CONTROL TEMPERATURE COOLING TOWER
Introduction
Depending on the number of cells and configuration, cooling-tower systems can be designed with a wide variety of capacity-control schemes. There are control schemes for parallel towers, each having multiple cells, and single towers with a single cell. This analysis deals with a single cell having one of four control schemes:
Variable-speed drive (VSD)
2-speed motor
2-motor assembly (primary/pony)
Multiple small fans
Inspection Step 1
Review your design documents to see if a water-side economizer is part of the system design. This would include a heat exchanger to transfer heat directly from the chilled-water system to the condenser-water loop without having the chiller online. The presence of a water-side-economizer loop indicates that the chilled-water system has loads that are not dependent on OSAT, but on some other criteria. These are usually process loads such as computer-room air-handling units (AHUs) that require 24-hour cooling. If this is the case, the condenser-water temperature required at the heat exchanger is the basis for controlling the tower fans.
Inspection Step 2
Inspect the fan starter, or VSD if present. Verify that the unit is in automatic mode. If it is in manual or bypass mode, find out who made the change and ask why before resetting any controls.
There are situations where the cooling-tower fan can run at full speed and not reduce the LWT low enough to trip the chiller offline. The LWT can vary with both load and OSA conditions, especially the wet-bulb temperature.
If a 3-way diverter valve is installed in the system piping, the valve could be properly controlling the condenser-water-supply temperature to the chiller, offsetting or masking the problem at the cooling tower.
Inspection Step 3
Make sure the sensor for the cooling-tower LWT is properly located and installed, and provides an accurate value.
Inspection Step 4
Inspect the override logs of the DDC system to verify that the control logic for LWT or condenser-fan speed have not been altered. If the setpoints have been overridden, find out who changed them and ask why before resetting them. Determine the original intended setpoints either as designed or as specified in the operating plan. If the reason these settings were overridden has been resolved, you can reset the system to the proper setpoints.
Inspection Step 5
If a two-speed motor is used, make sure it can operate at low speed and does not default to high speed due to a winding failure.
If a pony motor is used, make sure it operates properly. A failed pony motor can force the primary motor to operate at all times.