Knowledge · Overview

Air-to-air heat exchangers: how they work, designs, sizing

How gas-to-gas plate heat exchangers recover heat from extract air: the principle, cross-flow and enthalpy, materials, and the route to a sizing.

An air-to-air heat exchanger transfers heat from a warm air or gas stream to a cold one - without the two streams mixing. In ventilation terms: the warm exhaust air from the building or process preheats the cold outdoor air before both go their separate ways. This page explains the principle, the designs, and what a sound sizing needs.

How does a gas-to-gas plate heat exchanger work?

At its heart is a stack of thin plates. Between the plates the flow paths alternate: level by level, exhaust air and outdoor air flow in turn. The heat travels through the plate wall from the warm to the cold stream; the streams themselves stay completely separate. There are no moving parts, no drive and - unlike a rotary heat exchanger - no rotating storage mass that could carry extract air over into the supply air.

What counts for the transfer is the area of the plate stack, the spacing of the plates and how the streams are led relative to each other. A larger area means more recovery, but also more volume and more flow resistance

• sizing is always a trade-off between temperature recovery and pressure drop.

Which designs are available?

Two designs are held in our configurator:

• Cross-flow plate heat exchanger: the streams cross at right angles. The classic for heat recovery in ventilation and process plants; robust, easy to clean, available in many edge lengths.
• Enthalpy plate heat exchanger: instead of a solid plate, a vapour-permeable membrane separates the streams. Moisture is transferred along with the heat - which keeps the supply air less dry in winter.

Which design suits which case is covered in the comparison cross-flow or enthalpy?

Which materials can be chosen?

The catalogue covers four plate materials: aluminium as standard, coated aluminium for corrosive condensate, stainless steel for high temperatures and aggressive atmospheres, plastic for particularly corrosive applications. The choice depends on the composition of the extract air and on whether and how much condensate occurs - more on that in the article condensate and the risk of freezing.

What does a sound sizing need?

Four values per side are enough to start: volume flow, its reference (standard or operating conditions), temperature and relative humidity. To these comes the most important boundary condition: the permissible pressure drop per side, because it limits how densely and deeply the plate stack may be packed. Optionally, the installation altitude (air pressure), space constraints and preferences for material and design narrow the search further.

From these values the sizing calculates the air conditions (density, dew point, enthalpy), checks the catalogue across all edge lengths from 300 to 2400 mm and works out proposals with temperature recovery, pressure drop and notes on condensate and freezing. Our team reviews them and comes back to you with the result. Volume flows from a few hundred up to 100,000 m³/h are covered; large air volumes are resolved across several modules.

Where are the limits?

A plate heat exchanger only recovers heat while a driving temperature difference exists between the streams - it does not replace a heating coil, it relieves it. With very humid exhaust air and hard frost the condensate can freeze inside the unit; a defrost or bypass strategy is then required. And every additional bit of recovery costs pressure drop, which means fan power. These are exactly the trade-offs the sizing makes visible rather than hiding.

And not every heat transfer duty is an air-to-air one: for liquid-to-liquid or steam, AWS-Technik GmbH supplies gasketed and welded plate heat exchangers as well as shell and tube units under its sister brands - talk to us if your case belongs there.