DspEdu 2.1

Author's Home Page

List of Articles
 

Introduction

Executive Summary

General Description of the System

Energy Management

Monthly Historical Data of Energy Consumption, Energy Cost & Production

Specific Energy Consumption & its Monthly Variation

Electrical Energy Audit

Introduction

Layout of Power Supply

Reactive Power Management in Motors/System

Cable and Line Losses

DG Set Operation

Transformer Loading and Loss Analysis

Motor Loading Analysis

Lighting System

Thermal Energy Audit

The Crumb Drying Operation
 

Drying Operation Description

Energy Balance in the Drying Oven

Estimation of Drier Efficiency during peak production

Drier Efficiency during lean months

Recommendation for reducing energy consumption in Drier
 
 

Introduction

      [ I conducted an Energy Audit Study at a Rubber Produce Factory located in Kerala State in India in 2002. This Plant received Electrical Power from K.S.E.B Grid (the Kerala State Electricity Board ,referred to as KSEB in this article)  at 11 kV level with a  Contract Demand of 200 kVA . The Connected Load of the Plant was 259.5HP motor load, 21 kW lighting and 90 kW heater load. The Recorded Maximum Demand varied between 178 kVA and 190 kVA.The firm maintained a 110 kVA D.G. Sets and a 55 kVA D.G. Set as stand by units.This article covers the energy consumption details and conservation opportunities in this Plant as in 2001.]

    The Electrical Energy Audit at a Rubber Produce Factory is considered in this article.An abstracted version of the Energy Audit Report follows.

1. Executive Summary

1.1 Summary of Recommendations
 

Total Annual Energy Savings = 87.3 MWh

(12900 units of Electricity, 7247 lits of Diesel)

1.2 Savings Anticipated by Proposed Schemes

Total Expected Annual Financial Savings = Rs. 2.27 Lakhs

Total Annual Energy Savings = 87.3 MWh

1.3 Investment Required and Payback Period

Total Expected Investment = Rs. 3.90 Lakhs

Total Expected Annual Financial Savings = Rs. 2.27 Lakhs

Overall Payback Period = About 2 Years
 

1.4 Savings Through Schemes Already Implemented

The last audit was conducted in 1994 and covered 1989-1992. The audit found that SEC value for CENEX was 0.072 unit/kg, for Skim Rubber it was 0.025 unit/kg and for Crumb Rubber it was 1.13 unit/kg. The major energy conservation measures proposed by that audit were (i) reconnect some identified motors in Star Connection (ii) Replace the existing 55kVA DG Set by a 25kVA Set (iii) Convert the existing Drier into a Through Circulation type. The first recommendation was carried out. The second one could not be carried out since the Plant wanted this DG Set to be capable of starting the 50HP Hammer Mill.The third recommendation was not carried out since the original equipment manufacturer was not keen on taking up the work on reasonable amounts.

On this, the firm started experimenting with subjecting the wet crepe to natural air drying for days before it is milled and electrically dried. The firm commissioned an independent study which confirmed the Crumb SEC figure arrived at by the 1994 Audit in the case of operation without natural air drying and showed that with complete natural air drying the Crumb SEC will fall to 0.565 unit/kg. This was in Feb 2001. The firm fully implemented this policy of full natural drying before milling and electrical drying since then.

The present Audit arrived at the figure of 0.57unit/kg for Crumb Rubber based on audit measurements and this tally well with the figure mentioned above. Moreover this audit notes that the average SEC of Crumb Rubber was 0.59 unit/kg for the period 2001 April to 2002 July; thereby re-confirming the effectiveness of natural-air-drying policy. Moreover, this policy helped the plant to maintain lower MD and made it possible to get its CD revised from 300kVA to the current level of 200kVA.

At an average annual Crumb production of 225125 kg this represents a savings of 225125 x (1.13 – 0.59) = 121567 units per year and amounts to Rs. 3.04 Lakhs financial savings from energy bill alone. Further savings are also realised due to decreased Maximum Demand level.

This audit found that the SEC levels of CENEX and Skim Rubber continue without change at the same values observed in 1994.

1.5 Impact of Proposed Schemes

The Current SEC of Crumb Rubber = 0.59unit/kg Expected Reduction in SEC = 0.057unit/kg

The Current Energy Cost/kg of Crumb = Rs. 3.5/kg

Expected Energy Cost/kg of Crumb = Rs. 3.1/kg

The Current Energy Cost/kg of CENEX = Rs. 0.71/kg

Expected Energy Cost/kg of Crumb = Rs. 0.41/kg

(Due to reduction in Diesel Cost.Cenex is the most affected by Diesel Cost)

2.1 General Description of the System

The Firm generates raw latex from its plantation and produces Centrifuged Latex (CENEX) and Skim Rubber from the raw latex. It also generates cup lumps and tree lace from its rubber plantation and produces Crumb Rubber from this input. The Company does not buy raw material from outside.

The production in the factory is highly seasonal. February to August represents the lean season and September to January represents the peak season.

All the products are accounted in terms of a common product expressed as Kilograms of dry rubber content (DRC) in the product. The current average annual production level is about 715,600 kg of produce. The product mix is observed to be relatively fixed over the years at 60% of CENEX, 31.5% of Crumb Rubber and 8.5% of Skim Rubber.

2.2 Process Description

(i) Centrifuging

The raw latex collected from the plantation is introduced into 4 Nos Collection cum Sedimentation tanks from which it is drawn into 4 Nos mixing tanks (bulking). From mixing tanks the prepared latex is fed in a controlled manner under gravity flow into Centrifuging machine (ALFA LAVAL TRH 410A driven by a 15HP motor with controlled starting and stopping). The centrifuge runs continuously for 4hours and delivers 700 litres of CF.NEX into Cenex tanks in 4 hours. After 4 hours running, the centrifuge is cleaned and remounted (cleaning & remounting takes about 30 to 45 minutes) for next 4 hour running session.

The CENEX from cenex tanks pass through a Weighing Balance into Drums which hold 2205 litres per drum.

(ii) Skim Rubber Processing

The refuse from Centrifuge, termed as Skim Serum is mixed with 907. Sulphuric Acid in Collection/Coagulation tanks to prepare Coagulurn which is processed in Skim Rollers (There are two, driven by 20HP motors, both are in use now). The mats are prepared by giving three passes between rollers. The Skim Crepe prepared thereby is sent to Crepe shed for natural drying and after drying is packed in the form of 50-kg bundles.

(iii) Crumb Rubber Milling & Rolling

The Cup lumps and tree laces collected from Rubber Plantation are soaked in the soaking tanks for 24 to 48 hours. The raw material is collected from the tank in trays manually and is fed into Crumb Rollers (5 in number, driven by 25 HP, 25HP, 25HP, 20HP and 10HP motors respectively) successively with various roller gaps and differing number of passes in each roller. Generally only 4 rollers (3 x 25HP+20HP) are employed.

After rolling, the mats are fed into a 50 HP Hammer Mill through 20HP driven feed rollers. The wet crumb rubber from Hammer Mill goes to washing tanks (SNos) where water jets effect washing of Crumb Rubber.

Aluminum Trays, each holding about 20 Kg wet crumb (after dripping) are employed to collect wet crumb from tank and to prepare loaded trolleys to be fed into crumb Drier unit.

iv) Crumb Rubber Drying

The Aluminum trays holding 20 Kg Wet Crumb are pushed into a tunnel drier (Electrical heater, closed air circulation, essentially parallel air flow, but partial cross flow achieved by Aluminium reflectors inside the dryer) after allowing sufficient dripping time. The pusher mechanism is hydraulical1y operated and time controlled to effect pushing of one trolley holding two trays once in 14 minutes. Starting with an empty drier it takes about 12 to 13 hours of drier operation to produce 1000 Kg of dry Crumb Rubber starting with 1600 Kg of wet Crumb Rubber after dripping under normal voltage conditions.

Heater is rated for 90 kW in three identical units of 30 kW each and circulation of air is effected by a 10 HP driven blower. Automatic ON/OFF control of heater based on sensed temperature at 4 points in the airflow path is employed.

The hot trays coming out of drier are subjected to cooling by a suction fan and after cooling the Crumb Rubber goes to weighing balance and from there to Baling Press where the final product in the form of 25 Kg bales is prepared.

2.3 Energy Consumption Profile

Electrical Energy from KSEB received at 11KV level through a 400kVA transformer and Diesel used in two DG sets of 110kVA and 55kVA ratings are the energy inputs into the plant.

The electrical system has 286 HP motor load, 21 kW lighting load and 90kW-heater load connected in it. The current contract demand is 200 kVA and current Maximum Demand level is about 180kVA in the peak production months and below 150kVA in the low production months.

The annual electrical energy consumption is about 200 MWh of which about 91% is from KSEB and 9% is from Self Generation in DG Sets. The factory is located in a region where the KSEB supply quality is poor with low voltage problems, frequent long duration interruptions etc Factory is forced to use its standby DG sets almost daily due to poor quality of the supply. Plant uses about 11,300 litres of Diesel on the average in its DG Sets in a year. 

The plant requires only electrical energy. But it is forced to consume Diesel in considerable proportion due to poor quality of KSEB supply both in terms of low voltage and frequent interruptions.

Energy consumption profile, Energy flow analysis and Specific Energy consumption will show a distorted picture if total energy input in a common unit is employed due to following reasons.

1. It is the low quality of KSEB supply which forces the firm to run its standby DG sets almost like in-plant generating stations. 
2. The consumption of Diesel varies erractically based on the quality of supply.

84% of Plant Electrical Consumption from KSEB is during the 5:00AM-6: 00PM time block and 15% of consumption is during the 10:00PM-5: 00AM block. The plant consumes virtually no energy from KSEB during peak hours and uses 55kVA DG Set during this period for lighting and centrifuging.
 

The products manufactured are Centrifuged Latex (CENEX), Crumb Rubber and Skim Rubber.

The production figures for the three products during the study period are summarised below
 

The average product mix by DRC weight is 60% of CENEX, 31.5% of Crumb Rubber and 8.5% of Skim Rubber.

The diagrams for Average Annual Electrical Energy Consumption and cost are given in Fig 2.1, 2.2, 2.3, 2.4 and 2.5.

2.4 Methodology of Energy Audit

(i) The Energy Audit team visited the factory on two days (16.7.2001, 9.9.2002) to observe the plant operations and to carry out detailed audit measurements.

(ii) Load analysis of all electrical equipments and all busses was done using Clamp-on Audit meters and Digital Clamp-on meters and Energy Analysers.

(iii) Detailed discussions with Plant Personnel were held in order to gain insight into the plant scheduling, operational procedures, operation of major equipments, loading practice, Seasonal variation in raw material volume and quality, Seasonal variation in production, Variations in operational practice for Dryer etc.

(iv) Dryer operation was critically looked into and surface temperature was verified.

(v) Monthly energy consumption data and production data were obtained and analysed.

(vi) Further data acquisition and discussions took place through correspondence with the Plant Engineering Manager.

2.5 Areas Covered in the Report

All Fuels, Systems, All forms of Energy.

2.6 Equipment Used for the Energy Audit

(i) ITT Make Clamp-on Digital Power Audit Meters

(ii) Krykard ALM1 Single Phase Energy Analysers

(iii) Krykard ALM3 Three Phase Energy Analysers

(iv) DUCATI SIM50 Three Phase Energy Analyser

(v) Digital Temperature Meter

2.7 Energy Audit Study Period

April 1999 - March 2002
 

3.1 Introduction

The Firm receives KSEB supply at IlkV through a 400kVA, 1 1kV/433V Outdoor transformer. The Company also maintains two Standby DG Sets of 110 kVA and 55 kVA rating respectively.

The current contract demand is 200kVA and current MD level is about 180 kVA in the peak production months and below 150kVA during the remaining months. The system consumes about 1600 units of electrical energy per day during peak months and about 600 units of electrical energy per day during lean months.

The factory is located in a region that suffers very poor quality of supply with frequent and long interruptions. The region also is subjected conditions. Thus the Company is forced to operate Sets for extended durations during supply outages during low voltage periods.

3.2 Monthly Historical Data of Energy Consumption, Energy Cost & Production.

The firm has three products at present-Centrifuged Latex, Crumb Rubber and Skim Rubber.Tables 3.1, 3.2 and 3.3 give the monthly energy consumption and energy cost data for the study period. Tables 3.4, 3.5 and 3.6 give the monthly production figures for the same period.

 
 

 
 

 
 

Figs 3.7,3.8 and 3.9 show monthly production figures in the form of bar charts. Figure 3.10 shows the variation of Specific Electrical Energy Consumption of Crumb Rubber in Units/kg for the 36-month study period.

These charts bring out the seasonal nature of production with clearly identifiable peak and lean seasons.

3.3 Specific Energy Consumption & its Monthly Variation

The production of CENEX (Centrifuged Latex) involves only one process-Centrifuging. Production of Crumb Rubber involves two processes-Milling & Rolling (crushing in hammer mill, creping in crumb rollers) and drying (drying the wet crumb in an Electrical Tunnel Drier). Production of Skim Rubber involves only one energy consuming process- Creping rollers.

There is no energy metering available at bus levels or process levels in the plant. Only the total monthly energy consumption data is available. Since the process energy consumption was not metered separately in the past, historical data on specific Energy Consumption for the various processes for 3 years can not be prepared. However estimated SEC values for various processes based on measurements during audit are furnished later in this section.

In the absence of individual process metering, it becomes difficult to separate out SEC values of individual products from the data of total energy consumption alone. The major product, both from DRC weight and contribution to turnover points of view is CENEX. But the Crumb Rubber consumes the major portion of energy used up in the plant. Hence the three products converted into a common unit of DRC weight will lead to misleading conclusions as to their relative contributions to energy consumption. Hence the three products are kept separate in the SEC analysis that follows.
 

The Latex Centrifuging machine operates continuously for 4 hours producing 700 litres of Centrifuged latex ie 658 Kg of latex. 658 Kg of latex has a Dry Rubber content of 395 Kg. The machine has a controlled start lasting about 8 minutes during which it takes 13 kW input and during continuous running it takes 6.3 kW. Thus Centrifuging Machine takes 26.1 units of energy to produce 395 Kg of CENEX.

Two fans taking a total of 350 Watts and two flourescent lamps taking about 110Watts are employed continuously in the Centrifuging Room.

A latex agitator, consuming about 1 kW is employed for about 30 minutes during a 4-hour session.

Thus total energy consumed to produce 395 KG of CENEX is 28.43 units.

SEC of CENEX = 0.072 unit/kg

The chemical & physical quality of raw latex obtained from the Company's Rubber plantation varies slightly over various months in a year. This results in minor variations in monthly SEC of CENEX. These minor variations may be ignored and SEC of CENEX (as well as that of the Centrifuging Process) may be taken as a constant at 0.072 unit/kg.

Also, once the Centrifuge is cleaned and mounted it runs for 4 hours continuously at its rated output. This removes the possible dependence of SEC on the workforce efficiency.

The same value of SEC of CENEX was arrived at last audit conducted in 1994 too. The power measurements were verified during this audit too and it is verified that the SEC value of this product has not varied over the years.
 

Skim Rubber is produced in Skim Rollers of 20HP rating. Each mat is given 3 passes ie 2 single mat passes and one double mat pass. The roller motors works at 60% duty ratio as estimated by timing the operations at site. Both the roller motors were found to take same power under loaded as well as unloaded conditions. The production rate is 175 kg/hr. During single mat pass the motor takes 4.7 kW input, during double mat pass it takes 8.1 kW and during no load it takes 2kW. Based on these, the SEC of Skin rubber is worker out to be 0.025 unit/kg. Relatively constant nature of raw material for skim rubber justifies the assumption of a fixed SEC for this product.

SEC of Skim Rubber = 0.025 unit/kg

The same value of SEC of Skim Rubber was arrived at last audit conducted in 1994 too.The SEC value of this product has not varied over the years.
 

The monthly production figures for CENEX and Skim Rubber are used alongwith the above fixed SEC values to calculate the total energy consumption by these products on a monthly basis. Energy Consumption by these products is deducted from total electrical energy consumption to get the energy consumption for Crumb Rubber production alone.

The SEC of Crumb Rubber thus calculated and averaged on a yearly basis is given below.
 

 

Year	Crumb Rubber Production in kg	SEC in units/kg
1999-2000	225200	0.83
2000-2001	200525	0.85
2001-2002	249652	0.59
Average	225125 (265500 in the last Audit)	0.75 (1.13 in the last Audit)

The value of SEC averaged over the three years covered in the study is recommended as the base figure for SEC of Crumb Rubber. This value is given below.

Average SEC of Crumb Rubber in 1999-2002 Period = 0.75 units/kg
 

Milling & Creping = 241 units 

Drying (Drier + Blower) = 877 units 

Thus the SEC figures for Crumb Rubber in this case will be 

SEC of Crumb Rubber = 1.12 Units/kg

SEC of Milling And Creping = 0.24 Units/kg of dry Crumb

SEC of Drying Process = 0.88 Unit/kg of dry Crumb

Milling & Creping = 320 units 

Drying (Drier + Blower) = 250 units 

Thus the SEC figures for Crumb Rubber in this case will be

SEC of Crumb Rubber = 0.57 Units/kg

SEC of Milling And Creping = 0.32 Units/kg of dry Crumb

SEC of Drying Process = 0.25 Unit/kg of dry Crumb

A detailed analysis of the Electric Drier operation and its inefficiency was included in the Energy Audit Report in 1994 and it was pointed out that the wet crumb rubber that goes into the Drier had close to 40% moisture content – 1600 kg of wet crumb going into Drier resulted in 1000kg Dry Crumb those days. Thus evaporating the 600kg or so of water was the major power-consuming job in the plant. Some recommendations aimed at improving the Drier efficiency were included in the report. But the firm reported subsequently that the original equipment manufacturer was not keen on taking up the modification works proposed within a reasonable cost estimate. 

That was when the Plant personnel started experimenting with subjecting the crepe to a long period of natural drying in air before it is milled and pushed into the electrical drier. Though the firm practised this off and on since 1996, a thorough evaluation of its impact was carried out only in Feb 2001. An independent agent conducted measurements at the plant and arrived at figures close to the ones estimated above. 

The first figure given above is based on the measurements carried out in 1994 by this auditor and second set of figures comes from measurements during this audit. The independent agent, in Feb 2001, arrived at 0.96 units/kg as the SEC value for crumb air dried for 2.5 days and 0.565 units/kg as the SEC value for fully naturally dried case. These values are in close agreement with the values estimated by Energy Audit in 1994 and this audit. 

The firm adopted the policy of drying the crepe fully under natural conditions before hammer milling subsequent to the study in in Feb 2001.This is the reason why there is marked difference in SEC value in the year 2001-2002 compared to the value in the earlier years. The SEC figures in the 1999-2000 and 2000-2001 are higher (but less than the value corresponding to no-natural-drying case) possibly due to insufficient natural air drying time allotted. There was some hesitation on the part of Quality Control to let the crepe dry completely under natural conditions those years and detailed experimentation was necessary to ensure that the Crumb quality will not suffer due to complete natural drying. These considerations would have prevented the firm from drying completely under natural conditions in 1999-2001 period.

The SEC value in 2001-2002 was 0.59 unit/kg and is very close to the value of 0.565 measured by the factory's consultant in Feb 2001 and 0.57 estimated by this audit in Sep 2002. Though this audit covers data till Mar 2002, it was verified that SEC for April, May, June and July 2002 were also in the range of 0.5 to 0.55 units/kg from the data available in the plant in Sep 2002. This corroborates the effectiveness of "natural-drying-to-minimum-moisture" policy.

This policy of fully drying the crepe before milling and electrical drying has two other beneficial effects – it makes it possible to reduce the Maximum Demand recorded at the plant and it smoothes out the impact of seasonal variations in the raw material supply on the electrical system. The natural drying process takes many days and this delay (i.e the plant is under no compulsion to mill and electrically dry the crepe generated on a particualr day on that day itself) smoothes out the load variations in the electrical system and results in more uniform monthly production rates for Crumb Rubber.

Naturally dried crumb will require only lower heating rate in the Electrical Drier and thus the Drier will run with many elements switched off most of the time (the temperature control system will see to that), thereby contributing less than its full rating to the MD. Moreover it makes it possible to control MD actively by switching off some elements when that is needed to keep MD under the desired level. The Drier operates at 50% duty ratio with naturally dried crepe whereas it used to run at 75-77% duty ratio when the wet crepe used to be processed directly. In fact, this precisely is the reason why the firm could maintain its RMD at an average value of 165kVA in 2000-2001 and 2001-2002 against the average RMD of 200kVA or more in the past. Without this the firm could not have got its Contract Demand revised to 200kVA in 2000 Jan.
 

The monthly SEC figure of Crumb Rubber for the 36-month study period is shown in Fig. 3.10.

Monthly SEC for this product shows some variation. There seems to be three distinct periods in an year with three distinct values of SEC. Broadly Feb, March, April form the first period charactersied by low volume production and high SEC. Second -period is formed by May, June, July, Aug, characterised by moderate volume of production and high SEC. Third part of the year-formed by Sept, Oct, Nov, Dec, Jan is the peak production period with lowest SEC.

The reasons for monthly Variation of SEC of Crumb Rubber are

i. The period Feb, March, April represent a low volume raw material period. The raw material quality in terms of moisture content and size of agglomerates will be good during this period. However, plant runs at very low capacity utilisation level in this period.

The workforce in milling & rolling sections is managed on a task basis. They are free to go when they have finished the day's quota of wet crumb. This partially offsets the possible energy inefficiencies in this section due to slow pace of work during this period.

But the major effect of low volume production is on the dryer. Low volume production makes the dryer operation all the more discontinuous and unsteady. Dryer efficiency decreases in the discontinuous and unsteady mode. The Dryer operation is dealt with in detail in Chapter V. Inefficient operation of dryer is the major reason for high SEC in this period in the first 24 months. In the last 12 months even this tends to get smoothed out due to the natural air drying of crepe adopted. This policy a sort of delinks the creping process from milling and drying and thus effects a smoothing of load throughout the year.

ii. The period May, June, July, Aug is characterised by moderate volume of input. But the raw material in this rainy season is of high moisture content and moreover it comes in the form of sticky mass of large size. This leads to excessive hammering requirement in hammer mill, extra passes in crumb rollers, loss of energy & production time due to frequent tripping of machinery under jamming load etc. Thus the milling & rolling process involves more energy expenditure due to higher degree of effort needed, increased number of starting, and decreased duty ratio due to increased time spent in mannual material handling thanks to sticky nature of material.

Moreover, increased moisture content results in higher duty ratio on dryer and increased drying time (despite natural drying – the season is a wet one with little sun and air already full of moisture). Hence even with more efficient dryer operation made possible by moderate processing volumes, the dryer takes more energy due to increased moisture content. This results in highest SEC in this period.

iii. The period Sept, Nov, Dec, Jan is characterised by high volume, good quality input and thus has lowest SEC. In particular, the SEC in Dec (peak production month) at 0.8 unit/kg is commensurate with SEC figure achieved by other crumb manufacturing units that run round the clock using electrical dryer.

Thus reasons for monthly variation of SEC are variations in input volume, variations in input quality and underutilisation of plant machinery and electrical dryer unit.

3.4 Specific Energy Consumption of Processes

The Plant does not employ separate metering at process level. Thus it is not possible to draw up monthly variation of individual process Specific Energy Consumption .

3.5 Target Daily Energy Consumption Level

1. Only about 10% of total energy consumption is due to Latex production. The major component of SEC of Latex comes from Centrifuging machine. The 15 Hp motor driving the Centrifuge is fully loaded during starting and 50% loaded during running. No further improvement in SEC of latex is hence possible. Target SEC level for Latex is the current level of 0.072 unit/kg itself.
2. SEC o-f Skim Rubber can be improved by tighter loading practice. But this product has a low SEC already and consumes only about 1% of total energy. Hence impact of any change in SEC of Skim Rubber, (achieved through closer supervision of worker in the Skim Rubber Section) will not affect the overall SEC as well as energy consumption materially. Hence target SEC level for skim Rubber is the current level of 0.025 unit/kg itself.
3. The current SEC level of Crumb Rubber is 0.75 unit/kg.

4.1 Introduction
 

The firm receives electrical energy through a 400 kVA, 11kV/433V, DY11 Outdoor transformer with a Contract Demand of 200 kVA and an M.D level of about. 180 kVA in the peak production months and less than 150kVA in the other months. The firm has installed two DG Sets of 110 kVA and 55 kVA each. The total connected load is 259.5HP motor load, 21 kW lighting and 90 kW heater load. The average annual electrical consumption is about 200 kilo Units of which 91% is from KSEB and 9% is from DG Sets consume about 11,300 litres of diesel in an average year.

The plant production is highly seasonal and hence energy consumption is also seasonal. The monthly energy consumption is about 15 kilo Units in lean months (average) and about 25 kilo Units in peak months (average). It can be as low as 8 kilo units in lean months and as high as 30-33 kilo units in certain peak months.

Thus the daily consumption will be in the range 500 - 600 Units in lean months and in the range 800 -900 units in peak months.

All the loads were power analysed by suitable meters and energy analysers during the audit. Based on measured loading in various equipments and the data obtained from plant records & personnel on the production rate, number of hours of operation of various equipments etc, electrical energy flow during a peak production day is estimated. 

In a typical production day at the current level of production 2050 Kg of CENEX, 1125 kg of Crumb Rubber and 250 Kg of Skim Rubber will be produced.

This will require about 4 hours of operation of Hammer Mill & its Feed roller drive, 6 hours of Crumb Rolling Operation, 5 hours of Drier Operation, 2 hours of Skim Roller Operation and round the clock operation of Centrifuge.

The results of the analysis are summarised below:

Total Daily energy consumption for a peak production day = 1035 units

Total input into Motors = 682 units

Total loss in Motors = 170 units

Total Drier Input = 278 units

Total lighting energy = 40 units

Drier loss = 192 units

Cable Loss = 10 units

Transformer Loss = 25 units

Overall Electrical Energy Efficiency = 61.6%

Energy Efficiency of Motor loads = 75%

Energy Efficiency of Drier = 31%

4.2 Layout of Power Supply

Refer to Fig 4.1  for a simplified Electrical Schematic Diagram of the system. Fig.4.2 gives the kVA Load Curve of the system as measured on 9-09-2002.

3. Reactive Power Management in Motors/System

The maximum RMD is registered during one of the peak production months and minimum RMD is registered during lean months.The Contract Demand was revised from 300 kVA to the present level of 200 kVA in Jan 2000. The variation of the chargeable MD over the 36-month study period is shown in Fig. 4.3. The Firm registers a MD above the minimum chargable level of 150 kVA during the peak months. The average RMD after Contract Demand was revised to 200 kVA is around 165 kVA and thus firm pays excess MD charge to the tune of about Rs 50,000/ per year.

2. A 45-kVAr Capcitor bank is employed at Main SwitchBoard and is functioning partially. It was found to deliver 30kVAr of reactive power during audit.

1. There is no capacitor across any other motor or at any other bus. 

Contribution to Demand From Hammer Mill & its feed roller, 3 Crumb Rollers, Skim Roller, Centrifuge, Water Pump, Latex Agitator, Blower, Hydraulic pump, Aerator etc - 75 kW, 100 kVAr at 0.6 lag PF 

Contribution to Demand from lighting - 7 kW, 0 kVAr at unity pF 

Contribution to Demand from Heater - 90 kW, 0 kVAr.

Total Demand by Loads - 172 kW, 100 kVAr, 199 kVA. 

Case (i) The 45 kVAr capacitor at MSB is not ON.

Demand on Secondary side of Transformer = 199 kVA

Demand of Transformer = 2kW, 8 kVAr

Demand of HT Side = 205kVA at 0.85 P.F

Case (ii) The 45 kVAr Capacitor at MSB is ON, but delivers only 30kVAr

Demand on Secondary Side = 172kW,70kVAr,186kVA

Transformer Demand = 2kW, 8 kVAr

Demand at HT side = 174kW,78kVAr,191kVA

Comments

1. The reactive power management is not satisfactory with only 30 KVARs of capacitor power available in the system. The 10kVAr in the Centrifuge section and the damaged 15kVAr portion of the 45kVAr at the Main SwitchBoard will have to be replaced. This will result in a reduction of MD by 6kVA and will result in about Rs. 10,000/- savings in an year. The investment required will be about Rs. 15,000/-.

2. The estimated MD is about 191kVA and the system actually recorded 178-188kVA in the peak months in the second and third years of study. The minimum chargeable MD is 150kVA and this level will be crossed in peak months since the power demand itself will be around 170kW.Heater control may be needed to keep MD below 150kVA during peak production months.

4.4 Cable & Line Losses

The daily copper losses occurring in the major cables are worked out and are found to be at a tolerable level. The total cable loss is around 10 units/day that is around 1% of total consumption.

The resistance factor in these calculations have been calculated by using the per km resistance value of the corresponding cable. The Loss equation is Cable Loss = (kVA)²x L x RF x LLF x HRwhere kVA is the maximum kVA which goes through the cable, L is the length of cable in meters, RF is the resistance factor, LLF is the load loss factor and HR is the number of hours over which the energy loss is calculated. LLF is calculated from load factor in the cable as LLF = 0.7 x LF + 0.3 x LF x LF where LF is the load factor. 
No cables need replacement from loading and loss point of view.

4.5 Transformer Loading and Loss Analysis

The 11KV/433V, /Y, DY11, 4OOkVA transformer has a no load loss of 8OO watts and load loss of 6 kW. It has 4.37% impedance and 4.04% reactance. Its no load current is 1.1% of its rated current.
 

Refer Fig 4.2 for load curve on the secondary side of the transformer during a peak production day. It is seen that the kVA loading in the transformer is about 35% for about 6 hours, about 11% for 6 hours and about 5% for the remaining 12 hours. The peak loading takes place between 8.30 AM to 11.30 AM when hammer mill, Crumb rollers and Drier are operated together. But transformer is underloaded even during this peak load period.

The maximum efficiency of the transformer will be at about 37% load ie, at about 150 k'v'A. Transformer gets loaded to this level only For about 5 hours even during a peak production day.

The net energy loss per day in the transformer is worked out to be 25 Units of which no load loss accounts for 19.2 Units.

4.6 Analysis of DG Set Operation

The firm employs two DG sets of 110 kVA and 55 kVA capacities. The 110-kVA set is used to feed the plant during power failures and low voltage conditions with the full plant load ON ie hammer mills, crumb & skim rollers, centrifuge and drier running.

The 55 kVA is used during power failures and low voltage conditions if the plant load conditions do not justify the operation of 110kVA unit.

From the load analysis during audit it was found that the plant will draw about 90 to 110 kVA with crumb & skim Rollers, centrigue, and drier running. Hence the 110-kVA generator is loaded to its optimum efficiency level when it is employed. However, it is rarely employed.

However, the 55 kVA DG set is underloaded most of the time. The plant is subjected to frequent power failures. Also, it to use this 55 kVA DG set every day between 6.00pm to 10:30PM due to extremely low voltage (330V) it receives from KSEB during period. The only load on this 55 kVA is that of Centrifuge, water pump, and lighting amounting to kW, 15 kVA for this 4 to 5 hour period every day. During peak months crumb processing will extend to the evening & night periods and hence two crumb rollers (25 HP & 20 HP) will also come on this DG set, taking its loading to about 35 kW, 50 kVA. But this happens very rarely due to slightly reduced production levels in the recent years.

The yearly data on self-generation of electricity is given below: 
 

Averaged over the three-year study period, the yearly diesel consumption is about 11,300 litres and average Specific Fuel Consumption is 0.644 lit/kWh (ie 1.55 units/litre).

Thus, the DG set operation is inefficient and the reason is found in under loaded operation of 55-kVA set during evening time (due to low voltage of KSEB supply) for most of the year.

Records maintained by the firm reveal that 10% of total Diesel consumed was used to run the 110 kVA set and 90% was used o run the 55 kVA set during the three year study period. The firm also records that the 55kVA DG set operated at about 10 to 15 kW level for 80% of its duration of operation and it operated at about 35 kW level for the remaining period.

The average yearly total Diesel consumption in both sets put together is 11,300 lit of which 1130 lit goes into 110 kVA set running at, maximum efficienct loading level and 10170 lit goes into 55 kVA set which runs at optimum loading for 20% time and at about 30% loading level for 80% time.

During 1994 Audit it was found that this 55kVA set had an average Specific Fuel Consumption of 0.5 lit/unit i.e 2units/lit and the sets consumed 13,500 lit of diesel per year generating about 27000 units in the average.The DG Set had about 20% efficiency then.The loading pattern was the same then.But now the 55kVA set has only about 15% efficiency for the same loading.It has become more inefficient and that is only to be expected after a gap of 8 years.

From the above data it is deduced that the 55 kVA set runs for 256 hours/yr at about 35 kW load, and for 1024 hours/yr at about 12-15 kW load level. It runs for 4 hours per day n the average. At 35kW output the DG set has about 25% efficiency now and 2.4 unit/lit generation rate. Thus 55 kVA Set consumes about 3733 lit Diesel for 256 hours of operation at 35 kW load in an average year. Hence it consumes about 7980 lit Diesel for 765 hours of operation at 12-15 kW level" in an average year. (ie at a generation rate of 1.13 unit/lit, SFC of 0.9/kWh and efficiency of 10%). Thus operation of 55 kVA DG set is inefficient.

A proposal to replace the 55kVA by a 25kVA was included in the earlier audit.It was not implemented on the ground that it may be required to start the 50HP Hammer Mill on this set at times and it can not be done on a25kVA set.The 55kVA set has become more inefficient over the years and it is a sheer waste of diesel to run this already inefficient DG Set with loading level at 15% or lower.With milling-after-natural-drying scheme implemented there is no reason why the Hammer Mill should be started on 55kVA set at all. Hence the following proposal -

4.7.1 Proposal to replace the 55 kVA DG set by a 25 kVA DG set

If the 55 kVA DG set is replaced by a 25 kVA DG set, the Centrifuge, Waterpump & lighting load will keep its loading percentage close to the maximum efficiency loading level for its entire duration of operation.At least 32% efficiency & 3.2 units/ lit can be expected of the 25 kVA set.

However, this will result in about 20 kW Crumb roller & other load going back to KSEB supply for 256 hours in an year ie an increase of 5120 units per year in the consumption from KSEB.

Diesel consumption by 25 kVA DG set for 1280 hours of operation at 32% efficiency and 3.2 units/lit with 12 to 15 kW load = 3873 Lit

Reduction in Diesel Consumption = 7427 lit

Increase in KSEB Consumption = 5120 units/year

Additional KSEB Bill Amount = Rs. 12,000/- per year

Annual Savings in Diesel Cost = Rs. 1,48,540/- 

Net Annual Savings = Rs. 1,36,540/-

Investment Required = Rs. 1,75,000/-

Recommendation

Analysis of above proposal shows that replacing the 55kVA DG set by a new 25 kVA DG set is attractive in terms of savings in energy and payback. It is recommendd that the 55-kVA DG set be replaced by a new 25-kVA DG set.

4.7 Motor Loading Analysis

The major motors were studied in detail by recording the loading parameters and energy consumed by them using Energy Analysers and Audit Meters.The detailed tabulation of motor loading analysis is given in Table 4.1.

4.8 Oversizing of Motors

None.

4.9 Refrigeration & Air Conditioning

There is no refrigeration and a/c load in the plant.

4.10 Compressed Air Leakage.

The firm does not use compressed air for its operations.

4.11 Industrial Machinery Losses.

- The condition of belt drives was verified to be satisfactory during audit

- The lubrication practice for the side gears and main reduction gearing of Crumb roller & skim roller drives were inquired into and were found to be satisfactory. The frictional losses in the various roller drives were measured to be between 1 to 1.5kW by metering the idle running power input in to the motors.

- The surface temperature of various gearboxes was found to be normal indicating satisfactory lubrication and maintenance practice.

4.12 Lighting System

The total connected Load in the lighting Switch board is 21 kW. This load comprises the lights, fans, coolers, hospital steriliser load etc in the factory, office & recreation club, hospital and Doctor's Bung law etc. The conncected load splitup is as shown:

Factory

1. Flourescent Lamp 40W 42 Nos 

2. 60W Bulbs 13

3. MV lamp (HP) SOW 1

4. Ceiling fans 70W 6

5. Ind/pedestal fan 150W 2

6. Little cooler 125W 1

On a normal working day 15 tube lights for 12 hours, 6 bulbs for 9 hours, 2 ceiling fans for 7 hours, one pedastal fan for 10 hrs and cooler for 3 hours will constitute the actual consumption.

Thus actual energy consumption in factory Lighting = 13.3 unit/day

Office & Recreation Club

1. Flourescent Lamp 40W 23 Nos

2. 60W Bulbs 15

3. Ceiling Fan 70W 17

Actual use - 7tube lights for 7 hours, 4 bulbs for 7 hours and 4 fans for 7 hours.

Energy Consumption ~ 5.6 units/day

Hospital & Doctor's Bunglaw

1.Floures cent Lamp 40W 13 Nos

2.Bulb 60W 21

3.Bulb 403 8

4.Ceiling fans 70W 17

5.Steriliser 1500W 2

6.Fridge 220W 2

Actual Use - 5 tube lights for 3 hrs, 5 x 40W lamps for 3 hrs, 5 x 60 W lamps for 3 hours, 7 fans for 5 hours, 1500W sterilisers for 0.5 hr, one fridge for 9 hrs

Energy consumption = 8 units/day

Total lighting energy = 27 units/day

This will reach 40 units/day during a peak production day due to increased hours of operation in the night in the factory.

The plant already uses energy saving eletronic ballasts for the flourescent lamps.
 
 

5.1 The Crumb Drying Operation

The Crumb Rubber after processing in Hammer Mill get collected in Circulating Tank where it is washed by water jets. The washed crumb is scooped up from the tank by Aluminium trays with perforated bottom and filled in to Aluminium trays kept in a trolley. Each trolley carries two Aluminium trays and each tray holds about 20 kg wet Crumb after sufficient dripping time has been allowed. The natural air-dried Crumb Rubber has 5.7% moisture content (by weight).

The trolleys are pushed into the Drying Oven once in 14 minutes by a timer controlled hydraulic pusher mechanism. The oven length is such that it can hold 23 trays inside at a time. At normal pushing rate, a tray spends about 4 hours inside the oven.(All this for wet crumb processing).

5.2 Drying Operation Description

The drying oven is a rectangular chamber of 9.62m length, 2m breadth and 1.02m height with 2-inch thick glass wool insulation of sidewalls and top surface with both ends semi-open. A rail structure and suitably designed trolley allows movement of trolley through the inside of oven. A hydraulical1y operated pusher mechanism pushes a trolley inside and last trolley inside the oven comes out at the other end with dried crumb in it.

Drying is accomplished by ciculating hot air inside the oven. Air is heated by passing it through a heater box carrying 3 x 30kW electrical heating elements. Heater box is outside the oven.

A 10 HP driven axial fan is used to draw air out of the chamber through a 1m x 0.2m suction duct, pass it through the heater box chamber box and discahrge into the oven chamber through a 1m x 0.2m discharge duct.

The hot air is guided by Aluminium baffles inside the oven and is forced to flow through the material in the trays partially and parallel to the material partially there by effecting drying.

The end openings of drying chamber are usually covered with a metal plate and opened only during pushing of trolleys.

Thus, the oven is a closed air circulating, electrically heated, counter flow, drying oven. There is no intentional exhaust of humid air from inside. However both ends of oven chamber are not fully closed and there is considerable leakage of hot air from one end and seepage of cold air from the other end. Also leakage points exist at all joints of the duct and heater box assembly.

Some extent of exhaust is necessary for proper operation of drying oven. Otherwise the air in closed circuit will get the saturated (since temperature of air is controlled between 102-104 deg C by a thermostat control) and no further moisture removal can take place. The moisture removed from rubber in a continuous manner has to go out of the chamber through hot & moist air exhaust. But hot air exhaust from the closed air system will automatically result in seepage of an equivalent quantity .of cold air in to the circuit and hence results in loss of heat.

Borne of the moisture as well as heat of air get removed by air coming into contact with the relatively cold bottom surface of heater basement. See Fig 5.1 and Fig 5.2 for  sketches of drying oven.

5.3 Energy Balance in the Drying Oven

The only heat generation in the system is in the electrical heater. The heat absorption mechanisms are:

1) Heat spent in raising the body temperature of drier body due to heat capacity. This heat is spent during the heating up transient and is lost when the drier cools down. But this is not a constant heat dissipating mechanism.

2) Heat spent in raising the tempeerature of crumb rubber, trays and trolley room temperature to approximately 100 deg C

3) Heat spent in sensible heating of water contained in the wet rubber

4) Latent heat of evaporation of water content

5) Heat lost through hot air exhaust

6) Heat spent in sensible heating of fresh air inlet

7) Heat loss from surfaces of oven chamber, heater chamber and ducts

8) Heat lost to the basement (earth) by heat transfer from hot air to relatively cold bottom surface in the oven chamber.

5.4 Estimation of Drier Efficiency during peak production

Heater (Drier) operates for about 6 hour during peak production and dries about 1189 kg Naturally Dried Crumb Rubber producing 1125 kg of Dry Crumb Rubber. The initial moisture content is about 6% of dry weight of rubber.

Outside dimensions of heater chamber is 1.5M x 0.95M x 0.95M and outside dimensions of drying chamber will be 9.615 M x 2M x 1.Q2M. Heater surface temperature is about 60 C and drying chamber surface mean temperature is 40 C with an ambient temperature of 30 C. Thermal insulation is 2-inch glass wool throughout. Heat transfer coefficient for exposed surfaces is taken to be 5w/sq.m deg C

Heater surface area = 7.5 m
Heater surface heat loss = 5 x 7.5 x (60-30) = 1.12 kW = 6 units/day
Drying chamber surface area = 43 sq.m
Drying chamber surface loss = 2.15kW = 13 units/day
Total surface heat loss = 19 units/day 
Heat required to raise the temperature of 64 kg water to 100 deg C from 30 deg C = 64x70 x 4.2 KJ = 5.2 units
Heat required to raise the temperature of 1125 Kg rubber from 30 deg C to 100°C = 39 units
Heat required to evaporate 640 Kg water = 64x 540 x 4.2 KJ = 40.3 units

Since it is not possible to evaporate out the water contained in the rubber without heating up the rubber also, the useful output of dryer is taken as heat required to heat up water and rubber to 100 deg C plus heat required to evaporate water.

Dryer output = 5.2+39.5+40.3 = 85 units/day
Dryer input is found to be = 278 units
Total loss in the dryer = 193 units
Surface loss = 19 units

The remaining losses ie 174 units comprise the heat required to heat up trolley, trays, heater structure, duet structure and drying chamber structure to higher temperature levels, heat lost through hot air exhaust, heat loss to the bottom surface of the chamber.

Dryer efficiency = 85/278 = 30.6% 

5.5 Drier Efficiency during lean months

The estimated drier efficiency of 30.7% is achieved only during peak months. The reason for lower drier efficiency during lean months is examined below.

1.The number of trolleys to be processed comes down drastically during lean months. Hence dryer is working underloaded during these months. Some of the trolleys containing partially dried material will be left inside the chamber in the night and this requires switched on next day. Also pushing rate becomes highly variable during this season and throughput rate of Dryer comes down; loss mechanisms remaining the same through out. 

Some times the dryer is operated only during some days rather than every day due to lack of material to be processed. This leaves sufficient time for the dryer to cool down entailing greater energy expense the next time it is switched on.

2. Operation of dryer at low voltage is anoother mechanism responsible for loss of efficiency of the dryer. The firm gets low voltage from KSEB ( 300 to 330V) in the evening period on all days. But during peak production days, the 110 kVA DG set which will be switched on to the dryer also. During lean months, there won't be crumb rolling to be performed in the evening and hence firm will switch on only 55 kVA DG set and will continue to supply the KSEB low voltage to heater.

At 300 to 330V, the heater will generate only 50 to 60% of its rated heat. With heating reduced to this level the air temperature comes to 60 -- 70 C instead of 100 - 105°C. At this reduced temperature, the air gets easily saturated and the differential between dry bulb temperature and wet bulb temperature of air decreases drastically resulting in a very low rate of moisture removal. Hence operating heater at this voltage is virtually useless and results in excessive drying time and thoroughly inefficient operation of dryer.

3. The system frequency is only about 48.2 Hz and the dryer blower motor is in star connection. These two factors bring about a 10% reduction in speed of operation of blower resulting is lesser airflow rate. This also makes Dryer inefficient.

4. The Crumb Rubber contains more moisture during rainy season (lean season) and thus requires more drying time. This also results in higher consumption by drier during this season.
 

5.6 Recommendation for reducing energy consumption in Drier 

The drier is essentially a parallel flow type at present. It is recommended that it be converted into an essentially "through circulation' drier by suitable changes in air ducting. 

In the existing arrangement both the suction duct and discharge jduct for the air are located on the same side of drying chamber at the same height from ground level. Hot air is delivered into the chamber in a direction perpendicular to the direction of material movement and drawn out similarly. In between the entry and exit, the air flows parallel to the surface of wet material there by effecting heat & mass transfer. (See fig. 5.2). Some of 

the air tends to get shortcircuited through the space not occupied by trolleys and this is prevented to a certain extent by mechanical wier arrangements. A trolley contains two trays and by the very nature of airflow the tray farther from air inlet & outlet side gets only less air volume than the other tray. This results in under drying of the material in the farther tray if it is loaded to the same extent as the other one. Hence the firm is forced to reduce the material in that tray. But full compensation for the reduction of material in one tray can not be obtained by increasing the other tray content due to insufficient moisture removal due to increase in material depth. This reduces the throughput rate of dryer.

Semiquantitative analysis was performed to estimate the differences between parallel air flow and through circulation (in which the air is forced to go through the material, there by increasing the heat transfer area) in the dryer.

The relevent theory & formula are obtained from "Chemical Enginees's Handbook1, (Perry & Chilton, Fifth Edition, Mc-Graw Hill Company, Section 20 - Gas Solid Systems).

The heat transfer coefficient, for parallel flow drying

h_cp = 0.01 Gp^0.8/D_r^0.2 

Where h_cp is in Btu/hr.sq.ft deg F

Gp = Mass velocity of air per unit area of heat transfer area in Ib/hr.sq.ft

Dc = 4 x Cross sectional area of heat transfer/Perimeter of flow channel

Using the dimensions of heater Dc is estimated to be 0.037 ft and 

h_cp = 0.0193 Gp^0.8   (1) 

The heat transfer coefficient for through circulation drying is

h_ct = 0.11 G_t^0.59 / D_p^0.41

Dp = diameter of sphere having same surface area as the particle.

The crumb rubber comes in the form of lumps at the input of drier. The size of the lumps vary; however in the average, the arithmetic mean of largest dimension & smallest dimension of a lump is estimated to be close to 7cm. Due to highly irregular & corrugated nature of the surface, this lump is estimated to have 4 to 6 times the surface area of a sphere of same diameter.

Thus Dp = 14 to 17.5 cm = 0.47 ft to 0.58 ft
h_ct = 0.14 G_t^0.59 to 0.15 G_t^0.59

Where G_t = mass velocity of air per unit heat transfer area

Usually the drying proceeds in two stages - a stage of constant rate of drying and a stage of/variable rate of drying which is controlled by the internal moisture movement mechanisms of the object being dried. During the constant rate stage, the rate of moisture removal depends only on heat transfer coefficient between dry bulb & wet bulb temperatures of air and the heat transfer area.

Rate equation for parallel flow.

(Rate)p = hcp(A/l )(t-t_s)

Rate equation for through circulation 

(Rate)t = hct(A^' / l ) (t-t_s)

where 

A = heat transfer area for parallel flow

A^' = heat transfer area of through circulation 

l = Latent heat of evaporation of moisture 

t = drying air dry bulb temperature 

t_s= drying air wet bulb temperature.

The heat transfer area for through circulation is given by A6( 1-f)/Dp

where F=void fraction

a=crosssectional area of flow

and d=depth of bed. 

Void fraction is estimated to be 50% in the present case. It is estimated that the trolleys contained in 1m length of drying chamber will come effectively under the through circulation if the same duets are rearranged for through circulation. The depttj of bed is 1 ft. Thus A & A^' are estimated to be 26.9 sq.ft and about 5 sq.ft respectively. Using these values

(Rate)t / (Rate)p = 8.8 G_t^0.59 /G_p^0.8

The present mass flow rate is estimated to be about 16000 Ib/hr.

Gp ~ 2700 Ib/hr sq.ft

and Gt ~ 3700 Ib/hr sq.ft for same air flow.

Therefore (Rate)t / (Rate)p = 2

The moisture removal rate in the variable rate period is dependent on the moisture diffusion mechanisms inside the material and heat transfer coefficient as well. Since the change proposed is in the mode of air flow only, the same rate ratio can be expected during this period also.

The following conclusions can be arrived at based on the above analysis 

(i) Through circulation will remove twice the amount of moisture removed by parallel circulation in a given time.

(ii) The pushing rate can be doubled and drier operation time halved using, through circulation with same airflow rate and temperature.

(iii) The air temperature can be maintained at a lower level if pushing rate is kept the same using through circulation.

(iv) Both reduction in time of operation and reduction in air temperature setting will result in reduction of drier losses.

Payback Analysis

The average SEC of Crumb Rubber now (in 2002) is 0.57 units/kg as estimated in Chapter 3. The Drier contributes 0.25 unit of this. Only 0.076 unit out of this 0.25 unit will constitute useful output of drier.

If the dryer is converted into a through circulation type the effective moisture removal rate is predicted to be double. But a pure through circulation is difficult to achieve in a drier with side injection of air and only a mixture of parallel and through currents can be expected.

Thus, conservatively, one can expect a 50% increase in moisture removal rate as a result of conversion. This will make it possible to reduce the drier operating hours by 33%, if temperature is kept at its present level. An optimisation between pushing rate and temperature setting should be attempted experimentally after installing an energy meter in the heater panel.

Reduction in losses due to reduced operation time = 0.33x193 units 

(For 1125 kg of dry crumb production) 

= 0.057 units /kg

Average annual Crumb Rubber production = 225125 kg

Annual Energy Savings = 12900 units

It will be possible to maintain MD under 150kVA under these circumstances due to easier scheduling of heater operation made possible by faster processing in the oven. Now the firm is paying about Rs. 50000/- per year as extra MD Charges and this amount will be saved.In addition @Rs. 2.5 per unit there will be about Rs. 32,250/- per year savings from unit reduction.

Total Annual Savings = Rs. 80,000/- approx.

The proposal is viable on a simple payback basis if the proposed modification can be carried out at around Rs. 2 lakhs.

[It may be required to change the blower motor to 15 HP rating. The investment required includes cost of civil works, reinsulation & extension of duets, motor cost etc. But the required rating of motor can not be estimated at this stage and can be decided only after the drier is converted to through circulation type and trail runs are carried out.]

Steps to be followed in carrying out the Recommendation

(i) Shift the hot air entry port to the other side (ie opposite to the side at which ports are located now) of the chamber and locate it at a height such that air inters the chamber at 5 cm level above the trolley.

(ii) Shift the suction port slightly (if needed) such that it is below the trolley bottom level. But keep it in the same sidewall as it is at present.

(iii) It will be desirable to shift the heater box on to the top drying chamber if it will reduce the total ducting required.

(iv) The hot air in let port should have 1.5 times the length that it has now. The outlet (ie suction) port length should be kept unchanged. The width in both cases should be left unchanged.

(v) The hot air inlet should be moved into in the direction opposite to direction of material flow and outlet should be moved away from material entry side such that centre to centre distance of the ports is reduced to 2m.

(vi) After completion of conversion find the optimum setting for pushing time and temperature by trail runs. (An energy meter in the heater parel will be necessory for this). Also monitor the loading in the blower motor and decide on replacing it by a higher HP motor if necessary. If ducting can be reduced in length by mounting heater box on top of the chamber a change in rating of the motor will be unnecessary.

Note: -- Payback has another favourable aspect. Due to reduced drying time (ie faster pushing rate), it will be possible to finish the drying process before the low voltage period in the evening during the lean and low production months. This will reduce the inefficient energy consumption by drier at low voltages during lean months. Thus the actual energy savings will be more than the projected value.

Notes- 1. All possible points of hot air leakage and cold air seepage should be sealed properly during the conversion stage. The vent has to be located near the material exit door on the top surface of drying chamber. A hood connected to exhaust should be used at material exit door to convert unintentional exhaust into an intended one.