Innovative Energy Engineering



Ventilation & Exhaust

Pollutants or noxious gases such as CO2, VOCs, and odors need to be removed and fresh air with O2 needs to be supplied. Good Indoor Air Quality (IAQ) is required to protect health and life safety.

Supply air requirements typically are based on occupancy loads and exhaust requirements typically are based on pollutants produced. For most spaces these requirements don't balance. However, it is best to balance the supply and exhaust flows because any imbalance is balanced by uncontrolled infiltration or exfiltration. In some cases this imbalance is desired to control pressure between spaces. An example would be a garage being kept at negative pressure compared to an adjacent office to prevent any pollutant transfer to the office. In balanced systems exhaust and supply flowrates must be the higher of either requirement.

Air from spaces requiring exhaust must be "exhausted" to the outside and must not be recirculated. Few exceptions may be that locker room air can be used as makeup air for bathrooms etc.

Energy Recovery and Dedicated Outside Air Systems

In most cases it is of advantage to pre-treat the fresh air before it enters the space or an AHU. For example fresh humid air should be cooled and dehumidified before entering an AHU or make-up air for a heated garage should be heated. This requires energy, which can be reduced by air-to-air heat exchangers. These can be standalone devices with their own fan and filters or can be part of an AHU.

The most common type is the Energy Recovery Ventilators (ERV) that recover latent (moisture transfer) and sensible energy. In winter they preheat the outside air (OA), and in summer they pre-cool and dehumidify OA. Heat Recovery Ventilators (HRV) only recover sensible energy. Besides energy savings, costly downstream equipment can be reduced. In cases of dirty air energy recovery may not be economical due to filtration requirements. some exhaust, like grease-laden air, can not be routed through the heat exchanger.

Wheel ERV are more efficient, but more costly than Plate ERV. Especially cooling efficiency is significantly higher. Typically when we have cooling and heating, wheel ERV are more economical. Plate ERV are almost as efficient in heating and more economical in heating-only applications. Plate ERV typically are less efficient in cooling. Another type is the sensible-only runaround loop that uses water or glycol coils in in supply and exhaust air flows. Those are less efficient and require a pump. another sensible-only energy recovery device are heat pipe systems.

All ERV have minor cross-contamination (1-5% depending on type and pressure differences). This is not critical in most building applications but may be a problem in biohazard and other applications. When "0" cross-contamination is required a Heat Recovery (sensible heat only) plate heat exchanger or runaround loop can be used.

Air leaving the heat-exchanger is pre-treated, but not at the required condition. In most cases it is beneficial to further treat the air before it enters spaces or other equipment. this can be done by addition of cooling, heating and other energy recovery devices. Especially for dehumidification this treatment is beneficial. The system for this treatment of air typically is referred to as Dedicated Outside Air System (DOAS).

Any imbalance in supply and exhaust air leads to infiltration or exfiltration that can lead to building damage, IAQ problems and robs the ERV/DOAS of the opportunity to efficiently treat the air or recover exhaust energy.

Standards

Opportunities for Improvement

Specialty Applications

Many applications are proprietary to certain industries and follow industry-specific guidelines. The National Institute for Occupational Safety and Health and OSHA also provide reference material:

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