What is a heat exchanger?

Heat exchangers transfer energy in the form of heat from one medium (e.g, a gas or liquid) to another. They are found in industry, in motor vehicles and in the home, including in refrigerators and storage heaters. Heat exchangers that are specifically designed to recover heat from waste, exhaust gases and liquids and then reuse it by transferring it directly to another medium are known as recuperators. Recair produces the core for air-to-air heat recovery ventilators.

How does it work?

In an air-to-air recuperative heat exchanger, the incoming and outgoing airflows are separated by a solid barrier (i.e, a duct wall). Providing that there is a difference in temperature between the two airflows, heat from the warmer airflow will be transferred through the barrier to the cooler airflow in accordance with the second law of thermodynamics. This states that heat will move from a hotter region to a colder region. In this way, the heat from the warm, stale air leaving the building is transferred to the cool, fresh air entering the building or vice versa.

How do you know when a heat exchanger is performing well?

What most people want to be sure about when installing a heat-recovery ventilation system is that they won’t feel cold or hot draughts: i.e, the air coming out of the exchanger into the house should be more or less the same temperature as the air already in the house. But people could demand more. Just because a heat exchanger is delivering air at more or less the right temperature, that does not mean that it is working 100% efficiently. If it is not recovering as much heat as possible, it is not saving you as much money as it could! Over the lifetime of a heat exchanger, even the smallest improvements in efficiency can result in significant savings on your energy bills. That is why Recair is continuously striving to make its products maximally efficient, so that all heat is transferred from the exhaust air to the incoming air, with the waste air leaving the heat exchanger at the same temperature as the air outside, and the fresh air entering the room at the same temperature as the air inside.

How does Recair achieve maximum performance?

To achieve this ideal, we have worked with our Research & Development department to understand what conditions a heat exchanger needs to meet in order to ensure the maximum possible thermal effectiveness for both airflows. Through scientific testing, we have found that a maximally effective heat exchanger needs to meet certain criteria.


The criteria for maximum
How the Recair matches up
The two airflows should be in a pure counter-flow arrangement (i.e, with the flows moving in opposite directions). The airflows in all Recair’s cores are in a pure counter-flow arrangement.
The mass flows of the air should be in perfect balance (i.e., the mass of air passing in each direction over a given period of time should be the same). Recair advises manufacturers of HRV units to ensure that this condition is met so that Recair cores installed in their units can deliver their superior performance.
There should be no leakage, either between the airflows, or between either of the airflows and the exterior of the heat exchanger. Recair cores show negligible leakage (average <0.5%), making them among the best on the market.
To ensure that the two airflows are in as much contact as possible (through the duct walls), the surface area available for heat exchange should be as large as possible. Thanks to their unique triangular-duct design, Recair cores have a larger heat exchanging surface area than other products of comparable size (heat exchanging capacitance >50% higher than square ducts at same pressure loss).
For maximum heat transfer, the distribution of the airflows should be uniform. The small triangular ducts of Recair cores ensure counter-flow with uniform flow distribution.
All heat should be conveyed directly through the duct wall: no heat should be conducted along the length of the duct wall. To limit heat conduction, the duct walls in Recair cores are made of polystyrene, a material with low heat conductivity.
To ensure easy use, the heat exchanger should be as compact as possible. Recair cores offer the best ratio between heat-exchanging surface and intrinsic volume.
To ensure economic viability, the costs of materials, manufacture, installation and use should be kept to a minimum. Recair’s products are cost-effective, need little material, are light to transport and are easy to handle.
Too avoid draught the temperature difference between the incoming fresh-air and the room-air may not exceed 1,5º C. At a temperature difference of 30º C between in- and outside only the Recair core reaches enough effectiveness (>95% including fan heat) to reach this goal.

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