Compressor selection and sizing is one of the most critical yet frequently misunderstood aspects of process plant design. Whether you’re specifying a centrifugal compressor for high-flow, moderate-pressure applications, or a rotary screw unit for steady baseline compression, getting it right impacts capital cost, operating efficiency, and plant reliability.
Many process engineers rely on rules of thumb or vendor-supplied curves without understanding the underlying physics. This leads to oversized units (wasting millions in capital), undersized equipment (creating operational bottlenecks), or selection of the wrong compressor type entirely.
Why Compressor Sizing Matters
Compressors are often the largest power consumers in chemical plants. A poorly sized unit can increase energy costs by 15-25% annually. Oversizing is equally damaging—extra capacity sits unused, elevated discharge temperatures cause reliability issues, and maintenance costs spike.
Key consequences of poor sizing:
• Over-design: Capital cost increase, higher operating pressure leading to increased energy and heat rejection
• Under-design: Insufficient capacity at peak demand, throttling losses, elevated discharge temperatures causing seal and bearing failures
• Wrong type selected: Centrifugal vs. reciprocating vs. rotary screw each has distinct efficiency curves; wrong choice degrades performance
• Unstable operation: Compressors operating far from design point exhibit vibration, noise, and shortened life
The Compressor Sizing Fundamentals
Proper sizing requires understanding three key parameters:
1. Mass Flow Rate (kg/s or m³/min)
This is the compressor’s primary duty. Defined at inlet conditions (temperature, pressure, composition).
Common error: Using volumetric flow at discharge instead of inlet. Always work in terms of inlet volume or mass flow.
2. Inlet and Outlet Pressure
Determine the pressure ratio: PR = P_discharge / P_inlet
Pressure ratio is critical because it defines:
– Compression work required (and hence power)
– Compressor type selection (centrifugal for low ratios < 4, reciprocating for high ratios > 8)
– Intercooling requirements (for high ratios, reducing each stage to 3-4 reduces power significantly)
3. Inlet Temperature and Gas Properties
Hotter inlet gas = higher volume = larger compressor needed
Composition (molecular weight, heat capacity ratio) affects polytropic work calculation
Compressor Type Selection Matrix
Centrifugal Compressors:
– Best for: High flow, low to moderate pressure ratio (1.5–4), continuous duty
– Advantages: Lower capital cost at high flow, excellent efficiency over design range, compact footprint
– Disadvantages: Surge risk at low flow, requires steady inlet conditions, lower pressure ratios per stage
– Typical range: 100–10,000 m³/min, pressure ratios 1.2–4.0 per stage
Reciprocating (Piston) Compressors:
– Best for: High pressure ratios, variable flow, small to medium sizes
– Advantages: High pressure capability (single-stage up to 10 bar, multi-stage to 300+ bar)
– Disadvantages: Higher capital cost, more maintenance, lower efficiency, larger footprint
– Typical range: 10–1,000 m³/min
Rotary Screw Compressors:
– Best for: Baseline steady load, medium flow, simplicity
– Advantages: Lower initial cost, simple operation, good part-load efficiency
– Disadvantages: Lower full-load efficiency than centrifugal, limited pressure ratio per stage
– Typical range: 10–500 m³/min, pressure ratios 4–9 per stage
The Design Point and Operating Envelope
Every compressor has a design point: the flow, pressure ratio, and speed at which it operates most efficiently. Operation near the design point equals maximum efficiency and reliability.
Operation far from design point creates problems:
– Surge (centrifugal): Flow reversal at low capacity; causes vibration, noise, damage
– Throttling losses: Artificially reducing discharge flow to match lower downstream demand
– Elevated discharge temperature: Operating at higher flow or pressure than design point overheats the compressor
– Cavitation risk: Inlet pressures too low relative to design
How to Calculate Polytropic Work and Power
The polytropic work accounts for real compressor behavior (not isentropic):
W_poly = (Z_avg × R × T_inlet / (n_poly – 1)) × [PR^((n_poly – 1) / n_poly) − 1]
Where:
– Z_avg = average compressibility factor
– R = specific gas constant
– T_inlet = inlet temperature (K)
– n_poly = polytropic efficiency (typically 0.75–0.85 for real compressors)
– PR = pressure ratio
Brake power required = W_poly / motor_efficiency
Note: Isentropic work always underestimates actual power; polytropic efficiency factor corrects for real-world losses.
Real-World Example: Why Sizing Matters
A plant needs 500 m³/min of air at 8 bar gauge. Simple approach: buy a screw compressor rated 500 m³/min at 8 bar.
Better approach: Analyze demand profile.
– Peak demand: 500 m³/min, 4 hours/day
– Average demand: 250 m³/min, 20 hours/day
– Idle: 0 m³/min, valve unloaded
Result: Install a 300 m³/min screw compressor with VFD plus storage tank. VFD matches flow to demand, saving 30% energy. Payback in 18 months. Wrong sizing would waste 50% energy during light-load periods.
GrowMechanical Compressor Tools
GrowMechanical’s Centrifugal Compressor Calculation Sheet and Multi-Stage Compression Design Templates provide:
– Automated polytropic work calculation
– Compressor type recommendation based on duty
– VFD energy savings analysis
– Capital vs. operating cost trade-off studies
– Integration with downstream equipment
Need professional compressor sizing for your project? Contact GrowMechanical at contact@growmechanical.com to discuss custom calculation templates or detailed design support.
