Hazardous HVAC in industrial environments

Two criteria rise above others when designing for industrial environments: safety and functionality. In the industrial processes, materials being processed and applicable codes largely describe the design applications, but researching many variables related to hazardous materials and properly conceptualizing the project are fundamental for success.

11/01/2007


Two criteria rise above others when designing for industrial environments: safety and functionality. In the industrial processes, materials being processed and applicable codes largely describe the design applications, but researching many variables related to hazardous materials and properly conceptualizing the project are fundamental for success.

Power, lighting, grounding and controls, in addition to building architecture and structure, need to be considered when designing for the storage, use and handling of hazardous materials. Air flow, however, is paramount. Essentially, the full design team must work together to decide strategies for creating a safe and effective solution that also meets requirements from the authority having jurisdiction. Incidentally, early coordination with the building department to explain one's approach to codes and occupant safety may facilitate permits as the project nears construction.

Code check

One critical aspect of hazardous facilities design is understanding the codes, particularly the 2003 and 2006 International codes. Andy Harlow, Cleanroom/BioPharma sales manager for the Northeast at Camfil Farr, New York, said, “The difference between achieving classification at various occupancy states can be enormous and extremely process dependent. Many variables are absolutely critical and must be understood and accounted for to ensure successful outcomes.”

Three factors help determine whether an area is considered an H occupancy by code:

  • The types of hazardous materials used and stored in the facility

  • The NFPA classification and hazard rating of the hazardous materials

  • The quantities of each type of hazardous material that can be stored or used in each control area, which is a building or portion of a building for the storage, use and dispensing of allowable quantities of hazardous materials.

This last point is a balancing act. To a great extent, the industrial process, along with the owner's preferences and practices, influences the optimized amounts. While either the industrial architect or the mechanical engineer programs the control areas—itemizing the materials, their quantities and their use classifications—it is up to the engineer to research, develop and recommend a code-safe ventilation and exhaust design solution.

Furthermore, different quantities and classifications necessitate different design approaches, which mean different first costs and operations costs. International Fire Code (IFC) Table 2703.1.1 states that basic quantities of materials are allowed in control areas without taking extraordinary design measures. However, if the standard allowable quantities are exceeded, the design needs to comply with a host of requirements to meet code.

That being said, quantities of allowable materials are classified in three ways: storage, use-closed systems, and use-open systems. In general, larger quantities of materials are permitted in storage and use-closed than in use-open systems. To classify an installation as storage or use-closed, the system or containers must not be opened at any time during normal activities. Depending on the material, the maximum allowable quantities can be limited, particularly for the open-use classification.

According to IFC Table 2703.1.1 footnotes, quantities of some materials may be doubled with installation of an approved automatic fire suppression system. In some cases, storage classification quantities may be doubled again when the hazardous materials are kept in an approved storage cabinet, gas cabinet, exhausted enclosure, or safety cans.

Deflagration venting may need to be specified when a hazardous material listed in IFC Table 911.1 exceeds the Table 2703.1.1 quantities. Also known as blast walls, deflagration venting prevents unacceptable structural damage and limits the potential for occupant injury, but adds significant cost to a project. Spaces equipped with deflagration venting resists a minimum internal pressure of 100 lb per sq. ft as the vented wall or roof relieves at a maximum internal pressure of 20 lb per sq. ft.

Limiting quantities may save on first costs, but ongoing production and maintenance costs may be incurred due to more frequent transporting, stocking, and handling activities. Also, disconnection and connection of tanks and piping will likely classify the system as open-use and constrain the quantities allowed for production.

At the same time, workarounds may be possible. For example, in a solvent cleaning lab removable flammable liquid storage tanks were classified as closed systems by using dry-link couplings. These couplings provide fa drip-free connection to supply piping and are manufactured from a variety of materials for chemical compatibility with the hazardous material. By classifying the solvent supply tanks as a closed system, the team increased the allowable quantities. Those quantities were doubled again by protecting the area with an automatic sprinkler system. In the end, this approach also avoided costly deflagration venting of the room.

The code also allows for other approaches to increase allowable quantities within a control area. For instance, a mixture of different solvents can be collected and pumped to solvent waste containers after the solvents have been applied to the manufactured product. These containers are considered open-use because they require an unobstructed vent as they are filled. As with the solvent supply containers, the waste containers also need to be disconnected, removed, and reconnected in the control area. Locating the waste containers in a room with automatic fire suppression can increase the allowable quantities by as much as 100%. Thus, it is advisable to capture open-use container fumes by placing containers in a vented enclosure, which can be either off-the-shelf or custom-fabricated to meet the constraints of the area.

In either case, an average 100 fpm air velocity must be maintained across the face of the hood boundary. This velocity can equate to a significant amount of air depending on the size of and needed accessibility to the containers. To reduce the required air flow, which has a direct relationship to operating energy costs, sliding doors can reduce the face area of the hood. See Figure 1.

Incidentally, hazardous occupancies require a significant amount of air to maintain a clean and safe environment. In accordance with code, with the minimum ventilation requirements relating to six air changes per hour with a 10-ft ceiling, control areas exceeding the allowable quantities of hazardous materials must be ventilated at a minimum of 1 cfm/sq. ft. Also, ventilation may need to be increased to account for capture hoods, other process equipment, and if the area is a classified clean space. Ventilation requirements based on local exhaust hoods and process loads generally meet the requirements for a clean space, depending on the cleanroom class one is trying to achieve.

As a point of reference, typical room velocities and air change rates for different classes of cleanroom are shown in Table 1. If contaminates are captured and exhausted from the space effectively, then recirculated air may be acceptable. Air dilution calculations will help determine if the amount of ventilation and recirculated air will dilute the environment to safe levels. If there is a risk that vapors may not be effectively captured or if hazardous materials are used openly in the space that will equate to 25% of the lower explosive limit, one will need to increase the ventilation requirements. One effective approach in such a scenario is a combination of low exhaust pickup points around the perimeter of the room and uniform supply air from the ceiling.

Exploring exhaust systems

Exhaust systems falls under the International Mechanical Code, Section 510. The engineer should consider the density of each hazardous material in the area. For flammable solvents heavier than air, low-point exhaust systems pick up hazardous vapors that collect at floor level. Collection points are then required to be a minimum of 12 in. above the finished floor elevation. Similarly, exhaust duct velocities should be high enough to collect and convey hazardous fumes. For solvent fumes, duct velocities range from 1,000 fpm to 2,000 fpm. Heavier molecules and particles require even higher velocities, up to more than 4,500 fpm.

When selecting the exhaust diffuser, the effective open area of the capture grille must be calculated to ensure a high capture velocity at the grille face. Unlike commercial applications, exhaust noise is a much lesser consideration than face velocity.

Separate, independent exhaust systems also are required for each material, and they must exhaust directly to the outdoors. One's building department may make exceptions if the engineers show that different solvents are compatible with and can safely share an exhaust system. According to the International Mechanical Code, a hazardous exhaust system is required for normal operating conditions under the following circumstances:

  • To limit flammable vapor, gas, fume, mist or dust to concentrations below 25% of the lower flammability limit

  • If a NFPA health-hazard rating of 4 is present at any time

  • If a NFPA health-hazard rating of 1, 2 or 3 exceeds 1% of the median lethal concentration for acute inhalation toxicity.

The International Mechanical Code also mandates that fire suppression be installed in hazardous exhaust ductwork larger than 10 in. diameter. Of course, the intent of this requirement is to protect against hazardous materials such as paints and other flammables that can deposit residue on the duct interior and cause duct fires. Solvents and some other materials actually clean the inside of the duct and are evaporated readily. With no potential for buildup of flammable substances inside the ductwork, one may be able to persuade the building department to waive the requirement for sprinkler protection.

Exhaust design continues through the discharge stack. It is important that the hazardous vapors exhausted directly to the atmosphere for dilution not be recirculated back into the building Mathematical modeling, reduced-scale wind-tunnel studies or manual calculations are used to predict discharge stack behavior so recirculation cannot occur. Rain caps are inadvisable on exhaust stacks, because exhaust must discharge freely to the atmosphere. A minimum discharge velocity of 2,600 fpm will prevent rain from entering the stack when the fan is operating. Because rain has a terminal velocity of about 2,000 fpm a stack velocity of 3,000 fpm is recommended to resist downwash, increase effective stack height and allow a more optimum selection of the centrifugal exhaust fans to provide a stable operation point of the fan curve. High exhaust velocity is a poor substitute for stack height and effective stack design, unless one is specifying a high-plume exhaust fan that is designed for that application.

With all these design parameters, don't forget that energy usage must be considered as well. As such, the guidelines provided in Chapter 44 of the 2007 ASHRAE Applications are an ideal source to manually predict stack exhaust behavior. A depiction of the design procedure for determining the required stack height, included in the ASHRAE Applications, is shown in Figure 2.

At the end of the day, designing facilities with hazardous requirements can be challenging, but working out details through the design phase results in occupant safety and operations efficiency. Furthermore, involving the end user to understand the constraints of the control areas as they relate to the hazardous materials is vital in defining the requirements of the project as they relate to code regulations.

ISO Class

FS209 Class

Velocity (fpm)

8

10

12

16

Air changes per hour for ceiling height (ft)

If contaminates are captured and exhausted from space, then recirculated air may be acceptable. Air dilution calculations help determine if the amount of ventilation and recirculated air will dilute the environment to safe levels. Source: The RMH Group

2

-

85-100

638-750

510-600

425-500

319-375

3

1

70-85

525-638

420-510

350-425

263-319

4

10

60-70

450-525

360-420

300-350

225-263

5

100

45-55

338-413

270-330

225-275

169-206

6

1,000

25-35

188-263

150-210

125-175

94-131

7

10,000

8-16

60-120

48-96

40-80

30-60

8

100,000

4-6

30-45

24-36

20-30

15-23

9

1,000,000

2-3

15-23

12-18

10-15

8-11





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