Sunday, 26 June 2016: 1:30 PM-3:00 PM
Fundamentals and Applications
Paul Turnbull, Siemens Building Technologies, Inc.
Air velocity can impact a number of situations including the minimum duct wall thickness required for handling the reduced commercial kitchen exhaust velocity now allowed by NFPA 96. The results of an important CFD study on the impact of make-up air velocity used to control smoke in atriums is reported. Full scale fire tests show the influence of pressure compensating systems for stairwells. The results of a CFD study on ventilated patient isolation rooms show that portions of a room are not well ventilated when the standard ventilation rate is used. A simpler and more precise method has been developed to predict the necessary separation distance between a variety of exhaust configurations and make-up air inlets.
1.00 Analyzing the Performance of a Kitchen Exhaust Air Duct with Regards to Recent Standards: A CFD/Thermal Stress Simulation (ST-16-001)
Recent standards have defined a lower minimum air velocity in kitchen exhaust air ducts. While the aim remains in place to maintain an acceptable in duct air velocity and expel exhaust air-borne particles or fluids towards filters/exhaust outlet. Steel ducts are usually specified to a standard steel gauge thickness, capable of handling the extreme load conditions of high temperatures, possible corrosive conditions, and negative pressure levels within exhaust duct. This paper simulates a typical kitchen exhaust air duct performing under extreme temperature conditions and the low air duct velocities. Lower exhaust air velocities should correspond to lower in duct negative pressure values and therefore, possibly a reduction in steel duct wall thickness.
A CFD/thermals tress analysis was carried out under the most extreme load conditions specified under recently issued standards. This analysis has demonstrated that a lower steel duct thickness is more than sufficient than what is specified in recent standards, and therefore, a lower steel thickness gauge can be used. Provided a comprehensive simulation is carried out demonstrating that the reduced exhaust duct sheet thickness is well within the steel duct mechanical material properties as explained in this paper.
2.00 A CFD Study to Identify Methods to Increase Maximum Velocity of Make-up Air for Atrium Smoke Control (RP-1600) (ST-16-002)
The primary purpose of this study is to develop engineering methods to assess the impact of increased make-up air velocity in atria. The current restriction defined by NFPA 92 states that make-up air must not exceed 1.02 m/s (200 fpm) during the operation of a mechanical smoke exhaust system. This limitation not only limits creative and aesthetic atria designs but may also represent a significant cost. The present study analyzes the effect of make-up air injected by a variety of vent sizes at elevations at or below the limiting elevation of the flame through numerical simulations. This study focuses on identifying worst-case scenarios for the interaction of make-up air with an axisymmetric plume, by modeling multiple configurations, observing the results, and adapting further simulations to elicit the most extreme cases. A mass flow rate diagnostic is used to assess the increase in entrainment, i.e. smoke production. This mass flow diagnostic is developed to provide a comparative analysis, assessing the increase in the rate of smoke production with a specified make-up air velocity with that produced with no mechanical make-up air. The proportional increase in entrainment is defined as an alpha factor. The most significant smoke production increase and smoke layer stabilization descent is associated with the 1 MW fire, when lesser increases observed for the 2.5 MW and 5 MW fires. As the make-up air is introduced further from the edge of the flame, the apparent effect of the airflow velocity is reduced.
3.00 Performance of Stairwell Pressurization System with Open Stairwell Doors (RP-1447) (ST-16-003)
The National Research Council of Canada (NRC) conducted full-scale fire experiments to investigate whether pressure compensating systems are needed to maintain tenable conditions within pressurized stairwells. Ten tests were conducted in the NRC 10-storey test facility with the stairwell in the facility pressurized. The tests were conducted with the stairwell door on the fire floor closed and selected stairwell doors on the other floors open. Two fire scenarios with a shielded sprinklered fire and non-sprinklered fire were tested with varying number and location of open stairwell doors. Tenability analyses were conducted with experimental test results to investigate the performance of pressurized stairwell with and without pressure compensating systems. Without compensating for pressure losses, the pressure difference across the stairwell door on the fire floor decreased considerably with open stairwell doors. However, a non-compensated stairwell remained tenable for 30 minutes as long as the door on the fire floor was closed both for the shielded sprinklered fire and the non-sprinklered fire scenarios. It is concluded that if the base pressurization system meets the requirement of the design pressure difference with a proper arrangement of air injection points, the stairwell remained tenable as long as the door on the fire floor is closed for both sprinklered and non-sprinkled fire scenarios used in the tests.
4.00 Assessing Effectiveness of Ceiling Ventilated Mock Airborne Infection Isolation Room in Preventing Hospital Acquired Influenza Transmission to Health-Care Workers (GIA 15-16) (ST-16-004)
Exposure to airborne influenza from patient’s cough and exhaled air causes potential flu virus transmission to the persons located nearby. Flu virus can be transmitted through air by patient’s cough creating aerosols containing flu virus. Hospital acquired influenza is a major airborne disease that occurs to health care workers (HCW).
The present study examines the air flow patterns and influenza-infected cough aerosol transport behavior in a ceiling-ventilated mock AIIR and its effectiveness in mitigating HCW’s exposure to airborne infection. The Computational Fluid Dynamic analysis of the air flow patterns and the flu virus dispersal behavior in a Mock AIIR is conducted using the room geometries and layout (room dimensions, bathroom dimensions and details, placement of vents and furniture), ventilation parameters (flow rates at the inlet and outlet vents, diffuser design, thermal sources, etc.), and pressurization corresponding to that of a traditional ceiling mounted ventilation arrangement observed in existing hospitals. The measured data showed that ventilation rates for the AIIR is about 12 ACH (Air changes per hour). However, the numerical results revealed incomplete air mixing, and that not all of the room air was changed 12 times per hour. Two life-sized breathing human models were used to simulate a source patient and a receiving HCW. A patient-cough cycle is introduced into the simulation, and the AI dispersal is tracked in time using a multi-phase flow simulation approach.
5.00 Simplified Procedure for Calculating Exhaust/Intake Separation Distances (RP-1635) (ST-16-005)
This research was sponsored by ASHRAE Technical Committee (TC) 4.3. The purpose of this Research Project is to provide a simple, yet accurate procedure for calculating the minimum distance required between the outlet of an exhaust system and the outdoor air intake to a ventilation system to avoid re-entrainment of exhaust gases. The new procedure addresses the technical deficiencies in the simplified equations and tables that are currently in Standard 62.1-2013 Ventilation for Acceptable Indoor Air Quality and model building codes. This new procedure makes use of the knowledge provided in Chapter 45 of the 2015 ASHRAE Handbook—Applications, and was tested against various physical modeling and full-scale studies. The study demonstrates that the new method is more accurate than the existing Standard 62.1 equation which under-predicts and over-predicts observed dilution more frequently than the new method. In addition, the new method accounts for the following additional important variables: stack height, wind speed and hidden versus visible intakes. The new method also has theoretically justified procedures for addressing heated exhaust, louvered exhaust, capped heated exhaust and horizontal exhaust that is pointed away from the intake.