IAQ: Ventilation

Ventilation Is Important

The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) underscores the importance of ventilation and air filtration in reducing the transmission of COVID-19 through the position statement: “Transmission of SARS-CoV-2 through the air is sufficiently likely that airborne exposure to the virus should be controlled. Changes to building operations, including the operation of heating, ventilating, and air-conditioning (HVAC) systems, can reduce airborne exposures. Ventilation and filtration provided by heating, ventilating, and air-conditioning systems can reduce the airborne concentration of SARS-CoV-2 and thus the risk of transmission through the air. Unconditioned spaces can cause thermal stress to people that may be directly life threatening and that may also lower resistance to infection. In general, disabling of heating, ventilating, and air-conditioning systems is not a recommended measure to reduce the transmission of the virus.” Source ASHRAE

The amount of air required to be delivered to a given space by an HVAC system is based primarily on the number of people occupying the space, the type and amount of equipment, and the overall size of the space. Proper distribution of ventilation air throughout all occupied spaces is essential. When areas in a building are used differently than their original purpose, the HVAC system may require modification to accommodate these changes. For example, if a storage area is converted into space occupied by people, the HVAC system may require alteration (balancing) to deliver enough conditioned air to the space. Source CDC/NIOSH: INDOOR ENVIRONMENTAL QUALITY

ASHRAE Position Document on Infectious Aerosols April 14, 2020

Statutory Standards for Ventilation in Texas

Ventilation Methods and Standards

Nonexistent Mechanical Ventilation | Commonly found in the majority of older residences whether single-family structures, apartments, or condominiums.

Natural Ventilation | Open windows and doorways, the great outdoors—natural ventilation is unpredictable and uncontrollable.

Constant-Air-Volume (CAV) and Variable-Air-Volume (VAV) Mixing Systems employ an outside air intake, air-conditioning equipment, and an exhaust air path. CAV and VAV systems can be identically configured, with respect to the ventilation mechanisms. The exception: Due to the nature of a VAV system, the actual ventilation rate is most often unpredictable due to the absence of metrics (airflow rate) and, due to the fact the ventilation rate changes as the fan speed changes; a fixed-position outdoor-air damper cannot be relied on to introduce adequate ventilation at all times, with the exception of application of the demand-control ventilation strategy.

The Common Variable-Volume Air Handling System

Restaurant CAV systems manage ventilation in both the dining area and the kitchen. Kitchen exhaust hoods over stoves, ovens, dishwashers, and food preparation stations remove heat, water vapor, and undesirable odors. Kitchen make-up air-handling units replace the exhausted air with conditioned and filtered outdoor air at an airflow rate less than the exhaust airflow rate in order to created a negative pressure in the kitchen relative to the dining area. This prevents kitchen odors from infiltrating the dining room. The dining room ventilation air supply includes sufficient volume to satisfy the state-required ventilation rate and to replace air moved to the kitchen due to the difference in pressure between the two areas.

Typical Restaurant Constant-Volume Air Handling System

ASHRAE 62.1 published the Demand-Control Ventilation (DCV) standardin 1973 as a means to conserve energy while maintaining a healthy indoor air quality. Initially, a costly, multi-port carbon-dioxide (CO2) sensor-controller modulated the outdoor-air damper to maintain indoor CO2 concentration at an acceptable level for personnel safety and comfort. The ASHRAE standard has been revised over the years and has become somewhat complicated. ANSI/ASHRAE Standard 62.1-2019 is the most up-to-date version.

Over the years, ASHRAE 62.1 has become more complex, especially since 2004 when it was totally rewritten, and, in my opinion, become too complex. Simple systems are durable systems. If your organization is risk-averse, you will want to learn more about the Ventilation Rate Procedure. Start with this professional news article: CSE magazine, 14 Sep 2018 | ASHRAE 62.1: Using the Ventilation Rate Procedure.

Experimentally, my company settled on a 600 ppm (parts per million) CO2 concentration. This level of concentration provides a comfortable and healthy indoor environment that automatically adjusts itself on the basis of human occupancy. The CO2 concentration generally linearly tracks human population. The benefit is good ventilation at the best operational economy.

This simple DCV approach has been successfully applied to government buildings, commercial office buildings, and Texas detention centers.

Simplified Demand-Control Ventilation—CO2 sensor can be either duct-mounted or wall-mounted

Naturally, the introduction of unconditioned ventilation air represents a cost. The greater the ventilation rate, the greater the energy expended to temper (condition) outside air taken in to manage indoor air quality. The Honeywell-brand CO2 sensors (wall-mount, duct-mount, and SYLK network) provide fast response to changes in human population.

A Dedicated Outside Air system (DOAS) is a ventilation method superior to a return-air/outside-air mixing configuration because it consumes less energy than the CAV or VAV primary air-handling system and can be managed as either constant ventilation rate or demand-control ventilation.

Dedicated Outside System on a Variable Refrigerant Flow (VRF) System

A DOAS performs all ventilation functions separately from air-conditioning units. Energy-recovery heat exchangers and energy wheels conserve all but about 20% of the energy to heat or cool incoming outdoor air. The DOAS can be amended to an existing air-conditioning system through installation of ventilation ducting and the addition of one or more DOAS units.

The Displacement Ventilation System (DV) is applied to structures requiring high ventilation rates:
surgical suites, conference rooms, office spaces, classrooms, etc. Displacement ventilation has
been in use for several years but most people are generally unfamiliar with this method. Among
the advantages of DV are energy efficiency and effective ‘social distancing’.

“By strategically designing the height of air ventilation, displacement ventilation can enhance air change effectiveness. Displacement ventilation supplies air at very low velocity levels at or near the floor level, which then rises to the ceiling level. Since heat in a room is naturally stratified, displacement ventilation not only ensures that air is not delivered and pushed through the return air path (often the dirtiest portion of the air stream), but also tends to concentrate pollutants near the ceiling. Once there, the pollutants are out of the breathing zone and can be more easily removed.” International Well Building Institute: Displacement
Ventilation

Learn the functionality and the benefits of Displacement Ventilation (Price Industries) 20:32

Unlike mixing HVAC systems with overhead conditioned-air diffusers, DV systems only supply conditioned air to the occupied zone. This means HVAC units can be substantially lower capacity than an overhead mixing system.

View the video on the page titled IAQ: Airborne Pathogens—The Problem to see the plume effect.

Off-The-Shelf Products

The new dual adjustable* model CAR3 Constant Airflow Regulator is a modulating orifice that automatically regulates airflows in duct systems to constant levels. The passive control element responds to duct pressure and requires no sensors or controls.

The CAR3 compensates for changes in duct pressure caused by: use of demand control solutions, thermal stack effect and dust-clogged filters as an example. The CAR3 provides a low-cost solution to balancing forced-air systems for ventilation, heating, and air conditioning, eliminating the need for on-site balancing. The CAR3 will regulate airflow in supply, return, or exhaust duct systems. The CAR3 is designed to complement ALDES register assemblies or can fit inside standard rigid round ducting, as well as fittings such as take-offs, tees, etc. with a double lip gasket around the circumference ensuring a tight, no-leak fit.

For more information and/or to purchase, go to the manufacturer’s website https://aldes.us/commercial-ventilation-product/car3/

Review installation, operation, and product specifications https://aldes.us/commercial-ventilation-product/car3/

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Published by John White

A lifetime (over 50 years) of experiences with automation and control systems ranging from aerospace navigation, radar, and ordinance delivery systems to the world's first robotic drilling machine for the oil patch, to process-control systems, energy management systems and general problem-solving. At present, my focus is on self-funding HVAC retrofit projects and indoor air quality with a view to preventing infections from airborne pathogens.

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