Near Optimal Open Loop Cooling Tower Control - Johnson Controls - Metasys - LIT-12011147 - Software Application - Controller Configuration Tool - 13.1

Controller Tool Help

Brand
Johnson Controls
Metasys
Product name
Controller Configuration Tool
Document type
User Guide
Document number
LIT-12011147
Version
13.1
Revision date
2019-12-04
Language
English

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.

Table 1. Near Optimal Open Loop Cooling Tower Control Block Inputs and Outputs

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:

Table 2. Near Optimal Open Loop Tower Control Block Equation Constants

Fan Type

Constants

C1

C2

One Speed Fan

1.0

1.0

Two Speed Fan

Three Speed or Variable Speed Fan

0.5

1 This input or output is the Default Element of the block.