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