Pneumatic Flow Rate Calculator

Pneumatic Flow Rate (SCFM) Calculator

Calculation Result

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Understanding Pneumatic Flow Rate and Cv

In pneumatic systems, the Flow Coefficient (Cv) is a standardized metric used to describe the efficiency of a valve or component in allowing fluid (air) to pass through it. Unlike liquid flow, compressed air is compressible, meaning the flow rate depends heavily on the inlet pressure, temperature, and the pressure drop across the device.

How the Calculation Works

This calculator uses the industry-standard formula for compressible gas flow through an orifice. The behavior changes depending on whether the flow is Sub-critical or Critical (Choked).

• Sub-critical Flow: When the pressure drop is small, flow rate increases as the pressure drop increases.
• Critical (Choked) Flow: When the downstream pressure is less than approximately 53% of the absolute upstream pressure, the air reaches sonic velocity. At this point, increasing the pressure drop further will not increase the flow rate.

The Mathematical Formula

The standard formula for SCFM (Standard Cubic Feet per Minute) used here is:

If ΔP < 0.1 * P1 (Low pressure drop):
SCFM = 22.48 * Cv * sqrt((ΔP * P2) / (G * T))

*Where P1 is absolute inlet pressure, P2 is absolute outlet pressure, G is Specific Gravity (1 for air), and T is absolute temperature (Rankine).

Practical Example

Imagine you have a pneumatic valve with a Cv of 1.5. Your compressor provides 100 PSIG, and you allow a 15 PSI pressure drop to operate a cylinder. At a standard 70°F:

1. Absolute Inlet Pressure (P1) = 100 + 14.7 = 114.7 PSIA.
2. Absolute Outlet Pressure (P2) = 114.7 – 15 = 99.7 PSIA.
3. Temperature (T) = 70 + 459.67 = 529.67°R.
4. The resulting flow would be approximately 54.8 SCFM.

Why Pressure Drop Matters

Designing for a high pressure drop allows for smaller valves but wastes energy and results in lower pressure at the tool. Most pneumatic engineers aim for a 5 to 10 PSI pressure drop across valves for optimal efficiency. If your pressure drop is too high, your system components will not receive the force (pressure) they need to perform work effectively.

function calculatePneumaticFlow() { var cv = parseFloat(document.getElementById('cvValue').value); var p1_g = parseFloat(document.getElementById('inletPressure').value); var dp = parseFloat(document.getElementById('pressureDrop').value); var tempF = parseFloat(document.getElementById('airTemp').value); var resultDiv = document.getElementById('flowResult'); var resultText = document.getElementById('resultText'); var flowStatus = document.getElementById('flowStatus'); if (isNaN(cv) || isNaN(p1_g) || isNaN(dp) || isNaN(tempF) || cv <= 0 || dp <= 0) { alert("Please enter valid positive numbers for all fields."); return; } // Constants var p1_abs = p1_g + 14.7; var t_rankine = tempF + 459.67; var g = 1.0; // Specific gravity of air var p2_abs = p1_abs – dp; if (p2_abs <= 0) { alert("Pressure drop cannot be greater than or equal to absolute inlet pressure."); return; } var flowRate = 0; var isChoked = false; // Choked flow check (Critical pressure ratio for air is ~0.528) // If P2/P1 < 0.528, the flow is choked if ((p2_abs / p1_abs) <= 0.528) { isChoked = true; // Choked flow formula flowRate = 13.61 * cv * p1_abs * Math.sqrt(1 / (g * t_rankine)); } else { isChoked = false; // Sub-critical flow formula flowRate = 22.48 * cv * Math.sqrt((dp * p2_abs) / (g * t_rankine)); } resultText.innerHTML = flowRate.toFixed(2) + " SCFM"; if (isChoked) { flowStatus.innerHTML = "Status: Critical (Choked) Flow. The air has reached sonic velocity. Further increasing pressure drop will not increase flow."; flowStatus.style.color = "#d9534f"; } else { flowStatus.innerHTML = "Status: Sub-critical Flow. Flow is efficient and stable."; flowStatus.style.color = "#5cb85c"; } resultDiv.style.display = "block"; }