Johnson Controls York VERSECON YSWD 012, Installation Operation & Maintenance

Page: 53 / 128

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JOHNSON CONTROLS
FORM 145.05-NOM1 (708)
Compressor Staging
The number of compressors and steps of capacity varies
with the tonnage of the equipment. Each of the stages
of operation is assigned a window of operation based on
the percentage of the total operating range 0 to 100%.
To determine the window of operation for each stage
divide 100% by the number of stages. Table 14 gives
the window of operation for each stage.
The compressors are staged on and off based on Table
14 and the variable percentage output as outlined in
either the Supply Air Temperature Control, Return Air
Temperature Control, Dew Point Temperature Control,
and Coil Leaving Temperature Control section above.
We will use a 4-compressor 4-stage system in the
following example:
When the percentage output reaches 25%, the control
will initiate a demand for the first stage of operation.
If the percentage output drops to 10% or below, the
demand for first stage operation would continue for
two minutes. During this two minute off delay, if the
percentage output is equal to or greater than 25% again,
the first stage demand would continue and the two
minute off delay would be turned off. If at any time,
the cooling demand is lost for five seconds; the demand
for first stage operation would be turned off. Once
the cooling demand is lost, the demand for first stage
operation cannot be reestablished for three minutes.
When the percentage output reaches 50% or above for
three minutes, the control will initiate a demand for
the second stage of operation. If the percentage output
drops to 35% or below, the demand for second stage
operation would continue for two minutes. During this
two minute off delay, if the percentage output is equal
to or greater than 50% again, the second stage demand
would continue and the two minute off delay would be
turned off. If at any time, the cooling demand is lost
for four seconds; the demand for second stage operation
would be turned off. Once the cooling demand is
lost, the demand for second stage operation cannot be
reestablished for three minutes.
When the percentage output reaches 75% or above for
3 minutes, the control will initiate a demand for the
third stage of operation. If the percentage output drops
to 60% or below, the demand for third stage operation
would continue for 2 minutes. During this 2-minute
off delay, if the percentage output is equal to or greater
than 75% again, the third stage demand would continue
and the 2-minute off delay would be turned off. If at
any time the cooling demand is lost for 3 seconds, the
demand for third stage operation would be turned off.
Once the cooling demand is lost, the demand for third
stage operation cannot be reestablished for 3 minutes.
When the percentage output reaches 100% for three
minutes, the control will initiate a demand for the fourth
stage of operation. If the percentage output drops to
85% or below, the demand for fourth stage operation
would continue for two minutes. During this two
minute off delay, if the percentage output is equal to
100% again, the fourth stage demand would continue
and the two minute off delay would be turned off. If at
any time, the cooling demand is lost for two seconds;
the demand for fourth stage operation would be turned
off. Once the cooling demand is lost, the demand for
fourth stage operation cannot be reestablished for three
minutes.
If the unit is configured for 2-compressor 2-stages;
a first stage call will initiate a start sequence output
for compressor B. A second stage call will initiate or
maintain a start sequence output for compressor B and
A.
If the unit is configured for 4-compressor 4-stages;
a first stage call will initiate a start sequence output
for compressor B. A second stage call will initiate or
maintain a start sequence output for compressor B and
A. A third stage call will initiate or maintain a start
sequence output for compressor D, B, and A and a fourth
stage call will initiate or maintain a start sequence output
for compressor C, D, B, and A.
If the unit is configured for 2-compressors 3-stages;
a first stage call will initiate a start sequence for
compressor B; a second stage call will initiate a start
sequence for compressor A and turn off compressor B;
and a third stage call will initiate or maintain a start
sequence output for compressors B and A.
If the unit is configured for 4-compressors 6-stages;
a first stage call will initiate a start sequence for
compressor B. A second stage call will initiate a start
sequence for compressor A and turn off compressor B;
and a third stage call will initiate or maintain a start
sequence output for compressors B and A. A fourth
stage call will initiate or maintain a start sequence for
compressors D, B, and A; a fifth stage call will initiate
or maintain a start output for compressors C, B, and A;
a sixth stage call will initiate or maintain a start output
for compressor D, C, B, and A.
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