AIR VOLUME FOR DRYING GRAIN

INTRODUCTION

The practice of drying hay andgrain with unheated or tempered airin Western Canada is limited. Thereluctance to use a system that provides low cost drying is due to anumber of reasons, not the least ofwhich is the opinion that the climateof Western Canada is not suitable.Difficulties, especially if the seasonis late, cannot be overlooked. On theother hand Moysey and Wilde (1)concluded for the Saskatoon area thatnearly two batches of wheat at 18%moisture content could have beendried to 14% during September, 1959with as little as 3 cfm of unheatedair per bushel where 1959 was considered a poor harvest year. In addition, the opinion ignores the significant effect of tempering the air byadding small amounts of heat.

One of the main difficulties experienced by the author in the promotionof drying with natural and temperedair was the inability to advise on thesize of duct and fan system requiredwhich in turn is based on the quantity of air required. The common design figure is 3 cfm per bushel andwhich, as noted above, was used byMoysey and Wilde. The figure, however, must vary with the followingindependent variables:

—the amount of moisture to beremoved

—the rate of moisture removal

—the climate during the dryingperiod

—the amount of heat added (ifany).

This study was initiated in order todetermine the relationship of thesevariables and provide a guide to airvolumes required.

ANALYSIS OF WEATHER DATA

It is important to note that the required air volume is a weather dependent function and, therefore, different values could be expressed for

58

by

H. P. HarrisonMember CSAE

Deportment of Agricultural EngineeringUniversity of Alberta

Edmonton, Alberta

different levels of probability. Instead, an average was determined,which by definition, has a probabilitylevel of 0.5. The reason for makingthis simplification is the likelihood ofthe operator tempering the air underadverse conditions rather than overdesigning the system. In other words,a large fan and duct should not beconsidered in order to compensatefor the occasional poor dryingweather when it is easier to do so byadding some heat.

An examination of the dry bulbtemperatures and relative humiditiesfor the period of August through November suggested that the agricultural area of Alberta could be dividedinto four distinct, but arbitrary, climatic regions. These regions appeared to be represented by the Leth-bridge, Calgary, Edmonton and Fair-view weather stations. The cost ofsecuring the required weather datafrom the Climatological Division, Department of Transport, however, precluded obtaining it for more than onestation. In view of this, Edmontonwas selected because it seemed to bea climatic average of the province inso far as agriculture is concerned.

Brooker and McQuigg (2) definedt—tx when positive as an index of

drying potential and, when negative,as an index of wetting potentialwhere,

t = dry bulb temperature of airentering the grain or hay

tx = dry bulb temperature of airleaving the grain or hay

= a -)- btw where a and bare constants which varywith the moisture contentof the grain or hay

tw = wet bulb temperature ofthe air.

Natural Air

Indices were calculated for the Edmonton station using hourly values

of t and tw summarized for one week

intervals from August 1 to November27 for the period 1950-66. It wasnecessary to calculate the indices foreach 1% moisture content (wetbasis) as the constants a and b, andtherefore, tx change with the mois

ture content of the grain. The relationship of tx and the moisture con

tent may be noted from the psychro-metric chart in figure 1. In the adia-batic process of drying, sensible heatis changed to latent heat with nochange in the wet bulb temperature.Also in figure 1 the relationship between t—tx and AX (the amount of

water the air can take up) may benoted where:

AX = x' — x, and

x — the humidity ratio of airentering grain at t and twand

x' = the humidity ratio of airleaving the grain at tx andtw-

The amount of water to be removedby the air is:

Aw = w — w'

where w — weight of water heldby one lb. of grain orhay — initially, and

w' — weight of water held byone lb. of grain or hay—finally.

It is more useful, however, to substitute Mw, the initial moisture con

tent (wet basis) and Mw', the final,for w and w' or

Aw — Mw - Mw (1-Mw)/

1-Mw 1

Dividing Aw by AX or

Aw/AX = (lbs. water/lb.grain) / (lb. water/lb. dry air) or,

— lb. dry air/lb. grain.

CANADIAN AGRICULTURAL ENGINEERING, VOL. 11, No. 2, NOVEMBER 1969

SATURATION CURVE

WET BULB LINE

DRY BULB TEMP (°F)Figure 1. Psychrometric Chart

<

Q

X

TABLE I. TAX FOR SMALL GRAINS - EDMONTON.(MINUTE-LBS. WATER/LB. DRY AIR FOR ONE WEEK INTERVALS)

Date Moisture Ranges (percent - wet basis, drying to 14%)22

r-14

21

-14

20

-14

19

-14

18

-14

17

-14

16

-14

15

-14Aug. 1-7 8.6 7.9 7.3 6.7 6.0 5.3 4.7 4.1

8 - 14 8.2 8.1 7.5 6.6 6.1 5.6 4.7 4.2

15 - 21 8.1 7.6 7.1 6.2 6.1 5.2 4.6 4.1

22 - 28 6.2 5.7 5.3 4.9 4.1 3.7 3.1 2.6

29 - 4 6.1 5.7 5.2 4.9 4.2 3.7 3.0 2.6

Sept 5-11 6.5 6.0 5.5 5.1 4.7 3.9 3.4 2.9

12 - 18 6.9 6.4 5.9 5.5 5.0 4.3 3.8 3.2

19 - 25 6.5 6.1 5.6 5.2 4.5 4.1 3.6 3.0

26-2 5.6 5.2 4.9 4.2 3.6 3.0 2.5 2.0

Oct. 3-9 6.4 6.1 5.7 5.0 4.7 4.2 3.5 2.9

10 - 16 6.2 6.1 5.6 5.1 4.6 4.0 3.3 2.8

17 - 23 4.8 4.7 4.1 3.8 3.5 3.1 2.6 2.1

24 - 30 3.7 3.4 3.1 2.6 2.3 1.9 1.5 1.3

31-6 2.7 2.4 2.1 1.8 1.5 1.0 .7 .6

Nov. 7-13 1.1 .87 .7 .55 .28 .17 .06 • 01

14 - 20 .94 .82 .60 .41 .28 .17 0 0

21 - 27 .93 .74 .59 .37 .23 .08 .01 0

The air voluiq — 13

ne then isWwAw/TAX . 2

IfWw = lb./bushel, then q == cfm/

where bushel.

w.

13

— cu. ft./min. (cfm) of airrequired,

= weight of grain or hay(with moisture) - initial,

— number of minutes ofdrying per batch period,and

= number of cu. ft. per 1lb. of dry air.

The difficulty in applying equation2 is that AX changes as drying proceeds. In addition, the system shouldbe operating only when t —tx is positive for maximum efficiency and,therefore, T changes as drying proceeds. In view of this, a table ofTAX for small grains (see Table 1)is provided for the Edmonton station

CANADIAN AGRICULTURAL ENGINEERING, VOL. 11, No. 2, NOVEMBER 1969

which is the average for the rangeof moisture contents noted. The tableis based on a seven-day period,though other periods, either shorteror longer, can be easily calculated. Inessence this, and the foregoing, assumes that the air leaves the grain atequilibrium with the wettest grain.In order for this to be a valid assumption, the air flow must be moderateand the on-off of the fan must be controlled so that ventilation occurs onlywhen t — tx is positive.

An example of the use of the tableand equations 1 and 2 is as follows:

Drying wheat (60 lb./bus.) from18% moisture content to 14%during the last week in September (1 week drying period) requires 10. cfm/bus. or,

Aw — .18 — .14 (.82/.86) —.18 — .133 = .047 lb.water/lb. grain;

q _ 13 x 60 X .047/3.6 =10. cfm/bus.

As above, except last week ofSeptember and first week ofOctober (2 week period) or,

A.

B.

- 13 X4.7) =

60 X .047/(3.6 +4.4 cfm/bus.

It can be noted from equation 2that the air volume is inversely proportional to TAX. From Table 1,therefore, large air volumes are required late in the season when dryingto a low moisture content. In fact,drying to a safe moisture content afterthe third week in October withnatural air is virtually impossible. Onthe other hand, considerable moisturecan be removed from very wet grainafter this time without adding heat.Such economies of operation areavailable only if a high level of managerial skill is available.

The "lumpiness" of the data inTable 1 is due to:

—the limited summary period (15years)

—the step type selection of theconstants a and b (and therefore TAX)

—the constants a and b changeabruptly at 32°F.

Tempered Air

Tempering the inlet air by addinga small amount of heat affects theindex t — tx significantly. The great-

59

+ THERMOCOUPLESAT t/

x THERMOCOUPLESAT t°

+ THERMOCOUPLESTRANSITION FROM

» TO t,

4> THERMOCOUPLESAT tw

AIRPROBE

x̂*

TRANSDUCER WEIGH SCALE

WET

GRAIN

DRYINGZONE

DRY

GRAIN

est wetting potential (t —tx is negative) for the Edmonton station was4°F which occured on the week ofAug. 22 - 28 at 6:00 a.m. By increasing the dry bulb temperature (t) 10°,t — tx became positive. That is, by

increasing the inlet temperature 10°,drying could have taken place anytime day or night from Aug. 1 toNovember 27 and quite possibly theyear around. It is interesting to notethat the greatest wetting potential inNovember was only 2.2°F. The temperature rise to change t — tx positive

in this instance was only 5.5°F.

Tempering the air is accomplishedmost conveniently with a propaneburner. In fact, there are a numberof manufacturers offering fan-burnerpackages. If the source of power forthe fan is an internal combustion engine, this could be the source of-heat.The author, by suitable ducting, butexcluding the exhaust, found that,when the power of engine was matched to the fan, a 5° temperature risewas typical. If the engine was largerthan required, such as using a smalltractor, a 10° temperature rise waspossible. A British drier manufactureradvertises a temperature rise as muchas 25° using no other source of heatbut the engine used to drive the fan.

Tables of TAX for tempered air,or even hot air, could be developedand used with equation 2. For a largetemperature rise the seasonal anddaily variations of t and tw become

insignificant. It should be noted thatthe resulting values would be validonly if the air flow was of such magnitude that equilibrium of moistureoccurred before the air left the grain.

tures in the grain bin were useful inlocating the drying zone. The air flowin cu. ft. per min. and the loss (orgain) of water were recorded periodically. The latter was determined byweighing the bin and grain on astrain gauge type scale and was thebasis for termination of the drying. Inaddition, the moisture content wasdetermined by twenty randomsamples at the beginning and endingof drying as a check on the water lossdetermined above.

Results

Figure 4 illustrates the typical progression of the drying zone throughthe grain. It is interesting to note thatthe depth of the drying zone increased with time. It also can be seenthat the drying front barely reachedthe surface when drying was terminated. The location of the front attermination is supported by Figure 3.This illustrates the typical linear re- Figure 2. Grain bin for drying

DRYING EXPERIMENTS

A number of barley samples weredried to check the validity of the assumptions made and their effect onequation 2. Hie grain used had beenharvested late in the Fall of 1968 atapproximately 19% moisture content.This was subsequently dried to 13 or14% in batches of 60-70 bushels.While drying, the air temperature atvarious levels in the grain (See Figure2) as well as t and tw of the air (un

controlled) entering the grain, wererecorded using thermocouples and asequential recorder. The air tempera

60

toto

O

3I—

O

O

LU

I—

<

z

u.

o

O•—i—

oCD

o

UL

LU

U

z<I—to

a

Figure 3. Rate of water loss

GRAIN SURFACE

DRYING TERMINATED"Figure 4. Approximate location of drying zone

CANADIAN AGRICULTURAL ENGINEERING, VOL. 11, No. 2, NOVEMBER 1969

~ 300

u

^ 200O

0£

<

100-

J.

10 20 30 40 50 60 70 80

TIME (hours)

Figure 5. Change in air flow

TABLE II. RESULTS OF DRYING BARLEY

Trial NumberAw

(lbs/lb grain) <°F)T

(hours)

w

Uo3)Q» (est.)

(ft3 x 106)Q (actual)

(ft3 x 106) QVQ

1 .072 9.5 •4 3.1 1.3 1;4 .9

2 .075 9.0 95 3.4 1.5 1.1 1.4

3 .060 11.4 •4 3.2 1.1 1.2 .9

4 .063 11.6 51 2.9 1.0 .90 1.1

5 .071 10.2 83 3.3 1.4 1.3 1.1

6 .056 10.7 51 3.2 .88 .75 1.2

7 .067 9.0 66 3.3 1.4 .9 1.6

8 .069 7.6 '4 3.3 1.6 .9 1.8

9 .063 7.5 >4 3.2 1.5 .9 1.7

lation found between the water lossor moisture removal and the amountof air pumped through the grain. Ifthe drying period had been extended,the rate would have been reducedand become curvilinear after the drying front intercepted the grain surface. The water added (negativewater loss) during the first few hourswas due to the initial grain temperature being lower than the wet bulbtemperature. The drying period wasconsidered to be the interval betweenthe time that the water loss was zero(for the second time) until the grain,was in the 13 to 14% moisture range.

The change in air flow with respectto time, Figure 5, is partially attributed to an increase in static pressuredue to a reduction in volume or gainin density as the grain dried.

The significant result of the dryingexperiments is the ratio of the estimate, Q', (calculated from the recorded values of t and tw) to the totalquantity of air pumped, Q, whichmay be noted in Table 2. Values ofQ'/Q much greater than one are attributed primarily to errors in Q,which were calculated from measurements of the air velocity. The grad

ient across the inlet duct was steepand a small change in position of theair probe would alter the measuredair volume Q significantly. The possibility of values greater than one dueto Q' seems remote. This would occuronly if the air picked up more moisture than that specified at equilibrium. In this regard, equilibrium wasclose to the saturation curve. In viewof this, and the foregoing, reasonablevalues of air flow apparently can beestimated from the amount of moisture to be removed and the prevailingwet and dry bulb temperatures, provided the air flow is not large.

CONCLUSIONS

An equation was developed to assist in estimating the quantity of airrequired to dry grain and hay. Thewet and dry bulb temperatures of theEdmonton area, on a per week basisfrom August to November inclusive,were processed to simplify the use ofthe equation for small grains.

The validity of estimating the required air flow from the wet and drybulb temperatures appears reasonable, provided the air flow is notlarge.

CANADIAN AGRICULTURAL ENGINEERING, VOL. 11, No. 2, NOVEMBER 1969

Tempering the air by as little as5° to 19°F provides for drying underadverse climatic conditions and/oron a 24-hour basis.

REFERENCES

1. Moysey, E. B. and Wilde, D. H.1965. Drying grain with unheatedair. Cdn. Agr. Eng. 7: 12-13.

2. Brooker, D. B. and McQuigg, J. D.1960. Analysis of weather datapertinent to grain drying* Trans.Amer. Soc. Agr. Eng. 3: 116-119.

EVAPOTRANSPIRATION . . .

continued from page S3

of California. First Annu. Rep.USAEPG Contract CA-36-039SC-80334, 74-107.

11. Rijtema, P. E. 1959. Calculationmethods of potential evapotranspiration. Inst, for Land andWater Manage. Res. Tech. Bull7.

12. Stephens, J. C. and Stewart, E.H. 1963. A comparison of procedures for computing evapora-t i o n and evapotranspiration.I.A.S.H. Pub. 62: 123J33.

13. Stern, W. R. and Fitzpatrick, E.A. 1965. Calculated and observedevaporation in a dry monsoonalenvironment J. of Hydro! No.314: 297-311.

14. Suomi, V. E., Franssila, M. andIslitzer, N. F. 1954. An improvednet radiation instrument. J. Met-eorol. 11: 276-282.

15. Tanner, C. B. and Pelton, W. L.1960. Potential evapotranspirationestimates by the approximate energy balance method of Penman,J. Geophys. Res. 65: 3391-3413.

16. Thornthwaite, C. W. 1948. An approach toward a rational classification of climate. Geophys. Rev.38: 55-94.

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