Plant engineers have many flow measurement options when selecting flow sensors or flowmeters for heating-ventilation-air-conditioning (HVAC) and process control systems. It’s therefore helpful to understand in some detail the available flowmeter and flow sensor technologies. These include differential-pressure (DP), positive displacement (PD), mass, turbine, vortex-shedding and ultrasonic.
HVAC systems and process control equipment that provide air flow, heating, curing, cooling or chilling consume large amounts of energy. The cost of climate control alone for a 100,000-square-foot building can be a plant’s greatest expense. In addition, many plants require more energy to operate process equipment responsible for heating, blowing, curing, cooling, chilling and other functions vital to product manufacture or processing.
Reducing energy costs
Energy utilities and the suppliers of HVAC and process equipment have invested heavily and worked diligently for many years to improve the energy efficiency of their systems. Also, many utilities and local and state governments have programs designed to help manufacturers audit, improve and reduce energy costs. However, even after installing the most efficient, up-to-date HVAC systems and equipment, sooner or later most plants reach a point where the next meaningful improvement requires monitoring the flow of plant air or natural gas.
In addition to plant climate control, careful location of flow sensors and flowmeters in the HVAC system or process control loops can provide additional energy cost savings as well as improve the efficiency of processes. The end result is often better quality at lower unit cost and sometimes proprietary processes with competitive advantages. There can be a significant and rapid return on investment (ROI) (Figure 1).
Flow measurement technologies
The first consideration in flow sensor and flowmeter selection is the appropriate technology. While numerous technologies are available, the choices will be narrowed with proper consideration for your plant’s layout, processes, installation environment and conditions, maintenance schedules, energy costs and ROI. Some flow technologies, for example, are designed for air and gas applications and not useful with liquids, while still others may represent the only effective solutions for steam. Before trying to increase the efficiency of HVAC systems or process equipment, it’s important to understand the various flow measurement technologies.
Differential pressure (DP) is a widely used flow measurement that relies on several sub-technologies including orifice plates, averaging pitot tubes, venturis, sonic nozzles and V-cones. DP flowmeters measure volumetric flow rate and, depending on the sub-technology, can be applied in most liquids, gases and vapors, including steam. They’re well understood and easy to use. They can, however, introduce pressure loss and require a pressure gauge, additional energy costs to boost pressure and sensors for temperature and pressure when mass flow measurement is required.
Postive-displacement (PD) meters measure the volumetric flow rate of a liquid or gas by separating the flow stream into known volumes and counting them over time. This group also includes sub-technologies -- each with its own application advantages and disadvantages. These mechanical devices include vanes, gears, pistons or diaphragms to separate the fluid. They’re one of the few choices for reliable viscous liquid measurement, but they introduce pressure drop and their moving parts are subject to wear.
Turbine flow measurement devices rely on a spinning rotor. The rotor speed is proportional to fluid velocity. Multiplying the velocity times the turbine’s cross-sectional area yields the volumetric flow rate. Turbines are suitable for clean liquids and gases, but they introduce pressure drop and their moving parts are subject to wear.
Electromagnetic measurements determine the velocity of a conductive liquid by passing it through a magnetic field and measuring the developed voltage. Velocity multiplied by area yields the volumetric flow rate. Electromagnetic devices have no moving parts and don’t obstruct the flow stream. They’re accurate in conductive liquids or slurries flowing into a full pipe, but the liners are subject to wear.
Ultrasonic systems center around transit-time or Doppler frequency shift to measure the mean fluid velocity. The line-of-sight ultrasonic meter requires either two or more sensors inserted into the pipe or a clamp-on device to mount the transducers to the outside of the pipe wall. Like other velocity-measuring meters, volumetric flow rate is determined by multiplying mean velocity times area. Ultrasonic units offer good to excellent accuracy with liquids. They present no flow obstructions and the clamp-on type is nonintrusive.
Vortex-shedding devices exploit that the frequency at which vortices are shed from a body placed in the flow stream is proportional to fluid velocity. Again, velocity times area gives the volumetric flow rate. Vortex flow devices are particularly effective and one of the few technologies applicable for steam applications. These units have no moving parts and are fouling-tolerant, but require flow rates high enough to generate vortices.
Thermal sensors measure mass flow rate directly by measuring a fluid’s temperature gain or loss. They also are inherently dual-function, providing both flow rate and fluid temperature measurements from the same device. They have no moving parts, no orifices to plug and are temperature-compensated. They are most effectively applied in air and gas applications and, because they install in a single tap point, they’re often the most effective and economical solution for large pipes or duct applications or stacks.