Coriolis devices measure mass flow by means of a vibrating flow tube. Fluid flowing through the tube produces a deflection that is proportional to the mass flow rate. Coriolis devices also provide density measurement. They are highly accurate in liquids, slurries, or gases. Because Coriolis mass flow meters provide extreme accuracy, 0.1% of reading typical, they have become very popular for use in monitoring fuel flow in custody transfer or fiscal metering applications. Additionally, the inherent mass flow reading compensates for any effect due to temperature or density differences in the fluid.
When optimizing HVAC systems or thermal processing equipment, there are a number of selection factors to consider. The first is always matching the flow technology to the media, such as air for HVAC systems or specialty process gases, such as nitrogen, in chilling systems.
Accuracy and repeatability: It’s important to understand the accuracy, repeatability and flow range of the sensor or meter. Most manufacturers state these specifications for water, air or a specific gas. For example, a typical air flowmeter operates over a flow range from 1.5 to 150 feet per second with an accuracy of 2% of reading, 0.5% of full scale, with a repeatability of 0.5% of reading.
Check that the accuracy, repeatability and flow range in the manufacturer’s specification matches your needs. Accuracy and range might differ in water, air or gas. Look at a flowmeter’s repeatability specification, which tells how reliably the device can maintain its specified accuracy.
Operating environment: In long duct runs, large stacks, air drying applications or nitrogen blanketing applications, match the plant’s physical layout and its temperature and humidity conditions with the flow measurement technology. The same is true for individual process control loops providing air flow handling, heating or cooling. Factors such as the number of shifts, climatic temperature extremes and specific process humidity requirements might mean some flow technologies are better suited to making accurate measurements at extremely low flows, dealing with high turndown ratios, or pressure drop. Packaging and electronics housings vary widely. Where a plastic housing may be fine for protected and climate-controlled indoor applications, a rugged, metal and appropriately rated NEMA/IP enclosure ensures longest service life in non-climate-controlled or outdoor applications.
Ease of installation: Some flowmeter installations are more straightforward than others. Ask if the device can be inserted directly into the process pipe or if it requires an inline configuration that means cutting and splicing pipe in multiple places. The more penetrations, the greater the risk of pressure drop as well as the complexity and overall cost of the installation. Some flow devices feature minimally invasive or non-intrusive sensing technology, which minimizes installation time and labor cost. Remember that pressure drop causes expensive system inefficiencies in HVAC systems and many process control operations requiring air or gas flow.
Flow measurement devices require a specified length of upstream and downstream unobstructed straight-run pipe to obtain a well developed flow profile for the sensor to achieve its specified accuracy. The length differs from technology to technology; some require little and some may require several lengths. This is especially true in retrofit projects, where additional plant real estate might not be available or the cost to reconfigure equipment will extend the project payback time.
Maintenance and life: Consider the maintenance requirements for your flow sensor or meter. Some devices need more frequent recalibration. Mechanical-oriented technologies can require cleaning, which can be time-consuming or, worse, require removal from service. For plant HVAC systems, the ideal flow measurement device has no moving parts and no routine cleaning requirements.
Look beyond the purchase price to determine the initial cost, the total installed cost and the life-cycle cost. Some inexpensive flow devices require frequent maintenance or have a short service life. Other higher-priced devices are easier and less costly to install, require less maintenance and have a longer service life that provides a much better ROI.
Energy savings check list
Selecting accurate flow measurement sensors and meters is critical to increasing the HVAC system and process equipment efficiency. Before you start, identify the flow measurement technology choices available. They have advantages and limitations that differ depending on your plant’s unique operations.
Develop a spreadsheet to tabulate accuracy/repeatability, operating environment, installation time and expected life. To determine the potential energy savings, extrapolate a small daily percentage improvement in efficiency of your plant HVAC system or process equipment into a year’s worth of savings. It might surprise you and justify the upgrade.
Allen Kugi is senior applications engineer at Fluid Components Intl. in San Marcos, California.