STCKM_TSS_06Sept13.doc
- 1. 1 ACOUSTIC COMFORT STRATEGIES IN AIR CONDITIONING AND MECHANICAL VENTILATION SYSTEM Tuan Suhaimi Salleh Mechanical Engineering Branch, PWD Headquarters, 18th -21st Floor, Centrepoint North, The Boulevard, Mid Valley City, 59200 Kuala Lumpur, Malaysia Phone : +603-9235-4521 ; Fax : +603-2287-3946 Email: tsuhaimi@jkr.gov.my ABSTRACT The comfort acoustical environment of a workplace is typically given very little attention or always overlooked during buildings project planning and design. The functionality and aesthetics of the workspace are usually the primary focus of the designer. Providing a comfortable environment for office occupants contributes significantly to their optimum performance and eliminates symptoms associated to sick building syndrome. In modern green office concepts, workplace comfort is really a combination of factors that includes day lighting and electric lighting, indoor environmental quality, temperature, humidity level and acoustics. The assault on ears in the workplace can come from traffic noise outside, mechanical equipment in adjacent spaces, air conditioning system, copiers, phones, and voices within the workplace. However this article only discusses issues relating to acoustic comfort control strategies in Air Conditioning and Mechanical Ventilation System (ACMV) that are usually encountered in buildings. Some success rectification works in ACMV system noise and vibration problems are presented as a case study. Keywords: ACMV system; acoustic comfort; workplace; noise; vibration. INTRODUCTION Thermal comfort is the main goals for mechanical engineers at Public Work Department, Malaysia (PWD) in the process of designing and preparing the technical specification of Air Conditioning and Mechanical Ventilation System (ACMV) in various buildings project. At the same time it is very important to keep the background noise of the ACMV system low to a level that ensures comfortable working environment for office building occupants. The degree of occupants’ satisfaction is determined by many factors, for example, meeting and conference rooms, auditoriums or recording studios only tolerate a low level of background noise. On the other hand, high levels of background noise are acceptable and even desirable in certain situations, such as open-plan offices where a certain amount of speech and activity masking is essential. The ideal background noise should be a balance distribution of sound energy over a broadband frequency range with no dominant band of noise, smooth and steady with no audible tones such as whine or rumble and few fluctuations in noise pressure level. Acoustic comfort means having the right level and quality of sound to use the space as intended especially for office buildings. People are more productive and happier when they are not distracted by noises from outside or from surrounding.
- 2. 2 Humans perceive to sounds and loudness is a subjective measure. We can create a comfortable environment by controlling objective measures like sound pressure level (in decibel), reverberation time, the sound reflection and damping properties of materials such as acoustic panels on ceilings and walls. Acoustic environment can manipulate a person’s ability to work. A person’s productivity will decrease when they are in a noisy and uncomfortable workplace and vice versa, it can enhance productivity when it supplies an environment which supports easy verbal communication (Hodgson, M, 2008). Noisy and uncomfortable working space also will create disturbance and break concentration and eventually result in stressful occupants (Evans, G. W., & Johnson, D, 2000). ACOUSTIC IN GREEN BUILDINGS Most of the literature reviews are the outcome of analysis, studies and surveys done on offices and educational buildings in the U.S. These buildings are rated as green building based on LEED (Leadership in Energy and Environmental Design) which was developed by U.S. Green Building Council (USGBC) in 1998. The literature studies reviewed mainly focus on occupants’ satisfaction in green buildings and comparing them to satisfaction from occupants in conventional buildings. The types of acoustical complaints were also being surveyed to determine the major acoustical problems occur. Rao, S.P. et al (2011) discovered only Hodgson (2008) reported the result of the acoustical measurement done. Table 1 shows the measurement parameters and acceptability criteria used in Hodgson’s study compared to ASHRAE Design Guideline for HVAC-Related Background Sound in Room. ACMV system noise always determined by NC curves which is correlate well with occupant satisfaction in office buildings without excessive levels of low frequency noise or disturbing tonal noises. NC values are defined by a series of curves that give noise level limits in octave bands across the frequency range of human hearing. Table 1. Measurement parameters and acceptability criteria (Hodgson’s study) Measurement Parameter Acceptability Criteria (Hodgson) ASHRAE Background Noise-level, NC NC 30-35 in meeting and conference rooms NC 35-40 in workspaces NC 25-35 in meeting and conference rooms NC 30-40 in workspaces The surveys by many researchers found huge occupant dissatisfaction on acoustic quality in green buildings compared to conventional buildings, especially in working spaces which utilizes the open plan office layout. The main acoustic problems established are the lack of speech privacy and the problem with intermittent noise. Major acoustic complaints made were people talking on the phone; people talking in neighbouring area, people overhearing their private conversation, getting caught up in others’ conversation and telephone ringing. Other complaints were also received on noises projected by office equipment, mechanical system, office lighting, outdoor traffic and people in corridors.
- 3. 3 It was found that there are four major green building design strategies which might contributes to the poor acoustical performance. They are natural ventilation, daylighting, reduced use of finishes and open plan office layout. Acoustic in Green Building Rating System The acoustic performance in Green building rating system generally consider the background noise in the space (Noise Criteria - NC, Preferred Noise Criterion - PNC, Room Criteria - RC), the noise isolation from adjacent spaces (Sound Transmission Class - STC, Noise Isolation Class - NIC) and from the exterior (Outdoor-Indoor Transmission Class - OITC), and the room acoustics (reverberation time, speech intelligibility). These acoustic parameters can be controlled by providing sufficient attenuation to the ACMV systems, by designing appropriate room boundaries, and by selecting suitable room finishes. The acoustic comfort should be tackled in many ways in green building rating system as mentioned above but only contributes a low point in overall weightage. Table 2 shows the acoustic percentage rating system available in sustainable design. Table 2. Acoustic credits of the overall credits available for sustainable design Rating System Total point Acoustic Point Acoustic Value GBI (Malaysia) 100 1 1.0% PH (Malaysia) 117 1 0.85% BCA Green Mark (Singapore) 190 1 0.5% LEED (USA) 100 1 1.0% A low credit doesn’t mean acoustic comfort shall be put aside but need to be tackled wisely or otherwise could create major annoying problems which had proved through our ACMV noise and vibration forensic cases. ACOUSTIC ISSUES IN ACMV SYSTEM Nowadays building structures are designed to be light so that careful coordination in structural design and mechanical layout is essential. Otherwise proper isolation for ACMV equipment located inside the mechanical room is a must to avoid structure- borne noise phenomena. In another word, the ACMV system excessive noise appears to arise more from lack of proper design process than from inadequate technology that contributes to the noise. Typical ducted ACMV system acoustics problems arise from its main components, they are; 1. Equipment noise and vibration a. Location (radiated noise) b. Isolation 2. Duct system a. Duct borne noise b. Duct break-in and break-out noise
- 4. 4 c. Airflow-self generated noise (terminal noise) The design of ducted ACMV system must address the two distinct points above to control the noise and vibration otherwise the design will fail. Mechanical Equipment Noise and Vibration The rotating or motor-driven machinery that incorporate in ACMV systems will generate vibration energy that can travel through a building's structure and spread from the walls, floors and ceilings in the form of structure-borne noise and radiate noise into the receiver spaces. It is essential to control vibration at its source, because once it is allowed to transmit into the building structure, vibration from building services equipment are widespread and extremely difficult to contain. Duct System (airflow) Duct borne noise propagates along the ductwork, follows all branches and bends and ultimately exits at the supply diffuser impacting the occupied space. The sound generated by the fan will travel along the ductwork both upstream and downstream of the Air Handling Unit (AHU) easily because the velocity of sound is much higher than the velocity of the air in the ducts. The noise is generated aerodynamically by the flow of air through the duct system. This regenerated noise is very much dependent on the velocity of the air and smoothness of the flow. It increases very rapidly as the flow velocity is increased by changes in cross-sectional area of the duct. Airflow generated noise is proportional to the sixth power of velocity. In practice for room not highly sensitive to noise, airflow generated noise can be ignored when velocities are below 8 m/s in the main duct and 3 m/s in branch duct. Duct break-in noise is radiated ACMV equipment noise that enters ductwork and propagates into rooms. A flexible duct is light in weight, flexible and ease for installation is commonly used in air conditioning systems. Noise can more easily penetrate flexible ducts because of their lightweight nature. Duct break-out also propagates along ductwork and transmits through the wall of the duct impacting the adjacent space. At frequencies above the transition between axial and multi-mode transmission of sound (f > c/(2*duct width/height)) in a duct, the sound field inside the duct may be considered as diffuse and the breakout of the noise from inside to the outside of the duct is protected by the sound reduction index or transmission loss of the duct wall. However since the duct runs over a significant length inside the receiving room, the total amount of the noise breakout increases as the length increases. The final points in the ACMV air distribution system are the terminal air devices. There are “grilles”, “diffusers”, “register” and “vent cover” that go over duct opening in the room. Air noise from diffusers and from transitions can cause additional noise in the receiving room. ACMV NOISE AND VIBRATION STATISTIC Noise and Vibration Control Specialist Unit of Mechanical Engineering Branch, Public Work Department was established in the year 2008. Since that year there have been 24 cases of noise and vibration problems entertained which include 5 cases on noise level measurement during testing and commissioning of shooting range belonged to the
- 5. 5 Royal Malaysia Police Department and The Maritime Institute of Malaysia, which were to make sure noise from guns shooting activities do not affect the neighbourhood. Out of the 24 cases, 19 were related to noise and vibration control on ACMV systems. Although there were least cases, graph on Figure 1 shows the details breakdown of dominant noise and vibration sources which were emanated from equipment and airflow based on 19 forensics studies. There were 12 cases or 63% of noise problems attributed from the equipment (chillers, cooling tower, pump, AHU and condensing unit), and the remaining 7 cases or 37% were excessive noise from airflow. Figure 1. Noise and Vibration Sources Based on CKM Forensics Data In addition to noise problem, the equipment of ACMV systems also triggered building structural vibration due to plant location and improper installation of isolator attached to the equipments. From sites investigation there were 3 cases or 25% of studied equipment vibration problems transmitted to equipment ancillaries such as pipe support/hanger and also to the building structure. ACMV BASIC ACOUSTIC DESIGN TECNIQUES Good space planning and location of mechanical room at the early stage of design process is the best way to avoid noise and vibration problem in the office building. It is a good practice to position particularly noisy equipment in mechanical room or place where they are located away from critical area such as meeting or office room. Mechanical room shall large enough to allow adequate turning before duct exit and defy any alleged floor space constraints if possible, avoid placing AMCV equipments enclosures close to walls or ceilings to avoid ‘close coupling effect’ where a small air space will conduct equipment vibratory motion to wall or ceiling. Air tight mechanical room construction is advantage since sound can leak through as small as 1 mm openings. Alternatively creating a buffer zone of less critical space such as store room between a mechanical room and noise-sensitive space allows simpler and less costly
- 6. 6 constructions to provide the desired isolation, and this minimizes the risk of noise leaks at penetrations. Floating concrete floors which are supported above the structure floor on resilient mounts are required to separate mechanical rooms from noise-sensitive spaces that are above or below the mechanical room. Floating floor is not generally designed as part of a vibration isolation scheme for mechanical equipment but their primary use is for airborne sound isolation. Addition to that, it is sometimes necessary to have special noise-isolation ceilings above or below mechanical rooms and ducts routes. Right sizing of ACMV equipments, distribution system and choosing the correct size (cooling capacity) is critical not only in getting the cooling comfort but also lowest maintenance and life cost cycle (LCC) of the system and the best acoustic efficiency. The basic duct design concepts to minimizing ACMV system noise should not be ignored. Some type of solution shows in Table 3 to minimize air flow noise Table 3. Ductwork for the limitation of air flow noise Strategy Action Sample of solution Minimize rumble noise Keep airflow velocity in the duct as low as possible (max. 7.5m/s) near critical noise areas. Optimum NOTE 1: Slope 1 in 7 preferred for 10m/s above Bad Locate elbows or duct branch takeoffs (A) at least 1.5 duct diameters apart (B). Good NOTE 3: Rugged turning vanes should extend full radius elbow Bad Minimize flow resistance and turbulence Use turning vanes in large 90° rectangular elbows and branch takeoffs Quietest Noisier solution NOTE 2: Minimum 15mm radius required
- 7. 7 Duct expansion angle not more than 15° Quietest Noisier solution Proper duct tee design Quietest Noisier solution CASE STUDIES Usually we will perform in-field forensic investigation regarding to noise and vibration problems when there is a request from other government bodies through our counterpart in JKR nationwide. Most cases appear during pre-occupancy of the building or during defect and liability period. For each case we need to investigate the causes of the problem, analyse and finally bind a technical report which consists of a few alternative solutions to be followed voluntarily by the requester. We will also perform a follow up inspection to verify the effectiveness of our suggestion followed by the client or building owner. In this paper three successful rectifications are presented as case study. Most of the suggested retrofit works to suppress equipment’s noise and vibration such as water and air cooled chillers are still under client consideration before proceeding to the retrofit work because of the cost and complexity. Thus these kinds of cases were not highlighted in this paper. Equipment Hand held Sound Level Meter Type 2; SoundPro DL Data logging SLM 1/3 Octave with QuestSuite Professional II Software are used for noise level assessment and Photon Pro Analyzer; Single Axis Accelerometer (3056B2- 50g range, 100 mV/g, 10-32 top) with RT ProPhoton Software are used to measure vibration level for our forensic studies. Noise Descriptors Noise descriptors such as Leq, L10, L90, Lmax and NC were recorded. Usually in steady state noise condition the noise level measurement was carried out for 1-5 minutes using SLM data logger. Definition for the noise descriptors are presented as followed: Leq : The equivalent continuous dBA level which has the same energy as original fluctuating noise for the same given period time. L10 : A specified dBA levels which is exceeded ten percent of the time during the whole period of measurement.
- 8. 8 L90 : A specified dBA levels which is exceeded ninety percent of the time during the whole period of measurement. Lmax : The root means squared maximum level of a noise source or environment where peak is the maximum level of the raw noise source. NC : Noise Criteria curves define the limits that the octave band spectrum of a noise source must not exceed in order to achieve a level of occupant acceptance. Vibration Criteria Vibration criteria can be specified relatively to three areas: (1) human response to vibration, (2) vibration levels associated with potential damage to sensitive equipment in a building, and (3) vibration severity of a vibration machine. Equipment and structural vibration severity ratings are based on measured RMS velocity (mm/s). Case Study 1: Noise From Duct Airborne In this case we investigated causes of the annoying noise from air conditioning system which exceeded standard NC30 for Operation Theatre (OT) room during pre-occupancy stage. The main contributors to this noise were high velocity airflow and air turbulence in duct system serving to OT room. This ductborne noise occurred because of existing supply duct size did not follow the design parameter causing airflow velocity inside supply duct slightly higher and created annoying noise. a) Before Rectification Work Rating: NC37/ NC20 (ambient) Leq: 46.4dBA/27.1dBA (ambient) Airflow velocity: 6.3 m/s b) After Rectification Work Rating: NC30/ NC21 (ambient) Leq: 38dBA/30.1dBA (ambient) Airflow velocity: 5.4 m/s Figure 2. Operation Theatre Room NC Curve
- 9. 9 The smoothness of air flow in duct also could not be achieved because of air turbulence at the Air Handling Unit (AHU) outlet since the original design did not attach a turning vane at the first bend of the AHU outlet duct. With this finding we suggested to project superintending officer to advice the responsible consultant to replace existing duct with a new one according to the design parameter. We also recommended them to install turning vanes inside the AHU outlet duct to reduce air turbulence. The retrofit works was done accordingly and as a result OT room noise rating NC30 was achieved. Figure 2 show the comparison result before and after retrofit works. Although this is just a simple fault in ACMV system and could be find anywhere but the annoying noises will give critical impact to the end user if not tackled wisely during early stage of construction especially for room that required low level background noise. Case Study 2: Vibration Cause Condenser Water Pipe Leak In this case we were requested to investigate the cause of condenser water pipe leak while in operation. These piping’s system are connected to a cooling tower and the pumps to circulate condenser water in the air conditioning system. Although this client has their own internal maintenance team, they still need our technical advice to solve this problem since we are among the technical advisors to the Malaysia government. Figure 3. Vibration Measured on Pump Set
- 10. 10 Figure 4. Vibration Measured on Building Floor Figure 3 and 4 shows the vibration level measured based on RMS velocity level for condenser water pump set and floor near to the condenser water pump set located on the rooftop. The peak RMS velocity for one of condenser water pump set was 6.4227 mm/s and RMS velocity of resonance interaction between pump sets and floor was 0.6047 mm/s respectively. Both values exceeded the maximum allowable RMS Velocity Levels as recommended in 2003 ASHRAE Application Handbook. These situations occurred due to lack of maintenance causing excessive bearings wear which contributed to the condenser water pump sets being operated unbalanced. Assuming that impurity inclusion or defects existed inside pipe material and/or inside arc welded portion, the severity pump vibration had initiates fatigue crack, propagates and finally condenser water pipe leaked, spoiling the ACMV operation. From this finding, we suggested to the building owner to perform major overhaul and re-alignment the entire condenser water pump sets. As a result, after rectification works done, both situations show very low vibration effects and almost negligible; the peak RMS velocity for the same condenser water pump set is 0.0759 mm/s and RMS velocity for floor is 0.0652 mm/s. Case Study 3: Excessive Exhaust Fan Noise Investigation of this case began because of an official complaint received by building management from adjacent resident due to annoying noise released by the kitchen exhaust system. Neighbourhood apartments are located 10 meters away from the building perimeter fence or 40 meters from the exhaust fan outlet. The complainant would bring this matter to court and sue the building owner if this problem is not resolved. To avoid this matter prolong, building management has requested the State PWD Mechanical to solve this problem since their internal maintenance team does not
- 11. 11 have the expertise. Finally, our state counterpart has sought technical advice on these matters from us. From our observation, although this premise location can be designated as mixed area development with high traffic movement, the noise from exhaust fan is obviously annoying residents nearby. Noise pressure level measured at affected apartment showed that it exceeding maximum permissible sound level (60 dBA) as specified in Department of Environment Malaysia (DOE) guideline. As shows in Figure 6 (a) the noise pressure level measured at balcony of affected apartment, Leq = 66.3 dBA. The background noise at that time should be same as measured ambient noise, Leq = 53.9 dBA. We decided to re-design back the kitchen exhaust duct and changed the fan to a new one based on a backward design process since they have no as-built drawing or specification document of the existing system as a reference. All needed measurements and raw data were taken based on actual site condition for calculation and analysis before preparing a complete specification and technical drawing of the kitchen exhaust system, enabling our PWD state counterpart to complete their procurement tendering process. Table 4 shows important data or parameters of existing and re-designed condition that most influences noise level of the kitchen exhaust system. Re-calculation has been made based on onsite measurement shows that the existing fan size is oversized, as a result the actual contaminated air volume to be exhaust out is slightly less, hence the new design needs a fan with much lower speed to lower the airflow speed and contributes in reducing airflow noise and more economical. Table 4. Design Data and Parameter Factor Existing Condition Re-design Condition Contaminated air volume 127,425 m3 /hr 95,144 m3 /hr Fan Speed/Diameter 1,450 rpm/1,000 mm 960 rpm/1,400 mm Motor Rating 40 kW 30 kW Other improvements that have been made to assist the exhaust system reducing the background noise are introduced hereof: i) Acoustic wall barrier facing to affected apartment. ii) Acoustic louvers facing exhaust fan outlet, so that exhaust air disperse and at the same time acts as a noise filter to the adjacent office room. iii) Oversized duct to reduce contaminated airflow velocity complete with internal insulation and turning vanes inside duct bend. iv) Introduced the plenum chamber concept to reduce air turbulent. Figure 6(b) shows the result after rectification works done. Noise pressure level tremendously fell to ambient level 55.5 dBA and noise criteria rating also show a lot of improvement from NC63 reduced to NC50, hence noise emission level diffused by kitchen exhaust fan system is lower than DOE maximum permissible sound level. Although the new installed exhaust fan outlet facing only 15 meters away from the adjacent office room, noise pressure level and noise criteria inside that office room remain at a comfort level, where Leq = 43.5 dBA and NC33 respectively.
- 12. 12 a) Before Rectification Works Rating: NC63/NC51 (ambient) Leq: 66.3dBA/53.9dBA (ambient) b) After Rectification Works Rating: NC50/NC48 (ambient) Leq: 55.5dBA/55.7dBA (ambient) Figure 6. Noise Data Measured at Affected Apartment’s Balcony CONCLUSION It can be concluded that without careful implementation in overall ACMV system design strategies, acoustic quality is easily compromised. The designers should not overlook on these influence as it could jeopardize the acoustical environment. It is hoped that this article demonstrated a simple guide in predicting the potential acoustical problems that might occur in office building. The selected three case studies of noise and vibration problems in office building claims the immediate countermeasure action i.e. minimizing error is a must or no compromise to occurrence of mistakes during the initial stage of design processes such as ACMV equipments sizing, suitable location or positioning to avoid any noise and vibration effects from ACMV system especially to sensitive rooms. Full attention on supervisory work during ACMV system installation promises high quality products in the sense of human comfort. Perform without fail the scheduled predictive maintenance on ACMV equipments is very important to eliminate system failure creating side effect problem such as tendency of crack initiation on floor structure due to excessive equipment vibration while in operation. On top of that, for long term corrective measure, new specifications and technical drawing for noise and vibration control details have been drafted and incorporated into documents known as i) Standard Technical Specification for Air Conditioning and Mechanical Ventilation System and ii) Standard Mechanical Design Detail Drawing For Designers/Contractors. With these documents, noise and vibration control designs can be implemented in new projects and retrofits installation to meet the JKR standard of requirements.
- 13. 13 REFERENCES Istvan L.Ver and Leo L.Branek. 2006. Noise and Vibration Control Engineering, Principles and Applications: John Wiley and Sons. N.Y. Gergeo, G.A. Sehrndt and W.Parthey. 1992. Noise Sources. Proceedings 6th International FASE-Congress, Zurich. 104-124 2001 ASHRAE Fundamentals Handbook (SI). Chapter 7, 7.1-7.18, 2003 ASHRAE Application Handbook (SI). Chapter 47, 47.1-47.50 A.R. Ismail, M.J.M. Nor, M.F.M. Tahir, H.Mohamad, R.A.Bakar, R. Zulkifli, 2010 Environmental Noise Impact To Urban Area In Malaysia: A Case Study Due To Aircraft Noise. 3rd Regional Conference on Noise, Vibration and Comfort (NVC), pp. 443-451 The Planning Guideline, Environmental Noise Limits and Control, 2007. DOE of Malaysia pp 10 B.Atzori, P.Lazzrin and G.Meneghetti. 2005. A unified treatment of the mode I fatigue limit of components containing notches or defects. International Journal of Fracture, 133:61-87 HVAC (HVAC Noise Control). 2012. http://www.PDHcentre.com (Accessed 04 January 2013) Sreenivasaiah Purushothama Rao1, Asrul Mahjuddin Ressang Aminuddin, Hong Wan Thing, Nurul Amira Abd Jalil, Nazli Bin Che Din and Nila Inangda Manyam Keumala Daud, 2011. Thermal and acoustic environmental requirements for green building in Malaysia, Journal of Design and Built Environment Vol.11, Dec 2011. JKR Technical Report. 2011. CKM/SD/LV/KJ/11/30 (in Malay) JKR Technical Report. 2011. CKM/SD/LV/KJ/11/50 (in Malay) JKR Technical Report. 2012. CKM/SD.NV/LT/12/06 (in Malay) JKR Technical Report. 2012. CKM/SD/LV/LT/12/8 (in Malay) JKR Technical Report. 2012. CKM/SD/LV/LT/12/21 (in Malay) JKR Technical Report. 2012. CKM/SD/LV/LT/12/26 (in Malay) JKR Technical Report. 2013. CKM/SD.NV/LT/13/01 (in Malay)