RU2448273C2 - Rotary screw machine - Google Patents

Abstract

FIELD: machine building.

SUBSTANCE: rotary screw machine includes housing 3, drive 1 and driven 2 rotors located in it. Screw surfaces of rotors 1, 2 have two adjacent sections which differ by strokes of coils, within the limits of each of which the surface profile remains constant, binding numbers of screw entries, external radii, length and profiles of screw surfaces of rotors.

EFFECT: higher capacity and efficiency of machine and lower noise level.

3 cl, 4 dwg

Description

The inventive rotary screw machine is designed to compress gaseous media and can be used in various industries where gases with high pressure are required. It can also be used as a vacuum pump. The inventive device is reversible and can be used as an expander (expander) for converting the energy of compressed gas into mechanical work during its expansion. The combination of a compressor and an expander with intermediate heating or cooling of gas allows the use of devices in units operating in the direct or reverse thermodynamic cycle (in internal combustion engines and refrigerators).

A rotary screw machine is a volumetric gas compression device, where the role of the pistons is played by the teeth of one rotor rolling over the grooves of another. Most of the screw machines currently used in technology are based on the Lishholm design - see, for example, Amosov P.E. and others (Screw compressor machines, Handbook. Leningrad: Engineering, 1977, p.67-68) [1]. The device has helical rotors with convex teeth for the driving rotor and concave cavities for the driven rotor. The number of teeth of the leading rotor is four, the driven one is six. Thread profiles in the plane perpendicular to the axis of rotation of the rotors are complex, asymmetric; the length of the screw surface is from 0.75 to 0.86 turns of the turns of the driving rotor P _Z (from 270 to 310 ° in the rotation angle).

The disadvantages of this compressor design are:

1) pressure pulsations and intermittent supply of compressible gas;

2) the backflow of gas from cavities with a higher pressure back into the cavity with a lower pressure through the gaps between the moving surfaces of the rotors and the housing.

The first problem, and partly the second, is related to the fact that the screw compressor delivers compressible gas in separate portions. Cavities containing portions of gas are formed in the grooves of the screws and are limited along the periphery of the rotors by the cylindrical surfaces of the housing and by the ends of the rotors by the planes of the housing (spacers) with windows for gas inlet and outlet. Gas compression (changing the volume of the cavity) occurs when the cavity is not connected to the windows and is introduced by inserting the opposite rotor tooth into the groove of this rotor and rolling it along the groove until, at a certain phase of rotation, the cavity is aligned with the window through which the compressed gas is squeezed out into injection communication. During compression, there is no gas emission from this groove. For example, when the compression ratio in volume is equal to three, compression occurs during ~ 3/4 of a rotor’s turn and only in 1/4 of a turn - output. The total flow rate is determined by the sequential approach to the cavity window in different grooves, offset by the angle of rotation of the rotors. With appropriate coordination of the compression ratio and the number of teeth (grooves) on the rotors, the total flow rate is equalized, however, local pulsations (discontinuities) of the gas flow in such a design are inevitable.

The second problem, the reverse flow of gas, is entirely due to the wrong (incorrect) chosen geometry of the rotors - see below. All this reduces the consumer characteristics and durability of the device.

The weak point of Liesholm screw machines, therefore, is the presence of case windows at the gas inlet and outlet, without which it is impossible to change the volume of the cut-off cavity in this design, i.e. gas compression. This is associated with noise and increased hydraulic losses, local discontinuity of output, flow pulsation and pressure, often requiring the use of receivers to smooth out pulsations. Variable forces act on the rotors both in the radial and axial directions, which complicates the work of the supports of the rotors and gears, coordinating the rotation of the rotors. In cavities adjacent in the angle of rotation of the rotors located in different phases of the compression process, the pressure is different; the presence of gaps between the ends of the rotors and the housing leads to the fact that the pumped medium through these gaps under the influence of a pressure differential partially flows from cavities with a higher pressure back into the cavity with a lower pressure. This reduces the performance of the screw machine, worsens the efficiency, leads to an increase in the temperature of the pumped medium and the device itself. Reducing the gaps reduces the intensity of the overflow, however, due to thermal expansion or manufacturing inaccuracies, the probability of mechanical contact of surfaces (up to jamming) increases.

In order to reduce backflows, liquids (oils) are added to the pumped gas, blocking gaps. This requires the creation of devices for capturing liquids at the outlet and supplying them to the inlet; In addition, the pumped gas is contaminated to some extent by the liquid.

The desire to get rid of end spacers with windows, i.e. from intermittent gas and leakage over end gaps, it has become necessary to create designs in which only screw surfaces of rotors and cylindrical surfaces of the housing participate in the formation of portions of the pumped medium, and gas is compressed inside the rotors by reducing the volume of the cavity when moving along the axes of the rotors from gas inlet to outlet.

Known designs with continuously or stepwise changing along the axis of the rotors geometry - decreasing along the length of the step of cutting screws, which are described in the following patents:

US patent No. 3807911 (IPC class F04C 18/08, priority date 05/14/1973) [2];

USSR copyright certificate for invention No. 861738 (IPC F04C 18/16, priority date 12/25/1979) [3];

RF patent for invention No. 2143590 (IPC class F04C 18/16, priority date 09/21/1998) [4].

So, in the application for US invention No. 2007/0041861 (IPC class F01C 1/16, priority date 08/22/2005) [5] a screw rotor and a vacuum pump are described. The device contains the same (symmetrical) rotors, the helical surfaces of which have two different turns of adjacent sections, within each of which the surface profile remains constant. The tooth profiles are rectangular, the number of screw starts n / n, where n≥1. Oil is used to block overflows through gaps.

Known screw pump with increased suction efficiency, described in US patent for the invention No. 7484943 (IPC class F04C 18/08, priority date 08/11/2006) [6]. This device contains the same (symmetrical) rotors, the helical surfaces of which have three differing in the course of the turns P _Z adjacent sections. Along the length of the rotors (from entrance to exit) in the first section, P _Z continuously decreases, within the second and third it remains constant. Tooth profiles mated. The number of calls of screws n / n, where n≥1; length of sections from 1 to 4 · P _Z ; the ratio of the lengths of the input and output sections 3/1; the ratio of P _Z sections of the order of one and a half tens. At the gas outlet, housing spacers with windows are left.

Known screw compressor according to the patent of the Russian Federation for the invention No. 2143590 (IPC class F04C 18/16, priority date 09/21/1998) [7], adopted as a prototype. In this compressor design, the rotors have screw parts formed by single-thread or double-thread, containing two adjacent sections - forming and compressing. The angle of inclination of the forming turns is continuously decreasing (P _{Z of} turns is increasing), and the angle of inclination of the compression turns is constantly increasing (P _{Z of} turns is decreasing) along the length of the thread towards the discharge side. The length of the first section 2 · P _Z ; the length of the second is determined by the required compression ratio of the compressor.

Each rotor has two screw parts at a structural distance from each other, which are in antiphase. Gas is sucked in from the two ends of the rotors and is fed into the middle, which allows unloading the bearings from axial forces.

The analysis showed that in a screw compressor without inlet and outlet windows at the ends, a closed cavity occupies the entire length of the screw groove within the turn of the turn; the pressure in it is almost the same over the entire length, regardless of what law changes the geometry of the groove in this section. Compression of gas inside the rotors is carried out by reducing the volume of the cavity when it moves along the axis of the rotors from the entrance to the exit and is determined by the ratio of the volumes that the cut-off cavity has at the inlet and outlet of the gas.

The use of sections with a length less than the length of a fully formed (cut off) cavity leads to incomplete compression and uneven flow at the outlet. An incorrectly selected ratio of the number of visits of cutting of the leading and driven rotors and the shape of the tooth profile leads to unrecoverable clearances even with absolutely accurate manufacturing and does not completely isolate portions of the pumped medium. Some gaps appear and disappear at a certain stage of rotation of the rotors. The uneven flow, pressure pulsations and support forces, increased noise remain, albeit to a lesser extent, than in Lysholm. The arbitrarily selected ratios of the lengths of adjacent sections on the rotors do not allow matching the internal compression ratio obtained on the rotors with the required external compression ratio, which is a disadvantage of the above devices.

The objective of the proposed technical solution is to optimize the design, which allows to reduce gas flow through the gaps, flow pulsations and noise, increase efficiency, which is achieved by

- creating a theoretically gapless design of a rotor screw machine, in which only permissible technological gaps between moving surfaces would take place;

- ensuring the greatest possible uniformity in the delivery of compressed / expanded gas, reducing pressure pulsations;

- coordination of the internal compression ratio on the rotors of the compressor or expansion of the expander with the calculated (most often used during operation).

The problem is solved due to the fact that in a rotary screw machine for compressing / expanding gaseous media containing a pair of rotors placed in cylindrical bores of the housing, the screw surfaces of which have two adjacent sections differing in the course of the turns, within each of which the surface profile remains constant, at the same time The following necessary and sufficient conditions are satisfied:

the number of visits of the leading rotor n ₁ is associated with the number of visits of the driven rotor n _{2 by the} ratio n ₂ = n ₁ +1;

для ведомого ротора и

для ведущего ротора;the radii of the vertices of the teeth of the rotors are related to the interaxal distance a _{w by the} relations

for driven rotor and

for the driving rotor;

the tooth surface of each section of the driving rotor is an equidistant displaced by Δ in the rotor body relative to the surface, the profile of which in the axial section of the rotor is described by a curve

where P _Z is the course of the turns of the screw surface of a given portion of the leading rotor, z is the coordinate along the axis of the rotor, ρ ₁ ≤R ₁ is the distance from the point on the curve z ₁ (ρ ₁ ) to the axis of the rotor, Δ is justified from the design and technological requirements in depending on the accuracy of processing of screw surfaces, the size and material of the rotors, the permissible differences in the heating of the rotors and the housing;

the tooth surface of the driven rotor is associated with a surface whose profile in the axial section is z ₁ (ρ ₁ );

, где b - параллельный оси размер прямолинейного участка вершин зубьев роторов;the minimum length of each section, providing the possibility of placing a closed cavity on it, is determined by the expression

where b is the size of the rectilinear portion of the tops of the teeth of the rotors parallel to the axis;

the ratio of the values of P _{Z of} adjacent sections of the rotor is equal to the calculated (most often used during operation) degree of compression / expansion of the rotor screw machine in volume.

These are the conditions that the optimal design of a screw machine must satisfy. If they are followed, the formation of theoretically completely isolated cavities at once on two rotors is ensured with a volume changing during rotation of the rotors (without using case windows at the ends of the rotors, periodically opening to allow intake of suction gas and release of compressed). Only technological clearances between moving surfaces remain in the design. Thus, pressure pulsations and overflow of the pumped medium through gaps from cavities with a higher pressure in cavities with a lower pressure are minimized.

An additional task of optimizing the device is to expand the use of the rotary machine.

According to the RF patent for invention No. 2143590 [7], dual rotors are used to balance axial loads - on each rotor there are two antiphase separated screw parts.

The symmetrical rotors proposed in the patent allow the device to be used only as a compressor (or only as an expander). In addition, the volumetric flow rates, temperature and pressure at the gas inlet (outlet) in two parts of the device can be different - full compensation of axial loads is not obtained.

радиан, что позволяет увеличить частоту выдачи сжимаемого газа, тем самым повысить равномерность работы роторной винтовой машины.A variant of the design of the rotor screw machine is the possibility of using the leading and driven rotors in it, and the screw parts of the rotors can be both in antiphase and can be deployed at a certain angle, for example,

radian, which allows to increase the frequency of delivery of the compressible gas, thereby increasing the uniformity of operation of the rotor screw machine.

This allows you to make the design more optimal and expand the conditions of its operation - to increase the frequency of delivery of portions of compressible gas, thereby increasing the uniformity of operation of the rotor screw machine.

The variant of using a twin rotor with the same direction of turns of both separate screw parts of the rotors in the inventive device allows combining the functions of a compressor and an expander (expander) in one rotary screw machine when it is necessary to compress the gas, pass it through some technological device, and then expand it. Naturally, in this case, the compressor and expander parts should be separated by a partition. The expansion work transmitted directly through the rotor shafts is partially or fully used to drive the compressor part. The difference in the work of compression and expansion with a deficiency is replenished with an electric motor or, in excess, is output to a generator. The need for such a combination of compressor and expander functions exists, for example, in refrigeration units, internal combustion engines (both directly and in turbocharging systems), in electrochemical current generators, heat pumps, in gas dehydration devices and air conditioners.

The design of the claimed device and its variants is a sectional view of the mating screws of the driving and driven rotors shown in the figures:

Figure 1 - design of the inventive rotor screw machine with two rotors having two adjacent screw sections with different travel turns.

Figure 2 - profiles of the leading and driven rotors of the inventive rotor screw machine in axial section.

Figure 3 is a variant of the inventive rotary screw machine with two twin, balanced multidirectional rotors.

Figure 4 is a variant of the inventive rotor screw machine with two twin equally directed rotors separated by a partition. A design combining a compressor and an expander.

Figure 1 shows the design structure of the inventive rotor screw machine with rotors having two adjacent screw sections, includes a leading (1) and driven (2) rotors, a housing (3) with covers (4) and (5), gears ( 6), coordinating the rotation of the rotors, suction chamber (7) and discharge (8), two adjacent screw sections - a section with a large stroke (9) and a section with a lower stroke (10), the plane dividing the sections with a larger and smaller stroke (11) .

Figure 2 shows the axial section of the rotors — a design diagram explaining the geometry parameters of the axial profiles of the turns of the lead (1) and driven (2) rotors used in the formula of the inventive design of a rotor screw machine, where a _W is the center distance;

ρ, z (ρ) are the coordinates of the profiles in the axial section in a cylindrical system;

R ₁ is the outer radius of the driving rotor (1);

R ₂ is the outer radius of the driven rotor (2);

b ₁ = b ₂ = b - parallel to the axis, the size of the rectilinear section of the tops of the teeth.

радиан. На каждом из симметричных участков смежные нарезки - с большим ходом (9) и с меньшим ходом (10).Figure 3 shows an embodiment of the inventive rotor screw machine, which consists of a leading (1) and driven (2) rotors, a housing (3) with covers (4) and (5), gears (6) matching the rotation of the rotors, the chamber suction (7) and discharge (8). The rotors are double, balanced through the use of symmetrically located sections, the screw surfaces of which can be of different diameters and lengths and are rotated, for example, at an angle

radian. In each of the symmetrical sections, adjacent cuts - with a large stroke (9) and with a lower stroke (10).

Figure 4 shows a variant of the inventive rotary screw machine, combining a compressor and an expander. The design includes a housing (3), in which dual leading (1) and driven (2) rotors are installed, having the same direction of turns, covers (4) and (5), gears (6), which coordinate the rotation of the rotors, the suction chamber ( 7) and discharge (8) of the compressor. Accordingly, a partition (12), separating the gas cavities of the compressor and expander with different pressures, and chambers (13) and (14) for entering the compressed gas into the expander and expanding the outlet are additionally needed here.

The inventive device operates as follows. The leading and driven rotors (1) and (2) form closed cavities with portions of the pumped medium and, rotating, move them in the axial direction. Due to the fact that the screw surfaces have two adjacent sections (9) and (10) with different strokes, the volume of a portion of the pumped medium gradually changes as it moves from one section to another. The full cycle - filling the cavity with its cut-off, compression and delivery - takes place over 3 turns of the rotor. The cavities go continuously, one after another, and on average, for each revolution of the rotor, one portion is issued from each groove. The channel is clamped by the teeth of the adjacent rotor in two places along the length (in extreme positions - in three). The flow pulsations are short-term (only for the transition time of the clamping section through the end plane of the rotor at the outlet) and make, for example, 1/3 of the flow rate with a ratio of the number of calls on the rotors 2/3, and with a dual design (Fig. 3) - 1/6 of the flow .

The proposed method for forming closed cavities under the conditions indicated above, and the inventive design of a rotary screw machine allow to obtain high performance without the use of liquid to block gas flows through gaps and devices for trapping it at the outlet and feeding to the inlet.

The values of the diameters of the rotors and the course of the turns of the sections (9) and (10) are selected based on the required volumetric productivity and the degree of compression of the device.

If the stroke of the screw surface of section (9) (in the direction of movement of the pumped medium) is greater than the course of section (10), the pumped medium is compressed — volume decreases with increasing pressure. Such rotors can be used in screw compressors and vacuum pumps. If the course of the section (9) of the screw surface is less than the course of the section (10), the pumped medium expands with a decrease in its pressure. Such rotors can be used in screw expanders (expanders) and engines that convert the energy of compressed gas into mechanical work.

The design with unidirectional sections can be used as an expander compressor for compressing a gaseous medium and its subsequent expansion after processing. A similar problem arises, for example, in refrigeration technology, where the refrigerant circulating in a closed circuit is first compressed, then (after cooling to ambient temperature) it is expanded to its initial pressure (with adiabatic temperature decrease) and fed to the cooled chamber. The compressor expander allows you to partially use the work of gas expansion for the compression process, which leads to an overall reduction in energy costs for the refrigeration cycle and allows you to achieve lower refrigerant temperatures.

The above-mentioned variants of the inventive rotary screw machine can expand the range of its application.

Rotary screw machines of the claimed design are used in production, while their volumetric capacity (by entry conditions) reaches up to 500 m ³ / h, and the compression ratio in pressure is up to 10. The version of the air compressor expander used on the basis of the design of the inventive rotary screw machine is shown figure 4, allows to obtain volumetric performance (according to the conditions of entry), for example, up to 50 m ³ / hour, and the compression ratio in pressure up to 3.

Claims (3)

1. A rotary screw machine for compressing / expanding gaseous media, comprising a housing, leading and driven (driven) rotors located therein, screw surfaces of which have two adjacent sections differing in the course of the turns, within each of which the surface profile remains constant, characterized in that that the number of visits of the leading rotor n ₁ is related to the number of visits of the driven rotor n _{2 by the} ratio n ₂ = n ₁ +1; the radii of the vertices of the teeth of the rotors are related to the center distance a _{w by the} relations

for driven rotor and

for the driving rotor; the tooth surface of the driving rotor is an equidistant displaced by Δ in the rotor body relative to the surface whose profile in the axial section of the rotor is described by a curve

where P _Z is the course of the turns of the helical surface of the driving rotor, z is the coordinate along the axis of the rotor, ρ ₁ is the distance from the point on the curve to the axis of the corresponding rotor, Δ is justified by the design and technological requirements depending on the accuracy of machining of the screw surfaces, sizes and materials of the rotors permissible differences in heating of rotors and housing; the tooth surface of the driven rotor is associated with a surface whose profile in the axial section is z ₁ (ρ ₁ ); the minimum length of each screw section is determined by the expression

where b is the size of the rectangular portion of the tops of the teeth of the rotors parallel to the axis; the ratio of the values of P _Z - adjacent sections of the rotor is equal to the calculated degree of compression / expansion of the rotor screw machine in volume. 2. The rotor screw machine according to claim 1, characterized in that when the leading and driven rotors are doubled, the screw parts of the rotors can be both in antiphase and can be rotated at an angle π / n radians. 3. The rotor screw machine according to claim 2, characterized in that the driving and driven rotors are made with the same direction of the turns and are separated by a partition.

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