1112.4 Evaporators

Heat exchanger coils located downstream, or upstream within 18 inches (457 mm), of a heating source and capable of being isolated shall be fitted with a pressure-relief device that discharges to another part of the system in accordance with Section 1112.5 through Section 1112.5.2 or outside any enclosed space in accordance with Section 1112.10. The pressure relief device shall be connected at the highest possible location of the heat exchanger or piping between the heat exchanger and its manual isolation valves.

Exceptions:

1. Relief valves shall not be required on heat exchanger coils that have a design pressure more than 110 percent of refrigerant saturation pressure when exposed to the maximum heating source temperature.
2. A relief valve shall not be required on self-contained or unit systems where the volume of the lowside of the system, which is shut off by valves, is more than the specific volume of the refrigerant at critical conditions of temperature and pressure, as determined in accordance with Equation 1112.4.

(Equation 1112.4)

V₁ / [W₁ — (V₂ — V₁)/V_gt]
Shall be more than V_gc

Where:

V1	=	Lowside volume, cubic foot (m3).
V2	=	Total volume of system, cubic foot (m3).
W1	=	Total weight of refrigerant in system, pounds (kg).
Vgt	=	Specific volume of refrigerant vapor at 110°F (43°C), cubic feet per pound (m3/kg).
Vgc	=	Specific volume at critical temperature and pressure, cubic feet per pound (m3/kg). [ASHRAE 15:9.4.4]