Practical Ventilation and Air Conditioning Design for Recording Studios, Control Rooms and Auditoria
by Mike Hardy
- Running Costs and ‘Free Cooling’
- Minimum Outside Air
- Air Distribution
- Comparison of Mixed Flow and Displacement Systems
- V.A.V. Systems (Variable Air Volume Systems)
- Chilled Beams and Panels
- Ductwork - Studios and Control Rooms
- Thermostatic Control
- De-humidification and Humidification
- Small Single Studios
- Computer Modelling
- Legionnaires’ Disease
With capital cost as important as ever these days, the trick always is to use standard equipment which is produced for the massive world-wide air conditioning market. Like cars, mass production drives costs down and specials push costs up. In most cases it is possible to avoid using specials and, for most studios, manufacturers may be found with suitable mass produced equipment within their range.
As the problem in most studios is to remove unwanted heat, most of the observations made are related to cooling. The air volume required for studios is generally always related to the cooling cycle as a greater volume of air is required.
Running Costs and ‘Free Cooling’
In the U.K. and much of Europe, the external ambient temperature for most of the year is usually lower than the temperature required in a studio or control room. It is surprising therefore that the obvious fact is not often taken advantage of - and this does not just apply to studios. Many office blocks in London, quite unnecessarily operate refrigeration plants twelve months of the year because ‘free cooling’ has not been designed into the systems.
As most studios have to have ducted air conditioning systems, for noise control, these systems are tailor made for the addition of ‘free cooling’ and it is always worth consideration.
Minimum Outside Air
All occupied air conditioned environments require a proportion of outside air to be mixed with re-circulated air (typically 8 litres/second/person with no smoking to 43 litres/second/person with all occupants smoking). The use of 100% outside air systems with recuperators or ‘run around’ coils are becoming increasingly popular in offices. These, however, are specials and increase capital outlay costs.
However, television studios with high ceilings and a high lighting load should always be considered for 100% outside air systems with recuperators in order to remove the lighting load at source and exhaust to atmosphere.
Generally there is little requirement for noise control within a machine room. A cooling/heating unit may therefore be mounted within the machine room without adverse effect. Standard units may generally always be used and makes the air conditioning capital costs in machine rooms considerably less expensive than for studios.
In order to provide the correct noise levels within sound studios and control rooms, it is generally not possible to locate an air conditioning unit within the studio or control room (unless the discipline of turning off the unit at critical times is accepted). A remote unit, metal ductwork and attenuators have to be provided or alternatively a remote unit with fabric lined ductwork may be used if circumstances allow. One interesting exception to this rule has become available from one manufacturer that has developed the quietest mass produced cassette (room unit) on the market. This unit would be suitable for small control rooms and would reduce capital costs by as much as 50%.
The bible for the air conditioning engineer for noise is the N.R. curve which defines the decibel levels required for studios at each frequency - most studios and booths are designed to N.R. 20 and below and most control rooms to N.R. 25. The lower the N.R. level the more expensive the installation so it is wise not to over specify. Other equipment within a control room may operate at more than N.R. 25 so, if this were the case, it would not make sense for the air conditioning plant to be specified to operate at less than N.R. 25.
As most machine rooms are unoccupied, air distribution within machine rooms is generally not critical and a suitably sized wall unit or ceiling unit re-circulating air will usually suffice.
Mixed flow or displacement systems are the methods of air distribution employed in all ventilation and air conditioning systems. The method of introducing supply air into a studio will determine the room air patterns to a far greater extent than exhaust air.
Mixed Flow Ventilation Using Diffusers/Grille and Wall Grilles
Mixed flow distribution is the traditional method of supplying air to ventilated spaces. Cool air is blown in through the ceiling or wall and dilutes the room air in an attempt to provide an even temperature and contaminant level throughout the space.
In studios/control rooms in order to achieve satisfactory noise levels, air has to be introduced at a very low velocity compared with more conventional ventilation/air conditioning (typically 1.3 m/s to 2 m/s). Ceiling diffusers/grilles quite often have bulk head light fittings placed underneath them to prevent ‘dumping’ of cold air and also in order to help optimise the directivity index related to sound level at each frequency.
It is obviously less practical to do this with wall grilles and is the reason why ceiling grilles/diffusers are often preferred. However, wall grilles may be used if careful consideration is made of air distribution and attenuation.
Exhausted air in terms of air patterns is less critical (the analogy of blowing a match out rather than sucking it out is often made here) but, if practical, should be positioned over heat producing equipment. The quantity of air required for a mixed flow system is based on the formula:
Room sensible heat gain (watts) = air quantity (L/s) × (tr - ts) × 1.2
where tr - ts is the temperature difference between room and supply air temperature, typically 10 - 12°C.
With displacement systems, air is introduced at low level at low velocity through floor terminals or floor diffusers. Cool air floods the floor in much the same way as water would. The room heat sources lift the air up and the air passes through the occupied zone and is exhausted at high level.
As the displacement units are provided at low level, a considerable vertical temperature gradient naturally occurs between the floor and ceiling. The volume of air supplied to a room also therefore becomes height dependent and for a low ceiling the volume of air supplied is considerably more than for a mixed flow system.
Displacement systems are excellent for studios and auditoria with high ceilings because the higher temperatures at high level (out of the occupied zone) are generally of no consequence.
Socks are normally circular or semi-circular ducts formed from polypropylene or polyester based permeable fabrics and often provided with zips for removal and washing.
Vertical socks are excellent for high studios and allow a large volume of air to be delivered at low velocity with a small surface area. They are easy to install, filter the air and are able to be washed regularly. As it is possible to provide a high volume of air through socks at a low velocity, the supply air temperature on the cooling cycle may be increased considerably and are for this reason widely used in the food processing industry. However, the ability to deliver an increased supply air temperature during the cooling cycle depends on the plant selected. If socks are to be used for heating as well, they act as mixed flow distribution.
Socks use part displacement ventilation when used vertically (when cooling) and when used horizontally at high level mixed flow ventilation takes place.
The main disadvantage of socks is that they are highly obtrusive.
Comparison of Mixed Flow and Displacement Systems
Mixed flow is the traditional method of air distribution within buildings. As a result, most mass produced air conditioning equipment is produced for the mixed flow market. Most standard packaged air conditioning units will provide air at far too low a temperature for displacement systems, which during the cooling cycle generally needs to supply air at no more than 3 - 4°C below the room temperature compared with a mixed flow supply air temperature being 10°C lower than the room temperature.
Induction displacement units help partly to overcome the problem of a small temperature difference and air may be supplied 7°C lower on these systems.
If chilled water is ‘on tap’, then this may be ideally used with an air handling unit for displacement. But to provide a chilled water system for a small studio is expensive. With larger television studios, chilled water systems become much more viable and so too therefore does displacement, which is a natural for television studios with high ceilings.
Displacement systems approximate to mixed air flow air distribution during the heating cycle.
- Mixed Flow System Advantages:
- Generally always less expensive on small to medium sized systems and may use standard equipment.
- Requires much less air volume at low room heights (typically 2.7m and below), therefore smaller ducts, attenuators etc.
- Grilles/diffusers not obtrusive and use smaller surface areas.
- Displacement System Advantages:
- Draughts or dumping of cold air rarely occurs.
- Room contaminants removed from occupied zone continuously at optimum studio height (typically 3m and above).
- Should always be considered for high studios because the air temperature above the occupied zone is of no consequence and lighting gains may be removed at source.
- Requires less cooling plant on high studios.
The equipment available through the mass produced market applicable to most studios are as follows:
Central air cooled packaged equipment providing supply air to a studio(s). This equipment would normally be positioned externally and ductwork connected to and from the unit to the studio.
External and internal units normally known as ‘splits’, where purpose installed refrigeration pipework is installed between the internal and external unit. The indoor units serving studios would then have to be ducted to and from the studio to provide suitable sound attenuation. As previously described, the machine room would normally have the fan coil unit placed directly in the room.
Multi-split systems are also available, i.e. one external unit serving several indoor units. For the smaller studios i.e. perhaps two studios and a machine room, multi-split systems would normally be much more expensive than ‘one to one’ split systems. For larger or multi-studio installations, multi-split systems are worthy of consideration but careful consideration has to be given to zoning of the multi-split system.
British Standard 4434 describes the method for evaluating the maximum mass of refrigerant permitted to pass over a given room volume in order to minimise possible hazards in the eventuality of pipe fracture.
Central air cooled chillers providing chilled water distribution, which is the most flexible yet, has the most expensive capital costs except when considering larger studios.
Chilled water distribution systems should always be considered for medium to large systems. Chilled water distribution systems are generally far more versatile than refrigeration systems and units being added or modified in the future may be accommodated much more easily and is no different to modifying a low pressure hot water system as in an office or house. Chilled water also has the advantage of varying the ‘off coil’ supply temperature infinitely to the studio or machine room and would therefore be quite suitable for displacement distribution. In contrast, most direct expansion refrigerant systems will either supply air at approximately room air temperature (say 22°C) or 10°C lower i.e. 12°C when the compressor is running - generally speaking there is no in-between supply air temperature. (The use of hot gas bypass overcomes this but many manufacturers invalidate their guarantee with the use of hot gas-bypass).
V.A.V. Systems (Variable Air Volume Systems)
Several companies manufacture V.A.V. systems which vary the volume of distributed air proportional to the sensible heat gain by means normally of a localised thermostat. These systems are generally used on large buildings or may be available to a sound studio if the rest of a building is served by V.A.V. For a V.A.V. system to be dedicated to a studio the capital costs would be expensive and would not be competitive unless many studios were involved. V.A.V. may, however, be used for studios if for example the existing office systems has V.A.V. These systems provide mixed air flow distribution.
Chilled Beams and Panels
These incorporate chilled water pipework and are located within the ceiling. Chilled beams/panels would only be used for large installations with high heat gains and are used normally in conjunction with a displacement ventilation system. The chilled water inlet temperature has to be selected extremely carefully so that condensation will not occur.
Recent research has indicated that with the use of displacement ventilation systems in conjunction with ceiling beams/panels that the air flow tends to act as mixed air ventilation except in the vicinity of the heat sources.
Ductwork - Studios and Control Rooms
Air is normally distributed through pre-fabricated metal ductwork. Attenuators normally have to be inserted into ductwork to provide the necessary insertion loss at each sound frequency to provide the correct N.R. level.
Generally speaking, metal ductwork provides very little useful attenuation and is the reason attenuators have to be used.
Fabric lined metal ductwork or fabric constructed ductwork provides better attenuation at certain frequencies and on small systems with careful selection and a long enough run may even be used without attenuators.
The filters provided on most mass produced equipment are not normally the quality required for studios or control rooms.
As pre-fabricated ductwork is required anyway, it is a simple matter to provide a good filter in the return air system to filter both return and outside air. Air should pass through the filter at low velocity and care must be taken that the maximum pressure drop does not exceed the performance of the manufacturer’s equipment.
Air filters are classified between EU grades between 1 (the least effective) and 14 (the most effective) which define the arrestance and efficiency of air filters. An EU grade of 5 or better is suitable for most studios.
Activated Carbon Filters
These remove most odours and gaseous elements associated with environmental pollution. These would not normally be appropriate for studios unless the external air quality were poor. They have a considerable pressure drop and care must be taken to ensure the supply fan is able to cope with this pressure drop. Activated carbon filters are normally installed with pre-filters.
If a fixed percentage of outside air is used e.g. no ‘free cooling’ and smoking is likely to occur, electrostatic filters are worth considering and are excellent for removing smoke. As electrostatic filters have a minimal pressure drop, i.e. resistance to air flow, they may therefore be fitted to most existing equipment. With new installations, an electrostatic filter would normally be provided with a pre-filter to minimise the cleaning of the electrostatic filter and an after filter to trap dust blown off the electrostatic plates. Most electrostatic filters ‘crackle’ when dirty and have to be cleaned very regularly.
In theory it is possible to provide a duct sensor within the return air systems ductwork which will determine the functioning of the plant and the supply air temperature. In practice and due often to other localised influences, it is often a poor method of controlling the air temperature within a room. A displacement system should never be controlled on return air due to the temperature gradient.
Direct room sensing in a suitable position is generally always more desirable for both mixed flow and displacement systems as is individual room occupant adjustment if possible. Supply air sensing may also be considered for displacement systems.
De-humidification and Humidification
De-humidification or the reduction of moisture content of the air in summer takes place as part of the cooling process through the cooling coil. Most mass produced air conditioning equipment has an approximate sensible heat to latent heat removal of about 70% to 30%. That is approximately 70% of the cooling coils effort is directed to reducing temperature (as measured on a thermometer) and 30% is directed to reducing moisture content in the air. Most manufacturers will provide the ‘sensible to total ratios’ of their equipment at different ambient conditions. It is worth noting that when selecting equipment, manufacturers generally always quote total cooling capacities.
When comparing a sensible heat gain to a room, i.e. output of equipment, lights, occupants and outside air etc., the total capacities must always be de-rated by at least 30% due to latent cooling included in the total output figure. In almost all studios, the 30% latent reduction will provide a relative humidity of less than 55% at U.K., European and Asian conditions in summer. (With the use of ‘free cooling’ achieved by motorised dampers, in summer it is important the control system is designed to operate on minimum outside air at high external ambient to minimise the cooling load with the exception of systems designed for 100% outside air and recuperators).
In short, equipment is selected for sensible output, and generally the de-humidification achieved will be acceptable without the use of dedicated de-humidification control.
In the cases where the design sensible gain is exceptionally high compared with latent gain, high sensible units are used e.g. 90% sensible to 10% latent. This sort of ratio is typical for main-frame computer rooms. But to most manufacturers these units are ‘specials’ and need not be used unless there are exceptional circumstances e.g. if humidification was also a requirement, as with some main-frame computer rooms, then the removal of the least possible moisture would be certainly most important.
Humidification or the addition of moisture content is not normally a requirement in studios. If air dryness within the studio is critical in winter, then a humidifier may be added, normally in the form of steam injection humidifiers. Water spray humidifiers are not popular these days because of the possibility of humidifier fever and the need for expensive Legionella precautions because of the use of an often stagnant sump of water over which the air passes. As most chemical biocides are toxic, the alternative treatment for Legionella would normally be ultra-violet light which has an expensive capital cost. (There are many spray humidifiers throughout the U.K. which are switched off and drained down because of this very reason).
Small Single Studios
It is not easy to select mass produced equipment for small single studios, particularly if the heat gain is in the region of 2 - 3kW, without significantly oversizing the equipment. With oversized equipment the capital cost penalty is high because not only is the equipment oversized but the ductwork distribution system also has to be oversized to be compatible with the selected equipment. (If the air volume circulated is significantly less than the air conditioning equipment is designed for then the cooling coil section will freeze up when using direct expansion equipment.)
There are however several manufacturers that produce small air handling units primarily for use above false ceilings in offices or retail areas and intended to be used with limited ductwork. These units may be used although great care has to be taken with the pressure drop of the system to ensure that it is not greater than the static pressure available from the fan in the air handling unit. The low velocity distribution usually required for optimum attenuation for studios helps greatly to minimise pressure loss and with careful selection these units are quite suitable for small studios, and indeed with some units it is possible to dispense with attenuators altogether and use only lined ductwork.
Computational fluid dynamics (CFD) techniques are a useful tool available to a designer. They are particularly useful on medium to large studios/auditoria with a seated audience. The simulation may be used to assess occupant comfort and facilitate a better understanding of complex air patterns.
Legionnaires’ Disease is most commonly identified with the use of cooling towers. As air cooled equipment is usually used for all but very large studios, this is not normally a risk. However, condensate drip trays and condensate drains should be cleaned regularly as the Health and Safety Executive Guidelines H.S.(G)70.
The circuits identified as ‘at risk’ circuits by H.S.(G)70 are typically cooling tower circuits, hot and cold water open circuits, spas and spray coils. None of these circuits are usually associated with studio systems so ‘risk assessments’ as defined by H.S.(G)70 are not usually appropriate with the exception of drip trays and drains.
Studio designers and specifiers should be urged to think in terms of free cooling. The use of free cooling decreases dependence on refrigeration and reduces running costs as well as providing a high proportion of outside air for much of the year which is a particular advantage in stale or smoky atmospheres. Most studios and control room systems lend themselves very well to the addition of free cooling. With a limited budget, a system may be designed entirely on free cooling and without refrigeration. Such a system inevitably would overheat in summer but this is a limitation some studios would accept. The system may be designed so that a cooling coil could be added at a later date.
The designer/specifier should always try to design using mass produced equipment and not ‘specials’ in order to keep capital costs down. In small control rooms one manufacturer has produced a room unit which could be suitable with consequent reduction in capital costs. The unit is not suitable for ‘free cooling’ however.
The designer/specifier should understand the use of the system, i.e. potential heavy smoking or offgassing from other sources to identify the filtration requirements. This is particularly critical when ‘free cooling’ or 100% outside air is not used.
Careful consideration should be given to air distribution and considerations such as the height of a studio will be a major factor in determining the method of air distribution and therefore selection of plant.
100% outside air systems with recuperators or ‘run around’ coils should always be considered for larger television studios because of the potential to remove the lighting load at source and reduced cooling loads.
The writer is greatly indebted to the valuable input of Clive Glover of Munro Associates and David Hawkins of Eastlake Audio.