A Success Story

Screw compressor is one of the great success stories of the last quarter of the twentieth century in which the increase in the number of units manufactured has been spectacular, due principally to the development of advanced manufacturing techniques, the fact of which had limited the attractiveness of this machine in the first period of its modern existence [1].

Screw compressors are rotary positive displacement machines of simple design capable of high speed operation over a wide range of operating pressures and flow rates with high efficiencies. They are both reliable and compact and consequently they comprise 80% of all positive displacement compressors now sold and 50% of those currently in operation. A high accuracy profile milling and grinding make it now possible to reduce linear tolerances to below 10 m m. This permits rotors to be manufactured with interlobe clearances of 30-50 m m at an economic cost. Internal leakages have thus been reduced to a small fraction of their values in earlier designs and as a result twin screw compressors are more efficient than other types of compressors as well as being smaller.

Screw machines are used today for different applications both as compressors and expanders. They operate on a variety of working fluids which may be gases, dry vapour or multi-phase mixtures with phase changes taking place within the machine. They may operate without internal lubrication, oil flooded or with other fluids injected during the compression. For optimum performance from such machines, a specific design and operating mode is needed for each application. Hence, it is not possible to produce efficient machines by the specification of a universal rotor configuration or set of working parameters, even for a restricted class of machines.

A field of air compression, both dry or oil free is almost exclusively covered by screw compressors. The situation is becoming the same for the case of the process gas compression. In the field of refrigeration, reciprocating and vane compressors are continuously replaced by screw ones and a dramatic increase in needs of refrigeration compressors is expected in the next few years.

A commercial range of screw compressor is covered today by compressors whose outer diameter of the male rotor is between 75 mm and 620 mm. They produce between 0.6 m³/min and 600 m³/min of the compressed gas. A pressure ratio is 3.5 for dry compressors up to 15 for oil flooded ones. A normal pressure difference is up to 15 bars, but maximal pressure difference sometimes exceeds 40 bars. Typically, for oil flooded air compression applications, the volumetric efficiency of these machines now exceeds 90 % and the specific power input has been reduced to values which were regarded as unattainable only a few years ago.

Hidden Details

A Swedish company SRM was a pioneer in the field of a screw compressor art. Some other companies, like Compair of U.K, Atlas-Copco in Belgium, Ingersol-Rand and Denver Gardner in USA, GHH in Germany follow them closely. York, Trane and Carrier are leaders in the screw compressor applications in refrigeration and air conditioning. British company Holroyd overnumber all them in a screw rotor manufacturing, they also are tool design and tool machine manufacturer.

Inspite of the increasing screw compressor popularity, a public knowledge on screw compressors is still limited. Three classical screw compressor textbooks were published in Russian. [2] 1960 gives a full scale profile generation of circular, elliptic, cycloidal, and the most important, a reproducible presentation of a russian asymmetric profile later named SKBK. The profile generation in this book was based on an envelope approach. [3] 1961 practically repeats the excercise on screw profiles giving his contribution in the filed of a rotor tool profile generation. [4] 1977, 17 years after gives for the first time a reproducible presentation of SRM unsymmetric profile together with classical Lysholm profile. Two textbooks were published in German: [5], 1979 presented a profile generation method based on gear theory to reconstruct SRM unsimmetric profile 10 years after it was patented. [6] 1988, published some engineering aspects of screw compressors. There are two textbooks in English dealing with screw compressors: [7] 1993, on industrial compressors and [8] 1994, on rotary twin shaft compressors.

There is a substantial number of patents dealing with various aspects of screw compressors, specially their profiles. Only SRM claims 750 patents, 1946 (symmetric), 1970 (unsymmetric) and 1982 ("D" profile) are classical reference literature presenting state of the art in screw compressor profile generation. Succesfull profiling patents are also applied by Atlas-Copco, Compair, Denver Gardner, Hitachi, Fu Sheng, Ingersol-Rand. All patented profiles were generated by some suitable procedure, but useful information on methods used were hardly disclosed in patents or in accompanying publications. Information in patents are so hidden or tedious to reproduce directly that [9] 1978 publishes a derivation of a symmetric circular profile which was patented 32 years earlier. [5] used a classical gear conjugacy condition to reproduce a SRM unsymmetric profile 9 years after the patent was granted. A SRM licensee 1995, after 13 years of manufacturing, used a full scale academic potential to reproduce SRM "D" profile analytically.

Journals surprisingly lack of screw compressor literature. Lysholm classical papers [13] 1942 and [10] 1966 were exceptions in mid of the century, but he did not mention any of profiling details introduced into his successful screw profiling practice to reduce a blow-hole area.

Three compressor conferences deal exclusively or partly with screw compressors: Purdue compressor technology conference in U.S.A, IMechEng conference on industrial compressors in England and "VDI Schraubenkompressoren Tagung" in Germany. Inspite of abundance of screw compressor papers, a limited number of conference papers reveal useful information on screw compressor rotor profiling. All conference papers state that something was done, but nearly none of the papers give any reproducible information which means an insight into a profiling procedures used. Typical of these papers are the frequently cited publications of [11] and [12], also [14], [15], and [16], from which a reader will gain little on profile generation. [17], and [18] indicate that they used an envelope theory to calculate some geometry features of their rotors. [19] 1984 presents a rack generation of a screw rotor profile giving a fully reproducible pattern based on the method presented in [5], but unfortunately, although based on sound ground, the resulting profiles were not commercially successful because of their poor tightness.

Many reference textbooks in gears are useful background for a screw rotor profiling, but all of them are limited to a classical gear conjugate action condition. The exception is work of Litvin who broadcasts his gearing knowledge after he moved to West [20].

The search for new profiles has been both stimulated and facilitated by recent advances in mathematical modeling and computer simulation. These analytical methods may be combined to form a powerful tool for process analysis and optimization and thereby eliminate the earlier approach of intuitive changes, verified by tedious trial and error testing. As a result, this approach to the optimum design of screw rotors lobe profiles has substantially evolved over the past few years and is likely to lead to further improvements in machine performance in the near future. However, the computer models and numerical codes reported in the open literature often differ in their approach and in the mathematical level at which various phenomena are modeled. A lack of comparative experimental verification still hinders a comprehensive validation of the various modeling concepts. In spite of this, computer modeling and optimization are steadily gaining in credibility and are increasingly employed for design improvement.

Ground for Further Developments

The efficient operation of screw compressors is mainly dependent on proper rotor design. An additional and important requirement for the successful design of all types of compressor is an ability to predict accurately the effects on performance of the change in any design parameter such as clearance, rotor profile shape, oil or fluid injection position and rate, rotor diameter and proportions and speed.

Now, when tight clearances are introduced and internal compressor leakage rates become small, further improvements are only possible by the introduction of more refined design principles. The main requirement is to improve the rotor profiles so that the internal flow area through the compressor is maximized while the leakage path is minimized and internal friction due to relative motion between the contacting rotor surfaces is made as small as possible. Although it seems that everything was done in the rotor profiling, there is a lot of room for substantial improvements. The most promising appears to be a rack profile generation which gives stronger but lighter rotors with higher throughput and lower contact stress. The later enables a lower viscosity fluid than oil to be used for lubrication.

Rotor housings with better shaped ports can be designed using a multivariable optimization technique. This reduces flow losses enabling higher rotor speeds giving more effective compressors. A fascinating improvement in the compressor bearing design is achieved in recent years enabling a process fluid lubrication. Also seals are more efficient today. All these gives a good foundation for more effective and more efficient screw compressors.

Rotor Profiles

An efficient screw compressor needs a rotor profile which has a large flow cross section area, a short sealing line and a small blow-hole area. The larger the cross section area the higher the flow rate for the same rotor sizes and rotor speeds. Shorter sealing lines and a smaller blow-hole reduce leakages. Higher flow and smaller leakage rates both increase the compressor volumetric efficiency, which is the rate of flow delivered as a fraction of the sum of the flow plus leakages. This in turn increases the adiabatic efficiency because less power is wasted in the compression of gas which is recirculated internally.

As precise manufacture permits rotor clearances to be reduced, despite oil flooding, the likeliehood of direct rotor contact is increased. Hard rotor contact leads to deformation of the female rotor, increased contact forces and ultimately rotor seizure. Hence the profile should be designed so that the risk of seizure is minimized.

Fig. 1 shows a pair of a rack generated rotors [21]. The selection and distribution of primary curves on a rack which was used to create these rotors give a larger cross section area with stronger female rotor lobes than any other known screw compressor rotor.

Two additional favourable features characterize these rack generated rotors. Firstly they maintain a seal over the entire contact length while maintaining a small blow-hole. Secondly, the two contact bands B-C and G-H are straight lines on the rack which form involutes on the rotors. Hence the relative motion between the rotors is the best it could be.

Compressor Design

Although advanced rotor profiles are a necessary condition for a screw compressor to be efficient, all other components must be designed to take advantage of their potential if the full performance gains are to be achieved. Thus rotor to housing clearances, especially at the high pressure end must be properly selected. This in turn requires either expensive bearings with smaller clearances or cheaper bearings with their clearances reduced to an acceptable value by preloading.

A screw compressor, especially of the oil flooded type, which operates with high pressure differences, is heavily loaded by axial and radial forces which are transferred to the housing by the bearings. Rolling element bearings are normally chosen for small and medium screw compressors and these must be carefully selected to obtain a satisfactory design. Usually two bearings are employed on the discharge end of the rotor shafts in order to absorb the radial and axial loads separately. Also the distance between the rotor centre lines is in part determined by the bearing size and internal clearance. An assembly drawing of the compressor is shown in Fig. 2 in which the bearing arrangement can be seen.

The same oil is used for oil flooding and for bearing lubrication, but the supply to the bearings is separate to minimize the friction losses. Oil is injected into the compressor chamber at the place where thermodynamic calculations show the gas and oil inlet temperature to coincide. The position is defined on the rotor helix with the injection hole located so that the oil enters tangentially in line with the female rotor tip in order to recover as much as possible of the oil kinetic energy.

To minimize the flow losses in the suction and discharge ports, the suction port is positioned in the housing to let the gas enter with the fewest possible bends and the gas approach velocity is kept low by making the flow area as large as possible. The discharge port size is determined by estimating the built-in-volume ratio required for optimum thermodynamic performance. It is then adjusted in order to reduce the exit gas velocity and hence obtain the minimum combination of internal and discharge flow losses. The casing should be carefully dimensioned to minimize its weight, containing reinforcing bars across the suction port to improve its rigidity at higher pressures. It must be added that recent advances in the development of advanced low friction rolling element bearings highly contribute to the efficiency and reliability of the modern screw compressor.

The rotors and compressor designed following such principles are presented in Figs. 3 and 4. More information about that design can be found in [22]

  Literature

                Fig. 1 "N" Rotor Profile
 
 

                Fig. 2 Screw Compressor layout
 

                Fig. 3 "N" Screw Compressor Rotors
 

                Fig. 4 Oil Flooded Screw Compressor
 
 
 
 
 
 
 
 

  Published on the Internet