Refrigeration Machinery Room Ventilation (ASHRAE 15)

ScopeFrom ASHRAE 15's Q = 100 x sqrt(G) with G the mass of the LARGEST SINGLE SYSTEM's refrigerant charge in pounds. Two features of that relation carry the engineering. The square root means doubling the charge multiplies the requirement by 1.41 rather than 2, because the rate is aimed at diluting a credible release to a survivable concentration rather than at handling the entire charge. And it is the largest single system that governs rather than the sum of everything in the room, because the design event is one system failing, not all of them at once. The number that actually fails inspections is makeup air. A large exhaust fan in a tight room simply does not move its rated flow: it depressurizes the room, the fan rides up its curve, and the delivered cfm is a fraction of the design. Louver free area sized for the exhaust rate is part of the ventilation system rather than a detail, so the free area and the gross louver it implies at the entered free-area fraction are reported next to the fan. Detection is the other half and it is not computed here -- ventilation that has to be started by a person who has already been overcome is not a safety system, so the refrigerant detector, its setpoint, and the alarm are part of the same design. For an A1 room the air-change figure is reported for contrast; for AMMONIA, IIAR 2 sizes the room by air changes, 30 per hour on the gross volume, which on a typical room is about twice 100 x sqrt(G). This does not size the continuous ventilation rate, which is a separate and much smaller requirement for occupied heat removal, and it does not set detector locations or setpoints, evaluate the discharge location, or determine whether a machinery room is required at all. ANSI/ASHRAE 15, IIAR 2, and the mechanical code in force govern.

Formula or tablerequired emergency exhaust Q = 100 x sqrt(G) in cfm for A1 refrigerants, with G the mass in pounds of the largest single system's charge, or 30 air changes per hour on the gross room volume for ammonia (IIAR 2); air changes = Q x 60 / room volume; louver free area = Q / face velocity, and the gross louver is that over the free-area fraction; the charge an installed fan covers is (cfm/100) squared.

Source and editionThe emergency mechanical ventilation rate as ANSI/ASHRAE 15 states it for A1 refrigerants, and IIAR 2 (2014, 2021 6.14.7.1) at 30 air changes per hour for ammonia, which ASHRAE 15 has excluded since 15-2016 Addendum a. A2L rooms (ASHRAE 15-2022 Table 8-3) are not computed. Louver face velocity and free-area fraction are ENTERED (louver-free-area relates them). It does not size the separate continuous ventilation rate for occupied heat removal, set refrigerant-detector locations or setpoints, evaluate the discharge location, or determine whether a machinery room is required.

Free accessOne square root and one area; no standard text is reproduced.

What governsEstimate. AHJ and licensed professional govern.

API field nameslargest_system_charge_lb, room_length_ft, room_width_ft, room_height_ft, louver_face_velocity_fpm, louver_free_area_fraction, installed_fan_cfm, required_exhaust_cfm, room_volume_ft3, air_changes_per_hour, louver_free_area_ft2, gross_louver_area_ft2, charge_covered_lb

• Louver face velocity and free area are entered they depend on the louver selected and its pressure dropthe louver manufacturer
• The continuous rate is separate occupied heat removal is a different and much smaller requirementANSI/ASHRAE 15
• Detection is not computed the detector, its setpoint and the alarm are part of the same designASHRAE 15 and IIAR 2