In the past decade, microprocessor controller technology has advanced to provide optimum functionality, improved microprocessor capacity and more sophisticated software algorithms—all at a reduced price. Because of these advancements, pump users can now expand pump control capabilities into mechanical systems by incorporating multiple control algorithms for different system components using only one controller. What was previously defined as the local loop controller now can be expanded to achieve additional cost savings.
One area in which intelligent pump control can be expanded is the condenser loop of the water-cooled chiller. Every plant operator’s goal is to operate the condenser loop efficiently while protecting the chiller and adding to the system’s useful life.
Condenser Loops
A condenser loop of a water-cooled chiller consists of condenser water pumps and a cooling tower fan. In the past, plants considered the cooling tower a separate system with its own controller. The fan controller’s purpose was to maintain the temperature of the water entering the chiller condenser at the chiller manufacturer’s recommended setpoint. Typically, 85 F is considered an acceptable temperature for the water as it enters the condenser and passes to the chiller, though this may not be the most efficient setpoint. The cooling tower controller must either cycle the cooling tower fans or modulate the speed of the fans using a variable frequency drive (VFD), while delivering this 85-degree water.
Usually condenser water pumps are designed to provide consistent gallons per minute (gpm) through the chiller’s condenser barrel. This system is designed for the gpm the chiller manufacturer denotes and requires little control.
These older cooling tower control systems do not allow the different components of the system to work together to achieve the best possible energy savings. While using a VFD on the cooling tower fans can achieve some level of energy savings when applied to a system designed to maintain condenser water temperature, even more potential savings are available with an control strategy.
Sophisticated Control Systems
With only a few additional input/output (I/O) points into the new controller, the chiller alone would be able to achieve an average of 30 percent energy savings annually. The system could reduce the head pressure of the chiller by decreasing the temperature of the condenser water leaving the cooling tower and entering the condenser of the chiller. Lower head pressure results in lower lift, which is the difference between the high-pressure and the low-pressure sides of the refrigerant circuit. This phenomenon reduces the amount of work the compressor must perform, minimizing the amount of energy the chiller consumes. Most of the energy savings is a direct result of reducing the lift on the compressors.
The monitoring and control I/O for the new controller would include:
- Cooling tower basin water temperature
- Entering chiller condenser water temperature
- Leaving chiller condenser water temperature
- Outside air wet-bulb temperature
- Cooling tower fan motor speed
- Cooling tower fan motor status
- Condenser pump motor speed
- Condenser pump motor status
- Condenser pump gpm
- High side refrigerant pressure
- Low side refrigerant pressure
All of this information is fed into the condenser loop controller, not just a cooling tower controller. It has the information it needs to monitor and control this loop to optimum efficiency while ensuring that the system meets operational parameters for the chiller. This in-depth system controls the cooling tower’s speed, resets the condenser water temperature, varies the speed of the condenser water pump and monitors key operational requirements. The system can also calculate and report in real time the co-efficiency of performance (COP).
Figure 1 demonstrates the difference in energy consumption when continuous floating head pressure is applied with VFD operation of the cooling tower fan. This control algorithm coupled with additional monitoring and control I/O can wring out the hidden savings from the entire condenser loop. The red line represents the system before the application of floating head pressure, and the blue line represents the system after the application of floating head pressure with VFD fan control.
Figure 1. The difference in energy consumption when continuous floating head pressure is applied with VFD operation of the cooling tower fan (Courtesy of Schneider Electric)The algorithms (AFBs) that reside within the controller’s software use all of this I/O to float the head pressure to the most efficient value based on outside air temperature and chiller load. The speed of the condenser pump motors can also be reduced if proper condenser water difference in temperature (Δt) is maintained through the chiller’s condenser. The new controller software should also include monitoring and trending capabilities to ensure that specific parameters such as minimum gpm through the chiller barrel match the chiller manufacturer’s requirements.
If the system has an air-cooled condenser, the same strategy regarding the floating head pressure and pump speed control can be applied to control the speed of the air-cooled condenser fans using the same I/O (with the exception of the associated condenser water temperatures). The system would also use an outside air dry-bulb sensor instead of a wet-bulb sensor.
If managers want to monitor or control additional parts of their systems, they can expand the I/O to include those additional monitoring and control points. All controller information can be reported to the building management system (BMS) in real time via Bacnet, Lonworks or Modbus. Cloud-based technology allows operators to access this information using mobile devices.
The Cost of Pumps
While energy use is a primary concern in the design of optimal pump systems (40% of lifetime cost), pump operations and maintenance account for the next greatest portion of total cost of ownership, amounting to 35 percent. What are the root causes of these costs, and how can a system monitor and maintain pump operations? While dedicated sensors and instrumentation have been the traditional solution, the VFDs that save energy can also provide monitoring and feedback regarding many pumping issues.- Cavitation – Proportional-integral-derivative (PID) feedback can monitor and signal cavitation events.
- No/low-flow protection – The VFD can detect pump operation in no/low-flow mode to prevent the pump from being destroyed or severely damaged.
- Dry run protection – This function can detect pump operation in dry running condition to prevent pump damage.
- Low-pressure protection – This abnormal operating point can damage the pump.
- Cyclic start protection – Start limitations can be programmed to limit the number of starts and reduce the wear on the pump.
- Sleep/wake up functionality – Protect the pump from operating under no-flow conditions.
- Anti-jam protection – This capability will prevent materials from clogging the pump.
- Pipe fill protection – This function can prevent water hammer, which occurs when the pipes fill too fast.
- PID feedback – This function is typically used to detect faulty installation.