White paper: Flow measurement

Flow measurement solutions for a more energy-efficient world.

By Marisa Fedele, Siemens AG Industry Sector August 20, 2011

There has been no shortage of studies, discussions and debates surrounding the concept of energy efficiency over the past 40 years. In the early 1970s, when two international petroleum crises led to dramatic rises in the cost of energy, the major industrial countries were forced to consider a sobering possibility: that the world’s supply of energy resources might be finite. Throughout the same decade, scientific studies began to suggest a relationship between greenhouse gas emissions and global climate change, strengthening the belief that human activity was harming the environment.

However, while efforts to reduce energy usage may not be new, energy efficiency has risen to even greater prominence in recent years as energy costs and environmental concerns continue to increase exponentially. No one is more aware of this trend than the Heating Ventilation and Air Conditioning (HVAC) industry, which must contend with an ever-expanding list of regulations, guidelines and initiatives emphasizing the importance of energy-efficient heating, cooling and ventilation systems in buildings.

As facility managers are now being held more accountable for the overall energy consumption of the buildings they oversee, having access to highly accurate and reliable flow metering technology has become vital. Flow meters make it possible for facility managers to measure the performance of various HVAC systems and ultimately make the best possible decisions to optimize efficiency and manage energy consumption. Several different flow measurement technologies can adequately meet the needs of the HVAC industry. After reviewing a few of the most important regulations that necessitate the use of flow meters as part of HVAC systems, this article will examine four types of flow metering technologies that are particularly well suited for HVAC flow measurement, including a recent application example in New York City.

Growing appreciation for energy efficiency

Reducing energy consumption offers a variety of benefits to individuals, businesses and industries alike. From an economic standpoint, energy efficiency leads to reduced energy costs, which in turn enhances a country’s ability to remain commercially and industrially competitive. In addition, efficient energy use improves the security of a country’s energy supply by decreasing the need for energy imports. It also reduces energy shortages, which can be a boon to economic development. Of course, energy efficiency has major environmental advantages as well – namely a significant decrease in greenhouse gas emissions.

There is a growing global appreciation for the far-reaching benefits of energy efficiency, a trend reflected by the substantial number of laws and initiatives cropping up each year in countries throughout the world. Although there is still much potential for further progress, these efforts have already made a significant impact. In fact, according to a recent report from the World Energy Council, since 2004 energy consumption has grown much less rapidly than the economic activity in all world regions except the Middle East. Additionally, the reduction in energy intensity between 1990 and 2008 in most world regions resulted in a large savings of both energy and carbon emissions, estimated at 3.6 GTOE (Gigatons of Oil Equivalent) of primary energy and 8 GTOE of carbon emissions.1

HVAC systems within buildings have the potential to consume a tremendous amount of energy and generate a great deal of air pollution. For example, the American Council for an Energy-Efficient Economy estimates that buildings account for more than a third of energy use and carbon emissions in the United States.2 This is why so many recent regulations and initiatives, several of which are summarized below, have centered on energy-efficient buildings and have had a direct impact on the HVAC industry.

Kyoto Protocol

Aimed at reducing the effects of global warming, the Kyoto Protocol was adopted in 1997 in Kyoto, Japan, and entered into force in 2005. Under the Protocol, 37 industrialized countries and the European community make a binding commitment to decrease emissions of four greenhouse gases and two groups of gases (including hydrofluorocarbons, commonly used by the HVAC industry as refrigerants) by 5.2% against 1990 levels between 2008 and 2012.

Energy Policy Act of 2005

Passed by the US Congress and signed into law by former President George W. Bush, the Energy Policy Act of 2005 focuses on reducing energy consumption in a number of ways, including establishing tax incentives and loan guarantees for energy efficiency measures on new buildings and HVAC systems. The Act also requires that all federal buildings install advanced metering systems capable of measuring energy consumption on a daily basis by October 2012.

Leadership in Energy and Environmental Design (LEED)

Developed by the US Green Building Council in 2000, LEED is an internationally recognized green building certification system providing third-party verification that a building or community meets strict environmental standards in such categories as energy efficiency, greenhouse gas emissions and indoor air quality.

American College and University Presidents’ Climate Commitment

The Presidents’ Climate Commitment was officially launched in 2007 as an effort by a network of US colleges and universities to address global climate disruption. Each participating institution has made a commitment to reduce emissions from specific campus operations and ultimately become “climate neutral” (eliminate all emissions) by a target date.

Clinton Climate Initiative

In 2006, former President Bill Clinton and his charitable foundation established the Clinton Climate Initiative with the goal of cutting global greenhouse gas emissions. One of the major programs stemming from this initiative is the Energy Efficiency Building Retrofit Program, which operates in large cities worldwide to retrofit existing buildings with energy-saving products, technologies and systems.

Additional guidelines for energy-efficient HVAC systems are set forth by technical organizations such as the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), which boasts an international membership of more than 50,000. ASHRAE regularly publishes updated building standards that are widely accepted by building engineers and HVAC professionals, including standards related to energy efficiency in HVAC system design and operation.

How flow meters can help

It is clear that energy efficiency is not a passing trend, but rather a sweeping influencer of worldwide social, political and economic thought. Accordingly, legislation related to the more efficient use of energy by buildings is likely to increase in both abundance and stringency in the coming years. This makes it crucial for facility managers to carefully monitor the performance of the various HVAC systems responsible for maintaining the environmental comfort of a building – whether it is a small residential apartment complex or an immense international airport.

Today, a variety of flow meters with advanced capabilities are available for use by the HVAC industry to measure flow in hot or chilled water systems and provide the baseline and load profile information necessary to evaluate – and ultimately improve – the efficiency of a system. Certain flow meters demonstrating a very high rate of accuracy can also be commissioned as revenue-grade thermal energy meters, used to monitor a customer’s energy usage for billing purposes and provide an accurate picture of their energy consumption behaviors.

Differential Pressure

The most traditional method for measuring the flow of liquids, gases and steam, differential pressure flow meters enjoy a longstanding reputation for reliability. This tried-and-tested technology has been utilized by the HVAC industry for many years to measure flow in a variety of applications, depending on which primary elements (pitot tubes, Venturi tubes, orifice plates, etc.) are paired with the meter. Appropriate applications include natural gas boilers, air ducts, combustion intakes, boiler stacks and chilled water. One differential pressure flow transmitter currently on the market, the Sitrans P DS III from Siemens, is durable enough to withstand a very wide range of temperatures and pressures and can be paired seamlessly with an assortment of primary elements.


Suitable for measuring the flow of almost all electrically conductive liquids, pastes and slurries, and an appropriate replacement for traditional mechanical flow meters when greater functionality is required, electromagnetic flow technology is the right choice for the vast majority of hot and chilled and water applications, including new installations and small line sizes below 12 in (30.5 cm).

Electromagnetic flow meters offer several important benefits to the HVAC industry, including ease of operation and maintenance due to a lack of moving parts, a high level of durability, and the flexibility for either compact or remote installation using the same sensor and transmitter. The Sitrans F M line from Siemens offers the additional benefit of a Sensorprom memory unit that stores sensor calibration data and transmitter settings for the lifetime of the product. Should the transmitter ever need to be replaced, the new transmitter uploads all previous settings and resumes measurement without any need for reprogramming – saving facility managers valuable time and money.


Vortex flow technology can provide accurate volumetric and mass flow measurements of steam, gas and liquid flow independent of conductivity, viscosity, temperature, density or pressure, and is not negatively impacted by high moisture content. This makes vortex flow meters ideal for measuring energy consumption in HVAC applications that experience fluctuating temperatures and/or pressures, including burners, boilers and compressed air systems.

Like electromagnetic flow meters, vortex meters have no moving parts to wear or foul, ensuring maximum functionality with minimal required maintenance. Another significant benefit is that these meters comprise one of very few technologies currently capable of measuring steam flow, including both saturated and superheated steam. For added accuracy and convenience, the Sitrans F X line of vortex flow meters from Siemens offers two-wire technology with integrated pressure and temperature sensors, eliminating the need for cables. The Sitrans F X bluff body obstructs flow considerably less than an orifice plate meter, resulting in significantly lower pressure drop and energy consumption.

Clamp-on Ultrasonic

While other flow technologies often have difficulty coping with low load periods and low flow, clamp-on ultrasonic flow meters are capable of accurate flow measurement at any velocity. Clamp-on meters are particularly useful for retrofit projects in which the pipeline cannot be isolated, as well as for hot and chilled water sub-metering. Overall, clamp-on technology is optimal for measuring flow in a wide range of building management, power plant, university and district energy heating and cooling applications, including condenser water, potable water, glycol, thermal storage, river and lake water, lake source cooling, chemical feed, and ammonia feed.

Clamp-on ultrasonic flow meters also offer the benefit of being quick, easy and cost-effective to install and maintain as part of an existing HVAC application. This is due to their externally mounted sensors, which require no cutting of pipes, interruption of flow or periodic cleaning. Such a feature makes clamp-on flow meters among the most versatile on the market, as they can be installed anywhere along a hot or chilled water line. By using a dual-channel model, it is even possible to measure two pipes simultaneously in order to manage the distribution of energy throughout a system.

For facility managers needing an accurate way to charge an individual customer or tenant for measured energy usage, clamp-on ultrasonic flow meters can serve as revenue-grade thermal energy meters for billing. In this type of application, a high-accuracy meter is paired with precision-matched clamp-on or insert temperature sensors. The meter is then able to calculate BTUs by measuring the flow rate of hot or chilled water as well as the supply and return temperatures.

Siemens offers a clamp-on ultrasonic flow meter designed specifically with the unique and challenging needs of the HVAC industry in mind: the Sitrans FUE1010. The accuracy, wide bidirectional rangeability and high level of sensitivity of this meter improve the energy efficiency of most HVAC applications with no pressure drop, while avoiding the performance and readability problems that negatively impact the performance of conventional intrusive thermal energy flow meters. Optional analog input capability allows for calculation of cooling load (kW/ton), coefficient of performance (COP) and energy efficiency ratio (EER). The Sitrans FUE1010 can also serve as a high-precision thermal energy flow meter with 1000 Ohm platinum RTD clamp-on or insert sensors combined with four-wire temperature cables.

Case study: NYC building complex

A commercial real estate corporation that owns, develops and operates premier properties throughout North America was designing a prestigious four-tower building complex in New York City. Rather than installing four individual HVAC systems to regulate temperatures inside each tower, the company elected to install a single central district cooling plant consisting of 10 chillers to satisfy the cooled water needs of all four buildings. The cooling plant was supported by a 3.3 million gallon chilled water thermal storage system consisting of 13 tanks with a capacity of 250,000 gallons of water each.

Once the plant was completed, the real estate corporation sought a flow metering solution that could accomplish two vital tasks: allowing for the individual metering of tenant utility usage and ensuring that the cooling plant was operating at optimal efficiency. The company required flow meters that could be installed, repaired and, if necessary, replaced without ever having to cut open the pipes, which would simply be too expensive and labor intensive. The chosen meters also needed to be low-maintenance and demonstrate a high level of accuracy and reliability. After careful consideration and testing of several options, the right choice became clear: clamp-on ultrasonic flow technology.

To accommodate the requirement for individual tenant billing, multiple Sitrans FUE1010 clamp-on ultrasonic flow meters were installed in strategic locations around the complex. Since the cooling plant distributed chilled water to each tower individually, a single-channel meter measured the amount of water leaving the plant while additional meters monitored what was received at each tower. Another device tracked the amount of energy stored in the thermal storage tanks during the charging cycle and monitored the amount drawn down during peak hours.

With this setup, facility managers were able to keep track of the exact amount of water flowing from the chillers to the tenants and the difference in temperature between the supply and return water, which is all they needed to accurately calculate how much energy was being consumed by each tenant. The system also served as a way to monitor flow between the pumps and the chillers, which is a prerequisite for determining energy efficiency level and detecting performance issues.

A smart decision

As energy consumers are compelled by legislation, social pressure and the desire for a better quality of life to seek innovative new methods of reducing their “carbon footprint,” the demand for energy-efficient buildings will only continue to grow. To remain in compliance with regulations and building codes while satisfying the needs of environmentally conscious customers, the HVAC industry must take steps now to ensure that the systems they build and maintain are operating at maximum efficiency. Installing flow meters specially designed to work with HVAC systems is an ideal way to accomplish this important goal – and one of the smartest decisions a facility manager will ever make.


  1. World Energy Council. (2010). Energy Efficiency: A Recipe for Success – Executive Summary. London. Retrieved from https://www.worldenergy.org/documents/energyefficiencyexsum.pdf
  2. Prindle, W., Dietsch, N., Elliott, R.N., Kushler, M., Langer, T., and Nadel, S. (2003). Energy Efficiency’s Next Generation: Innovation at the State Level (Publication No. E031). Washington, DC: American Council for an Energy-Efficient Economy. Retrieved from https://www.aceee.org/files/pdf/e031full.pdf