Soft Loading; Figure 16. Soft Load Limit Function - McQuay MicroTech II Operation Manual

How Load Balancing Works
The CSM continually reads the percent load (% RLA) from each chiller that is running. It then selects the lowest of these
percent load values and adds the Load Balancing Capacity Difference Limit variable (default is 8%) to this minimum. The
result is the Load Balancing Load Limit Group X (Load Limit screen). If this value is less than the demand limiting
function and the soft load function has expired, the CSM sends it to every chiller in the group. Each chiller then inhibits
loading or unloads as required to keep the load within 5% of this limit.
The Load Balancing Capacity Difference Limit effectively defines a range of acceptable chiller percent load values. This
range floats up and down as the minimum percent load value floats.
A chiller that has just started is not included in the calculation to determine the lowest percent load until one of two
conditions are met:
1. The Load Balancing Startup Timeout Period has expired
2. The percent load of the startup chiller has reached the Group Load Balancing Load Limit
The CSM can maintain up to 6 separate Load Balancing Groups. If a chiller has a Load Balancing Group # = 0, that chiller
will not be balanced with any other chillers.
As an example, consider a system with two older, inefficient chillers and one new, efficient chiller. The new chiller is
Chiller #3, the CSM's Load Balancing Capacity Difference Limit variable is set to 8%, and the chilled water setpoints in
each chiller controller are the same. When Chiller #3's load is 55% RLA, the load on Chiller #1 and Chiller #2 is prevented
from exceeding 64% RLA. (Loading is inhibited at 63% through 67%; unloading occurs at 68% and higher.)
To set up load balancing control
1. Set the Load Balancing Flag to "Yes."
2. Set the Load Balancing Capacity Difference Limit as required.
3. Enter the same Load Balancing Group # to all chillers which you want to be balanced together.

Soft Loading

Soft loading control can be used to prevent the lead chiller's load from rising too fast during chiller system start-up when
the return chilled water temperature is high. The CSM provides soft loading by writing a value to the chiller's capacity
limit. The value written to this variable ramps up from an initial value when the first chiller starts to run.
How Soft Loading Works
If the Soft Load Flag is set to Yes, the CSM generates the Soft Load Limit function based on the Initial Soft Load Amps
variable and the Soft Load Ramp Time variable.

Figure 16. Soft Load Limit Function

Whenever a chiller starts running, the CSM checks to see whether any other chillers are already running. If any other chiller
is running, it does nothing. If no other chillers are running, and if the value of Soft Load Limit is less than the capacity
limits produced by the demand limit function, the CSM sends the Soft Load Limit to every chiller in the system.
Immediately after the first chiller enters the run mode, the Soft Load Limit is continually sent out to the chillers every 15
seconds until it reaches 100% or it bumps into another load limiting control (such as demand limit).
When soft loading is used, the first chiller will start normally, but no other chillers will be enabled until the Soft Load
Ramp Time has been reached.
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100
Initial Soft Load
Amp= 40%
80
60
40
Soft Load Ramp
Time= 5min
20
First Chiller Starts
0 2 4
Time (Min)
6 8
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OM 780-2