Installation of a heat exchanger
How to correctly install a counterflow plate heat exchanger in an air handling unit?
(General recommendations for unit design, new EN 308)
What are the main performance parameters of the heat exchanger?
These are their temperature (humidity) efficiency, pressure drop and leakage at a given flow rate, temperature, humidity and pressure in the supply and exhaust duct. These parameters are measured in laboratory conditions according to the revised EN 308:2022 standard in an ideal test box.
The box has ideal thermal insulation and also ideal flow routing to and from the heat exchanger. These values are then processed into the rSelect design software, which is the basis for the designers of the ventilation units.
It is important to note that the specific incorporation of a heat exchanger into a ventilation unit often results in a deterioration of these performance parameters, where the causes of these deteriorations are deviations of the real ventilation unit from the ideal test box.
What is required for good recuperator performance?
Its own tightness, but also the tightness of its installation in the ventilation unit. Leakages between the supply and extract air as well as with respect to the surroundings can distort the real measurement results. The new standard EN308:2022 therefore introduces a so-called gross thermal efficiency, which is measured and which is corrected (worsened) by the measured leakage to a so-called net thermal efficiency, which expresses the efficiency of the heat exchanger itself unaffected by internal leakage. It is therefore desirable to seal the heat exchanger perfectly in the ventilation unit enclosure.
Other assumptions
The direction of flow at the inlet to the heat exchanger should be as perpendicular as possible to the inlet face of the heat exchanger. If the inlet direction is too oblique, the pressure drop at the inlet will increase and the flow in the heat exchanger will change with less active heat exchange areas in the airflow plenum.
The distribution of air velocities over the inlet area of the heat exchanger should be as uniform as possible so that all heat exchange areas are as active as possible. This can be achieved by a straight gradual expansion of the air duct without curves towards the inlet of the recuperator without local anomalies and asymmetries of the air flow. The maximum angle of expansion of the inlet duct from the air inlet to the heat exchanger should be up to 15°.
Effect of pressure drop
The heat exchanger plates are very thin for good heat transfer and so are never perfectly rigid and are subject to pressure differences between inlet and outlet. The pressure differences vary according to the location in the heat exchanger and cause different deformations of the plates at different points in the heat exchanger. Thus, the heat exchanger may locally ‘inflate’ or ‘deflate’ the inlet against the outlet and vice versa. Pressure differences in the heat exchanger are caused not only by the pressure loss itself, but also and above all by the pressure ratio between the supply and extract branches inside the ventilation unit.
Position of the heat exchanger
It is a good idea to always have the plates vertically in the direction of gravity to reliably drain condensate from the heat exchanger, or to keep the heat exchanger defrost cycle as short as possible in winter conditions. Ideally, the entire heat exchanger should be tilted towards the air outlet so that the condensate leaves in the direction of flow as quickly as possible and does not linger inside the heat exchanger and block the airflow.
Temperature sensors and thermal insulation
It is also worth remembering that the temperature distribution at the outlet of the recuperator is significantly uneven by the principle of the heat exchanger function, making it difficult to obtain an average outlet temperature. A single temperature sensor is not sufficient here, as the position of the sensor greatly influences the result. It is necessary to use multiple temperature sensors or to mix the air perfectly throughout the entire cross-section of the duct with perfect external thermal insulation.
The thermal insulation of the unit is also important, as if it is undersized, there may be leaks or heat gains outside the unit, which may distort the performance of the heat exchanger.
Counterflow arrangement
It is also always necessary to control the direction of flow in the heat exchanger so that the supply and extract streams are always in a counter flow arrangement. A parallel flow arrangement has a significant effect on reducing the heat recovery efficiency by up to half.