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When processing metal for cast-iron needs with air plasma cutting devices basic characteristics are:
- cutting speed;
- energy consumed;
- maximum of handled metal;
- use of consumables;
- amortization costs;
- total costs, including rent, transport and salary to workers.
These total costs won't be examined in present technical and economic assessment due to wide range of prices. Consumer can assess these costs on his own.
"Cutting speed"
Features which influence cutting speed:
- handled metal;
- gage of handled metal;
- voltage of supply network;
- cutter's (operator's) experience.
For PURM-180М device connection of cutting speed and gage of the handled metal is presented on graph 1.1.
Graph 1.1. Connection of metal cutting speed and gage of the metal:
Graph shows minimum values of cutting speed for carbon steel.
You can increase cutting speed (to approach maximum value) by means of the following:
- automation of the process (uniform feeding of plasmatron, fixed distance between plasmatron and metal);
- continuous power supply due to network voltage not less than 380V.
"Maximum gage of the handled metal"
Maximum 56 mm gage of the handled metal tells us that starting to cut "from an edge" an operator can cut through the metal with this gage applying minimum cutting speed (about 1 mm/sec). Here thermal conductivity of the metal should be comparable with thermal conductivity of structural steel. For example, maximum gage of handled metal for copper and its alloys - 30-40 mm. Such reduction happens because of quicker heat abstruction out of the cutting area. Maximum gage of handled metal is very sensitive to the voltage of power supply (power on arc). The lower the voltage the lower the lesser is the metal gage which can be cut through by the device.
"Use of consumables":
Use (consumption) of consumables depends on:
- cutter's (operator's) experience;
- purity and humidity of air;
- metal gage.
Below is graph 1.2 which can approximately assess cutting length with one cathode (electrode) depending on metal gage.
Graph 1.2. Connection of cutting length with one cathode and gage of the handled metal:
Cathode's life time can be calculated in two ways:
- 100 minutes of continuous cutting with 160-180 А current, pure and dry air (50% humidity) and one activation of plasmatron;
- 360 activations of plasmatron (starting current exceeds rated current several times thus reducing cathode's life time).
Apparently when working, an operator often switches plasmatron on and off (transitions, technological stops) thus reducing cathode's life time, and weather conditions are not regular. Therefore actual time of continuous cutting with one cathode is about 40 min. (i.e. this is a sum of cutting time intervals). From practical experience one cathode is worth of 2-2.5 hours of actual working time. A nozzle wears out three times slower than that. Therefore, due to practical experience, 3 cathodes and 1 nozzle are consumed during 8 hour shift.
Example:
Let's calculate prime cost of 1 meter of the handled metal, prime cost of the 1st ton of finished metal waste and productivity of metal waste processing when handling a steel sheet 12x2,5 m and 20 mm in gage into 3A metal waste (maximum piece size is 1500x500x500 mm) with device PURM-180М (without rent, salaries and transport).Standard sheet 12x2,5 m is taken because length of metal's cut will be maximum in regard to the weight of the cut off piece. Sheet's cutting ticket is shown on pic.1.
Picture 1. Cutting ticket for sheet 12*2,5 м into 3A format (lines of the cut are dotted).
Input data:
1) energy price 1.5 roub./kW*h;
2) metal gage - h=20 mm;
3) voltage of supply network - stable, 380V;
4) handled metal - carbon steel;
5) working mode - one shift (8 hours).
Calculation:
1. Using graph 1.1 let's set the cutting speed V as about 12.5 mm/sec.
Using graph 1.2: length of metal's cut with one cathode and metal gage L of 20 mm amounts to about 30 meters.
As a result costs per one meter of cut metal amount to:
where Scath - price of cathode,
Snozzle - price of nozzle,
Samort.m - device amortization for 1 meter of metal's cut (roub./m),
Splasm.m - plasmatron amortization for 1 meter of metal's cut (roub./m),
Sel.m - energy cost for 1 meter of metal's cut (roub./m).
Device amortization per hour per 8 hour work will be:
where Sdevice - price of the plasma cutting device PURM-180М,
P - life time of device in years,
N - amount of working days per year,
T - amount of working hours per day.
leq - length of metal's cut per 1 hour with cutting speed v = 12,5 mm/sec = 45 m/h and operator's labor utilization rate Q = 30% amounts to:
Operator's labor utilization rate is taken as net cutting time. The remaining 70% of the working time is reserved for transitions, handling operations.
Therefore, device amortization for 1 meter of metal's cut is:
Time of guaranteed work of a plasmatron (cutter) is 360 hours of continuous work (time, when arc is being burned), price of plasmatron is 2950 roubles.
Therefore, plasmatron amortization per hour will be:
Plasmatron amortization for 1 meter of metal's cut will be:
"Energy consumption".
Maximum power consumed by device PURM-180М without compressor is - 30 kW. Power adjustment is not.
where Sel - energy cost in the area (roub.),
h- efficiency rate of energy consumption.
Price of 1 kW per hour of Sen .energy is taken as 150 kopecks of 1 kW per hour.
30% of device working time consumes 30 kW at full capacity. About 30-40% of the time device works in idling mode consuming almost no energy. There we add 5% to the device working time at full capacity and efficiency rate of energy consumption will amount to 30 + 5 = 35%.
Total prime cost of 1 meter of metal's cut:
where Scath - price of cathode К-01U,
Snozzle - price of nozzle S1-180М,
L - length of metal's cut with one cathode (30 meters).
2. To calculate prime cost of 1 ton of metal waste cut into 3A format let's get total length of metal's cut:
Using cutting ticket on pic.1 we get:
Total weight of the cut sheet:
where A - length of a sheet (m),
B - width of a sheet (m),
h - width of a sheet (m),
m - specific weight of a sheet (tons/m3.).
Costs of sheet cutting:
Costs per 1 ton of metal's cut:
Time to cut a sheet will be:
Cutting productiveness per hour:
Cutting productiveness per shift:
All prices of this technical and economical assessment are taken from currentprice list of "Faktor", LLC.
Prices are in roubles including VAT (18%).
When operator's qualification level is raised the cutting speed can be increased, thus productiveness level will be higher and cutting costs will reduce. If production process is set efficiently the operator's labor utilization rate can be also increased thus increasing productiveness level and will reduce cutting costs considerably. When this rate is to 50% and cutting speed is raised to 16-17 mm/sec the cutting length per hour will amount to 30 meters, thus causing 125% productiveness growth. Productiveness growth will be 17.5 tons per shift thus reducing cutting prime cost almost twice.
When using PURM-180 and PURM-180М devices at metal waste processing sites practical experience has shown that 9-10 cathodes and 2-3 nozzles are burnt during 24h at the end of 3 shift work with 2 hours break. During this time 18 to 30 tons of finished metal waste with 5-15 mm are processed.
PURM-180М device is used to cool plasmatron and as an air gas plasma generator. Therefore it can be used in field conditions by low temperatures and high humidity. 100% duty cycle allows to use this device in continuous mode.
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