Installation manual

22 Magnitude™ Frictionless Centrifugal Chillers – Model WMC IM 1029-2
Sizing Vent Piping (ASHRAE Method)
Relief valve pipe sizing is based on the discharge capacity for the given evaporator or condenser and the
length of piping to be run. Discharge capacity for R-134a vessels is calculated using a complicated equation
that accounts for equivalent length of pipe, valve capacity, Moody friction factor, pipe ID, outlet pressure and
back pressure. The formula and tables are contained in ASHRAE Standard 15-2001.
The Daikin Magnitude centrifugal units have a relief valve setting of 200 psi.
U
sing the ASHRAE formula and basing calculations on the 225 psi design yields a conservative pipe size,
which is summarized in Table 5. The table gives the pipe size required per relief valve. When valves are
piped together, the common piping must follow the rules set out in the following paragraph on common
piping.
Table 5. Relief Valve Piping Sizes
Equivalent length (ft) 2.2 18.5 105.8 296.7 973.6 4117.4
Pipe Size inch (NPT) 1 1/4 1 1/2 2 2 1/2 3 4
Moody Factor 0.0209 0.0202 0.0190 0.0182 0.0173 0.0163
NOTE: A 1-inch pipe is too small to handle these valves. A pipe increaser must always be installed at the valve outlet.
Common Piping
According to ASHRAE Standard 15, the pipe size cannot be less than the relief valve outlet size. The
discharge from more than one relief valve can be run into a common header, the area of which cannot be less
than the sum of the areas of the connected pipes. For further details, refer to ASHRAE Standard 15. The
common header can be calculated by the formula:
D
Common
D D D
n
= +






1
2
2
2 2
0 5
....
.
The above information is a guide only. Consult local codes and/or latest version of ASHRAE Standard 15 for
sizing data.