Use of Air-System Components as Orifice Meters for Airflow Control Systems.
Airflow monitoring stations (AFMSs) are commonly used for continuous monitoring of airflow as well as its quantitative regulation. Using an AFMS allows HVAC systems to be controlled more efficiently and with improved building pressure control and reduced fan energy consumption.
Currently, there are two main types of devices for continuous measurement of airflow in HVAC systems (ASHRAE 2013a): pitotmetric measuring stations and orifice meters.
Pitotmetric measuring stations are used widely in the industry and are primarily installed on outdoor air, supply, and exhaust main ducts. These measuring stations can differ significantly in design, size, and other characteristics, but they use the same general principle: measuring the dynamic pressure of the airflow in the cross-section of the duct.
The calculated velocity values are determined according to the following relation (ASHRAE 2013b):
[mathematical expression not reproducible] (1)
V = velocity of air, fpm (m/s)
C = unit conversion factor
[P.sub.w] = velocity pressure in the duct, in. w.g. (Pa)
g = gravitational constant
[rho] = density of air, lb/[ft.sup.3] (kg/[m.sup.3])
The information on air velocities and airflows is sent to a main control board, which ensures the implementation of the given algorithm for the operation of the air-handling unit (AHU).
The advantage of this method of measuring airflow is the minimal interference within the measured airstream. Its disadvantages are the following:
* A sizable decrease in the accuracy of the measurements due to an abrupt decrease in pressure associated with decreasing airflow rates.
* For every duct size, it is necessary to use a corresponding-size measuring station, which complicates the preliminary ordering of the equipment.
* The need for installation of additional straight ductwork upstream and downstream of the measuring device for its qualitative calibration.
* High cost.
The second approach is the installation of an orifice plate meter. In this case, airflow is obtained from the pressure drop across an orifice plate according to the following formula (ASHRAE 2013b):
[mathematical expression not reproducible] (2)
Q = discharge flow rate, cfs ([m.sup.3]/s)
k = coefficient, a function of duct geometry and the Reynolds number
A = orifice cross-sectional area, [ft.sup.2] ([m.sup.2])
[P.sub.1] - [P.sub.2] = pressure drop obtained by pressure taps, lb/[ft.sup.2] (Pa)
This is a fairly simple and low-cost method for measuring airflow, but it is also not devoid of a number of disadvantages, including:
* An orifice plate is an additional resistance and increases the pressure loss in the system.
* As with an AFMS, it is necessary to have straight sections of duct before and after the orifice plate, which is not always feasible.
In lieu of the methods above, we propose another method of continuous control of the airflow rate: using a quasiorifice meter. The basis of this recommendation is that the elements of an AHU possess stable aerodynamic characteristics that can be used as an effective orifice meter. Heat exchangers and heating coils, as well as cooling coils for AHUs operating in cooling mode without air drying, can be used as measuring elements.
The true airflow can be determined using the difference in static pressure before and after the measured element combined with calculations using aerodynamic characteristics obtained from manufacturer's data. If manufacturer's data are unavailable, a calculation can be made based on measurements obtained by the calibration of the components. Unlike a conventional orifice plate, these measuring devices do not create additional pressure losses and do not disrupt the airflow regime.
Similar ideas have been discussed in previous publications (ASHRAE 2011, 2013b) where it was suggested that, given the aerodynamic characteristics of a component, measurements based on these characteristics may be possible. However, in practice the previously mentioned characteristics are seldom obtained because most practitioners are interested solely in component resistances in units.
Our practical research has demonstrated that the absence of a priori data is not an impediment to the proposed method of measurement and calibration. Acquiring the required parameters is completely analogous to the calibration of the measuring stations. Accounting for the fact that practically all measuring stations are subject to calibration as a part of startup, using the components of the AHU as de facto meters does not present any increased effort or additional man-hours. Furthermore, measuring pressure losses in several components in immediate proximity of one another (e.g., heat exchangers, heaters, and coolers) provides for more accurate measurements because of the greater magnitudes of measured values. Therefore, we propose the use of one or several components of an AHU as quasiorifice meters in lieu of traditional measurement stations.
The advantages of using this method are:
* Decreased cost.
* Removal of the expenditures associated with the acquisition and installation of measurement stations.
* Elimination of the need for additional straight ducts designated to accommodate the measuring element.
* Absence of additional pressure losses arising from the introduction of an orifice plate to the system.
* Improved accuracy of measurements.
Improved accuracy is explained by the following: when measuring airflow with the use of an AFMS, as the airflow rate decreases, for example, from 600 to 200 fpm (3 to 1 m/s), it leads to a decrease in the measured dynamic pressure by a factor of nine. The accuracy of the measurement decreases substantially.
A similar result is obtained using a conventional orifice plate. The quasiorifice method is free of this disadvantage, because the measured difference in static pressures inside the supply unit before and after the quasiorifice meter usually is not less than 0.5 in. w.g. (125 Pa). Therefore, even with the sharpest decline of airflow, the value of the measured pressure difference remains very significant, which ensures a higher accuracy.
EXAMPLES OF PRACTICAL IMPLEMENTATION OF THE PROPOSED METHOD
We have used the quasiorifice method of controlling airflow described above several times in the past two years.
For a more detailed explanation of the method, two cases of using this method of measurement are presented in the subsections "Example 1" and "Example 2".
The proposed measurement and control method was implemented on a unit, AHU-1, with an air capacity of 13,510 cfm (22,953.64 [m.sup.3]/h). The heating coil, which has a design resistance of 0.61 in. w.g. (152 Pa), was used as the measuring station.
We tested this measurement system by varying the airflows from 100% to 27% of full capacity. With each setting, airflow in the main duct was measured with a thermoanemometer in accordance with the standard measurement method described in ASHRAE Handbook--Fundamentals (ASHRAE 2013b) while pressure drops across the measurement system were measured by a differential pressure transmitter with full scale of 2 in. w.g. (500 Pa).
To measure the pressure, plastic measuring tubes with open ends located inside the intermediate sections of the AHU before and after the measured element were used. The second ends, terminating in the union, were connected by a rubber hose with a fitting to a differential manometer. Eleven measurements were conducted. The results are presented in Table 1 and depicted in Figure 1.
The analysis of this information can be described by the following relation:
Q = 18,000[delta][P.sup.0.58] (3)
Here, 18,000 is a system-dependent numerical coefficient, [delta]P is the dynamic pressure difference in in. w.g. (Pa), and the exponent 0.58 depends on the system's geometry and Reynolds number.
The proposed method for measuring and regulating airflow was also implemented on AHU-2 supply and exhaust units, which serve a multizone air-conditioning system with quantitative regulation of airflow by the carbon dioxide (C[O.sub.2]) sensors in each zone.
Based on the specific layout of AHU-2, a set of two adjacent components was selected as a quasiorifice meter: a rotary heat exchanger and an air heater. The total resistance of these two components in the design mode is 1.45 in. w.g. (360 Pa).To obtain the aerodynamic characteristics of this device it was calibrated with a change in system performance from 100% to 23% of the designed capacity.
In each case, the airflow was measured in the main duct using a thermal anemometer according to standard procedure (ASHRAE 2013b), and the pressure drop was measured and transmitted to a control panel by a differential pressure sensor with a scale of 2.0 in. w.g. (500 Pa). Seven measurements were conducted. The results are presented in Table 2 and depicted in Figure 2.
The analysis of this information can be described by the following relation:
Q = 14,470[delta][P.sup.0.75] (4)
Here, 14,470 is a system-dependent numerical coefficient, [delta]P is the dynamic pressure difference in in. w.g. (Pa), and the exponent 0.75 depends on the system's geometry and Reynolds number. The deviation of the measured points from the calculated values is within [+ or -]5%.
It does not escape notice that the magnitudes of the exponents in Equations 2, 3, and 4 are significantly different. This is explained by differences in airflow characteristics that may be observed in a regular orifice plate versus by those existing in the channel of elements used as the quasiorifice (Nudelman et al. 2017).
The measurements were incorporated into the algorithm of the computer program for controlling the operation of the AHUs and were used to monitor and regulate the airflow in the systems in conjunction with zonal regulating air valves controlled by C[O.sub.2] sensors.
Implementation of this algorithm allowed us to obtain a quantitative estimate of the annual savings of heating, cooling, and electricity. It should be noted that the need for calibrating the quasiorifice meter arises only in the quantitative or quantitative-qualitative regulation of the supply system.
If the task is less complicated--for instance, to register and maintain a constant flow of air in the system despite any external influences--then the solution to the problem is much simpler: it is enough to measure pressure losses on the quasidiaphragm at a design airflow only once. The control system will maintain the required parameters of the airflow by changing the fan speed based on the differential pressure transmitter.
The proposed method, however, does not preclude the use of standard flow stations in cases when airflow systems do not include elements with flow characteristics suitable for the role of quasiorifice meters. This remark is in reference to exhaust and many types of recirculation systems.
Use of a quasiorifice meter allows a system to effectively and accurately monitor airflow, provide control, and help to provide economic efficiency analysis of an airflow system.
ASHRAE. 2011. Chapter 38, Testing, adjusting, and balancing. In ASHRAE Handbook--HVAC Applications. Atlanta: ASHRAE.
ASHRAE. 2013a. Chapter 3, Fluid flow. In ASHRAE Hand-book--Fundamentals. Atlanta: ASHRAE.
ASHRAE. 2013b. Chapter 36, Measurement and instruments. In ASHRAE Handbook--Fundamentals. Atlanta: ASHRAE.
Nudelman, M., E. Kernerman, and N. Muscolino. 2017. Accounting for non-quadratic behavior of AHU systems in determination of airflow rate. ASHRAE Transactions 123(1):3-7.
Mikhail Nudelman, PE Member ASHRAE
Edward Kernerman, PhD, PE
Nicholas Muscolino Associate Member ASHRAE
Mikhail Nudelman is an HVAC engineer and Nicholas Muscolino is operations manager at Aero Building Solutions, Franklin Park, Illinois, USA. Edward Kernerman is a professor at Novosibirsk State Architecture and Construction University, Novosibirsk, Russia.
Table 1. Pressure Losses in Heating Coil of AHU-1 at Variable Airflows Number of AHU-1 Capacity, Heating Coil Measurement cfm ([m.sup.3]/h) Resistance, in. w.g. (Pa) 1 13,641 (23,176) 0.620 (154) 2 12,055 (20,482) 0.501 (125) 3 10,533 (17,896) 0.397 (99) 4 9057 (15,388) 0.306 (76) 5 7280 (12,369) 0.210 (52) 6 6090 (10,347) 0.154 (38) 7 5593 (9503) 0.133 (33) 8 5114 (86,890) 0.120 (30) 9 4646 (7894) 0.103 (26) 10 4186 (7112) 0.083 (21) 11 3724 (6327) 0.070 (17) Table 2. Pressure Losses in Heat Exchanger and Heating Coil of AHU-2 Heat Exchanger and Number of AHU-2 Capacity, Heating Coil Measurement cfm ([m.sup.3]/h) Resistance, in. w.g. (Pa) 1 19540 (33200) 1.452 (360) 2 16480 (28000) 1.157 (287) 3 14360 (24400) 0.968 (240) 4 11770 (20000) 0.730 (181) 5 9590 (16300) 0.560 (139) 6 7000 (11900) 0.395 (98) 7 4580 (7780) 0.242 (60)
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|Author:||Nudelman, Mikhail; Kernerman, Edward; Muscolino, Nicholas|
|Date:||Jul 1, 2018|
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