The Near Optimal Open Loop Cooling Tower Control block uses an open loop control equation in terms of the total chilled water cooling load. The cooling tower airflow is proportional to the chilled water cooling load. The Near Optimal Open Loop Cooling Tower Control block only exists within a Control Sequence or Control Activity within a Control Sequence.
This logic block is used within the Open Loop Tower Control with Low Limit Output Controller module. For more information on the module, see Open Loop Tower Control with Low Limit in the Central Plant category of the Output Control Modules section.
The following table describes the inputs and outputs of the Near Optimal Open Loop Cooling Tower Control logic block.
|
Name |
Data Type |
Exposed for Connection by Default |
Description |
Default Value |
|---|---|---|---|---|
|
Inputs |
||||
|
Chiller Plant Capacity |
Float |
Yes |
Provides the name plate chiller capacity. You may enter the sum of all the chillers in the plant, or the Chiller Selector may supply the sum of the active chillers. |
100.0 kW, 28.4 Tons |
|
Chiller Plant Load |
Float |
Yes |
Indicates the amount of energy added to the chilled water by the plant. |
100.0 kW, 28.4 Tons |
|
Chiller Plant Sensitivity |
Float |
Yes |
Indicates the incremental increase in chiller power for each degree increase in condenser water temperature as a fraction of the power. Typically, the sensitivity factor is between 0.01 and 0.03. |
0.02 Minimum: 0.01 Maximum: 0.03 Display Precision: 100ths |
|
Chiller Rated Power |
Float |
Yes |
Provides the name plate chiller power. You may enter the sum of all the chillers in the plant, or the Chiller Selector may supply the sum of the active chillers. |
100.0 kW |
|
Enable |
Boolean |
Yes |
Enables or disables the block. When disabled, the Percent Cmd is 0.0%. |
True |
|
Fan Type |
Enum |
Yes |
Defines the type of fans used in the cooling towers. This block supports 1-speed, 2-speed, 3-speed, and variable speed fans. It does not support vernier fans. |
VFDSet Name: Tower Fan Type |
|
Tower Design Approach |
Float |
Yes |
Indicates the designed cooling tower approach, which is the expected temperature difference between the condenser water temperature leaving the cooling tower and the outdoor air wet bulb temperature under design conditions. This value is typically 7 °F, but may be found in the cooling tower manufacturer’s literature. |
3.9 °C, 7 °F |
|
Tower Design Range |
Float |
Yes |
Indicates the designed cooling tower range, which is the expected condenser water temperature drop from inlet to outlet under design conditions. This value is typically 10 °F, but may be found in the cooling tower manufacturer’s literature. |
5.6 °C, 10 °F |
|
Tower Rated Power |
Float |
Yes |
Provides the name plate cooling tower power. You may enter the sum of all the cooling towers in the plant, or the Cooling Tower Selector may supply the sum of the active cooling towers. |
10.0 kW |
|
Outputs |
||||
|
Percent Cmd 1 |
Float |
Yes |
Indicates the desired fan speed. |
0.0% |
|
Properties |
||||
|
Anti Spike |
Boolean |
N/A |
Enables or disables the anti-spike sub-filter (True = enabled, False = disabled). If disabled, the sub-filter input passes directly to the sub-filter output. |
True |
|
Lowpass Filter Enable |
Boolean |
N/A |
Enables or disables the low pass filter (True = enabled, False = disabled). If disabled, the sub-filter input passes directly to the sub-filter output. |
True |
|
Minus 20 dB Frequency |
Float |
N/A |
Indicates the rejection frequency for the low pass filter. A sinusoid signal at that frequency passed through the filter is reduced 10 times (higher frequencies are reduced more, lower frequencies are reduced less). The -20 db frequency is limited to 0.01 Hz. |
0.033 Hz |
|
Period |
Float |
N/A |
Indicates the execution frequency of the block. If the value is 60 seconds, the block executes once per minute. |
60 seconds |
|
Spike Fraction |
Float |
N/A |
Defines the initial width of the anti-spike filter clipping window. This value is used to calculate the Spike Window value. |
0.025 |
Near Optimal Open Loop Tower Control Equations
The Near Optimal Open Loop Tower Control block (which is used within the Open Loop Tower Control with Low Limit module) uses the following equations:
When the part load ratio is greater than 0.25, the following equation is used:
When the PLR is less than 0.25, the following equation is used:
And the C1 and C2 constants are defined as follows:
|
Fan Type |
Constants |
|
|---|---|---|
|
C1 |
C2 |
|
|
One Speed Fan |
1.0 |
1.0 |
|
Two Speed Fan |
|
|
|
Three Speed or Variable Speed Fan |
|
0.5 |