System components
1.
2.
3.
4.
5.
6.
7.
Pressure gauges
Compressors
Oil separators
Condensers
Liquid receiver
Liquid indicator
Evaporators
8. Expansion valves
9. High pressure cut-out
10. Low pressure cut- in/cutout
11. Oil pressure cut-out
12. Room temperature
control
13. Solenoid valves
14. Drier
Pressure gauges
• The pressure gauge on the
compressor suction and
discharge shows the gas
pressure and also has
marked on it the relative
condensing temperature. It
helps to check correct
pressure-temperature
relationships for
refrigerants. Pressure gauge
in the lub oil line is normally
of differential type.
Compressors
• Refrigeration compressors are usually
reciprocating type for marine refrigeration. It
may be of the rotary screw displacement type
or of the scroll type.
Reciprocating compressors
• Reciprocating compressors for systems cooling
domestic store rooms are usually of the
vertical in-line type. The larger reciprocating
compressors have their cylinders arranged in
either V or W formation with 4,6, 8, 12 or
even 16 cylinders. Compressor speeds have
been increased considerably over the years
from 500 rev/min to the high speed of 1500
to 2000 rev/min.
HEAD
PISTON
HEAD GASKET
VALVE PLATE GASKET
VALVE
PLATE
RINGS
CRANKCASE PRESSURE
EQUALIZING TUBE
WRIST
PIN
SPINNER TUBE
ASSEMBLY
CONNECTING
ROD
OIL
PRESSURE
REGULATOR
MAIN BEARINGS
OIL PUMP
ASSEMBLY
OIL
DRAIN
OIL PICKUP
TUBE & SCREEN
ROTOR
OIL RETURN
CHECK VALVE
STATOR
V-belt drive
• Compressors are usually
driven by V-belts. Most
are driven at less speed
than the motor speed.
Pulleys must be in
perfect alignment and
the pulley shafts
(compressor and
motor) must be parallel
to each other.
• Each crank of the crankshaft for the compressor
shown carries three bottom ends. The aluminium
alloy pistons operate in cast iron liners, which are
honed internally. Piston rings may be of plain cast
iron but special rings having phosphor-bronze inserts
are sometimes fitted. These assist when running in.
Connecting rods are H section steel forgings with
white metal lined steel top end bushes. The
crankcase and cylinders comprise a one-piece iron
casting. Main bearings are white metal lined steel
shells.
• Gas from the evaporator passes through a
strainer housed in the suction connection of
the machine. This is lined with felt to trap
scale and impurities scoured from the system
by the refrigerant particularly during the
running-in period. Freons tend to clean the
circuit but the impurities will cause problems
unless removed by strainers. Any oil returning
with the refrigerant drains to the crankcase
through holes in the diaphragm plate.
Suction and Discharge Valve
• There is a valve plate under
cylinder head with plate type
of suction and discharge valve
located in it. Large diameter
and very small lifts of plates
offer the least resistance to the
flow of refrigerant gas. Heavy
springs on the discharge valve
cage permit a greater valve lift
to protect compressor in case
of severe liquid refrigerant or
oil pumping.
Lubrication
• Oil is supplied to the bearings
and crankshaft seal by means of
a gear pump driven from the
crankshaft. Oil pressure is about
2 bar above crankcase pressure
and the differential oil pressure
gauge is necessary to compare
oil pressure with that of the gas
in the crankcase. There is a
relief valve in the oil system set
to about 2.5 bar above
crankcase pressure. Protection
against oil failure is provided by
a differential oil pressure
switch.
Crankcase heaters
• A certain amount of refrigerant will always be dissolved in the
lubricating oil. However, large amounts of refrigerant in the oil
are undesirable. Excessive dilution can result in inadequate
lubrication. In addition, during compressor start-up, the
lowering of the crankcase pressure will cause oil foaming due
to the vaporization of the refrigerant. In severe cases, this can
disrupt lubrication and can cause carryover of the liquid
refrigerant and oil into the cylinder. Since marine systems
typically operate on the pump-down cycle, the low crankcase
pressure at shutdown limits refrigerant absorption by the oil.
Crankcase heaters which come on automatically during the
compressor off cycle are used to keep the oil warm and
reduce refrigerant absorption.
Shaft seal
• A mechanical seal is fitted around
the crankshaft at the drive end of
the crankcase. This prevents
leakage of oil and refrigerant from
the crankcase. All seals use two
rubbing surfaces. One surface turns
with the crankshaft and is sealed to
the shaft with an O-ring of synthetic
material. The other surface is
stationary and mounted on the
housing with gasket. The rubbing
surfaces can be hardened steel and
bronze or ceramic and carbon. The
mechanical seal is lubricated from
the compressor system.
Unloader mechanism
• The most common method
of varying the capacity of
multi-cylinder compressors
is to vary the number of
active cylinders by holding
the suction valves open.
The capacity control system
unloads cylinders (i.e., cuts
cylinders out of operation)
in response to decreases in
refrigeration load based on
suction pressure.
• A bellows device, actuated by suction pressure can
serve to cut out one or more cylinders. Under high
loads (high suction pressures) none of the suction
valves are held open, and all the cylinders are in
operation. As the load decreases (and the suction
pressure falls off), the cylinders are cut out in
sequence. If the suction pressure continues to fall
off, the compressor will stop on the low pressure
switch. Compressor lubricating oil is used to operate
the valve lifting mechanism. Since oil pressure is
required to load the cylinder, the compressor will
start with all controlled cylinders unloaded, thus
reducing the starting load on the compressor motor.
Oil separators
• Oil separators may be fitted in
hot gas discharge lines from the
compressor. It is a closed vessel
fitted with a series of baffles or a
knitted wire mesh through which
the oil-laden vapour passes. The
reduction in velocity of the
vapour as it enters the larger area
of the separator allows the oil
particles, which have greater
momentum, to impinge on the
baffles. The oil then drains by
gravity to the bottom of the
vessel where a float valve
controls flow to the compressor
crankcase.
Manual Valves
• Manual valves are installed at
various locations in the
refrigeration system to
facilitate system operation, to
isolate components for
maintenance and for other
purposes. Most of the valves
used in refrigeration systems
are of the packed valve type.
They are of the backseating
type. When in the open
position, the valve is
backseated to minimize the
possibility of leakage.
Condensers
• Most marine refrigeration condensers are of the watercooled, multipass, shell-and-tube type. Seawater is circulated
through the tubes, and the hot gas from the compressor
discharge is admitted to the shell and condenses on the outer
surfaces of the tubes. The condenser is typically constructed
of a steel shell, copper-nickel tubes and tube plates, and
bronze waterheads. Gas inlet, liquid outlet, purge, and water
regulating valve control connections are provided.
Liquid Receiver
• The receiver collects the liquid refrigerant draining
from the condenser. It consists of a steel shell with
steel dished heads welded on each end. Sight glasses
or a liquid level indicator is installed to permit
determination of the amount of liquid refrigerant in
the receiver. The receiver will typically have sufficient
capacity to hold the entire system refrigerant charge
and will retain a small liquid level during full load
operation. High levels indicate overcharge and low
levels indicate undercharge.
• Sometimes bottom part of condenser serves as the
liquid receiver
Liquid Indicators (Sight Glasses)
• Liquid indicators or sight glasses are
commonly installed in the liquid line
to indicate a proper refrigerant
charge. Bubbles appearing in the
liquid stream are an indication of a
shortage of refrigerant. Some
indicators also include a moisture
indicator. A portion of the indicator
will change color based on the
relative moisture content of the
refrigerant.
Refrigerant drier
• Shell is filled with a
desiccant such as activated
alumina or silica gel which
absorbs moisture and also
acts as a filter. Even small
amounts of moisture can
cause problems such as
frozen thermostatic
expansion valves, so it is
important to remove
sufficient moisture to
prevent the release of water
in the low pressure portions
of the system.
Evaporators
• Marine evaporators are usually forced
convection evaporators. They are in common
use in ship’s provision refrigeration and air
conditioning systems. The units consist of a
cooling coil with finned tubes, a motor-driven
fan, and drain pan, all enclosed in a sheet
metal casing. Units designed for sub-zero
temperature applications are usually fitted
with an electric resistance defrost system.
Expansion device
• The basic functions of an expansion device used in
refrigeration systems are to:
1. Reduce pressure from condenser pressure to evaporator
pressure, and
2. Regulate the refrigerant flow from the high-pressure liquid
line into the evaporator at a rate equal to the evaporation rate
in the evaporator
• Under ideal conditions, the mass flow rate of refrigerant in
the system should be proportional to the cooling load. It is
desirable that liquid refrigerant should not enter the
compressor. In such a case, the mass flow rate has to be
controlled in such a manner that only superheated vapour
leaves the evaporator.
Thermostatic Expansion Valve
• While there are a number of devices available to control the
flow of refrigerant to the evaporator, such as the capillary
tube, the float valve, and the constant-pressure expansion
valve, the thermostatic expansion valve is the device most
commonly found in marine systems. The thermostatic
expansion valve responds to the evaporator temperature and
pressure and maintains a constant superheat at the outlet of
the evaporator. As refrigerant is fed to the evaporator, the
liquid boils off into a vapour. By the time the refrigerant gas
reaches the end of the evaporator, it is superheated. Feeding
more refrigerant to the evaporator will lower the superheat
temperature, while feeding less refrigerant will raise the
superheat temperature.
• This consists of a feeler bulb
that is attached to the
evaporator exit tube so that
it senses the temperature at
the exit of evaporator. The
feeler bulb is connected to
the top of the bellows by a
capillary tube. The feeler
bulb and the narrow tube
contain some fluid that is
called power fluid. system.
To ensure the correct
operation of the valve, the
bulb must be securely
clamped to the evaporator
outlet line.
• Pb - Bulb pressure on the upper
side of the diaphragm, tending to
open the valve, where Pb is the
saturation pressure of the
refrigerant in the bulb,
corresponding to the
temperature of the gas at the
evaporator outlet.
• Po - Evaporator pressure on the
lower side of the diaphragm,
tending to close the valve, where
Po is the saturation pressure of
the refrigerant at the evaporator
inlet, and Δp is the pressure drop
between the evaporator inlet and
outlet.
• Ps - Pressure exerted by the
regulating spring, tending to close
the valve. The spring tension, set
by the regulating spindle, controls
the degree of superheat; a typical
superheat value is 4°C to 6°C.
• At any constant operating condition, these forces are
balanced and Pb = Po + Ps
• If the superheat starts to rise, the bulb pressure
increases, Pb > Po +Ps and the valve is moved in the
opening direction, admitting more liquid and
restoring the constant operating condition.
• If the superheat falls, Pb < Po + Ps and the valve is
moved to the closing position, reducing the supply of
liquid.
• In practice, to achieve the desired degree of
superheat at the evaporator outlet, dry expansion
evaporators require up to 20 per cent of their cooling
surfaces to be available to superheat the gas, the
precise area varying with demand.
TEV with external equalizing connection
• There is an appreciable
pressure drop in the large
evaporators. Additional
control is introduced by
incorporating a pressure
equalising connection. This
connection eliminates
further increase in the
superheat temperature to
compensate for the
reduction in pressure, and
so allows an increase in the
effective area of the
evaporator.
Back pressure valve in vegetable room
• If the evaporator in
vegetable room is kept at
common pressure of meat
room and fish room, then
ice formation will take place
in this evaporator also. This
ice will be formed from the
moisture of fruits and
vegetables stored in the
room and they will become
desiccated.
• The bellows pressure
area and the valve area
are equal. The
adjustment spring and
the evaporator pressure
change can operate the
valve motion. The valve
has a gauge opening to
check pressure of the
evaporator. For R22
system, this pressure
may be 4 bar gauge.
Solenoid valve
• The solenoid valve is
thermostat controlled valve
which provides automatic
opening of and closing of
liquid line to the evaporator.
When the coil (3) is
energised, the pilot orifice (4)
is opened and the diaphragm
(1) moves into open position
(vice versa when coil is deenergised).
Thermostats
• Thermostats are temperature-controlled electric
switches. It is used to control the temperature in a
refrigerated space by ‘opening and closing’ a
solenoid valve in the liquid line.
• Three types of element are used to sense and relay
temperature changes to the electrical contacts.
1. A fluid-filled bulb connected through a capillary to a
bellows.
2. A thermistor
3. A bi-metal element
High Pressure Cut-out
• There are a number of faults which cause high
discharge pressure, including loss of
condenser cooling, air in the system and
overcharge.
• In the event of overpressure on the condenser
side of the compressor the high pressure cutout will cause the compressor to shut down.
The device is re-set by hand.
• The bellows in the cut-out is
connected by a small bore pipe
between the compressor discharge
and the condenser. The bellows tends
to be expanded by the pressure and
this movement is opposed by the
spring. The adjustment screw is used
to set the spring pressure. During
normal system operation, the switch
arm is held up by the switch arm catch
and holds the electrical contact in
place. Excessive pressure expands the
bellows and moves the switch arm
catch around its pivot. The upper end
slips to the right of the step and
releases the switch arm so breaking
the electrical contact and causing the
compressor to cut-out. The machine
cannot be restarted until the trouble
has been remedied and the switch reset by hand.
Low pressure controller
• The low pressure control stops the
compressor when low suction pressure
indicates closure of all cold compartment
solenoids. When the pressure in the
compressor suction rises again due to one or
more solenoids opening, the low pressure
control restarts the compressor.
• The controller is operated
through a bellow which
monitors pressure in the
compressor suction. A
pressure differential between
cut out and cut in settings is
necessary to avoid hunting.
The push pin operates the
switch through a contact
which is flipped open or
closed through a coiled spring
plate. With the contacts open
the spring is coiled as shown.
Outward movement of the
pin compresses the spring
and this then flips the contact
to close the compressor
starting circuit.
Oil pressure safety cut-out
• This is used to protect against
too low oil pressure in forced
lubrication systems. It is a
differential control, using two
connections. One side
responds to the suction
pressure of compressor and
the other responds to the oil
pressure. If the differential oil
pressure fails, or falls below a
minimum value, the control
stops the compressor after a
certain time delay.