i. Outside design conditions 39°C DBT, 28°C WBT

ii. Inside design conditions 25°C DBT, 50% RH

iii. Solar heat gain through glass 5.52kW

iv. Solar heat gain through walls, roof and floor 5.87kW

v. Occupants 32

vi. Sensible heat gain per person 57W

vii. Latent heat gain per person 57W

viii. Internal lighting load 10 lamps of 100 W 7 lamps of 60 W 9 fluorescent tubes of 40 W

ix. Infiltrared air 15 CMM

If 25% fresh air and 75% recirculated air is mixed and passed through the conditioner coil find

a) The amount of total air required in m3/hr

b) The DPT of the coil

c) The condition of supply air to the room

d) The capacity of conditioning plant

Subject:- Refrigeration and Air Conditioning

Topic:- Design of air conditioning systems

Difficulty:- Low

$t_d1=39℃,t_w1=28℃, t_d2=25℃, RH_2=50%,BPF=0.2$

Locate points 1 and 2 on psychrometric chart, and draw vertical from 1 and horizontal from 2 to obtain point A.

Let $Q ̇_si$ be rate at which sensible heat is added by infiltrated air=$m ̇_i ×(hA-h2)$

$m ̇_i=\frac{V ̇_ai}{v_1} =\frac{0.25}{0.92}=0.2717 kg/s$

∴$Q ̇_si=0.2717×(65.5-51)=3.94 \frac{kJ}{s}$

Let $Q ̇_li$ be rate at which latent heat is added by infiltrated air= $m ̇_i ×(h1-hA)$

∴$Q ̇_li=0.2717×(94.5-65.5)=7.8793 \frac{kJ}{s}$

Now RSH=$5.87+5.52+\frac{(32×57)}{1000}+\frac{10×100+7×60+9×40}{/1000}+Q ̇_{si}$=18.934 kW

RLH=$\frac{(32×57)}{1000}+Q ̇_li$=9.7033 kW

RSHF=$\frac{RSH}{RSH+RLH}$=0.66

Now draw RSHF line. To locate point 3, we know it lies on line 1-2

$\frac{m ̇_1}{m ̇_3} =\frac{length(23)}{length(12)}$

$\frac{m ̇_1}{m ̇_3}$ =0.25 (given)

length(23)=1.7 cm

Now mark point 3 and assume coil ADP as 5°C and mark point C on saturation curve. Let 4 be the point of intersection of C-3 line and RSHF line.

BPF=$\frac{length(C4)}{length(C3)}$ =1.7/8.4=0.202 ,hence we proceed with this

Calculations:

By energy balance of room

$m ̇_4×h4+RSH+RLH=m ̇_2×h2$

But from the layout, $m ̇_4=m ̇_2$

$m ̇_4$=1.1932 kg of dry air/sec

Also,$v_4=0.81 \frac{m3}{kg}$

∴ $m ̇_4=\frac{V ̇_a4}{v_4}$

$V ̇_a4$=total air required in $\frac{m3}{hr}$=3479.43 $\frac{m3}{hr}$

DPT of coil= 5°C (by trial and error approach)

Supply air conditions i.e. point 4

$t_d4=9.8℃,t_w4=9℃, RH_4=90%$

Capacity of conditioning plant,

$Q ̇_a=m ̇_3×(h3-h4)$

Also, $m ̇_3=m ̇_4$

3.5×capacity=$m ̇_4×(62-27)$

Capacity=11.932 TR