Handbook of air conditioning and refrigeration / Shan K

As defined in Sec. 3.4, a sorption isotherm is a constant-temperature curve that indicates the relationship
between the moisture content of the desiccant X des, or moisture absorbed as a percentage
of its dry mass, and the relative humidity of the surrounding air � sur. Figure 29.5 shows the sorption
isotherms of silica gels, lithium chloride, and molecular sieves. LiCl has a much higher water-holding
capacity than silica gels and molecular sieves.
Rotary Desiccant Dehumidifiers
A rotary desiccant dehumidifier is a rotary wheel that removes moisture from the airstream flowing
through it. A rotary desiccant dehumidifier has similar construction and operating characteristics to
the rotary total heat exchanger, as discussed in Sec. 12.5. Both are rotary wheels. The main differences
between a rotary desiccant dehumidfier and a rotary total heat exchanger or rotary heat exchanger
are as follows:
● The purpose of using a rotary desiccant dehumidifier is to remove moisture from an airstream,
while the purpose of using a rotary total heat exchanger is to transfer heat energy from one
airstream to another airstream.
● In a rotary desiccant dehumidifier, there is a process airstream and a regeneration airstream. The
regeneration airstream has a very high temperature, whereas in a rotary total heat exchanger, there
is an outdoor airstream and an exhaust airstream at indoor temperature.
● A rotary desiccant dehumidifier rotates at a speed of 6 and 10 r/h (revolutions per hour), whereas
a rotary heat exchanger rotates at a speed of 10 to 25 r/min.
29.10 CASE STUDY: A DESICCANT-BASED AIR CONDITIONING
SYSTEM FOR A SUPERMARKET
Loads in Supermarkets
AC SYSTEMS: PACKAGED AND DESICCANT-BASED SYSTEMS 29.27
A desiccant-based air conditioning system is operated at an open cycle when the process air is entirely
outdoor air. If the process air at the inlet to the rotary desiccant dehumidifier is a mixture of
recirculating air and outdoor air, the desiccant-based system is said to be operated at a closed cycle.
For supermarkets with many frozen food refrigerators, especially single-deck well types and single-deck
island food refrigerators with considerable area exposed to indoor air, there are three types
of loads:
● Refrigeration load. The refrigeration load of the frozen food refrigerator depends mainly on the
temperature of the foods in it, the design ambient conditions (the dew point temperature of the
conditioned space air), and the frozen surface area exposed to the space air. The higher the space
temperature and space relative humidity, and therefore the dew point, the greater the amount of
moisture that may condense on the frozen surface, the larger the accompanying latent heat of condensation,
and the greater the refrigeration load.
● Space cooling load. A considerable portion of the sensible space cooling load is removed by the
cooler surfaces of the frozen food refrigerator and the exfiltrated cold airstreams from the frozen
refrigerators through their openings, which results in a space cooling load with a heavy latent load
that may vary from 50 to 65 percent.
● Coil load. The coil load or refrigeration load of the desiccant-based air conditioning system in a
packaged unit or sometimes an AHU to maintain a required space condition in the sales area is
the sum of the space cooling load, outdoor ventilation air load, and system heat gain. During summer,
the outdoor ventilation load is again mainly latent load, and it is affected by the space temperature
and relative humidity.