Application Engineering |
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B U L L E | T | I | N |
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Introduction
The 3 to 15 ton Copeland Scroll Digital™ compressors described in this bulletin include the follow compressor model numbers:
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ZPD34 to ZPD54K5 | ZRD36 to ZRD81KC |
ZPD61 to ZPD91KC | ZRD94 to ZRD125KC |
ZPD103 to ZPD182KC |
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ZPD and ZRD digital scroll compressors are variable capacity compressors that can modulate down to 10% of full load. Digital scrolls are suitable for a variety of applications where a variable capacity compressor is useful, such as VAV applications, dedicated outside air units, units that typically used hot gas bypass for capacity control, and applications that require accurate control of temperature and humidity. Other applications include multiple compressor systems where modulation is required over the entire operating range of the system and in applications where compressor starting and stopping is unacceptable. Typical digital scroll model numbers are
Nomenclature
The model number of the Copeland Scroll Digital compressors includes the approximate nominal 60 Hz capacity at the AHRI high temperature full load air conditioning rating point. An example is the
Digital Compressor Operation
The digital scroll is capable of seamlessly modulating its capacity from 10% to 100%. A normally closed
© 2012 Emerson Climate Technologies, Inc. | 4 |
Printed in the U.S.A. |
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full capacity, or loaded state. When the solenoid valve is energized, the two scroll elements move apart axially, or into the unloaded state. During the unloaded state, the compressor motor continues running, but since the scrolls are separated, there is no compression. During the loaded state, the compressor delivers 100% capacity and during the unloaded state, the compressor delivers 0% capacity. A cycle consists of one loaded state and one unloaded state. By varying the time of the loaded state and the unloaded state, an average capacity is obtained. The lowest achievable capacity is 10% which equates to 1.5 seconds of pumping during one 15 second cycle.
An example for the 15 second controller cycle: In any 15 second cycle, if the loaded time is 10 seconds and the unloaded time is 5 seconds, the average capacity is 66% or if the loaded time is 5 seconds and the unloaded time is 10 seconds the capacity during that 15 second period is 33%. See Figure 1 for a graphical representation of the digital cycle, and Figure 6 for
agraph showing solenoid
How it Works
The digital scroll compressor unloads by taking advantage of the Copeland Scroll compressor's axial compliance. All Copeland Scroll compressors are designed so that the compression elements can separate axially a few thousands of an inch. The 3 through 7.5 ton compressors described in this bulletin use a lift piston mechanism to separate the scrolls during the unloaded state. When the solenoid is energized the volume on top of the piston is vented to the low side allowing the piston and fixed scroll assembly to move axial away from the orbiting scroll. When the solenoid is
The 8 ton and larger digital scroll compressors employ a solenoid valve that is mounted on the side of the compressor that vents the intermediate cavity to the low side of the compressor during the unloaded state. During the loaded state the solenoid valve is de- energized and the intermediate cavity is pressurized to load the floating seal and scrolls axially.
Please refer to Figures 2 and 3 for cross sectional pictures of the two digital modulation mechanisms.
APPLICATION CONSIDERATIONS
Operating Envelope
The operating envelope of the digital scroll compressors for all loading conditions is shown in Figures 4 and 5.