Surface Finishing Facility Ventilation Design
PROCESS VENTILATION OVERVIEW
Process ventilation is required in surface finishing facilities to comply with OSHA and environmental regulations to protect worker health and safety, and prevent corrosive and moisture laden vapors from accumulating in the shop atmosphere. Process ventilation is also required to protect adjacent process solutions from airborne contaminants and protect real assets (building systems and process equipment) from corrosive mists and vapors. Process ventilation can result in large energy consumption due to the need to heat, cool, filter, and/or dehumidify makeup air that is removed by the ventilation system and discharged outside the building. Efficient process ventilation design practices will reduce capital equipment costs, energy consumption, and overall operating costs.
Inefficient and oversized process ventilation systems increase the size and operating cost of the following:
- Process tank heating equipment
- Ventilation equipment and pollution control equipment
- Makeup air units (MAU’s)
MAUs are used to balance building air systems by replacing the air removed by the exhaust ventilation system. Effective integration of makeup air and ventilation system design is critical to the performance of the ventilation system.
The optimum time to improve the efficiency, effectiveness and operating costs of ventilation systems is during the initial design of the surface finishing system and facility; however, existing systems can almost always be effectively renovated and improved.
EXISTING PROCESS VENTILATION SYSTEM ASSESSMENT
Existing ventilation systems may be inadequate to properly protect workers or assets due to the following:
- Improper ventilation system design (capture efficiency, sizing, materials of construction, structural, and/or condensate drainage)
- Improper system balancing
- Design criteria changes:
- Lower personal exposure limits (PEL) for specific chemistries
- New chemistries have replaced the original chemistry in process tanks
- Parts, racks, and fixtures that interfere with the ventilation system
- Modifications to ventilation systems by the addition of new processes
- Lack of proper maintenance
- Inadequate makeup air supply
We recommend that existing surface finishing facilities have experienced technical staff or consultants review potential exposure issues and ventilation designs. The following questions should be answered to characterize the effectiveness of existing facility ventilation systems:
- What are the principal hazards (See Tables 1.0 and 2.0)?
- What hazards require stricter control?
- Are there any hazards that require periodic review for regulatory compliance?
- Has the ventilation system been designed to adequately mitigate the identified hazards?
- Has the ventilation system been tested and validated for proper air flow?
- Are odors present?
- Is mist or steam observed escaping from process tanks?
- Is there evidence of corrosion of process equipment or the building due to air emissions?
- Is there evidence of staining near process tanks from condensed vapors?
Table 1.0 Determination of ACGIH Hazard Potential Standard Based Upon Hygienic Standards
(See ACGIH Ventilation Handbook – Appendix A)
|Hazard Potential||Gas & Vapor (ppm)||Mist (mg/m3)||Flash Point °F|
Table 2.0 Determination of ACGIH Rate of Gas, Vapor or Mist Evolution Standard
Boiling Point °F
|Relative Evaporation Rate*
(Hours for 100% Evaporation)
*Dry Time Relation (See ACGIH Ventilation Handbook – Appendix B) <5 Fast; 5-15 Medium; 15-75 Slow; >75 Nil
**Rate of gassing depends upon rate chemical or electrochemical activity, which is dependent upon the material treated and solution chemistry. This activity tends to increase with (1) amount of work in the tank, (2) concentration of solution in the tank, (3) temperature of solution in the tank, (4) current density applied to work in the tank in electrochemical tanks.
Some materials commonly found in surface finishing facilities are targeted for more stringent control. Periodic reviews of processes, with constituents such as chrome, nickel, and cobalt, are recommended. Proper ventilation of these processes is essential; robust design approaches and/or increased ventilation rates may be required.
Some process solutions, such as chromic-sulfuric etches, aluminum etches and bright dips, and hard chromium plating solutions are notoriously difficult to effectively ventilate due to vigorous generation of mists and may warrant added devices such as enclosures, shields, and/or tank covers to isolate process tanks from workers and adjacent processes.
Observation of mist escaping or staining near the process tanks may indicate that the ventilation system is not performing adequately. These signs may indicate a worker exposure problem that testing can quantify. These same signs typically mean that the facility building and process equipment will require more frequent maintenance and repair, as corrosive vapors or mists, that are not effectively captured, may attack susceptible components of the building and equipment.
VENTILATION SYSTEM DESIGN REVIEW
A highly effective ventilation system contains many elements. A systematic review looks at each of the elements to ensure effective operation and should include the following:
- Process Conditions
- Solution chemistry & operating parameters (concentration, temperature, and agitation)
- Parts, racks & fixtures
- Materials of construction (chemistry and temperature)
- Ventilation hoods, push air, covers, ducts, drains, structure, fans, blowers, and scrubbers, etc.
- Facility Conditions
- Makeup air delivery
- Air balance
- Open doors & windows
- Cross current air-flow
- Airborne contaminants from adjacent manufacturing processes (dust, oil mist, etc.)
- Equipment and facility condition and maintenance
- Personnel and material traffic patterns
Review of the basic design analysis is important in order to determine if the system is capable of operating at a level sufficient to protect workers and to identify where changes are needed based upon the identified hazards. Design review includes the following:
- Process specific hazards
- Checking ventilation requirements based on hazard rating and the physical nature of the process (i.e. temperature, agitation chemical activity, tank size)
- Checking ventilation design factors based on ventilation rates and a number of ventilation design parameters, including:
- Hood slot velocity
- Hood slot size
- Hood plenum depth
- Duct velocity
- Duct sizing
- Push air rates
- Push air manifold design
- Cross currents in facility
- Condensate drainage in hoods and ducts
Varying ventilation rates, as a function of operating conditions, enhances energy efficiency. Process solution temperature and agitation can be controlled differently under active, standby, and inactive operating conditions which impact solution air hazard ratings and required ventilation rates. Automated covers, ventilation hood duct dampers, and automated push air manifolds can be integrated with process tanks to reduce ventilation rates based upon cover position (open or closed). Variable ventilation rate control may require ventilation and makeup air system fans to be equipped with variable frequency drives (VFD’s) to meet process ventilation demands and proper building pressure requirements.
Conducting a review and assessment of the ventilation system operation is paramount to improve system performance and reliability. A typical review would address the following areas:
- Start-up procedures
- Operating procedures
- Work load and production schedules
- Process demand
- Shut-down procedures
- Redundancy or back-ups
- Periodic testing
- Preventive maintenance program
Comparing operation to design is important to assess process-specific functionality and to identify gap areas where operation and design do not match. A key part of the assessment is to examine the preventive maintenance program and look at data logging, operation outside the allowable limits, and down-time issues.
ITI is a global consulting, engineering, and design-build firm based in Burlington, Vermont. We specialize in manufacturing processes, water and wastewater treatment, recycling, and ventilation applications for the metal and surface finishing industry.