Multi-purpose means multiple challenges
By ROBERT BEANForced Air HVAC Systems Building Mechanical Electrical HVAC Systems Indoor Air Quality
Points to consider when designing industrial and commercial systems.
One of the challenges facing designers of ventilation systems is developing appropriate application-based strategies. Clients generally have a limited understanding of occupant exposure risks and a preconceived idea of what is an acceptable budget. This is particularly true of multi-purpose buildings containing both commercial and industrial spaces. The approach for one is not suitable for the other – the same is true of the capital and operational cost budgets.
In commercial buildings there will be general HVAC systems of mixed (recirculated) air for heating and cooling and exhaust systems, often with heat recovery devices. Occupant density is high relative to industrial facilities and enables proxies such as CO2 levels to be used for occupancy load and ventilation control. Pollutants are not explosive and are of a lower health and safety risk. The primary concern will be for the quality of make up air and control over the release and management of internally generated pollutants from people, office systems, equipment and materials of construction. These systems are well served by using ASHRAE Standard 62.1 Ventilation for Acceptable Indoor Air Quality as a reference.
In the case of industrial buildings, the designer may require general systems with or without recirculation, with or without heat recovery, plus local and often specialized exhaust systems. Unlike commercial buildings, occupant density can be low and use of CO2 as a proxy for occupancy load and ventilation control can be misleading. Dilution air is not often recommended as a solution, particularly in spaces with hazardous compounds that could be explosive and present a higher health and safety risk. These pollutants are generated from materials, processes, operations and equipment and are industrial in nature, including toxic vapours, gases, liquids, fumes, dusts and noise, as well as heat and cold.
ASHRAE Standard 62.1 might apply to the general ventilation systems in these spaces but the designer also needs to look at threshold limit values (TLV), short- and long-term time weighted averages (TWA) of occupational exposure limits (OEL) to contaminants that may be present in the space. Due to this higher level of concern, very little flexibility in the indoor air quality systems is offered to workers in these environments. As such, greater care is required in their design.
One useful strategy employs a hybrid approach separating the thermal comfort system from the ventilation system, which enables the designer to engineer solutions that are dedicated exclusively to addressing indoor air quality.
Thermal comfort systems, such as radiant floor cooling and heating, are ideal in industrial facilities. The floor as a heat exchanger is not an explosion risk and the system does not disturb the flow of particulate and gases, as is the case with an air-based system. In addition, thermal systems promote drying of wet surfaces and enable worker comfort conditions not afforded by other options. Using radiant floors eliminates gas lines that would be required for space heaters, as well as electrical power to switches and motors that may contribute to explosion risks.
Dedicated indoor air quality systems are comprised of in-space, general purpose exhaust ducting, which is connected to externally located fans and motors. These systems extract common air from the space. Localized and specialized exhaust systems are then used to extract polluted air away from the workers’ breathing zone. Such systems are found in wood working shops, welding and fabrication shops and vehicle service and maintenance bays.
Due to the characteristics of pollutants found in these types of spaces, composition, density and buoyancy are concerns. Therefore, air velocities are a big deal. Velocities can vary from 1000 to 2000 fpm for some vapours, gases and smoke, to as high as 4500 fpm for heavy and moist dusts.
To relieve negative pressure created by these exhaust systems, make up air is frequently introduced to the space at ambient conditions using direct fired make up air units. In this equipment, a gas burner is installed directly in the incoming air stream and where the products of combustion are supplied to the building or process. Because there is no heat exchanger involved, the efficiency approaches 100 per cent discounted for the production of water vapour. Since there is no combustion exhaust, strict adherence to regulations and codes, such as ANSI Z83.4-2015/CSA 3.7-2015 – Non-recirculating direct gas-fired industrial air heaters, is required. These devices are not for every project but they are useful in buildings with large open spaces having high exhaust and infiltration airflows and no low-humidity control requirements.
Designers involved in these types of spaces should become familiar with the content found in the most current version of Industrial Ventilation, A Manual of Recommended Practice for Design published by the American Conference of Governmental Industrial Hygienists.<>
A note from Robert to HPAC readers: For those who are in the know, know that I know that there are strict definitions for pollutants, contaminant and toxicants, though I have taken executive authority to use them loosely and interchangeably in the article.