Handbook of air conditioning and refrigeration / Shan K

AC SYSTEMS: PACKAGED AND DESICCANT-BASED SYSTEMS 29.23
during sorption.The desiccant dehumidification process line deviates from the thermodynamic wetbulb
temperature line and is represented by line di-do on the psychrometric chart in Fig. 29.3.
The decrease in humidity ratio during the desiccant dehumidification process is given by
Difference in humidity ratio � w di � w do
(29.21)
where w di, w do � humidity ratio at dehumidifier inlet and outlet, lb/lb (kg/kg). The reduction of latent
heat Q l, in Btu/h (W), can be calculated as
Q l � 60V˙ o� o(w di � w do)h fg
where volume flow rate of air entering dehumidifier, cfm [m3 /(60 s)]
�o � air density of entering air, lb/ft3 (kg/m 3 V˙
o �
)
hfg � latent heat of vaporization, Btu/lb (J/kg)
(29.22)
Desiccant dehumidification is a dehumidification and heating process on the psychrometric chart,
and it is illustrated by a straight line inclined downward, deviating slightly from the thermodynamic
wet-bulb temperature line, as shown in Fig. 29.3. It has a flatter negative slope.
The acute angle between the desiccant humidification process and thermodynamic wet-bulb
temperature depends mainly on the desiccant material and the water-holding capacity of the desiccant.
When the desiccant approaches saturation, angle � is smaller than that when a new desiccant
first absorbs moisture from the surrounding air. For a rotary desiccant dehumidifier impregnated
with lithium chloride, if manufacturer’s data are not available, angle � can be estimated at 5°.
The regeneration process removes the absorbed moisture from the desiccant. For the regeneration
airstream, it is also a humidifying and cooling process. Molecular sieves require a higher regeneration
temperature than silica gels and lithium chloride.
FIGURE 29.3 Desiccant dehumidification and sensible cooling.