Building Services Engineering 5th Edition Handbook

144 Ventilation and air conditioning
The mass flow balance for the junction is:
mass flow of stream 1 + mass flow of stream 2 = mixed mass flow
or,
Q 1 ρ 1 + Q 2 = Qρ
The enthalpy balance, taking the specific heat capacity as constant, is:
Q 1 t 1 + Q 2 t 2 = Qt
Dividing through by Q gives,
Q 1
Q t 1 + Q 2
Q t 2 = t
The mixed air temperature and moisture content lie on the straight line connecting the two entry
conditions and can be found by the volume flow rate proportions as indicated by the equation.
Humidification
In winter, incoming fresh air with a low moisture content can be humidified by steam injection,
banks of water sprays, evaporation from a heated water tank or a spinning disc atomizer. A
preheater low-pressure hot-water coil usually precedes the humidifier to increase the waterholding
capacity of the air. This also offsets the reduction in temperature of the air owing to
transference of some of its sensible heat into latent energy, which is needed for the evaporation
process. Figure 5.8 shows such an arrangement.
A temperature sensor in the humidified air is used as a dew-point control by modulating
the preheater power to produce air at a consistent moisture content throughout the winter. For
comfort air conditioning, the room percentage saturation will be 50% ± 10%. This permits a
wide range of humidifier performance characteristics.
The humidification process often follows a line of constant wet-bulb temperature. The water
spray temperature is varied to alter the slope of the line on the psychrometric chart. A complete
psychrometric cycle for a single-duct system during winter operation is shown in Fig. 5.9. The
preheating and humidification stages have been omitted, as close humidity control is deemed
not to be needed in this case. A typical summer cycle is shown in Fig. 5.10.
Some reheating of the cooled and dehumidified air will be necessary because of practical
limitations of cooling coil design. Part of the boiler plant remains operational during the summer.
Reheating can be avoided by using a cooling coil bypass which mixes air M and air C to produce the
correct supply condition. Heating and refrigeration plant capacities are found from the enthalpy
changes and specific volume, read from the chart, and the air volume flow rate:
heat transfer rate = Q m3
s
× kg
v s m 3 × (SE 1 − SE 2 ) kJ
kg × kWs
kJ