How It Works In Detail - Dometic SMX II Description And Installation Manual

in the condenser coil. The seawater pump circulates rela-
tively cool seawater through the inner tube in the condenser
coil. The heat from the refrigerant is exchanged to the
seawater and discharged overboard. This cools the refriger-
ant and condenses it into a liquid. The liquid refrigerant is
then passed through the evaporator coil and the cycle
repeats. Removing heat and moisture from the cabin air
lowers its temperature and humidity levels. In the electric
heat mode, a solenoid valve on the cooling/heating unit
closes and stops the flow of refrigerant through that one unit
while the blower remains on. The electric heater energizes,
warming the cabin air as it is drawn over the heating ele-
ments. The conditioned air is blown through the ducting and
out the supply air grill(s).
The Modulating system can operate up to five independently
controlled cooling/heating units at one time. Unlike the typical
"split" system, the Modulating system adjusts to the number
of cooling/heating units that are on line and operational. The
control and setpoint of any zone can be set to the desired
temperature, heating or cooling, and the Modulating system
will condition that zone accordingly. Note that the cooling
units must have the optional electric heaters to heat the
cabin.
The cabin controls are independent of each other and relay
a signal to the condensing unit, turning it on in the cooling
mode. If the cabin control is set for heating, a solenoid valve
stops the flow of refrigerant through the cooling/heating unit's
evaporator coil and energizes the electric heater. The
Modulating system is designed to provide the most flexibility
in independent zone control of any split refrigerant-based
system by providing heating or cooling at the same time, but
in different cabins.

How it Works in Detail

The Condensing Unit
The modulating condensing unit consists of the compressor,
the receiver, the heat exchanger or condenser coil, a hot gas
by-pass valve and a "de-superheat" valve, the associated
electrical components, and the system service valves. These
parts work together to compress the expanded refrigerant,
flowing back from the cooling unit to the compressor, into a
high-pressure state. The compressed refrigerant then
passes through the heat exchanger (condenser coil) where it
gives up the heat that was absorbed in the cooling (evapora-
tor) coil. It is then condensed to a liquid state as it flows to
the liquid receiver. The process is repeated as the refrigerant
flows back to the cooling unit. The bypass valve senses the
increased pressure when a cooling unit is removed from the
system, such as when an area is thermostatically satisfied,
and allows the refrigerant to flow back to the suction side of
the compressor.
L-0952 Introduction
When cooling is called for by the thermostat control of one of
the cooling/heating units, a signal is sent to the compressor
relay . This signal relays power to the compressor allowing it
to operate. As the system comes on, low-pressure refrigerant
gas flows back from the cooling/heating units to the accumu-
lator. The accumulator is a storage place for the refrigerant
and ensures that only gas, and not liquid, refrigerant makes
it back into the compressor. The compressor then com-
presses the refrigerant into a high-pressure gas and dis-
charges it into the condenser coil. The high-pressure hot
refrigerant gas is cooled and condenses into a high-pressure
liquid as it passes through the outer tube of the condenser
coil. The heat from the refrigerant gas is exchanged to the
seawater, which flows through the inner tube of the con-
denser coil, and is pumped overboard. The refrigerant then
flows into the receiver that stores the liquid refrigerant on its
way back to the cooling/heating units.
If all cooling/heating units (up to five) are on and running
then the system operates just like standard remote DX
equipment. However, as cooling/heating units cycle off and
the suction pressure coming back from those units starts to
get lower, the Modulating System starts to compensate for
the changes in refrigerant pressure and temperature
extremes. The two unique components on the condensing
unit that serve to balance the system are the Hot Gas
Bypass Valve and the "De-Superheat" TX Valve .
The Hot Gas Bypass Valve is positioned between the suction
line leading into the accumulator and the discharge line
coming out of the compressor. If the suction pressure gets to
low (<55psi) then the Hot Gas Bypass Valve opens and
helps to raise the suction side pressure.
The TX Valve is positioned between the receiver and the
accumulator and senses the temperature of the suction line
leading into the compressor. If the suction temperature gets
too high then the TX Valve opens and flashes cool refrigerant
from the receiver to the accumulator, lowering the tempera-
ture of the gas going into the compressor. If the suction side
pressure drops below 35psi then the low-pressure switch will
open. The condensing unit is also equipped with a high-
pressure switch that opens when the discharge pressure
rises above 425psi. (See Figure 12)
The Refrigerant Lines and Manifolds
The refrigerant lines are made of copper tube that is sized
for the BTU load of the system. The suction lines are always
larger than the discharge lines. The suction lines must be
insulated. They supply both liquid and gas refrigerant to and
from the evaporators and the condensing unit. Typically, the
condensing unit lines are larger and will connect to a
refrigerant manifold. At the manifold, the lines will reduce in
size to supply the independent evaporators. (See Figure 11.)
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