The Role of Flow Monitoring in Process Cooling Applications
Flow monitoring equipment can help detect inefficiencies and maintenance issues, while lowering costs.
When most people think of industrial process cooling systems, they consider the cooling equipment like chillers, heat exchangers or refrigerators as well as a network of pumps and pipes to move cooling fluid through the facility. But, these systems also require a number of peripheral devices to ensure that the system is performing as it should. Flow-monitoring devices in key locations throughout the installation can help to detect inefficiencies in the chiller system. Increasing the chiller efficiency reduces facility costs and also provides early detection of potential maintenance issues.
Flow monitoring in process cooling systems offers several benefits. First among them is the ability to quantify the efficiency of the chiller. As much as 30 percent of the total energy consumed by an industrial facility is used by the process cooling systems, so increasing the chiller efficiency is always a worthwhile exercise. Any chilled-water system that utilizes variable primary and secondary loops can reduce the bottom line with the use of flowmeters. After all, it is flowmeters that help control the loop flows. Being able to control the loop flows on an as-needed basis in response to energy/HVAC demands helps with electricity cost savings. Pumps are big users.
To calculate the chiller efficiency, use a flowmeter and a pair of temperature sensors for measuring a temperature differential. Mount the flowmeter and the first temperature sensor on the inlet pipe to a chiller. Mount the second temperature sensor on the chiller outlet (figure 1).
With the flow rate and temperature differential known, the efficiency is calculated using the following definitions:
- Chiller tonnage is defined as the amount of heat required to melt 1 ton of ice in 24 hours.
- 1 ton is equal to 12000 BTU/hr.
- Chiller tonnage is equal to 500 multiplied by the flow rate in gallons per minute multiplied by the temperature differential (?T), where ?T is the temperature differential between the chiller inlet and outlet. It is expressed as: Chiller tonnage = 500 x Flow Rate x ?T
- Efficiency is defined as the ratio of energy consumed (kW) to the rate of heat removal in tons. It is expressed in kW/ton, where kW is the electrical input in kilowatts. A lower kW/ton value indicates higher efficiency.
To improve the efficiency of the process cooling system, first calculate the initial efficiency of the chiller. Then, look for ways to optimize performance. Replace weak or restrictive valves and fix leaky seals. Make sure that all equipment in the system is well maintained to avoid component failure.
After putting improvements in place, calculate the efficiency again to see how much it has increased. As long as the flow rate is kept at a constant value, there are two ways to compare efficiency:
- Compare the kW/ton values.
- Compare the ?T values at the chiller.
If the calculations show either a lower kW/ton value or a higher ?T, this indicates increased efficiency.
Frequently re-calculating the chiller efficiency allows a facility manager to ensure that the cooling system is operating at peak performance all the time. Tracking this data over time allows performance trends to be identified so that managers can anticipate the needs of the cooling system and respond proactively to maintenance issues, which can become costly if neglected.
Monitoring flow in cooling systems provides other benefits as well. Most types of industrial equipment have a specified rate of flow that is needed to adequately cool the equipment. If the flow rate is inadequate, the equipment is at risk of overheating. Installing flowmeters near sensitive equipment is the best way to monitor whether the equipment is being cooled properly.
Monitoring the flow rate also helps to detect leaks and failing pumps. When the flow rate drops suddenly in one section of the cooling system, it could mean that a substantial leak in the cooling fluid system has formed or that a pump has failed. Quick detection of these problems allows the facility to take steps like redirecting the flow of cooling fluid or reducing the use of heat-sensitive equipment while the leak or the pump is repaired.
Integrating Flow and Energy Data into a BAS System. In many facilities, the peripheral devices are integrated into a single control system. Traditional building automation system (BAS) peripherals have analog outputs (e.g., 4 to 20 mA) that send a single data element to the controller. However, multiple data elements — kilowatts consumed, flow rate and temperature differential, for example — are required to calculate energy and efficiency. To transmit multiple data values to the controller, many systems use protocol transmitters. Sensors and metering devices are connected to the transmitter, which then sends the data to the control system using a building protocol language such as Modbus or BACnet. These languages are compatible with most standard building control systems and allow the collection and integration of all data required to quantify energy-related values.
Choosing a Flow-Monitoring Device for Your Industrial Process
Choosing the best flowmeter for industrial process cooling systems is not as complicated as it sounds. The most important variables in this selection are the size of the pipe and the type of fluid used in the chiller system. Many flow-monitoring technologies exist such as impeller, ultrasonic and electromagnetic models, and each type of device can be purchased for a variety of pipe sizes.
For a basic, water-cooled chiller system, a simple impeller-style flowmeter is usually sufficient. They are cost effective and easy to install in almost any system, and they are compatible with a range of pipe sizes. The device is simply installed so that the impeller is in line with the direction of water flow. Many impeller-style meters are available with temperature probes that connect directly to the meter, allowing quick calculation of the chiller tonnage and the overall efficiency.
Ultrasonic flowmeters are mounted on the outside of the pipe. Transducers are mounted opposite each other, and ultrasonic waves that pass between the transducers are used to measure the flow rate. Ultrasonic meters are useful in applications that involve fluids other than water because some chemicals can damage impeller units. Ultrasonic meters also can measure flow in either direction.
Electromagnetic flow devices measure flow in conductive fluids. When installed, the magnetic coils in the meter generate a magnetic field around the pipe. An electrode is inserted into the pipe to measure the voltage of the fluid. Flow rate is calculated as a function of this voltage. Electromagnetic flowmeters are useful for slurries and other fluids that contain solids that would foul an impeller or disrupt an ultrasonic transducer.
Many cooling system designers decide to use high accuracy flow devices to ensure that all calculated values represent the system as closely as possible. While high accuracy in measurements is always a good idea, in process cooling applications, the reality is that repeatability is more important than accuracy. Putting too much emphasis on flow and temperature accuracy can lead to overly expensive solutions. Repeatability is more important. Time-based comparisons provide the information that is valuable and the basis for measurement and efficiency. When using the flow rate in efficiency calculations, if the flow rate is inexact but consistent, the tonnage values can still be compared, and changes in efficiency can still be tracked.
In conclusion, with all of the energy consumed by a process cooling system, improvements to efficiency have a direct impact on a facility’s bottom line. Flow monitoring is a critical variable in efficiency calculations. To best manage your facility’s energy consumption, more information is always better, so invest in good building peripherals such as durable, high quality flow-metering devices.