Scroll Technology Starts to Take Off
However, until recent computer-aided manufacturing advancements became practical, it was impossible for engineers and designers to successfully manufacture the scroll compressor design in mass quantities and at low costs.
Computer-aided design and manufacturing techniques have made it possible for today’s engineers to produce low-cost scroll compressors in high volumes even with the scroll’s complex geometry. It is the actual scroll forms that gave engineers these problems (Figure 1).
For many years, piston-type reciprocating compressors led the residential market for both air conditioning and heat pump applications. A demand for a quieter, more reliable, and efficient compression system was forced on compressor designers because of rising energy costs, federal regulations, and much more consumer awareness for lower sound qualities in the past 15 years.
Although the demand started in the residential markets, the scroll now has applications in low-temperature refrigeration.
Scroll RefresherThe scroll compressor’s operation is relatively simple.
Two spiral-shaped parts (scrolls) fit inside one another (Figures 1 and 2). These two mating parts are often referred to as involute spirals. One of the spiral-shaped parts stays stationary while the other orbits around the stationary member. This orbiting motion is created from the centers of the journal bearing and the motor shaft being offset.
It is a true orbital motion, not a rotational motion. This orbiting motion causes “continuous” crescent-shaped gas pockets to be formed.
The orbiting motion draws gas into the outer pocket and seals it as the orbiting continues. This continuous orbiting motion causes the crescent-shaped gas pocket to become smaller and smaller in volume as it nears the center of the scroll form (Figure 3). Once at the center, the gas pocket is fully compressed and is discharged out of a port of the non-orbiting (fixed) scroll member (Figures 3 and 6).
Several crescent-shaped gas pockets are compressed at the same time, which provides for a smooth and continuous compression cycle. Thus, the scroll compressor conducts its intake, compression, and discharge phases simultaneously.
The scroll compressor always takes in a fixed volume of gas at suction pressure, then decreases that same gas volume which increases its pressure.
In fact, during the discharge phase, the scroll compressor compresses the discharge gas to a zero volume, eliminating any carryover of trapped discharge gas characteristic of piston-type compressors. Because of this, the scroll compressor is often referred to as a “fixed compression ratio” compressor.
Unlike piston-type compressors, the scroll compressor has no re-expansion of discharge gas which can be trapped in a clearance volume. This is why piston-type compressors are often referred to as “variable compression ratio” compressors. This contributes to low volumetric inefficiencies in piston-type compressors.
On some scroll compressors the mating scrolls or scroll pump have an operating frequency very close to the operating frequency of the compressor drive motor. Since scroll compressors are usually ridged mounted, a low beating sound may be heard while the scroll is running from the two frequencies interacting.
Also, during shutdown, some scroll compressors may run backwards for a few seconds as the scroll’s internal pressures equalize. A “burp” may be heard while this is happening. A check valve in the discharge line of the compressor will prevent the compressor from running in reverse too long.
Scroll AdvantagesThe methods in which the two scroll members interact and operate have several advantages.
When liquid refrigerant, oil, or small solid particles enter between the two scrolls, the mating scroll parts can actually move apart in a sideways direction. This is referred to as “radial” movement. In some manufacturers’ compressors, radial movement is accomplished by allowing a swing link to pivot on the motor shaft crank pin.
A dowel pin traveling in a slot governs the amount of pivot of the swing link. This pivoting action allows for the two scrolls to separate a bit in the sideways radial direction when oil or liquid refrigerant is present. This eliminates high-stress situations and allows for just the right amount of contact force between mating scroll surfaces.
This action allows the compressor to handle some liquid. When a liquid slug is experienced, the scroll’s mating parts will separate slightly and allow for the pressurized gas to vent to suction pressure. This allows the liquid slug to be swept from the mating scroll surfaces to suction pressure and vaporize. A gurgling noise may be heard during this process; the compressor may even stall briefly and restart as the excess liquid is purged out of the scrolls.
In some applications, this will eliminate the need for a suction accumulator and/or a crankcase heater. Often, crankcase heaters are used to prevent migration on-off cycles or long shutdowns, which will help prevent liquid slugging on start-ups.
Also, in specific applications there is no need for a start kit because the compressor will start under any load. Due to the internal design of the scrolls, internal compressor pressures are always balanced at start-up.
As the orbiting scroll orbits, centrifugal forces on the sides of the mating scrolls, along with some lubricating oil, form a seal that prevents gas pocket leakage. This is often referred to as “flank” sealing, a major contributor to the scroll’s higher efficiencies. A small amount of lubricating oil is usually entrained in the suction gases, and along with the centrifugal forces, provides for flank sealing.
Tight up-and-down mating or sealing of each scroll’s tips (also referred to as “axial” sealing) also prevents any compressed gas pocket leakage and adds to the efficiency. Some manufacturers use tip seals for the axial seal.
Scroll tip seals act the same as piston rings in a reciprocating piston-type compressor. These tip seals ride on the surface of the opposite scroll and provide a seal so gases cannot escape between mating scroll parts at the tips of the scrolls.
The scroll requires no valves, so it does not have valve losses that contribute to inefficiencies as piston-type compressors do. The scroll has no re-expansion of discharge gases, which can be trapped in a clearance volume. This contributes to low volumetric inefficiencies in piston-type compressors. This is why the scroll compressor has a very high capacity in high-compression-ratio applications.
A considerable distance separates the scroll compressor’s suction and discharge ports or locations. This greatly reduces the transfer of heat between the suction and discharge gases. Because of this, the suction gases will see less heat transferred into them and will have a higher density. This will increase the mass flow rate of refrigerant through the scroll compressor.
Because of the scroll’s continuous compression process and the fact that it has no valves to create valve noise, the scroll produces very low gas-pulsation noises and very little vibration when compared to piston-type compressors.
Finally, the scroll’s compressor’s simplicity requires only the stationary and the orbiting scroll to compress gas. Piston-type compressors require about 15 parts to do the same task.
What’s NewThis summer, Copeland Corp. is introducing a “horizontal” scroll compressor for height-sensitive applications in supermarkets (Figure 4).
The horizontal scroll compressor was shown at the Food Marketing Institute (FMI) show in Chicago this May. It will be available in 2-, 3-, 4-, 5-, and 6-hp models for low-, medium-, and high-temperature applications.
The horizontal scroll will be compatible with HFC-404A and HCFC-22 applications. This model will meet the needs of the low height, quiet operation, and compact design in self-contained refrigeration systems for supermarket applications.
Some of these low-profile condensing units are mounted right on top or very near to the case lineups they are serving. This leads to shorter piping runs, which means minimum pressure drop and minimal suction line superheats to keep energy savings and capacities high.
An example of this application is the Hussmann Corp. low-profile Protocol LP2000 (Figure 5), which uses horizontal scroll compressors to aid in its low profile, energy efficient configuration.
Tomczyk is a professor of hvac at Ferris State University, Big Rapids, MI, and author of the book Troubleshooting and Servicing Modern Air Conditioning and Refrigeration Systems, published by Business News Publishing Co. To order, call 800-837-1037.