IV. PERFORMANCE OF STOVES
In the present cooking operation there is bound to be minimum water carry-over along with the cooked cocoon, spillage and drainage losses depending on the procedures adopted which vary from unit to unit. As the process duration is hardly 2–3 minutes and cocoons have to be stirred continuously in water with a stick, in order to locate the thread ends, some evaporation loss is unavoidable. The heat loss due to spillage, carry over of hot water and evaporation loss is not actually useful heat but can be part of it depending on the type of process adopted. A water boiling test (WBT) usually establishes the thermal efficiency of a stove-pot combination, but such an entity as “thermal efficiency” cannot be defined for silk reeling units in a straightforward manner. Probably useful energy and minimum energy requirements can be worked out by a detailed energy and water balance.
In the silk reeling unit the useful heat includes evaporation from the cooking vessels, the water quantity exchanged from the cooking basins to reeling basins and also unavoidable drainage loss at the end of each batch to replace dirty hot water with fresh clean water, The unutilized part of heat consists of various heat streams representing loss of heat by different routes. These heat streams include:
Hence in order to understand the performance of the ovens, detailed energy balance experiments were undertaken for calculation of the above heat streams and water balance for the cooking process at few selected ovens in the field.
4.1 Water Balance Experiments
Since the cooking basin is the component of the silk reeling unit where the majority of heat input energy is given through fuel consumption, a water balance exercise was carried out considering cooking basins as ‘control volume’. All the energy and water balance is done for this control volume. The water balance chart for the cooking basin is shown in Figure 3.
In order to conduct the water balance studies on the ovens the following parameters were monitored:
Figure 3: Water balance chart for cooking basins
Total water consumption for the batch was monitored by weighing the known quantity of water before the start of the batch. The weight of fresh cocoons was recorded every time the person cooking was about to put it into the basin. Along with this the recycled cocoons coming from the reeling basins were also weighed. This was done for a batch in order to know the ratio of fresh cocoons to recycled cocoons entering the cooking basin. Cooked cocoons were again weighed, immediately after taking out from cooking basin, before taking them to reeling basin. This gives the quantity of water carryover from cooking basin to reeling basin. The pupae from the cooking basin were collected separately in order to know the free water going out with them. At the end of the batch the drained-off water was weighed as were the cocoons remaining for recycling in the next batch. The main difficulty encountered during the experimental work was to monitor or measure the spillage losses and evaporation losses as there was no definite means to measure them. Therefore evaporation loss was calculated by using formulas from the water temperature and surrounding air condition (temperature and humidity) and spillage was calculated by using the difference between these two.
A summary of the water balance experiments is given in Table 6. Detailed sample calculation procedures for water balance are given as Annexure A.
The findings of the water balance exercise can be summarized as follows.
Table 6 : Summary of water balance experiments on Cottage Basin Ovens
|No of pans||1||1||1||1||1||1||6||6||6||4|
|Cocoons processed (kg)||2.0||2.5||2.5||2.6||2.6||2.6||50||31||14.5||8.25|
|Fuel used||Wood chips||Tamarind husk||Paddy husk+coffee bean shell mixture||Tamarind wood||Acacia wood|
|Fuel consumed (kg)||3.99||4.00||3.50||5.25||4.98||5.01||71.5||57.5||21.5||17|
|Out/in water ratio for||—||—||—||—||—||—||2.16||3.93||1.27||1.27|
|Water temp ©||87.6||88.5||88.5||88.5||88.5||88.5||96||96||89||89|
|Water in (kg, 100%)||24.6||21.0||27.0||17.5||21.0||20.5||324||271||66.6||55.13|
|Water carryover (%)||—||—||—||—||—||—||33.3||45.0||27.6||19.0|
|Energy consumption (kcal/kg cocoon)|
|Q-use (kcal/kg cocoon)||1715||1058||1048||1374||1448||1508||861||137||879||1052|
|Q-in (kcal/kg cocoon)||9277||7200||6300||8142||7723||7769||6928||898||6480||9005|
4.2. Energy Balance Experiments
In order to conduct the energy balance, experiments following parameters were monitored per batch of silk production:
On the basis of various operating parameters recorded, heat balance calculations were carried out to estimate the heat losses in different heat streams. This also helps in assessing the magnitude of each type of loss and scope for its reduction. Sample energy balance calculations are given in Annexures B & C. The results of the energy balance are summarized in Figure 4, in the form of a Sankey diagram, showing various heat streams.
(i) CTSRI economic
(a) Charka ovens
(ii) CSTRI economic
(b) Cottage basin ovens
(i) Open (basket) type
(ii) Closed (barrel) type
(c) Stifling ovens
Figure 4 : Sankey Diagram showing various heat stream of stoves
From the experimental data analysis it can be seen that CSTRI charka ovens give better performance than the traditional oven as total losses are less and therefore efficiency by difference is higher (16.8–19.4%) against 14.7–16.6%). Unaccounted heat loss cannot be identified as useful heat alone. The improvement in the CSTRI charka can be attributed to less radiation losses (16–18% against 12–23%) from fuel feed port opening, lower flue gas loss, less heat loss due to lower thermal mass (3–4% against 3–7%), etc. Compared to the CSTRI charka, there is a large variation (scatter) in heat stream values in traditional charkas. This can be due to wide variations in traditional charka design and also operating procedures. The heat stream values for the CSTRI charka vary in a arrow band owing to standardization of design. It should be kept in mind here that the range of values given here is not for the same fuel type (tamarind husk and saw mill waste for traditional ovens and paddy husk + coffee bean mixture for CSTRI ovens). Therefore, comparison of performance just gives an insight into the magnitude of different heat streams for a given heat input.
Cottage basin ovens
From the results of the experiments, it can be seen that the flue gas losses are a little lower in the case of CSTRI ovens, due to effective recovery of heat from flue gases and lower burning rates. In the case of cottage basin ovens also, the losses from the fuel feeding port are less (6–7% against 12%) than in the case of the CSTRI ovens. But due to more recovery of heat (low burning rate, less excess air, more residence time), the heat accumulated by the CSTRI oven structure is greater. This also leads to higher surface temperatures resulting in higher surface (7.6–14.2% as against 9.3%) loss. In the case of cottage basin ovens due to frequent removal of charcoal for re-reeling operations, there is a substantial (30–41% portions of unaccounted loss.
It can be observed that there is a wide variation in the heat losses, which may be due to inherent variation in operational procedures. Also, here the heat streams are expressed in kcal per kg cocoon processed, which gives wide variation in energy consumption per kg of cocoon as the quantity of cocoon processed per batch varies widely between barrel and open stifling and also from batch to batch.
Flue gas loss forms the major fraction of heat loss (35–59%) as there is no heat recovery from the hot flue gas. Due to massive structure, the thermal mass loss is higher (4.8–7.2%) in barrel stifling as compared to open stifling (1.3–3.4%), but due to lower surface temperatures achieved surface loss is lower in the case of barrel stifling (5.1–9.1%) as compared to open stifling (6.1 – 11.7%).
For open stifling the overall specific energy consumption is 3–4 times higher than barrel stifling. This is due to the fact that the quantity of steam escaping from the basket is much higher than that in the case of barrel stifling which is magnified by latent heat of steam in terms of heat loss, though it forms part of useful heat in the process.
V. POTENTIAL FOR ENERGY SAVING IN COCOON REELING
The specific fuel consumption levels in the charka unit are higher for loose faster burning fuels (2.44 and 3.26 kg/kg cocoon respectively for groundnut shell and eucalyptus leaves as fuel) compared to about 1.5–2.0 kg wood/kg cocoon in the case of the cottage basin oven and therefore offers larger scope for improvement. Similarly, open (basket) stifling consumes 4–5 times more fuel compared to closed (barrel) stifling (1.0–1.2 kg as against 0.28–0.36 kg) in order to stifle 1 kg of cocoon. For basket stifling, generally charka ovens are used. The flue gas losses are higher (32–37%) in the case of charka ovens than that for the cottage basin ovens (24 –34%) offering larger scope for energy saving by way of reducing flue gas loss.
Based on the water balance exercise, it was found that the useful heat requirement to cook 1 kg of cocoon, under the present operating practice, is about 1300 kcal in the case of the charka oven and 875 kcal in the case of the cottage basin oven. The reason for the higher value of the charka oven can be attributed to the longer period of operation and larger cooking vessel area. The energy consumption can be brought down if cooking and reeling processes are carried out separately, perhaps on the same platform, in two vessels and utilizing flue gas to heat the reeling basin water. This will make the operation faster, as cooking and reeling can be carried out simultaneously, and the cooking pot dimensions can be made smaller to reduce evaporation loss.
From the present energy use pattern, it can be observed that, the majority of charka units use locally available loose biomass as fuel. Seasonal variation in type of fuel used is thus expected making it difficult to develop one common design for all charka ovens as the burning characteristics drastically change for each fuel type. Therefore, retro-fitting of ovens to reduce heat loss (such as flue gas, fuel port opening, evaporation) by controlled burning will be an appropriate way to achieve energy efficiency.
As they consume lot of fuelwood logs energy saving in the cottage basin ovens, can contribute significantly to reducing deforestation. Also, as the cottage basin reelers have comparatively good financial stability compared to charka reelers, they can go for retro-fitting or even for newer designs of stove if they are economically attractive. Retrofitting of ovens by way of controlled burning rate, maximum flue gas heat recovery, reducing other losses can result in marginal energy saving (about 25%) and therefore are less likely to attract reelers. Hence, there is a need to develop alternate designs suitable for meeting the energy demand of silk reeling units with substantial fuel saving so as to make them economically viable.
5.1. Integrated Biomass Gasifier System for Silk Reeling Unit
Considering the unavoidable evaporation losses and necessity to have controlled burning, the gasifier system appears to be promising alternative option for meeting the energy requirement of a silk reeling unit, because of the following reasons:
Biomass gasification is the process of incomplete combustion, achieved by supplying an insufficient quantity of air (about 30% of stoichiometric requirement), which converts the solid biomass into producer gas consisting of CO,H2 and other hydrocarbons. This producer gas having calorific value of 1100 – 1200 kcal/NM3 gives the advantage of easy handling and more precise control. The producer gas thus obtained can be used either for cooking or stifling operations. This is expected to reduce the fuel consumption by 50–60%.
Further, the available heat in the flue gas, which amounts to about 40% of heat input can be utilized for drying of pupae or even for stifling purpose. The only additional investment for this will be modifying the flue gas path so as to divert gases over the trays containing pupae to be dried or cocoon to be stifled. This will help in drying of pupae in the silk reeling unit itself. This will help the reeler to gain more bargaining power to decide the selling price of pupae (gaining additional income from it). This allows pupae to be collected only once a week and also reduces the weight of the pupae to be transported. The integrated gasifier system for silk reeling unit is shown in Figure 5.
At TERI's Gual Pahari campus, a prototype unit for testing this concept is being developed (Photographs 25–28). Preliminary trial runs are very encouraging as for maintaining the same power of the stove (for given oven dimension and water quantity), the fuel consumption observed is less than half as compared to the traditional wood combustion based ovens.
From the extensive survey and energy balance exercise of charka and cottage basin ovens carried out during the course of this study the following facts emerge:
Figure 5: Integrated gasifier system for silk reeling unit
Development of gasifier based silk reeling unit at TERI
Integrated gasifier based proto-type cottage basin reeling unit
Close-up of cottage basin oven showing producer gas pipe network
Producer gas burner in operation (one below each cooking vessel)
Proto-type drying unit for pupae drying or cocoon stifling utilizing exhaust gas H2O
Work described in this case study was carried out under the project sponsored by the Swiss Development Cooperation, New Delhi. The authors are grateful to Dr. Urs Heierli and Dr. Veena Joshi of SDC for their sustained encouragement and guidance throughout the course of this study. The authors also wish to thankful to Mr. P. Jaboyedoff, SDC consultant, for fruitful discussions and valuable suggestion during the course of the study. The support provided by CSTRI personnel in carrying out the field survey and experiments was crucial for the succesful and fruitful completion of the study. Special thanks are due to Dr. T.H. Somsekhar, Director, CSTRI for his support and encouragement throughout the course of this study. The support and encouragement of Dr. R.K. Pachauri, Director, TERI, and the help provided by Mr. H.V. Dayal, Dean, TERI Bangalore office, is gratefully acknowledged. Thanks are due to Mr. R.B. Guptha for his help in conducting the survey.
Annexure - A
Calculation of flue gas flow rate through chimney
In the thermal analysis of any chulha, the thermal losses through the chimney in the form of hot flue gases in the major fraction. During the operation of chulha/oven, a lot of excess air is supplied to the fuel (i.e. more than the stoichiometric air requirement for complete combustion of the fuel). This is to make up for the draft created by the chimney. The quantity of air flow is quite low and therefore, difficult to measure with conventional techniques of flow measurement, like pilot tube or anemometer. The best and simplest way of estimating the flue gas flow rate in the field is to measure the volume fraction of easily measurable flue gas components such as CO2 or O2 along with flue gas temperature and then estimate the excess air and hence the flue gas flow rate. During the course of study a simple software was developed using lotus 1–2–3 spreadsheet to estimate the flue gas quantity for the burning of fuel from its composition, and by knowing the flue gas temperature and volume function of either CO2 or O2 The basic calculation procedure used is described below through sample calculation for tamarind wood fuel by measuring the O2 fraction in the flue gas using a fyrite kit.
Composition of the fuel used, tamarind wood, on weight basis is
Moisture = 7.23%; Ash = 8.43%
Composition of the majority of the biomass fuel on a moisture and ash free weight basis is more or less consistent as C = 50%, H = 12% and O = 38%, therefore the actual weight of constituents (C,H,O) in the fuel works out to be:
C = 0.4217 kg = 0.0351 kmol
H = 0.1012 kg = 0.0506 kmol
O = 0.3205 kg = 0.0100 kmol
Theoretical oxygen requirement
From the basic combustion reaction equations for complete combustion of 1 kg of fuel, the theoretical oxygen required will be:
O2 = (C × 1) + (H × 0.5) - (0) = 0.0504 kmol
Knowing the fact that air consists of 21% O2 (oxygen) and 79% N2 (nitrogen) by volume, the theoretical air required for complete combustion of 1 kg of fuel works out to
Air required = 0.2401 kmol
Actual combustion reaction
In actual practice, the amount of air supplied during the combustion of fuel is greater than the theoretical air requirement of the fuel. This excess air is responsible for high flue gas losses through the chimney. If ‘X’ is the excess air factor, the chemical combustion reaction becomes,
Reactants = Products
Reactants = 0.0351 C + 0.0506 H2 + 0.0100 O2 + (1+X) 0.0504 (O2+3.762 N2) + (0.0723/18) H2O
Product = 0.0351 CO2 + 0.0546 H2O + 0.1897 (1+X) N2 + 0.0504 × O2
Balancing the various constituents C,H,O,N on both the sides, one gets a set of equations in the form of X. Thus, the value of X can be calculated if one knows any one fraction of gas in the product of combustion. As CO2 and O2 can be absorbed quickly, its volume fraction in the flue gas can be measured easily using the fyrite kit. Here the case is considered where O2 volume fraction in the flue gas is measured using a J N Marshal Fyrite Kit. It gives the volume fraction in the moisture free gas.
O2 volume fraction in the moisture flue gas = 10.0% (measured)
Thus through oxygen balance in the combustion reaction equation one gets.
Solving for X, one gets
X = 0.8665 = 86,65 %
Thus quantity of flue gas under given operating conditions can be determined by substituting the value of X in the products.
Flue gas = 0.4329 kmol on moisture free basis
= 0.4875 kmol including moisture
Knowing the molecular weight of different flue gas components (CO2 = 44, O2=32, H2O= 18, N2 = 28), the quantity of flue gas can be calculated as,
Flue gas = 13.8421 kg (including moisture) per kg of fuel
Therefore, flue gas flow rate for burning rate of 14.3 kg/hr works out to be
= 197.97 kg/hr
Annexure - B
Sample calculation for energy balance of cooking oven
In this annexure the procedure for carrying out an energy balance of a cooking oven, used in silk reeling units, is described in detail. The energy balance of stifling units and charka ovens can be carried out using similar procedures. The cooking oven performance was monitored for a batch operation.
|Type of oven||: Traditional cottage basin|
|No. of cooking basins||: 6|
|Type||: Tamarind wood|
|Calorific value||: 4845 kcal/kg|
|Ash content||: 8.43%|
|Moisture content||: 7.23%|
|Duration of batch||: 5 hrs|
|Oxygen (O2 %) in flue gas||: 10%|
|Flue gas temperature||: 390°C|
|Fuel consumption||: 71.5 kg|
|Cocoons processed||: 50 kg|
|Silk yarn produced||: 4.64 kg|
|Ambient condition, DBT||: 35.2°C|
|Feed water temperature||: 27°C|
|Cooking basin water temp.||: 96°C|
Complete oven details (geometric dimensions, construction material, etc.) required for the calculation of areas, weight of oven, heat transfer coefficient were measured and noted down. The dimensions used here in sample calculations are
|Width of fuel port opening||: 0.42 m|
|Length/depth of fuel bed||: 1.60 m|
|Height of fuel port opening||: 0.79 m|
Based on the above parameter, the calculation of various heat streams is described below
Heat input (Qin)
The total heat input through combustion of fuel in the oven can be calculated from the fuel consumption and its calorific value.
Qin = 71.5 (kg/batch) × 4845 (kcal/kg) = 346417.50 kcal/batch
Theoretically, the net heat input will be less, as the heat content of the balance char should be deducted. But since the char obtained is utilized from time to time for re-reeling operation as well as for igniting fuel in a stifling oven, it was difficult to quantify the exact amount of char obtained in a batch. Therefore this loss is put in the unaccounted heat loss.
Heat loss through flue gas (Qf)
From the calculation procedure described in the earlier Annexure-A the flue gas flow rate works out to be
|Flue gas flow rate (Mg)||= 197.94 kg/batch|
|Hence, Qf||= Mg Cpg(Tg-Tamb)|
|= 91297.85 kcal/batch|
|Mg||= flue gas flow rate (kg/hr)|
|Cpg||= sp. Heat of gas = 0.26 kcal/kg °C|
|Tg||= flue gas temperature (°C)|
|Tamb||= ambient dry bulb temperature (°C)|
Surface heat loss (Qs)
The heat loss from the oven surface takes place through two types of heat transfer mode,
Here the sample procedure for calculation of surface heat loss is given for one surface (right hand vertical side). Similarly, the heat loss from other oven surfaces can be worked out to arrive at total surface heat loss.
a) Convection heat loss (Qs,c)
|Wall surface temperature (Ts)||= 47°C|
|Surface area (As)||= 1.449 m2|
|Wall height (H w)||= 0.84 m = 2.7599 ft|
|Temperature gradient (T)||= Ts -Tamb = 11.8°C=21.24°F|
For air as fluid, the natural convection heat transfer coefficient (Hc) can be calculated using following equations
|Hc||= convective heat transfer coefficient = BTU/hrft 2°F|
|C & m||= constants|
|D||= geometric dimension (ft)|
The values of C, m and D for various oven surfaces are as follows (Ref. Process Heat Transfer by Kern): Thus for the right vertical side surface the heat transfer coefficient can be calculated as,
Hc = 0.46653 Btu/hr ft2°F = 2.2766 kcal/hr m2°C
Qs,c = hc A T
= 38.9172 kcal/hr = 194.58 kcal/batch
b) Radiation surface heat loss (Qs,r)
Heat loss from the surface through radiation mode (Qs,r), by virtue of temperature difference between wall surface (Ts) and surrounding (T amb), can be calculated as
Qs,r =ó A s (Ts - Tamb)
|Ó||= Stefan Boltzman constant =4.88× 10-8 kcal/hr m2 K4|
|= Emissivity of wall surface|
|Ts' Tamb||= Surface and ambient temperature in °K|
Qs,r = 67.5 kcal/hr = 337.5 kcal/batch
Therefore, total surface loss from the right hand side vertical surface becomes,
Qs = Qs,c + Qs,r = 532.12 kcal/batch
In a similar manner the surface heat loss from other oven surfaces can be calculated. Thus total surface heat loss becomes
Qs = 32398.60 kcal/batch
Heat loss from oven opening (Qo)
In order to estimate the amount of heat loss from red hot fuel bed on the grate, through oven fuel port opening, it is necessary to calculate the view factor (also called as radiation shape factor) between the fuel bed (grate) area and fuel port opening of the oven. This can be determined using the following two ratios.
|Ratio 1 = depth/width||= 1.6/0.42||= 3.80|
|Ratio 2 = height/width||= 0.79/0.42||= 1.88|
Then, referring to the graph given in figure 4.9 of ‘Process Heat Transfer’ book by Kern and Krause, for the above two ratios, the view factor (VF) works out to be 0.085. With fuel bed temperature (Tbb) of 1300 °C (1573 °K), the radiation heat loss through oven opening becomes
Qo = ó × A × VF × (Tb - Tamb)
Qo = 8402.9 kcal/hr = 42014.66 kcal/batch
Heat loss due to thermal mess of oven (Qm)
During the operation of the oven throughout the day, the oven gets heated up by absorbing the heat due to the thermal capacity property (specific heat) of its various structural components. This accumulated heat energy is liberated back to the surrounding atmosphere when the oven is not operational. Hence it is assumed, and this normally happens in many chulhas, that the oven temperature returns to the original temperature by liberating all the accumulated heat at the time of start by calculating weight of each component of the oven, and by knowing its specific heat. Here a sample calculation is done for the right side vertical wall of the oven.
|Mass of the wall||= volume × bulk density|
|= (Area × thickness) + bulk density|
|= (1.449 × 0.25) × 2000 = 724.5 kg|
|Qm,rhs||= Mwall × Cp, wall × T|
|= 724.5 × 0.596 × (47-35.2)|
|= 5095.26 kcal|
This heat is lost throughout the day. Since the oven under consideration operates for 10 hrs/day, and the batch under study is of 5 hrs duration, the contribution of heat loss due to thermal mass for the batch under study can be calculated as,
Qm,rhs = 2547.63 kcal/batch
Similarly, the heat loss due to thermal mass of other oven components can be calculated. The total heat loss due to thermal mass of the oven under consideration amounts to
Qm = 9216.97 kcal/batch
Useful heat (Qu)
The useful heat, for cooking/stifling the cocoon, is calculated by a detailed water (and heat balance) exercise as described in Annexure -C. Thus the useful heat for the cooking process works out to be
Qu = 864.3 kcal/kg cocoon = 43065 kcal/batch
Heat recovered in water preheater drum (Qdr)
The amount of heat recovered from the gases for heating the water in the drum is calculated separately as follows.
|Average water temperature in drum||= 49°C|
|Sensible heat of water (Qsensible)||= Quantity of water used/batch × Cp × T|
|= 140 × 1 × (49-27) = 3080 kcal/batch|
Rate of evaporation of water from drum surface (gm/hr m2) can be as
M = constant × 45.8 × (vapor press differences at water and surrounding air temperature in millibar)
|Constant||= 0.81 for faster relative movement of water|
|= 0.55 for still water|
|Saturation vapor pressure at 49°C||= 153 millibar|
|Saturation vaporpressure at DBT||= 52.153 millibar|
|Vapor pressure of air||= Relative humidity × Psat|
|= 0.65 × 52.153 = 33.9 millibar|
Relative humidity of air can be calculated from the measured values of DBT and WBT measured values using psychometric charts.
|Drum area||= (p/4) D2 = (3.14/4) × (0.572) = 0.255 m2|
|Rate of evaporation||= 0.81 × 45.8 × (153-33.9) gm/hr m2|
|= 3 kg/hr m2|
|= 0.765 kg/hr|
|Qevap||= evaporation rate × (latent heat + D T)|
|= 0.765 × 540 + (49-27)|
|= 429.93 kcal/hr = 2149.65 kcal/batch|
|Qdrum||= Qsensible + Qevap|
|= 5229.65 kcal/batch|
Various heat streams calculated are summarized in table B-1.
Table B-1: Summary of energy balance calculation
|Heat stream||Quantity (kcal/batch)||Percentage|
|Heat input (Qι)||346417.50||100.00|
|Flue gas loss (Qφ||91297.85||26.35|
|Surface loss (Qσ)||32398.60||9.35|
|Fuel port opening loss (Qo)||42068.74||12.14|
|Thermal mass loss (Qμ)||9234.14||2.67|
|Useful heat (Qυ)||43065.00||12.43|
|Drum heating (Qδρ)||5229.65||1.51|
|Total heat loss (Qτoτ)||223293.98||64.46|
|Unaccounted (Qυα) (including ash + char)||123123.52||35.54|
Annexure - C
Sample calculation for water balance of cooking basin
As described in earlier section 4.1, to estimate useful heat of a cooking oven, it is essential to estimate the various water streams. This is not an easy task as water carry-over takes place from cooking to reeling basins and vice versa. Therefore, in order to arrive at a net quantity of water going out from cooking basin to reeling basin, the weight of cocoons before putting into the cooking basin and after cooking (before taking it to reeling basin) was monitored continuously for one batch. For the traditional cottage basin under consideration (for sample calculation earlier and in this annexure), the ratio of water taken out from cooking basin to that going into it worked out to be 2.16.
In order to monitor water flow, the main water supply was stopped and reelers were forced to take water from the tank with graduations for monitoring water consumption. Reelers were asked to give the cocoons for weighing before and after cooking. Also the pupae waste, water drained, jute waste produced, pupae recycled, etc. were measured during the batch. The summary of the data collected during monitoring is given below for one batch.
|Cocoons processed||= 50 kg|
|Water consumption||= 324 kg|
|Water out to in ratio for basins||= 2.16 kg/kg cocoon|
|Duration of batch||= 5 hr|
|Cooking basin water temperature||= 96°C|
|Feed water temperature||= 27°C|
|Pupae waste||= 13.25 kg|
|Water drained||= 64.18 kg|
|Water spillage (by diff)||= 97.72 kg|
|Jute waste produced||=7.7 kg|
Water evaporated from the open surface of the cooking basins is calculated as follows:
|Rate of evaporation = Constant × 45.8 × DP gm/hr m2|
|Constant||= 0.81 for faster relative movement|
|= 0.55 for still water|
|D P||= vapor pressure difference at water surface and surrounding air in millibar|
|Vapor pressure at water surface||= Psat at water temp. (96°C)|
|= 883.75 millibar|
|Saturated vapor pressure at DBT||=56.16 millibar|
The relative humidity for prevailing air temperatures (DBT = 27.5°C, WBT = 30.3°C) works out to be 65%
|Vapor pressure at air temperature||= 0.65 × 56.16|
|= 36.50 millibar|
|Rate of evaporation||= 0.81 × 45.8 × (883.75 - 36.5)|
|= 31431 gm/hr m2 = 31.43 kg/hr m2|
There are 6 cooking vessels of 235 mm dia each. Thus the total area of all cooking basins works out to be equal to 0.26 m2.
|Rate of evaporation||= 8,17 kg/hr = 40.85 kg/batch|
Various water streams coming and going into the control volume (cooking basins) are shown in Figure C-1.
Figure C-1: Water balance of cooking basins
Assuming that fresh cocoons are at 27°C (feed water temperature) and specific heat of pupae waste, cooked cocoon same as that of water (considering the fact that moisture content is the major fraction in it) the heat balance across the control volume can be carried out as follows to arrive at the value of useful heat.
|Qin||= Qcocoon + Qwater in|
|= (50 + 324) × 27 = 10098 kcal|
|Qout||= Qcarryover + Qdrain + Qpupae + Qspillage + Qevap|
|= (108 + 64.18 + 13.25 + 97.72) × 96 + 40.85 × (540 + 96)|
|= (27182.4 + 25980.6 = 53163 kcal/batch|
|Quse||= Qout - Qin|
|= 43065 kcal/batch = 861.3 kcal/kg cocoon □|