Question 1

What is FAD (Free Air Delivery) and how is it different from displaced volume?

Accepted Answer

FAD (Free Air Delivery) is the volume of air delivered by a compressor, measured at standard atmospheric conditions (typically 1 bar absolute, 20°C) rather than at compressed pressure. This is the correct specification to use when sizing a compressor. Displaced volume (or swept volume) is the theoretical volume the piston/rotor displaces per unit time — it is always larger than FAD because of leakage, heat, and non-ideal compression. Volumetric efficiency = FAD / Displaced volume, typically 75–90% for reciprocating compressors and 80–95% for screw compressors. When comparing compressors, always compare FAD values. Beware of manufacturers quoting displaced volume — a 500 L/min displaced volume compressor may deliver only 380–450 L/min FAD. In South Africa, compressor specifications should follow SANS 1616 (reciprocating) or ISO 1217 (rotary screw).

Question 2

How do I calculate the total compressed air demand for multiple air tools?

Accepted Answer

Total compressed air demand: 1. List all tools and their FAD consumption at the specified operating pressure. 2. Multiply each tool's FAD by its duty cycle (what fraction of time it runs simultaneously): Effective FAD = FAD_tool × (duty cycle / 100). 3. Sum all effective FAD values: Total FAD = Σ(FAD_i × duty_i). 4. Apply a safety/diversity factor: Required compressor FAD = Total × (1 + safety factor / 100). Typical safety factors: 20–25% for general workshop. 30–40% for mining and heavy industry (leakage and future expansion). 15–20% for clean process air. Tool diversity factor: In practice, not all tools run simultaneously. A diversity factor of 60–80% is applied in large industrial setups. South African standards: SANS 10140 requires that compressed air systems be designed by a competent person for pressures above 1 bar.

Question 3

How do I calculate the required receiver (air tank) size?

Accepted Answer

The receiver acts as an energy buffer, smoothing out demand peaks and allowing the compressor to cycle rather than run continuously. Receiver sizing formula: V = (Q_compressor × t_on) × P_atm / (P_max − P_min), or equivalently for cut-in/cut-out sizing: V = (FAD × t) / (P_max − P_min), where V = receiver volume (litres), FAD = compressor output in L/min, t = maximum allowable off-time or cycle time (minutes), P_max = maximum pressure (cut-out, bar absolute), P_min = minimum acceptable pressure (cut-in, bar absolute). Practical rule: A receiver should store at least 1–3 minutes of average air demand. For piston compressors, a minimum of 10–15 seconds off time per cycle prevents overheating. SANS 10140-4 governs pressure vessel design and inspection in South Africa. Receivers above 50 litre × bar must be designed to SANS 347 and inspected periodically by an approved inspection authority (e.g., SAISI).

Question 4

How much motor power does an air compressor require?

Accepted Answer

Theoretical (isothermal) compression power for single-stage: P_iso = P1 × Q × ln(P2/P1), where P1 = inlet pressure (bar abs), P2 = outlet pressure (bar abs), Q = FAD in m³/s. For adiabatic (polytropic) compression (more realistic): P_adiabatic = (n/(n−1)) × P1 × Q × [(P2/P1)^((n−1)/n) − 1], where n ≈ 1.3–1.4 for air. For two-stage compression (more efficient): Two-stage power ≈ single-stage power × 0.85. Shaft power = adiabatic power / (compressor efficiency × mechanical efficiency). Motor power (kW) = Shaft power / motor efficiency × 1.15 (service factor). Rule of thumb: A well-sized screw compressor requires approximately 5–7 kW per 100 L/min FAD at 7 bar(g). Always size the motor for maximum ambient temperature, altitude derating, and startup torque requirements.

Question 5

How does altitude affect air compressor performance in South Africa?

Accepted Answer

At altitude, atmospheric pressure is lower, so the air is less dense. This directly reduces compressor FAD: FAD_altitude = FAD_rated × (P_altitude / P_sea_level). Atmospheric pressure at altitude: P = 101.325 × (1 − 0.0000226 × h)^5.256 kPa, where h = altitude in metres. South African altitude examples: Durban (sea level): 101.3 kPa — no derating. Cape Town (sea level): 101.3 kPa — no derating. Johannesburg (1753m): 82.5 kPa → 81.5% of sea-level FAD. Pretoria (1340m): 86.5 kPa → 85.4% of sea-level FAD. Kimberley (1197m): 87.8 kPa → 86.7% of sea-level FAD. Polokwane (1230m): 87.5 kPa → 86.4% of sea-level FAD. Also consider: Higher altitude = higher discharge temperature for the same pressure ratio, which can increase wear. Mining applications at high altitude must apply MHSA altitude corrections.

Question 6

What is the difference between oil-flooded screw and reciprocating compressors?

Accepted Answer

Reciprocating (piston) compressors: Intermittent duty (50–70% cycle). 2–50 bar range easily. Lower initial cost. Higher maintenance (valves, rings, seals). Better for low duty cycle and small workshops. Oil-flooded rotary screw compressors: Continuous duty (100%). Quieter. 5–13 bar typical. Higher initial cost but lower total lifecycle cost for base load. Better for continuous production. Typical sizes: 7.5 kW (small workshop) to 315 kW (large industrial). Oil-free screw / centrifugal compressors: Required where oil contamination is unacceptable (food, pharma, instrumentation air). Higher cost. Dryer + filter requirements: All compressed air systems need after-coolers, separators, dryers (refrigerated or desiccant), and particulate filters. For instrument air in South Africa, ISO 8573-1 Class 2 (≤ 0.1 mg/m³ oil, pressure dew point ≤ −40°C) is standard.

Question 7

How do I size compressed air piping to minimise pressure drop?

Accepted Answer

Compressed air piping pressure drop should not exceed 0.3–0.5 bar across the distribution system (from compressor to point of use). Pipe sizing steps: 1. Convert air demand to actual volumetric flow at line pressure: Q_actual = FAD × P_atm / P_line. 2. Use Darcy-Weisbach or the simplified approximation: ΔP ≈ 7.57 × Q² × L × f / (d⁵ × P_abs), where Q = flow in m³/min, L = length in m, f = friction factor, d = internal diameter in mm, P_abs = absolute pressure in bar. 3. Check velocity: should not exceed 6–10 m/s in mains, 15 m/s in high-pressure branches. South African guidelines: SANS 10149 covers installation of compressed air systems. Typical distribution pressure: 7–10 bar at compressor, 6.5 bar at tool. Ring main systems reduce pressure drop by halving the worst-case pipe length. Galvanised or aluminium alloy piping is preferred; avoid PVC (UV degradation).

Question 8

What safety standards apply to compressed air systems in South Africa?

Accepted Answer

Key South African standards for compressed air: SANS 10140-4: Pressure equipment — design and construction. Applies above 200 kPa (2 bar). SANS 347: Categorisation and conformity of pressure equipment. Vessels above Group 2 pressure require third-party inspection and approval. OHS Act (85 of 1993) + Pressure Equipment Regulations (2009): Employers must ensure pressure systems are safe, maintained, and operated by competent persons. Compressors above 50 kJ stored energy require a certificate of fitness. MHSA (29 of 1996): For mining operations, underground compressed air systems require engineer approval and regular inspection. Blasting operations use compressed air; these systems are closely regulated. ATEX / NEC classification: In hazardous areas (refineries, chemical plants), compressor and motor must be ex-rated. Inspecting authorities: SAISI (South African Iron and Steel Institute), Lloyd's Register, Bureau Veritas.

Tool	L/min
½" Impact wrench	170
¾" Impact wrench	280
Angle grinder (4")	140
Die grinder	110
½" Drill	100
Air chisel / hammer	200
Air spray gun	240
Nail / brad gun	50
Orbital sander	170
⅜" Ratchet	100
Blow gun / cleaning	120
Pneumatic cylinder (50mm bore)	60
Sandblaster (5mm nozzle)	600
Jackhammer (mining)	1200
Rock drill drifter	2000

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