Sizing a vacuum pump is a two-axis problem, and taking the axes in the wrong order is the most common way to buy the wrong pump. First fix the vacuum level the process needs, then size the flow (CFM / m³/hr) you need at that level — not the free-air number on the front of the datasheet. Get the order right and the pump almost picks itself.
Axis 1: vacuum level — how deep
The vacuum level is set by the physics of your process, and it is not negotiable:
| Duty | Typical vacuum needed | Family |
|---|---|---|
| Sheet feeding, hold-down, gripping | Moderate (≈600 mmHg gauge) | YELV / YELVD |
| Packaging, conveying, clamping, filtration | Deep (−850 mbar / 750 mmHg) | YBDVP, ACME |
| Vacuum drying, freeze-drying, lab | Fine (≤6×10⁻² Pa) | DSVP two-stage |
Overshooting wastes money and energy — a fine-vacuum two-stage pump on a gripping duty is capacity you will never use, running hot to make vacuum the job throws away. Undershooting simply fails: the pump never reaches the level the process needs. If the units in that table are unfamiliar, read ultimate vacuum explained before going further — mbar absolute, mmHg and torr describe the same thing and confusing them is the classic spec error.
Axis 2: flow — and the derating trap
Here is the concept that trips up most first-time buyers: the free-air displacement is not the flow you get at your operating vacuum.
The displacement figure — say 63 m³/hr on an ACME-063 — is the swept volume with the inlet open at atmosphere. As the pump pulls the system deeper, the trapped gas gets thinner and some slips back past the vane seal, so the effective pumping speed falls as vacuum deepens. Near the pump's ultimate, effective speed approaches zero — by definition, at the ultimate vacuum the net pumping speed is zero.
So the pumping-speed curve slopes down from the full free-air figure at atmosphere to zero at the ultimate. Your operating point sits somewhere on that slope, and that is the flow you actually get.
What this means in practice
- Never size on the headline free-air number if your duty runs at meaningful vacuum. You will get less.
- Never pick a pump whose ultimate is barely deeper than your operating vacuum — you would be running on the steep, near-dead end of the curve where the pump delivers almost nothing. Pick a pump with ultimate comfortably beyond your operating point so you sit on the healthy, flat part of the curve.
- Keep margin. Size so the pump works at 70–80% of its capability at the operating point, not 100%.
The flow you need: two duty types
Continuous-flow duty (conveying, gripping, hold-down with leakage). The pump must supply the steady leakage / process flow at the operating vacuum, forever. Size the effective speed at the operating vacuum to exceed that demand with margin.
Pump-down duty (chambers, packaging, evacuation). The pump must pull a known volume from atmosphere to the target vacuum inside the cycle time. This is the pattern behind most vacuum conveying and packaging lines. Larger volume, deeper target, or faster cycle → more pump. Then multiply by cycle rate: a machine doing many pump-downs a minute turns an intermittent pull into a continuous duty. The packaging worked example is in vacuum pumps for packaging machines.
A worked selection
Say you need to hold a conveying line at roughly −700 mbar with a continuous make-up flow of about 45 m³/hr to cover system leakage.
- Vacuum level: −700 mbar is deep → dry YBDVP (ultimate −850 mbar) or oil ACME (750 mmHg). Both have ultimate comfortably beyond −700 mbar, so neither runs on the dead end of its curve. Good.
- Flow at operating vacuum: you need ~45 m³/hr at −700 mbar, not free-air. Because effective speed at −700 mbar is below the free-air rating, you step up: a YBDVP 100 (100 m³/hr free-air) leaves healthy margin so its derated speed at −700 mbar still clears 45 m³/hr.
- Margin check: the pump loafs rather than running at its limit → quieter, cooler, longer-lived. Done.
Notice the free-air figure (100) is roughly double the required operating flow (45). That gap is the derating allowance plus margin — it is not over-buying, it is correct sizing.
Reference: Yash vacuum ranges
| Series | Free-air range | Vacuum | Notes |
|---|---|---|---|
| YELV / YELVD | 8–72 m³/hr | 600 mmHg + 10–12 psi | Printing, folding; also gives blow air |
| ACME | 10–302 m³/hr | 750 mmHg | Deep single-stage, oil |
| YBDVP | 10–360 m³/hr | −850 mbar | Oil-free, clean processes |
| DSVP | 240–1800 LPM | ≤6×10⁻² Pa | Two-stage, fine vacuum |
Accounting for altitude, temperature and leakage
Three real-world factors move the sizing that a clean datasheet calculation ignores.
- Altitude. Atmospheric pressure falls with elevation, so the available pressure difference for a vacuum job is smaller at a hill-station plant than at sea level. A pump that hits a target gauge vacuum easily in Chennai works harder for the same job in a high-altitude facility. On deep-vacuum duty at altitude, keep extra margin.
- Inlet temperature. Hot process gas is less dense and expands, so a pump handling warm air moves less mass per stroke than its cool-air rating suggests. Drying and hot-pack duties should be sized with this in mind rather than on the nominal figure.
- System leakage. Every real system leaks — through fittings, seals, porous product and worn cups. On a continuous-flow duty the pump spends its life making up that leakage, and leakage grows as the plant ages. Size for realistic, aged leakage, not for a leak-free ideal you had on commissioning day.
None of these is exotic; they are the difference between a pump that meets spec in the brochure and one that meets spec in your plant, in year three.
Turndown and part-load
A duty whose demand varies — more grips at some moments, fewer at others; faster and slower cycles — is easier to serve with a pump sized for the peak and allowed to loaf at the average. Vacuum pumps tolerate part-load running well, so sizing for the worst case rarely wastes much energy at the average. Where demand swings widely and runs continuously, a vacuum reservoir smooths the peaks and lets a smaller pump cover them — the same reservoir trick used in vacuum forming.
Common sizing mistakes
- Sizing on free-air for a high-vacuum duty. The number one error — you will be short.
- Matching ultimate vacuum too tightly to the operating point. Leaves you on the dead end of the speed curve. Always keep depth headroom.
- Forgetting cycle rate. An intermittent pump-down at high frequency is a continuous duty.
- Ignoring pipe and filter losses. An undersized inlet line throttles the pump regardless of its rating — the restriction sets the flow.
Summary
Choose vacuum level first — it is fixed by the process — then size flow at that level with margin, never on the free-air figure, because pumping speed derates as vacuum deepens. Pick a pump whose ultimate is comfortably beyond your operating point so you sit on the healthy part of the speed curve. Match the range to duty: YELV for printing, YBDVP for clean deep vacuum, ACME for deep oil-sealed, DSVP for fine lab vacuum.
Send us your vacuum level, required flow and cycle and we will size against the operating point. WhatsApp +91 9311693322 or email sales@yashblowers.org.