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One of the most important decisions to be made as one considers plasma equipment is the choice of gas to be used in the spray process. Yes, equipment from all manufacturers can be used with any of the plasma forming gases. The question is, whose system works best with which gases. To make that decision there are certain facts with which you should be aware. Listed below are facts to enable you to decide from an economic, safety and operational point of view which conditions are the best.

  • FACT: Argon is a monatomic gas. It costs more than Nitrogen, produces lower noise levels, and longer nozzle life.
  • FACT: Nitrogen is a diatomic gas. It costs less than Argon, produces higher noise levels and significantly shorter nozzle life.
  • FACT: Comparing the relative costs of the two principle plasma gases, Argon and Nitrogen the cost of Argon is approximately 3 times that of Nitrogen.
  • FACT: Under normal spray conditions the consumption of Nitrogen is approximately 1- 1/2 times that of Argon.
  • FACT: When using Argon as an operating media, electrode life is approximately 10 times that compared to when using Nitrogen.
  • FACT: When operating with energy efficient Argon, approximately 1/3 less power is required as compared to that of Nitrogen operation.
  • FACT: Nitrogen operation requires a power supply having 1 1/2 to 2 times the capacity of that required or Argon operation.
  • FACT: The noise level when operating with Argon under standard conditions is approximately 90 dBa as compared to 110 dBa when operating with Nitrogen.

When analyzing the above facts it should be noted that we are comparing only those facts to which a value can be assigned. There are others of indeterminate value, such as the increased effects of the heat of recombination, which are present when Nitrogen is used as the plasma forming gas and which necessitates cooling the part during spraying and requires a greater degree of operator skill and attention, to which a value cannot be assigned. When making the total decision, obviously, one must take into consideration all three factors.

As we take a step by step approach to the analysis, it is obvious that on the basis of initial gas cost Argon is more expensive. The higher initial cost is somewhat offset by a higher cubic foot usage when using Nitrogen. Both of these factors can be directly convertible to a per hour operating cost. Using local costs and recognizing these vary dependent on local conditions we calculate $8.40 per hour for Argon vs. $4.31 per hour for Nitrogen. Another fact which falls into the same category is power consumption which is directly related to kilowatt-hour costs. Again, using a local cost of $.08 per kilowatt we calculate power costs of $1.68 using Argon vs. $2.60 using Nitrogen. Combining the above costs, the hourly rate for Argon operation is approximately $10.08 per hour as compared to a rate of $6.91 per hour of Nitrogen.

Comparing the costs based on electrode life and initial equipment cost the calculation is not as straight forward. Computing the cost of electrodes and particularly nozzles on an hourly basis, a lot depends on care, severity of usage and other factors.

Being extremely conservative and using 5 times the nozzle life factor and increased operation, the hourly cost for Nitrogen operation is $8.42 per hour vs. $1.32 per hour for Argon operation. Adding these costs to those above for gases and power we find the per hour cost of operation is $11.40 for Argon and $15.33 for Nitrogen. These are the basic raw costs that can be calculated and do not reflect other incidentals such as higher installation costs, etc. When these factors are combined with the above hourly cost it is obvious that there is a cost advantage using Argon.

The tabulation which follows summarizes the cost factor discussed above.

Argon Operation

 

Nitrogen Operation

Gas:

Volume - 70 cfh

 

105 cfh

 

Cost - $.12/cubic foot

 

$.041/cubic foot

 

Typical cost/hr.

$ 8.40

Typical cost/hr.

$ 4.31

Power:

21 kwh

 

32.5 kwh

 

Typical electrical cost @.08/kw

$1.68 

Typical electrical cost @.08/kw

$2.60 

 

Total Gas & Power:

$10.08

Total Gas & Power:

$6.91

Nozzle:

Life 75 hours (5X)

 

15 hours

 

Typical cost - $ 99.00

 

Typical cost - $ 120.00

 

Typical cost/hr.

$1.32

Typical cost/hr.

$8.00

Power Supply:

40Kw

 

60/80Kw

 

Cost difference - =

 

 Cost difference - =

 

Amortized over 10,000/hrs.

 0      

Amortized over 10,000/hrs.

.42     

 

Calculated cost/hr.

$11.40

Calculated cost/hr.

$15.33

Finally we come to a factor that cannot be expressed in dollars and cents. The lower noise level obviously costs less in noise abatement programs as well as operator safety requirements. Noise levels not only effect the immediate area in which spraying is being conducted but also in terms of worker efficiency in adjacent areas. The per hour cost in terms of noise reduction programs and reduced efficiency might be significantly higher than anyone realizes. 

When combining all the above facts, one can easily come up with very real justification for using argon as the plasma forming gas. This is particularly so if the equipment has been designed to produce equivalent coatings using argon to those produced using nitrogen. This is the case with the Bay State plasma equipment which utilizes the patented boundary layer control nozzle.  The boundary layer control nozzle is the subject of a technical report, BL-1. With the boundary layer control nozzle and the option to select the use of an energy efficient internal powder injection port, coatings are produced using argon which are equivalent to those produced with other manufacturers' equipment using nitrogen.

MIXING GASES

There are times when it is desirable to add a secondary or tertiary mixing gas to the primary gas. The reasons would include increasing plasma velocity, increasing plasma temperature or controlling the chemical content of the plasma effluent. The principle mixing gases are helium, hydrogen and nitrogen. These may be used singularly or if the equipment allows, in combination. Recommended spray parameters for a given spray material will normally advise when and how much mixing gas is advisable. Because of design peculiarities, some manufacturers almost always require mixing gases to obtain satisfactory coating results.

Ordinarily, some reduction in nozzle life can be anticipated by the introduction of a mixing gas. The degree of reduction is dependent on the quality and type of mixing gas used. Just as in the case with primary plasma gases, the diatomic gases cause more nozzle wear than do monatomic gases.

 

 

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