Air Flow Rates

2/10/14 Air Flow Rates

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Air ConditioningAir Flow Rates

When determining air flow rates for rooms that are air conditioned, the followingprocedure should be adopted;

1. Calculate heat gains. 2. Complete psychometric chart. 3. Determine mass flow rate of supply air from the following;

m = H / (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) from psychometric chart.

4. Convert mass flow rate to a volume flow rate:

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

(kg/m3)

5. Convert this to an Air Change rate for comparison.

Volume flow rate (m3/h) = Volume flow rate (m3/s) x 3600

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

Volume (m3)

6. Check out the recommended air flow rate from CIBSE Guide B2 (Ventilationand Air Conditioning) Section 3 - Requirements.

Use the higher value of air change rate for design purposes.

Example 1



Determine the supply, fresh air and recirculation air flow rates for the Sports Centre(Fitness Suite No.1) shown below.See Assignments section – Drawings.The room is to be fully air conditioned.



DATAOccupancy = 80 people

Room volume = 3740 m3.

Room temperature = 18oC from table 3.19 CIBSE guide.Sensible heat gain = 70 kW from calculations (not shown) See Heat Gains section of these notes for further details.

Supply air temp. = 13oC from psychrometric chart (not shown) See Psychrometrics section of these notes for further details.

Supply air density = See Science section – Properties of air – density at 13oC and

50% = 1.225 kg/m3. Supply Air Rate m = H / (Cp x (tr –ts)) m = 70 / (1.005 x (18 -13)) m = 70 / 5.025 m = 13.93 kg/s

Volume flow rate (m3/s) = mass flow rate (kg/s) / density of air (kg/m3) = 13.93 / 1.225

= 11.37 m3/s

Volume flow rate (m3/h) = Volume flow rate (m3/s) x 3600

= 11.37 m3/s x 3600

= 40,932 m3/h

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

= 40,932 m3/h / 3740 = 10.9 AC/h Table 3.1 Summary of Recommendations (Guide B2) directs to Table 3.19.From Table 3.19 for Fitness Centre the air flow rate is 10 to 12 AC/h. Use higher value of 12 AC/h supply air rate as found from above Table.This higher value is then used to size all plant and ductwork. Fresh Air Rate



Section 3.2.1.3 Body odour (CIBSE guide B2) gives the following information;Therefore in the absence of further information, it is recommended that 8 litre.s-1 per personshould be taken as the minimum ventilation rate to control body odour levels in rooms withsedentary occupants. Since the level of activity is higher than sedentary in the Fitness Room, we will adopt afresh air rate of 24 l/s per person. Fresh air rate = 24 l/s x 80 people = 1920 l/s

= 1.92 m3/s

= 1.92 x 3600 = 6912 m3/h

= 6912 m3/h / 3740 = 1.9 AC/h Percentage fresh air; 1.9 / 12.0 (x 100%) = 15.8% Recirculation Air Rate Recirculation Air Rate = Supply air rate - Fresh air rate = 12.0 - 1.9 = 10.1 AC/h Percentage recirculation air; 10.1 / 12.0 (x 100%) = 84.2% The air flow rates are shown on the diagram below.





A slight negative pressure may be provided in the Fitness Room .This is so that contaminated air does not enter the adjoining Reception / Foyer area.The Return air amount may be increased to 110% of the previously calculated value; that

is, 13.2 AC/h or 13.72 m3/s.

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Air Flow Rates