3. GENERAL OPERATING
INSTRUCTIONS
While designing, constructing and installing of
refrigeration plant user must consider the
requirements related to safety of persons, property
as well as local and global environment.
3.1 OPERATION OF COMPRESSOR.
When the compressor is at rest, the suction valves are
held off their seats by a valve lifting mechanism. As
rotation commences the oil pump comes into operation,
supplying low pressure lubricant to the moving parts,
also making available, under the control of a time delay,
a high pressure supply which actuates the valve lifting
mechanism. The time delay ensures that normal running
speed in first attained before allowing the compressor
cylinders to be progressively brought on load. In
machines not fitted with the capacity control, all
cylinders come into operation simultaneously. But,
where capacity control is fitted, a proportion of the
cylinders only are controlled directly by the time delay.
The remaining being brought on load by means of the
three way solenoid valves. When starting, if the suction
valve is held clear off its seats, as the piston descends on
its suction stroke, gas is drawn into the cylinder in the
normal manner. But, as the piston rises on the
compression stroke, the gas is merely recirculated back
into the suction manifold through the open suction
valve. Thus, the only energy required to drive the
compressor is that necessary to overcome its inertia,
friction and pumping losses.
This condition of start up referred to as 'no-load' starting,
and is effective in reducing peak starting electrical
requirements and heavy wear of the starter, motor and
drive gear. In the normal operation, the piston traveling
down the cylinder, reduces the pressure and therein. Gas
in the suction manifold, at a higher pressure, lifts the
suction valve from its seat against the pressure of the
valve springs, and flows into the cylinder. As soon as the
gas pressure in the cylinder and the suction manifold is
equal, the suction valve springs, force the valve ring
back onto its seat. As the piston rise on its compression
stroke the gas pressure is raised until the delivery valves
are lifted from this seats and the gas is allowed to flow
through into the delivery manifold (see fig. 3)
In very low temperature installations, the suction
pressure is also extremely low, and it is not practicable to
compress the gas to condensing pressure in one stage.
For this reason, two-stage machines are used, where the
above sequence of events is effected twice, the first
compression raising the gas to an intermediate level, the
second the condensing pressure.
3.2 LIQUID STROKE
The term liquid stroke refers to the stage wherein an
amount of wet saturated or even liquid refrigerant is
taken into the cylinders and forced out by piston, the
valve mechanism lifting from its seal audibly. This
constitutes an overload of the mechanism as liquid is not
compressible. Generally, the compressors without a
liquid stroke protection will suffer damage when liquid
carry over and compression occurs. They are designed
for "superheated" operation, i.e. the suction gas should
have a. certain amount of superheat say 3°C to 8°C.
While designing a refrigerating plant, it must be ensured
that the suction gas should reach the compressor in
perfectly dry or little superheated condition. However,
KPCL does not take any responsibility for any damage
arising due to liquid stroke, but in order to safeguard
against a minor liquid stroke, a protection device is
fitted to all KIRLOSKAR PNEUMATIC compressors.
This arrangement, however, only allows the passage of
relatively small quantities of wet refrigerant through the
compressor without causing serious damage. When
large amount of liquid are drawn into the cylinders, the
delivery valve assembly and the suction valve stroke
limiter are lifted to such an extent that they cannot return
to their normal position, thus permanently retaining the
valves off their seats and restricting the damage suffered
by the compressor.
Other factors also play an important part in case of liquid
stroke, for even when only a small amount of wet gas is
drawn in, it may still have harmful. Results, even though
liquid stroke proper does not occur. viz :
The operating temperatures of the gas at the delivery
valves will drop abruptly, this imposing an additional
thermal load on the valve assemblies, over and above
the existing mechanical load. If this happens frequently,
it will lead to early failure of the valve assemblies.
A quantity of liquid refrigerant can enter the crankcase
and get mixed with the lubricating oil. The oil
characteristics will be consequently impaired, which
can cause damage to the bearings, crankshaft and
cylinder liners or even total seizure of the piston(s).
In this case the compressor must be inspected for
damage immediately after liquid stroke occurs.
3.3 SAFETY RULES: AMMONIA:
(Characteristics, Storage, Leakage)
Ammonia (NH ) causes watering eyes, coughing and
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very painful skin inflammation, especially of the
mucous membranes and the eyelids. Inhalation may
cause inflammation of the respiratory organs, choking,
cramp and even death in severe cases.
The liquid ammonia stock should be stored in a properly
designed store, outside the engine rooms where no other
gas bottles stored. Full and empty ammonia bottles
should be kept in a horizontal position in order to avoid
their failing over. It is permissible however to store the
bottles, in a vertical position, provided that each bottle
can be separately fastened in such away that the
possibility of its failing over is absolutely excluded.
Ammonia bottles should not be exposed to direct
sunlight or to temperatures exceeding 59°C or lower
than about (-)10°C. They should be carefully handled.
Before use, the contents should be tested to be, sure that
it is ammonia and not some other liquid or gas. Even
when the bottle carries label indicating its contents, this
check should not be omitted. The ammonia should be
obtained from reliable firms in order to be sure that the
refrigerant does not contain water and / or alcohol.
As it is possible that leak might suddenly develop so at
least one gas mask must be placed in a conspicuous and
accessible spot. The mask must completely cover the
eyes. nose and mouth. It should be occasionally checked
for correct functioning and the filter should be renewed
periodically for its effective long life. The mask should
be a good fit. A gas mask and a pair of leather or rubber
gloves are essential. While working on the refrigerant
plant, the possibility of ammonia leaks exist. The gloves
CAUTION
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