putting units in series to fix a pressure drop problem. Series raises pressure drop. If the complaint was already "too much ΔP", series makes it worse.

Assuming the piping drawing matches the thermal model. A unit selected as single-pass counter-current and installed with U connections is not the unit that was modelled. Check the port layout on the drawing against the selection.

Ignoring the intermediate point. In a series pair, the temperature between the units is a real operating condition. If a stream crosses a viscosity or deposition threshold there, the second unit will not behave as calculated.

Common questions

Can I put two different plate patterns in series? Yes, and it is sometimes the right answer — for example a wide-gap unit first to handle solids, followed by a standard unit for the close approach. The two units should then be selected as a pair, not independently.

Does parallel piping halve the pressure drop? It halves the flow through each unit, and since pressure drop falls roughly with the square of flow, each unit sees about a quarter of the single-unit pressure drop. The header and valve losses are added on top, and they are often what actually limits the arrangement.

Why is my multi-pass unit not reaching the duty? Most often because the selection used a single-pass LMTD without correcting for the pass arrangement. Recomputing with the correct correction factor usually explains the shortfall.

Is a series pair harder to control? Somewhat. There are two units to balance rather than one, and the intermediate temperature gives the control loop an extra variable. For a steady process duty it is manageable; for a fast-cycling duty, consider a single larger frame first.

Arrangement before area

Arrangement is the cheapest performance lever available, and the easiest to get wrong silently — because a badly arranged installation still looks correct on the datasheet. Decide the pass arrangement and the connection layout before the plate count, draw the piping as it will actually be built, and check the thermal model against that drawing.

Reapter supplies gasketed plate heat exchangers and plate-and-shell units with the connection layout matched to the installation, and can supply replacement plates for existing frames where the original arrangement is fixed. Send us the duty, the pressure drop available on each side and a sketch of the pipe route, and we will propose the arrangement that meets the duty without asking the pump for head it does not have.