Principle Of Operation; Freezing Cycle - Scotsman B 21 WS Manuel De Service

Page 6
How it works
In the ice makers the water used to make the ice
is kept constantly in circulation by a water pump
which primes it to the spray system nozzles from
where it is diverted into the inverted cup molds of
the evaporator (Fig. A).
A small quantity of the sprayed water freezes into
ice; the rest of it cascades by gravity into the
sump assembly below for recirculation.
FREEZING CYCLE (Fig. B)
The hot gas refrigerant discharged out from the
compressor reaches the condenser where, being
cooled down, condenses into liquid. Flowing into
the liquid line it passes through the drier/filter,
then it goes all the way through the capillary tube
where it looses its pressure.
Next the refrigerant enters into the evaporator
serpentine (which has a larger diameter then the
capillary tube) and starts to boil off; this reaction
is emphasized by the heat transferred by the
sprayed water.
The refrigerant then increases in volume and
changes entirely into vapor.
The vapor refrigerant then passes through the
suction accumulator (used to prevent that any
small amount of liquid refrigerant may reach the
compressor) and through the suction line. In both
the accumulator and the suction line it exchanges
heat with the refrigerant flowing into the capillary
tube (warmer), before to be sucked in the
compressor and to be recirculated as hot
compressed refrigerant gas.
The freezing cycle on18-21-31-40 is controlled
by only the evaporator thermostat which has its
bulb in contact with the evaporator serpentine
while in 50-65-90 there is an second phase
controlled by a timer.
The electrical components in operation during
the freezing cycle are:
COMPRESSOR
WATER PUMP
FAN MOTOR (in air cooled version)
On 18-21-31-40 air cooled versions the refrigerant
head pressure is gradually reduced from a value
of approx. 11 bars (155 psig) at the beginning of
the freezing cycle with the unit at 21°C (70°F)
ambient temperature, to a minimun value of
approx. 7 bars (100 psig) just at the end of the
freezing cycle few seconds before the starting of
the defrost cycle.
On 50-65-90 air cooled versions the refrigerant
head pressure is kept between two pre-set values
÷ ÷
(10 8,5 bar - 140 120 psig) with the unit a 21°C
(70°F) ambient temperature.
The declining of the pressure is relied to the
reduction of the evaporating pressure, caused
by the progressive growth of the ice thickness
into the inverted cup molds and to the flow of air

PRINCIPLE OF OPERATION

drown through the air cooled condenser by the
fan motor. The above values are in relation as
well to the ambient temperature of the ice maker
site and they are subject to rise with the increase
of this temperature.
On 18-21-31-40-50 water cooled versions the
refrigerant head pressure ranges between 8.5
and 10 bars (120
an automatic hi pressure control that energizes a
water solenoid valve located on the water line to
the condenser, which rates the cooling water to
the condenser.
On 65-90 water cooled versions the head
pressure is constant at 9.5 bar (135 psig)
controlled by a water regulating valve.
At starting of freezing cycle the refrigerant suction
or lo-pressure lowers rapidly to 1.0 bar - 14 psig
then it declines gradually - in relation with the
growing of the ice thickness - to reach, at the end
of the cycle, approx. 0
the models 18, 21, 31 and 40 and 0.2÷0.3 bar
(3÷4 psig) on models 50, 65 and 90 with the
cubes fully formed in the cup molds.
The total length of the freezing cycle ranges from
23 to 25 minutes.
DEFROST OR HARVEST CYCLE (Fig. D)
On 18-21-31-40 the temperature of the evaporator
thermostat, in contact with the evaporator
serpentine, drops to a pre-set value it changes its
electrical contacts energizing the herebelow
shown components.
(On 50-65-90 when the timer microswitch drops
down into the lower portion of the cam it changes
its electrical contacts energizing the same
components).
COMPRESSOR
WATER INLET SOLENOID VALVE
HOT GAS SOLENOID VALVE
The incoming water, passing through the water
inlet valve and the flow control, runs over the
evaporator platen and then flows by gravity
through the dribbler holes down into the sump/
reservoir (Fig. C).
The water filling the sump/reservoir forces part of
the surplus water from the previous freezing
cycle to go out to the waste through the overflow
pipe. This overflow limits the level of the sump
water which will be used to produce the next
batch of ice cubes.
Meanwhile the refrigerant, as hot gas discharged
from the compressor, flows through the hot gas
valve directly into the evaporator serpentine by-
passing the condenser.
The hot gas circulating into the serpentine of the
evaporator warms up the copper molds causing
the harvest of the ice cubes. The ice cubes,
released from the cups, drop by gravity onto a
slanted cube chute, then through a curtained
opening they fall into the storage bin.
÷
140 psig) being controlled by
÷ ÷
0.1 bar - 0 1.5 psig on