. The room condition is 26°C DBT and 50% relative humidity. The outdoor condition is 40°C DBT and 50% relative humidity. The ventilation requirement is such that on mass flow rate basis 25% of fresh air is introduced and 75% of supply air is recirculated. The bypass factor of cooling coil is 0.15. Determine:

i. Supply air flow rate

ii. Outside air sensible heat

iii. Outside air latent heat

iv. Grand total he

Subject:- Refrigeration and Air Conditioning

Topic:- Design of air conditioning systems

Difficulty:- Low

$t_d2=26℃,∅2=50%, t_d1=40℃, ∅1=50%,$

RSH=25 kW,RLH=7.5 kW, BPF=0.15

Locate points 1 and 2 on psychrometric chart.

RSHF=$\frac{RSH}{(RSH+RLH)}$=0.77

Mark RSHF on sensible heat factor scale as ‘a’ and join with alignment circle. Draw line 2-5 which is RSHF line parallel to line ab.

Since 25% of fresh air is mixed with 75 % of supply air before entering cooling coil, therefore mark this point 3 on line 1-2 as length(23)=length $(12)*0.25$

Mark 3 and through 3 draw a line 3-6 called GSHF line intersecting RSHF line at point 4 and saturation curve and point 6 such that

BPF=0.15=$\frac{length(46)}{length(36)}$

Hence mark 4.

i) Supply air flow rate

$m ̇_4=\frac{V ̇_4}{v_4} =\frac{V ̇_4}{(0.825 m3/kg)}$……..(1)

By energy balance of room,

$m ̇_4*h4+RSH+RLH=m ̇_2*h2 ……. (m ̇_4=m ̇_2)$

And h2= $53\frac{kJ}{kg}$ of dry air , h4=$\frac{36.5kJ}{kg}$ of dry air

m ̇_4=1.97 kg/sec, substituting in equation (1)

$V ̇_4=1.625 \frac{m^3}{sec}=97.5 \frac{m^3}{min}$

ii)Outside air sensible heat

Outside air flow rate = $0.25 * 97.5=24.375 m^3/min$

OASH=$ 0.02044*24.375*(t_d1- t_d2) =6.975 kW$

iii)Outside air latent heat

From chart, ω1=0.0234 $\frac{kg}{(kg \ of \ dry \ air)}$ and ω2=0.0108$\frac{kg}{(kg \ of \ dry \ air)}$

OALH= $50*24.375*(ω_1- ω_2) $=15.35 kW

iv)Grand total heat

We know, total sensible heat = RSH+OASH=31.975 kW

Total latent heat = RLH+OALH= 22.85 kW

Grand total heat=TSH+TLH=31.975+22.85=54.825 kW