Chapitre 12 Traitement d’air et climatisation
beaucoup de contenu dans les diapos du CM M1
12.1 Air humide
12.1.2 Transformations
HVAC systems operate transformations that change the conditions of the moist air. These transformations can be represented on the psychrometric chart (Fig. 12.1). In order to produce air at the required supply conditions, an air handling unit may use one or several elementary transformations. Supposing that the air flow being processed has a mass (or mass flow rate) of dry air \(m_{da}\):
Figure 12.1: Transformations
- Simple heating: increase of temperature at a constant absolute humidity. The heating power is directly related to the enthalpy change, which is only a difference of sensible heat because \(w\) is constant (see eq. (??)): \[\begin{align} P_h & = m_{da} \Delta h \\ \Delta h & = c_{da} \Delta T \end{align}\]
- Vapour humidification: increase of humidity \(w\) at constant dry bulb temperature. The vapour flow rate is directly related to the humidity change, and the power is only a difference of latent heat: \begin{align} Q_ & = m_{da} w \ P_ & = m_{da} h \ h & = l_v w \end{align
- Adiabatic humidification: increase of humidity and decrease of temperature, at constant enthalpy. The evaporation rate \(Q_\mathit{vap}\) generates a decrease of temperature \(\Delta T\): \[\begin{align} Q_\mathit{vap} & = m_{da} \Delta w \\ \Delta h & \approx c_{da} \Delta T + l_v \Delta w = 0 \\ \Delta T & = -\frac{l_v}{c_\mathit{da}} \Delta w \end{align}\]
- “Dry” cooling: decrease of temperature at constant absolute humidity. (par exemple dans un échangeur) \[\begin{align} P_c & = m_{da} \Delta h <0 \\ \Delta h & = c_{da} \Delta T < 0 \end{align}\]
- Cooling and dehumidification: allows cooling the air below its dew point. The absolute humidity decreases as a consequence of condensation on the cold exchanger. (batterie froide dont la surface est plus froide que la température de rosée) \[\begin{align} P_c & = m_{da} \Delta h <0 \\ \Delta h & = c_{da} \Delta T + l_v \Delta w< 0 \end{align}\]
12.2 Centrales de traitement d’air
étapes du dimensionnement d’une CTA
12.2.1 Evaluating air conditioning needs
In order to maintain required indoor conditions (temperature and humidity), the AC system (air handling unit or other) must supply air at specified conditions. These conditions are entirely specified by: the dry bulb temperature \(T_s\) and absolute humidity \(w_s\) of the supply air (or any two variables from the psychrometric chart), and the supply mass flow rate \(\dot{m}_{da,s}\).
The first step is the estimation of the loads of the building, i.e. all influences on the indoor heat and moisture balance besides the HVAC system.
- Sensible loads (in W) are all heat inputs that directly impact the indoor temperature: heat loss (or gain) through the envelope, air infiltration, solar gain, occupants and equipments, lighting… \[\begin{equation} \mathrm{SL} = H\left(T_e-T_i\right) + \dot{m}_\mathit{inf} c_{da} \left(T_e-T_i\right) + \Phi_\mathit{sol} + \Phi_\mathit{in} + ... \end{equation}\]
- Latent loads (in W) are the energy content (latent heat) of the humidity inputs: air infiltration, indoor vapour production… \[\begin{equation} \mathrm{LL} = l_v \left( \dot{m}_\mathit{inf} \left(w_e-w_i\right) + Q_\mathit{vap,in} + ... \right) \end{equation}\]
The supply conditions should be set so that the temperature of the air supply compensates the sensible loads, and the moisture content compensates the latent loads: \[\begin{align} \dot{m}_{da,s} c_{da} (T_s - T_i) & + \mathrm{SL} = 0 \\ \dot{m}_{da,s} l_v (w_s - w_i) & + \mathrm{LL} = 0 \end{align}\]
This is a system of two equations and three unknown variables \(\left\{ \dot{m}_{da,s}, T_s, w_s \right\}\). It can be solved by choosing the value of one of them: for instance, an imposed supply flow rate \(\dot{m}_{da,s}\) or an imposed supply temperature.