Selection of induction furnace in foundry

2017-11-30

  Abstract
  The intermediate frequency induction furnace is going to be applied increasingly in modern foundry.Discussion of the influencing factors during the selecting process of the induction furnace to be taken in this article:furnace size and power capacity and the relationship of process requirements,the type of power supply,the configuration between power and furnace configuration and its matching with process needs.The discussions on the melting rate and productivity for induction furnace,which to be confused frequently during selecting of induction furnace in foundry,was taken in detail.Hence the concept of power utilization coefficient K was introduced.In addition,the integrity of the system functions need to be considered in the selecting process,the security,advanced,economical and environmental adaptability also discussed in detail.
  Key words
  Foundry,induction furnace,selection of furnace type,intermediate frequency power supply
  1.The Intermediate frequency induction furnaces are applied widely in modern foundry
  With the rapid development of solid-state intermediate frequency(shorted as IF below)power supply technology,the conversion efficiency has been gradually increased to the current 96 to 97%,even to 98%.Its operational safety is fully able to meet the needs of production,and its investment cost has decreased year by year to have been lower than the mains frequency(50 or 60 Hz,shorted as NF below)power supply;Taking into account in its many advantages of making it since the late 1980s of the last century been widely applied in Europe,America and other developed countries,basically replaces the conventional NF induction furnace.Since the mid-1990s,with the rapid development of China's electronic technology,the domestic high-power solid-state IF power supply has been the success of the development and application.Therefore,the conventional NF induction furnace in China has disappeared from the sales market,the existed NF induction furnace in foundry also gradually substituted by IF induction furnace during technical transformation.
  Because the advantages of IF power supply:low-cost,easy to control,small footprint,possibility to connect with computer-controlled management system and others(see Table 1),even the traditional NF power supply(50Hz)of channel induction furnace was substituted by solid IF power supply(output 50 to 200 Hz)since the mid-1990s abroad.The first set of channel induction furnace equipped with solid state IF power supply(50~150 Hz)has also come out in China in 2002,this channel furnace was also equipped with computerized automatic melting process control management system,all the furnace running status,lining sintering process and its erosion status,power input,and the melting temperature can be monitored and controlled.
  Table 1 Comparison between NF and IF induction melting furnace(for iron/steel melting)
 

No.

Comparing item

IF furnace

NF furnace

Comments

1

Power density

600 ~ 1400 kW/t

300 kW/t

The allowed configurable power density depends upon the working frequency. Detail see table 2

2

Melting process

By batch melting

by residual liquid

See Note.1

3

Requirement to charge size

Less of requirement

Larger charge size

 

4

Melting power consumption

500 ~ 550 kWh/t

540 ~ 580 kWh/t

Lower consumption owing to higher power density, higher melting rate and lower heat loss for IF furnace

5

Power regulation range

0~100% step-less regulation

Step regulation

NF furnace also needs three phases regulation when power regulation to be taken

6

Power auto adjusting

available

difficult

 

7

Stirring effect of liquid

Adjustable

Higher and fixed

The stirring effect depends upon working frequency and the higher frequency, the lower stirring effect

8

Occupied space of power supply cabinet

30 ~ 40%

100%

 

9

Maintenance of power supply

less

frequently

 

10

Fault Diagnosis and protective function

Available

Partial available

 

11

Possibility to link with computerized managing system

available

difficult

 

12

Ration of investment

~ 90% or less

100%

 

 
  Note.1 The batch melting process in IF furnace means that the molten liquid in IF furnace could be emptied each time.Its advantages are no need of starting block in cold starting process,less of limitation to charge size and less time of overheating for liquid.The starting block to be needed in NF furnace,thus each time 1/3 of molten liquid needs to remain for next melting process.
  Table 2 Allowed power density in different working frequency(for iron/steel)
 

Working frequency (Hz)

1000

500

250

125

50

Furnace size (t)

0.2~1.5

0.6~6

1.1~18

2.5~60

8~100

Power density (kW/t)

1345

945

670

475

300

 
  Thus we can have the conclusion that the higher working frequency,the higher allowed power density.
  The higher power density of 600~800 kW/t(even 1000 kW/t for smaller size furnace)are selected in the IF furnace manufactured by Western furnace manufacturers,and 600 kW/t of power density were selected by Chinese furnace manufacturers.The main two factors are considered:the service life for lining and the production management,because the higher power density,the more severe corrosion to lining.
  As can be seen from the above analysis,either from the technical and operating performance,or from the investment terms,the IF induction furnace posses the undisputed position as a melting equipment in modern foundry.
  2.To determine the furnace size and power capacity
  There are many technological factors should be considered when determine the furnace size.Basically two conditions need to meet:the maximum casting weight and matching with the hot metal demand in production.Due to the higher power density of the IF induction furnace configuration,basically the melting period can be done in less than one hour if its configuration power density is greater than 600 kW/t.
  The furnace power capacity is basically determined based on productivity.Once the furnace size and productivity is determined,the configuration power of the induction furnace could be calculated according to the following formula(1).
  3.Selection of types for power supply
  There are two types of categories IF power supply:the solid state power supply with parallel inverter circuit and that with series inverter circuit.
  Because these two types of inverter power supply have different performance,so there are also different performance for those induction furnace equipped different types of inverter power supply..
  XMEI**can provide two types of power supply---PS type solid state power supply with full-bridge inverter in parallel circuit and CA type solid state power supply(SPS)with IGBT half-bridge inverter in serial circuit.The table 3 below shows the comparison of both types power supply(shorted as SPS below).
  So,the user in foundry can take right selection of IF induction furnace equipped different SPS according to their individual process demand.Table 4(Application of both types of power supplies)will show you their application in foundries.
  Table 3 Comparison of power supply with parallel circuit and serial circuit
 

Item

PS typeSPS
with parallel syntonic circuit

CA type SPS
with serial syntonic circuit

Capacity

160 ~ 8000 kW

50 ~ 4000 kW

COS φ in network side

Approach 1 at full power, Cosφ will go down if power down

Approach 1 at any power

Interfere of harmonic

same

same

Coefficient of SPS.

Same when middle power
Lit bit lower when larger power

Same when middle power
Lit bit higher when larger power

Adaptability with load

Limitation

Excellent

Capability of constant power output

Power is lower when cold charge starting, constant power output is possible but need higher technology

Constant power output during whole melting period

Range of working frequency

Normal 200~ 2500 Hz
Mainly for melting operation

Wider range of working frequency,
Max. 100 KHz
Mainly for melting, holding, quench, heating

Stability

High stability.
Higher Anti-disturb

Less than PS type

Current of components and its protection

Lower current (1/4 of coil current)
Good protection

Larger current (equal to coil current)
Expensive protection measurement

Safety factor of transformer

1.2~1.25

1.1

Possibility of power sharing

No

Yes

Coil voltage

lower

Higher

Price

100%

120 ~ 130%

Price of spare parts

Lower

Higher

 
  So,there are different application because their different performance,table 4 will show their application in foundries.
  Table 4 Application of both power supply
 

Furnace

Suggested SPS

Comments

Melting furnace with lower, medium power

CA type SPS

Higher performance

PS type SPS

Lower price

Melting furnace with larger power

PS type SPS

Higher reliability

Power sharing system

CA type SPS

Only selection

Holding furnace

CA type SPS

Higher COS φduring lower power

 
  4.The configuration of induction furnace with power supply
  In order to adapt to different casting process needs,also in order to fully improve the power utilization efficiency(discussed in detail in the next section),there have been many schedules of IF power supply and induction furnace configuration program,which is set out in Table 5 below.
  Table 5 Configuration of furnaces with power supply

No.

Schedule of configuration

Comments

1

One set of PS type SPS matches with one set of FN

It is most simple configuration of SPS and FN.
In case of lower productivity it is used suitably.
It is applied in smaller capacity FN and in ferrous industry.
The power utilization coefficient K2 in operation process is lower.

2

One set of PS type SPS matches with two FNs through switch

It is an economic configuration for SPS and FN.
One FN is running and another FN is doing maintenance or lining reinstallation.
In some case both FNs can be running in melting/pouring and charging operation alternately to providing metal liquid to casting production line.
The power utilization coefficient K2 in operation process is more high than that in No.1.

3

Two sets of PS type SPS (one melting SPS “M” and one holding SPS “H”) match with two FNs through two sets of switch. Thus one FN is running for melting and one FN is running for holding/pouring

In this case two FNs are running in melting and holding/pouring operation separately. It is applied in foundry widely and can reach the same effect as No.4 but lower investment.
The switching of FNs are done by Switches.
The power utilization coefficient K2 in operation process is higher.

4

One set of CA type SPS matches with two FNs.

CA SPS is a power supply with power sharing system, that means one power can provide power to both FNs by different power ratios at same time. It is a update type of SPS.
The advantages of CA type SPS are as follows:
1. Each FN is running at proper power to matching with its working condition;
2. There is no mechanical switch and is running reliably;
3. Theoretically, the power utilization coefficient K2 in operation process is 1.0, thus increase the FN productivity greatly;
4. Comparing with No.3, there is only one set of transformer and one set of water cooling tower to be needed, thus less of space to be occupied.

 
  5.The relation between Melting rate(M)and Productivity(N)
  It should be noted that the generally the induction furnace manufacturer provided the malting rate as the furnace melting performance or its melting capability in their catalogue or technical specifications.The melting rate is the performance of the induction furnace itself,It is related to the power capacity and type of power supply and it has nothing to do with the melting operating system.And the furnace productivity related to the furnace melting rate,but also with the melting operating system.Generally,the melting operating cycle covers a certain time of no-load auxiliary time,such as:feeding,skimming,sampling and the waiting time for the test results as well as the time waiting for pouring.These non-load auxiliary time reduced power input,ie a reduction of the furnace melting capacity.
  For the sake of clarity of statement,we introduce the concept of the power utilization efficiency in furnace K1 and the power utilizationcoefficient in operationK2.
  Power utilization efficiency in furnaceK1 is the proportionof the output power of SPS toits rated powerin the entire melting period,it is relevant to the type of power supply.Usually,K1 value is around 0.8 for those IF induction furnace configured with SCR full-bridge inverter in parallel circuit.XMEI improve its performance and increase the inverter control(usually rectifier control only),this value(for PS type SPS)is close to about 0.9.And K1 value(for CA type SPS)can reach 0.95~1.0 theoretically for those IF induction furnace configured with theIGBT half-bridge inverter in serial circuit.
  Power utilizationcoefficient in operationK2depends upon many factors such as process design in melting plant,the management level and the configuration schedule of furnaces with SPS.Its value is equal tothe ratio of actual output power and output power of SPS in the entire operating cycle.Typically,power utilization coefficient in operation K2 value is between 0.8 to 0.9,and the shorter of non-load furnace auxiliary operation time(for example:feeding,sampling,pending laboratory test,waiting for pouring etc.),the higher the K2 value.In the table 4 schedule 4(two running furnaces configured with CA type of SPS)K2 value theoretically up to 1.0,and actually it is about 0.9 in the case of lower non-load auxiliary time.
  The formula listed below shows the relation between melting rate and productivity:
  M=P·K1/p(t/h)
  N=M·K2(t/h)
  Among formula:
  M–-melting rate of furnace(t/hr)
  N–-productivity(t/hr)
  P–-rated power of SPS(kW);
  K1–-power utilization coefficient in furnace,generally,its value to be 0.9 for PS type SPS and 0.95~1.0 for CA SPS;
  K2–-power utilization coefficient in operation,generally,its range to be 0.8~0.9;
  p––-power consumption(kWh/t).
  Take example,one set of 2500kW SPS to match with 5t furnace.
  The type of SPS is PS series,power consumption is 520 kWh/t when melting iron,K1 is 0.9,K2 is 0.85,Thus the productivity will be:
  The melting rate M=P·K1/p(t/h)=2500·0.9/520=4.33t/h and
  The productivity N=M·K2(t/h)=4.33·0.85=3.68t/h
  Therefore,the productivity is lower than melting rate duo to the power utilization coefficient in operation.User has to pay attention to this points during selection of induction furnace.
  5.Evaluation of furnace selection
  As an important facility in foundry,the invest to melting furnace occurs a larger percentage among the total investment in foundry,therefore in addition to the above mentioned factors,some other function evaluation to furnace system such as its security,advanced nature,economy,and adapt to environmental protection should be taken during selecting process of furnace system,of course this evaluation should carry out on basis of the background of the investment scale for foundry,management level,technology level etc..
  Following is a brief discussion on these aspects:
  l Security of furnace system
  The full protection function to the mechanical section in furnace system should contain:application of closed water cooling circulating system,the monitoring and alarming system to the outlet temperatures and flow rate of cooling water,emergency cooling water tank and piping,the security measurements of hydraulic system(the protection measurement in case of broken of hydraulic rubber hoses(speed limited valve),dual hydraulic pumps,application of flame retardantoil),sturdyfurnace steel structure and application of coil grout etc.;the full protection functions to the electrical section in furnace system includes:full-digital control master board with real-time multiple monitoring and protective functions to all electrical parameters,fault self-diagnosis function,lining leakage monitoring/alarming system,reliable cooling measurements to SPS including capacitors.
  l Advanced nature of furnace installation
  The advanced nature of furnace installation should adapt with the advanced degree of overall equipments and the management level in foundry.The SPS configured with full digital control board will improve the stability and reliability to the furnace operation(lining service life and melting operation)greatly.In addition,the application of many advanced technology and facilities,such as power sharing system,remote control technology,computerized melting managing system,worn-lining quick removing device,pneumatic lining installation tools,liquid auto-weighing system,auto-sintering system of new lining etc.,will also improve the stability and reliability to the furnace operation greatly.The application of all these advanced technology provides the effective means of quality managing system for casting production.
  l System economy
  The pros and cons,which to be caused bythe higher primary investment duo to the selection of advanced furnace installation,the lower running and maintenance cost because of the selected advanced furnace system and its increasing of productivity,should be evaluated comprehensively and rationally.The evaluation could be taken from following aspects:
  a)To evaluate the recovery cycle of increased invest duo to the selection of advanced technology by calculating the saving of running cost in whole year on basis of power consumption decreasing.
  b)To evaluate the economic benefit in aspect of increasing productivity duo to the increasing of power utilization efficiency in operation if the power sharing system to be selected.
  c)To evaluate the benefit in aspect of saving in dairy maintenance cost duo to application of furnace system with advanced technology and good security.
  d)To evaluate the benefit of lining running cost duo to application of advanced auto-lining sintering system and pneumatic lining installation device.
  l Environmental improvement
  There used to have larger size induction furnace configured with higher power capacity in modern foundry,which used to have greater productivity.The result is possible harmonic pollution to network.The harmonic interference degreeof the electrical network caused by SPS mainly is accepted by whether the harmonics pouring into the network exceeds that required by the national standard regarding to the electrical network harmonic interference degree(see GB/T 14549-93 about the quality of electric energy,the harmonics of the civil electric network)
  To limit or reduce the harmonic current caused by SPS,which enters into common joint point in network,the following measurements could be taken:
  n To increase the rectifying phase that may eliminate the 5th,7th.17th and 19th harmonics
  n To equip with the resonance filters that may filter out the major harmful harmonics,such as to install the 5th,7th and 11th harmonic filters
  n To change the feeding point on which the furnace is connected to the network,thus link the SPS to a civil power network with bigger short circuit capacity.
  ________________________________________________________
  Present article was published on《Foundry Technology》2003 No.2,in China

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