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Monitoring by Control Technique - Capture Systems

Description

Capture efficiency is defined as the percentage of emissions captured and vented to a control device. Various capture systems may be used to capture emissions and direct them to a control device. These systems include enclosures and local exhaust ventilation measures.

Types of Enclosures
There are basically two types of enclosures:
(1) total enclosures, referred to as permanent total enclosures (PTEs), and
(2) nontotal, or partial, enclosures.
 
A PTE is an enclosure that completely surrounds a source such that all volatile organic compound (VOC) emissions are contained and directed to a control device. The EPA has established a set of criteria that must be met for an enclosure to qualify as a PTE; these criteria are contained in Method 204 - Criteria For And Verification of a Permanent or Temporary Total Enclosure (40 CFR 51, Appendix M). If the criteria set forth in this method are met, the capture efficiency may be assumed to be 100 percent and need not be measured. The following summarizes the PTE criteria:
 
Permanent Total Enclosure Criteria
  1. Any natural draft opening (NDO) shall be at least four equivalent opening diameters from each VOC emitting point;
  2. The total area of all NDOs shall not exceed 5 percent of the surface area of the enclosure four walls, floor, and ceiling;
  3. The average face velocity (FV) of air through all NDOs shall be at least 3,600 m/hr (200 ft/min);
  4. All access doors and windows whose areas are not included in the calculation in item No. 2 shall be closed during routine operation of the process; and
  5. All VOC emissions must be captured and contained for discharge through a control device.

Other specific information about PTEs can be found from the following Air Pollution Control Technology Fact Sheet.

You can view a training video on capture systems here: Pollution Controls and Available Monitoring Techniques Part 2 - High Resolution (WMV).

Partial Enclosure Criteria
An enclosure that does not meet the minimum criteria for a PTE is not a total enclosure; it is a non-PTE, or partial enclosure (PE), and capture efficiency is determined by measurement. Typically, this determination of capture efficiency includes constructing a temporary total enclosure (TTE), except where a building enclosure is used, and conducting measurements with some combination of the following procedures:
  • Method 204 A - Volatile Organic Compounds Content in Liquid Input Stream
  • Method 204 B - Volatile Organic Compounds Emissions in Captured Stream
  • Method 204 C - Volatile Organic Compounds Emissions in Captured Stream (Dilution Technique)
  • Method 204 D - Volatile Organic Compounds Emissions in Uncaptured Stream from Temporary Total Enclosure
  • Method 204 E - Volatile Organic Compounds Emissions in Uncaptured Stream from Building Enclosure
  • Method 204 F - Volatile Organic Compounds Content in Liquid Input Stream (Distillation Approach)

Alternative protocols and test methods for determining capture efficiency also may be used. EPA has established data quality objectives that must be met for alternative protocols and test methods; these are presented in Appendix A to Subpart KK to Part 63 - Data Quality Objective and Lower Confidence Limit Approaches for Alternative Capture Efficiency Protocols and Test Methods.

The second type of control measure used to capture emissions and vent them to a control device is the application of local exhaust ventilation. Local exhaust ventilation systems typically consist of a hood, or hoods, that captures the contaminant at the point of generation and a duct system and exhaust fan that move the VOC-laden air to the control device.

For both types of capture systems (enclosures and local exhaust ventilation systems), maintaining the integrity of the capture device (i.e., enclosure, hood) and the airflow (ventilation) through the system are the critical operating/maintenance parameters with respect to maintaining capture system performance.

Appendix M to Part 51—Recommended Test Methods for State Implementation Plans.

(The official version of the methods is available from the GPO's Electronic Code of Federal Regulations, Subpart BB—Data Requirements for Characterizing Air Quality for the Primary SO2 NAAQS:Appendix M to Part 51—Recommended Test Methods for State Implementation Plans)

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Monitoring Information

Maintaining the integrity of the capture device (i.e., enclosure, hood) and the airflow (ventilation) through the system are the critical operating/maintenance parameters with respect to maintaining capture system performance. The indicators of performance for capture systems relate to these two parameters and, for purposes of discussion, monitoring approaches can be divided into two subcategories:

  1. Indicators of capture by the enclosure or hood (e.g., enclosure differential pressure, NDO velocity, hood face velocity); and
  2. Indicators of system air flow (e.g., fan rpm, duct pressure differential) measured downstream of the capture device combined with a system integrity inspection.
Industry Specific Information
To search for additional monitoring information specific to an industry, see the monitoring information by industry type page.
 

Costs

Information on the costs of Permanent Total Enclosures is presented in the EPA Air Pollution Control Cost Manual*, Section 2, Chapter 3 - Permanent Total Enclosures(41 pp, 125 K, About PDF). Costs of monitoring systems, both Continuous Emission Monitors and parametric monitoring systems, are addressed in the EPA Air Pollution Control Cost Manual*, Section 2, Chapter 4 - Monitors.(42 pp, 542 K, About PDF).

*EPA is currently updating the Control Cost Manual. The most recent versions are available on the Economic and Cost Analysis for Air Pollution Regulations website.

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