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Not all systems are created equal (Part 2)

It is, hence, essential to embrace a well-thought-out approach to specifying and installing meters for greater efficiency and proper O&M

| | Dec 15, 2017 | 1:16 pm
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In the November issue, I spoke on how metering and sub-metering of energy and resource use is a critical component of any proper and professional comprehensive O&M programme. I also discussed what to meter and how meters can be installed to measure data for several objectives. Here, let’s look at specific meters…

BTU meters

BTU meters are designed and configured to send calculated BTU data, optionally along with individual temperature and flow measurement data, to the EMS/BMS or other data-collection system for monitoring. The meters may also have a display for manual reading of internally stored energy usage data. The main advantage of BTU meters is that temperature sensors constitute a factory-matched set to minimise temperature difference calculation error. However, they generally cost more than using individual sensors connected to the EMS/BMS. Nevertheless, BTU meters are strongly recommended due to their improved temperature measurement accuracy and stability and ease of data collection, particularly if energy is being metered for revenue purposes (allocating costs of chilled water usage per building or for efficiency benchmarking and O&M of the chilled water plant).

A typical BTU meter will have the flow meter and temperature sensors, all field-mounted. The temperature sensors are provided with the BTU meter, so that they can be factory-matched and calibrated for improved accuracy. The flow meter can be of any type, depending on the desired accuracy. The output of the BTU meter can be a pulse or analogue one connected to an EMS/BMS or other data-collection system. Modern BTU meters also include the ability to directly connect to common control networks, such as BACnet/MSTP, Modbus/EIA-485 and LonWorks, and various proprietary networks. This allows, at low cost, not only the BTU data but also the flow and temperature data to be monitored by the EMS/BMS.

Flow meters

The most common flow meters used for chilled water metering applications are Turbine, Full-bore magnetic, Single-point magnetic, Vortex-shedding and Transit-time ultrasonic meters.

The Turbine meter is perhaps the most common for this application, owing to its low cost, but it is prone to clogging on open systems, such as condenser water systems. Because of the moving parts, routine maintenance is required.

Full-bore magnetic flow meters are the best from an accuracy point of view; they are regarded as being very accurate even at very low flow rates. They also score high from an operational standpoint, as they come with the promise of low maintenance costs and are longer lasting. That said about their virtues, they are expensive. Until recently, they were extremely expensive, because most manufacturers designed the meters for the more demanding industrial market, but commercial quality meters are now available at much lower cost. Since the full-bore meter senses the entire water flow, and not just a single point, it is much less sensitive to installation problems; as long as turbulence does not cause reversing eddy currents within the flow tube, the meter will be accurate.

Single-point magnetic meters are often used for large piping, when the cost of full-bore meters becomes prohibitive, but because they measure flow only at a single point in the pipe, they are much less accurate than full-bore meters.

Vortex-shedding meters were more common before magnetic meters came down in price. They are now more commonly used on gas and steam flow. A significant limitation is that they are not very accurate at low flow.

Ultrasonic meters are non-invasive. That is, they do not require any openings into the pipe, and were initially used for ad hoc flow measurements, such as for test and balance. Installation details are critical. Manufacturers provide jigs and assemblies to ensure the sensors are accurately installed, but they are still prone to inaccuracies from installation error. Since they are non-invasive, ultrasonic meters are particularly applicable to retrofit applications.

For metering chilled water flows at buildings, particularly for revenue purposes, the full-bore magnetic flow meter is strongly recommended. The pipe sizes are generally small enough at building connections, which make these meters affordable. Full-bore magnetic flow meters are also strongly recommended for metering total central plant output of variable-flow chilled water systems.

Temperature sensors

The most common temperature sensing devices used in chilled water applications are Thermistors, Resistance Temperature Detectors (RTDs) and Integrated Circuit (solid-state) Temperature Sensors. These all use materials whose resistance or impedance changes with temperature.

Thermistors are generally the least expensive and are fairly accurate (±0.4ºF for standard thermistors and ±0.2ºF for extra-precision thermistors). Historically, they had problems staying in calibration, but this is not a problem with modern thermistors, which drift less than about 0.04°F over a five-year period. Their temperature range is very broad, so a single thermistor sensor can be used for virtually all HVAC applications; specific ranges do not have to be specified. Their signal is non-linear with respect to temperature changes, so signal conditioning is required, but this capability is standard for most modern EMS/BMS.

RTDs were once among the most common temperature sensors in HVAC applications, but they recently have been mostly displaced by less expensive thermistors. RTDs have low resistance, so transmitters are required and, unlike thermistors, they must be ordered for the specific temperature range required by the application. Accuracies of RTDs vary widely from ±0.02°F to ±1.0°F, depending on the material – platinum is most common – and construction.

Integrated circuit temperature sensors are not commonly used in HVAC applications, except with BTU meters. They are not very accurate (±1.0°F), but they are extremely repeatable and linear. They also do not require calibration. Hence, they are excellent for differential temperature measurement, once the two sensors are matched and calibrated at the factory. Differential temperature measurement accuracy is typically about ±0.15F.

Differential temperature accuracy and sensor recommendations for metering purposes of a chilled water plant are ±0.15ºF. Thermistors will not be able to meet this accuracy requirement. Ultra-high accuracy RTDs, or matched integrated circuit sensors supplied with a BTU meter, must be used. The accuracy and long-term stability of the BTU meter-integrated circuit sensors is one of their main advantages and why BTU meters are recommended for chilled water flow metering.

Domestic, Treated Sewage Effluent (TSE), Tower Make-Up (MU) Water

Water meters can be classified into two basic types: Positive displacement and velocity. Each of these meter types has variations, leading to the perception that there are several different kinds. Meters that feature both positive displacement and velocity are known as compound meters. The unit of measurement is typically in gallons or cubic feet.

Positive Displacement Meters

In this type of meter, a known volume of liquid in a tiny compartment moves with the flow of water. Positive displacement flow meters operate by repeatedly filling and emptying the compartments. The flow rate is calculated based on the number of times the compartments are filled and emptied. The movement of a disc or piston drives an arrangement of gears that registers and records the volume of liquid exiting the meter.

There are two types of positive displacement meters: Nutating disc and piston. Nutating disc meters have a round disc that is located inside a cylindrical chamber. The disc is mounted on a spindle. The disc nutates, or wobbles, as it passes a known volume of liquid through the cylindrical chamber. The rotating motion of the disc is then transmitted to the register, which records the volume of water that went through the meter. Piston meters have a piston that oscillates back and forth, as water flows through the meter. A known volume of water is measured for each rotation, and the motion is transmitted to a register through an arrangement of magnetic drive and gear assembly.

Positive displacement meters are sensitive to low-flow rates and have high accuracy over a wide range of flow rates. Positive displacement meters are used for smaller-flow applications.

Velocity Meters

Velocity meters operate on the principle that water passing through a known cross-sectional area with a measured velocity can be equated into a volume of flow. Velocity meters are good for high-flow applications. Velocity meters come in different types, including Venturi, Orifice, Turbine, Ultrasonic and Magnetic meters.

Venturi meters have a section that has a smaller diameter than the pipe on the upstream side. Based on a principle of hydraulics, as water flows through the pipe, its velocity is increased as it flows through a reduced cross-sectional area. The difference in pressure before water enters the smaller diameter section and at the smaller diameter “throat” is measured. The change in pressure is proportional to the square of velocity. The flow rate can be determined by measuring the difference in pressure. Venturi meters are suitable for large pipelines and do not require much maintenance.

Orifice meters work on the same principle as Venturi meters, except that, instead of the decreasing cross-sectional area, there is a circular disc with a concentric hole. The flow rate is calculated similarly to the Venturi meter by measuring the difference in pressures. The pressure drop is very high through these meters and, consequently, they are seldom used in modern applications.

Turbine meters have a rotating element that turns with the flow of water. The number of rotor revolutions measures the volume of water. Magnetic meters have an insulated section through which water flows. The flow of water induces an electrical current that is proportional to the velocity and, hence, the flow rate.

In some cases, it is necessary to have a combination meter – a positive displacement meter and velocity meter installed together – to be able to measure extremely high flow rate applications. Low flows are measured through positive displacement while high flows are measured by velocity. A valve arrangement directs flows into each part of the meter.

For flows up to 160 gpm, positive displacement meters in sizes of 1”, 1½” or two inches are commonly used. In sizes of 2-3 inches, either displacement or turbine types of meters can be used. In the 3-4-inch-size range, the meter type depends on the average flow rate. If the flow rate is between five and 35 per cent of maximum flow rate, the positive displacement type is better. If the flow rates are going to be 10-15% of the maximum capacity, a turbine type should be used. If close accuracy at low flows is important, but large flows also have to be measured, a compound meter is best. Venturi meters can be considered as an option for a large range of flows, such as metering a main, if you need a meter that will not interfere with the flow, if the meter fails. Magnetic meters offer high accuracy with no head loss but are the most expensive option for metering water.

Sewer/blowdown metering

Owing to the expense and potential problems, measuring sewage flow should only be considered when water use for irrigation or evaporative processes (such as cooling towers, evaporative condensers and direct/indirect evaporative cooling) are large and the local utility provides a financial credit for the reduced sewer burden. Even in these cases, it is usually more cost effective to sub-meter the water flow to these applications and deduct the flows from overall water usage.

Meter installation

Though it has been said before, it’s not sufficient to focus only on the quality of equipment; installation is as important as the selection process. The entire installation exercise must be verified in detail by a commissioning process containing checklists of all items and processes necessary to ascertain compliance to the installation criteria provided by the meter manufacturer. Different types of meters have very specific installation requirements that are necessary for proper function. The meter manufacturer installation procedures and recommendations should be followed in detail. Meters should be installed by qualified contractors under the supervision of the project commissioning agent, the project inspector, the project manager and the engineer of record.

Commissioning and calibration of meters is the process that contributes to the proper installation and provision of adjustments necessary to generate accurate meter data. The accuracy and reliability of the data obtained from meters largely depends on whether they are properly installed and calibrated. Installation details must strictly follow the meter manufacturer’s instructions. Once a meter is properly installed, a process must be conducted to confirm accuracy and calibration.

Accuracy is the ability of a measurement to match the actual value being measured. Calibration is the act of checking or adjusting the accuracy of meter by comparing it with a known standard. The known standard in the case of most meters can be a portable meter of greater accuracy with calibration traceable to the National Institute of Standards and Technology (NIST). All revenue-grade meters should have a calibration certificate indicating measurements traceable to the NIST or equivalent institution. Meter manufacturers and suppliers must produce the criteria for installed accuracy validation. Documentation affirming that the meter was properly installed and providing accurate data should be specified to be a part of the commissioning process.

In closing, I would recommend that any building or chilled water plant have a meter programme in place. However, do always be sure to use the right meter for the job, and to ensure that it is installed as per instructions. Also, it is vital to have a calibration/re-commissioning programme in place, and you will be on your way to energy savings.


The writer is the Managing Partner of Al Shirawi US Chiller Services. He can be contacted at dan@uschillerservices.com


CPI Industry accepts no liability for the views or opinions expressed in this column, or for the consequences of any actions taken on the basis of the information provided here.

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