SCROLL COMPRESSORS

Although the scroll mechanism has been known for many years, having been patented in France in 1905, it was not until the latter part of the last century that it first appeared in commercially available compressors. Manufacturing technol­ogy had by this time developed sufficiently to enable the precision spiral forms to be made economically.

Scroll compressors are positive displacement machines that compress refriger­ants with two inter-fitting spiral-shaped scroll members as shown in Figure 4.24. One scroll remains fixed whilst the second scroll moves in orbit inside it. Note that the moving scroll does not rotate but orbits with a circular motion. Typically

Two to three orbits, or crankshaft revolutions, are required to complete the com­pression cycle.

The scroll has certain common features with the screw. Both types have a built in volume ratio and therefore the scroll exhibits IE curves similar in shape to those of the screw (Figure 4.18). There is no clearance volume and hence no re-expansion loss. However, there is a very important difference in the sealing of the compression pockets. The screw relies on clearance between rotors and casing whereas the scroll can be built with contacting seals, i. e. the scrolls touch each other at the pocket boundaries. This is possible because the orbiting motion gives rise to much lower velocities than rotating motion, and also the load on the flanks and tips of the scrolls can be controlled. Additionally there is no direct path between the discharge port area at the centre of the scrolls and the suction. The result of this is very low leakage and heat transfer losses, giving better VE char­acteristic than most other types (Figure 4.16). This enables the scroll to function efficiently in much smaller displacements than the screw (Figure 4.2), with the upper size limit being effectively determined by the economics of manufacture.

Almost all production scrolls are of the hermetic type and a typical con­figuration is shown in Figure 4.25 . These compressors have advantages over similar sized piston hermetics in air-conditioning applications and this has encouraged investment in production facilities, building millions worldwide.

The flat volumetric curve enables the scroll to deliver more cooling and heat­ing capacity at extreme conditions, the compression process is smoother and quieter, and there are many fewer moving parts, ensuring very high reliability. Additionally the scroll has excellent resistance to fault conditions such as liq­uid floodback, and compliance mechanisms can deliver unloaded starting and extreme pressure protection (Elson et al., 1991).

Whilst no oil injection into the compression process is needed, bearing and thrust surface lubrication is vital. Oil can be fed to the upper drive bearings and other surfaces using the centrifugal forces generated by an offset drilling along the length of the shaft. Capacity control using variable speed inverter drive is possible for many scrolls. More recently a method using intermittent and fre­quent scroll separation has been introduced (Hundy, 2002) . When the scrolls are separated axially the capacity is zero. The motor continues to run at normal speed but with very low power and back flow of gas from the high-pressure side is prevented by a discharge valve. When the scrolls are brought together normal pumping is resumed. The axial movement of the fixed scroll is pow­ered by a hydraulically actuated piston, in response to a pulse width modu­lated signal from a controller. The total cycle time is typically 20 seconds, and the duration of the loaded period within that cycle time is infinitely variable. Because the cycle time is relatively short, the thermal inertia of the system has the effect of damping the fluctuations so that the effect is very similar to con­tinuous operation at reduced capacity.

The ‘take-off’ of air-conditioning scrolls has prompted the introduction of many variants, the most important of which is the refrigeration or low- temperature version. As with the screw, use of a smaller discharge port ena­bles the compressor to be optimized for the higher pressure ratios applicable to lower temperature applications. By introducing a discharge valve, similar to those employed in reciprocating compressors, the effects of under compression can be minimized. Liquid injection is used for cooling where necessary, and the economizer cycle can be used to boost capacity and efficiency (see Chapter 3). These developments have enabled the refrigeration scroll to compete with pis­ton types in a wide variety of commercial applications.

The upper size limit for a single air-conditioning scroll has been considera­bly extended with the introduction of a dual scroll that has a scroll set mounted on each end of a horizontal shaft (Pirenne, 2007). About 50% capacity reduc­tion is achieved by idling one of the scroll sets.

Posted in Refrigeration and Air Conditioning