NFPA 37 fire separation & API RP 500 area classification
Use the airflow available in the worst-case operating mode (e.g. one fan running). API RP 500 "adequate" = ≥1.0 cfm/ft² and ≥6 ACH.
A gas compressor building > 1,000 ft² is commonly Group H-2 → IBC §415.6.4.2 requires a 30 ft setback.
Sets the IBC Table 705.5 exterior-wall fire-rating required below 30 ft.
How NFPA 37 separation and API RP 500 area classification combine to set compressor-building spacing
NFPA 37 §4.1.2.2.2 requires detached engine/compressor buildings to be at least 1.5 m (5 ft) apart, measured from openings and combustible walls. That minimum drops to 0 ft (buildings may be adjacent) if a facing wall has at least a 1-hour fire-resistance rating or a building is protected by an automatic fire-protection system. API RP 500 does not mandate a separation distance but classifies the electrical area in the gap.
NFPA 37 governs fire separation of stationary combustion engine buildings, and API RP 500 governs the electrical area classification (Division 1/Division 2) around and between them. Owner or insurer facility-siting standards (and 49 CFR 192 for U.S. gas stations) may impose additional spacing.
If the buildings are adequately ventilated, each is Class I, Division 2 with an exterior Division 2 zone extending about 15 ft (4.5 m) for lighter-than-air natural gas (API RP 500 Figure 23). Buildings spaced closer than twice that extent have overlapping envelopes, so the entire gap is Division 2 — acceptable, but all electrical in the gap must be Division 2-rated.
API RP 500 §6.3.2.4.2 defines adequate ventilation as at least 1.0 cubic foot per minute of air per square foot of floor area AND at least 6 air changes per hour. Meeting it keeps the interior Class I, Division 2; failing it classifies the interior Division 1 and adds a 10 ft Division 1 transition zone outside the openings.