Feasibility of using thermal response methods for nonintrusive compressed air flow measurement
Introduction
This paper examines a novel approach to nonintrusive flow meters for use in compressed air systems. Nonintrusive flow meters are of interest in multiple application areas. While greater accuracy is generally achieved with an insertion flow meter, many applications make intrusive measurements impractical. These industries generally fall into two categories. The first is the one in which a one-time spot measurement is desired and the time and cost of installing an inline flow meter is inappropriate. For example, fault detection and diagnosis equipment may be used only for a short period. The second category is applications where the fluid inside the pipe is corrosive. Inline sensors quickly fail in these applications, and there is a strong maintenance and economic incentive for a nonintrusive meter. A few identified applications for an improved nonintrusive flow meter are listed in Table 1.
This paper evaluates the feasibility of using a nonintrusive thermal response method for flow measurement. Specifically, the focus is on nonintrusive flow measurement of compressed air. Compressed air systems are one of the most common systems found in industrial facilities. The widespread use of compressed air in industry has led many people to term compressed air the fourth utility. Some estimates report that compressed air constitutes approximately 10% of all industrial electricity [14]. Due to inefficiencies in compression, the relative cost of compressed air is high [15,16]. Because of this high cost, any losses in the system result in substantial wasted electricity. There are several points of loss and inefficiencies in a compressed air system, including oversized compressors, high operating pressure, and inefficient compressed air drying [[14], [15], [16]]. These inefficiencies in energy use result in waste, where losses in the system can range up to 60% [17,18].
One of the most common and worst contributors of losses is leaks in compressed air lines. Leaks in an unmaintained plant typically consume 10–40% of the total system usage [19]. The large amount of lost energy presents an opportunity for cost savings by repairing leaks. Additionally, plants often run the compressed air system at higher pressures to compensate for leaks in order to maintain downstream pressure at equipment. The higher pressure exacerbates compressed air leakage, inducing additional losses. Reducing these leaks has demonstrated significant cost savings for industrial plants [20].
Fig. 1 displays data from the Texas A&M Industrial Assessment Center on recommendations related to repairing compressed air leaks. The graph displays data taken from approximately 50 industrial compressed air systems and shows the average leakage rate for the facilities. The annual cost of the compressed air leaks depends on the cost of electricity, compressor efficiency, and operating hours of the plant. For a two-shift plant operating 16 h/day, 5 days/week with a cost of electricity and demand of $0.06/kWh and $4/kW·mo, respectively, a leakage rate of 0.047 m3/s (100 SCFM) would result in an annual cost of $6800. If the compressor continues to run when the plant is shutdown, the resulting cost of compressed air leaks is over $12,500. Fig. 1 demonstrates that most systems have an opportunity to significantly reduce utility expenditures in compressed air systems by reducing the leakage rate of air. Repairing leaking systems requires minimal capital investment and expertise, resulting in short payback periods.
The primary measurement in estimating potential energy and cost savings is the quantification of the air flow rate due to leaks. Once the flow rate is determined, conversion factors are used to estimate the required electricity for compression. These other factors and values are obtained from nameplates and utility information. The current methods for quantifying leakage rate either estimate total system rate or estimate flow from individual leaks. In estimating total system losses, the four primary methods are performing a pressure decay test, measuring compressor power, using an ultrasonic flow meter, and installing an inline flow meter. All of these methods require production to be halted, so that all air flow in the compressed air lines comes from leaks in the system. Measuring individual leaks can be done while production is still running. Each of the listed methods has disadvantages that create high uncertainty in the air flow measurement. The following paragraphs describe each method along with the relative advantages and disadvantages of the method. Table 2 summarizes these advantages and disadvantages.
A pressure decay test records the time for pressure to drop a predefined amount when the compressor is shut off. Using the volume of the entire system, ideal gas laws can be applied to estimate total flow rate [21]. Determining the volume of all major compressed air lines and tanks can be difficult, resulting in large uncertainties.
Commercially available clamp-on ultrasonic flow meters use ultrasonic waves to estimate the speed of the air flowing inside of the compressed air line. They directly measure the speed in the pipe and rely on knowledge of system pressure. This method requires extensive setup and calibration along with knowledge of several parameters, some of which are difficult to obtain [22,23]. Additionally, strict requirements are given on the location of the meter and correct attachment to the pipe. Small variations in this setup create large uncertainties in flow measurement.
Voltage and current measurements can be used to directly measure the electric power required for compression [14]. Combined with the estimated capacity of the compressed air system, the leakage rate can be predicted. This method is common in professional energy assessments, but it requires electrical certifications and a significant time investment to locate and isolate the desired electrical lines. Additionally, this method can only estimate total system flow; leakage rate of individual lines cannot be obtained directly from the power meter.
A final approach to flow measurement is to identify and quantify individual air leaks. In identifying individual leaks, the most common method for finding leaks is with ultrasonic leak detection [14]. Compressed air leaks are loud and can be identified audibly when other equipment is stopped. When production is still running, ultrasonic leak detectors can aid in identifying leaks among other loud equipment. Although some work has been performed to estimate sizes of individual leaks based on ultrasonic measurements, typically the size of each leak is estimated based on laboratory calibration of hole size [24]. However, this laboratory calibration assumes a smooth circular hole, which creates large uncertainties in the actual leakage rate from the hole. The uncertainty in leak size, together with the challenge in determining all leak points, makes using this method to determine total system losses difficult.
Given the practical challenges and potentially large uncertainties of the existing methods, the potential for using thermal response signatures for determining compressed air flow rate noninvasively warrants further examination. Several initial simulation study of this type of approach has been proposed previously [[25], [26], [27]]. For example, in Ref. [27] the authors provide an analytical model is developed for a heated pipe, and a simulation study is used to examine the correlation between the temperature responses and the flow rates.
Section snippets
Proposed design of the flow meter
The proposed nonintrusive flow meter uses the thermal response of a compressed air line to derive the flow inside of the line. The setup of the system is diagrammed in Fig. 2. The device is wrapped around a compressed air line with the heaters and a thermocouple. The inner pipe surface temperature is estimated by measuring the outer surface temperature. Lumped capacitance can be used to estimate the inner pipe surface temperature from the outer pipe surface temperature because of the high
Simulation setup
The proposed method was tested in two separate simulation programs. The first program used was a basic commercial finite element program. To test the sinusoidal method of flow estimation, a separate numeric model was developed independently. Each model from both programs is built as a two-dimensional axisymmetric model. The setup of each of these simulations are described in the following sections.
Preliminary field test
Having verified the initial concept experimentally for the temperature decay rate method, the system was tested in an industrial setting. This test was done to assess the feasibility of performing a test in an industrial environment and to compare the method with other standard flow rate estimation methods. The system in the test case was run by a 11.186 kW (15 hp), reciprocating compressor. A receiver tank was located directly under the compressor. The main header for the compressed air system
Conclusions
As discussed previously, existing nonintrusive gas flow meters are designed to work under specific conditions and require extensive information about pipe and flow conditions to ensure accuracy. There are many accurate intrusive flow measurement technologies, but the necessary installation time and expense make these prohibitive in many applications. In contrast, the proposed thermal method in this work does not require extensive installation, production down-time, or detailed information about
Credit author statement
Trevor J. Terrill: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Resources, Data Curation, Writing – Original Draft, Writing – Review & Editing, Visualization. Taeksoo Kim: Resources, Writing – Original Draft, Writing – Review & Editing. Bryan P. Rasmussen: Conceptualization, Methodology, Formal analysis, Writing – Review & Editing, Supervision, Project administration, Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was supported by the US Department of Energy, Office of Energy Efficiency and Renewable Energy, Advanced Manufacturing Office, IAC program under award number DE-EE0007700. Any opinions, findings, conclusions or recommendations expressed in this paper are those of the authors and do not necessarily reflect the views of the US Department of Energy.
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