Manual

55
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Head Pressure Control Valve Operation ORI/ORD
EXAMPLE: Our example calls for a compressor equipped with
unloaders Since the compressor would unload at the low ambients
this must be taken into consideration This is necessary since as the
compressor unloads, the condenser’s capacity increases and additional
ooding is required Using the same roof-top unit as in the earlier
example (40°F evaporator and minus 20°F minimum ambient), a
multiplier of 79 is shown in Table-2 And since we have unloaders
(50%), this 79 is used to enter Table-3 to nd a multiplier of 91 This
nal multiplier is applied to the 119 pounds calculated earlier to arrive
at the nal extra charge requirement: 119 x 91 = 108 pounds This is
added to the normal system charge to arrive at a total system charge
Table 1
Table 2
Percent of Condenser to be
Flooded
High Suction Temp.
A/C Chillers
Evaporating Temperature
o
F
Condenser
Ambient
Temperature
o
F
35 40 45 50
80 0 0 0 0
70 0 0 0 0
60 26 20 10 4
50 45 40 33 28
40 56 52 46 42
30 64 60 55 51
20 69 66 62 59
0 76 73 70 68
-20 80 79 76 73
Table 3
Condenser Flooding
with NO Unloading
Percent of Full
Compressor
Capacity
= 50%
05 50
35 01
75 51
06 02
36 52
66 03
96 53
27 04
47 54
67 05
97 55
18 06
38 56
68 07
88 57
19 97 *
29 08
69 58