Specifications

Catalog WSC/WDC-4 59
d = inside diameter of pipe
ln = natural logarithm
P
2
= absolute pressure at outlet of discharge piping, psi (kPa)
P
o
= allowed back pressure (absolute) at the outlet of the relief valve. psi (kPa), 15% of set
pressure,
P
o
= (0.15P) + atmospheric pressure
McQuay centrifugal chillers have the following relief valve settings and discharge capacity:
WSC evaporator and condenser = 200 psi, 75.5 lb of air/min
WDC evaporator = 180 psi, 68.5 lb of air/min
WDC condenser = 225 psi, 84.4 lb of air/min
Since the pressures and valve size are fixed for McQuay chillers, the above equation can be
reduced to the simple table shown below.
Table 9, Relief Valve Piping Sizes
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
Equivalent length (ft) 2.2 18.5 105.8 296.7 973.6 4117.4
NOTE: A 1-inch pipe is too small to handle these valves. A pipe increaser must be installed at the valve outlet.
Per ASHRAE Standard 15, the pipe size cannot be less than the relief device. The discharge from
more than one relief valve can be run into a common header, the area of which shall not 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:
5.0
2
....
2
2
2
1
¸
¹
·
¨
©
§
+=
n
DDD
Common
D
The above information is a guide only. Consult local codes and/or latest version of ASHRAE
Standard 15 for sizing data.
Standby Power
The McQuay dual compressor chillers have a considerable advantage over the same capacity
single compressor chillers when it comes to installations incorporating engine-driven standby
generators for emergency power. Starting with only one compressor greatly reduces the load on
the power plant and can reduce its size, saving considerable plant first cost.
It is essential that any centrifugal chiller connected to standby power come to a complete stop and
then be restarted with the standby power. Attempting to switch from regular line power to
auxiliary power while the compressor is running can result in extreme transient torque that will
severely damage the compressor.
Electrical Data
Wiring and Conduit
Wire sizes must comply with local and state electrical codes. Where total amperes require larger
conductors than a single conduit would permit, limited by dimensions of motor terminal box, two or
more conduits can be used. Where multiple conduits are used, all three phases must be balanced in
each conduit. Failure to balance each conduit will result in excessive heating of the conductors and
unbalanced voltage.
An interposing relay can be required on remote mounted starter applications when the length of the
conductors run between the chiller and starter is excessive.
Note: On WDC dual compressor units, dual power leads are standard, requiring separate power leads
properly sized and protected to each compressor starter or VFD. Separate disconnects must be used.