ELSEVIER

Ayrtncu/rttru/ Engineering 14 (1995 I 1 - 14 0 1994 Elsevier Science Limited

Printed in Great Britain. All rights reserved 0 I U-8609/95/$9.X) + 0.00

Impact of Friction, Flow Rate and Loading Density on Automated Soft-shell Crawfish Separation

Shulin Chen, Douglas G. Drennan II & Ronald F. Malone

Department of Civil Engineering, Louisiana State University, Baton Rouge. Louisiana 70803-6405. USA

(Received 8 May 1993; accepted 12 September 1993)

ABSTRACT

The separation of soft-shell crawfish from the hard-shell (or intermolt) population is the key step within soft-shell crawfish culture. Automated soft-shell crawfish separation in a culture tray is accomplished by utilizing hydraulic force and electrical inhibition gates. The impacts of water flow rate, surface roughness of the raceway tray, and crawfish loading density upon the operation of the automatic soft-shell crawfish separation process were evaluated. The flow rate which resulted in the desired hydraulic force for optimal per$ormance ranged from 075 to 090 liter/s. The magnitude of the frictional and drag forces acting on a crawfish were identified to be of the order of 0001-0-01 N. The hard-shell crawfish escape rate from the separation unit can be approximated as a first-order process with respect to the loading density.

INTRODUCTION

Production of soft-shell crustaceans, such as crabs and crawfish, has attracted the attention of the aquaculture industry in recent years. The current production technology is labor intensive. Labor cost generally account for 26-36% of the total operating costs (Culley & Duobinis- Gray, 1990). The hand separation of the soft-shell crawfish from the intermolts accounts for the majority of the labor costs. Thus, an automated separation process was developed for soft-shell crawfish production (Malone & Culley, 1988) as an effort to reduce production costs. The key component of the automated separation (production) unit is a culture tray consisting of a series of raceways to which a specified water velocity is provided. Two electric inhibition gates installed at the

2 Shulin Chen, Douglas G. Drennan II, Ronald F. Malone

end of the raceways serve to reduce the escape rate of hard-shell (inter- molt) crawfish from the tray before they molt (Chen et ai., in press). Soft- shell crawfish separation with such a unit is achieved by confining the intermolt crawfish within the culture (or separator) tray until the crawfish molt and become soft. Upon molting, the soft-shell crawfish lose their ability to stay in the culture tray because they can no longer resist the drag force created by the water flow. Consequently, the soft-shell craw- fish are flushed out of the separation tray into a collection box by the water current and thus separated from the intermolt population in the separation tray. Attempts by inter-molt crawfish to escape while in the tray are discouraged by an electrical potential across the discharge end of the raceways, i.e. electrical inhibition gates. Thus, the unit separates the soft-shell crawfish from the intermolt population. The basic principle of the separation tray operation is described elsewhere (Malone & Chen, 199 1; Robin, 1992; Rondelle, 1992). This paper focuses on the develop- ment of the design criteria for the process and discusses the implications.

Ideally, the separation unit should keep all the intermolts within the tray and release immediately only those which have molted. The hydraulic forces employed in the separation process can be very effec- tive in flushing the soft-shell crawfish out of the tray as long as a suffi- cient velocity is provided. Therefore, the major parameter which controls the separation efficiency is the intermolt escape rate. The lower the escape rate, the bet?er the system performs because most of the craw- fish coming out of the tray are soft instead of a hard-soft mixture. Thus, the major design task in the separation process is to control the hard- shell crawfish escape rate while effectively flushing out the soft and dead crawfish.

The probability of a crawfishs escaping from a separation tray is determined by three factors: (1) the force balance acting on the crawfish, (2) the social interaction among the crawfish themselves, and (3) the effectiveness of the electronic inhibition gates. The force balance on the crawfish is between the drag force, created by water velocity and the friction force between the crawfish and the inner surface of the tray. The social interaction is determined by the behavior of the crawfish after being put in the separation tray. The effectiveness of the inhibition gate is a function of its configuration and the applied voltage (Chen et al., in press). Therefore, the escape rate of the intermolt crawfish can be con- sidered a function of the roughness of the tray surface, flow rate (or water velocity), inhibition gate configuration, and crawfish loading density. The objectives of this study were: (1) to define the magnitude of the drag and frictional forces; (2) to examine the effects of loading density on the escape rate; (3) to evaluate the retention time of dead

Studies on automated sofr-shelf crawfish separation 3

crawfish as affected by flow rate; and (4) to develop a simple model to describe the separation process. The behavioral responses of the craw- fish to the inhibition gate and the engineering design of the gate is reported elsewhere (Chen et al., 1993).

MATERIALS AND METHODS

This study was conducted with red swamp crawfish (Procumbanu clarkii) over a 2-year period during 1990 and 1991. Crawfish were obtained from the Louisiana State University Agricultural Experiment Stations Ben Hur Research Farm. The crawfish used were harvestable size with an average carapace length of 39 mm. Crawfish were brought into the laboratory where they were placed in a recirculating holding system before being used in the experiments. Experiments conducted included the investigations on force balance, the impacts of water flow rate and crawfish loading density.

Force balance

The purpose of the force balance investigation was to evaluate magni- tudes of the drag and frictional forces acting on a crawfish in the separa- tion tray. The measurement of drag force was conducted using a vertically suspended, metal wire with a diameter of 1 .19 mm and a length of 89 cm. One end of the wire was mounted on a frame above the tray; the other end was L-shaped and attached to a crawfish immersed in the water current in the tray. The drag produced on the crawfish by the current deflected the suspended metal wire from the starting vertical position. The larger the drag force acting on the crawfish, the larger the deflection formed. After calibrating the wire with standard weights, the drag force acting on the crawfish was measured from the corresponding deflection of the metal wire.

During this measurement, care was taken to ensure that the crawfish were as close to the tray surface as possible without touching. Because the drag force varies with the cross-sectional area of the craw-fish, the drag force was measured in two directions: with the crawfish parallel to the current and with the crawfish perpendicular to the current. Ten replications were made for each flow rate.

Associated with the drag force measurement, flow measurements were also conducted. Volumetric flow rate was measured using a flow testing bucket within which two water level detectors were connected to a data acquisition unit. The data acquisition unit consisted of a Kaypro-

3 Shrrlin Chen, Douglas G. Drennan II, Ronald F. Malone

2x computer and an analog to digital converter (ADC-1, Remote Measurement Systems, Inc., Seattle, WA). The data acquisition unit recorded the time it took the water to reach the two level detectors and then calculated the flow rate knowing the water volume between the two levels. In addition to flow rate measurements, the data acquisition unit also monitored and recorded water temperature which was maintained at approximately 20C. Water current velocity measurements were con- ducted by using floating media (Malone et nl., in press).

The friction between the crawfish and plates with different surface roughness was also investigated. The plates tested included acrylic plates, fiberglass plates (the same material and surface treatment as the separation tray), plates coated with no.40 sand, and plates covered by 60 grit sand paper. The plates were positioned in an open-top box and were capable of being adjusted to form different angles between the plate surface and bottom of the box. A water depth of 20 mm was maintained in the box and water was also applied to the surface of the plates during testing. Live crawfish were tested in the box to determine the angles at which crawfish could no longer crawl up and the angles at which they began to slide down the various plates. The effective crawling friction coefficient, corresponding to the maximum angle at which a crawfish could crawl, and effective sliding friction coefficient, corresponding to the minimum angle at which a crawfish started sliding, were then deter- mined. After obtaining the crawfish weight and volume, the frictional force between a crawfish and different surfaces was calculated by assum- ing that the maximum sliding angle corresponds to the sliding friction coefficient and that the maximum crawling angle corresponds to the maximum crawling friction coefficient as defined by analogies to friction between two rigid bodies (Sears et al., 1982).

Flow rate and detention time

The detention time of a molted or dead crawfish is defined as the time period it can stay in the separation tray before being flushed out. The experimental system used to evaluate the effects of flow rate upon deten- tion time and the effects of loading density upon the escape rate consisted of a water sump tank, a biofilter, and a separation tray. The majority of the water pumped from the sump was directed to the separa- tion tray and recirculated back to the sump for reuse. Water quality in the system was maintained by diverting a small portion of the water through an upflow sand biofilter for biofiltration and solids removal (Malone & Burden, 1988).

The separation tray was made of fiberglass and measured 2.4 m X 1.2 m x 0.1 m with four internal raceways O-3 m in width (Fig. 1). Primary

Studies on alrtomated so/?-shell crawjkh separation 5

Position n1 I CraWbOUt Position #2 waca flow port

2.44 m inhibition gate

Fig. 1. Schematic diagram of the prototype experimental tray used for automatic soft- crawfish separation

12 V XC Power Supply

2.44 m 4 u

Fig. 2. The configuration of the electroshockin g gates and other related components

and secondary electronic inhibition gates were installed at one end of the tray. Base plates were installed in front of the primary inhibition gate to maintain the water level in the event of a power failure and to facilitate contact between the crawfish and the inhibition gate (Fig. 2). The addi- tion of the secondary inhibition gate and a crawl-out port made linkage

6 Shulin Chen, Douglas G. Drennan II, Ronald F. Malone

between trays possible by facilitating vertical distribution among stacked trays (Rondelle, 1992) and improved separation efficiency (Malone & Chen, 1991). Due to the limited scope of this study, experiments were conducted using only the primary inhibition gate (Figs 1 and 2).

During the experiments, the separation tray was loaded, with 800 craw- fish unless otherwise specified. Water was delivered to the head of the tray (Fig. 1). The water flowed through the raceway where crawfish were loaded and exited the tray through the outlet where the secondary inhibi- tion gate is normally located (Fig. 1). Soft and dead crawfish, and shed shells were then flushed out into the collection box where escaped inter- molt crawfish were also collected. In a commercial facility, soft crawfish are further separated from the dead crawfish and shells via the secon- dary separator, and the escaped inter-molt crawfish are recycled back into the system via a discrimination conveyor which removes the dead crawfish and shed shells before the primary conveyor recycles the craw- fish back into the separation trays (Robin, 1992).

Dead crawfish detention time was tested by placing dead crawfish in the fully loaded tray and recording the time required to flush them out. Twelve dead crawfish were placed in the tray at three different positions (Fig. 1) in groups of four. Position 1 was at the end of raceway no.l, position 2 was at the head of raceway no.3, and position 4 was at the end of raceway no.3 (Fig. 1). These positions were about 2.4 m apart from each other and 2.4 m apart from the water inlet and outlet. The crawfish were labeled prior to placement in their respective positions, and the time period it took for each dead crawfish to be flushed out of the system was recorded.

Intermolt escape rate as affected by water flow rate was investigated by examining the number of crawfish that escaped within a given period. These tests were conducted as 10-h trials. Data collection was conducted twice during each of the 10-h experiments. Dead and soft crawfish were replaced with crawfish from the acclimation system and crawfish were also fed at the time of data collection. The number of escaped intermolt crawfish in the collection box was converted to an average loss rate per hour which neglects the effects of population changes within the period between data collections.

Effects of loading density on the escape rate

Experiments were also conducted to determine the relationship between the intermolt crawfish escape rate and the loading density. Five loading densities ranging from 200 to 1100 crawfish in the tray were tested. Each experiment lasted three days during which a specific density of

Studies on automated soft-shell cran@h separation 7

acclimated immature crawfish were maintained in the separation tray. The intermolt crawfish loss rate was recorded every 12 hours, and the dead and visibly weak crawfish were replaced. The experimental results were used to develop a relationship between escape rate and loading density.

Two additional experiments were conducted to test the validity of such a relationship. In the first experiment, 1000 crawfish from the holding system were loaded in the separation tray 3 days prior to data collection. During this time, the crawfish escape rate was monitored to ensure stabilization of the population. The number of crawfish escaping from the separation tray was recorded, and the escaped crawfish were removed from the system. This resulted in a continuous decrease in the experimental population.

In the second experiment, 400 crawfish from the holding system were allowed to be acclimated in the separation tray for 3 days. The crawfish were removed from the separation tray and then immediately reloaded at the head of the tray at which time data collection began.

RESULTS AND DISCUSSION

Force balance

The sliding and crawling frictional coefficients of crawfish on surfaces with different roughness are given in Table 1. The data indicate that the frictional coefficients increased with the roughness of the surface. The smallest frictional coefficient corresponded to the acrylic plate while the largest value corresponded to the plate covered with no.40 sand. It is

TABLE 1 The Effective Sliding and Crawling Friction Coefficients of Crawfish on Surfaces with

Different Roughness

Sut$ace Sliding friction coefficient (STD)

Crurvlingfriction coeficient

(STD)

Acrylic Tray bottom. sanded No. 60 grit sand paper No. 40 sand

0.84 (0.18) 0.33 (0.02) 1.04 (0.15) 0.61jO.07) 1.88 (0.54) 1.27 (0.14j 3.25 (1.74) 1.36 (,0.331

STD. Standard deviation.

8 Shulin Chen, Douglas G. Drennan 11, Ronald F. Malone

evident from Table 1 that the crawling frictional coefficient is smaller than the sliding frictional coefficient. While the sliding frictional coeffi- cient increased from 0.84 to 3.25, the crawling coefficient increased from 0.33 to 1.36. The smaller crawling frictional coefficient is due to the limited ability of the propodus of the walking legs to grip the surface. When a crawfish walks, the friction is produced only between the propodus of the walking legs and the surface, whereas when a crawfish slides, friction exists between the crawfishs body (including legs) and the surface.

Frictional force between a crawfish and the surface of the tray is deter- mined by the frictional coefficient and the weight of the crawfish in water corrected for buoyancy. The measurement of two 20-crawfish groups with a size range of 70-90 mm in total length indicates that the specific gravity of these crawfish ranged from 1.04 to 1.09 and the average volume of these crawfish ranged from 13.9 to 14-O cm3. Therefore, the corresponding frictional forces calculated ranged from 0.006 to O-014 for the fiberglass plate based on Table 1. It is evident that the frictional force, especially the fractional force of walking crawfish, would change dramatically with the roughness of the surface.

Drag forces act to counter the frictional forces on the crawfish. The drag forces acting on the crawfish increased with the water flow rate as shown in Fig. 3. Figure 3 depicts the drag forces acting on a crawfish in

Water Current Velocity (m/s)

0.151 0.174 0.197 0.2: 0.06 :

I

5 0.04- u 5 0 Parallel to flow E 0.03 -

u ?? Perpendicular to flow

20

0.6 0.8 1.0 1.2

Water Flow Rate (I/s) Fig. 3. Drag force acting on crawfish versus flow rate in the tray at two different orienta-

tions

Studies on automated soft-shell cra@sh separation 9

two orientations: (1) parallel to the water flow; and (2) perpendicular to the water flow. At a given velocity, the drag force was directly related to the effective cross-sectional area of the crawfish. Therefore, a crawfish which was perpendicular to the flow was subject to a drag force 3-4 times greater than a crawfish oriented parallel to the flow.

The drag force results (Fig. 3) coincide with those obtained for the frictional forces acting on the crawfish. As mentioned earlier, the fric- tional forces acting in a direction parallel to the flow ranged from O-006 to O-0 13, while the drag forces ranged from O-0 10 to O-01 8 with a flow in the range of 0.17-O-69 liter/s. According to these results a uniform flow rate of less than l-04 liters/s (16-5 gpm) is sufficient to move (or flush) the crawfish with the water current.

It should be pointed out that in an actual operation, the crawfish rests on the tray surface, not just above it as in these experiments. Thus, the actual drag forces acting on the crawfish are smaller than the values suggested in Fig. 3 due to a decrease in flow velocity near the bottom of the tray surface. Additionally, it was observed that a walking crawfish was more easily pushed by the current, supporting the experiment results that the crawling frictional force is smaller than the sliding frictional force as illustrated in Table 1. This phenomenon was even more signifi- cant when crawfish were oriented perpendicular to the current. Another factor which might have contributed to this phenomenon is that a stand- ing walking crawfish was subjected to a stronger drag force than a craw- fish resting on the bottom.

Flow rate and detention time

The selected flow (0.88 liter/s), based on the force balance discussion above, was experimentally verified by the change in crawfish detention time and crawfish escape rate with respect to the change in water flow rate (Fig. 4). Figure 4 shows that although the crawfish escape rate increased, the detention time of the crawfish in the tray decreased with an increase in the flow rate. Flow rates greater than 0.75 liter/s are required to ensure a short detention time of a dead crawfish, while exces- sive flow rates ( > O-90 liter/s) result in an unacceptable escape rate of intermolt crawfish. Ideally, the flow rate should be selected so that it will provide enough drag to push the dead and soft-shell crawfish out of the system while not being great enough to flush an excessive number of intermolt crawfish out of the system. Therefore, the working flow rate should be in the range 0*75-O-90 liter/s, corresponding to a water current velocity of approximately O-1 7-O-l 9 m/s.

10 Shulin Chen, DougraS G. Drennan 11, Ronald F. Malone

Water Current Velocity (m/s)

0.151 0.162 0.174 0.289 0.197 0.208

6 0-l . I . I - I . I to 0.6 0.7 0.8 0.9 1.0 1.1

Water Flow Rate (l/s)

Fig. 4. Hard-shell (intermolt) crawfish escaping rate and dead crawfish detention time versus flow rate delivered to the separation raceway tray

Effects of loading density on the escape rate

The crawfish loading density in the separation tray is another important factor affecting the escape rate of the crawfish. Experimental results indicate that the escape rate of crawfish is linearly related to the number of crawfish in the tray as illustrated in Fig. 5. The linear regression of the data for Fig. 5 results in the equation below:

K= - 2.42 + 0.012 x N (1)

(?=0_93,SE= 1~43,SB=0*0019,n=5)

where:

K = crawfish escape rate, number of crawfish/h, N= number of crawfish in the separation tray, SE = standard error of the regression model, SB= standard error of the coefficient estimated by regression, n = number of observations.

In fact, if every crawfish was assumed to have the same probability of escaping within a certain period, the crawfish escape rate in a unit time must be proportional to the crawfish population in the tray. This is

Studies on automated soft-shelf crawfish separation 11

Number of Crawfish in the Tray

Fig. 5, Hard-shell (intermolt) crawfish escape rate from the separation raceway tray as a function of loading density

verified here by Fig. 5 and was also verified under different experimental conditions by Rondelle ( 1992).

lModel for escape rate prediction

Figure 5 suggests that the escape rate of the crawfish in the separation unit can be treated as a first-order process and can be expressed as:

dN/dt= - kN (21 where:

N= number of crawfish in the tray, t= time (h), k = rate constant ( 1 /h).

Integration of eqn (2) results in:

N= N, exp( - kt) (3)

where N[ = the initial number of crawfish in the tray. From eqn ( l), the value of the rate constant k is 0.0 12/h by neglecting

the effect of the constant. Substituting the k value obtained from the linear regression above into eqn (3). results in the following expression:

N= N, exp( - O-0 12 t). (4

12 Shulin Chen, Douglas G. Drennan 11, Ronald F. Malone

Equation (4) then can be used as a simple model to describe the inter- molt crawfish escape rate from the separation tray. This equation will serve a key role in modeling efforts to simulate a commercial scale operation where many separation trays are linked together (Rondelle, 1992).

A comparison of the crawfish population change between that predicted by the model and actual data are presented in Fig. 6. Predic- tion 1 represents the first experiment and Prediction 2 represents the second experiment. Similarly, the actual data obtained from the two experiments are labeled Experiment 1 and Experiment 2, respectively. Experiments 1 and 2 both exhibit smaller escape rates than that predicted. There are two possible explanations for this: (1) The actual rate constants (k) were lower than predicted by the model due to the acclimation of the crawfish to the separation tray prior to data collection. As reported elsewhere (Malone et al., in press), the crawfish escape rate was higher in the first 3-5 days after loading; it then decreased and stabi- lized at a lower rate. Because new (non-acclimated) crawfish were added after each data collection in the first experiment, the k value obtained (0*012/h) tended to be higher. (2) The crawfish used in these two experi- ments were mostly mature crawfish, and this might have contributed to the difference between the predicted and actual crawfish escape rate.

1100-

1ooo

900-

800-

700-

600-

sco-

400

300-

200-

loo-

. Q .

??

: . .

Q ExpcliIncnr1

?? prdictcd 1 Expclialcnt2 ?? Rediacd2

I

a 0 ??

. ?? 0

. .

. . . .

. . . . .

. . e e 0: 1 1 . 1 . 1 .

0 20 40 60 80 100

Time sine Experiment Started (hour)

Fig. 6. The comparison of model predictions and actual data for hard-shell (intermolt) crawfish escape rate (prediction is based on rate constant k = 0-O 12.)

Studies on automated sofr-shell crawfish reparation 13

CONCLUSION

(1) The magnitude of the frictional and drag forces acting on a crawfish in the raceway separation tray has been identified to be of the order of 0.00 l-0.0 1 N.

(2) The flow rate required to provide enough drag force to assure a short detention time without an excessive intermolt crawfish escape rate was in the range O-7 l-0.90 liter/s, corresponding to water current velocity of 0.17-o-19 m/s.

(3) The intermolt crawfish escape rate can be approximated as a first- order process whose constant rate was determined by the roughness of the tray surface and the water flow rate.

ACKNOWLEDGMENTS

This research was supported by the Louisiana Education Quality Support Fund under contract number LEQSH 1987- 1990)-RD-B-5. The authors thank Dr D. Culley in School of Forestry, Wildlife, and Fisheries at Louisiana State University for arranging a crawfish supply for the experiments. The authors also thank several students for their contributions to this research, including Babu Chitta, Hongzheng Lu, JoAnn Kurt, and Yongau Zhang. Additionally, the authors thank MS Pamela Rupert for her help in preparing this manuscript.

REFERENCES

Chen. S., Rusch, K. A. & Malone, R. F. (in press). Partial preliminary design of electrical inhibition gate for soft-shell crawfish separation. J. Appl. Aqun- culture.

Chen, S., Rusch, K. A. & Malone, R. F. (1993). Use of electrical stimulation in the automatic separation of soft-shell crawfish. Progressive Fish-Culrurist, 55(2), 114-20.

Culley, D. D. & Duobinis-Gray, L. (1990). Cuihire offhe Louisiana Soft Craw- fish - A Production Manual. Louisiana Sea Grant College Program, Louisiana State University, Baton Rouge, LA, 4 lpp.

Malone, R. F. & Burden, D. G. (1988). Design of recirculating soft crawfish shedding systems. Louisiana Sea Grant Publication, Louisiana State University, Baton Rouge, LA.

Malone, R. F. Rc Chen, S. (1991). Automated production of soft-crawfish. In Proc. Louisiana Aquacultural Conference, February 7-8, 1991. Baton Rouge, Louisiana. Louisiana State University Agricultural Center, pp.27-3 1.