Critical equipment

Published:  05 June, 2014

Amin Almasi examines integrally-geared compressors for HVAC and plant applications for PWE

An integrally-geared centrifugal compressor is a multi-shaft compressor where each stage is a 3D impeller operated at its optimum speed; such a compressor consists of a gear unit with a central bull-gear and different gear pinions which drives impeller stages (3D impellers). Integrally-geared compressors have been used in different plant applications such as HVAC, refrigeration and other plant services.

Integrally-geared centrifugal compressors have provided many advantages such as advanced high-speed 3D impeller technologies, relatively low costs (compared to conventional compressors) and higher efficiencies compared to other turbo-compressor types. They can also offer nearly isotherm compression with an intercooler after one or two impellers (each stage or sometimes two stages). In other words, currently they can provide nearest possible operation to isotherm compression compared to many other compression options, therefore a really high efficiency could be provided by them. Again, the high efficiency is because all stages are intercooled (isotherm compression), and the speed increasing gear allows all stages to operate in a higher specific speed.

Operation and Design

Compressors in plants are normally un-spared, and are considered as critical equipment items. Compressor reliability is usually considered a high priority, and can be directly proportional to the plant’s profit. Integrally-geared machines use multiple bearings, seals, gear meshes and operate at high speeds. In standard machines, all these should be managed to achieve a high reliability.

The reliability records of integrally-geared centrifugal compressors have been improved over years and particularly modern integrally-geared centrifugal compressors have introduced high reliability and availability over the last decade.

Traditionally, some specialists have suggested using integrally-geared centrifugal compressors only for spared compressor applications. However, the reality is that many compression services in plant applications are un-spared and it is not feasible to provide two machines instead of one. Because the gas is cooled after each impeller stage (or two impellers), the compressor’s performance characteristics and reliability are dependent on intercooler conditions. These intercoolers should be designed and manufactured in a compact and cost-effective manner with suitable reliability and operation. Many manufacturers’ standard integrally-geared compressors are typically supplied with only an overall surge protection system and not individual impeller stage protection, they are prone to surging if intercoolers do not attain design heat removal requirements. Special care is needed for checking and verification of the intercoolers. The reference check is an important consideration for integrally-geared compressors. Details of seals, varying flow-rates and the possibility of fouling can significantly reduce the reliability of this type of compressor opposed to traditional applications in inert gas applications. As a very rough indication, manufacturer standard integrally-geared compressors from reputable manufacturers with proper references can now offer above 98% of availability, which is adequate for many HVAC and plant’s services.

For integrally-geared centrifugal compressors like many other types of centrifugal compressors, the sectional head-capacity characteristic curve of each compressor section should rise continuously from the rated point to the predicted surge. The compressor, without the use of a bypass, should be suitable for continuous operation at any capacity at least 10% greater than the predicted surge capacity.

Thrust loads from impellers and gears should be absorbed by individual thrust bearings on pinions, or transmitted to the bull-gear thrust bearing by means of thrust rider rings fixed to the pinions and bull gear. All specified operating conditions and start-up conditions should be evaluated for residual thrust loads. Balance pistons are normally not used. Thrust balancing maybe achieved by helix thrust force direction of the gearing and offsetting impeller aerodynamic thrust forces.

The bearing design and operation in an integrally-geared compressor need great attention. The speed is usually very high, often for last stages above 40,000 rpm. Specially designed bearings are needed which should be tightly controlled in the manufacturing, installation and operation. As a design criterion, bearing metal temperatures should not exceed 100°C at all specified operating conditions. The oil inlet temperature range should be adjusted with great care. The geometry and the design of advanced 3D impellers can have significant effects on the overall performance of an integrally-geared centrifugal compressor because the flow development inside each 3D impeller not only determines the aerodynamic performance and efficiency of the 3D impeller itself, but it also strongly affects the performance, operation and efficiency of the downstream and upstream systems such as IGV (inlet guide vanes), diffuser (volute), integrate facilities and complex piping systems of each stage.

3D Impellers in Integrally-Geared Compressors

The flow inside an integrally geared compressor package is three-dimensional (3D) and complicated. The blade angle distributions in each 3D impeller (from the inlet to outlet of each impeller) have significant influence on the impeller flow characteristics and overall integrally-geared compressor performance and operation. The blade angle distributions can affect compressor performance, loss generation, operating range, and surge/stall limits. The variations of the hub meridional blade angle distribution have usually greater effects on operation and efficiency than varying the shroud meridional blade angle distribution. The efficiency, operation and performance vary with the blade angle distribution.

The comparison of the wall skin friction, blade loading, and hub-to-shroud loading losses should be done for different operating scenarios such as design (rated) mass flow-rate, reduced (part-load) flow-rates, etc. The wall skin friction loss usually increases as the variation of blade angle increases because the increase in the blade surface area.

The impeller blades which have relatively small variations of blade angle and are relatively radial, have been found to exhibit relatively small wall skin friction loss due to relatively small surface area but a relatively high blade loading loss. Further, the flow separation near the hub at the suction side and the tip might occur and all these could result in relatively higher energy losses and also relatively small margins to stall for these blades.

In contrast, the blades which have large variations of blade angle, exhibit relatively large wall skin friction and hub-to-shroud loading losses, but they represent relatively low blade loading loss because the flow from the inlet to the outlet is usually well guided. However, as the flow coefficient increases, the performance of such blades (blades with large variations of blade angle) could steeply decrease. To properly design a 3D impeller, relationships between the blade angle distributions and the flow characteristics, loss generation, and performance of these impellers should be carefully studied and proper optimisation should be done for that impeller. The blade distribution angles and meridional contours of a 3D impeller should properly be optimised.

Gear Systems in Integrally-Geared Compressors

The gear system is one of the key systems in an integrally-geared centrifugal compressor. The complex configuration, high accuracy, and high-load operating conditions of gear systems in an integrally-gear compressor require very stringent design and manufacturing criteria. The gear system is usually manufactured by a special sub-vendor (usually a well-known gear unit manufacturer) and then sent to the compressor vendor for further works and the completion of the compressor unit. The natural modes, vibration responses, and generated noise levels should be properly investigated for gear systems. Gear system noises and their dynamic forces are becoming increasingly important topics for integrally geared compressors. Particular noise is a critical factor in an HVAC application.

There are two primary types of dynamic excitations in gear systems, which can cause excessive vibration and noise responses. One type is the gear meshing dynamic excitation. This type of dynamic excitation is unique to geared systems, and arises from a combination of the periodic variation in the meshing tooth number, tooth impact forces, and transmission error because of elastic tooth deformation, gear tooth profile manufacturing error, and misalignments. The frequency of this dynamic force is directly related to the tooth-to-tooth time period and therefore it shows up at mainly the mesh harmonics in the response spectrum.

The second type of dynamic excitation in gear systems represents an external set of dynamic shaft loads that typically occurs much lower in frequencies compared to the mesh harmonics. The sources of this dynamic excitation includes shaft rotational imbalance, shaft geometrical eccentricity, and dynamic loads coming from other sources such as compressor stages, the driver, and torque fluctuations under loaded conditions. Because of the fact that there are two types of dynamic excitations in an integrally-geared compressor, as long as one is significant, the gear system of an integrally-geared compressor can experience undesirable levels of vibration and noise responses. However, depending on the relative strength of each type of excitation, the dominant response may occur at lower shaft frequencies or higher mesh frequencies. Furthermore, if the degree of nonlinearity is strong enough, there may also be dynamic interactions between the gear meshing and shaft dynamic excitations.

In other words, the noise is generated mainly at gear rotational frequency, as a result of unbalance, eccentricity and swash, and at tooth contact frequency (TCF) and its harmonics. The tooth contact frequency (TCF) and its harmonics are caused by variations in the tooth contact forces resulting from geometric errors and variable elastic mesh deflections.

Total power loss in gear systems can be broken down into the contributions of friction between the teeth, lubrication, and gear wind age effects. Regarding the lubrication losses in gear systems, they are usually because of churning and jet lubrication which could induce gas–oil trapping in inter-tooth spaces. Power losses in high-speed gear systems come from the friction between the teeth (sliding and rolling), the lubrication process (dip or jet lubrication), the pumping of a gas–lubricant mixture during the meshing, and the losses associated with wind age effects.

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