Ventilation Bldg 3001 Tinker.doc

Issue:

Can air from the Low and High Bay areas of Building 3001 be used to ventilate office spaces constructed within those bay areas?

Assumptions:

The bay areas are kept clean, open use of chemicals and solvents is minimal and major sources of contaminates are exhausted outside.

Ventilation and makeup air is distributed evenly throughout the bays.

Since the facility is considered industrial, it is monitored by base bioenvironmental to ensure compliance with AFOSH standards. Consequently the air in the space is considered safe and does not pose a threat to human health or occupancy.

This analysis looks only at acceptability of air quality for human occupants as based upon ASHRAE Standard 62.1-2004, Ventilation for Acceptable Indoor Air Quality. Issues of acceptability regarding health are governed by AFOSH and OSHSA standards.

Analysis:

In determining the acceptability of transferring air from the bay area to office space constructed within the bays, section 5.17 of ASHRAE Standard 62.1 provides subjective criteria for establishing air-quality classification based on relative contaminant concentrations. For example office space is classified as Class 1 air. Class 1 air has low contaminant concentration, low sensory-irritation intensity, and inoffensive odor. Whereas Class 2 air has moderate contaminant concentration, mild sensory-irritation intensity, or mildly offensive odors, though not necessarily harmful or objectionable, it is considered inappropriate for transfer to spaces used for different purposes, such as a Class 1 space. The Standard identifies air classes according to occupancy category based on how the space is used. The bay areas are considered to be similar in use as the occupancy category identified in the Standard as Wood/metal shops in educational facilities. This type of space is classified as Class 2. Though the bay areas are considered industrial, taking into consideration the extensive exhaust systems utilized to capture contaminates at their sources, high air exchange rates and significant volume of dilution ventilation, I would classify the air in this space as a Class 2 or better. I base this determination upon discussions with the engineering staff (Mark Hill), information provided and the following:

1. Sources of contamination in the bay areas are captured at their source and exhausted directly outside.

2. A significant amount of air is exhausted from the bay areas and is replaced by an equal or greater amount outside air drawn in through the air handlers. This provides make up air to the space and dilution of any contaminates not captured by the exhaust systems. This general dilution ventilation is more than adequate to minimize contaminant concentrations and prevent the accumulation of odors. In the Low Bay Area the Outdoor Air Rate is .55 SCFM/sq ft which is more than three times the rate recommended by AHRAE of .18 SCFM/sq ft for a similar occupancy category. In the High Bay Area the Outdoor Air Rate is .33 SCFM/sq ft. This make up air also provides ventilation air for the occupants. Based upon a building occupancy of 2,675 people, distributed evenly, the low bay area has an outdoor ventilation rate of 310 SCFM/person and the high bay area has a rate of 184 SCFM/person. The recommended rate is 15 to 20 SCFM/person. This is determined as follows:

a. Low Bay Area

i. Ventilation air supplied by 38 air handlers supplying 2,563,000 SCFM of which 556,200 SCFM is outside air. Floor Area: 1,011,000 sq ft. This represents an outdoor air rate of .55 SCFM/sq ft.

ii. Outdoor ventilation: 556,200 SCFM. Occupancy: 1790 people. Outdoor ventilation air rate: 310 SCFM/person.

b. High Bay Area

i. Ventilation air supplied by 22 air handlers supplying 960,000 SCFM of which 163,000 SCFM is outside air. Floor Area: 499,500 sq ft. This represents an outdoor air rate of .33 SCFM/sq ft.

ii. Outdoor ventilation: 163,000 SCFM. Occupancy: 885 people. Outdoor ventilation air rate: 184 SCFM/person.

3. Air in the bay areas is turned over at a high rate of 6.1 Air Changes per hour (AC/Hr) for the low bay area and 3.3 AC/hr for the high bay area. It is continuously filtered for entrained dust and particulates.

a. Low Bay Area

i. Area Volume: 25,275,000 cu ft. Total air flow to space: 2,563,000 SCFM. Turn over rate: 6.1 Air Changes per hour resulting in the total air volume in the space being filtered once every 10 minutes.

b. High Bay Area

i. Area Volume: 17,482,500 cu ft. Total air flow to space: 960,000 SCFM. Turn over rate: 3.3 AC/Hr resulting in the total air volume in the space being filtered once every 18 minutes.

4. The assessment by base engineering that the air within the bay areas was acceptable and was without offensive odors or irritation.

With the determination of the air in the bay areas being classified as class 2 or better, one needs to determine whether the air is appropriate for use in ventilating the office areas constructed inside the bay areas.

Air from a Class 2 space would normally not be transferred to a Class 1 space because the spaces are used for different purposes and have different occupancy category classifications. However, in this case the office spaces are to be constructed inside the bay areas for the purpose of placing engineering and quality control staff “on the production floor” to provide direct support of floor operations. Consequently, the spaces share the similar purpose. ASHRAE 62.1 section 5.17.3.2 allows such spaces that are normally Class 1 be identified as “Spaces ancillary to Class 2 spaces” and as such classified as Class 2 space.

Recommendations:

Design the office area HVAC system as a single pass through system that does not recirculate or return air back to the air handler. This is to ensure adequate outside ventilation air is provided to the occupants of the office space. Equip space with relief dampers to avoid over-pressurization of the space and connect to a short section of lined duct to attenuate sound transmission from bay area. Note: Since conditioned air is taken from and returned to the bay area this system does not suffer an energy penalty from not reusing air from the office space.

The following example illustrates utilizing a return air system may result in inadequate outdoor air ventilation for the office space.

Office space: 10,000 sq ft installed in bay. Occupant load: 50 people. Cooling load: 8-10 watts per sq ft. (Approximately 25 tons total). Supply Air needed to condition space: 10,000 CFM. If 80% of the air is returned, (i.e. only 20% supplied from bay area) the actual amount of outside air supplied would be: (10,000CFM x 20% (air from the low bay area) x 22% (proportion of outside air contained in low bay air) = 440 CFM. The 50 occupants would need approximately 1000 CFM of outside air.

A single pass system would supply 22% (proportion of outside air contained in low bay air) x 10,000 CFM supply air = 2200 CFM.

Also recommend the office space air handlers be equipped with a MERV 7 or 8 filters to remove particulates that may become re-entrained in the process area. Also locate air intakes away from possible contamination sources and 8 to 10 feet above the finished floor.

K. Quinn Hart, P.E.

AF Subject Matter Expert, HVAC Systems