Carrier 48MA - Manual (Page 4)

Fig. 1 — Refrigerant Piping Schematic (10-Zone Units Shown, 8- and 12-Zone Units Similar)
partial load operation, difficulties arise in conven­
tional hot deck/cold deck units. When some zones
are at full heat, some at partial heat, some at
partial cooling, conventional multizones must
operate the hot and cold decks simultaneously at
high energy cost. The Carrier Modular design
satisfies each zone’s demand by a discrete
module(s). There are no hot decks, cold decks or
zone air mixing dampers to waste energy. The only
energy expended is that required to heat or cool
the individual zone. Since there is no mixing,
energy is saved and operating costs are significantly
lowered. In addition, the control system provides
excellent humidity and temperature control. Multi­
stage cooling is available on larger zones where 2 or
more modules are used for efficient control of
zone space requirements.
The following features and safety devices are
provided on the refrigerant cycle:
1. Suction line accumulator
2. Crankcase heaters
3. High- and low-pressure switches
4. Discharge line thermostat
5. Time Guard® circuit
6. Airflow switch for indoor fan motor
7. Internal motor protection thermostats em­
bedded in compressor motor windings
8. Hot gas bypass capability
9. Compressor unloading capability
10. Filter-driers
PSYCHROMETRICS ^ The 48MA/50ME units
differ psychrometrically from the conventional
multizones due to the operation of the outdoor air
coil. The coil in the Carrier units cools and
dehumidifies the outdoor air entering the unit thus
assuring that raw outdoor air is not passed along to
the zones. This air treatment by the outdoor air
coil (and also by the zone module evaporator coils)
provides excellent low load performance and
precise temperature control to the conditioned
space. The only large load variation occurs on the
outdoor air coil where a thermal expansion valve is
used. This allows the use of simple capillary tube
expansion devices on the zone coils. The zone coils
cool and dehumidify a mixture of return air and
outdoor air — outdoor air at the approximate dew
point temperature of the return air.
The psychrometric chart (Figure 2) illustrates
this air treatment for a typical set of conditions. As
an example: 1000 cfm of outdoor air at 95 F/75 F
having 99 grains moisture content enters the
outdoor air coil and is cooled and treated so that
the air leaving the coil has 68 grains of moisture
content. The outdoor air coil under these con­
ditions has a capacity of 60,000 Btuh of which
39,000 Btuh is sensible. This is a coil sensible heat
factor of 0.65. By examining the room conditions,
it is evident that the outdoor air coil is very
effective in removing the latent load. At 75 F/50%,
the room content is 64 grains of moisture. The
percent moisture removed with respect to room
conditions is: