Plate heat exchangers: function, types, sizing and selection

A plate heat exchanger transfers heat between two liquids, or between a liquid and steam, without mixing the media. Compared with a classic shell-and-tube unit it delivers far higher capacity in a smaller footprint, because a stack of thin, profiled plates packs a very large heat transfer area into a minimal volume. This guide explains how plate heat exchangers work, the main construction types, the basics of sizing, typical applications, and closes with a practical selection checklist.

How does a plate heat exchanger work?

At its core a plate heat exchanger consists of a pack of pressed metal plates arranged so that two separate flow channels form in alternation: one for the hot medium, one for the cold. The two media flow past each other in adjacent channels, separated only by the thin plate wall. Heat passes from the warmer to the colder medium without any direct contact between the fluids.

The plate pack is clamped between frame and pressure plate; every second channel carries the hot or cold medium.

Three effects make this design so effective. First, the plate wall is very thin, often well below one millimetre, so the material’s thermal resistance stays low. Second, the embossed herringbone or wave profile forces the flow into turbulence even at low velocities. Turbulence thins the insulating boundary layer at the wall and raises the heat transfer. Third, the plate stack creates a very large area per unit of volume.

The interplay of these effects determines the overall heat transfer coefficient (k). It describes how much heat is transferred per square metre of area and per kelvin of temperature difference. Plate units reach a multiple of what a shell-and-tube exchanger achieves under comparable conditions.

The flow arrangement is decisive for efficiency. In counterflow the hot and cold media move in opposite directions. This keeps the temperature difference as uniform as possible across the entire plate length, and the driving mean temperature difference is larger than in parallel flow. Plate heat exchangers are therefore almost always operated in counterflow, because it reduces the required area.

Which construction types exist?

Gasketed plate heat exchangers

In a gasketed unit the plates are clamped between a frame and a pressure plate and sealed by inserted elastomer gaskets. The big advantage is flexibility: plates can be added, replaced and fully opened for cleaning. The gaskets, however, limit temperature and pressure. This type dominates building services and processes that require frequent cleaning. A side-by-side look at the construction types is available under construction types compared.

Welded plate heat exchangers

Welded units are fully welded and need no elastomer gaskets at all. This makes them more resistant to pressure and temperature, suited to aggressive media, and low in maintenance. They cannot be opened, however, and only allow limited mechanical cleaning. They are typical in refrigeration, heat pumps and process applications with demanding media.

Double wall plate heat exchangers

In the double wall design, each heat transfer surface consists of two separately pressed plates that are welded together only at the port openings. An open gap remains between the two layers: if a plate develops a leak, the medium drips visibly to the outside instead of mixing with the second circuit. This design is the first choice wherever mixing of the media must never occur, for instance in domestic hot water heating or food and pharmaceutical processes. Its heat transfer coefficient is lower than that of a single-wall plate, so the design needs a larger area.

How is a plate heat exchanger sized?

Sizing answers the question of how much plate area a given task requires. The starting point is the heat duty Q that has to be transferred. It results from the mass flow rate, the specific heat capacity and the temperature spread, that is the temperature difference between the inlet and outlet of a medium.

From this duty, the k value and the logarithmic mean temperature difference, the required area follows. A larger temperature spread reduces the required flow rate and with it the pressure drop, but it calls for careful matching of area and plate profile. How the governing equation Q = k * A * dt_log works in detail, and which orders of magnitude the k value takes, is shown in the in-depth guide sizing a plate heat exchanger.

Besides the thermal duty, the permissible pressure drop and the nominal pressure belong to every sizing exercise. The nominal pressure sets the pressure rating the unit is approved for; the maximum operating pressure must not exceed it at the relevant temperature.

Where are plate heat exchangers used?

Plate heat exchangers cover a broad field of use. In building services and heating they separate hydraulic circuits, prepare domestic hot water and connect district heating. In refrigeration and air conditioning they serve as evaporators, condensers and intermediate-circuit exchangers. In industrial processes they handle cooling, heating, heat recovery and the tempering of process media.

Which construction type and sizing suit your task depends strongly on the medium, the temperatures and the operating pressure. An overview of concrete use cases is offered on the applications page.

What should you look for when selecting?

Before selecting a plate heat exchanger, the following points should be clear:

Media and fluid data: which liquids or vapours, with what specific heat capacity, viscosity and fouling tendency?
Duty and temperatures: what heat duty is required, and which inlet and outlet temperatures are demanded on both sides?
Spread and flow rate: how large is the temperature spread, and which flow rate results from it?
Pressure drop: what pressure drop is permissible on each side without breaking the pump design?
Nominal pressure and temperature limit: which pressure rating and maximum temperature must the unit withstand permanently?
Type and maintenance: is opening and cleaning required (gasketed), or do pressure resistance and low maintenance count (welded)?
Material: are the plate and gasket materials compatible with the media?

Answering these points up front leads to a robust sizing faster. The next step - from the key data to a concrete unit size - is taken in the guide sizing a plate heat exchanger.