# Defining fresh air rates for sizing heating coils

dear StackExchange colleagues;

If I want to find the fresh air rate in kg/s and the supply rate in kg/s during winter and summer for the case below:

DATA:

• Indoor condition all year = 22oC dB temperature, 50% saturation.
• Outdoor condition summer = 28oC dB temperature, 80% saturation.
• Outdoor condition winter = -3oC dB temperature, 80% saturation.
• Maximum occupancy = 1000 people.
• ADP of the cooling coil = 8oC
• Fresh air requirement = 12 l/s/person
• Supply air rate = 8 air changes per hour.

Fresh Air Flow Rate

Fresh air rate = 12 l/s/p x 1000 people = 12,000 l/s = 12.0 m3/s

The specific volume at the outside condition may be determined from a psychrometric chart.

It is approximately 0.88 m3/kg.

Mass flow rate = Volume flow rate / specific volume

Mass flow rate (Fresh Air) = 12.0 / 0.88 = 13.64 kg/s.

Supply Air Flow Rate

The ventilation supply air rate is taken from table B2.3 and is given as 8 air changes per hour; the mass flow rate can be calculated.

Volume flow rate (m3/h) = Volume of room (m3) x air change rate (ac/h)

Volume of room (m3) = 40 x 20 x 8 = 6400 m3

Volume flow rate (m3/h) = 6400 (m3) x 8 (ac/h)

Volume flow rate (m3/h) = 51,200 m3/h

Volume flow rate (m3/s) = 51,200 / 3600 = 14.22 m3/s.

Mass flow rate = Volume flow rate / specific volume

The specific volume at the supply condition may be approximated at this stage from a psychrometric chart. It can be taken as 0.834 m3/kg.

Mass flow rate (Supply Air) = 14.22 / 0.834 = 17.05 kg/s.

right this procedure is for the air flows during summer, I did all the calculations for winter and found:

Fresh Air Flow Rate: 12/0.77 [0.77 from the chart with -3oC and 80%] Supply air Flow Rate: 14.22/0.88 [ from chart with room temp conditions plus 8, 30oC and 50%]

I have this example that is very close to my doubt: http://www.arca53.dsl.pipex.com/index_files/psy21.htm

it contradicts my thinking, he uses the same flow for winter and summer, can I do that?

thanks a lot