Chapter 1
CASE STUDIES

Case Study - India

STUDY OF STOVES USED IN SILK REELING INDUSTRY

Sanjay Mande, B.R. Pai and V.V.N. Kishore
Tata Energy Research Institute
Habitat Place, Lodi Road
New Delhi - 110 003 (INDIA)

1. INTRODUCTION

Silk holds a unique place in the textile world and is regarded as the “Queen of Textiles”. India is a traditional sericultural country and ranks only next to China in silk production. Mulberry silk production accounts for the major (about 90%) share which is mainly concentrated in the states of Andhra Pradesh, Karnataka, and Tamil Nadu. While, the states of Assam, Jammu & Kashmir, Uttar Pradesh and West Bengal produce nonmulberry silks like Tasar, Muga and Eri. India produced around 14,500 metric tons of natural silk during the year 1993–94. The majority of the silk is reeled either in charka or in cottage basin ovens; both can be called small scale cottage industries. At present there are about 35,155 charka ovens and 25,937 cottage basin ovens installed in different states. Various fuels used are mainly firewood for cottage basin and local available loose biomass (such as groundnut shell, tamarind husk, rice husk, coffee beans, etc) for charka units. It is estimated that about 105,000 tons of loose biomass and 120,000 tons of fuelwood are being consumed every year for the production of silk yarn. The distribution of silk reeling units in different states is given in Table 1.

Table 1: Distribution of silk reeling units in India

(Source: Biennial Statistics Journal of the Central Silk Board, India - 1992)

1.1. Process description

The worm rearer feeds the silkworms (pupae) with mulberry leaves and they grow rapidly. After the fifth instar (stage of worm growth), the worms are mounted on bamboo mounts to allow them to spin the cocoons. After the spinning is complete, the cocoons are sold to the reelers (Photograph-1)

The reelers buy the cocoons from the government regulated cocoon markets and produce raw silk yarn. The reeling industry is predominantly a cottage based one. There are various steps involved in the production of raw silk yarn from cocoons.

1. stifling
2. cooking
3. reeling
4. re-reeling
5. skeining
6. bookmaking and bundling

Flow Chart of Silk Reeling Industry

Thermal energy is needed for the first four steps; the first two steps are more energy intensive and account for the major share of total energy required. Stifling is a process for killing of the pupa and drying the cocoons for storage. There are two ways of stifling viz. with the help of steam (either in a basket or in a barrel)(Photographs-2 & 3) or with the help of hot air (Photograph-4). The stifled cocoons are stored till they are subjected to cooking. The processing of stifled cocoons, consisting of cooking, reeling, and re-reeling, is the main activity of the silk reeling units. Prior to reeling of silk from cocoons, the cocoons are cooked to unwind the continuous silk filament. Cooking is the process of locating the end of the silk baves by subjecting the cocoons to boiling water. The sericin and part of the gum is dissolved in this operation. There are two types of ovens which are used for cooking viz, the charka oven (Photograph - 5) and the cottage basin oven (Photograph - 6). The process of cooking consists of keeping the cocoons in boiling water contained in aluminium or copper cooking vessels for about 1–2 minutes, with continuous, vigorous stirring with wooden rods. This process allows release of gum from the cocoons and makes them amenable for continuous unwinding of silk filament in reeling basins.

Photograph - 1	Photograph - 2
Raw cocoons received at reeling unit	Barrel (closed) steam stifling
Photograph - 3	Photograph - 4
Basket (open) steam stifling	Hot air stifling in ‘Ushnakoti’
Photograph - 5	Photograph - 6
Cocoon cooking in charka ovens	Cocoon cooking in cottage basin ovens
Photograph - 7	Photograph - 8
Reeling in a cottage basin oven unit	Re-reeling in a cottage basin oven unit

The cooked cocoons are then subjected to reeling wherein, the located ends are reeled onto the reels (Photograph - 7). Three types of reeling units exist, viz, charka reeling units, cottage basin/domestic basin reeling units and multi-end reeling units. In re-reeling, the already reeled raw silk is reeled on to the standard sized reels (Photograph - 8). The raw silk is then skeined and bundled.

II. STOVE DESIGNS

2.1. Stifling

Cocoons can be stifled by several methods such as sun drying, steam stifling, hot air drying. The steam stifled cocoons can be stored for short duration (1–2 weeks) whereas hot air stifled cocoons can be safely stored for several months. Therefore steam stifling is preferred for multi-voltine cocoons whereas hot air drying is recommended for bi-voltine cocoons. The duration of stifling depends upon the number of days the cocoons are to be stored and the quality of the cocoons. In cottage industries stifling is done in batches in one of the following ways:

Photograph - 9	Photograph - 10
Stove used for barrel stifling	Ushnakoti stove for hot air stifling
Photograph - 11	Photograph - 12
Idle traditional charka oven unit	Traditional charka unit in operation

Barrel stifling

In this method, commonly used in cottage basin reeling units, a metal barrel is embedded into the masonry of the oven. A platform is provided inside the barrel to keep the basket of cocoons. The barrel is also provided with a close fitting lid to prevent the steam from escaping thereby reducing evaporation loss. Water is poured into the barrel to about two thirds height of the platform and boiled over a fire in the oven below (Photograph - 9). When the water starts steaming, a bamboo basket loaded with about 10 to 12 kg of cocoons is placed on the platform in the barrel and the lid is securely closed (Photograph - 2). The average duration of one stifling batch operation is 10 to 15 minutes.

Basket stifling

In this method, commonly used in charka reeling units, about 7 to 10 kg of cocoons are loaded into a bamboo basket in which the sides are closely woven but the bottom is loosely woven in order to allow the steam to pass through the cocoons easily. A thick gunny bag is tightly stretched over the top, leaving the sides and the bottom free (Photograph - 3). The basket is placed over the mouth of a vessel in which water is boiled. Sometimes, the cooking/reeling basin itself is used for this purpose, as in the case of charkas. The hot steam fills the basket and stifles the pupae in the cocoons. The average duration of one operation is 30 minutes to 1 hour. A lot of energy is wasted in this method in the form of latent heat of steam escaping through the basket.

Ushnakoti: hot air stifling

Recently, CSTRI has developed hot air stifling chamber, called an Ushnakoti. It is a room with wiremesh trays on which raw cocoons are spread. Fuel is burned in one corner of the room and hot flue gas obtained is circulated inside the room through a metalic pipe which heats the surrounding room air by natural convection (Photograph - 4 & 10). The system is still in the development stage.

In the northern region, black cloth sun drying is being experimented with. In this method cocoons are spread on the ground and covered with the black cloth which helps to raise the temperature so as to stifle the cocoon. Preliminary test results are encouraging.

2.2 Cooking and Reeling

Charka Oven

The majority of the charka units use non-woody biomass fuels such as groundnut shell, paddy husks, etc. This is the simplest mechanism for silk reeling, in which both the cooking and reeling operations are carried out in the same basin (Photograph - 5, 11 & 12). First, the water in the basin is brought to boiling by allowing higher burning rates of the fuel. The cocoons are then cooked for a few minutes, with simultaneous stirring and mixing. After the cooking is over, the temperature in the basin is reduced by adding cold water and simultaneously reducing the burning rate of the fuel by cutting off the air supply by closing a damper. The manual reeling operation starts with the reeler rotating the charka by hand and the other person attending to the process of feeding the silk threads to the charka. In some units, where several charka ovens are installed under the same shed, the reel is power driven by a common shaft. After all the cocoons are reeled, the cooking operation starts again by increasing the burning rate and the process continues. The quality of yarn produced in charka units seems to be poor but the charka units account for nearly 50% of raw silk produced in India.

Although, charkas are not made according to any specifications of measurements, they are all similar in design and constructional details. Generally each charka establishment installs five to six charkas and each consists of three distinct parts, namely the mud platform, distributor and reel.

(a) Mud platform

This is rectangular in shape and usually measures about 60 to 75 cm high and 90 cm wide and 120 cm long. The forepart has a built-in fire place with a basin fitted over it. The other part of the platform is solid with a flat top intended for the reeler to sit and attend to the reeling operations. The basin is of thick copper sheet and is generally of circular shape measuring about 50 cm in diameter and 20 cm in depth. Occasionally, instead of a copper basin which is comparatively costly, one half of a vertically bisected earthenware pot is used. Even the use of an aluminium basin is quite common. The basin is buried up to its brim in the mud platform with the spacious part of the basin's underside exposed to the fire place below. The basin is used for both cooking and reeling operations.

The fire place, which is not provided with a grate or ash pit, is generally built for burning firewood or dry twigs. In some places it is designed for burning paddy husk or groundnut shells. At the opposite end of the opening there is a chimney provided for the flue gases to escape.

(b) Distributor

This is a simple device consisting of a crudely made eccentric wheel which in operation revolves on its vertical axis and drives the wooden traverse rod to and fro. The eccentric wheel is driven by an endless cord -belt from the reel passing over its constricted part. The traverse rod is provided with loops of wire along its length at regular intervals. These serve as thread guides for the threads passing through onto the reel. During the reeling operation, the traverse moves briskly to and fro in front of the reel thereby distribution the threads onto the reel.

(c) The reel

Only one long reel made of thick sections of seasoned wood is provided for each basin. Its circumference is not standardized and ranges from 150 cm to 225 cm in different charkas. It can accommodate four to five ends, the reel is rotated manually or power driven.

Cottage basin oven

In the cottage basin system the cooking operations and the reeling operations are carried out separately. The cocoons are first cooked in the cooking unit and then taken to the reeling basins for reeling.

(a) Cooking unit

The cooking unit consists of a masonry structure of a convenient height in which several, usually four cooking pans are embedded in rows. The basins are normally made of copper. They are 20 to 22 cm in diameter and about 20 cm in depth. The oven is well constructed with ash pit, grating and chimney for the flue gases to escape. In the path of exhaust gases a fairly large metalic water drum is embedded to serve as a hot water source to the reeling basins (Photographs - 13 to 16). The cooking unit is located a little distance away from the reeling unit to prevent the heat and smoke from causing disturbance to the reelers. The average water temperature in the cooking vessels is 87–96° C. A handful of cocoons are taken each time and put into the cooking pans. A stick is used to brush the cocoons to separate the floss and locate the ends of the filaments. The cocoons are now ready for reeling. The whole operation takes about 1 to 2 minutes.

Photograph - 13	Photograph - 14
Idle traditional cottage basin oven unit	Traditional cottage basin oven unit in operation
Photograph - 15	Photograph - 16
Traditional cottage basin oven with step grate for loose biomass fuels	Use of wood logs in a cottage basin oven

(b) Reeling unit

The reeling unit consists of four to five reeling basins. Each basin is embedded into a table, the table top is usually spacious and provides sufficient accommodation for keeping the necessary reeling equipment. The reeling basin is usually made of copper. It has dimensions of about 45 × 25 × 7.5 cm (Photograph - 7). The average temperature of water in the reeling basin is maintained at around 40°C in order to facilitate reeling. Taps are provided to draw water from the hot water drum of the cooking unit.

Each basin is designed to reel from four to six ends. To facilitate easy attachment of filaments porcelain button thread guides are provided for each basin. Each basin has its independent croissure frame designed for application of travellite croissure. The reels are driven by corresponding drive wheels fitted on a common transmission shaft. At one end of the transmission shaft, the traverse mechanism consisting of required gears, and at the other end, one handle is provided. Each reel is served by a braking mechanism of simple design.

Photograph - 17	Photograph - 18
Use of two chimneys in a traditional cottage basin oven	Idle CSTRI economic charka oven unit
Photograph - 19	Photograph - 20
CSTRI economic charka unit in operation	CSTRI economic cottage basin oven with step grate for loose biomass fuels
	Photograph - 22
	CSTRI economic cottage basin oven in operation
Photograph - 21
Idle CSTRI economic cottage basin oven
Photograph - 23	Photograph - 24
Close-up of step grate in oven used for loose biomass fuels	3-pan cocoon cooking technique being developed at CSTRI

The reeled raw silk is then sent for re-reeling onto bigger reels of standard size and weight. In another version of the cottage basin, namely, the domestic system of reeling, also called the Italian system of reeling, the re-reeling is avoided. Instead, the silk is reeled onto a standard reel in one operation. After that the yarn is skeined and bundled to make it ready for dispatch.

2.3. Economic Ovens

The traditional ovens (charka and cottage basin) used in silk reeling units consume lot of fuel on account of their ill designed fireplace and chimney. In many traditional cottage basin ovens use of two chimneys is common (Photograph - 17). The reason for it may be removal of smoke coming out through the fuel port opening or distributing the fire on all cooking vessels. Hence, the Central Silk Technological Research Institute, Bangalore (CSTRI) developed economic ovens aimed at reducing the fuel consumption. The first version of the economic oven could not use locally available fuels like husk & cobs and required fuelwood to be fed in pieces. Therefore, it found limited acceptability despite subsidies. These factors led to the development of a new version which is multi-fuel in nature (Photographs - 18 to 24). Under the National Sericulture Programme (NSP), a subsidy of 50% in the case of charka ovens and 30% in the case of cottage basin ovens is offered. The cost of the charka and cottage basin oven is Rs. 1,120 and Rs. 6500 respectively. The detailed technical drawings of charka and cottage basin ovens are given in Figures 1 and 2 respectively. Table 2 gives the details of the material required for CSTRI oven construction. The programme seems to be gaining momentum.

III. ENERGY USE PATTERNS

A detailed field survey was carried out in order to establish the energy use pattern in the various types of ovens used in the silk reeling industry. Traditional and improved charka and cottage basin ovens and stifling units spread in traditional mulberry silk states of Andhra Pradesh, Karnataka, and Tamil Nadu were covered in the survey. In all, 236 cooking ovens (113 charka and 123 cottage basin) and 94 stifling ovens (37 barrel and 57 basket) were surveyed.

Table 2 : Construction material requirement for CSTRI's economic oven

Figure 1 : Technical drawing of CSTRI charka oven

Figure 2 : Technical drawing of CSTRI cottage basin oven

3.1. Charka Ovens

In all, 72 traditional and 41 CSTRI charka ovens, spread over three major southern states, were surveyed. It was observed that different types of locally available biomass fuels are used in different regions, e.g. tamarind husk (Kanakpura, Ramnagar), paddy husk + coffee bean shell (Kollegal), paddy husk and groundnut shell (Chintamani, Kolar), groundnut shell (Madanapally), Eucalyptus leaves (Hossur and Palacode). Most of the reelers in the charka sector are poor and depend solely on the returns of the proceeds of the previous day for purchasing fresh cocoons. It is quite common in this sector for the reelers to take up piece jobs from bigger and affluent reelers. Also, many reelers concentrate on processing second grade cocoons. All these factors contribute to the variation in the cocoon consumption. The survey findings for charka ovens are summarized in Table 3. The following broad observations can be made:

• The charka ovens do not use firewood
• Daily fuel consumption in charka units ranges from 17 to 27 kg for slow burning fuels and 26–37 kg for faster burning fuels.
• Variety of low cost, locally available loose biomass fuels are used.
• The specific fuel consumption seems to be higher (2.44 to 3.20 kg/kg cocoon) for faster burning fuels such as groundnut shells or eucalyptus leaves compared to slow burning fuels (1.58 to 1.93 kg/kg cocoon) such as paddy husk or saw mill waste.
• The variation in specific fuel consumption is low for slow burning fuels (3–10%) and high for fast burning fuels (21–34%).
• For comparison between traditional and economic ovens, there is sufficient number of data only for groundnut shell fuel. For this case the specific fuel consumption in economic ovens is marginally higher (by about 13.5%) compared to that in traditional ovens. However, as the confidence limit of specific fuel consumption for both the ovens overlap, the t-test is carried out to know the confidence level of the statement that ‘specific fuel consumption of traditional oven is lower than economic oven’. It is found that there is 92% level of confidence in the above statement.

Table 3 : Summary of survey findings; charka ovens

Note: Number in brackets indicate standard deviation

Table 4 : Summary of survey findings: cottage basin ovens

Note: Number in brackets indicate standard deviation and no. of pans

3.2. Cottage basin ovens

During the course of the study, 113 traditional and 10 CSTRI cottage basin ovens were surveyed. The surveyed traditional ovens had 2–6 cooking vessels. The CSTRI ovens covered were mainly of 4,5,6 pan type. Table 4 summarizes the survey findings of cottage basin ovens for different types of fuels used. The following broad observations can be made:

• Woody biomass in the form of wood logs is predominantly used as fuel.
• Daily fuel consumption in cottage basin units ranges from 50 to 285 kg.
• If comparison is made, for same fuel use, the cottage basin consumes 50–55% less for groundnut shell (faster burning fuel) and about 25–30% less for saw mill waste (slow burning fuel).
• For the same fuel type, tamarind wood, CSTRI cottage basin ovens consume about 21% less fuel as compared to the traditional ovens. Again, as confidence limits of specific fuel consumption of two stoves overlaps, a ‘t-test’ was carried out. It showed confidence level of 95% in the survey findings for the survey data.

3.3. Stifling Units

In all, 37 barrel stifling units and 57 open stifling units were surveyed. The findings of the survey are summarized in Table 5. The following broad observations can be made:

• Generally, basket stifling is used for charka ovens as cottage basin units use barrel stifling.
• Daily fuel consumption values for basket stifling units in charka ovens and for barrel stifling is about 30 kg and for basket stifling in wood fired stoves is about 100 kg.
• Due to much less evaporation loss in barrel stifling its specific fuel consumption values are much lower than that of open or basket stifling.
• When a comparison is made with common fuels, it can be observed that reduction in specific fuel consumption in barrel stifling is 51% in case of loose biomass (tamarind husk) and 66% in case of solid biomass (tamarind wood).
• Specific fuel consumption for stifling accounting for about 35–50% (open stifling vs. charka cooking) and 20–25% (barrel stifling vs. cottage basin cooking) of that required for cooking in charka and cottage basin reeling units respectively.

Table 5: Summary of survey findings: stifling units

Note: Number in brackets indicate standard deviation

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:

1. flue gas loss (heat carried away through the chimney in the form of hot flue gas);
2. heat loss through oven openings (heat transfer by radiation mode from fuel bed and flame to the ambient air, through fuel port and other oven openings, if any;
3. surface heat loss (loss of heat by conduction, convention and radiation due to temperature gradient existing between hot oven surface and cool surrounding air);
4. heat loss due to oven thermal mass (loss of heat accumulated by oven structure, due to heat capacity of oven material, resulting rise in oven temperature, which is liberated back to the ambient air; when oven is not in operation;
5. unaccounted portion of heat loss including loss through hot ash and heat content of charcoal formed during operation which could not be monitored as it was used in reeling operation or for igniting stifling oven.

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:

1. Total water consumption for the batch
2. Water quantity in and out of the cooking basin
3. Water quantity drained after the batch for refill with fresh water
4. Cocoon consumption for the batch
5. Quantity of pupae (secondary cocoons) recycled for next batch
6. Fuel consumption for the batch
7. The temperature of the cooking basin and the feed water
8. Quantity of waste silk produced.

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.

• The specific water consumption for the cocoon cooking operation is higher (6–12 kg/kg cocoon) for the charka oven than the cottage basin (4.5 to 9 kg/kg cocoon) ovens.
• Drainage loss (50–67%) and loss due to water carry-over from cooking to reeling basin (19–45%) are a major fraction of the water consumption for the charka and cottage basin oven respectively.
• In some cottage basin ovens spillage loss also forms a significant (as high as 30%) portion of water consumption due to the poor operating practice followed.
• Under the present operating practice followed in the reeling units, the useful heat required for charka and cottage basin cooking operations works out to be about 1300 and 875 kcal/kg cocoon respectively. The reason for higher energy requirement for charka ovens can be attributed to slower operation and large evaporation area available resulting in lot of heat loss due to evaporation. Also water consumption per kg of cocoon, and hence heat loss due to drainage of hot water, is greater in the case of charka ovens.

Table 6 : Summary of water balance experiments on Cottage Basin Ovens

4.2. Energy Balance Experiments

In order to conduct the energy balance, experiments following parameters were monitored per batch of silk production:

1. Oxygen, carbon dioxide (volume %) in the flue gases
2. Flue gas temperature
3. Cooking basin temperature
4. Drum water temperature
5. Reeling basin temperatures
6. Dry bulb and wet bulb temperatures
7. Temperatures of the oven surfaces
8. Cocoon consumption
9. Silk production
10. Ovens dimensions

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.

Figure 4 : Sankey Diagram showing various heat stream of stoves

Charka ovens

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.

Stifling units

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:

• Efficiency of conversion of biomass into producer gas is very high (70%);
• use of gaseous fuel entails higher thermal efficiency and better control;
• gas can be efficiently used for stifling as well as pupae (worm) drying.

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.

VI. FINDINGS

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:

• The majority of the charka ovens use locally available biomass as fuel, whereas cottage basin ovens use large wood logs as the major source of heat energy.
• The thermal performance of the charka stove, which uses several biomass residues in loose form, is affected significantly by its burning characteristics. Lower burning rates, and hence lower flue gas losses, are associated with slow burning fuels like rice husk, saw mill waste, etc. whereas, faster burning fuels such as eucalyptus leaves or groundnut shell, etc. result in higher burning rates and hence higher flue gas loss.
• An important constraint of the cooking and reeling processes is that the cooking and reeling basins can not be covered with lids, thus allowing continuous, and significant heat losses due to evaporation. Also drainage loss due to need to change the dirty water with fresh water after every batch is unavoidable.
• In the present practice of cooking there is bound to be a minimum heat loss from the cooking basin due to hot water carry-over (along with the cooked cocoons) from cooking to reeling basin (27 – 30%), drainage of water after every batch (charka: 50–67%, cottage basin: 19–45%), evaporation (15–25%) from continuously stirred boiling water in the cooking pots, hot water wasted along with pupae waste and spillage losses depending on operating procedures which vary from unit to unit.
• Another important factor is the time pressure under which cooking takes place. The cooking operation has to be completed within 2–3 minutes, which leads people to increase the burning rate resulting in higher flue gas temperature and hence higher flue gas loss.
• In terms of specific fuel consumption (kg/kg cocoon) CSTRI cottage basin ovens perform better (21% fuel saving for same fuel, tamarind wood, with confidence level of 95%). However for groundnut shell as fuel CSTRI charka ovens consume 13.5% more fuel (92% confidence level as per t-test).
• Barrel stifling is far more efficient than open or basket stifling due to substantial reduction in evaporation loss resulting in lower specific fuel consumption (by 65% for loose biomass and by 75% for wood).
• Specific fuel consumption for stifling accounts for about 35–50% (open stifling vs. charka cooking) and 20–25% (barrel stifling vs. cottage basin cooking) of that required for cooking in charka and cottage basin reeling units respectively.
• Water balance experiments indicate that for the current practice of cocoon cooking the amount of useful heat required for cooking each kg of cocoon works out to be about 1300 and 875 kcal for charka and cottage basin ovens respectively.
• Biomass gasifier based silk reeling ovens show significant (more than 50%) fuel saving potential.
• Integrated biomass gasifier silk reeling units can help in achieving all the operations viz stifling, cooking, pupae drying with substantial reduction in fuel consumption, thereby improving the economic viability of the silk reeling industry as a whole.

Figure 5 : Integrated gasifier system for silk reeling unit

Development of gasifier based silk reeling unit at TERI

Photograph - 25	Photograph - 26
Integrated gasifier based proto-type cottage basin reeling unit	Close-up of cottage basin oven showing producer gas pipe network
Photograph - 27	Photograph - 28
Producer gas burner in operation (one below each cooking vessel)	Proto-type drying unit for pupae drying or cocoon stifling utilizing exhaust gas H2O