Heat exchanger, an essential piece in heat recovery processes

Do you know the importance of the heat exchanger in ventilation systems? The heat exchanger is a key component in the operation of heat recovery systems and, consequently, in double flow mechanical ventilation systems. Next, we will review the implications of the Spanish regulations regarding these elements.

The CTE DB HE, Energy Saving indicates that the use of buildings by their occupants must be done with a rational use of the necessary energy. Thus, energy consumption should be reduced to sustainable limits, which will, in turn, be ensured to come from renewable sources. These guidelines should be contemplated and followed both in the design process and during the construction and useful life of the building itself (use and maintenance).

What objective does a heat recovery system pursue?

The heat recoverers of the extracted air in ventilation and air conditioning systems allow for maintaining appropriate indoor air quality without energetically burdening the systems for adjusting the hygrothermal conditions of the air supplied to the building's rooms. The use of heat recoverers in ventilation and air conditioning installations allows for the utilization of both the residual sensible and latent heat from the process to achieve a reduction in the energy central (investment costs) as well as a decrease in operational energy consumption (operating costs).

What parameters determine the amount of energy recovered by the recoverer?

According to the registry of documents recognized as public and informative of the RITE, “Energy Saving and Recovery in Air Conditioning Installations”, the equation that allows calculating the energy that a heat recoverer is capable of recovering (E) is the result of multiplying the mass flow rate (mas), the difference in enthalpy ( abla h) of the air between the outlet and the inlet of the recoverer, and the operating time (t):

E = mas abla h t

Thus, in a building, the energy recovered from the extracted air will be greater if:

• The external extraction air flows are high.
• The number of hours the installation is operating is high.
• The summer season is characterized by a high number of hours with relatively high dry bulb and wet bulb temperatures, and the winter season, by a high number of degree-days.

On the other hand, the thermal jumps abla W and abla T are directly related to the efficiency of the recovery systems (specifically, it is the minimum efficiency that is prescribed in the regulation).

The efficiency ( abla) of any exchanger will be the result of dividing the amount of energy recovered by the maximum recoverable energy:

abla = Amount of energy recovered / Maximum amount of recoverable energy

Depending on the type of energy exchanged, we talk about sensible energy (associated with dry bulb temperature), latent (associated with specific humidity, which is the mass of exchanged water vapor), or total (associated with specific enthalpy content).

Types of recoverers

To recover energy from the air expelled from a building, various types of recoverers can be used in air ventilation and conditioning installations:

• Plate recoverer.
• Rotary recoverer.
• Heat tubes (heat pipe).
• Double water battery.
• External battery.
• Loop water battery.
• Active recovery by refrigeration circuit.

While all the equipment used allows for energy exchange between the supply air and the extracted air, they also all introduce an additional pressure drop, causing an increase in the energy consumption of the fans that seek to maintain the same airflow in circulation in the installation. Only the installation of an energy recoverer will prevent this additional pressure drop.