Refrigeration Machinery Room Ventilation (ASHRAE 15)

From 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. The air-change figure is reported only for contrast: sizing a machinery room by air changes rather than by charge is the common error, and it usually gives a much smaller fan. 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.

required emergency exhaust Q = 100 x sqrt(G) in cfm, with G the mass in pounds of the largest single system's charge; 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.

The emergency mechanical ventilation rate as ANSI/ASHRAE 15 states it, carried by IIAR 2 for ammonia machinery rooms. Louver face velocity and free-area fraction are ENTERED. 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.

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

Estimate. AHJ and licensed professional govern.

Field names used by the API: largest_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