Compressor FAD Site Conversion

1Reference Bases

“Normal”, “standard” and “free air” are not one condition — they are a family of them, and the difference between 0 °C and 20 °C alone is 7 % on mass. Declare the basis the process requirement is written to and the basis the vendor quoted to; they are frequently not the same.

2Site Conversion & Selection Check

Yellow cells are inputs, green are calculated. Leave the inlet loss at zero for a first pass and add the vendor's filter and silencer figure once it is confirmed.

3Screening Checks

Sanity checks on the inputs and the outcome. They are a first filter, not a substitute for the vendor's certified selection.

4Reference Conditions in Common Use

The presets above are drawn from this list. Where a datasheet says only “FAD” with no basis, ask — the spread across this table is more than 10 % on mass.

Capacity loss shown is the dry-air mass reduction at constant swept volume, 20 °C dry, relative to sea level — altitude alone, before any temperature or humidity effect.

5Method & Assumptions

1. The conversion is a dry-air mass balance. Water vapour is a passenger — it occupies inlet volume but is not the product. Both states are reduced to the partial pressure of dry air, p_d = p_atm − Δp_inlet − RH · p_sat(T), and dry-air mass is held constant between them. That gives the single conversion factor K = (p_d,ref / p_d,site) · (T_site / T_ref), with temperatures absolute.
2. Demand runs one way, supply runs the other. A process requirement stated at reference conditions is a mass duty, so the inlet volume the compressor must pass at site is V̇_site = Q_ref · K — always larger than the reference figure at a hot, high site. A quoted FAD is a volume the machine can pass, so the mass it actually delivers at site, expressed back in the demand basis, is FAD / K — always smaller. The two errors compound, which is why an apparently comfortable margin on the datasheet can disappear at site.
3. Atmospheric pressure from elevation uses the ISA barometric relation p = 101.325 · (1 − 2.25577×10⁻⁵ · h)^5.25588 kPa a, with h in metres. It assumes the ISA temperature lapse and a sea-level base of 101.325 kPa; for a site with a measured barometric record, switch the control to Entered directly and use the site minimum, not the annual mean.
4. Saturation vapour pressure uses the Buck equation over water, p_sat = 0.61121 · exp((18.678 − T/234.5) · T/(257.14 + T)) kPa with T in °C — better than 0.1 % against steam-table values from −20 °C to 50 °C. Legacy sheets often carry a rounded constant instead (9.8 kPa at 45 °C against a true 9.59 kPa); switch to Entered directly if you need to reproduce one.
5. Inlet loss is the pressure drop across the intake filter, silencer and any ducting, referred to the compressor suction flange. It is a direct debit against inlet density and is frequently omitted at the enquiry stage. Ask the vendor for it — on a heavily filtered outdoor package it is not negligible.
6. Ideal-gas treatment. At atmospheric intake pressure the compressibility of moist air is within roughly 0.1 % of unity, so no Z correction is applied. Densities use R_dry = 287.058 and R_vapour = 461.495 J/kg·K.
7. What this sheet does not do. It converts capacity only. It does not correct shaft power, discharge temperature or specific energy, and it takes no account of the vendor's own altitude and temperature limits — many packages carry a maximum ambient above which the drive is derated or the machine is simply not offered. Those limits are the vendor's to state, and they govern.
8. Margin. The design margin entered against the requirement is applied to the demand before conversion. A margin on capacity is not a margin on the machine: check that the resulting selection still sits inside its turndown and does not spend its life cycling.