Screw diameter is an important parameter of screw compressors. Under the requirements of series standardization and generalization, the principle of determining the screw diameter series is: within the range of the optimal peripheral speed, the number of screw diameters as few as possible can satisfy the widest possible range of air displacement. my country stipulates that the screw diameter series is (ram); (63), (80), (100), 125, 160, 200, 250, 316, 400, 500, 630, (800). The diameter with brackets is only suitable for asymmetric tooth shapes, of which 160, 200, 250, and 316 are the most commonly used.
Because the air displacement is proportional to the square of the screw diameter, the air displacement values of adjacent series of screw diameters are quite different, especially when the screw diameter is large. To this end, under each screw diameter, several aspect ratios are listed to change the exhaust volume range, so that the exhaust volumes of adjacent series of screw diameters can be staggered.
1. The aspect ratio refers to the ratio of the axial length l of the compressor screw to the screw (nominal) diameter D0. And denoted as λ≤l/Do. The general design selection range is between λ = 0.9 ~ 1.5, and it develops in the direction of small relative length. When the exhaust volume is the same, the machine with a smaller length-diameter ratio λ has a larger screw diameter and a larger area of the suction and exhaust orifices, so the gas flow loss is small. The machine with the smallest aspect ratio makes the screw short and thick, and its inertia moment increases, so that the rotor has good rigidity and increases the reliability of operation. In addition, from the overall structure point of view, with a shorter screw rotor, it is possible to connect two screw sections in series on one shaft to make the overall structure compact.
It should be pointed out that for compressors with large displacement, a higher ^ value is used. This is particularly evident for oil-injected machines, where the screw diameter is inconvenient to be too large. In order to obtain the required displacement, individual machines even have to use extra long lead (the aspect ratio is as high as 2.0 or more).
2. The twist angle of the screw The twist angle of the screw is a necessary condition to ensure that the compressor space between the teeth is fully inhaled and the internal compression is realized. In order to fully inflate the inter-tooth volume, the torsion angle rz of the screw should not be greater than the suction angle As. For the male screw, it is r1z≤a1s, and the suction angle can be determined by the basic relationship: a1s≈2π(1-1/Z1)-β. If the screw has a symmetrical circular arc tooth shape, β≈0 in the above formula, and for the standard tooth Z1=4, the suction angle of the male screw is spit a1s≈270°. When the lead of the screw is short, the twist angle r1z of the male screw is often larger than the suction angle. It is permissible as long as the excess value is not more than 30°.
If the screw is asymmetrical, then β=β01≈35. . For the standard tooth Z1=4, the male screw a1s≈235°, the torsion angle of the female screw r2z=l60°, it is beneficial to obtain a large actual torsion angle coefficient by taking a smaller torsion angle for the asymmetrical tooth shape. Generally, the range of torsion angle of the screw is:
r1z=240°～300°
r2z=160°～200°
List the lead T, aspect ratio λ and torsion angle rz of my country's screw series standards in Table 4-5. The diameter, length, relative length, lead, aspect ratio, and twist of the screw air compressor can be seen from the table:
a. After the screw diameter and lead are determined, the aspect ratio λ is related to the torsion angle rz. proportional;
b. The variation range of the torsion angle of the screw is r1z=240°～300°. When the screw diameter is a fixed value, there are only a few combinations of lead and aspect ratio. to obtain the above-mentioned twist angle values. This is why several leads need to be used for the same screw diameter.
In consideration of machining, in order to reduce tool size, the number of leads per screw diameter is limited. my country's screw compressor series stipulates that each screw diameter has two types of long and short leads. Only in individual cases (such as two-stage oil-injection machines, low-pressure stages of oil-free machines), in order to keep the screw diameter from being too large, it is allowed to use an extra long lead.