General Comparison between Centrifugal and Reciprocating Compressor Reference: What's Correct for My Application - A Centrifugal or Reciprocating Compressor By: Paul Gallick, Senior Applications Engineer, Elliott Company, Greg Phillipi and Benjamin F. Williams, Ariel Corporation Parameter Maximum Flow Centrifugal Compressor Reciprocating Compressor Capacity is limited by cylinder size, the number of throws available, 3 They can be sized for an inlet flow of 680,000 actual m /h in a single and the available driver speeds. A "throw" is a location on the body. Actual means at given suction pressure and temperature. The crankcase where a compressor cylinder can be attached. maximum flow through a centrifugal compressor is limited by the choke point, which is the point at which the flow through some part of the compressor nears a velocity of Mach 1. Minimum Flow Similar to the maximum flow, the minimum flow in a reciprocating 3 It is recommended that for flow rates of actual 300 m /h and above , compressor is limited by the cylinder size, stroke, and speed. centrifugal compressors be critically evaluated for suitability. Unlike Reciprocating compressors of capacities of a few m3/h are available. a reciprocating compressor where flow is solely a function of compressor geometry and speed, the minimum flow for a centrifugal compressor is limited by an aerodynamic condition known as surge, which is a function of compressor geometry, speed, aerodynamic gas conditions, and system resistance. Minimum Suction (Inlet)Pressure This can be atmospheric or sub-atmospheric (vacuum). For subCan be atmospheric or vacuum. Where suction conditions involve sub-atmospheric pressures, adequate measures must be taken to atmospheric suction conditions, special seal and buffering designs are employed to prevent atmospheric air from being drawn into the prevent atmospheric air leaking into the cylinder through the piston rod packing. compressor. Maximum Discharge (Outlet) For horizontally split compressors discharge pressures up to 100 Typical reciprocating compressors in the process industry are used Pressure barg are common. For radially split (barrel) compressors discharge to generate discharge pressures as high as 800 barg. Special pressures could go as high as 1000 barg. compressors known as hypercompressors used in the low density polyethylene manufacture will generate pressures as high as 3500 barg. Minimum Suction (Inlet) Standard Centrifugal compressor materials are typically suitable for - The common compressor cylinder materials, cast gray iron and cast Temperature 20 to -50 deg C. Refrigeration compressors in ethylene service ductile iron are acceptable for use at temperatures as low as -40 deg typically have temperatures as low as -100 deg C which require C which typically occur in refrigeration applications. The lowest special low temperature alloys. The lowest temperature suction temperatures required typically are in LNG boil-off requirement for centrifugal compressors is typically found in LNG applications with requirements as low as -170 deg C and there are boil-off gas applications. Minimum temperatures up to -170 deg C very limited manufacturers for this application. are required to be accommodated for this service and low temperature alloy steels are employed as materials. Low temperature seals and O-Rings are also required. Maximum Discharge (Outlet) Temperature Maximum discharge temperatures are typically 200 to 230 deg C. Centrifugal compressors with higher temperatures can be manufactured but would require special designs such as center supported diaphragms, less efficient seal materials, and high temperature O-rings and sealants. Discharge temperature limits will depend on the application (gas compressed) and the seal element materials selected. In hydrogenrich gas applications, API 618 (2007) limits discharge temperatures to 135 deg C. For natural gas service the maximum discharge temperature limit is 175 deg C. However, a more practical limit followed is 149 deg C. Air compressor discharge temperature limits may be as high as 200 deg C. Flow Range (turndown) Flow range of a centrifugal compressor is determined by the surge and choke points. Typical turndown for a fixed speed, multi-stage centrifugal compressor is approximately 20-30%. With variable speed drive or adjustable inlet guide vanes the turndown can be increased to 40-50%. Reciprocating Compressors have the ability to change flow through speed control, the addition of fixed clearance to a cylinder (fixed or variable volume clearance pockets), cylinder end deactivation, and gas recycle. Typical flow range might be from 100%, down to 20%, or even lower. The application will determine what type of capacity control method is required and used. On low compression ratio applications (compression ratio less than 1.6, such as pipeline transmission of natural gas) adding fixed clearance will hardly change the flow. Such an application may require speed control or cylinder end deactivation. In other applications with higher compression ratios, clearance pockets and cylinder end deactivation are commonly used to regulate flow. Compression Ratio For centrifugal compressors compression ratio is a function of gas molecular weight, compressibility factor, stage geometry, speed, and the number of compressor stages. For a specific gas, the limits to compression ratio are the mechanical and rotodynamic limitations on speed and the number of stages that can be accommodated in a single body. High discharge temperatures due to high compression ratios can usually be controlled by inter-cooling between a compression stage. The maximum compression ratio that a reciprocating compressor can handle in one stage is limited mostly by gas discharge temperature. The piston rod load generated by the compression ratio may also be a limit. Typical compression ratios for one stage are 1.2 to 4.0. Compressed Gas Molecular Weight Compression ratio is highly depended on gas molecular weight. Head is developed by increasing gas velocity to create kinetic energy and then converting the kinetic energy to pressure in the diffuser. The amount of kinetic energy is a function of the gas velocity and gas molecular weight. Centrifugal compressors are used for a broad range of molecular weights including low molecular weight applications such as hydrogen recycle and high molecular weight application using refrigerant gases with molecular weights over 100. Reciprocating compressors are not limited by gas molecular weight. Both light and heavy gases are compressed very well. Over the range of molecular weight different application configurations may be required. For example, very low molecular weight gases may present seal challenges and very high molecular weight gases may present challenges related to compressor efficiency. Efficiency Polytropic efficiencies are used for centrifugal compressors rather than adiabatic efficiencies. In applications involving air compression adiabatic efficiencies are used. Typical polytropic efficiencies range from 70% to 85%. Efficiencies approaching 90% are possible. Efficiencies are primarily affected by internal leakage and mechanical losses. Reciprocating compressors have a very characteristic adiabatic efficiency curve. Refer the figure. As compression ratio drops, adiabatic efficiency drops. Efficiency changes with molecular weight too. Other factors also impact efficiency, most significantly the compressor cylinder's ratio of valve flow area to main bore diameter and piston speed. Multiservice Capability Materials of Construction Cost - Capital and Operating Typically centrifugal compressors are not designed to handle a multitude of gases. Customized designs would be required which could handle different gases simultaneously. Reciprocating compressors are very adaptable to a multitude of gases and can handle different gases at either the same stage or at different stages in the same machine. The number of different services on a given compressor crank case (frame) is only limited by the throws available and the number of stages required for each service. 8, 10, and even 12 frames are not uncommon. Materials for major components such as casings, nozzles, shafts, impellers etc. are primarily carbon steel, stainless steels and / or alloy steels. Components may be cast, forged, or machined. Cast iron maybe used for some stationary components. Material selection is primarily dependent on temperature, stress (pressure / torque), and gas composition (corrosive / erosive). Reciprocating compressors are made of very common materials such as gray iron, ductile iron, carbon steel, stainless steel, and alloy steel. This could be in cast, forging, or bar stock form. Some compressor pistons and covers maybe made of aluminum. For corrosive applications it is common to use stainless steel such as 17-4PH or 400 series for piston rods and compressor valve seats and guards. The capital cost of a centrifugal compressor is typically higher than a Generally a reciprocating compressor will have a lower capital cost but a higher operating cost (excluding power consumption). For the reciprocating compressor operating at the same conditions. This is primarily due to the fact that centrifugal compressors require parts same operating conditions a reciprocating compressor will consume less power per unit volume flow. The reason for higher operating with more complex geometry such as impellers and diaphragms. cost is due to more wearable parts requiring frequent maintenance However, a centrifugal compressor has fewer wearing parts, and leading to higher machine downtime. Compressor valves resulting in lower operating costs in terms of replacement parts, happen to be one of the most wearable parts in a reciprocating compressor. repairs and downtime. For gas pipeline compression service where large centrifugal compressors (>7500 kW) are employed using gas turbine drivers becomes economical compared to electrical motors when doing a cost evaluation in terms of capital and operating expenditure. Reliability Reliability / availability of centrifugal compressors is typically 98 to 99%. Reliability / availability of reciprocating compressors is typically 95 to 98%. Since reciprocating compressors have many more parts and more rubbing seals (pressure packing, piston rings, and rider rings) that wear and require more frequent replacement, they are considered somewhat less reliable than centrifugal compressors. Another reciprocating compressor component are compressor valves (simple spring-loaded check valves) which require frequent maintenance and replacement. In clean gas service and without much variation in operating conditions a centrifugal compressor can operate continuously for 10 years or longer. Maintenance requirements are typically limited to replacing bearing pads and seal wearing parts. Maintenance requirements for reciprocating compressors vary significantly with the application and follow maintenance patterns very much based on what has been described in the reliability section. Compressor valve and seal elements may require to be maintained in durations as short as a few months and as long as 3-5 years. Major machine overhaul including bearing replacement may be required after 10 years of operation or longer. Installation Time and Complexity The installation time varies widely depending on the size of the compressor. The number of main casing nozzle and the type of driver (electric motor / gas or steam turbine) also affect installation time. Location can also be a factor. Remote or offshore locations can add to installation time. The compressor and driver are typically packaged on a base plate complete with oil piping and wiring to junction boxes. Process equipment such as scrubbers and coolers and process control valves are typically installed at site. Auxiliary systems such as lube oil consoles, control panels, and seal buffer systems may also be installed separately. Piping and wiring from these auxiliary systems and process equipment to the compressor train are typically done at site. Installation time for a typical motor / gear driven compressor package is 2-3 weeks. For very large compressor or a gas turbine driven compressor the installation time could be as high as 6-8 weeks. Installation time for a reciprocating compressor varies significantly with site and location, and whether or not the compressor is packaged. Packaged compressors up to 3.4 MW and of a high-speed short stroke design are common today. Installation time for these might vary from a few days to a couple of weeks. Larger slow speed long stroke compressors assembled at site might require 3 to 4 weeks to install. Typical Maintenance Intervals Lead Time Prepared by: Lead time for a centrifugal compressor train range from 35 to 75 Lead time for a bare compressor will vary from 14 to 40 weeks weeks. Often the lead time is governed by the driver (electric motor depending on size and manufacturer. Electrical motor driven reciprocating compressors may require longer lead times specifically / turbine) since these are generally made to order. Special if large high horsepower motors are required. For reciprocating gas metallurgy and / or special design requirements of compressor engine driven large compressors the lead times may be shorter. components significantly adds to the lead time. Ankur Srivastava Chemical Engineer ankur_2061@hotmail.com