Cryopump Capacity Calculator

Calculator Inputs

Formula Used

This tool uses a constant-load pumpdown model where the chamber has volume V, an effective pumping speed S, and a total gas throughput Q.

• Total gas load: Q = Q_leak + Q_outgas + Q_flow + Q_extra (mbar*L/s)
• Process flow conversion: Q_flow = P_stp * F, with P_stp = 1013.25 mbar and F = sccm * (1e-3/60) L/s
• Pumpdown pressure: P(t) = P0*exp(-(S/V)t) + (Q/S)*(1 - exp(-(S/V)t))
• Ultimate pressure: P_inf = Q/S
• Time constant: tau = V/S
• Captured mass over time: integrate throughput Q to get mbar*L, then convert with the ideal gas law PV = nRT and m = n*M.
• Film capacity estimate: m_film = A * (d/10) * rho, where A is cm^2, d is mm, and rho is g/cm^3.

Tip: If your target pressure is near P_inf, reduce gas load or increase speed.

How to Use This Calculator

1. Select the dominant gas to estimate mass and condensate density.
2. Enter chamber volume, initial pressure, target pressure, and pumpdown time.
3. Add leak, outgassing, and any process flow or extra throughput.
4. Leave effective speed blank to compute a required value.
5. Set operating duration to estimate captured mass between regenerations.
6. Use surface area and thickness to estimate a frost-limited capacity.
7. Optionally override capacity if you know a rated limit.
8. Click Calculate, then export CSV or PDF if needed.

Technical Article

1) Understanding Cryopump Capacity

Cryopump capacity is the amount of gas the pump can capture before performance degrades and regeneration is required in practice. Speed drives pumpdown time, but capacity is often limited by frost buildup that reduces conductance and capture efficiency. For water vapor and CO2, capacity can be consumed faster because frost is bulky and can choke conductance.

2) Key Inputs and Units

The model uses volume V (L), initial pressure P0 (mbar), target Pf (mbar), and pumpdown time t (s). Total gas load Q (mbar*L/s) is the sum of leaks, outgassing, process flow converted from sccm, and any safety margin.

3) Effective Speed at the Chamber

Effective speed S (L/s) is the speed seen at the chamber after inlet restrictions and line conductance. A high flange rating can deliver a much lower chamber speed if plumbing is narrow or long.

4) Constant-Load Pumpdown Behavior

The pressure evolution is P(t) = P0*exp(-(S/V)t) + (Q/S)*(1 - exp(-(S/V)t)). The term Q/S sets the ultimate pressure P_inf. If Pf is near P_inf, the last decade of pumpdown becomes slow unless Q is reduced or S increases.

5) Typical Gas-Load Numbers

Outgassing strongly affects both speed and capacity planning. Clean, baked metal systems can be near 1e-4 to 1e-2 mbar*L/s, while moist or polymer-rich setups can reach 1e-2 to 1e-1. Even a few sccm of flow can dominate Q.

6) Throughput to Captured Mass

Captured mass is estimated from integrated throughput: (mbar*L) = Q * t_oper. Using PV = nRT, 1 mbar*L equals 0.1 Pa*m^3, which is about 4.1e-5 mol at 293 K. Multiply moles by molar mass to get grams. For N2, this is about 1.1 mg per mbar*L at 293 K, which helps translate throughput into practical capacity.

7) Frost/Film Capacity Estimate

A practical capacity proxy is deposited frost mass: m_film = A*(d/10)*rho, with A in cm^2, thickness d in mm, and density rho in g/cm^3. Apply a usable fraction (for example 0.8) to plan regeneration before blockage and conductance loss.

8) Using the Output for Decisions

Use required S to compare against vendor performance and to evaluate plumbing improvements that raise effective speed. Use captured mass and hours-to-limit to schedule regeneration and to document trends over time. Export CSV/PDF to share results. If the tool warns that Pf is near P_inf, focus on lowering Q through bakeout, leak checks, and reduced process flow rather than only increasing pump size.

FAQs

1) What does "effective speed" mean?

It is the net pumping speed at the chamber, after inlet and line conductance losses. It can be much lower than the pump's catalog speed because restrictions limit the delivered flow.

2) Why does the pumpdown slow near the end?

As pressure falls, the constant gas load term approaches the ultimate pressure P_inf = Q/S. When Pf is close to P_inf, the exponential term becomes small and the remaining gas load dominates.

3) How is sccm converted to throughput?

The calculator uses STP: Q_flow = P_stp * F, with P_stp = 1013.25 mbar and F = sccm*(1e-3/60) L/s. This estimates added gas load from a process flow.

4) Which temperature should I enter?

Enter a representative gas temperature for mass conversion, often near room temperature for chamber gas. It does not change the pumpdown equation; it only affects the grams calculated from integrated throughput.

5) When should I use capacity override?

Use it when you know a vendor-rated capacity for your gas or a measured allowable load between regenerations. The override replaces the surface/thickness estimate for the hours-to-limit output.

6) Why is water vapor different from nitrogen?

Water often dominates outgassing and can create heavy frost on cold surfaces, reducing conductance. Its effective capture behavior may differ, so conservative margins and good bakeout practices are important.

7) What if my system has multiple gases?

Select the dominant gas for a first estimate, then repeat with other likely species. Mixed gases can change deposition and capacity, so treat the results as planning values and validate with operating data.