Optimal heat exchanger parameters

Recovery heat exchangers used in air-conditioning units have two key parameters: heat recovery efficiency and pressure drop. These parameters oppose each other; therefore, their values must be balanced so that the heat exchanger’s overall result and performance in the recovery unit are optimal.

Net Energy Performance ratio

To monitor and evaluate both quantities at the same time, we have established a Net Energy Performance ratio (NEP ratio), i.e. the ratio between the thermal energy gained by the heat exchanger’s heat recovery, and the energy supplied to ventilators in order to overcome the heat exchanger’s pressure drops. To choose the optimal recovery unit, the NEP ratio should be in the range 9–18.

How to calculate the NEP ratio

The value of the Net Energy Performance ratio can be expressed using the following formula:

NEP ratio = P_rout/ P_rin

P_rout calculation

The thermal energy gained by the heat exchanger’s heat recovery can be calculated as:

P_rout (W) = ϱ_air q_v c_air Δt

ϱ_air = 1,2 kg⋅m^-3 – specific air weight
q_v (m³⋅s^-1) – standard air flow
c_air = 1020 J⋅kg^-1⋅K^-1 – specific thermal air capacity
Δt (°C) – air temperature increase at the supply passage via recovery unit

P_rin calculation

The energy supplied to ventilators necessary to overcome the heat exchanger’s pressure drops can be calculated as follows:

P_rin (W) = 2 [ (q_v dp) / η_fan ] pef

q_v (m³⋅s^-1) – standard air flow
dp (Pa) – standard pressure drop of the recovery unit with a standard flow rate
η_fan = 60 % – average ventilator efficiency (electrical energy transfer to air motion through the recovery unit)
pef = 2,5 – primary energy factor for electrical energy acquisition

The value of the NEP ratio can be easily worked out using our rSelect software. The suitability of your selected recovery unit is displayed by an emoticon with a brief description. Possible outputs are divided into four intervals.

Explanation of NEP ratio values

NEP < 5 🙁 – the recovery unit you have chosen is too small for the entered amount of air
NEP = 5 – 9 😐 – the recovery unit is still slightly undersized for the entered amount of air
NEP = 9 – 18 🙂 – this is the ideal range, the best values are closer to the upper interval limit
NEP > 18 😐 – the heat exchanger is too big for the entered amount of air, please change its size or shorten its length