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Issue : January-March 2003

HVAC SYSTEMS for Theatres, Auditoriums & Cinema Halls

The art and science of sysytem design

By Sudhir K. Sinha
General Manager
(Operations & Technology)
Voltas Ltd., Mumbai

and

R. K. Dastur
Chief Operations Manager
Voltas Ltd., Mumbai

S K Sinha, a mechanical engineer from IIT Kharagpur, has 25 years of experience in the HVAC industry. He has worked as a practicing engineer in various functions with Voltas. He has been an active ISHRAE member and was president of Kolkota chapter as well as regional director of Eastern region. He is also an active ASHRAE member and currently he is the president elect of ASHRAE Western India chapter. He can be contacted at sksinha@voltas.com

R.K. Dastur is a mechanical engineer from VJTI Mumbai. He has 23 years of experience in the HVAC industry and has worked in various capacities in Voltas. He has hands-on experience of design and has installed the HVAC system at one of the most prestigious theatres in Mumbai, NCPA or National Centre for Performing Arts.

The design consideration for any air conditioning system depends largely on its application. It is a known fact that Theatres, Auditoriums, Concert Halls or Motion Picture Theatres (Cinema Halls) are high occupancy (persons per sq.ft) areas. Consequently, the fresh air requirement is high in order to dilute the CO2 concentration. High occupancy also leads to a greater amount of latent load emitted by the human beings. All these considerations need to be factored into, by the designers while calculating the air conditioning load. The inside design condition is as per ASHRAE standard for human comfort. Typically, the Comfort band on Temperature varies between 70 to 76°F (21.1°to 24.4°C) with RH not exceeding 65%.

Another important criterion is the noise and vibration control. The desired Noise Criteria (NC) varies with the type and quality of the facility. The need for noise control is lower in Cinema Halls but it is important in Theaters, Auditoriums, and Concert Halls. In most cases, sound and vibration control is required both, for mechanical equipment and the air distribution system.

Table 1 summarises a list of important Theatres, Auditoriums and Cinema Halls air conditioned by the company, showing the plant capacity, occupancy, fresh air quantity, duct velocities and NC level maintained, and will give the reader a fair idea of existing practises in the industry.

Table 01 - Click to view clear picture
Click to view clear picture

Air Distribution

Generally, the seating area of any Auditorium or Cinema Hall is isolated from the exterior by lobbies, corridors, etc. In the seating area, people generally remain in one place throughout the show. A good air distribution system ensures that air reaches the entire seating area without creating drafts and noise. There are multiple options of locating the supply air ducting and return air (ducted or otherwise). The most common and possibly the best method is to locate the supply air duct at the ceiling level and distribute the air uniformly through ceiling diffusers, the design of which will depend on the height at which duct is located. The return air can be located below the seat and ducted back to the air handling unit room through trenches in the floor. This layout of air distribution would ensure uniform distribution of air without draft and noise. In a lownoise requirement, it is advisable to acoustically line the supply and return air duct completely with fiberglass wool of adequate thickness.

Another alternative is to locate the supply air duct along the sidewalls of the hall. The duct can be concealed in boxing and the supply air grilles can be either continuous or conventional. The grille is to be designed for adequate throw so that air reaches even the central portion of the seating area.

However, very large halls require larger throw from the sidewalls and such a requirement could vary between 50 to 150 ft. In such installations, selection of the grille is very important to meet the throw requirement. Generally, this results in a high air discharge velocity leading to noise as well as draft. It is therefore, recommended that supply air throw should be designed for a maximum of 30 ft throw from the side and to ensure it reaches the central portion of the hall, the return air should be collected below the seat.

Another alternative to take the return air in directly below the stage, and this method is quite commonly followed.

Recommended design velocity for supply air in the main duct should not exceed 1200 fpm and of return air 600 fpm. The recommended velocity at the outlet of each grille / diffuser is to be maintained between 150 to 200 fpm.

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Fig.01

Mechanical Equipment

Location of mechanical equipment such as Chillers, Air Handling Units, and Pumps etc. is quite critical, particularly, where low noise criteria is a key requirement. It is advisable to have the AC plant room and the air handling unit room in a separate area, which is structurally isolated from the Theatre or Auditorium. The locational constraints sometimes force us to accept an air handling unit room within the same structure as that of the Auditorium and sometimes very near the Auditorium. It is essential to ensure, that in such installations, the air handling unit (AHU) is mounted on correctly selected vibration isolators and the chilled water pipe is provided with a flexible connection to avoid transmission of vibrations to the structure. The design of the AHU has to be for low rpm i.e. 650 rpm with outlet velocity not exceeding 1800 fpm. It is preferable to have AHU fans of backward-curved type and the bearing of nylon pillow block. A double-skin type of AHU is always preferred for low noise application. It is important to acoustically line the walls of the AHU room. Sound attenuators should be provided both in the supply air duct as well as return air path. Wherever return air is not ducted, it is recommended to have the return air attenuator on the return air opening of the Auditorium wall.

Cinema Halls or Motion Picture Theatres

Cinema Halls are the simplest of the auditorium structures. They have continuous operation of 8 to 12 hours in case of commercial Cinema Halls or 4 to 8 hours in case of Multiplexes. They operate between full occupancy to low occupancy and require plant performance to be controlled at low load. Noise criteria is not so important in this application. Lobby and passages have very dense occupancy in-between shows or during intervals. Lighting load is very low during the show and generally smoking is not permitted.

Projection Booths

Large theatres using high-intensity lamps require careful handling of design for projection rooms. Normally the projection equipment manufacturer recommends exhaust out of the projection room and thus the projection room is maintained at negative pressure. Exhaust is normally taken through the housing of the projectors. The other heat source includes sound and dimming equipment, which require controlled environment. A similar theatre using 16mm films is less complex in its requirement for the projection booths. It is good practice to air-condition the projection room giving a filtered supply air to avoid soiling of lenses.

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Stage

In a performance theatre, the stage air conditioning has many complexities:

Controlled Ozone Injection

Considering the increasing cost of energy and also the increasing awareness of Indoor Air Quality, controlled and engineered Ozone injection has been found to be an ideal solution to achieve both. Ozone, when injected in the indoor air through the supply air duct in controlled quantities, oxidizes various pollutants including volatile organic components such as carbon monoxide, toluene, formaldehyde, etc. which are commonly found in indoor furnishings. Ozone also removes all odours and eliminates fungi. It is important that injection of Ozone is within acceptable limit as per international norms to avoid illeffects of excessive Ozone.

In applications where Ozone injections are applied to improve indoor air quality, the fresh air requirement also gets considerably reduced from the ASHRAE standard of 15 cfm per person. This leads to a considerable reduction in air conditioning load thus saving capital and operating costs.

Regal Cinema in Mumbai had the problem of fish odour getting into the cinema hall, whenever a fish truck from the docks passed by. Regal Cinema then installed Ozone injection and since then the indoor air quality has improved and can be experienced as a testimony of Ozone’s effectiveness. There are many installations in India and abroad, where the public areas, in particular have been treated with Ozone injection for improvement of indoor air thus reducing the air conditioning load and operating cost.

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Noise

We will deal with this topic of ‘Noise’ and ‘Noise Control’ in great detail, since this is one of the most critical parameters in such applications. The major sources of noise in any air conditioning system are:

• Supply and Return Fans
Fans generate the maximum noise. Centrifugal fans produce noise across the entire audio frequency band having maximum value between 31.5 to 250 Hz. Thus, it is evident that these values are maximum in the low frequency band. The lowest noise levels are produced when the fan is operated in the region of its peak efficiency on its performance curve. The following formula can be used to calculate the efficiency.


where K = 6345 (a constant in the FPS system)

The sound power level of the noise generated by the fan can be obtained from the manufacturer for the entire octave band frequency 63 Hz to 4000 Hz. The increase in the sound power level on account of the fan not operating at its peak efficiency can be predicted using Table 2. Emperical data is available to predict the increase in dB based on efficiency of fans. Table 2 shows how to determine noise power level for the fan selected for the application.

Table 2 : Increase in noise level based on efficiency
Aero Foil Centrifugal Fan & Vaneaxial Backward Curved
Centrifugal Fan
Forward Curved Fan
Efficiency Increase
in dB
Efficiency Increase
in dB
Efficiency Increase
in dB
72 to 80
68 to 71
60 to 67
52 to 59
44 to 51
0
3
6
9
12
67 to 75
64 to 66
56 to 63
49 to 55
41 to 48
0
3
6
9
12
58 to 65
55 to 57
49 to 54
42 to 48
36 to 41
0
3
6
9
12

• Smooth Air Flow

Fig.02

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Ducting, Dampers, Air Terminal Devices
Deflectors such as vanes, fins, etc used to deflect air for even distribution of the same throughout the airconditioned space leads to some generation of noise. The greater the deflection, higher is the noise level produced.

The sound power level (Lw) generated by air flowing through diffusers increases by 16 dB for octave bands around 500 Hz and 16 to 24 dB for octave bands between 500 to 1000 Hz, in case the velocity of the air passing through diffusers is doubled.

Dampers, which are integral parts of grilles or diffusers, to control the volume of airflow, increase/ radiate the noise frequency in the range of 1000 to 8000 Hz. More the restriction to the airflow, higher the noise level. Thus, if the airflow is reduced by half using dampers, there will be an increase in the sound power levels by 10 to 20 dB depending on the design of the air outlet.

It is therefore, recommended not to use dampers in the grilles for such sensitive applications. Instead dampers can be installed in the duct for achieving the same objective.

Fig.03

Figure 3 predicts the noise power level generated using different types of grilles and diffusers at various frequency bands. This table can be used to predict the noise power level of a single grille or diffuser. The next step is to predict the noise pressure level (Lp) from noise power level (Lw). The formula below can be used for calculating the sound pressure level. This formula is applicable only for Auditorium and Lecture Hall applications. The volumes of such spaces are in excess of 20,000 ft3 and are usually characterised by many seats, people and sound absorptive materials.

Lp = Lw – 5 log10X – 28 log10h + 1.3 log10 N –3log10f + C

Hence,
Lp – Lw = –5 log10X – 28 log10h + 1.3 log10 N –3log10f + C

Where: -

A misaligned diffuser can create noise eg misalignment of 1 dia in 2 dia length can increase the sound power by 15 dB

Figure 4 illustrates the correct installation of a diffuser with respect to the duct outlet. The duct connection shown in the sketch is a flexible duct but the same is true also for a rigid duct.

Fig.04

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Noise Control

The control of noise in a duct air distribution system can be achieved through attenuation of sound in ducts using sound attenuators or sound absorptive plenums. The systematic approach to attenuation is to reduce the noise level in the various components of the air distribution system.

Sound is attenuated in the duct system by insertion losses in:
• Unlined ducts
• Lined ducts
• Lined elbows
• Branch points resulting from division of power
• Result of end reflection

Unlined ducts. Attenuation provided by unlined rectangular sheet metal ducts in dB/metre is shown in Table 3.

Table 3 : Attenuation provided by unlined rectangular duct
Perimeter/Area
ratio (cm/sq.cm)
Hz
  63 125 250
0.31
0.13-0.31
Under 0.05 to 0.13
0.3
1.0
0.3
1.0
0.3
0.3
0.3
0.3
0.3

If the ducts are externally lined, values must be doubled.

Transmission losses depend on the level of sound within a duct, cross sectional area of the duct and length of duct in occupied space. The same is represented in Figure 5 and in Table 4.

Table 4 : Attenuation provided by unlined ducts
Rectangular Duct (unlined)
Dimensions Perimeter / area ratio  
cm x cm (in x in) cm/cm2 in/in2 dB/metre
10 x 20
15 x 30
20 x 30
25 x 41
30 x 30
30 x 61
30 x 91
38 x 38
38 x 76
46 x 71
46 x 91
61 x 91
61 x 122
76 x 76
76 x 152
91 x 91
91 x 182
107 x 107
122 x 122
(4 x 8)
(6 x 12)
(8 x 12)
(10 x 16)
(12 x 12)
(12 x 24)
(12 x 36)
(15 x 15)
(15 x 30)
(18 x 28)
(18 x 36)
(24 x 36)
(24 x 48)
(30 x 30)
(30 x 60)
(36 x 36)
(36 x 72)
(42 x 42)
(48 x 48)
0.30
0.20
0.17
0.13
0.13
0.10
0.09
0.11
0.08
0.07
0.07
0.05
0.05
0.05
0.04
0.04
0.03
0.04
0.03
(0.75)
(0.50)
(0.42)
(0.33)
(0.33)
(0.25)
(0.22)
(0.27)
(0.20)
(0.18)
(0.17)
(0.14)
(0.13)
(0.13)
(0.10)
(0.11)
(0.08)
(0.10)
(0.08)
0.7
0.6
0.6
0.5
0.6
1.2
1.2
1.2
1.2
1.2
1.2
1.1
1.1
1.1
0.4
0.4
0.4
0.4
0.4
2.1
1.9
1.8
1.7
1.7
0.8
0.8
0.9
0.8
0.8
0.7
0.7
0.6
0.7
0.6
0.7
0.6
0.6
0.6
3.3
2.6
2.4
2.1
2.3
1.8
1.6
2.0
1.6
1.5
1.4
1.3
1.3
1.4
1.3
1.3
1.0
1.2
1.1
8.2
6.3
5.8
4.9
5.4
4.1
3.6
4.7
3.5
3.4
3.1
2.9
2.6
3.0
2.2
2.7
2.0
2.4
2.2
22.4
17.3
15.9
13.5
14.8
11.1
9.8
12.7
9.5
9.3
8.5
7.8
6.3
8.2
4.4
6.6
3.3
5.2
4.3
21.2
15.7
16.3
13.3
14.9
9.5
5.8
12.6
8.1
8.8
7.1
7.3
5.8
7.6
4.9
6.7
4.3
5.9
5.4
8.4
7.6
8.7
7.9
8.8
6.5
4.2
8.3
6.1
7.0
5.8
6.6
5.5
7.0
5.1
6.7
4.9
6.5
6.2

Circular Duct (unlined)
Diameter cm (in) :              
15.2 (6)
30 (12)
61 (24)
122 (48)
0.7
0.5
0.3
0.1
1.6
1.0
0.7
0.3
3.3
2.3
1.6
1.0
5.9
4.9
3.3
2.0
7.2
7.2
5.6
2.0
7.2
7.2
3.0
1.6
6.6
4.9
1.6
1.6

 

Fig.05

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Table 5 : Attenuation of lined elbows
Width of duct in plane of turn in inches 63
125 250 500 1000 2000
  --------------------Hz------------------------
5 to 10
11 to 20
21 to 40
41 to 80
Unlined elbows all sizes
0
0
1
2
6
1
1
2
3
1
1
2
3
3
2
2
3
4
5
3
3
4
5
6
3
4
6
6
8
3

Lined duct. Attenuation of sound provided by lined duct with 1" and 2" thick insulation, is shown in Figure 6.

Lined Elbows. Attenuation of lined elbows, 90deg rectangular with/without fiberglass, 25 mm lining, is shown in Table 5.

Branch Points. Division of sound power at a duct branch results in attenuation that can be expressed as :


and is shown in Table 6.

End Reflection. Attenuation of sound in ducts resulting from end reflection loss in dB is shown in Table 7.

Table 6 : Attenuation at a lined duct branch due to division of power
Ratio area of branch/main duct 0.01 0.02 0.04 0.06 0.08
dB to be subtracted
Ratio area of branch/main duct
dB to be subtracted
20
0.1
10
17
0.2
7
14
0.4
4
12
0.6
2
11
0.8
1

 

Table 7 : Attenuation in ducts from end reflection
Hz 63 125 250 500 1000
6
12
24
48
18
13
8
4
12
8
4
1
8
4
1
0
4
1
0
0
1
0
0
0

 

Table 8 : Attenuation/self noise generation for face velocity 1000 fpm and pressure drop 0.4"wg
Hz 63 125 250 500 1000 2000 4000 8000
Insertion loss db
Self noise Lw db generation
4
48
11
42
20
39
37
35
44
36
44
37
38
35
22
30

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Fig.6

Attenuation Due to Sound Attenuator or Duct Silencer

A sound attenuator has a cross section equal to or greater than the duct in which it is installed.

A sound attenuator has three important characteristics, which are a function of air velocity at the face of the attenuator :
• Insertion loss
• Pressure drop
• Self noise

Figure 7 shows a typical attenuator. Figure 8 shows a comparison of the insertion loss provided by a 2.1m long sound attenuator :
• A low-pressure drop unit
• A standard pressure drop unit
• Duct of the same length and cross section lined with a fiberglass blanket 25 mm thick
Note: In areas like concert halls/auditoriums, where air velocities are less than 1000 ft/min, the insertion loss is far greater than the self noise propagated at frequencies less than 500 Hz.

Fig.7

Fig.8

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Determination of Extent of Sound Attenuation

Attenuation of noise can be done by considering:

Primary Attenuation. Deals with the location of the source of noise (eg fan). It also considers the aspects of how the noise can be controlled from the source by reducing the rpm, outlet fan velocity, considering the materials in the bearing, and whether backward or forward curve. Fan type should be used depending on the critical nature of the noise criteria to be maintained within the air-conditioned space.

Secondary Attenuation. This considers the noise within the ducting work and air terminal devices and the design of these items has to be done in order to reduce the noise level. Additionally, whatever dB level is actually attained has to be matched to the criteria desired within the air conditioned area by the client, which would therefore call for additional attenuation to be carried out viz., by inserting a sound attenuater.

Step I:
We have obtained the sound power level of the fan Lw from the fan manufacturer.

Step II:
From this, we have to account for the insertion losses to arrive at the net sound power level.

Step III:
We have also obtained the value of Lw-Lp applicable for Air Terminal Devices. The actual sound pressure level thus can be arrived at using the following expression:
Lp = Lw – (Lw – Lp)

Step IV:
Using NC chart, we can arrive at the actual NC value without use of a sound attenuator. Hence, the difference between actual and desired NC levels has to be attenuated in the sound attenuator.

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Rating System for HVAC Noise

Single number rating systems in common use are
• Weighted sound level
• NC curves
• RC curves

These rating systems for HVAC are valid for continuous steady state noise that exhibits no fluctuations in sound level with time.

Failure to recognise the limitation leads to a serious error in rating noise produced by the system.

Fluctuating noise levels may occur as a result of unstable fan operation OR beats between 2 or more fans OR other rotating equipment which operates at the same speed.

Sound level meters are designed to measure ‘A’ weighted sound level of noise expressed as dB (A)

Fig.09

Table 9 : Noise Control Criteria for HVAC systems
Acceptability Criteria For Steady Background Noise In Unoccupied Rooms
Recommended Criterion Approximate
A-weighted
sound level *
Type Of Room Preferred Alternative dB
Recording Studios
Concert and Recital Halls
TV Studios, Music Rooms
Legitimate Theaters
Private Residences
Conference Rooms
Lecture Rooms, Classrooms
Executive Offices
Private Offices
Churches
Cinemas
Apartments, Hotel Bedrooms
Courtrooms
Open-plan Offices & Schools
Libraries
Lobbies, Public Areas
Restaurants
Public Offices (Large)
RC 10-20(N)
RC 15-20(N)
RC 20-25(N)
RC 20-25(N)
RC 25-30(N)
RC 25-30(N)
RC 25-30(N)
RC 25-30(N)
RC 30-35(N)
RC 30-35(N)
RC 30-35(N)
RC 30-35(N)
RC 35-40(N)
RC 35-40(N)
RC 35-40(N)
RC 35-40(N)
RC 40-45(N)
RC 40-45(N)
NC 10-20
NC 15-20
NC 20-25
NC 20-25
NC 25-30
NC 25-30
NC 25-30
NC 25-30
NC 30-35
NC 30-35
NC 30-35
NC 30-35
NC 35-40
NC 35-40
NC 35-40
NC 35-40
NC 40-45
NC 40-45
18-28
23-28
28-33
28-33
33-38
33-38
33-38
33-38
38-43
38-43
38-43
38-43
43-48
43-48
43-48
43-48
48-53
48-53

* A-weighted sound levels are not recommended for use in the design of HVAC systems
Note

Loudness of sound varies both, with sound pressure and frequency.

Noise induced annoyance is not directly proportional to loudness. ‘A’ weighted scale is unreliable because specified sound level in dB (A) is not necessarily acceptable to an occupant of a room, supplied by the system.

‘A’ weighted is not used as a diagnostic tool in analysis of noise complaints about an operating HVAC system as the problem may not only be loudness related. Knowledge of the spectrum of noise is required to discover and correct the reason for complaints.

Hence NC curves are used as shown in Figure 9 and recommended criteria for both NC and RC are shown in Table 9 for a variety of applications.

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