6 CASE STUDIES

6.1 Reduction in demand of steam in belt dryer system

The following case study is from a chemical industry belt dryer. Optimising airflow to the requirement helped in reducing the useful heat carried away by the excess airflow. Drying processes where falling rate of evaporation time is significant, reduction of airflow can be a good energy saving option. During the falling rate period of drying, the moisture diffusion from inside the material to the surface is predominant and this is a function of more of the material properties than external conditions like airflow

.

The energy inputs to the system are:

Steam for air heating	300 kg/hr
Electrical energy for FD	8.5 kW
Electrical energy for ID	3.4 kW

Initial performance parameters were established by field measurements and the performance was monitored for few batches. Following is summary of performance parameters at present conditions:

1. Air flow rate	17,000 kg/hr
2. Moisture evaporation load	27 kg/hr
3. Effectiveness of dryer	10.90%
4. Effectiveness of steam consumption	9.50%
5. Efficiency of FD fan	29.20%
6. Efficiency of ID fan	26.20%

Recommendation:

Start the circulating fans in each section and cut down the blower flow rate. Replace the blower with smaller size blower. Following is summary of performance parameters at after modifications conditions:

1. Air flow rate	5000 kg/hr
2. Moisture evaporation load	27 kg/hr
3. Effectiveness of dryer	40%
4. Effectiveness of steam consumption	38%
5. Efficiency of FD fan	60%
6. Efficiency of ID fan	26.20%

Energy savings:

The energy consumption of proposed system shall be:

• Steam for air heating : 90kg/hr

• Electrical energy for FD : 2 kW

• Electrical energy for FD

Steam Saving	215 kg/hr (i.e. 72 %)
Yearly fuel Saving	Rs. 3,87,000/-
Electrical saving	9 kW (i.e. 75%)
Yearly electrical Saving	Rs. 3,24,000/-

Comments: This is a typical example where reduction in demand by modification in the end application leads to a mammoth saving of 75%.

However, note that for a spin flash dryer, reduction in airflow should be done carefully, as the heat and mass transfer rates in SFDs are closely linked with the airflow.

6.2 Improvements in Cylinder drying- textile Industry

The study conducted on a 17 cylinder dryer (0.56 m dia and length 2.26 m each) is given below.

Case-A shows actual performance before modifications and Case-B shows the results. A 0.69 m wide cloth weighing 0.1322 kg/m on bone-dry basis was dried from 85.5% moisture to 6.5% moisture on bone-dry basis.

The modifications where

1. Stopping of steam leaks

2. Reducing machine stoppages

3. Insulating cylinder ends

Summary of dryer performance before and after the modification is given below.

Description	Case A	Case B
Machine run time, minutes	150	180
Machine stop time, minutes	30	Nil
(Machine stopped, steam ON)
Production time utilisation, %	83.8	100
Running speed, m/minute	40.8	40.8
Production
(i) Meters	12240	14688
(ii) kg	1617.7	1941.3
(iii) kg/h	539.2	647.1
Average evaporation, kg/h	426	512
Steam pressure, bar	2	2
Average steam consumption, kg/h	840	763
Specific steam consumption, kg/kg of cloth	1.558	1.179
% steam saving	-	24%
Actual steam saving, kg/h	-	231 kg/h
@Rs 0.5/kg steam, monetary savings	-	Rs 115/h
@3000 hours/annum, annual savings	-	Rs 3.45 lakhs

Investment required was minor for arresting steam leaks/repairing steam traps. Payback period was less than 4 months.

The following points are to be noted.

 Productivity of a machine influences specific energy consumption

 First priority should be given to stopping all live steam leakages through trap and rotary joints

 Steam consumption could have been further reduced if incoming moisture was reduced to 60 to 70% level instead of 85.5%.

 Practically about 1.6 to 1.8 kg steam/kg evaporation is required in cylinder drying

6.3 Improvements in hot air drying of fabric in Stenters- Textile Industry

In a Textile plant, Improving mechanical dewatering, before stenter drying, by retrofitting a suction slot was implemented.

The stenter-drying heater, fired by natural gas, gives a heat output of 967 kW (3.3 million BTUs/hour), in the form of heat transfer fluid at a maximum temperature of 377 °C, to serve the stenter’s heating requirements.

The plant modifications involved fitting the suction slot equipment to the top of the mangle assembly so that it came within the fabric path before the stenter. The suction slot is basically a system of dewatering by use of vacuum exhausters having capacity of 100 cfm and 12” mercury column.

Although the complete stenter range could be operated with or without the suction slot, it was immediately apparent that the production rate for one of the main quantities of fabric could be increased by about 50% with the suction slot operating.

Operation of the suction slot increases the electrical load used for drying by approximately 25 kW.

Energy savings of GBP 17,500/year (1989 prices)

• Benefits through increased productivity of GBP 99,200/year (1989 prices)

• Payback period of 3 months on all benefits

• Payback period of 19 months on energy savings alone

Table 6-2: Energy requirements per tonne of fabric produced

Fabric type	Mangle only, (GJ/te) average	Suction slot, (GJ/te) average	% Energy saving average
Polyester and nylon non-woven	28.15	14.02	49.6
Nylon woven	11.79	5.57	49.1
Polypropylene woven	11.19	9.49	12.9

6.4 Paper Machine Dryer Improvements

DRYER SECTION DESCRIPTION.

Inland's Maysville, Ky., mill produces 26-lb to 69lb 100% recycled linerboard on its No. 1 paper machine, which has a trim width of 289 in. at the reel. The dryer system has 55 dryer cans divided into six steam sections. The first section has five dryer cans, two of which are not heated. The remaining sections have ten dryer cans each. With the exception of the unirun section, the dryer drives are "silent-- drive" type, meaning the motor only drives the last four cans in each section and the remaining cans turn due to friction with the felts. Rotary siphons were installed on all dryers at startup in 1992. The third and fourth section dryer cans were also equipped with turbulence bars. The mill uses a recirculating thermocompressor differential pressure control system to remove condensate from the dryer cans. With this system, condensate is drawn out of the dryer cans, since a lower pressure is maintained in the condensate flash tanks.

To maintain the differential pressure, the blow through steam is recirculated into a thermocompressor where it is recompressed and injected into the steam header.When the thermocompressor valve is wide open and differential pressure is still below set point, excess steam is discharged to the off-machine silos for heating. When the silo valve is wide open and the differential pressure is still below set point, the blow through steam is then vented to the atmosphere.

Dryer Flooding Challenges. As the Maysville mill's No. 1 paper machine was optimized and production increased, dryer flooding became increasingly problematic. The mill's paper yield task team identified several issues associated with this flooding:

Energy loss. Dryer can flooding caused extensive energy loss. Differential pressures as high as 18 psi in the early dryer sections were required to evacuate the dryer cans.This resulted in excessive blow through steam. Also, venting was a serious problem during production of linerboard weights of 33 lb and lower.

Machine speed limitations. Dryer flooding limited machine speed on the No. 1 paper machine. When the dryer drive motors became loaded due to dryer flooding, the machine speed was reduced to regain normal operation.This occurred when producing linerboard weights of 42 lb and heavier. Also, when dryer cans flooded during operations, it could take as long as ten hours to drain them.

Delayed threading. Dryer flooding during sheet breaks delayed threading and extended the time required to bring the machine back up to speed. Leaking steam valves caused dryer flooding in all the sections. The second section flooded in minutes after shutdown, because the silent-drive allowed the rotary siphons to stop at positions other than six o'clock. This section took 30 minutes to one hour to drain during a shutdown. During startup, dryer flooding delayed threading and caused repeated drive trips while the section was brought up to speed.

Felt wear Dryer flooding caused excessive wear on the silent-drive felts of the second section. Flooded dryers in the second section required more force from the felts to turn them. By 1997, the felts on the second section that had an expected life of 200 days were splitting at the seams within 60 days.

Stationary Siphons. The use of rotary siphons in the silent-drive second section had been a concern since the 1992 startup of the No. 1 paper machine. The stationary siphons on the market at that time were considered unreliable due to siphon shoe erosion, vibration, and other mechanical problems. However, by 1998, stationary siphons were considered a reliable, efficient, and low-maintenance alternative to rotary siphons.

The paper yield task team investigated stationary siphons as a means to reduce the mill's dryer flooding problems. Because condensate does not have to resist centrifugal forces to exit the dryer cans through stationary siphon systems, less differential pressure is required to evacuate condensate out of the cans. Consequently, there is less blow through steam and steam venting.

The stationary siphons are particularly suitable for use on silent-drive sections, where rotary siphons may not stop in the six o'clock position during shutdowns. The paper yield task team estimated that significant savings could be achieved for second section felts by converting from rotary siphons to stationary siphons.

The paper yield task team evaluated and received good recommendations on several stationary siphon vendors. Because of factors such as low-maintenance design, ease of installation, and level of service, the team recommended conversion of the rotary siphons to Deublin stationary siphons. The conversion was approved in two phases. Stationary siphons and turbulence bars were installed in 13 dryer cans in August 1999.The next 30 dryer cans were converted for Phase II in March 2000.

Turbulator® Tube™ Bars are stainless steel axial bars, installed on the inside of dryers and used to induce turbulence in the condensate layer, to improve the uniformity and rate of heat transfer (drying rate) of the dryers. The increased heat transfer rate is also much more uniform in the cross-machine direction than any other internal dryer configuration.

A primary consideration for recommending stationary siphons was the maintenance required on the steam joints. The existing dryer system required extensive maintenance and one or more dryer cans were constantly valued out for repair. The paper yield task team saw the Deublin joint as an attractive alternative, because it allows seal face measurement to assess joint wear and predict remaining seal life,This measurement permits seal replacement as required, not after failure or earlier than necessary to avoid failure. The joints were also designed to minimize loading and friction on the two seal faces to extend seal life and minimize maintenance.

RESULTS. During Phase I of the installation, an unexpected bolt pattern on 12 of the 13 dryer cans required that the pattern be measured so the steam joints could be re-cut. For Phase II, the supplier provided joints with both bolt patterns, so the installation went more smoothly. The task team was very pleased with the results of the dual bolt pattern. Since the first conversion in August 1999, there has been only one stock issue for Deublin parts, and it was a result of the original incorrect bolt pattern and not a typical repair.Along with a noticeable reduction in required

maintenance, the following benefits from the conversion have been noted:

Energy savings. There was a substantial improvement in the lb steam/lb paper consumption after Phase I and further improvement after Phase II. There was a significant reduction in blow through steam and venting. Steam savings amounted to 200 lb steam/ton of paper produced.

Machine speed increase. After Phase IIi, there was a big reduction in steam required to dry the sheet. Before the conversion, the dryer can maximum allowable pressure limited the drying rate and machine speed. After the conversion, improved heat transfer from a minimized condensate layer resulted in the same drying rate at lower operating pressures. This enabled a significant machine speed increase.

No dryer flooding. There were no more instances of dryer flooding in the sections converted to stationary siphons. After Phase I, the maximum differential pressures required to evacuate cans was reduced from 18 psi to 10 psi. After Phase II, operators lowered differential pressures as low as 5 psi.The operators have been reluctant to reduce differential pressure further due to the risk of dryer flooding.

"Silent-drive" felt performance. Felt roll damage continued to limit the second section felt life until the damaged felt rolls were replaced in: March and August 2000. Since that time, the second section felts are running until scheduled off after the expected life is reached.

6.5 Paper machine dryer modifications and improved control system

A case history was presented on the No. 26 Stora Enso Biron paper machine DMS ( Dryer management System) installation (also profiled in the July 2004 issue of Pulp & Paper, p. 45). The No. 26 unit is a Voith lightweight coated machine with two on-machine coalers. The machine produces 30- to 45-lb (45- to 65-gsm) grades at speeds up to 4,200 fpm (1,280 mpm).

Phase I of the rebuild, completed in June 2003, included modifying the steam and condensate system and installing the DMS. Wet end steam joints and syphons were replaced in May 2004. The remaining steam joints and syphons were replaced in January 2005. Wisconsin Focus on Energy funded a Johnson Systems dryer section study to identify potential energy savings. Results from the system installation included:

• Consistency of steam and condensate system operation greatly improved
• Annual energy savings of $75,000
• Dryer flooding eliminated
• System startups are reliable; sheet break recovery time reduced by three to four minutes
• Reduced maintenance adds up to $90,000 savings annually
• Raw stock sheet threading has improved
• Less shift-to-shift variability of the paper produced
• Improved coater tail threading due to calculation of required steam pressure
• Total annual savings of $263,000 due to reduced energy consumption, lower maintenance cost, and higher production.
• Project payback was seven months.

6.6 Heat recovery from exhaust gas in a spray dryer- Chemical Industry

A spray dryer having 400 tons/h water evaporation capacity is used to dry inorganic salts. The input air is heated by direct gas firing to between 200 and 300 C. The feedstock was having initial temperature between 20 and 60 C and a moisture content of 40 to 60% by weight. The feedstock enters the dryer through a rotating disk atomiser.

The dried solid is separated from the exhaust air in a cyclone. The exhaust air has an average temperature between 100 to 112 C.

A heat exchanger was installed to recover heat from the exhaust air, to preheat the incoming air. A schematic of the system after modification is given below in fig 6.1.

Figure 6-1: Heat recovery- Spray Dryer

The heat recovery device used was a glass tube recuperator. The dryer exhaust air flows upwards through the inside of the tubes. The glass tubes were used essentially to prevent corrosion of tubes due to salty vapors.

It was found that after the installation of heat exchanger, the gas consumption in dryer was found to reduce from 60.7 m3/h to 40.6 m3/h. For an average production rate of 300 kg/hr dry products, the specific energy consumption reduced from 6.6 MJ/kg to 5 MJ/kg.

6.7 Waste Heat Recovery from CHP

The site consists of underground mines and a nickel concentrator. The plant utilised three diesel fired spray dryers for drying nickel concentrate from a moisture concentration of approximately 30% down to 0.5%. In 1996 approximately 250,000 tonnes of nickel concentrate was dried utilising around 8.5 million litres of diesel. This provided an efficiency

of drying of around 1.3 GJ per tonne of concentrate dried.

In 1997, the plant commissioned a 42 MW gas turbine and a project for utilisation of the waste heat available from the turbine exhaust gases for drying of nickel concentrate was commissioned.

Following the commissioning of the gas turbines and subsequent utilisation of the waste heat gas diesel usage dropped significantly. Natural gas was then used in place of diesel for the supplementary firing required beyond the heat available from the gas turbine exhaust. By 1998 the production throughput had increased to around 300,000 tonnes of concentrate. With the use of the available waste heat and the conversion to natural gas supplementary firing energy had been reduced to below 0.4 GJ per tonne of nickel concentrate dried. This represented a reduction

in fuel use of approximately 270 TJ of diesel.

6.8 Energy saving in Spin Flash Dryer System-Blower: Chemical Industry

The plant manufactures CPC blue powder. The cakes from the filter press are manually conveyed to the dryer. Heat source of dryer is thermic fluid circulated coils. Drying time was 4 hours.

The blower draws atmospheric air through a filter and the heating coils into the dryer and exhausts out through bag filters located after the dryer. The blower was rated for 75 HP. Airflow was measured to be about 22,000 m3/h and actual power input to the blower was 51.5 kW. The blower operating speed was 2400 rpm with a pulley diameter of 8.5 “and motor side pulley of 12” dia.

During the study, it was noted that the suction damper of the blower is partially closed. It was suggested to avoid damper control and reduce the speed of the blower to save energy.

Initially the blower speed was reduced to 1700 rpm. Operation of the dryer was observed to ensure production and quality parameters. It was found that that the drying time was increased by 20%. This was not acceptable. Clearly, airflow has reduced due to the fact that speed reduction was not optimum.

A 12” pulley was installed on motor and operation of the dryer was observed. Blower speed is now 2000 rpm. Power input was 41.5 kW. There was no change in production time or quality and hence the measure was accepted.

Total energy saving was 80000 kWh/annum. I.e. Rs 3.7 lakhs/annum. Investment for new pulley was only Rs 10,000/- with a payback period of 10 days.

6.9 Improved Mechanical dewatering to save energy in Rotary Dryer- Beet Sugar Industry

This case study from a British Sugar Mill shows that the energy requirements of removing moisture by mechanical dewatering techniques are generally insignificant compared with those needed to evaporate moisture. This means that as much moisture as possible should be removed mechanically (i.e. pressing, filtering, sedimentation, etc.) prior to entering a dryer,

especially if the initial moisture content is high. This principle can be illustrated by the use of screw presses in processing pulp, a by-product of the extraction of sugar from sugar beet that is used as cattle fodder.

Sugar beet to be processed was cut into thin slices before going into the diffuser where the sugar was extracted. The remaining pulp was sent to screw presses, which reduced its moisture content from 8.5 to 2.3 kg/kg (dry basis). After pressing, molasses were added to increase the nutritional value of the pulp which was then sent to rotary dryers to be dried to a final moisture content of 0.3 kg/kg (dry basis). The plant operated 6 presses and 3 rotary dryers

for this purpose.

The water expelled from the wet beet pulp by pressing was 8.69 kg/s. Each press required approximately 0.2 MW of electrical power. Hence the specific energy consumption of removing water using a screw press was 21.1 kJ/kg of water.

The evaporation rate in the rotary dryer was 6.88 kg/s. The evaporation of 6.88 kg/s of water in a single rotary dryer required 20 MW from a direct fired heater. Therefore the specific energy consumption of removing water in the rotary dryer was 2,840 kJ/kg.

The percentage energy saving by using mechanical dewatering was therefore:      8.69

6.88 + 8.69  = 55.8%