Copeland Compressors (now a Division of Emerson) has recently introduced their digital scroll compressor technology. This is one of the most interesting products to come out in a long time for the DX cooling market. But to understand why it is so cool, you first need to understand a little bit about scroll compressors in general.
Scroll compressors have essentially displaced the older reciprocating compressor designs for small-tonnage air conditioning systems. Which is much to the operator's benefit, because scrolls are inherently more reliable and require none of the maintenance that the piston-type reciprocating compressors required. But this advantage comes with a price: comprehension. Reciprocating compressors were so much easier to understand--since the compression stroke in a piston is easy to grasp and most people are familiar with this process from the similar function of pistons in gas engines.
Some smart guy had to come along and invent a highly efficient and low-maintenance compressor technology that no one can describe easily--even using curious arm gestures and words like "orbit"!
The secret to a scroll compressor is two high-precision spiral "scrolls" that are designed to mesh with each other to extremely close tolerances:
The upper scroll is stationary and the lower scroll 'orbits' in a rotary fashion:
Comparison of scroll to piston compressors showing relationship of upper and lower scroll (click for larger image)
The upper and lower scrolls continually 'pinch' off volumes of low pressure gas and move them towards the center of the scrolls, compressing the volume further and further as they work. This compression requires extremely close tolerances between the sides and ends of the scroll surfaces, since the only seal is the lubricating oil in the refrigerant circuit. If tolerances are to great, no seal is effected and the compression is lost.
Still hard to picture? This animation should make things a bit clearer:
So, great: We have a highly efficient compressor with two capacity settings: 'On' and 'Off'. If you are trying to meet a close control spec, this may not be close enough control. You would typically overshoot and then undershoot the required cooling capacity as the compressor kicks on and off. And since anti-recycle timers are required to prevent overheating the compressor motor, there is a limited number of times the cooling capacity can be switched on and off in an hour.
A new innovation allows two-step unloading to 66% capacity--but this can still be a pretty big step of control on a small refrigeration system--especially ones with only a single compressor. Wouldn't it be nice to get a fully modulating compressor with all of the advantages of the scroll compressor?
That's where the digital scroll comes in. Copeland's engineers were clever enough to realize that they could achieve this performance out of a scroll, not by modulating its capacity directly, but instead by modulating the time during which this capacity is provided. They found that if they quickly turn on and off the compression cycle, without having to turn off and on the compressor motor, they could modulate the output very closely to meet the needed capacity. The trick was finding a mechanism by which this rapid switching between active and inactive compression could be accomplished.
The solution they arrived at was elegant. They found that by merely moving the scrolls apart axially, they could defeat the oil seal between the scrolls, and turn off the compression. Then they simply needed to move the scrolls back together and compression would immediately restart.
The above graphic shows the scrolls separated to cancel out the compression cycle, and a visual representation of how the scroll would operate to provide 50% capacity--Operating 10 seconds on and then 10 seconds off in a repeating cycle.
What's the result? Very efficient operation down to 10% of full capacity:
(click for larger image)
See the savings noted above? That's compared to the commonly used DX modulation method of hot gas bypass (HGBP). It's important to note that the HGBP works by applying a false load to the compressor--it does not reduce compressor energy at all! As far as the compressor motor is concerned, it is doing just as much work as when the compressor is providing full output. In fact, the HGBP system is even more of an energy hog than is suggested by the graph above--since compressors with this device will operate at full load for extended periods of time, drawing full amps all the time, as opposed to a standard system where the compressor would turn on and off to match the load.
The digital scroll gives a DX system all the fine control capability of a chilled water system, without sacrificing energy performance like HGBP systems do. It allows effective operation of VAV airflow systems without frosting coils. It provides efficiencies unmatched in the DX market. For these reasons Aaon was quick to incorporate these compressors into their RM and RN rooftop packaged AHU lines. Unfortunately, digital scrolls are not available in all standard scroll compressor sizes and voltage ratings. And they are currently only available in R-22 compressors. This handy chart indicates where the digital scrolls are available in each RM/RN model size and for which voltages. This file is valid as of 9/22/07, and is definitely subject to change in if/when new digital scrolls are released. Additionally, R-410a compressors are expected out in the near future for 6, 7, 13, 15, 16 and 25 ton sizes in 460/3ph electrical services only. Stay tuned for the availability of those units!
Update: Aaon has rolled out units using R-410a digital scrolls, as well as new software to calculate the efficiency benefits of these compressors.
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