Ventilation  (and Air Conditioning)  of Large Rooms

 

Large rooms and buildings such as lecture rooms; theatres, conference rooms, exhibition halls, auditoria and halls for public gatherings need careful designing for ventilation or air conditioning.

 

Two of the most important design criteria are:

o       The ventilation rate.

o       The distribution of air in the space.

 

Ventilation Rate

 

The ventilation air change rate (Theatres) given in the CIBSE guide Table B2.3 is 6-10 air changes per hour.

This table is in the Ventilation Design section of these notes.

This is total air supply rate. 

We can choose a higher figure if necessary.

 

The amount of fresh air supplied per occupant is given in Table B2.2.

This is 8 litres per second per person for non-smoking areas and 16 litres per second per person for smoking areas.

If we assume that a building is non-smoking, therefore 8 l/s/p of fresh air is required for the fresh air amount.

These two figures can determine the air flow rates in the ventilation system.

 

The following formulae may be used:

 

Ventilation rate (m3/h)   =          Air Change Rate (/h)     x    Room Volume (m3)

 

Ventilation rate (m3/s)   =          Ventilation rate (m3/h)   /   3600

 

Example

 

The THEATRE building shown below requires mechanical ventilation.

Determine the air flow rates in the system.

CIBSE guide table B2.3 gives a supply ventilation rate of 6 - 10 air changes per hour.

Use a supply air ventilation rate of 10 air changes per hour for this example.

DATA:

Room width             =          20 metres

Occupancy   =          750 seats.

 

 















AIR FLOW RATES

Room Volume (m3)                        =          L   x    W   x   H

Room Volume (m3)                        =          25 x 20 x 9

Ventilation rate (m3/h)                 =          4500 m3

Ventilation rate (m3/s)                  =          12.5 m3/s

Ventilation rate (m3/h)                 =          Air Change Rate (/h)     x    Room Volume (m3)

Ventilation rate (m3/h)                 =          10                   x          4500

                                                            =          45,000  m3/h

 

Choose an Outdoor  Air Recommended minima Rate from Table B2.2 for a room with non-smoking        =  8   l/s/p

 

Fresh Air Rate   (l/s)           =   Number of occupants   x    Outdoor Supply air per Person (l/s)

Fresh Air Rate   (l/s)           =          750   x   8      =         6000  l/s

Fresh Air Rate (m3/s)        =          Fresh Air Rate(l/s)  /     1000

Fresh Air Rate (m3/s)        =          6000              /          1000 =          6 m3/s

 

For comparison convert Fresh Air Rate to an Air Change Rate.

 

Fresh Air Rate (m3/h)           =       Fresh Air Rate (m3/s)                    x  3600

Fresh Air Rate (m3/h)           =       6          x          3600              =          21,600 m3/h           

 

Fresh Air Rate (AC/h)           =       Fresh Air Rate (m3/h) /    Room Volume (m3)

Fresh Air Rate (AC/h)           =       21,600             /   4500           =     4.8   AC  /h

Fresh Air Rate (AC/h)           =       4.8 air changes of fresh air per hour

 

The supply air ventilation rate is 10 air changes per hour.

These figures can now be put on a drawing for clarity.

 

 

 

 

 

 

 

 

 


                                                                                                                       













 

Ventilation Rate if Air Conditioning is Used

 

The previous ventilation rate may change if air conditioning is necessary.

If air conditioning is to be incorporated into a design then the heat gains should be calculated to ascertain the supply air rate into the building.

The following formula may be used to calculate the supply air rate.

 

                        H         =          m   x   Cp    x    (tr   -ts)

Where;

            H          =          Sensible heat gain (kW)

            m         =          mass flow rate of air (kg/s)

           Cp         =          Specific heat capacity of air (1.005 kJ/kg K)

            tr          =          room temperature (oC)

            ts          =          supply air temperature (oC)

 

If the heat gain in the previous example  is 135 kW, the room air temperature is 22 oC and the supply air temperature in summer is 14oC, then the mass flow rate of supply air is

 

            m        =          H     /      (Cp   x   (tr –ts))

            m        =          135     /          (1.005   x  (22 –14))

            m        =          16.8   kg/s

 

Convert this to a volume flow rate:

 

Volume flow rate  (m3/s) =          mass flow rate (kg/s)   /   density of air (kg/m3)

Volume flow rate  (m3/s) =          16.8               /    1.2 kg/m3        =14m3/s

Volume flow rate  (m3/h) =          14         x  3600        =  50,400 m3/h

 

Convert this to an Air Change rate for comparison.

 

Supply Air Rate (AC/h)         =       Volume Flow Rate (m3/h)    /    Room Volume (m3)

Supply Air Rate (AC/h)         =     50400 / 4500   = 11.2     ac/h                         

This rate is more than the Air Change Rate given in Table 2.3 CIBSE guide of 10 ac/h.

For good practice we will use the higher of the two values in the actual design which is:    11.2     Air Changes per hour.




Air Distribution

 

Air distribution in a large hall poses some difficulties which must be addressed.

The object of good air distribution is to allow air to be supplied to all parts of the room, to avoid draughts and to have effective air mixing in a space.

This means that all the areas in a room should have the benefit of cool air if required or warm air if required. Also all occupants should have a supply of fresh air.

 

In a heating system warm air will naturally rise so supplying at low level and extracting at high level is a good way to employ buoyancy effects.

In an air conditioning system cool air may be supplied at high level and allowed to drop to low level to be extracted.

The drawing below shows upward ventilation in an Auditorium.

One advantage is that small adjustable louvres near seats can be used to give the occupants control over their environment.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


The velocity of air supplied at low level is about 0.5 m/s to reduce draughts at floor outlets.

Outlets may also be positioned at low level on side walls.

The upward system can be cheaper to install since propeller fans can be used in the roof to extract the air.

 

The drawing below shows a typical example of Downward ventilation in a Concert Hall.

One advantage of this type of air distribution is that there is no supply at seating level therefore no possibility of draughts and no difficulty in installing ductwork under seat areas. Another advantage is that if air conditioning is used, the cool supply air will tend to drop in the warmer room air, thus assisting distribution.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


One disadvantage however, is that low level extract grilles are required and these can be difficult to assimilate into the building fabric.

Also vertical ductwork is required from these low level grilles and this takes up floor and wall space and normally requires to be hidden from view.

 

In some halls with a very high ceiling the velocity of air supplied at high level needs to be high to get the air down into the occupancy space.

The problem with high velocity at diffusers is noise.

The diffusers must be carefully chosen to minimise the risk of noise yet maintain an adequate ‘throw’ to distribute the air down at low level.

The air diffusers become like nozzles in a jet so that adequate velocity is reached.

 

An outlet velocity of about 3-5 m/s is available from jet diffusers.

An example of this type of ventilation is used in Usher Hall Edinburgh where 65 jet diffusers are installed, each handling 280 litres/s at an outlet velocity of approximately 3 m/s.

 

Because of their aerodynamic design jet diffusers give a long throw even at high outlet volumes.

In some diffusers the jet can be adjusted over 360°.

As these jets handle different supply temperatures the jet can be oriented upwards or downwards for heating or cooling mode.

This can be achieved by hand or by electric motor.

 

The diagram below shows a sizing diagram and photo of a jet diffuser.