At a glance
- Type
- Dry oil-free screw
- Architecture
- Two-stage with intercooler
- Cooling
- Air-cooled (A) or water-cooled (W)
- Drive
- Variable-speed permanent-magnet
- Oil in compression
- None — oil-free by design
Overview
PUREFLOW DP is a two-stage dry oil-free screw compressor that splits compression across two airends with an intercooler between them. No oil enters the compression chamber, so the air is oil-free at the airend by design — not by downstream filtration. It is for continuous, high-load-factor duty where discharge pressure, site ambient temperature or running hours push single-stage machines to their temperature limit.
Model range
PUREFLOW DP is the two-stage dry oil-free screw family. Contact SIND for the current coverage.
Architecture
Two-stage dry oil-free screw
- Cooling options
- Air-cooled (A) / Water-cooled (W)
- Drive
- Variable-speed permanent-magnet
- Oil in compression
- None
How to choose
By duty point
- Start with
- Required FAD and pressure
- Then check
- Ambient temperature and load profile
- Before a quote
- Confirm the model row with SIND
When to specify DP
High pressure, hot ambient, continuous duty
- Pressure
- ≥ 10 bar(g) preferred
- Ambient
- ≥ 35 °C design summer
- Load profile
- Continuous full-load operation
Technology
Technology Platform
PureFlow DP uses two-stage dry oil-free screw compression. Ambient air enters the first-stage airend, where a pair of helical rotors compress the air to intermediate pressure. The air then passes through an intercooler, where its temperature is reduced before entering the second-stage airend for compression to final discharge pressure.
Why Two-Stage for Oil-Free?
Dry oil-free screw compressors are temperature-limited. The rotors and casing must withstand the full discharge temperature without oil for cooling. Single-stage dry screw machines can reach this temperature limit at high pressure ratios, high ambient temperatures or long continuous running periods.
Performance Boundaries
Compared with single-stage dry oil-free screw (PureFlow SP), two-stage compression (PureFlow DP) shows a significant advantage under the following conditions:
More technical detail
Technology Platform
The two-stage arrangement with intercooling brings the compression process closer to the thermodynamic ideal of isothermal compression. Single-stage compression deviates further from isothermal because of its larger temperature rise. By splitting the total pressure ratio across two stages and removing inter-stage heat, two-stage compression reduces specific power and extends the usable pressure band.
Why Two-Stage for Oil-Free?
Two-stage compression addresses this by:
Industry knowledge: two-stage dry screw compression reduces specific power by 10–20% compared with single-stage dry screw at the same pressure ratio. The actual figure depends on the specific model, pressure and operating conditions. This range represents category knowledge, not a SIND commitment for a specific model.
- Reducing per-stage temperature rise — each airend handles a smaller pressure ratio, so each runs cooler.
- Intercooling between stages — removes compression heat before the second stage, keeping the second-stage inlet temperature low.
- Extending the pressure band — two-stage machines can reach higher discharge pressures without approaching the temperature limit.
- Improving specific power — cooler compression is more efficient; the actual improvement depends on pressure ratio and load profile.
Performance Boundaries
- Discharge pressure ≥ 10 bar(g) — the higher the pressure ratio, the greater the benefit of intercooling.
- Site ambient temperature ≥ 35 °C — hot ambient reduces the temperature margin of single-stage machines; two-stage runs cooler.
- Continuous full-load operation — the return on investment for two-stage machines is realised fastest in high annual operating-hour scenarios, where the specific-power improvement translates directly into energy cost savings.
- Part-load efficiency matters — variable-speed drive on a two-stage machine matches air output to air demand across the full load range, eliminating unload losses and realising the thermodynamic advantage of two-stage compression at all loads.
Key Design Decision: Variable-Speed Permanent-Magnet Drive
Why does PureFlow DP use variable-speed permanent-magnet drive as standard?
Dry oil-free screw compressors carry a higher purchase cost than oil-injected machines of equivalent power. Running at fixed speed would introduce unload-cycle losses at partial load, undermining the efficiency advantage. Variable-speed permanent-magnet drive ensures that two-stage machines deliver their efficiency benefit across all load conditions — variable-frequency control varies motor speed to match air output directly to air demand, eliminating unload losses and realising the thermodynamic advantage of two-stage compression across the full operating range.
Permanent-magnet motors have higher efficiency at partial load than induction motors, which further improves part-load energy performance.
Air-Cooled vs Water-Cooled
PureFlow DP is available in air-cooled (A suffix) and water-cooled (W suffix) configurations. The choice turns on site conditions:
Selection advice: decide on design summer ambient, cooling-water availability, noise limits and whether heat recovery has a use on site. Water-cooled is typically specified for indoor installations with noise limits, hot climates, or where heat recovery offsets heating costs.
- Air-cooled (A) — rejects heat to ambient air via finned coolers and fans. Simpler installation, no water circuit required. Best where design summer ambient is moderate, cooling water is unavailable or expensive, and noise limits allow fan operation.
- Water-cooled (W) — rejects heat to a plant water circuit via a shell-and-tube or plate heat exchanger. Quieter operation (no fan noise), allows heat recovery from the cooling water circuit, and maintains performance at higher ambient temperatures. Requires a cooling tower or plant water source, and water treatment to prevent scaling and corrosion.
Selection Logic
PureFlow DP selection is based on three parameters that determine the duty point:
The intersection of these parameters defines the duty point. If the duty point falls in the high-pressure, high-ambient or continuous-duty region, PureFlow DP is the correct choice. If pressure and ambient are moderate and running hours are low, PureFlow SP may offer lower purchase cost with acceptable efficiency.
Selection advice: if a factory has both 8 bar and 4 bar air requirements, do not select a single two-stage machine and step the pressure down to 4 bar for all outlets. Instead, use separate pressure supplies — a high-pressure machine for high-pressure needs and a low-pressure machine for low-pressure needs — to avoid degrading high-grade energy.
- Required pressure (bar(g)): determined by the furthest endpoint’s minimum working pressure plus pipeline losses.
- Required flow (m³/min): determined by the sum of all simultaneous air consumers plus margin.
- Load profile and annual operating hours: continuous full-load operation favours two-stage; intermittent or part-load duty may favour single-stage (PureFlow SP).
Selection & quote checklist
- Required pressure (bar(g)): determined by the furthest endpoint’s minimum working pressure plus pipeline losses.
- Required flow (m³/min): determined by the sum of all simultaneous air consumers plus margin.
- Load profile and annual operating hours : continuous full-load operation favours two-stage; intermittent or part-load duty…
FAQ
SP or DP?
Take discharge pressure, site ambient and load profile. One dry stage is temperature-limited, so high pressure (≥ 10 bar), hot ambient (≥ 35 °C) or continuous full-load running points to DP. Moderate pressure, cool ambient and intermittent duty may favour SP for lower purchase cost.
Is the air oil-free?
At the airend, by design — not by certificate. Oil-free means the machine adds no oil; delivered air quality also depends on intake conditions and downstream treatment.
Do I still need a dryer and filters?
Yes. Drying is set by the required dew point; filtration handles particulate and whatever arrives with the intake air. Oil-free air is not dry air and is not particle-free air. The complete system includes intake filtration, compression, aftercooling, drying and downstream filtration.
Air-cooled or water-cooled?
Decide on design summer ambient, cooling-water availability, noise limits and whether heat recovery has a use. Air-cooled is simpler and requires no water circuit. Water-cooled is quieter, allows heat recovery, and maintains performance at higher ambient temperatures.
Are these model numbers orderable?
No. Zero issued SKU at this time. The coding format shown is the naming rule that will apply on release. Contact SIND for the current model row and availability.
More questions
What inlet air does it need?
Clean and dry, away from road dust, engine exhaust and process vents. Working drains at every low point are essential. Intake air quality affects rotor life and downstream air quality.
Is 60 Hz available?
50 Hz standard. 60 Hz configuration on request.
Ownership & service
Energy, maintenance and service
Target Industries and Markets
PureFlow DP is specified where oil-free air is mandatory and the duty profile justifies two-stage compression:
- Pharmaceutical and medical air — process air and instrument air where oil contamination is unacceptable.
- Food and beverage packaging — air that contacts packaging or product must be oil-free to meet food-safety standards.
- Electronics and semiconductor assembly — clean, dry, oil-free air for process and instrument applications.
- Photovoltaic manufacturing — solar cell and panel production requires oil-free process air.
- Textile air-jet weaving — oil-free air prevents fabric staining.
- Glass container forming — blow-moulding air for glass bottles and containers.
- Coating and finishing — spray air where oil causes surface defects.
- Instrument air in petrochemical, power and water utilities — control and instrument air where oil damages valves and actuators.
Energy and Total Cost of Ownership
Dry oil-free screw compressors carry a higher purchase cost than oil-injected machines. The total cost of ownership turns on energy cost, maintenance cost and the cost of oil contamination:
Selection advice: for continuous full-load operation at high pressure, the energy saving from two-stage compression typically offsets the higher purchase cost within 2–4 years, depending on electricity cost and annual operating hours. For intermittent or part-load duty at moderate pressure, single-stage (PureFlow SP) may offer lower purchase cost with acceptable efficiency.
- Energy cost — two-stage compression with intercooling reduces specific power by 10–20% compared with single-stage dry screw at the same pressure ratio. Variable-speed drive eliminates unload losses and matches air output to air demand. The actual energy saving depends on pressure ratio, load profile and annual operating hours.
- Maintenance cost — no oil separator elements, no chamber oil changes, no oily condensate disposal from the machine side. Dry compression removes these maintenance items. Downstream drying and filtration still require maintenance.
- Cost of oil contamination — in pharmaceutical, food, electronics and coating applications, oil in the air stream causes product rejection, batch loss, surface defects or regulatory non-compliance. The cost of oil contamination typically far exceeds the energy cost of compression. Oil-free compression removes this risk at the source.
Specifications vary by configuration; confirm with the datasheet. Dimensions and weights are indicative; request the installation drawing for the specific model before designing foundations or access routes.
