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become plugged causing oil starvation to the compressor bearings. The size of the accumulator
depends upon the operating range of the system and the amount of sub-cooling and subsequent
head pressure allowed by the refrigerant control. System modelling indicates that systems operating
down to and below -18 °C will require an accumulator that can hold around 70% to 75% of the system.
3.4
Filter screens
CAUTION
Screen blocking! Compressor breakdown! Use screens with at least
0.6 mm openings.
The use of screens finer than 30 x 30 meshes (0.6 mm openings) anywhere in the system should be
avoided with these compressors. Field experience has shown that finer mesh screens used to protect
thermal expansion valves, capillary tubes or accumulators can become temporarily or permanently
plugged with normal system debris and block the flow of either oil or refrigerant to the compressor.
Such blockage can result in compressor failure.
3.5
Mufflers
Gas flow through scroll compressors is continuous with relatively low pulsation. External mufflers
may not be required on Copeland Scroll compressors. Due to system variability, individual tests
should be conducted by the system manufacturer to verify acceptable levels of sound and vibration.
If adequate attenuation is not achieved, use a muffler with a larger cross-sectional area to inlet area
ratio. A ratio of 20:1 to 30:1 is recommended.
A hollow shell muffler will work quite well. Locate the muffler at minimum 15 to maximum 45 cm from
the compressor for the most effective operation. The further the muffler is placed from the
compressor within these ranges, the more effective. Choose a muffler with a length of 10 to
15 cm.
3.6
Sound shell
No sound shell attenuation for YHV*2P compressors is available from Emerson at this time. If a
sound shell is still needed, particular attention shall be paid to the electrostatic charge of the
insulation material, which could be a potential ignition source.
3.7
Insulation material
Insulation material is typically used in a system to insulate the suction line, suction accumulator,
expansion valve bulb or discharge line thermostat. When choosing the insulation material, particular
attention shall be paid to its electrostatic charge, which could be a potential ignition source.
3.8
Reversing valves
A variable speed scroll brings a significant benefit during the defrost cycle. By taking advantage of
the higher speeds and flow rates, the defrost time will typically be shorter than in a fixed-speed
compressor system, which will reduce the time electric resistance heat is used during the defrost
cycle.
Reversing valve sizing must be within the guidelines of the valve manufacturer. Required pressure
drop to ensure valve shifting must be measured throughout the operating range of the unit and
compared to the valve manufacturer's data. Conditions that generate low flow rates and low pressure
drop across the valve can result in a valve not shifting.
This can result in a condition where the compressor appears to be not pumping, i.e. balanced
pressure. It can also produce elevated compressor sound levels. During a defrost cycle, when the
reversing valve abruptly changes the refrigerant flow direction, the suction and discharge pressures
will go outside of the operating envelope. The condition will usually cross the diagonal line
representing the lower right-hand side corner of the envelope. The sound that the compressor makes
during this transition period is normal, and the duration of the sound will depend on the coil volume,
outdoor ambient and system charge.
Since Copeland Scroll compressors have a very high volumetric efficiency their displacements are
lower than those of equivalent capacity reciprocating compressors. As a result, Emerson
recommends that the capacity rating on reversing valves be no more than 1.5 to 2 times the nominal
AGL_HP_VS_YHV2P_E_Rev0
15
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