Carrier 48MA - Manual (Page 3)

CONSTRUCTION
General — Carrier Modular multizones are sturdy
and lightweight. The units are ideal for rooftop
applications where low silhouettes are required.
^|p Maximum height of any 48MA/50ME unit
mounted on a matching roof curb is less than 5
feet. Each unit is of one-piece design with extruded
aluminum frame and 26 ga steel top and side panel
construction. Panels are easily removed for access
to unit interior. Assembled, the insulated unit will
not sweat at 77 E wet-bulb on cooling days. The
unit insulation conserves heat energy in the winter,
keeping energy costs to a minimum.
The unit roof curb, constructed of 14 ga
galvanized steel, is National Roofing Con­
tractors Association (NRCA) approved. A
condenser run-off sheet is built into the curb and is
insulated to prevent heat transfer. The curb is
designed to be flashed to the roof and includes
wood nailers to aid installation. All duct and utility
connections are inside the curb perimeter.
Service access on side panels is accomplished by
removing latches on each side of panel. The side
panel gaskets provide complete perimeter sealing
when compressed against the base unit frame. Each
48MA/50ME unit has large, waterproof condensate
pans to prevent moisture leakage into the con­
ditioned space. Galvanneal steel panel surfaces are
bonderized and finished with Carrier Weather
Armor, a baked enamel finish.
Enters are 2-in. throwaway fiberglass with an
NBS efficiency of 10%. High efficiency (36.5%
NBS) throwaway filters are available as a factory-
installed option. With 41.5 square feet of zone
filter area standard, both the low- and high-
efficiency filters are extremely effective. With this
large area, filter velocity is low — 335 fpm in the
48MA/50ME040 and 145 fpm in the 48MA/
50ME016. A roll filter package is available as a
factory-installed option.
The package consists of 65 ft of 2-in. filter
media, automatic media advance switch, advance
motor and a runout switch. Outside air is drawn
into the unit thru louvered side panels and pre­
filtered by cleanable outdoor air filters in the
panels.
Electrical — The Carrier Modular multizones
include factory-installed power and control circuit
breakers which are suitable for use as disconnect
switches where local codes permit. A 350-va,
115-volt convenience outlet on the main control
panel allows use of a trouble light or small power
tools.
Etched solid copper circuit panels with inter­
changeable plug-in relays and marked terminal
boards are used in all units to improve reliability
and simplify the modular design. Conventional
commercial 24-volt, 2-stage heat/2-stage cool
thermostats are readily wired to marked terminals
on the zone control board. Modules combined to
make a single, large zone are controlled by a single
thermostat
by wiring the zone module control
terminals as illustrated in thermostat usage section
using factory-supplied jumpers.
Refrigeration System — The modular multizone
units incorporate individual zone evaporator coils
plus an outdoor air (Humidry®) evaporator coil
(see Eig. 1). The zone coils are controlled by room
thermostats thru a liquid line solenoid valve. The
metering device for the zone evaporators is a
capillary tube.
The outdoor air evaporator coil cools and
dehumidifies the outside air drawn into the unit.
This coil is a “free floating” coil; that is, after the
first zone cooling coil is activated by cooling
demand, the outdoor air coil is controlled by
suction pressure only. Since the outdoor air evap­
orator coil handles a varying load, a thermal
expansion valve is used to meter the correct
amount of liquid refrigerant to this coil.
The load on unit compressors varies depending
on outdoor air coil load and the number of zone
coils in operation simultaneously. Compressor
unloaders and hot gas bypass valves are employed
to compensate for the variation. The 15-ton unit
unloads to 1/3 or 5 tons, the 20-ton unit to 1/6 or
3.3 tons, the 25-ton unit to 1/5 or 5 tons, the
28-ton unit to 1/6 or 4.7 tons, the 30-ton unit to
1/6 or 5 tons, and the 37-ton unit to 1/4 or 9.3
tons. The unloaders operate from suction pressure
to maintain system suction temperature between
32 and 45 E. If the load is less than the minimum
step indicated above, a hot gas bypass valve meters
hot gas into the outdoor air coil to provide an
additional load to the system. This keeps the
compressor on the line and prevents the coils from
icing up due to low suction temperature.
Since the outdoor air coil would become a
condenser when the ambient temperature is below
the suction temperature, a thermostat closes the
outdoor air damper if the ambient drops below
32 E and compressors are operating. If there is no
airflow across the outdoor coil, there is no heat
transfer and the coil becomes an extension of the
refrigerant piping.
Head pressure is maintained by cycling one or 2
condenser fans with a condensing pressure switch
and modulating the remaining fan with a Motor-
master® solid state speed controller, permitting
operation of the refrigerant system to -20
ambient. The Carrier modular design is not de­
pendent on an economizer cycle for cooling at low
outdoor temperatures. (A factory-installed econo­
mizer option is available.)
All multizone units function satisfactorily in
the full cooling or full heating mode. However, at