14
SECTION 3 - Operation
3.1 SAFETY DEVICES
The Zanotti Z120 machines are equipped with system
components to protect from damage caused by
unsafe operating conditions that automatically shut
down the unit when such conditions occur. This is
accomplished by the following fuses and safety
devices.
3.2 REFRIGERATION CIRCUIT
The operation modes are Cool, Null, Defrost and Heat.
The machines cycle between Cool and Null to
maintain the box temperature at the set point.
The thermostat controls the operation of the unit by
energizing and de-energizing the Power Relays PR
(KMC or K1). When PR is energized it energizes the
evaporator fans, the condenser fans, and the motor
contactor.
3.2.1 Cooling
When cooling, the unit operates as a vapor compression
refrigeration system. The main components of the system
are the reciprocating compressor, air-cooled condenser,
thermostatic expansion valve, direct expansion
evaporator, and hot gas solenoid (two-way).
The compressor raises the pressure and temperature of
the refrigerant and forces it into the condenser circuit. The
condenser fan circulates surrounding air over the outside
of the condenser. Heat transfer is thus established from
the refrigerant gas (inside the circuit) to the air ambient
(flowing over the circuit). The condenser is designed to
improve the transfer of heat. This removal of heat causes
the refrigerant to liquefy; liquid refrigerant flows from the
condenser to the receiver.
The refrigerant then flows through the filter- drier, where
an absorbent keeps it dry and clean. The refrigerant then
flows through a sight glass with moisture indication.
The liquid then flows to the thermostatic expansion valve
that regulates the flow rate of refrigerant in the evaporator
to obtain maximum use of the evaporator heat transfer
surface. The evaporator tubes have aluminum fins to
increase heat transfer; therefore, heat is removed from
the air circulated through the evaporator. This cold air is
circulated throughout the refrigerated compartment to
maintain the box at the desired temperature. The transfer
of heat from the air to the low temperature liquid
refrigerant causes the liquid to vaporize.
The vapor at low temperature and pressure enters the
evaporator coil and then enters the compressor pressure
regulating valve (CPR) that regulates refrigerant pressure
entering the compressor, where the cycle starts over.
Unsafe Condition Safety
Device
Auto
restart
Ref.
Excess current on
fans
F6, F8 fuse
on CPU
NO
1.3.2
Excess
compressor
discharge press.
High-
pressure
switch (HP1)
YES
Excess
current
draw on road dc
motor
FGS Road
Supply Fuse
NO
1.3.2
Excess current
draw in defrost or
heat mode
FSS Fuse on
electric
board
NO
1.3.2
Excess
current
draw of the AC
motor
F1T in
control box
NO
1.3.3
Low
battery
voltage
Alarm Code
PAb
YES
3.5.3
Excess
current
draw on PC board
inputs
FB Fuse on
electric
board
NO
1.3.2
Road & Standby
Supply
Alarm Code
YES
AC Motor Klixon
F1t
Yes
