Compression ratio is one of the simplest compressor calculations in oil and gas, but it explains a surprising amount about how a compressor is operating.
The equation is:
Compression ratio = absolute discharge pressure ÷ absolute suction pressure
The important word is absolute.
Using gauge pressure directly is one of the most common mistakes in this calculation.
Compression ratio matters because a compressor working from 50 psi suction pressure to 300 psi discharge pressure is doing something very different from a compressor working from 250 psi suction to 500 psi discharge, even though both examples involve substantial pressure.
The ratio helps operators and engineers understand compressor loading, discharge temperature, staging, capacity, and changing field conditions.
What Is Compressor Compression Ratio?
Compression ratio compares the gas pressure leaving a compressor stage with the pressure entering it.
It is commonly written as:
CR = Pdischarge ÷ Psuction
Both pressures must be absolute.
For example, a compression ratio of:
3
means the absolute discharge pressure is three times the absolute suction pressure.
Compression ratio is dimensionless because one pressure is divided by another pressure using the same units.
Why Must Compressor Pressure Be Absolute?
Gas compression calculations depend on the physical pressure experienced by the gas.
A pressure gauge normally measures pressure relative to the surrounding atmosphere.
That is gauge pressure.
Absolute pressure begins from a true zero pressure reference.
For a simple near sea level approximation:
PSIA = PSIG + 14.7
Suppose compressor suction pressure is:
50 psig
Approximate absolute suction pressure:
50 + 14.7 = 64.7 psia
If discharge pressure is:
200 psig
Absolute discharge pressure:
200 + 14.7 = 214.7 psia
Compression ratio:
214.7 ÷ 64.7 = approximately 3.32
If someone incorrectly divided gauge pressures:
200 ÷ 50 = 4
They would report a compression ratio of 4 instead of approximately 3.32.
That is a significant error.
What Is the Difference Between PSIG and PSIA?
PSIG means pounds per square inch gauge.
PSIA means pounds per square inch absolute.
At approximately normal atmospheric pressure near sea level:
0 psig is roughly 14.7 psia
Therefore a compressor suction gauge showing zero does not mean there is zero absolute pressure in the gas.
This distinction becomes especially important when compressor suction pressure is relatively low.
Example: Calculate Compression Ratio
Suppose a compressor operates at:
Suction pressure = 100 psig
Discharge pressure = 400 psig
Convert suction pressure:
100 + 14.7 = 114.7 psia
Convert discharge pressure:
400 + 14.7 = 414.7 psia
Now calculate:
Compression ratio = 414.7 ÷ 114.7
Compression ratio = approximately 3.62
The compressor is therefore operating at an approximate compression ratio of:
3.62
Why Is Compression Ratio More Useful Than Pressure Increase Alone?
Consider two compressors.
Compressor A:
Suction = 50 psig
Discharge = 250 psig
Compressor B:
Suction = 500 psig
Discharge = 700 psig
Both increase gauge pressure by:
200 psi
But the compression ratios are very different.
For Compressor A:
Suction absolute pressure:
50 + 14.7 = 64.7 psia
Discharge absolute pressure:
250 + 14.7 = 264.7 psia
Compression ratio:
264.7 ÷ 64.7 = approximately 4.09
For Compressor B:
Suction absolute pressure:
500 + 14.7 = 514.7 psia
Discharge absolute pressure:
700 + 14.7 = 714.7 psia
Compression ratio:
714.7 ÷ 514.7 = approximately 1.39
Both machines add 200 psi of gauge pressure.
The first compressor is operating across a much larger pressure ratio.
That difference matters.
What Does a Higher Compression Ratio Do?
As compression ratio increases, the compressor generally has to perform more work on each unit of gas.
Higher compression ratio can contribute to:
Higher discharge temperature
Higher power requirement
Lower volumetric efficiency in reciprocating compressors
Greater mechanical and thermal stress
Reduced capacity under some conditions
Greater need for multiple compression stages
The exact effect depends on compressor type, gas composition, temperature, efficiency, speed, and equipment design.
Why Does Gas Get Hot When It Is Compressed?
Compressing gas requires work.
That energy increases the gas temperature unless enough heat is removed during the compression process.
This is why compressor discharge piping is usually hotter than suction piping.
The greater the compression ratio, the greater the potential temperature rise.
Gas temperature therefore becomes an important operating limit.
Can You Estimate Compressor Discharge Temperature?
Yes.
A simplified ideal gas relationship for isentropic compression is:
T₂ = T₁ × (P₂ ÷ P₁)^((k minus 1) ÷ k)
Where:
T₁ = absolute suction temperature
T₂ = ideal discharge temperature
P₁ = absolute suction pressure
P₂ = absolute discharge pressure
k = ratio of specific heats
This gives an idealized discharge temperature.
Real compressor discharge temperature is generally higher than the ideal isentropic result because real compression is not perfectly efficient.
Why Must Temperature Be Absolute?
Just as pressure must be absolute, thermodynamic temperature calculations require an absolute temperature scale.
For Fahrenheit:
Rankine = Fahrenheit + 459.67
For Celsius:
Kelvin = Celsius + 273.15
Do not insert ordinary Fahrenheit or Celsius temperatures directly into compressor temperature equations.
Example: Estimate Ideal Compressor Discharge Temperature
Suppose gas enters a compressor at:
80 degrees Fahrenheit
Compression ratio:
3
Assume for this simplified example:
k = 1.3
Convert suction temperature to Rankine:
80 + 459.67 = 539.67 degrees Rankine
The exponent is:
(1.3 minus 1) ÷ 1.3
0.3 ÷ 1.3 = approximately 0.2308
Now:
T₂ = 539.67 × 3^0.2308
The pressure ratio temperature factor is approximately:
1.288
Therefore:
T₂ = approximately 695 degrees Rankine
Convert back to Fahrenheit:
695 minus 459.67 = approximately 235 degrees Fahrenheit
The ideal discharge temperature is therefore roughly:
235 degrees Fahrenheit
Actual discharge temperature depends on compressor efficiency and real gas behavior.
What Is Isentropic Efficiency?
Isentropic compression is an ideal thermodynamic process.
A real compressor requires more work than an ideal compressor to reach the same discharge pressure.
Isentropic efficiency compares real compressor performance with that idealized process.
A simplified temperature relationship can be expressed as:
Efficiency = ideal temperature rise ÷ actual temperature rise
Rearranging:
Actual temperature rise = ideal temperature rise ÷ efficiency
This provides a useful way to understand why real discharge temperatures exceed ideal values.
Example: Include Compressor Efficiency
From the previous example:
Suction temperature = 80 degrees Fahrenheit
Ideal discharge temperature = approximately 235 degrees Fahrenheit
Ideal temperature rise:
235 minus 80 = 155 degrees Fahrenheit
Suppose isentropic efficiency is:
75 percent
Convert:
75 percent = 0.75
Estimated actual temperature rise:
155 ÷ 0.75 = approximately 207 degrees Fahrenheit
Estimated actual discharge temperature:
80 + 207 = approximately 287 degrees Fahrenheit
This simplified example shows how compressor inefficiency increases discharge temperature.
Actual engineering calculations should use appropriate gas properties and compressor methods.
Why Is High Discharge Temperature a Problem?
Excessive compressor discharge temperature can affect:
Lubricating oil
Valves
Seals
Packing
Gaskets
Cylinder components
Piping
Downstream equipment
Gas properties
Equipment reliability
Every compressor has operating limits established by its design and manufacturer.
A high discharge temperature alarm should not be treated as merely an inconvenient process alarm.
It can indicate that the compressor is operating outside desirable conditions.
What Causes Compressor Discharge Temperature to Rise?
One common cause is increasing compression ratio.
But other causes can include:
Higher suction temperature
Reduced compressor efficiency
Valve problems
Cooling system problems
Changing gas composition
Intercooler fouling
Low coolant flow
High ambient temperature
Recycle conditions
Mechanical problems
The temperature trend should be considered together with pressure, flow, load, and equipment condition.
Why Does Low Suction Pressure Increase Compression Ratio?
Suppose discharge pressure stays constant at:
400 psig
Initially suction pressure is:
150 psig
Convert to absolute pressure:
Suction = 164.7 psia
Discharge = 414.7 psia
Compression ratio:
414.7 ÷ 164.7 = approximately 2.52
Now suppose suction pressure falls to:
75 psig
New absolute suction pressure:
75 + 14.7 = 89.7 psia
Compression ratio:
414.7 ÷ 89.7 = approximately 4.62
Discharge pressure did not change.
But compression ratio increased from approximately:
2.52
to:
4.62
That is a major operating change.
Why Does Low Suction Pressure Matter in Mature Oil and Gas Fields?
As wells decline, gathering system pressure and available gas supply can change.
A compressor may be expected to pull increasingly low suction pressure while still delivering gas into a pipeline operating at a relatively stable pressure.
That increases compression ratio.
Eventually the compressor may become limited by:
Temperature
Horsepower
Cylinder loading
Capacity
Rod load
Valve performance
Other equipment constraints
A compressor installed years earlier may therefore become less suitable as field conditions evolve.
Can Higher Discharge Pressure Increase Compression Ratio?
Yes.
Suppose suction pressure remains:
100 psig
Initial discharge pressure:
300 psig
Absolute pressures:
Suction = 114.7 psia
Discharge = 314.7 psia
Compression ratio:
314.7 ÷ 114.7 = approximately 2.74
Now downstream pipeline pressure rises and compressor discharge pressure becomes:
450 psig
Absolute discharge pressure:
464.7 psia
New compression ratio:
464.7 ÷ 114.7 = approximately 4.05
The compressor now has to compress against a much more demanding pressure ratio.
Why Might Pipeline Pressure Change Compressor Performance?
A compressor does not operate independently from the system around it.
Its suction pressure depends on upstream conditions.
Its discharge pressure depends on downstream conditions.
If gathering pressure falls while sales pipeline pressure rises, the compressor gets squeezed from both sides.
Suction pressure decreases.
Discharge pressure increases.
Compression ratio can rise sharply.
This can reduce available capacity and increase discharge temperature.
What Is Single Stage Compression?
Single stage compression means the gas reaches its required discharge pressure through one compression stage.
For modest compression ratios, this can be practical.
As the total pressure ratio becomes larger, dividing compression into multiple stages can provide important advantages.
What Is Multistage Compression?
Multistage compression divides the total pressure increase between two or more compression stages.
Instead of compressing gas directly from low suction pressure to high final discharge pressure in one step, the gas is compressed partially, cooled, and then compressed again.
For example:
Stage 1 raises gas from low pressure to intermediate pressure.
An intercooler removes heat.
Stage 2 raises gas from intermediate pressure to final pressure.
Large compression systems can use several stages.
Why Use Multiple Compressor Stages?
Multistage compression can help:
Control discharge temperature
Reduce compression work
Improve equipment practicality
Manage pressure ratio per stage
Remove liquids between stages
Operate within compressor mechanical limits
The appropriate number of stages depends on the required pressures, gas properties, compressor design, and operating range.
How Do You Calculate Total Compression Ratio Across Several Stages?
Use the initial suction absolute pressure and final discharge absolute pressure:
Total compression ratio = final absolute discharge pressure ÷ initial absolute suction pressure
You can also multiply the individual stage ratios.
For a two stage compressor:
Total ratio = Stage 1 ratio × Stage 2 ratio
Example: Two Stage Compression Ratio
Suppose:
Stage 1 ratio = 3
Stage 2 ratio = 3
Total compression ratio:
3 × 3 = 9
The final absolute discharge pressure is nine times the initial absolute suction pressure, ignoring intermediate pressure losses for the simplified relationship.
How Do You Find Equal Compression Ratio Per Stage?
If total compression ratio and number of stages are known, an approximate equal ratio can be calculated.
For two stages:
Stage ratio = √total compression ratio
For three stages:
Stage ratio = cube root of total compression ratio
More generally:
Stage ratio = total compression ratio^(1 ÷ number of stages)
This is useful for preliminary calculations.
Actual compressor stage pressures can differ because of cooling, pressure losses, gas properties, equipment selection, and design constraints.
Example: Divide a Ratio of 9 Across Two Stages
Total compression ratio:
9
Number of stages:
2
Stage ratio:
√9 = 3
Each stage would have an approximate compression ratio of:
3
The first stage compresses by a factor of 3.
The second stage compresses by another factor of 3.
Overall:
3 × 3 = 9
Example: Divide a Ratio of 27 Across Three Stages
Total ratio:
27
Number of stages:
3
Stage ratio:
Cube root of 27 = 3
Each stage would operate at an approximate ratio of:
3
Overall:
3 × 3 × 3 = 27
How Do You Calculate Intermediate Pressure?
For two stage compression with approximately equal ratios:
Intermediate absolute pressure = √(initial absolute pressure × final absolute pressure)
Suppose:
Initial suction pressure = 50 psia
Final discharge pressure = 450 psia
Intermediate pressure:
√(50 × 450)
Multiply:
50 × 450 = 22,500
Square root:
√22,500 = 150 psia
Stage 1 ratio:
150 ÷ 50 = 3
Stage 2 ratio:
450 ÷ 150 = 3
Both stages have an equal pressure ratio of 3.
Why Is Intercooling Used Between Compressor Stages?
Gas becomes hot during compression.
An intercooler removes some of that heat before the gas enters the next stage.
Cooler gas is denser.
Reducing gas temperature between stages can reduce the work required by later compression stages and control discharge temperatures.
Intercooling is therefore a major advantage of multistage compression.
What Is an Interstage Scrubber?
Cooling compressed natural gas can cause liquids to condense.
These liquids may include:
Water
Hydrocarbon condensate
Compressor oil
Other entrained liquids
An interstage scrubber separates accumulated liquid before gas enters the next compressor stage.
This is important because many compressors are not designed to ingest significant quantities of liquid.
Why Can Cooling Gas Create Liquid?
Compressed gas can contain water vapor and heavier hydrocarbons.
After compression, the gas passes through an intercooler.
As temperature falls, the gas may no longer be able to hold the same amount of vapor.
Liquid condenses.
That is why compressor packages commonly combine:
Compression
Cooling
Liquid separation
The equipment works as a system.
Why Is Liquid Entering a Compressor Dangerous?
Compressors are designed primarily to compress gas.
Liquid is far less compressible.
Large liquid slugs can create severe mechanical forces in reciprocating compressors.
Even smaller amounts of liquid can cause:
Valve damage
Cylinder problems
Lubrication issues
Erosion
Corrosion
Unstable operation
Suction scrubbers and liquid shutdown systems therefore provide important protection.
What Is a Suction Scrubber?
A suction scrubber is a vessel installed upstream of a compressor to remove free liquid from the gas stream.
Gas enters the vessel.
Liquid droplets separate.
Gas leaves toward the compressor.
Collected liquid is removed through the vessel liquid handling system.
A high liquid level can trigger an alarm or compressor shutdown.
Why Would a Compressor Trip on High Suction Scrubber Level?
The shutdown protects the compressor from liquid carryover.
If the scrubber cannot remove liquid quickly enough, level rises.
Possible causes include:
Upstream separator carryover
Condensation
Failed liquid level control
Blocked liquid outlet
Sudden liquid slug
Changing gas conditions
Allowing the compressor to continue running could expose it to liquid ingestion.
What Is a Reciprocating Compressor?
A reciprocating compressor uses pistons moving inside cylinders to compress gas.
Gas enters through suction valves.
The piston reduces cylinder volume.
Pressure rises.
Discharge valves open when cylinder pressure exceeds discharge pressure.
The compressed gas then leaves the cylinder.
Reciprocating compressors are common where relatively high pressure ratios or flexible operating conditions are required.
What Is a Screw Compressor?
A screw compressor uses rotating helical elements to trap and compress gas.
Screw compressors are widely used in natural gas gathering and processing.
They can be well suited to certain low and moderate pressure applications and changing flow conditions.
Compression ratio still matters, but equipment behavior differs from a reciprocating compressor.
What Is a Centrifugal Gas Compressor?
A centrifugal compressor uses rapidly rotating impellers to add energy to gas.
The gas gains velocity through the impeller and pressure through the compressor flow path.
Centrifugal compressors are common in high volume gas processing and transmission applications.
Their performance is strongly connected to:
Flow
Speed
Gas properties
Pressure ratio
Compressor map
Surge
Stonewall or choke conditions
Is Compression Ratio Calculated the Same Way for Different Compressor Types?
The basic pressure ratio is still:
Absolute discharge pressure ÷ absolute suction pressure
That relationship applies regardless of whether the compressor is:
Reciprocating
Screw
Centrifugal
Another design
What changes is how the machine responds to that ratio and what operating limits become important.
What Is Compressor Surge?
Surge is primarily associated with dynamic compressors such as centrifugal compressors.
It occurs when the compressor cannot maintain stable forward flow under certain low flow and pressure conditions.
Flow can become unstable and may temporarily reverse.
Surge can create:
Strong vibration
Noise
Pressure fluctuations
Mechanical stress
Potential equipment damage
Anti surge control systems are used to keep centrifugal compressors away from this unstable region.
Does High Compression Ratio Cause Surge?
Not by itself.
Surge depends on the compressor operating point relative to its performance map.
However, changing suction pressure, discharge pressure, flow, gas composition, and speed can move the operating point toward the surge region.
Compression ratio is one part of that operating picture.
What Is a Compressor Performance Map?
A centrifugal compressor map shows how the machine performs across combinations of:
Flow
Pressure ratio or head
Speed
Efficiency
The map also identifies operating boundaries.
Operators and engineers use it to determine whether the compressor is operating within an acceptable region.
A pressure ratio number without flow information is not enough to completely describe centrifugal compressor performance.
What Is Volumetric Efficiency in a Reciprocating Compressor?
Volumetric efficiency describes how effectively the compressor cylinder fills with new suction gas.
A reciprocating compressor does not replace every cubic inch of cylinder volume with fresh gas during each cycle.
One reason is clearance volume.
Some compressed gas remains in the cylinder after discharge.
That trapped gas expands during the next suction stroke before fresh suction gas can enter.
Why Does Higher Compression Ratio Reduce Reciprocating Compressor Capacity?
At higher compression ratio, gas trapped in the cylinder clearance space is compressed to a higher pressure.
During the suction stroke, that gas must expand farther before cylinder pressure falls enough for the suction valve to open.
The suction valve therefore opens later.
Less fresh gas enters the cylinder.
Volumetric efficiency decreases.
This is one reason falling suction pressure can reduce compressor capacity even when the compressor continues running at the same speed.
What Is Clearance Volume?
Clearance volume is the space remaining inside a reciprocating compressor cylinder when the piston reaches the end of its compression stroke.
Some clearance is necessary because the piston cannot physically contact the cylinder head and valves.
Gas trapped in this space affects compressor capacity.
Clearance can also be intentionally adjusted on some compressors to control capacity.
Can Adding Clearance Reduce Compressor Capacity?
Yes.
Increasing clearance leaves more compressed gas inside the cylinder.
That gas expands during the next suction stroke.
Fresh gas enters later.
The cylinder takes in less new gas.
Adjustable clearance pockets are therefore one method of capacity control on some reciprocating compressors.
Why Does Low Suction Pressure Reduce Gas Throughput?
Gas density decreases as suction pressure falls, assuming other conditions remain similar.
Each cubic foot entering the compressor then contains less gas mass.
At the same time, increasing compression ratio can reduce volumetric efficiency in a reciprocating compressor.
These effects combine.
The machine may continue moving a similar physical displacement while processing fewer standard cubic feet of gas.
Why Does High Suction Temperature Reduce Compressor Capacity?
Hot gas is less dense than cooler gas at the same pressure.
That means each unit of actual suction volume contains less gas mass.
A compressor with fixed physical displacement therefore handles fewer standard cubic feet when suction temperature increases.
This is why suction cooling conditions can influence capacity.
How Does Gas Molecular Weight Affect Compression?
Natural gas is not one single substance.
Its composition can include:
Methane
Ethane
Propane
Butanes
Carbon dioxide
Nitrogen
Hydrogen sulfide
Heavier hydrocarbons
Changing composition affects gas properties such as:
Density
Compressibility
Specific heat ratio
Required horsepower
Discharge temperature
Compressor performance
A compressor that handles one gas composition well may behave differently when the gas becomes significantly richer or leaner.
Why Can a Compressor Start Tripping After Field Conditions Change?
The compressor may not have developed a new mechanical failure.
The operating conditions may have moved.
Possible changes include:
Lower suction pressure
Higher discharge pressure
Higher suction temperature
Different gas composition
More liquid entering the scrubber
Higher required flow
Reduced cooling
Increasing compression ratio
A compressor designed around one operating envelope can eventually encounter very different conditions.
How Should an Operator Troubleshoot Rising Compression Ratio?
First determine which side changed.
Did suction pressure fall?
Did discharge pressure rise?
Did both happen?
Then look at the surrounding system.
For falling suction pressure, investigate:
Upstream well production
Gathering restrictions
Suction valves
Filters or scrubbers
Suction piping
Other compressors
For rising discharge pressure, investigate:
Sales pipeline pressure
Discharge valves
Coolers
Check valves
Downstream restrictions
Facility operating changes
Compression ratio tells you that conditions changed.
The individual pressures tell you where to start looking.
Example: Compression Ratio Trend
Suppose a compressor normally operates at:
Suction = 150 psig
Discharge = 400 psig
Absolute pressures:
164.7 psia suction
414.7 psia discharge
Ratio:
414.7 ÷ 164.7 = approximately 2.52
Several months later:
Suction = 100 psig
Discharge = 425 psig
Absolute pressures:
114.7 psia suction
439.7 psia discharge
Ratio:
439.7 ÷ 114.7 = approximately 3.83
Later:
Suction = 75 psig
Discharge = 450 psig
Absolute pressures:
89.7 psia suction
464.7 psia discharge
Ratio:
464.7 ÷ 89.7 = approximately 5.18
Looking only at discharge pressure might make the change seem modest.
Looking at compression ratio reveals how dramatically the compressor operating condition changed.
Can Compression Ratio Be Too High?
Every compressor has operating limits.
There is no single compression ratio that is acceptable for every compressor.
Limits depend on:
Compressor type
Number of stages
Gas composition
Discharge temperature
Cylinder design
Rod loading
Valve design
Driver horsepower
Cooling
Lubrication
Manufacturer requirements
Never use a general rule of thumb as a substitute for the compressor operating limits supplied for the actual equipment.
Why Is There No Universal Maximum Compression Ratio?
A small reciprocating compressor and a large centrifugal pipeline compressor operate very differently.
Even two reciprocating compressors can have different:
Cylinder sizes
Clearance
Speed
Valve arrangements
Cooling
Gas composition
Pressure ratings
Horsepower
A ratio that is reasonable for one machine may be unacceptable for another.
How Can Multiple Stages Reduce Discharge Temperature?
Suppose the total required pressure ratio is:
9
One stage would compress through the entire ratio of 9.
Two approximately equal stages would each have a ratio of:
3
Gas can be cooled between the stages.
Instead of entering the second stage at the very high temperature created by continuous compression, it enters after intercooling.
This reduces the final temperature and can reduce compression work.
Why Is Equal Compression Ratio Per Stage Only an Approximation?
Real compressor systems contain pressure losses.
Gas loses pressure through:
Intercoolers
Scrubbers
Piping
Valves
Filters
Other equipment
Gas temperature may also not return to the same value after each cooler.
Different compressor cylinders or stages may have different limits.
Therefore equal pressure ratio is a useful starting concept, not a universal final design.
How Do You Calculate a Two Stage Compressor Example?
Suppose gas enters at:
50 psig
and must leave at:
500 psig
Convert to absolute pressure:
Initial pressure:
50 + 14.7 = 64.7 psia
Final pressure:
500 + 14.7 = 514.7 psia
Total compression ratio:
514.7 ÷ 64.7 = approximately 7.96
For two approximately equal stages:
Stage ratio = √7.96
Stage ratio = approximately 2.82
Approximate intermediate absolute pressure:
64.7 × 2.82 = approximately 182.5 psia
Convert to gauge pressure:
182.5 minus 14.7 = approximately 167.8 psig
So a simplified equal ratio arrangement would place intermediate pressure near:
168 psig
Stage 1:
64.7 to 182.5 psia
Stage 2:
182.5 to 514.7 psia
Each stage has an approximate compression ratio of:
2.82
Actual design would account for interstage pressure losses and equipment requirements.
What Happens If One Stage Has a Much Higher Ratio Than Another?
That stage may experience:
Higher discharge temperature
Greater work requirement
Different capacity
Greater mechanical loading
Reduced efficiency
Potential operating limit problems
An uneven stage pressure split can sometimes indicate an operating or equipment issue, although the expected stage pressures depend on the actual compressor design.
Why Can Intercooler Problems Affect Stage Pressures?
An intercooler creates some pressure drop.
If it becomes fouled or restricted, pressure loss can increase.
This can change the suction condition for the next stage.
Poor cooling also increases next stage suction temperature.
That warmer gas is less dense and can affect compressor capacity.
An intercooler problem can therefore show up as more than a high temperature.
It can alter the balance of the compressor package.
What Happens If an Interstage Scrubber Becomes Restricted?
A restriction creates pressure loss between stages.
The upstream stage may see increased discharge pressure.
The downstream stage may receive lower suction pressure.
Both effects can increase the pressure ratio experienced by the affected stages.
This is why differential pressure and stage pressure trends can be valuable troubleshooting information.
Can Compressor Valves Affect Compression Ratio?
Failed or leaking valves in a reciprocating compressor can reduce effective compression performance.
A leaking discharge valve can allow high pressure gas to return into the cylinder.
A leaking suction valve can push gas backward toward the suction side.
Symptoms can include:
Reduced capacity
Abnormal temperatures
Changed pressure behavior
Valve temperature differences
Increased power consumption
Unusual sound
Compression ratio calculations alone cannot diagnose a valve failure, but they help establish the operating conditions around the problem.
Why Are Temperature Measurements Useful on Reciprocating Compressor Valves?
Valve problems often create abnormal heat.
Gas leaking repeatedly through a damaged valve can generate a localized temperature increase.
Comparing valve temperatures between similar cylinders or ends can help identify abnormal behavior.
Operators often gain more information by trending temperatures than by looking at one isolated reading.
What Is Compressor Recycle?
Recycle routes some discharge gas back toward the compressor suction system.
It can be used for:
Capacity control
Maintaining minimum flow
Preventing surge on centrifugal compressors
Startup
Process stability
Recycle does not create free compression capacity.
The compressor spends power recompressing gas that has already passed through the machine.
Why Does Recycle Gas Need Cooling?
Discharge gas is hot.
If hot discharge gas is recycled directly to suction, suction temperature can rise.
Higher suction temperature reduces gas density and can increase discharge temperature.
Recycle systems therefore often include cooling or route gas through equipment where temperature can be controlled.
How Does Compression Ratio Affect Compressor Horsepower?
Higher pressure ratio generally increases the work required to compress each unit of gas.
However, compressor horsepower cannot be calculated accurately from compression ratio alone.
You also need information such as:
Gas flow
Suction temperature
Gas composition
Compressibility
Efficiency
Compressor type
Operating conditions
Compression ratio tells you how large the pressure change is relative to suction pressure.
It does not tell you how much gas is being compressed.
Can a Compressor Have a High Compression Ratio but Low Horsepower?
Yes.
A small compressor moving a small gas flow can operate at a relatively high ratio while requiring modest total power.
A large pipeline compressor operating at a lower ratio can require thousands of horsepower because it moves enormous quantities of gas.
Power depends on both the work per unit of gas and the amount of gas being processed.
Can a Compressor Have a Low Compression Ratio but High Horsepower?
Absolutely.
Large transmission compressors often move very high gas volumes.
Even a modest pressure ratio can require enormous power when the mass flow is large.
This is another reason compression ratio should not be treated as a direct horsepower measurement.
What Is the Most Common Compression Ratio Calculation Mistake?
Using gauge pressure.
Suppose:
Suction = 20 psig
Discharge = 100 psig
Incorrect method:
100 ÷ 20 = 5
Correct approximate method:
Suction absolute:
20 + 14.7 = 34.7 psia
Discharge absolute:
100 + 14.7 = 114.7 psia
Compression ratio:
114.7 ÷ 34.7 = approximately 3.31
The incorrect answer is 5.
The approximate correct answer is 3.31.
At low pressures, the difference can be enormous.
Why Is the Error Smaller at Very High Pressures?
Atmospheric pressure becomes a smaller fraction of the total pressure.
For example:
1,000 psig is approximately 1,014.7 psia.
Adding 14.7 changes the number by only a small percentage.
But:
10 psig is approximately 24.7 psia.
Here, atmospheric pressure represents a very large portion of the absolute pressure.
The lower the gauge pressure, the more dangerous it is to ignore the absolute pressure conversion.
What Should You Record When Evaluating Compressor Performance?
Useful operating information includes:
Suction pressure
Discharge pressure
Suction temperature
Discharge temperature
Gas flow
Compressor speed
Driver load
Recycle position
Interstage pressures
Interstage temperatures
Scrubber levels
Cooler temperatures
Ambient temperature
Gas composition when available
One measurement rarely tells the whole story.
Trends are usually more valuable.
A Practical Compressor Calculation
Suppose a two stage field gas compressor has:
First stage suction = 40 psig
Final discharge = 400 psig
First convert to absolute pressure.
Initial suction:
40 + 14.7 = 54.7 psia
Final discharge:
400 + 14.7 = 414.7 psia
Total compression ratio:
414.7 ÷ 54.7 = approximately 7.58
Estimate equal ratio per stage:
√7.58 = approximately 2.75
Approximate intermediate absolute pressure:
54.7 × 2.75 = approximately 150.4 psia
Convert to gauge pressure:
150.4 minus 14.7 = approximately 135.7 psig
A simple theoretical pressure split is therefore:
First stage suction = 40 psig
Interstage pressure = approximately 136 psig
Final discharge = 400 psig
Both stages would operate at roughly the same pressure ratio before accounting for real equipment pressure losses and design considerations.
Why Is Compression Ratio Worth Trending?
Because it summarizes changes on both sides of the compressor.
If suction pressure falls, ratio increases.
If discharge pressure rises, ratio increases.
If both happen, ratio can increase dramatically.
Tracking compression ratio alongside discharge temperature and gas flow can make gradual changes easier to recognize.
A compressor that once operated comfortably may slowly move toward a difficult operating condition even though no single pressure changes dramatically from one day to the next.
Frequently Asked Questions
What is compressor compression ratio?
Compression ratio is the absolute discharge pressure divided by the absolute suction pressure.
What is the compression ratio formula?
Use:
Compression ratio = absolute discharge pressure ÷ absolute suction pressure
Should compressor compression ratio use PSIG or PSIA?
Use PSIA or another absolute pressure unit.
How do you convert PSIG to PSIA?
For a simple near sea level approximation:
PSIA = PSIG + 14.7
Use the appropriate atmospheric or measurement basis for the actual application.
What does a compression ratio of 3 mean?
It means the absolute discharge pressure is three times the absolute suction pressure.
Does low suction pressure increase compression ratio?
Yes, if discharge pressure remains similar.
Does high discharge pressure increase compression ratio?
Yes, if suction pressure remains similar.
Why does high compression ratio increase discharge temperature?
More work is required to compress the gas through the larger pressure ratio, which increases gas temperature.
Why are compressors divided into stages?
Multiple stages can control temperature, reduce compression work, and keep each stage within practical operating limits.
How do you calculate total compression ratio across two stages?
Multiply the two stage ratios or divide final absolute discharge pressure by initial absolute suction pressure.
How do you estimate equal compression ratio for two stages?
Use:
Stage ratio = √total compression ratio
How do you estimate equal compression ratio for three stages?
Use:
Stage ratio = cube root of total compression ratio
How do you calculate intermediate pressure for two equal stages?
A simplified relationship is:
Intermediate absolute pressure = √(initial absolute pressure × final absolute pressure)
Why is an intercooler installed between compressor stages?
It removes heat from compressed gas before the gas enters the next stage.
Why is an interstage scrubber needed?
Cooling can condense water and hydrocarbons. The scrubber removes liquid before gas enters the next compression stage.
Why does low suction pressure reduce reciprocating compressor capacity?
Lower suction pressure reduces gas density and can increase compression ratio. Higher ratio also increases reexpansion of clearance gas, reducing the amount of fresh gas entering the cylinder.
Is compression ratio the same as pressure difference?
No. Pressure difference is discharge pressure minus suction pressure. Compression ratio is discharge absolute pressure divided by suction absolute pressure.
Can two compressors have the same pressure difference but different compression ratios?
Yes. The suction pressure can make their compression ratios very different.
Is there one maximum safe compression ratio for every gas compressor?
No. Acceptable limits depend on compressor type, gas properties, stage arrangement, temperatures, mechanical limits, horsepower, and manufacturer requirements.
Does compression ratio tell you compressor horsepower?
No. It helps determine compression work, but horsepower also depends on gas flow, temperature, composition, efficiency, and other factors.
What is the most common mistake when calculating compression ratio?
Using gauge pressures directly instead of converting both suction and discharge pressure to absolute pressure first.
What compressor measurements should be reviewed with compression ratio?
Review suction and discharge temperature, gas flow, driver load, speed, interstage conditions, cooler performance, scrubber levels, and recycle position.
What is the main equation to remember?
Use:
Compression ratio = absolute discharge pressure ÷ absolute suction pressure
Then remember the practical interpretation:
Lower suction pressure increases the ratio.
Higher discharge pressure increases the ratio.
A higher ratio generally makes the compression job more demanding.