Heat Transfer Principles

Rotor matrix
Rotary Heat Exchangers

A rotating heat exchanger or rotary heat exchanger, also known as a thermal wheel, enthalpy wheel, and heat recovery wheel, 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.

Heatex Model E Drawing

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.

Rotor airflowThe wheel is built up by a matrix which consists of two foils, one flat and one corrugated, together they create channels for the air to pass through. The well height is defined as flat foil thickness plus total wave height.

Well height definitionWith just a few well heights a wide range of efficiency and pressure drop requirements can be met. The diameter of the wheel can be modified to either increase efficiency and/or airflow rate or lower the pressure drop.

The selected material of the matrix depends on the application requirements. For heat transfer only, we recommend plain aluminum. In corrosive environments, the matrix is usually protected by a layer of epoxy. Commonly used materials for humidity transfer are silica gel and 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.

Plate heat exchangers
Plate Heat Exchangers

A plate heat exchanger works in a rather straight forward process. Two neighboring 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 type of plate air to air heat exchangers are crossflow and counter flow. In a cross flow heat exchanger the cold and the warm air flow perpendicular to each other.
Cross flow airflowIn counter flow exchangers, the two airstreams flow in opposite direction to one another. The longer the airstreams flow next to each other the better efficiency, which generally makes counter flow more temperature efficient than cross flow exchangers.
Counter flow air configuration

However, the cross flow exchangers can be placed in two-step configurations which give results equivalent to those of the counter flows.

Twin cross configurations

Depending on the application, rotary and plate heat exchangers have their own advantages.

Rotary Heat Exchangers Plate Heat Exchangers
Low risk of freezing Very low leakage rate
Humidity transfer No moving parts
Leaving a small footprint in air direction Minimal service and long lifetime
High flow rate capacities Rapid and easy cleaning
Applications

Air-to-air heat exchangers are used in a wide variety of applications.

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