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Safety and Health Topics > Mineral Processing Dust Control > Dust Control Handbook > Ch. 6 - Estimating Costs of Dust Control Systems
  
Chapter 6

Estimating Costs of Dust Control Systems


Necessity for Cost Estimates

Since adequate control of dust emissions can usually be achieved by more than one dust control method, a considerable economic burden may result if the appropriate method is not selected.  This burden can be higher capital costs, higher operating and maintenance costs, or both.  Preliminary cost estimates can prevent this unnecessary economic burden by-
  • Characterizing the costs to be incurred and determining whether they are feasible
  • Comparing expected costs of alternative dust control techniques to help identify the optimum control technique

Cost Estimating Procedures

Several methods, with varying degrees of accuracy, are available for estimating costs.  They range from presenting an overall installed cost on a per-unit basis to presenting detailed cost estimates based on preliminary designs, schematics, and/or vendors' cost estimates.  The lease accurate method is to equate overall costs to a basic operating parameter such as tons per hour or cubic feet per minute.  This approach is not recommended.  Where possible, detailed cost estimates should be arrived at by preparing preliminary designs and schematics.  However, if time does not permit this approach, equipment vendors may be contacted.  Based on their current knowledge of the technology and experience in the industry, they can provide reasonably accurate cost estimates.
 

Cost Components

To prepare and analyze cost estimates, a basic knowledge of the cost components and their relationship to the total cost of the system is essential.

Total costs for any system can be divided into-
  • Capital costs
  • Operating and maintenance costs
Capital Costs

Capital costs consist of the delivered costs for major control equipment, auxiliary equipment and accessories, and field installation.  Capital costs can be grouped as follows:

Summary of Capital Costs
  • Major equipment (35%)*
  • Auxiliary or accessory equipment (15%)
  • Field installation (20%)
  • Project management and engineering (13%)
  • Freight, taxes, subcontractor, and so forth (17%)
  • Start up cost, working capital, and other capitalized costs (15-20%)**
_____________________

* Average percent of the capital investment
** Additional costs, expressed as a percentage of total capital costs
  • Major Control Equipment


    • - Baghouses
    - Electrostatic precipitators
    - Scrubbers
    - Cyclones
    - Water-spray bars
    - Nozzles
     
  • Auxiliary or Accessory Equipment


    • - Air-moving equipment
     
      -- Fans and blowers
      -- Electrical motors, starters, wire, conduit, switches, etc.
      -- Hoods, ductwork, gaskets, dampers, etc.
    - Liquid-moving equipment
     
      -- Pumps
      -- Compressors
      -- Electrical motors, starter, wire, conduit, switches, etc.
      -- Piping and valves
      -- Settling tanks (for wet scubbers)
    - Storage and disposal equipment
     
      --Dust storage bins and hoppers
      --Sludge pits
      --Drag lines, trackway, etc.
    - Supporting construction
     
      -- Structural steelwork
      -- Cement foundation
      -- Thermal insulation
      -- Vibration and antiwear materials
      -- Protection covers
    - Instruments to measure or control the following:
     
      -- Air and liquid flow
      -- Temperature and pressure
      -- Operation and capacity
      -- Power
      -- Opacity of flue gas
      -- Dust concentration
  • Field Installation Costs


    • - Labor required for delivery, assembly, removal or relocation of equipment
    - Freight, taxes, and subcontractors' fees
    - Engineering supervision
    - Startup and performance testing
    - Extending or increasing utilities
Baghouses

The capital costs of a baghouse depend on the following:
  • Type of filtering media used (cotton, dacron, glass, Teflon, etc.)
  • Type of fabric used (felted vs. woven)
  • Adopted air-to-cloth ratios
  • Type of cleaning mechanism (mechanical shaker, pulse jet, reverse air)
  • Type of baghouse (suction type vs. pressure type; continuous duty vs. intermittent)
  • Design and construction (standard design vs. custom design; carbon steel vs. stainless steel)
  • Temperature of exhaust gases (high temperature vs. low ambient temperature)
Electrostatic Precipitators

The capital costs of an electrostatic precipitator depend on the net plate area (NPA).  The NPA, in turn, depends on the efficiency required of the precipitator.

Following are the factors that affect the cost of an electrostatic precipitator:
  • The electrical characteristics of the dust may have a significant effect on the collection efficiency and the plate area.
  • The resistivity of the dust varies with the temperature and moisture content of the bag.  Therefore, in some cases, auxiliary equipment may be required to precondition the gas stream before it enters the precipitator.
  • The addition of moisture to the gas stream, in combination with a low operating temperature, may require insulating the precipitator to prevent condensation and corrosion.
Scrubbers

The capital costs of a scrubber depend, generally, on the following three factors:
  • Volumetric airflow rate
  • Operating pressure
  • Construction
The volumetric airflow rate is the most important factor because the size of the scrubber and its cost are determined by the actual gas volume at the scrubber's inlet.

Operating pressure also affects scrubber efficiency and price.  The higher the air volume and/or operating pressure, the greater the plate thickness of the shell must be to prevent buckling of the shell.

The cost of a scrubber can also increase if it is constructed of special materials, such as stainless steel or fiber-reinforced plastics to protect against corrosion or lining the scrubber shell with PVC, rubber, or refractories to protect against erosion.

Cyclones/Multiclones

The capital costs of a cyclone or multiclone are a function of the particulate-removal efficiency, which, in turn, depends on the inlet gas velocity and inlet diameter.  Theoretically, the higher the velocity or the smaller the inlet diameter, the greater the efficiency and pressure drop.

The material of construction also affects the cost. For handling highly abrasive dust, the cyclone/multiclone may have to be constructed of abrasion-resistant material or lined with ceramic material.  For a highly corrosive gas stream, stainless steel or fiber-reinforced plastic may be necessary.

Fans and Motors

The capital costs of a fan are based on-
  • Construction 
  • Class
  • Volume handled
  • Pressure developed
The capital costs of a fan motor are related to-
  • Fan speed
  • Total system pressure
  • Gas volume flow rate
  • Selected motor housing
Pumps

Although pump prices vary with the design of the pump, they are generally a function of-
  • Pump head developed, ft
  • Pump capacity, gal/min
  • Pump speed, r/min
The selections of revolutions per minute for these pumps should be based on the design flow rate:
 
Flow rate (gal/min) Pump (r/min)
0 - 1,000 3,550
500 - 5,000 1,750
2,000 - 10,000 1,170

Note: Generally, the capital cost of the pump and motor combination varies inversely with the revolutions per minute; however, maintenance costs may be higher as the revolutions per minute increase.

Operating and Maintenance Costs

Operating and maintenance costs consist of direct expenses of labor and materials for operating and maintenance, the cost of replacement parts, utility costs, and waste disposal costs.  They may also include indirect costs of overhead, taxes, insurance, general administration, and capital recovery charges.  However, only direct costs are discussed here.

Operating Costs

Operating costs include-
  • Direct labor and materials
  • Utilities

Direct Labor and Material Costs - Labor and materials costs for operation and maintenance of dust control systems vary substantially among plants due to factors such as the degree of automation, equipment age, and operating periods.  Generally speaking, labor costs can be reduced by increased system automation.

For small- to medium-size systems with an installed cost of approximately $100,000 or less, the total cost of maintenance is approximately 5% of the installed cost.


Estimated Labor Hours per Shift

Utility Costs - the utility costs for equipment such as pumps and electrical motors are a function of power/energy requirements, which can be calculated as follows:


Fan Power Equation


Pump Power Equation

Horsepower requirements for baghouse shaker motors, reverse-air fan motors, etc. can be estimated at approximately 0.5 hp per 1,000 ft2 of cloth area.  Power usage depends on dust loading and cleaning frequency.  Assuming a 50% usage factor, power requirements are approximately 0.2 kW·h for 1,000 ft2 of cloth area.

Electrostatic Precipitator Power

The power requirements for an electrostatic precipitator are approximately 1.5 W/ft2 of collection plate area. The range varies from 0.3 to 3 W/ft2.

Once the power requirement is known, the annual power costs can be calculated using the following equation:


Annual power cost ($) =

Power usage    x    Cost of power    x    Total annual

   (kW·h)                  ($/kW·h)              operating hrs.

Waste Disposal Costs

The cost of waste disposal included the removal and hauling of dry contaminants to a nearby site.  This cost varies with the particular plant and available landfill site.

Water Costs

Water costs vary in different areas.

Maintenance Costs

Maintenance costs include-
  • Labor and materials for preventive and routine maintenance, such as lubrication, surface protection, cleaning, and painting
  • Replacement of worn-out equipment, parts, or structures due to wear, abrasion, or corrosion
The annual cost of replacement parts represents the cost of the parts or components divided by their expected life.  Replacement parts are components such as filter bags and spray nozzles, which have a limited life and must be replaced periodically.
 

Cost Justification

A 10-15% return on investment is necessary to justify and capital investment.  However, when dust controls are considered, such a return is not always practical.  The following are some tangible benefits that may assist in economic justification of a dust control system:
  • Industrial taxes amount to about 50% of net income, which means business investments result in a tax savings equivalent to approximately half the expenditure.
  • Return on investment before taxes is approximately twice the value of the return calculated after taxes.
Various federal and state governments provide the following tax relief benefits for industries that install dust control or pollution control systems:
  • Section 169 of the Internal Revenue Tax Reform Act of 1969 permits a faster tax write-off of pollution control facilities.  If the facility is certified by both the Environmental Protection Agency and the appropriate state agencies, facilities installed after 1968 can be amortized over a 60-month period.  An exception to this is any control system that recovers the costs by generating a profit in some manner.
  • Thirty-seven states provide tax incentives for pollution control facilities, such as-
- Property tax exemption
- Sales and use tax exemption
- Income tax credit
- Accelerated amortization
  • All but four states (California, Idaho, New Jersey, and Texas) authorize the use of industrial revenue bonds to finance pollution control equipment.  These are normally 15-year bonds that provide tax-free interest to the holders.  Although the interest rates offered by these bonds are about 2% lower than most other bonds, they can attract investors because of their tax-free status.
Other intangible benefits may further assist in justifying a dust control system:
  • Reducing health hazard possibilities
  • Reducing risk of dust explosion and fire
  • Reducing equipment wear and damage
  • Increasing visibility
  • Reducing or eliminating unpleasant odors
  • Improving relations with neighbors
  • Creating safer and more pleasant working conditions, thus improving employee morale and productivity
  • Possible product or byproduct savings

 
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