Defining fresh air rates for sizing heating coils

Question

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

Answer 1

Using the same volume flow rate for winter and summer is ok, since the fan will push the same amount of volume. It is true that the specific volume will change at different moist air conditions, so to be extremely exact you could adjust for this change in winter and summer.

However, HVAC uses approximations because being extremely exact has diminishing returns when you are buying equipment that has nominal sizes. (No sense being accurate out to the third decimal (ie. 4.567 tons) when nominal tonnages are 5,10,20 etc.)

You are correct that using slightly different mass flow rates for winter and summer would be more exact, but I am sure that you would arrive at similar answers in the end, or at least close enough to size your equipment.