Question

A room is maintained at 70 DB and 60% RH. Heating air leaves the heating cooling coils at 85 DB and 50 RH. If 3000 cfm return air are mixed together, with 7000 cfm of heated air find the resulting dry and RH temperatures of the mixture.

Answer 1

Ans) Given,

(CFM)₁= 3000

(CFM)₂ = 7000

(DB)₁ = 70 and (DB)₂ = 85

(RH)₁ = 60 and (RH)₂= 50

When the quantity of two identical fluids are mixed at different conditions the quantity of resulting mixture is the sum of the quantities of two mixing fluids .

Final Mixture rate (CFM)_F = 3000 cfm + 7000 cfm = 10,000 cfm

Resulting DB are RH are the weighted average of condition of the two mixing fluids. Therefore,

Resulting DB = [(DB)₁ (CFM)₁/ (CFM)_F] + [(DB)₂ (CFM)₂/ (CFM)_F]

= 70(3000/10000) + 85(7000/10000)

= 21 + 59.5

= 80.5 DB

Resulting RH = [(RH)₁ (CFM)₁/ (CFM)_F] + [(RH)₂ (CFM)₂/ (CFM)_F]

= 60(3000/10000) + 50(7000/10000)

= 18 + 35

= 53 RH

Hence, resulting mixture has Dry Bulb temperature (DB) = 80.5 F and Relative Humidity (RH) = 53 %