Application Engineering |
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B U L L E | T | I | N |
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Compressor operation during the loaded state should always be inside of the envelope. The envelopes represent allowable operation of the compressor with 20F° suction superheat at rated voltage. If the specified modulation ranges are exceeded, overheating of the compressor and tripping of the overload can occur because of inadequate motor cooling.
Solenoid Valve and Coil
The external solenoid valve and coil specified by Emerson must be used since this is a critical component for the proper functioning of this compressor. The solenoid valve and coil are designed for approximately 32 million cycles. Do not attempt to substitute replacement coils or valves; use only the replacement parts specified in Table 4. Refer to the Service Procedures section for information on changing the modulation valves.
Pressure Fluctuations
During scroll modulation the suction and the discharge pressure will fluctuate. This fluctuation should be observed during unit testing. The installation and setting of pressure controls should take this into account. During the unloaded state, the discharge pressure will decrease and the suction pressure will increase. This normal pressure fluctuation has had no observable effect on the reliability of system components, however, component manufacturers should be consulted to ensure the proper application of their products.
Piping
Unlike a variable speed compressor whose mass flow and gas velocity changes with its speed, the digital scroll’s pumping capacity is equal to its 100% capacity while it is pumping. For this reason the gas velocity remains high even during periods when the capacity demand is low. Because the mass flow and gas velocity remain high, piping may be designed as if it were designed for a non capacity controlled compressor. For vertical piping a trap every 20 feet should be sufficient to ensure proper oil return. This recommendation is based upon a minimum 1500 fpm velocity or higher. When the digital scroll compressor is part of a tandem, a double riser should be considered to assure that the velocity remains above 1500 fpm when only the digital scroll is running.
Start Up and Shut Down
To improve the starting characteristics of the digital scroll compressor, the the Emerson controllers delay loading the compressor for 0.1 seconds. Likewise, to eliminate the reverse rotation sound at shut down the compressor is unloaded 0.5 seconds before shut down.
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Compressor Cycling
Because of the digital scroll's seamless capacity modulation from 10% to 100%, capacity short cycling should not be a problem for single compressors. However, if the digital compressor is in tandem with a non modulated scroll, short cycling of the non modulated compressor may be a problem if the system control is not designed and set correctly. The Emerson digital controllers have a built in two minute
Sound Characteristics
The sound spectrum of the loaded state and the unloaded state are different. Special consideration should be given to the transition sound between the loaded and unloaded states. If the transition sound is unacceptable, a heavy sound blanket should be applied to the compressor. Fabricating Services (www.fabsrv.com) is a one source for scroll compressor sound blankets.
The Emerson controllers unload the compressor a fraction of a second before shut down allowing the scroll set to unload, ensuring a relatively quiet shutdown.
Internal Pressure Relief (IPR) Valve
WARNING
A high pressure control must be used in all
High Pressure Control
As mentioned above, not all digital scrolls have IPR valves, therefore high pressure controls are required in some applications. The recommended maximum cut out setting is 425 psig (30 bar) for