Comparison of Commercial Modulation Technologies
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Copeland Scroll Digital
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Utilizes axial scroll compliance to achieve modulation. By forcing the scrolls to separate, compression of refrigerant stops without stopping the compressor motor. In this “unloaded” state, compressor output is 0% capacity. When the scrolls are engaged in the “loaded” state, compressor output is 100% capacity. The scrolls are separated in a periodic cycle (20 seconds) to obtain a time-averaged compressor capacity based on the ratio of loading and unloading times. This allows the Digital scroll to achieve infinite capacity modulation between 10% and 100%.
| Advantage |
Disadvantage |
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Modulation Capability: Most precise capacity matching capability. Provides continuous modulation between 10% & 100% capacity. Widest available capacity modulation range available.
Electromagnetic Interference Issues: Negligible, since no motor frequency variation is needed
Oil Management Issues: Negligible, since compressor motor runs at constant speed
Reliability: High reliability due to simple and compact design. Comparable to Copeland scroll fixed capacity compressors
Efficiency: High system efficiency is obtained by matching capacity with cooling/heating demand
Temperature Control: Precise and smooth control is achieved
Humidity Control: Precise and smooth control is achieved
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Installation Costs: Higher than hot-gas bypass and Ultratech scroll compressor, less expensive than inverter systems
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Copeland Scroll UltraTech
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Typically used on split and packaged systems under 5 HP. The compressor runs at either 67% capacity or 100% capacity depending on cooling/heating demand. When partial load is needed, bypass ports inside the compression chamber open which partially unloads the compressor. This allows the compressor to pump only the refrigerant needed to achieve 67% capacity, making this more efficient than hot gas bypass.
| Advantage |
Disadvantage |
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Installation Costs: Less expensive than inverter systems or Digital scroll systems. Most cost effective method of modulation for systems under 5 HP.
Electromagnetic Interference Issues: None
Oil Management Issues: None
Reliability: Comparable to fixed scroll systems
Efficiency: More efficient than hot-gas bypass. More efficient than inverters or Digital scroll at 67% load. Less efficient than inverters or Digital scroll at full load.
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Modulation Capability: Only two capacity steps, 67% and 100%
Temperature Control: Less precise than inverter systems or Digital scroll systems
Humidity Control: Less precise than inverter systems or Digital scroll systems
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Multiple Compressor Systems
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System capacity can be modulated by using multiple refrigeration circuits or by using multiple compressors in single-circuit systems. In a four circuit system, commonly used in packaged rooftops, individual compressors can be turned of and off to achieve a specific output. For example, a 40 HP system using four 10 HP circuits can provide 0, 10, 20, 30, or 40 HP depending on how many compressors are on at a given time. Similarly, a 10HP single-circuit tandem compressor system can provide 0, 5, or 10 HP of output depending on whether both compressors are on at a given time.
| Advantage |
Disadvantage |
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Installation Costs: Least expensive form of modulation
Electromagnetic Interference Issues: None
Oil Management: No extra oil management hardware needed
Reliability: Comparable to fixed compressor systems
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Modulation Capability: Finite number of capacity steps
Efficiency: Better than single-compressor system, but finite capacity steps limits efficiency gains. For example, in a 40 HP system requiring 25 HP of output at a given time, the system must operate at 30 HP output.
Temperature Control: Precise and smooth control is not obtainable
Humidity Control: Precise and smooth control is not obtainable
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Inverter System
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Inverter systems modulate refrigerant flow by varying the speed of the compressor motor. The compressor motor speed determines the refrigerant flow speed. Therefore, by varying the motor frequency, capacity can be modulated. Capacity output increases and decreases with motor speed. Although this allows precise temperature and humidity control, oil management hardware and electronics are needed to ensure enough oil is in the compressor during slow motor conditions and to ensure that too much oil does not get pushed out of the compressor during fast motor conditions.
| Advantage |
Disadvantage |
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Modulation Capability: More capacity output possibilities than hot-gas bypass
Temperature Control: Precise control is achieved
Humidity Control: Precise control is achieved
Efficiency: High system efficiency is obtained by matching capacity with cooling/heating demand
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Installation Costs: Most expensive of modulation technologies
Modulation Capability: More capacity steps than hot-gas bypass or multiple circuit systems, but capacity range is limited by minimum motor speed. Capacity change is slow because motor frequency changes must be performed slowly to avoid damage due to insufficient compressor lubrication
Electromagnetic Interference Issues: High electromagnetic interference from varying motor frequency
Oil Management: Extra hardware and electronics needed to control compressor oil level.
Reliability: More complex electrical and mechanical hardware
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Hot Gas Bypass
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Hot gas bypass modulates refrigerant flow by bypassing a portion of high-pressure refrigerant discharge from the compressor back to the suction line without passing through the evaporator. Although this modulates capacity, a significant portion of refrigerant is compressed without providing any cooling or heating.
| Advantage |
Disadvantage |
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Installation Costs: Extra valves and piping required, but lower costs than inverter-driven or Digital compressor equipped systems.
Electromagnetic Interference Issues: None
Oil Management: No extra oil management hardware needed
Reliability: Less compressor cycling than that of multiple compressor systems
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Modulation Capability: Capacity can be quickly adjusted by opening or closing a valve, but number of capacity steps is finite
Efficiency: Energy is wasted compressing refrigerant that is not used to provide needed cooling or heating
Temperature Control: Precise and smooth control is not obtainable
Humidity Control: Precise and smooth control is not obtainable
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