Boletin copeland 21-1278

Copeland 21-1278

21-1278

Application Engineering Bulletin AE-1278-R5 Revised February, 1997

MODULOAD® CAPACITY CONTROL FOR 3D COMPRESSORS

On refrigeration and air conditioning applications where the refrigeration load may vary over a wide range, some means of compressor capacity control is often desirable for optimum system performance and control. In addition, compressor capacitymodulation can reduce power and energy consumption, provide better dehumidification, reduce compressor cycling, and decrease the starting electrical load. In order to achieve the above objectives, Copeland has developed Moduload, a unique and highly efficient method for unloading small Discus Copelametic compressors. Moduload will, for the first time, permit efficient unloading on Copelandcompressors with less than 4 cylinders for high, medium, and low temperature applications. Moduload assures smooth, vibrationfree operation in the unloaded mode by unloading all cylinders equally.
Theory of Operation

Since compressed gas is re-expanded to obtain unloading, the percent capacity reduction is a function of operating compression ratio. The higher the compression ratio, the greater thepercent capacity reduction during unloading.
Capacity Control Valve Construction

The unloading assembly is considerably different in design from either the blocked suction unloading used on Copeland 4, 6, and 8 cylinder compressors or the external type unloading (hot gas bypass) used on 9R or 9D 3 cylinder compressors. The unloading hardware consists of a spring and plunger assembly for eachcylinder housed in a chamber mounted on top of the valve plate. See Figure 1. In the fully loaded operating mode (with the solenoid valve de-energized), the solenoid needle valve is seated in the upper port, and the top of the unloading piston is exposed to discharge pressure through the discharge pressure port. The high pressure discharge gas on top of the unloader piston forces it to remain seated inthe Discus valve. When unloaded operation is required, the solenoid valve is energized. The needle valve seats in the lower port, and the unloading piston chamber (above the unloading piston) is exposed to suction pressure through the suction port. With suction pressure on top of the piston, the spring force underneath the piston is sufficient to move the piston to the top of the chamber. In thisposition, the unloader piston opens a passage in the center of the Discus valve through which high-pressure gas enters the expansion

Moduload compressors unload by allowing compressed gas to fill a storage chamber above each cylinder and re-expand back into the cylinder during the suction stroke. As the gas re-expands, both volumetric efficiency and compressor capacity are reduced. The energyconsumed during compression is almost completely recovered during re-expansion, resulting in high efficiency in the unloaded mode. Moduload uses a piston, cylinder, and spring mechanism which are solenoid actuated. All cylinders are unloaded equally, thus providing well-balanced operation at all times.

© 1986 Copeland Corporation

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NORMAL OPERATING CONDITIONS VALVEDE-ENERGIZED HIGH SIDE PRESSURE FORCING UNLOADING PISTONS DOWN

Figure 1

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Moduload Discus 3D Compressors Typical Unloader Performance Factors R404A & R-502 Low Temperature Applications

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Figure 2

Moduload Discus 3D Compressors Typical Unloader Performance Factors R404A & R-502 Medium Temperature Applications

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Figure 3

ModuloadDiscus 3D Compressors Typical Unloader Performance Factors R-22 High Temperature Applications

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Figure 4

Moduload Discus 3D Compressors Typical Unloader Performance Factors R-12 Medium and High Temperature Applications

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Figure 5

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chamber during the compression stroke. During the suction stroke, the gas contained in the...