~ ) Pergamon Renewable Energy, Vol. 13, No. 4, pp. 453~469, 1998

© 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain

P I I : S 0 9 6 1 ~ 1 4 8 1 (98)00030-5 0960-1481/98 $19.00 + 0.00

THE P E R F O R M A N C E OF F O R C E D C O N V E C T I O N G R E E N H O U S E D R I E R S

MIGUEL CONDOR[* and LUIS SARAVIA INENCO, Instituto de Investigaci6n en Energia no Convencional,

Universidad Nacional de Salta, Calle Buenos Aires 177, 4400 Salta, Argentina

(Received 15 December 1997; accepted 19 February 1998)

Abstract--An analytical study of the evaporation rate in two types of forced convection greenhouse driers, the single and the double chamber systems, is presented. A performance parameter defined to compare both driers and its dependence on the operational variables is studied introducing the concept of the characteristic function. The drying curve is calculated and a time scale change is introduced to reduce the curves obtained under different operational conditions to a single generalized drying curve. Simulation tests for red pepper are presented showing that the use of a double chamber drier improves the productivity by almost 90%. © 1998 Elsevier Science Ltd. All rights reserved.

1. INTRODUCTION

Greenhouse solar driers use regular greenhouse structures where the product is placed in trays receiving the solar radiation through the plastic cover, while the moisture is removed by natural convection or forced air flow. These systems have the following potential advantages :

(1) Direct radiation driers are more efficient in the use of the received solar energy. (2) The initial inversion is low. (3) The potential use of the structure along the year, both as a greenhouse or as drier,

improves the economic benefits.

The main purpose of this paper is to perform an analytical study of their energy perform- ance. Although a numerical analysis can take into account more details related to the energy balance, the analytical study gives a better feeling of the influence of the operational and design parameters, such as ambient temperature, ambient humidity, solar radiation, air flow, product moisture content and product initial weight. This study will be restricted to forced air flow driers.

Two greenhouse drier types will be studied, the single chamber and the double chamber systems. The second one is a proposal [1] that improves the energy consumption incor- porating very simple changes.

* To whom correspondence should be addressed.

453

454 M. CONDOR[ and L. SARAVIA

A scheme of the simplest and better known greenhouse drier, the single chamber, is shown in Fig. 1. The whole greenhouse structure is used as a single drying chamber while a fan circulates outside air through the chamber removing the water vapor. The drier is initially loaded with fresh product and several days are necessary to obtain the required drying. The maximum evaporation rate is obtained during the first day. For the following days, the evaporation rate decreases as the product drying resistance increases.

The double chamber greenhouse drier incorporates a simple change in the inner dis- position of the drier. A scheme including successive working stages is shown in Fig. 2. The greenhouse is divided in two chambers, A and B, and the fan is placed between both of them, in a transparent separation wall. In the initial stage, Fig. 2a, the product is loaded in chamber A and the air flows from B to A. Chamber B works as a solar collector preheating the ambient air. When the drying process reaches half the cycle the second stage begins. Fresh product is loaded in chamber B and the air flow is reversed from A to B, as shown in Fig. 2b. The product in A is in an advanced drying stage and a small amount of water is incorporated in the air. Chamber A practically works as a preheating collector for chamber B where a higher evaporation rate is being produced. At the end of the second stage the product in A is dried, then it is unloaded and replaced by new fresh product. The air flow is reversed again and the third stage begins as shown in Fig. 2c. Now, chamber B contains the partially dried product and works as a preheating collector for the chamber A, that contains the humid product. At the end of the third stage the dried product is unloaded from B and a new cycle begins alternating stages (b) and (c).

The proposed operation strategy for the second system increases the use of the available energy. The single chamber drier uses only a small percentage of the incoming energy during the last drying stage since the amount of removed moisture is very low. The double

HUMID AIR

SOLAR RADIATION

PLASTIC COVER

TRAYS WITH PRODUCT

AMBIENT AIR

Fig. 1. Scheme of a forced air flow single chamber drier.

The performance of forced convection greenhouse driers

A B

SOLAR

A B A B

SEMI DRIED I v/////~///////////////////~ (2b)

455

SEMI DRIED

~/////.///////. ///////~/////A (20)

Fig. 2. Scheme of a forced air flow double chamber drier showing successives working stages a, b and c.

chamber drier makes better use of the energy since the chamber containing the half dried product also works as a preheating collector for the other chamber improving the evap- oration rate and allowing a larger initial product load for the same floor area.

In the following sections an analysis of both driers is performed and a performance parameter is defined to compare both systems. The definition considers the two potential sources of drying: the solar energy and the humidity content of the incoming air. The notion of the characteristic function for a drier is introduced playing a similar role to the Whillier's equation in the case of solar collectors [2].

The product drying curve is also analyzed. A change of scale in the time variable is proposed reducing the drying curves measured under different meteorological conditions to a single generalized drying curve.

Finally, the performance of both drier types are compared in a simulated test. The comparison is performed using the conditions of an experimental set-up for red pepper drying in a 50 m 2 drier.

2. A N A L Y T I C A L T H E R M A L M O D E L FOR T H E S I N G L E C H A M B E R DRIER

A simple analytical model for the thermal behavior of the drier and product is proposed, obtaining an expression for the evaporation rate as a function of the operational variables.

The main simplifying hypothesis is that the chamber has a uniform distribution of temperature and humidity. The fan produces a constant air flow rh, during all the process.

If the incoming ambient air has an absolute humidity W, and temperature T~, and the

456 M. CONDORf and L. SARAVIA

air in the drier has a temperature T and humidity W, the rate of evaporation rnw satisfies the mass conservation law :

m w = l~Vla(W-- Wa) (1)

The rate of evaporation is determined by the product mass transfer equation. The potential is Ws(Tp) - W where Tp is the product temperature, and Ws the corresponding saturation humidity. A single coefficient Hpd will be used, incorporating both the product evaporation surface and the diffusion constant. This coefficient will depend on the drying stage deter- mined by the product moisture content X, using the dried product weight as a reference.

rn w = n p a ( X ) ( W s ( T p ) - W) (2)

Equation (2) is similar to Dalton's equation for a free water surface [3], but uses the absolute humidity instead of the vapor partial pressure. The use of the absolute humidity simplifies the analytical study.

Two thermal balance equations are used, one for the chamber and the other for the product.

The equation for the chamber is :

( re ) ,AchI = Cpr / ' t a (T- T , ) + q f m , v + h c A ~ ( T - Ta) (3)

I is the solar radiation on a horizontal surface. The solar energy received by the greenhouse is referred to the chamber soil surface Ach. z is the transmittance of the cover and e is the mean absorption coefficient for the chamber. If the geometry of the greenhouse is compli- cated, a detailed evaluation of the incoming radiation is necessary and the results can be incorporated in the values of (zc0~. A simple expression for the enthalpy of the humid air is used, considering the sensible and change of phase heat terms, qf is the water phase change heat and C v the air specific heat. Ac is the greenhouse cover surface and h~ its thermal loss coefficient. The heat stored in the soil and the product are not considered. This storage is important in the case of greenhouses since it increases the temperature during the most critical period at night. But in the case of driers the effect is less important and the average heat balance is only slightly altered since most of the stored heat during the day is returned to the chamber during the first night hours.

The equation for the product is :

('t0~v)2ApiI = qfrhw + HpT(Tp - T) (4)

where Api is the product cross-section area for the incident radiation I and ep is the mean absorption coefficient for the product. HpT is the product of the thermal exchange surface by the thermal convection coefficient.

There are four unknown variables in these equations: T, W, Tp and row. They can be calculated as a function of the known variables using eqns (1)-(4). The main purpose is to obtain an expression for the evaporation rate row, which is a measure of the drier productivity.

The chamber humidity W is eliminated using eqns (1) and (2) :

W s ( T p ) - - Wa mw = 1 1 (5)

- - A t - . Hpd ma

The performance of forced convection greenhouse driers 457

The chamber temperature T is eliminated using eqns (3) and (4), and an expression for the product temperature Tp is obtained :

(Z%) 2Api I - qfmw (zoo 1A~hI-- qfmw rp = r . + + (6)

HpT Cprna + h~Ac

Expressions (5) and (6) allow the elimination of the product temperature Tp if an analytical expression for Ws(Tp) is known. Since the temperature interval for Tp in a drier is rather small, a linear approximation for this function can be used.

Ws(Tp) = Wo +bTp (7)

The approximation should be centered around the product temperature range. This approximation is similar to one used by Pennman [4] in his studies of the evaporation rates for lakes and plants.

The final expression for the drying rate is :

where

Ach mw : 7 [ K , I + Ws(Ta)- I'~a]

A 0

A~h A+h /A~D A~h ma Cpa/~/a + hcAoJ

F( %)=Api ] K 1 = b L HpT + CpalTla"~hcAcJ

(8)

Equation (8) gives the drier instantaneous production as a function of the operational variables. The equation can be written in terms of a non dimensional parameter as is shown in the next section. K0 depends on X through Hpd and HvT. K~ does not depend on X since the relation HpT/Api does not change with the product shrinkage.

The predicted values for temperature and humidity in the drier are obtained using eqns (1) and (3):

('[~) 1 AchI-- qfrhw T = T a -4- (9)

c~m. + h~A~

W = Wa + m,v (10) ma

3. THE CHARACTERISTIC FUNCTION

Equation (8) shows that the drying process is produced by two possible sources: the solar radiation L and the deficit in the incoming air saturation humidity Ws(Ta)- Wa. It is useful to define a performance factor that indicates how the drier is using this two drying sources.

The instantaneous drier production can be defined in terms of thermal power as :

458 M. CONDORS and L. SARAVIA

Pd = qfmw (11)

The two drying sources are expressed in terms of the total solar energy received :

Pi = A~hI (12)

and the air drying potential :

ew = qf#la(Ws(Ta)-- l~a) (13)

This equation measures the power associated to the water that could be incorporated into the air when it is saturated at ambient temperature.

Using the previous definitions, an instantaneous performance factor fp can be defined as :

Pa fp -- p w + p i (14)

It gives an idea of the drier efficiency in the use of its two drying potentials. Values offp will be in the interval [0, 1]. It is possible to introduce a non dimensional variable replacing operational parameters such as I, Ta and Wa in the expression for the performance factor. I f a non dimensional variable Z is defined as :

Pw W s ( T a ) - Wa Z - P w + P i -- A~h (15)

Ws ( r a ) -- I~a + I qfr//a

fp can be expressed as the product of two functions :

fp = U(JO" V(Z) (16)

where

qfKl u(x) - K o

V ( Z ) = 1 - - (1 Ach \ qf~aaK, ) Z

V(Z) mainly depends on the meteorological variables through Z and U(X) depends on the product moisture content X through the Hpd and Hpx coefficients. The function fp(X, Z) will be named as the characteristic function of the p r o d u c t ~ t r i e r combination.

Values of Z are in the interval [0, 1]. Other no dimensional numbers could be used with different combinations of Pw and Pi, for example Pw/Pi, but this one has the disadvantage of changing in the interval [0, ~ ] .

Z = 0 when there is no saturation deficit and the ambient air does not contribute to the drying potential, in this case V(Z) = 1. Z = 1 when there is no solar radiation, in this case V(Z) = Ach/qcrJlaK 1. As the product begins to dry X decreases and K0(X) increases. That is, both drier productivity and performance will decrease their values along the process.

If the characteristic function is determined experimentally at some site, it is possible to use this data to evaluate the drier behavior at other places. It should be remarked that the characteristic function depends both on the product and the drier parameters and it is not

The performance of forced convection greenhouse driers 459

possible to separate their effect. Thz is, the function is characteristic for a given drier and a given product.

The characteristic function can be shown as a graph were fp is a function of Z with X as a parameter. This function has a role for drier similar to the role of the Whillier's equation for solar collectors. The parametersfp and Z correspond to the efficiency r /and AT/I in the case of collectors.

4. T H E D R Y I N G C U R V E

The instantaneous evaporation rate, given by eqn (8), depends on the product moisture content X(t) and some further considerations are necessary to obtain the drying curve and therefore the total drying time.

By definition, the drying rate mw is proport ional to the derivative of the product moisture content X:

tn w = _ M d - -

where M~ is the dried weight of the product.

dX dt (17)

I f the value for row, given by eqn (8), is replaced in (17) a differential equation for X is obtained. Variables X and t can be separated allowing the integration of the equation.

Ach K0 dX = - ( K l I + Ws(Ta) - Wa)dt (18) i

where L Ta and W, are known variables depending on time t and K0 depends on X. X~ is the initial moisture content of the product. The integration determines the function X(t), that is, the product drying curve.

The drying curve given by eqn (18) shows an implicit dependence on the operational variables L T~ and Wa. A new variable t*, having dimension of time, can be defined :

t*(t) = ; (KlI(t)q- Ws(Ta(t))-- Wa(t))dt (19)

and the drying curve will be given by eqn (18) as a new function X(t*). Drying curves determined experimentally under different conditions could be reduced to

a unique function if they are referred to the variable t* instead of t, since the dependence of (18) on the meteorological variables is incorporated in the definition of t*. A reduced set of experiments could be performed to obtain X(t*) and the result could be extrapolated to other meteorological conditions. Variable t* will be named as the equivalent drying time and the function X(t*) as the generalized drying curve.

A similar concept to this one was proposed by Mulet et al. in an experimental work [5] for the case of open sun drying, where only the solar radiation was considered as drying source and the final expression was divided by a constant reference energy to obtain the time dimension.

The dependence of Hpd on X is needed to perform the integral in eqn (18) and obtain the drying curve for a given product. It can be obtained in the laboratory measuring the

460 M. CONDORf and L. SARAVIA

variables considered in eqn (2). Usually one or more linear relations are enough to give a good reproduction of the experiments.

4.1. Stage with constant drying rate During the first drying stage, under controlled internal conditions, the drying rate and

consequently Hpd can be constant. In this case K0 may be taken out of the integral in eqn (18) and X(t*) becomes the following linear function.

Ach X = X~ - - - t* (20)

MdKo

4.2. Stage with a decreasing drying rate For some usual products, the published results [6-8] show that one or two linear relation-

ship fit the function Hpd, that is :

Hpd(X) = a* + b*X (21)

I f more than one single linear relation is necessary the results can be easily adapted. Then, the dependence of K0 on X can be written as :

Aeh Ko = K* + a* +b*X (22)

where K* is a constant

K,=Ach +bqf(A~h Ach m~- \HpT "]- CpaJT/a -~- h~Ao]

Resolving (18), the following expression for drying curve is obtained:

==-.Ach (a*+b*X~ _ A~h t* (23) K*(X-- XO + b ~ l n \ a*+b*~] Md

X~ will be the initial moisture content of product if a single line is adopted for all process, or it will be the critic moisture content if a constant initial stage is considered.

These results agree with those obtained by Treybal [9] for conventional driers. In this case, the drying potentials are constant and the dependence with the real time t is direct.

Function t*(t) can be interpreted as a scale change in the time that depends on the meteorological variables. The implicit relation

t*(X) = t*(t) (24)

determines the drying curve X(t), which can be obtained point by point on a plot using typical values for the local meteorological variables. Particularly, the required final value for X determines the total drying time.

5. D O U B L E CHAMBER G R E E N H O U S E DRIER

In this section, the notion of characteristic function is applied to the double chamber drier. The chambers are identified with capital letters A and B using the notation of Section 2.

The performance of forced convection greenhouse driers 461

The operational conditions of chamber A are similar to the single chamber drier and equation (11) for the power related to the water evaporation can be used.

1 ( Ac~ ) PdA - - K0 A qrK1Pi + - - Pw (25)

ma

where the drying potentials have been introduced and K l is referred to A~h, the area for one chamber, while Ko is referred to the total drier area Ach.

The input air conditions in chamber B are defined by the chamber A exit conditions, that is, the temperature TA and absolute humidity WA. Equations (1) to (4) are resolved again and the following expression for evaporation rate in chamber B is obtained :

Ach[ bCprha (TA -- Ta) ] (26) mwB = KoB K , I + Ws(Ta)-- WA+ Cpma+hcAc

The different subscripts, A or B, indicate that some drier parameters take different values in both chambers. Then, using eqns (9) and (10) and definition (11), the power used in the water evaporation process for chamber B is :

Ach F(KoA 1 qfbCp#ta ~ Ach qfbCp#ta("C°Ol Pi] PdB = K0 B L ~ A c h /~/a (Cpl~a-[-hcAc)2/pdA-]- Ac~h (Col~a-~hcAc) 2 (27)

Equation (27) is a generalization of equation (8) for the case where the input air variables are not the same as the ambient air ones. In the case of the double chamber drier, the relation A'~h/Ach = 1/2.

The total power produced by both chambers is :

PdT = P d A + P d B = A*Pw+B*Pi (28)

Equation (28) is again a linear relation on Pi and Pw variables where

1 Ach[ AEh(KoA 1 qfbCp147 a "~ A*(J(A'XB)--#taKoA I+KoB ~ch FF/a (Cpma+hcA~)2J]

_ " 1 Ach qfbCp#ls(,co01 B* (XA, XB) qfK~ rna A * + A~h 2 K0. (Cp'~a +h~A~) 2

The characteristic functionfp may be calculated again using definition (14).

fpT = B* + (A* -- B*)Z (29)

where B* and A* are functions both of XA and XB. The equivalent time notion must be applied separately in each chamber, because the

drying curves are different. For chamber A the result is similar to that obtained in eqn (19). For chamber B the equivalent time is obtained from eqn (26) :

Cp#t~, + hcAc ( TA -- T~,) dt

This expression can also be used in the case of a drier with a solar preheater.

462 M. CONDORI and L. SARAVIA

6. COMPUTER SIMULATION

Some experimental tests in a double chamber drier prototype with a total area equal to 50 m 2 were performed [1] during the summer season for three months. These experiments provided the operational information and the values for the different parameters used in the computer simulation detailed in this section.

The greenhouse structure, formed by wood arcs, had a length of l0 m and was oriented in the easwwest direction. This structure was covered with LDT type plastic film, 150/~m thick. The south wall was insulated using polystyrene sheets, covered with a black plastic film. Two axial fans with 0.75 hp total power and driven by a power controller were used to circulate the air through the drier. In each chamber, the product was distributed in trays placed in two rows with several levels and separated by a central path. The useful tray area for each level was 12 m 2.

Drying of different regional products was carried on. The products tested were red pepper, oregano, beans, grapes and alfalfa. In general, good results were obtained showing the operational feasibility of the double chamber drier.

Also, some measurements of water evaporation were performed using 600 small black 1 plastic trays. An average of 235 kg of water was evaporated in 3~ days. The drier inside

temperature reached 50°C at noon with an outside temperature of 30°C and a solar radiation on horizontal surface of 1100 W/m 2.

A simple computer program in QBasic was prepared using the analytical expressions obtained for both driers and a simulation test of small prototypes was performed, comparing their performance. A greenhouse plastic structure with a 50 m 2 soil area was considered and the values of meteorological variables corresponding to the first week of November/1994 in Salta were used as an input. The main parameters for the product and the driers are given in Table 1. They correspond to red pepper cut in halves, one of the products usually dried in the north of Argentine.

Considering that the lower trays are shaded by the upper ones and the shrinkage of the

Table 1. List of parameters and their values as used in the simulation program

Parameters Single chamber Chamber A Chamber B Units

Ach 50 25 25 m 2 Ac 100 50 50 m 2 Api 35 26 26 m 2 Ad 71 64 64 m 2 Hr,-r 708 637 637 W/°C he 10 !0 10 W/m2?C Hpd 0.0264 X 0.0264 X 0.0264 X kg water/s.kg fresh rha 0.5 0.5 0.5 kg/s (rct) l 0.6 0.6 0.6 (z%)2 0.36 0.36 0.36 qf 2270 2270 2270 kJ/kg Cpa 1 1 1 kJ/°C kg b 0.002702 0.002702 0.002702 1/°C W0 - 0.0463 - 0.0463 - 0.0463 kg water/kg air Mi 200 56 180 kg

The performance of forced convection greenhouse driers 463

product, a constant average value for Api was adopted in each case. A similar consideration was performed for the heat diffusion surface ApT and a constant average value for Api/Hpv ration was adopted.

Since the product loaded in each chamber of the double chamber drier is smaller than the one used in the single drier, Ap,, HpT and Hpd values were proportionally modified.

The values for Ac and hc were adopted considering the drier global losses and the insulation on the south wall.

Bigger values for the air flow FY/a improve the drying rate, but the consumed electric power also increases and an intermediate value should be adopted. Values around 0.5 kg/s provide an adequate behavior without an excessive electrical consumption.

The (rc~)~ parameter determines the amount of radiation that is really absorbed in the chambers. Values between 0.5 and 0.6 are usual, depending on cover plastic conditions. The (~'~p)2 value indicates the percentage of this radiation directly absorbed by the product.

In both drier, the initial load of fresh product was selected to obtain a relative humidity no larger than 60% during the day, to prevent possible fungus growth. Values of 200 kg for the single chamber drier and 180 kg in the chamber B, with 56 kg of half dried product in chamber A, were used considering a load density of 6 kg/m 2 for fresh pepper cut in halves.

An initial moisture content X = 9.65 was adopted for the fresh product and the process was considered as finished when a value equal to X = 0.1 was obtained. The product was considered half dried when X = 2.3.

The obtained characteristic function fp(Z) for both driers is shown in Fig. 3 with the

(a) 0.27

0.24-

0.21-

8 o.18- e-.

E 0.15-

I =

13- ,- 0.12-

" t - D

0.09- 0") t -

b5 0.06-

0.03-

X = 9 . 4

~j~XX= 1.9

X = 0 . 1 6

0 0 0.05 0'.1 0. 5 o'.2 o. ,s o'.3 o. s 014 0. ,5 o'.s 0. 5 0.6 Z = Pw/(Pw + Pi)

Fig. 3a. Characteristic function for a single chamber drier, parametrized with the product moisture content.

464 M. CONDORf and L. SARAVIA (b)

c~

t -

O "1=

#.

a

E t -

O

O a

0.25

0.2"

0.15-

0.1-

0.05-

+ 0 . 6

0.14

5.6

2.8

+

1.9

8.9

~o.2 o.~s o'.3 o.35 o'.4 o.:~5 0.5 Z=Pw/(Pw+Pi)

I • chamberA • chamber B + total drier / 3

Fig. 3b. Daily points obtained with the characteristic function for a double chamber drier, par- ametrized with the product moisture content.

product moisture content X as a parameter. Mean values offp and Z at noon for successive days are shown using different symbols. Typical values for the single chamber drier, under 25%, are shown in Fig. 3a. They decrease with the product moisture content as expected. Values for the double chamber drier are shown in Fig. 3b. In this case the calculation for each chamber is performed using the drying potential of all the drier as a reference. Chamber A behavior, indicated by triangles, is similar to the single chamber drier while chamber B performance, indicated by squares, stays almost constant as the process advances. The total drier performance, indicated by plus symbols, is the sum of both behaviors and values between 10-23% are obtained. Along the process, for both driers, the noon points for the Z variable are in an interval between 0.16 and 0.48, showing that larger changes in Z should not be expected since in each region there is a correlation between the air drying potential Pw and the solar radiation availability L

The detailed dependence of the performance with the time is given in Fig. 4, where it has been assumed that during the night the fans are working. The highest values for the single chamber drier, Fig. 4a, are obtained during the first day and the performance factor decreases considerably for the rest of the process. The performance during the night is high but the absolute amount of evaporated water is small since the power related to the air drying potential is low. In the double chamber drier, Fig. 4b, the total performance does not decrease below 10% and it will have a second peak when new product is introduced in the drier in the middle of the process. Performance in chamber A is low since the outside air is being introduced and the product humidity content is quite low. Performance in chamber B is higher since the product humidity content is larger and it is receiving preheated air from chamber A.

The performance of forced convection greenhouse driers 465

(a) 0.24"

0.21

0.18-

0

~ o.15-

0

0.12- 13_

O.09-

e--

0 . 0 6

0.03

0 8 16 24 32 4'o ~6 s'6 r~ ~2 60 6'6 96

j-

1(34 112 time (drying hours)

Fig. 4a. The instantaneous performance for a single chamber drier.

(b) 0.24

0.21 g,} 0

~ o.18 E

,Vi ~" \ i ",, dj

0.15-

~" \;"~,..-".., ,,i .... 0.12- -~_ ~ ' i)

o ",j' , - - L_/

'~ 0.09- E

0.06

o00

01 , 0 6 1'6 14 ~0

t ime (drying hours) 1'2 36 42 48 54

I - - chamber A - - chamber B - - total drier I

Fig. 4b. The instantaneous performance for each chamber for a double chamber drier.

466 M. CONDORf and L. SARAVIA

The accummulated instantaneous production expressed as the amount of evaporated water is shown in Fig. 5. At the end of the process, the single chamber drier dried 200 kg of initial load in 110 h, obtaining 18.8 kg of dried product. The double chamber drier needed 53 h to treat 56 kg of half dried product placed in the chamber A, originated in 180 kg of initial load, and it will be necessary an equal time to finish the drying of the chamber B load, obtaining 33.8 kg of dried product in a cycle, that is, a production improvement of 87% since the used time has been reduced significantly for a larger initial load.

Figure 6 shows the evolution of temperature and humidity of chamber B as well as the ambient temperature. Temperatures in chamber B are always below those corresponding to chamber A, because of the evaporative cooling produced by the fresh product. During the day, the gap relative to the ambient temperature increases at the end of process, when the product in chamber B is half dried.

The drying curve as a function of the real and the equivalent time for the single and double chamber driers are presented in Figs 7 and 8, respectively. The equivalent time is multiplied by a scale factor. In all cases, the equivalent curve is smooth and follow the behavior of real drying. In the case of chamber B this is a curve similar to the one obtained with a constant Hpd value, showing it has received a bigger air drying potential since the product drying resistance has increased. At the end of the process, the final product moisture content is X = 2.3 for chamber B and X = 0.1 for chamber A, consequently the behavior for the second half process will be similar to the first.

330-

300-

270-

~ 240

"5 210- v v 180-

t -

._o "5 150- "0 120- D.

.$ gO- L- -

a 60-

30- f 0 8 1'6 2'4 3'2

./ /

/

/ f

J

S

'~0 z~8 56 .6~4 72 8'0 88 96 1()4 112 time (drying hours)

- - single drier - - double chamber

Fig. 5. The accumulated instantaneous production for both driers.

The performance of forced convection greenhouse driers 467

43-

40

37-

31-

28- Q.. E 25 (11 1-

22-

19-

16-

13 o 6

.....,, ...... ,,,. , ",,.

:J" ' / i . / i\A / ~,,,,.. ~..j

1'2 1'8 2'4 3'0 36 4'2 4'8 t ime (drying hours)

-0.9

"0.8

-0.7

-0.6

-0.5 -r

0.4 I;E

0 .3

-0.2

-0.1

0 54

- - Tem. amb. - - Tern. cham. B - - Hum. Cham. B j

Fig. 6. Temperature and humidity for the chamber B of the double chamber drier.

10-

9- (/)

.Q 8- nO

"~- 7 "

t - O

0.}

"5

"O .o D_

5- \

0! 0 8 1'6 2'4 3'a 4'0 4'a 5'6 6~ 7'2 8'o 8'8 9'6 164 112

time (drying hours)

realt ime - - equivalent x65 I

Fig. 7. Generalized drying curve for the single chamber drier.

468

..Q

~3 E

0

--1

0

"6

2

M. CONDORI and L. SARAVIA

10

5- ~ ~ _ . . . ~ h a m b e r B

3-

0 0 6 12 l'a 2'4 3'o 3'6 & 4'a $4 time (drying hours)

- - real time - - equivalent x60 t

Fig. 8. Generalized drying curve for the double chamber drier.

7. CONCLUSIONS

A general and simple model for the air forced drier, were both the product drying kinetic and the drier design parameters are taken into account, has been developed. Two drying potentials were considered as available energy sources: the air saturation deficit and the incident solar radiation.

Two new concepts are introduced : the generalized drying curve and the drier performance curve. The first one uses as a reference a time variable including both the received energy and the drier operative parameters allowing the correction of the solar energy variability effect in the drying curve. The second curve shows the production efficiency referred to the two drying potentials, where the product water content and the meteorological variables are introduced using two non dimensional variables. The use of both concepts allows :

(1) An easy extrapolation of the results obtained in different places. (2) A better comparison of the different driers performance using the same energy potentials

as a reference. (3) The comparison of the drying curves for different products, considering both the energy

received and the product load used in the drier. (4) An easier extrapolation of the results obtained in laboratory under controlled con-

ditions to real drying systems.

The simulation results show that a higher production rate can be obtained improving the use of the drying potentials. Particularly, the productivity of the double chamber greenhouse drier compared to the single chamber type is increased by 87% for the same drier area. The

The performance of forced convection greenhouse driers 469

necessary changes needed to implement the double chamber drier are simple and inexpensive reducing significantly the drying cost.

Acknowledgements--This work was supported by the CONICET (Consejo Nacional de Investigaciones Cientificas y T6cnicas.

REFERENCES

1. Saravia, L., Echaz6, R., Zunino, L. and Quiroga, M., Secadero Invernadero de Doble C~mara. Actas del V Congreso Ib6rico de Energias Renovables, Madrid, 1990.

2. Duffle, J. A. and Beckman, W. A., Solar Engineering of Thermal Processes, 2nd edn. John Wiley and Sons, New York, 1991.

3. Threlkeld, J. L., Thermal Environmental Engineering. Prentice-Hall International, Spain, 1973.

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