Determine Required Discharge Pressure

The compressor discharge pressure must overcome the sum of: hydrostatic pressure at the sparger depth (0.01 MPa per metre of liquid depth), sparger and diffuser pressure reduction (0.01 to 0.05 MPa for fine-bubble diffusers), pipe distribution pressure loss from compressor to fermenter (0.02 to 0.05 MPa depending on distance and flow rate), and any control valve differential pressure reserve (0.05 MPa minimum). For a typical 5-metre-deep fermenter with fine-bubble spargers, total back-pressure is 0.15 to 0.25 MPa; the compressor discharge pressure should be 0.25 to 0.35 MPa. Add 0.05 MPa margin for pressure fluctuation during batch cycle changes.

Calculate Total Aeration Air Demand

Fermentation aeration rate (VVM — volumes of air per volume of liquid per minute) is typically 0.5 to 1.5 VVM for aerobic fermentation depending on the organism and product. Multiply VVM by total bioreactor working volume to get total volumetric air flow at the fermenter. Convert to free air delivery at atmospheric pressure using: FAD (m³/min) = Fermenter air flow (m³/min) x Absolute discharge pressure (MPa) / 0.1013 MPa. Add 15 to 20% spare capacity margin and allowance for simultaneous demand from multiple fermenters at different batch stages.

Choose Between Single-Stage and Two-Stage Models

For discharge pressures below 0.30 MPa, single-stage LW and DW models provide the best specific energy consumption. For discharge pressures of 0.30 to 0.50 MPa, two-stage models achieve better energy efficiency by splitting the compression ratio across two stages with inter-stage cooling. The crossover point where two-stage becomes more efficient than single-stage is approximately 0.28 to 0.32 MPa under typical ambient conditions. Where discharge pressure requirement is uncertain or may vary between products, two-stage models provide more flexibility to operate at varying output pressures without efficiency penalty.

Plan the Sterilisation Filtration System Downstream

For pharmaceutical fermentation, the oil-free compressed air must still pass through a sterilising-grade inlet air filter (0.2 micron absolute rating for bacteria and mould removal) installed on the fermenter aeration inlet after the after-cooler and moisture separator. Oil-free compressed air does not require oil coalescing or activated-carbon stages. A pre-filter of 1 to 5 micron rating upstream of the sterilising filter extends sterilising filter service life. Filter housings must be steam-sterilisable or disposable for GMP compliance. The compressor manufacturer can provide guidance on filter sizing matched to each compressor model flow rate.

Plan Cooling Water and Foundation Requirements

Oil-free reciprocating compressors require a reliable cooling water supply for cylinder jacket cooling and inter-stage cooling. Cooling water flow rates range from approximately 2 m³/h for small LW models to 30 m³/h for large DW models; detailed requirements are in the installation manual per model. A closed-circuit cooling tower or plate heat exchanger cooling system is standard practice. Foundation requirements follow the same reinforced concrete isolated slab principles as for lubricated reciprocating compressors. Foundation drawings are provided on request at time of order. Anti-vibration mounts between compressor baseplate and concrete foundation are standard.

Oil contamination of fermentation aeration air causes several problems. Even trace levels of hydrocarbon oil in the aeration stream inhibit aerobic microbial metabolism — oil droplets coat cell surfaces and interfere with oxygen transfer into the cell. In pharmaceutical fermentation, oil contamination triggers product rejection under GMP regulations because the oil cannot be separated from the fermentation broth after the event. In food-grade fermentation, oil in process air violates food safety regulations on permitted additives and contaminants. For wastewater treatment activated sludge processes, oil inhibits the biological treatment organisms and adds to the hydrocarbon loading of the treated effluent. Oil-free compressed air eliminates all these risks at source.

Under normal operating conditions with properly filtered inlet air and cylinder cooling water maintained at 30 to 40 deg C, PTFE composite piston ring life is typically 2,000 to 4,000 hours for LP stage rings and 1,500 to 3,000 hours for HP stage rings, where HP rings operate under higher thermal and mechanical loading. Service life is strongly influenced by inlet air quality — moisture and dust in the inlet air accelerate PTFE ring wear significantly. A high-efficiency inlet air filter rated at 5 micron and a post-cooler moisture separator before the compressor inlet are recommended to maximise ring life. PTFE valve seat discs in the suction and discharge valves typically achieve 3,000 to 6,000 hours before needing replacement.

Cooling water must be clean, non-scaling, and non-corrosive. Recommended inlet temperature is 20 to 35 deg C, with maximum inlet temperature of 40 deg C. Cooling water hardness should be below 180 mg/L as CaCO3 to prevent scale formation in cylinder water jackets and inter-stage cooler tubes. pH should be 7.0 to 8.5. A closed-circuit cooling water system using a cooling tower or plate heat exchanger and inhibitor dosing is strongly recommended over once-through mains water supply, both to control water consumption and to maintain consistent cooling water quality. Cooling water flow rates and pressure requirements per model are listed in the installation manual.

Yes. Parallel multi-unit operation from a common header is the standard configuration for large-scale fermentation air supply stations. Each unit has its own discharge check valve, isolation valve, and unloading system. A lead-lag control scheme — typically a simple pressure-switch cascade or PLC sequencer — automatically starts and stops individual units in response to total system air demand as fermenters are started, stopped, or have aeration rates changed during batch cycle transitions. Multi-unit stations also provide redundancy: if one unit goes offline for ring or valve maintenance, the remaining units continue to supply the critical fermentation demand. For large amino acid and antibiotic plants, it is common to have 4 to 8 units of DW-150 or DW-190 series running in parallel with one standby unit maintained ready for automatic start on demand.

Yes. The oil-free compressor ensures that the compressed air contains no hydrocarbon oil, but the compressed air still contains microorganisms (bacteria, mould spores, and viruses) drawn in from the atmosphere through the compressor inlet. For pharmaceutical fermentation, food-grade yeast cultivation, and other applications requiring sterile aeration, a sterilising-grade filtration stage rated at 0.2 micron absolute (or 0.01 micron for viruses in some vaccine applications) must be installed in the aeration line upstream of each fermenter. These sterilising filters must be integrity-tested before each batch cycle using bubble point or pressure decay testing per GMP requirements. Oil-free compressed air requires only sterilising filtration — the oil coalescing and activated-carbon stages required for oil-injected compressor installations are not needed.

At the same nominal flow rate, single-stage models use a larger cylinder bore and shorter compression stroke to achieve the lower pressure ratio in one stage. They are more energy efficient at discharge pressures below 0.28 MPa because the elimination of the second compression stage and inter-stage cooler reduces mechanical losses. Two-stage models use two smaller cylinders in series with inter-stage cooling and are more efficient above 0.30 MPa because inter-stage cooling reduces the work of the second compression stage. At 0.35 to 0.50 MPa, two-stage models are typically 8 to 12% more energy efficient than single-stage models at equivalent flow and pressure. Two-stage models also produce lower discharge air temperature at the final stage outlet, which is beneficial for PTFE ring thermal management and downstream moisture separation.

The distance piece is an open-vented chamber positioned between the crankcase and the compression cylinder that the piston rod passes through. The crankcase contains lubricating oil for the crankshaft, connecting rod bearings, and crosshead — this oil must not enter the compression cylinder. The lower rod packing (crankcase side) prevents oil from migrating up the rod into the distance piece. The upper rod packing (cylinder side) prevents compressed air from leaking down into the distance piece. The distance piece is vented to atmosphere so any small leakage past either packing seal is released to air rather than accumulating or crossing over to the other side. This two-barrier arrangement with atmospheric venting between the barriers provides a physical guarantee that oil from the crankcase cannot reach the compression space, regardless of the condition of either packing seal.

The compressor series itself is flow and pressure-rated rather than VVM-rated, as VVM is a bioreactor design parameter determined by fermenter volume and process requirement. As a sizing example: a fermentation plant with twelve 200 m³ working-volume fermenters operating at 0.8 VVM and 0.25 MPa discharge pressure would require a total free air delivery of approximately 12 x 200 x 0.8 x (0.25 / 0.1013 + 1) = 12 x 160 x 3.47 = approximately 665 m³/min FAD at the compressor discharge. This would typically be met by four to five HW-380/2.5 or HW-400/2.2 compressors in parallel with one standby unit. For specific bioreactor VVM and fermenter volume parameters, our technical team provides complete station sizing calculations on request.

Large models with 6 kV or 10 kV motors use reduced-voltage starting — typically auto-transformer or star-delta — to limit motor inrush current and voltage dip on the site medium-voltage bus. The compressor is always started in the unloaded condition (suction valve closed or bypass valve open to atmosphere) to minimise mechanical starting torque and allow the motor to reach full speed before loading begins. Once the motor reaches operating speed, the starting equipment transitions to the run configuration and loading sequence begins. For motors above 630 kW, a dedicated medium-voltage soft-starter panel is typically specified. Starting current curves, power factor data, and motor terminal box arrangement drawings are provided on request for electrical contractor use.

A standard 12-month warranty from commissioning covers manufacturing defects in materials and workmanship. An initial commissioning spare parts set — PTFE piston rings, valve plates and springs, gasket sets, inlet filter elements — is recommended to be ordered with each compressor to ensure availability for the first scheduled ring and valve maintenance event. Our spare parts catalogue covers all 50 models in the series with standardised part numbers and recommended holding quantities per unit. For large fermentation plant projects involving multiple units, site spare parts holding agreements can be arranged to guarantee critical PTFE ring sets and valve components are available for unplanned maintenance within 24 hours. Contact our technical team to discuss spare parts supply strategy for your project.