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FAHRenergy heat recovery ventilator HRV, Scandinavian Design

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Cross Flow Heat Recovery Explained
The cold and warm air, flow at a right (90 degrees) angle to each other.
Due to this a cross flow heat exchanger is of quadratic shape.
A Cross Flow heat exchanger has one advantage: The heat exchanger
is easier to manufacture and thus cheaper. 
The main disadvantage is the physical efficiency limitation.
It is difficult to obtain 50% efficiency. Two or more cross heat
exchangers in series are needed to improve the efficiency to 75%.
FAHRenergy Flow Breaker©
To reduce the size of the system modern flow methods may be
utilized to break up the natural, laminar flow. FAHRenergy's Flow
Breaker creates a duct system which appears much larger to the
air flow than it actually is.
The FAHRenergy Flow Breaker system reduces the size of the
system by a factor of two.
Half the volume = half the weight = half the installation cost

Counter Flow & Cross Flow

Below is shown an installed FAHRenergy Counter Flow Room unit.
By following the explanation and the arrows a good understanding
of the in/out flow of the fresh/stale air may be obtained.

Counter flow heat recovery for tenant and room

Counter Flow Heat Recovery Explained
As the cold air travels towards the warm end of the system, the warm air travels towards the cold end of the system. The cold air will become progressively warmer whereas the warm air will become progressively cooler as the heat (energy) is exchanged. If the system is sufficiently long, the entire heat will be exchanged = 100% efficiency.

The fresh, cold air will be heated to the temperature of the warm, stale air before it reaches the warm side of the system. At the same time the warm stale air is cooled to the temperature of the cold, fresh air, before it leaves the system.