A meter for low air flows

J. agric. Engng Res. (1966) 11 (4) 303-306

A Meter for Low Air Flows D. J.

GREIG*

1. Introduction In grain storage-drying systems the rate of air movement is very important because it determines the rate of moisture removal' the total energy requirement for drying" and the heat required for a designed temperature rise. The rates of air flow in this type of drier range from 5 to 20 ft/min through the grain, and in chilled storage systems from I to 10 It/min. An instrument operating on the hot wire anemometer principle was constructed to measure the rate of flow of air leaving the surface of the grain. It differs from that described by MacDonald and Hedlin' in the method of temperature measurement used, the present meter giving a reading of greater accuracy. Although a theoretical relationship has been developed" between the temperature of a hot body receiving a constant power input and the speed of air passing over it, the agreement between calculated and experimental determinations may not always be close. It is therefore more usual to determine this relationship from experimental observations. 2. Construction Mechanical The design of the instrument (Fig. 1) is based on the measurement of the change in temperature of a hot wire having a fixed power input. Hukill" has described the reduction in precision of measurement by thermocouple anemometers at air speeds below 4 It/min. A reducing section (A), 1 ft square at the base and 3 in dia at the measuring point, was therefore used to provide a 20-fold increase in the air flow over the hot wire. Hot and cold copper/constantan thermocouple junctions J1 and J2 made of 28 s.w.g. wire" are located in a 3 in dia tube (B). These

provide an output which is unaffected by the temperature of the air passing over them. Tube B is bolted to the upper outlet of A. A tightly fitting end cap (C) may be placed over the outlet of Tube B to prevent air movement when the adjustment of the instrument zero position is carried out. The complete measuring c

JI

B J2

2.1.

• Department of Agricultural Engineering, University of Newcastle upon Tyne

303

o

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8

Scale in inches

Fig. 1. The meter

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10 II

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304

A METER FOR LOW AIR FLOWS

circuit and the battery for the heating wire are located in the box D on the side of Section A.

3. Calibration The instrument was calibrated against air flows in the range 0'2-30 ft/min obtained by drawing air from the top of the instrument 2.2. Electrical The hot wire R6 (Fig. 2) is made from 7! in through an orifice plate." For air flows between I and 30 ft/min the of 26 s.w.g. enamelled constantan wire close calibration represented a straight line if log Q wound on to the hot junction Jl , This is series was plotted against dial setting. The equation connected to a 2 V battery B2, a 1 n W.w. representing this relationship is thus of the form : potentiometer R7, poles b of the on/off switch log Q = k + m log P SI and a milliammeter and shunt resistance R8. A regression analysis was carried out to In the measuring circuit, resistances RI and determine the constants m and k for values of R2 form a potential divider powered by the I! V Q against P. log 100 dry cell battery BI, controlled by pole a of the The resulting equation for values of P between on/off switch SI, and adjusted by the combina2·00 and 8·00 was log 100 Q = 4·167 - 0·254P tion of resistances R3 and R4. The sensitive, with a correlation coefficient of 0·97 for 54 low resistance, centre zero galvanometer G, points. protective short circuiting switch S2 and attenuatFor P > 8,00, representing very low air flows, ing potentiometer R5 may be connected either the plot of log Q versus P (Fig. 3) was not a to reference cell C, for standardising the current straight line but can be represented by 2 straight in RI and R2, or to the thermocouple output, lines, one for dial readings between 8·00 and for measurement, by the keyswitch S3. 9·10 and the other for readings between 9·10 and 9·70. Values of P in these two ranges were again used to compute the constants in the straight line equations giving the following results:R8 8·00 < P < 9'10; log 100 Q = 5·91 - 0·473 P; r

= 0·92
9·10 r

S2 RI R2 Sic

81

R3

R4

~~./VVVV'-

Fig. 2. Hot wire and measuring circuit Resistors: (R1) 1000 n high stability w.w.; (R2) 5 n 10 turnpotentiometer; (RJ) 430 n high stability W.w.; (R4) 100 n potentiometer W.w.; (RS) 10000 n W.w.; (R6) 7t in, 26 s.w.g, enamelled constantan wire close wound on to hot junction J1 with 1 in flying leads; (R7) 1 n potentiometer w.w.; (R8) To suit milliammeter Switches: (Sl) D.P.S.T. toggle; (S2) S.P.S.T. toggle; (S3) D.P.D.T. biased keyswitch Meters: (A) 1 mA f.s.d. milliammeter; (G) Centre zero pointer galvanometer, 10 n Batteries: (C) Reference cell. Mercury/cadmium sulphide, 1·0813 V e.m.f.; (B1) 1·5 V dry cell; (B2) 2 V, 8 Ah lead acid cell

9'70; log 100 Q

=

8·86 - 0·796 P;

4. Operation Closing switch SI connects dry cell BI to the heating wire R6. With S2 open and S3 in the standardizing position, the reference cell is connected across the series circuit of galvanometer and Rl. The current through RI and R2 is now adjusted by R4 until, with RS short circuited, there is no deflection of the galvanometer pointer. The voltage drop across Rl due to the current flowing in it from Bl is then equal to the reference voltage of 1·0813 V. With the top cap C in position to prevent air movement S3 is placed in the TEST position connecting the thermocouple output across R2. With RS fully in circuit on dial position 10·00, R7 is adjusted until again there is 00 deflection of the galvanometer pointer. The output of the thermocouples then equals the voltage drop across R2 due to the standardized current passing through it from BI.

305

D. 1. GREIG

Log 100 Q. 4·167 - O' 254 P

"

'E 5·0 .....


'·0 Log 100Q= 5,91- 0·473 0·5

p---------o.a

Log 100Q=8,86-0,796

p.---------"\((

Potentiometer diol setting, P

Fig. 3. Calibration linesand regression equations

The top cap is removed, the instrument placed in its measuring position and R2 readjusted until again there is no galvanometer deflection. The air flow rate can then be obtained from the regression equations or from the calibration chart. 5. Resistance to air flow It is important that the resistance to air flow offered by an instrument of this type exerts a negligible effect on the measured air flow. The ci

~ .5

0·150 0'125

REFERENCES

e ..,

"

0'100

~

0-075

~ Q.

-

resistance offered was measured for air flows up to 55 ft/min. The rate of air flow to a plenum chamber fitted with static pressure tappings was measured by orifice plate" and the pressure drop across the meter, by a micromanometer" (Fig. 4). For air flows up to 10 ft/min the static pressure drop across the instrument was 0·003 in W.G. rising to 0·01 in W.G. at 17 ft/min and to 0·04 in W.G. at 30 ft/min. These values of resistance would be offered by depths of clean barley of 0,33, 0·44 and 1·60 in respectively at the same air flow rates and were therefore considered negligible.

0'050

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Cii ',0,025

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35

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45

Air flow, It' min-IIf

Fig. 4. Static pressure drop across meter

50

55

, Boyce, D. S. Grain moisture and temperature changes with position and time during through drying. J. agric. 2 Engng Res., 1965, 10 (4) 333 Schaper; L. A., Isaacs, G. W.; Dale, A. C. Evaluating heat available for drying in naturalair. Trans. ASAE, 1961, 4 (1) 140 3 MacDonald, J. B.; Hedlin, C. P. Air flow meterfor crop drier. Agric. Engng, St Joseph, Mich., 1954, 35 (9) 658 • Faires, V. M. Thermodynamics. 3rd edn. MacMillan Publishing Co., N.Y., 1962, p. 509

306 • Hukill, W. V. Temperature. Amer. Inst. Phys., 1941 , p.666 • Reference tables for copper constantan thermal couples. Brit. Stand. Spec. 1828: 1961

A METER FOR LOW AIR FLOWS

Code for air flo w measurement. Brit. Stand. Spec. 1042: 1943 • Shedd, C. K. A micromanometer. Agr ic, Engng, St Joseph, Mich., 1953, 34 (3) p. 178 7