Geometry of a Scroll

By definition, a volution is a turn or twist about a center. A volute is a spiral. A spiral is a circular curve: the locus of a point moving with an ever increasing radius about a fixed center. A spiral may have one or more volutions, as shown in Figure 1. There are many different kinds of spirals, each defined by variations of a basic mathematical equation. The fundamental spiral form is the Spiral of Archimedes, which is defined by the simple equation: r = a, where r is the radius from a fixed center, a is a constant and is the angle (in polar coordinates). Other variants include the hyperbolic spiral, parabolic spiral, logarithmic

spiral and involute spiral. If we add a third dimension to a simple spiral, the result is a coiled plane similar in appearance to arolled strip of paper. Ancient scribes rolled written parchments onto wooden spools for storage and safekeeping, which is where we derive the term scroll.
Of particular interest to scroll compressor design is the involute spiral, shown in Figure 2. An involute spiral is a spiral with a continuously variable radius measured from the circumference of a base circle centered on a fixed axis. The curve can be visualized as the end point of a tightly pulled cord unwinding from a cylinder. The shape of an involute permits opposing machine elements to mesh so that the bearing faces roll against one another rather than slide (e.g., gear teeth). This reduces friction and wear, and produces a constant angular-velocity ratio during meshing. The involute geometry of identical meshing scroll vanes creates a rolling action at tangential points and minimizes sliding.

The Scroll Set
The scroll is the fundamental compressing element in a scroll compressor. Conceptually, it is a freestanding strip of metal machined into the form of

an involute spiral, and bound on one edge by a solid flat base. A scroll set uses two scrolls with identical geometry. One scroll is inverted, rotated 180 degrees, and inserted into the gaps of the second scroll as shown in Figure 3. In most scroll compressors, the unit frame holds the upper scroll stationary. An eccentric motor shaft moves the lower scroll in an orbital pattern. A specially designed coupling, called an Oldham coupling, holds the lower scroll at a fixed angular position, preventing rotation and allowing radial movement in an orbital path.

Compression Process

When assembled, the flanks of the upper and lower scroll vanes form crescent-shaped pockets. As the lower scroll orbits, the sealing points (tangent points) on the vane flanks migrate inward, pushing crescent-shaped pockets toward the involute center. As the pockets move, they decrease in

volume and consequently compress the trapped gas. Figure 4 (on page 5) shows a sequence of orbits, and the movement and variation of the trapped gas pockets. In Figure 4, the first orbit begins with the ends of both scrolls fully open, allowing the interstitial space to fill with low-pressure refrigerant gas(position A). The lower scroll’s orbit eventually closes the first pockets of refrigerant gas (Position C). As the first orbit ends, the first pair of crescent- shaped pockets have migrated inward to a middle position, and the scroll’s outer ends begin opening again to admit more low-pressure refrigerant gas (Position D). The second orbit pushes the first gas pockets toward the center of the scroll set, continually decreasing the gas volume and increasing the gas pressure. The third orbit begins with the crescent-shaped pockets just outside of the scroll set center. As the third orbit continues, the inner ends of the vanes break contact (Position J), admitting the compressed gas to the center discharge port. The third orbit continues the compression cycle, discharging high-pressure refrigerant gas (Position L). It is important to note the symmetry of the scroll and the crescent-shaped pockets. The shape and position of both pockets described in the above paragraph are symmetrical and diametrically opposed to each other through the complete compression cycle (e.g., 3 orbits). The natural symmetry in the scroll set balances radial gas forces against the vanes, providing a smooth compression cycle. Moreover, each orbit begins the compression cycle anew so that at any given time there are three pairs of symmetrical crescent-shaped pockets at low-, medium-, and high-pressure conditions, as shown in Figure 5 (on page 6). Between Positions A and L on Figure 4, compression is a smooth and continuous process without vibration or strong pulsations as in reciprocating compressors.

Compliance
Some scroll compressor manufacturers have adopted the term compliance to describe the orbital

path between the upper and lower scrolls in a compressor. A radially compliant compressor allows the orbiting scroll to follow a flexible path that is defined by its contact with the stationary scroll (much the same as a cam and follower). An“unloader” bushing installed between the orbiting scroll and the motor shaft absorbs variations in orbit radius created by machining and assembly discrepancies. Axial compliance refers to the ability of the orbiting and stationary scrolls to separate axially. In a non-compliant compressor,

Fig. 6. Radial leakage occurs between adjoining flanks of the scroll vanes, and axial leakage occurs between the vane tips and the base of the opposite scroll

the orbiting scroll follows a fixed path where the orbiting and the fixed scrolls never touch. Carrier and Danfoss refer to this as a controlled orbit design. The geometric relationship between the scrolls in a controlled orbit compressor is constant under all operating conditions.
The decision to make a compressor with scrolls that contact each other or scrolls in a contact-free controlled orbit follows the method used to seal the scrolls.

Sealing Techniques
Compressor performance is directly related to internal leakage and mechanical losses. Each crescent-shaped pocket of refrigerant gas trapped between the scroll vanes naturally tries to find a place of equilibrium. If the gas on one side of a vane is at a higher pressure than the gas on the other side, the high-pressure gas will seek a path to the low-pressure side. In a scroll compressor, there are only two leakage paths: radial and axial. Figure 6 shows that radial leakage occurs between the flanks of the scroll vanes where an advancing high-pressure crescent-shaped pocket attempts to leak back into the following pocket of lower pressure gas. Axial leakage occurs between the scroll vane tip (the free involute scroll edge) and the baseplate of the opposite scroll. Axial leakage is generally considered more critical than radial leakage (ASHRAE 2004).
Leakage increases power consumption, reduces compressor capacity, and diminishes efficiency.
Radial Sealing Compliant compressors use contact between the orbiting and the fixed scrolls as the sealing mechanism. However, radially compliant compressors may not have uniformly effective sealing at allcontact points when new. These designs require a“wear-in” period to equalize contact on all surfaces. Contact makes compliance mandatory. Controlled orbit compressors, in contrast, rely on an ultra-precise scroll profile to ensure scroll flank tightness. Computer-controlled machine tools create precise surface geometry, maintaining tolerances measured in microns (one micron is1x10-6 meters, or 0.000039 inches). The vane flanks never touch. Tolerances are so precise that a thin oil film seals the gap and provides a lubricatingsurface for the orbiting scroll to pass over with no friction or wear. Since the controlled orbit concept never allows mechanical contact between the flanks, compliance is unnecessary and the compressor maintains a fixed geometry over the life of the scroll set
Axial Sealing
Compliant designs depend on contact between the vane tips and the opposite baseplate. Axial flexibility is necessary to provide allowances for thermal growth and wear. Some manufacturers use gas pressure to load the stationary scroll against the orbiting scroll.

Controlled orbit compressors maintain dynamic contact between the orbiting vane tips and the stationary baseplate with floating seals. Grooves machined into the vane tips hold seal elements that“float” between the vane and the opposite baseplate as shown in Figure 7. Refrigerant gas pressure loads the seals against baseplate for proper dynamic contact during operation. Contact forces are very small, which, combined with reduced contact surface area, substantially reduces friction losses and increases efficiency

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1 Short radius or regular 45° elbow (copper sweat)
2 Long radius or sweep 90° elbow (copper sweat)

Pipe coupling (copper sweat

A combination pipe union and reducer fitting (brass threaded)

. Reducer fittings, bronze threaded (left) and copper sweat (right)

Pipe tee (copper sweat)

Pipe cap (copper sweat)

Control valves are imperative elements in any system where fluid flow must be monitored and manipulated. Selection of the proper valve involves a thorough knowledge of the process for which it will be used. Involved in selecting the proper valve is not only which type of valve to use, but the material of which it is made and the size it must be to perform its designated task.
The basic valve is used to permit or restrain the flow of fluid and/or adjust the pressure in a system. A complete control valve is made of the valve itself, an actuator, and, if necessary, a valve control device. The actuator is what provides the required force to cause the closing part of the valve to move. Valve control devices keep the valves in the proper operating conditions; they can ensure appropriate position, interpret signals, and manipulate responses.
When implementing a valve into a process, one must consider the possible adverse occurrences in the system. This can include noise due to the movement of the valve, which can ultimately produce shock waves and damage the construction of the system. Cavitation and flashing, which involve the rapid expansion and collapse of vapor bubbles inside the pipe, can also damage the system and may corrode the valve material and reduce the fluid flow.
There are four general types of valves. 1. Electronic, or electrical valves. The movement of the ball or flap that controls flow is controlled electronically through circuits or digitally. These types of valves have very precise control but can also be very expensive. 2. Non-Return valves. These valves allow flow in only one direction, and are common in various industries. When pressure in the opposite direction is applied, the valve closes. 3. Electromechanical valves. These valves have electro magnets controlling whether the valve is open or closed. These valves can only be fully open or fully closed. 4. Mechanical Valves. These valves use mechanical energy in the process of opening and closing the actual valve. Larger valves can be opened and closed using mechanical processes such as levers and pulleys, whereas smaller mechanical valves can be opened or closed via a turning wheel or pulling a level by hand.
There are four major valve types: ball valves, butterfly valves, globe valves, and plug valves. There is also an array of many other types of valves specific to certain processes. Selecting which type of valve to use depends on what task the valve must carry out and in what space said valve can fit to carry out the task.
Some general features that one can take into consideration when selecting a valve are the following: 1. Pressure rating 2. Size and flow capacity 3. Desired flow condition 4. Temperature limits 5. Shutoff response to leakage 6. Equipments and pipes connected 7. Material compatibility and durability & lastly the cost which would be number 8. Water hammer (or, more generally, fluid hammer) is a pressure surge or wave caused when a fluid (usually a liquid but sometimes also a gas) in motion is forced to stop or change direction suddenly (momentum change). Water hammer commonly occurs when a valve closes suddenly at an end of a pipeline system, and a pressure wave propagates in the pipe. It is also called hydraulic shock.
This pressure wave can cause major problems, from noise and vibration to pipe collapse. It is possible to reduce the effects of the water hammer pulses with accumulators, expansion tanks and other features.
Rough calculations can be made either using the Joukowsky equation, or more accurate ones using the method of characteristics.

Water Hammer
Water hammer is a specific plumbing noise, not a generic name for pipe clatter. It occurs when you shut off the water suddenly and the fast-moving water rushing through the pipe is brought to a quick halt, creating a sort of shock wave and a hammering noise. Plumbing that's properly installed has air chambers, or cushions, that compress when the shock wave hits, softening the blow and preventing this hammering. The chambers can fail, though, because water under pressure gradually absorbs the air.
If you never had hammering and then it suddenly starts, most likely your plumbing system's air chambers have become waterlogged. You can cure water hammer by turning off the water behind the waterlogged chamber, opening the offending faucet and permitting the faucet to drain thoroughly. Once all the water drains from the chamber, air will fill it again and restore the cushion. If the air chamber is located below the outlet, you may have to drain the main supply lines to allow the chamber to fill with air again.
The air chamber will not drain properly if it's clogged with scale or residue from chemicals or minerals in the water. The chamber always should be larger than the supply pipe to preclude such clogging. Since the chamber is simply a capped length of pipe, however, all you have to do to clear it is remove the cap and clean out the residue