Collection of questions with answers

Pagina 1

Building energy modelling – Exam questions Always explain terms in the formula and unit of measurement !! 1) Define and describe the moist air specific enthalpy. Write the mathematical formula. Highlight the latent and sensible contributions. The moist air can be treated as a mixture of two ideal gases: dry air and water vapour. Considering true that: - The gaseous phase obeys the ideal gas law - The condensed phase are incompressible and do not dissolve non-condensable gases à the vapour behaves as if it was present alone in the same volume and at the same temperature of the mixture However, we need to define a quantity in order to describe the exchanges of energy. For this reason, we introduce enthalpy (for which additive property is valid). We can define the enthalpy of the mixture of dry air and water vapour as: We define enthalpy of the moist air mixture, specific to dry air mass, the quantity: where ha (Ha/Ma) and hv (Hv/Mv) are respectively the specific enthalpies of dry air and water vapour at the same temperature of the mixture. To determine the enthalpy of any state, it is necessary to define a reference state to which assign arbitrarily the value of zero enthalpy; the enthalpy of any other state will then be calculated as a difference. We need now to define a reference state for dry air and one for water. 1 – Specific enthalpy of dry air ha (SENSIBLE) Dry air: ideal gas à we don’t need to specify pressure of reference state Tref = 0°C = 275,15 K 2 – Specific enthalpy of water vapour hv (LATENT) For water, we can not choose a state close to the limit state –> assume as reference state the liquid phase at triple point, for which enthalpy is equal to zero (Tref = 0°C) Hence, for the vapour component, the following relation is valid: where: - lLv (0°C) is the…