Heat Transfer Principles

Air-to-Air Heat Exchangers and Application Areas

Air-to-air heat exchangers are used both in heat and energy recovery in ventilation applications and in closed loop cooling solutions for thermal management. There are two categories of air-to-air heat exchangers, rotary and plate heat exchangers.

Rotor matrix
Rotary Heat Exchangers

A rotating heat exchanger is a type of energy recovery heat exchanger positioned within the supply and exhaust air streams of an air-handling system.

Heatex rotary heat exchangers consist of an aluminum wheel supported by a casing of galvanized steel. The wheel is rotated by a small electric motor and belt drive system. In one half of the rotation, the exhaust air from the inside space flows through the matrix. Its heat is stored in the matrix and in the other half of the rotation, it is transferred to the fresh supply air coming from the outside.

The wheel is built up by a matrix that consists of two foils, one flat and one corrugated, together they create channels for the air to pass through. The size of the channel is called well height.

Different well heights and diameters of the wheel give different efficiencies, pressure drops and airflow rates.

Airflow and Well Height

In corrosive environments, the matrix is usually protected by a layer of epoxy. For humidity transfer, the matrix is either coated partially or entirely with silica gel or molecular sieve.

Leakage between the exhaust and supply air is reduced by sealing and in some cases also by adding a purge sector. However, some leakage cannot be fully prevented.

Rotary heat exchangers have low freezing risk as to the wheels by definition defrost themselves while rotating. Freezing is only an issue at very low temperatures.

See all our rotary heat exchangers.

Plate heat exchangers
Plate Heat Exchangers

A plate heat exchanger is a type of energy recovery heat exchanger positioned within the supply and exhaust air streams of an air-handling system.

A plate heat exchanger works in a rather straight forward process. Two neighboring aluminum plates create channels for the air to pass through. The supply air passes on one side of the plate and the exhaust air on the other side. The heat in the exhaust air is transferred through the plate from the warmer air to the colder air.

A very important parameter for the performance of a plate heat exchanger is the spacing between the plates. A narrow channel leads to high-pressure drop but also to high efficiency. The later means that more heat is transferred to the cold side. If a lower pressure drop is required, it is better to use a higher channel spacing. The trade-off is lower efficiency.

Usually, the exhaust air is contaminated with humidity and pollutants, but with a plate heat exchanger, airflows never mix, leaving the supply air fresh and clean. To avoid leakage and contamination, Heatex plate heat exchangers are constructed with a double sealing concept. This means both gluing and a mechanical fold.

To protect the aluminum from harmful substances or corrosive environments its recommended to coat the plates with epoxy and paint both end-plates and profiles.

The two most common types of plate air to air heat exchangers are cross flow and counter flow heat exchangers. In a cross flow heat exchanger the cold and the warm air flow perpendicular to each other. In a counter flow exchangers, the two airstreams flow in opposite direction to one another.

Plate Heat Exchanger

See our cross flow and counter flow heat exchangers.

Ventilation Applications

All buildings require a ventilation system to maintain a good indoor air quality which enables man and machine to function efficiently.

Indoor air is affected by breathing people, which increases the concentration of carbon dioxide and machines which increases the number of particles. To ensure a healthy climate inside the building, the contaminated air must be replaced. Thus, the buildings need to be equipped with a device to replace the contaminated air with new and fresh air.

To improve overall building operating cost, a ventilation system often contains a heat recovery system (heat exchanger) which transfers heat and/or humidity between the supply and exhaust air.

Besides commercial and residential ventilation such systems also include industrial applications where e.g. high-temperature process air is ventilated from the system and is used to preheat new fresh air.

The most common application is ventilation for comfort applications i.e., air exchange in spaces populated by people. In northern Europe, its usually referred to as “heat recovery” where cold fresh air is preheated before it enters the building. In more hot and humid areas, it usually means precooling and dehumidifying fresh air before it enters the building. It all depends on the region, climate zone, or business you are in. The common factor is heat and humidity recovery, but they differ in requirements when it comes to capacity, efficiency, and material needs.

Thermal Management Applications

All electronic devices from tiny circuitry to large generators generate excess heat and thus require cooling to prevent overheating. Additionally, they need protection from moisture, dust, and other contaminants to improve reliability and prevent premature failure.

Effective, reliable and environmentally friendly thermal management solutions that offer a long service life, reduced operating costs, and minimal downtime are essential to successful system design.

Air-to-air heat exchangers are easy to implement, reliable, cost-efficient, and environmentally friendly and an excellent alternative to other more complicated cooling solutions.

Heatex offers complete customized air-to-air cooling systems for small electronic enclosures and cabinets used in photovoltaic systems to large nacelles used to house wind turbines.

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